Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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34118 Publications

The occurrences of Ca2UO2(CO3)3 in Fe(II) containing deep groundwater at Forsmark, eastern Sweden.

Tullborg, E.-L.; Suksi, J.; Geipel, G.; Krall, L.; Auqué, L.; Gimeno, M.; Puigdomenech, I.

Elevated U concentrations, most evident in a section ~500 mbsl, have been measured in deep Fe(II)-containing groundwater at Forsmark, eastern Sweden and have prompted detailed geochemical and isotopic investigations. The highest U contents (up to 175µg/L) are associated with HCO3- of 120-135 mg/L and Ca2+ of 900-1050 mg/L. Geochemical modelling shows that elevated dissolved U can be stabilized by Ca-uranyl-carbonate complexes. Indeed, time resolved luminescence spectrometry confirmed the Ca2UO2(CO3)3^0 complex, which is identified in deep reducing groundwater for the first time. U isotopes have been monitored in several sections with high U, and show stable but fracture specific activity ratios (ARs) around 1.5 to 2, although the U concentration varies. This is explained by mobilization of a solid phase with the same AR present in the fracture system close to the sampled sections. The AR >1 in this solid phase indicates a Quaternary age.

Keywords: Ca-uranyl-carbonate complexes; groundwater; uranium, luminescence spectrometry, 234U/238U AR; reducing condtions

Permalink: https://www.hzdr.de/publications/Publ-24974
Publ.-Id: 24974


In vivo examination of an injectable hydrogel system crosslinked by peptide-oligosaccharide interaction in immunocompetent nude mice

Tondera, C.; Wieduwild, R.; Röder, E.; Werner, C.; Zhang, Y.; Pietzsch, J.

Hydrogels can serve as matrices to mimic natural tissue function and be used for wide-ranging applications such as tissue regeneration and drug delivery. Injectable hydrogels are particularly favorable because their uses are minimally invasive. However, to create mouldable substance for injection often results in compromised function and stability. Here we report an injectable hydrogel system crosslinked by peptide-oligosaccharide non-covalent interaction. The dynamic network showed fast self-healing, a property essential for injectability. Injected hydrogels in immunocompetent mice and release of encapsulated compound were monitored up to 9 months by magnetic resonance imaging (MRI) and optical imaging. This surprisingly stable hydrogel did not cause adverse inflammatory response, as analyzed by measuring cytokine levels, immunohistochemistry, and MRI. Hydrogel degradation is associated with invasion of macrophages and vascular formation. The facile synthesis, high biocompatibility and stability of this injectable hydrogel could lead to various experimental and clinical applications in regenerative medicine and drug delivery.

Keywords: biomaterials; injectable hydrogels; magnetic resonance imaging; fluorescence imaging; immunohistochemistry

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Permalink: https://www.hzdr.de/publications/Publ-24973
Publ.-Id: 24973


Implementing fluid dynamics obtained from GeoPET in reactive transport models

Lippmann-Pipke, J.; Eichelbaum, S.; Kulenkampff, J.

Flow and transport simulations in geomaterials are commonly conducted on high-resolution tomograms (µCT) of the pore structure or stochastic models that are calibrated with measured integral quantities, like break through curves (BTC). Yet, there existed virtually no method for experimental verification of the simulated velocity distribution results.
Positron emission tomography (PET) has unrivaled sensitivity and robustness for non-destructive, quantitative, spatio-temporal measurement of tracer concentrations in body tissue. In the past decade, we empowered PET for its applicability in opaque/geological media – GeoPET (Kulenkampff et al.; Kulenkampff et al., 2008; Zakhnini et al., 2013) and have developed detailed correction schemes to bring the images into sharp focus. Thereby it is the appropriate method for experimental verification and calibration of computer simulations of pore-scale transport by means of the observed propagation of a tracer pulse, c P ET (x, y, z, t).
In parallel, we aimed at deriving velocity and porosity distributions directly from our concentration time series of fluid flow processes in geomaterials. This would allow us to directly benefit from lab scale observations and to parameterize respective numerical transport models. For this we have developed a robust spatiotemporal (3D+t) parameter extraction algorithm. Here, we will present its functionality, and demonstrate the use of obtained velocity distributions in finite element simulations of reactive transport processes on drill core scale.

Kulenkampff, J., Gruendig, M., Zakhnini, A., Gerasch, R., and Lippmann-Pipke, J.: Process tomography of diffusion with PET for evaluating anisotropy and heterogeneity, Clay Minerals, in press.
Kulenkampff, J., Gründig, M., Richter, M., and Enzmann, F.: Evaluation of positron emission tomography for visualisation of migration processes in geomaterials, Physics and Chemistry of the Earth, 33, 937-942, 2008.
Zakhnini, A., Kulenkampff, J., Sauerzapf, S., Pietrzyk, U., and Lippmann-Pipke, J.: Monte Carlo simulations of GeoPET experiments: 3D images of tracer distributions (18-F, 124-I and 58-Co) in Opalinus Clay, anhydrite and quartz, Computers and Geosciences, 57 183-196, 2013.

Keywords: positron emission tomography; segmentation; geochemical transport modelling; transport experiments

  • Poster
    EGU General Assembly 2016, 18.-22.04.2016, Wien, Österreich

Permalink: https://www.hzdr.de/publications/Publ-24972
Publ.-Id: 24972


Nonnormality of shear flows, transient growth, and its effects on helical and azimuthal magnetorotational instabilities

Mamatsashvili, G.

Magnetized shear flows are ubiquitous in nature and laboratory. The combined action of the magnetic field and velocity shear triggers various important instabilities and dynamical phenomena, ultimately determining transition to turbulence and global evolution of the flows. Here we focus on magnetorotational instability (MRI), and its non-ideal/resistive variants – helical and azimuthal magnetorotational instabilities (HMRI, AMRI) that arise as a result of combined action of magnetic field and differential rotation. It is one of the main instability responsible for turbulence and outward transport of angular momentum in magnetized astrophysical discs, which represent special case of shear flows. The discs are in Keplerian rotation with decreasing angular velocity and increasing angular momentum, which are Rayleigh-stable. However, it is well know that such spectrally stable hydrodynamical shear flows are non-normal or non-self-adjoint and as a consequence perturbations can undergo substantial transient, or non-modal (non-exponential) amplification in there. Since one of the main factors driving MRI in the magnetized case is also shear, the effects of non-normality inevitably influence the dynamics of MRI (i.e., the main linear equations describing MRI contain shear and are therefore non-normal) and should be taken into account. We investigate in detail the shear/non-normality-induced, or non-modal dynamics (growth) of HMRI/AMRI, which dominates at intermediate (dynamical/orbital) times, versus its modal growth that dominates at large times. We show that the non-modal growth of MRI can exceed its modal growth in a range of parameters. This can have implications for nonlinear transition. Interesting connection between the modal growth of HMRI and purely hydrodynamical non-modal growth was identified.

Keywords: Nonnormality of shear flows; transient growth; magnetorotational instability; transition to turbulence

  • Invited lecture (Conferences)
    Seminar at ZARM, University of Bremen, 05.-06.09.2016, Bremen, Germany

Permalink: https://www.hzdr.de/publications/Publ-24971
Publ.-Id: 24971


MHD turbulence in shear flows: self-sustenance via interplay of nonmodal growth and nonlinear transverse cascade

Mamatsashvili, G.

We find and investigate via numerical simulations self-sustained two-dimensional turbulence in a magnetohydrodynamic flow with a maximally simple configuration: plane, noninflectional (with a constant shear of velocity) and threaded by a parallel uniform background magnetic field. This flow is spectrally stable, so the turbulence is subcritical by nature and hence it can be energetically supported just by transient growth mechanism due to shear flow nonnormality. This mechanism appears to be essentially anisotropic in spectral (wavenumber) plane and operates mainly for spatial Fourier harmonics with streamwise wavenumbers less than a ratio of flow shear to the Alfv\'{e}n speed, k_y < S/u_A (i.e., the Alfv\'{e}n frequency is lower than the shear rate). We focused on the analysis of the character of nonlinear processes and underlying self-sustaining scheme of the turbulence, i.e., on the interplay between linear transient growth and nonlinear processes, in spectral plane. Our study, being concerned with a new type of the energy-injecting process for turbulence -- the transient growth, represents an alternative to the main trends of MHD turbulence research. We find similarity of the nonlinear dynamics to the related dynamics in hydrodynamic flows -- to the \emph{bypass} concept of subcritical turbulence. The essence of the analyzed nonlinear MHD processes appears to be a transverse redistribution of kinetic and magnetic spectral energies in wavenumber plane [as occurs in the related hydrodynamic flow, see Horton et al., Phys. Rev. E {\bf 81}, 066304 (2010)] and differs fundamentally from the existing concepts of (anisotropic direct and inverse) cascade processes in MHD shear flows.

Keywords: Nonmodal growth; MHD turbulence; shear flow nonnormality

  • Lecture (Conference)
    Max Planck Princeton Center (MPPC) workshop in Plasma Physics, 12.-15.01.2016, Berlin, Germany

Permalink: https://www.hzdr.de/publications/Publ-24970
Publ.-Id: 24970


Nonmodal dynamics of helical magnetorotational instability

Mamatsashvili, G.; Stefani, F.

The helical magnetorotational instability is known to work for resistive rotational flows with comparably steep negative or extremely steep positive shear. The corresponding lower and upper Liu limits of the shear are continuously connected when some axial electrical current is allowed to flow through the rotating fluid. Using a local approximation we demonstrate that the magnetohydrodynamic behavior of this dissipation-induced instability is intimately connected with the nonmodal growth of the underlying purely hydrodynamic problem. We also present preliminary results on the nonlinear development (saturation) of helical magnetorotational instability and characterize the saturation amplitude as a function of flow Reynolds number.

Keywords: helical magnetorotational instability; nonmodal growth; shear flow nonnormality

  • Lecture (Conference)
    17th MHD days, 30.11.-02.12.2016, Goettingen, Germany

Permalink: https://www.hzdr.de/publications/Publ-24969
Publ.-Id: 24969


The influence of interface curvature on solutal Marangoni convection in the Hele-Shaw cell

Mokbel, M.; Schwarzenberger, K.; Aland, S.; Eckert, K.

We study numerically the impact of interfacial curvature onto the Marangoni convection in a two-layer system of immiscible liquids with mass transfer of an alcohol. Assuming a parabolic velocity profile and constant solute concentration across the gap, the simulations solve Navier-Stokes equations coupled to advection-diffusion equations in both phases. Interfacial curvature imposes concentration gradients along the interface as soon as the mass transfer starts. This leads to an immediate interfacial convection which is superimposed later by the onset of the actual Marangoni roll cells. Prominent impact of interfacial curvature onto the Marangoni cells is the occurrence of a locking effect, i.e., the Marangoni roll cells adapt to the shape of the interface. Whereas mass transfer is enhanced by interfacial curvature compared to the planar interface in the beginning, locking drastically reduces the mass transfer rate. Even for small interface curvature significant differences to the planar case are found, which might explain the accelerated growth of cells in experiments compared to that in numerical simulation recently observed in Koellner et al. [EJP ST 224 (2015), 261-276].

Keywords: Hele-Shaw Cell; Marangoni Convection; Finite Element Method; Interface Curvature

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Permalink: https://www.hzdr.de/publications/Publ-24968
Publ.-Id: 24968


Perspektiven des neuen Dialyse-Membranverfahrens Aus dem Labor zu einer technischen Pilotanlage FCM GmbH: Galliumarsenid-Hersteller Dialyseverfahren für das Gallium-Recycling

Stelter, M.; Zeidler, O.; Scharf, C.; Eichler, S.

Entwicklung einer effizienten und wirtschaftlichen Trennungstechnologie von Ga aus GaAs-führenden Abwässern aus der Wafer-Herstellung

Der Verbrauch des Seltenen Metalls Gallium steigt seit Jahren exponentiell. Ein Großteil wird in der Produktion von Galliumarsenid eingesetzt, das als Halbleiterwerkstoff in der drahtlosen Kommunikation unverzichtbar ist.
Das Helmholtz-Institut Freiberg hat zusammen mit der TUBAF ein innovatives Recyclingver-fahren entwickelt, das die Recyclingquote des strategisch wertvollen Elementes in der Halb-leitererzeugung um 20% erhöhen kann. Beim Projektpartner Freiberger Compound Materials, einem Global Player für GaAs-Wafer, entspricht dies bereits 2-3 Tonnen Gallium pro Jahr mit ca. 0,5 Mio € Marktwert. Dafür wurde vom Bundesministerium für Wirtschaft und Energie (BMWi) der „Deutsche Rohstoffeffizienz-Preis 2014“ verliehen.
Das entwickelte Membranverfahren reinigt verbrauchte Ätzlösungen und Suspensionen, die bei der Fertigung von Wafern und ihrer Weiterverarbeitung von Mikrochips anfallen. Dies ermöglicht eine direkte Gewinnung aus der Lösung, welche bisher nicht ökonomisch war. Bei der Verfahrensentwicklung wurde von Anfang an auf die Ökonomie des entwickelten Pro-zesses geachtet. Da Störstoffe wie Arsensäure mit gewöhnlichem Wasser von der Gallium-lösung getrennt und wie gewohnt neutralisiert werden, entsteht kein zusätzlicher Energie- oder Chemikalienverbrauch. Dafür kommen spezielle Membranen zum Einsatz, die eine effiziente und selektive Trennung erzielen und chemisch hochresistent sind.
Eine Prefeasibility-Study ergab, dass mit Investitionskosten unter 100T Euro und geringen Membrankosten in einem Bereich von 5T € pro Jahr geringe Amortisationsdauern erzielbar sind. Das Projekt befindet sich momentan in der Pilotierungsphase und wird weiter technolo-gisch optimiert. Nach der Einführung in den technischen Maßstab könnte das Verfahren auch für das Recycling anderer Elemente weiterentwickelt werden.

  • Poster
    Woche der Umwelt 2016, 07.-8.6.2016, Berlin, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-24967
Publ.-Id: 24967


Application of Helium Ion Microscopy to study radiation damage

Hlawacek, G.; Veligura, V.; Bali, R.

Helium Ion Microscopy (HIM) is well known for its exceptional imaging and nanofabrication capabilities [1]⁠. After a brief introduction of this relatively young technique, I will focus on the localized creation of defects in metals and insulators.
Point defects created by the impinging He beam can be exploited to create sub–micron sized luminescent areas in appropriate materials (NaCl) [2]⁠. This combination of the nano sized beam of the HIM with ionoluminescence also allows to study fundamental processes of the defect formation and interaction [3]⁠. A technological relevant application of low fluence irradiation in the HIM is the formation of arbitrary shaped nano scale ferromagnetic areas in an otherwise para-magnetic matrix. In the particular case Fe60Al40 has been irradiated with fluences of only 6×1014cm-2 to create spin valve structures with a critical spacer length of only 20 nm [4]⁠.
Going beyond normally used ion doses allows to investigate defect agglomeration, blister formation and the subsequent surface restructuring [5]⁠. We present examples of materials modification at doses starting from 1×1017 cm−2 up to 1×1020 cm−2. Examples of surface structures formed under extreme ion fluencies at different temperatures will be presented for a wide range of materials including technological relevant materials for nuclear applications (Gold, Tungsten, Iron).

References:
[1] G. Hlawacek, V. Veligura, R. van Gastel, and B. Poelsema, J. Vac. Sci. Technol. B Microelectron. Nanom. Struct. 32, 020801 (2014).
[2] V. Veligura, G. Hlawacek, U. Jahn, R. van Gastel, H. J. W. Zandvliet, and B. Poelsema, J. Appl. Phys. 115, 183502 (2014).
[3] V. Veligura, G. Hlawacek, R. van Gastel, H. J. W. Zandvliet, and B. Poelsema, J. Phys. Condens. Matter 26, 165401 (2014).
[4] F. Röder, G. Hlawacek, S. Wintz, R. Hübner, L. Bischoff, H. Lichte, K. Potzger, J. Lindner, J. Fassbender, and R. Bali, Sci. Rep. 5, 16786 (2015).
[5] V. Veligura, G. Hlawacek, R. P. Berkelaar, R. van Gastel, H. J. W. Zandvliet, and B. Poelsema, Beilstein J. Nanotechnol. 4, 453 (2013).

  • Invited lecture (Conferences)
    CAARI - The Conference on Application of Accelerators in Research and Industry, 01.11.2016, Fort Worth, USA

Permalink: https://www.hzdr.de/publications/Publ-24966
Publ.-Id: 24966


Revealing crystal orientation and defects in the Helium Ion Microscope using channeling

Hlawacek, G.; Veligura, V.; Jankowski, M.

Helium Ion Microscopy (HIM) is well known for its exceptional imaging and nanofabrication capabilities [1]⁠. HIM has an unprecedented surface sensitivity, and channeling can be utilized to maximize the signal to noise ratio, obtain information on the crystal structure and reveal defects such as dislocation networks.
Using a poly crystalline gold sample we show how channeling can be used to obtain crystallographic information in the HIM [2]⁠.
We demonstrate the resolving power of this technique using a thin (2 ML) silver layer on Pt(111). This is is representative example of a surface confined alloy widely studied in the field surface science. The obtained HIM results are compared to results obtained by low energy electron microscopy, spot profile analysis low energy electron diffraction (SPA-LEED), and atomic force microscopy phase contrast. In HIM single atom layer high steps can be visualized as a result of a work function change—across the otherwise atomically flat terraces—of only 20 meV. By utilizing the dechanneling contrast [3]⁠ mechanism also the surface reconstruction of this thin surface layer can be revealed. We find a threefold periodic structure of channeling (fcc stacking) and dechanneling (hcp stacking) areas. The periodicity of this structure—measured along the <11-2> surface directions—is 6.65 nm [4]⁠. This is in excellent agreement with values obtained by SPA-LEED.

  • Lecture (Conference)
    CAARI - The Conference on Application of Accelerators in Research and Industry, 03.10.2016, Fort Worth, USA

Permalink: https://www.hzdr.de/publications/Publ-24965
Publ.-Id: 24965


Spatially controlled ripple formation in the HIM using Low Voltages and High Temperatures

Hlawacek, G.; Sottili, L.; Engler, M.; Nanda, G.; van Alkemade, P.; Facsko, D.

Ripple formation is a well known phenomenon that is observed for many materials under low energy ion bombardment. Often broad beam noble gas ion irradiation using energies of a few keV is employed to create these self-organized patterns on various metal, semiconductor and insulator surfaces. In addition to the fundamental interest in the formation and evolution of these structures they can be utilized in a number of new applications.
Creating nano scale periodic roughness can be of interest for various microfluidic applications or to control friction in new MEMS and NEMS devices. However, these applications are not realized at their full potential today as the required sub micron patterning which can not easily be realized using broad beams.
Here, we present for the first time ripple patterns that have been created on the GaAs(001) surface using 5 keV Ne ions and elevated temperatures of up to 600 K in a Helium Ion Microscope (HIM). We will present the home built sample heater that can be loaded through the load lock of the Carl Zeiss Orion NanoFab and describe the influence on the device performance, as well as HIM operation at 5 keV.
The evolution of the ripple wavelength changes from 30 nm at low 1e17 Ne/cm² to 80 nm at 1e18 Ne/cm². The orientation of the ripples with respect to the shape can be changed by rotating the pattern on the surface and the influence of the geometrical constrains of the irradiated area on the ripple pattern is studied.

  • Lecture (Conference)
    AVS - International Symposium and Exhibition, 10.11.2016, Nashville, USA

Permalink: https://www.hzdr.de/publications/Publ-24964
Publ.-Id: 24964


Helium-ion microscopy, helium-ion irradiation and post-irradiation nanoindentation of the 9%Cr steel Eurofer97 and oxide dispersion strengthened (ODS) Eurofer

Bergner, F.; Hlawacek, G.; Heintze, C.

Helium embrittlement poses a major challenge for the application of ferritic/martensitic steels in future nuclear devices. The strategy for an effective suppression of helium embrittlement is to reduce the mean free diffusion path of helium by way of introducing nanoparticles into the steel. The study is aimed at clarifying the role of oxide nanoparticles on the formation of helium bubbles and related hardening for the case of He-ion irradiations performed in the He-ion microscope (HIM).

  • Poster
    E-MRS 2016 Fall Meeting, 19.-22.09.2016, Warsaw, Poland

Permalink: https://www.hzdr.de/publications/Publ-24963
Publ.-Id: 24963


Charge Collection Efficiency in Segmented Semiconductor Detector Interstrip Region

Alarcon-Diez, V.; Vickridge, I.; Jakšić, M.; Grilj, V.; Schmidt, B.

Charged particle semiconductor detectors have been used in Ion Beam Analysis (IBA) for over four decades without great changes in either design or fabrication. However one area where improvement is desirable would be to increase the detector solid angle so as to improve spectrum statistics for a given incident beam fluence. This would allow use of very low fluences opening the way for example to increased time resolution in real-time RBS or analysis of materials that are highly sensitive to beam damage. In order to achieve this goal without incurring the costs of degraded resolution due to kinematic broadening or large detector capacitance, a single-chip segmented detector (SEGDET) was designed and built within the SPIRIT EU infrastructure project. In this work we present the Charge Collection Efficiency (CCE) in the vicinity between two adjacent segments focusing on the interstrip zone. Microbeam Ion Beam Induced Charge (IBIC) was used to perform X-Y mapping of CCE with different ion masses and energies, as a function of detector operating conditions (bias voltage changes, detector housing possibilities and guard ring configuration). We show the CCE in the active area edge region and have also mapped the charge from the interstrip region, shared between adjacent segments. The results indicate that the electrical extent of the interstrip region is very close to the physical extent of the interstrip and guard ring structure with interstrip impacts contributing less than 8% to the complete spectrum. The interstrip contributions to the spectra can be substantially reduced by an offline anticoincidence criterion through the list mode data analysis, which should also be easy to implement directly in the data acquisition software.

Keywords: silicon detectors; ion beam analysis; segmented detectors

  • Lecture (Conference)
    12th European Conference on Accelerators in Applied Research and Technology (ECAART12), 03.-08.07.2016, Jyväskylä, Finland
  • Poster
    12th European Conference on Accelerators in Applied Research and Technology (ECAART12), 03.-08.07.2016, Jyväskylä, Finland

Permalink: https://www.hzdr.de/publications/Publ-24962
Publ.-Id: 24962


Nanometer Scale Time of Flight Back Scattering Spectrometry in the Helium Ion Microscope

Klingner, N.; Hlawacek, G.; Heller, R.; von Bornay, J.; Facsko, S.

TOF-BS and TOF-SIMS

  • Poster
    EMC2016 – The 16th European Microscopy Congress, 28.08.-03.09.2016, Lyon, Frankreich
  • Poster
    Microscopy & Microanalysis 2016 Meeting (M&M2016), 24.-28.07.2016, Columbus, USA

Permalink: https://www.hzdr.de/publications/Publ-24961
Publ.-Id: 24961


Effects of baffles on bubble rising behaviour – extending bubble residence time and mass transfer

Herrmann-Heber, R.; Sommer, A.-E.; Reinecke, S.; Hampel, U.; Mohseni, E.

Up to 80 % of the total energy budget of wastewater treatment plants is consumed by the activated sludge process. To improve the energy efficiency the specific standard oxygen transfer rate (SSOTE) must be increased. This can be achieved by increasing the overall oxygen mass transfer inside the aeration tank. Oxygen mass transfer happens from the gaseous phase of the bubble to the liquid phase of the activated sludge. The amount of transferred oxygen depends on various parameters like interfacial area, surface active substances, concentration gradients, turbulences and bubble residence time. However, parameters such as the interfacial area cannot be changed without additional energy usage.

Our approach is to increase the length of the bubble rising path and to decelerate the bubbles by inserting baffles in the activated sludge tank. The overall oxygen mass transfer is then extended due to the longer bubble residence time in the aeration tank. Furthermore, the bubbles can be guided in their rise inside the aeration tank. By directing bubbles into less oxygenated sludge higher concentration differences occur which lead to higher local oxygen mass transfer.

Hydrophobic and hydrophilic materials were selected for the laboratory experiments to investigate their effects on the bubble rising behaviour. The target parameters were bubble size distribution, equivalent Sauter mean diameter of the bubbles, bubble rise velocity, bubble residence time, local and global mass transfer coefficient under the variation of wetting behaviour of the material (hydrophilic, hydrophobic), roughness profile of the material, inclination angle and length of plate. The wetting characteristics of these plates were also tested within activated sludge to analyse the influence of a developed biofilm to the surface characteristics of the plates. A variety of materials with different surface properties where investigated: glass, PVC, PTFE, stainless steel and various coated and treated stainless steel plates.

Keywords: waste water treatment; bubble rise; bubble residence time; baffles

  • Poster
    The 9th Eastern European Young Water Professionals Conference, 24.-27.05.2017, Budapest, Ungarn
  • Contribution to proceedings
    The 9th Eastern European Young Water Professionals Conference, 24.-27.05.2017, Budapest, Ungarn
    9th Eastern European Young Water Professionals Conference - CONFERENCE PROCEEDINGS Uniting Europe for Clean Water: Cross-Border Cooperation of Old, New and Candidate Countries of EU for identifying problems, finding causes and solutions, Budapest, 433-440

Permalink: https://www.hzdr.de/publications/Publ-24960
Publ.-Id: 24960


Tailoring magnetic nanostructures with neon in the ion microscope

Hlawacek, G.; Bali, R.; Röder, F.; Aleksandrov, Y.; Semisalova, A.; Wintz, S.; Wagner, K.; Schultheiss, H.; Fassbender, J.

In the HIM high fluences of Neon are often used to create structures on the nanoscale. This is not always necessary and certain properties of specific materials systems can be changed using much smaller fluences. The magnetic properties of specific materials as well as the properties connected to the shape of magnets can be influenced.

  • Poster
    1st International Conference on Helium Ion Microscopy and Emerging Focused Ion Beam Technologies (HEFIB 2016), 08.-10.06.2016, Luxemburg, Luxembourg
  • Lecture (Conference)
    Microscopy & Microanalysis 2016 Meeting (M&M2016), 27.07.2016, Columbus, USA

Permalink: https://www.hzdr.de/publications/Publ-24959
Publ.-Id: 24959


Helmholtz-Zentrum Dresden-Rossendorf Ion Beam Center

Hlawacek, G.; Heinig, K.-H.

Introduction IBC

  • Poster
    ECSEL Forum, 03.05.2016, Brüssel, Belgien

Permalink: https://www.hzdr.de/publications/Publ-24958
Publ.-Id: 24958


Writing nanoscale magnets with neon using a gas field ion source microscope

Hlawacek, G.; Röder, F.; Semisalova, A.; Bali, R.; Wintz, S.; Hübner, R.; Bischoff, L.; Lichte, H.; Potzger, K.; Lindner, J.; Fassbender, J.

Writing nanoscale magnets with neon using a gas field ion source microscope

  • Lecture (Conference)
    DPG-Frühjahrstagung 2016, 09.03.2016, Regensburg, Deutschland
  • Lecture (Conference)
    11th DACH FIB Workshop, 27.06.2016, Brünn, Tschechien

Permalink: https://www.hzdr.de/publications/Publ-24957
Publ.-Id: 24957


Gas Dispersion in Biological Wastewater Treatment – Smaller Bubbles for More Efficiency

Herrmann-Heber, R.; Mohseni, E.; Reinecke, S.; Hampel, U.

In Germany wastewater is treated in 10000 plants which use 4.400 GWh of energy per year. Up to 80% of the whole energy in these plants is consumed for the aeration of microorganisms in the so called activated sludge tank. Aeration is the essential part of the process since the microorganisms need sufficient amount of oxygen to degrade ammonia. Aerators with flexible membranes located at the bottom of the aeration tank are currently state of the art for this process. However the process suffers from some limitation such as pressure drop, insufficient mixing and underutilization of oxygen. These are mainly due to scarce knowledge about gas dispersion inside the tank. The type of aerator defines the initial size of bubbles dispersed into the tank. The typical bubble size generated by flexible membrane aerators has been determined to be between 2-4 mm by Hasanen et. al. However, this is considerably higher than the optimal bubble size. Motarjemi and Jameson have calculated the optimal value for the 95% oxygen transfer to water in 3-6 m tank depth to be in the range 0.7…1.0 mm.
A novel approach uses solid perforated stainless steel aerators for gas dispersion. In this work bubble formation of stainless steel aerators has been experimentally studied for comparison to membrane aerators. Preliminary results of the stainless steel membrane sparger showed a significant reduction in the bubble size by 47 % and consequently an increase in bubble residence time compared to flexible membranes. Moreover, uniform bubble size has been generated across the sparger which is not the case for membrane aerators. The pressure drop of these novel aerators is compared with flexible membranes. Mass transfer measurements were done in a large scale bubble column setup under realistic process conditions.

Keywords: waste water treatment; gas dispersion; mass transfer; bubble rise; bubble size

  • Poster
    Jahrestreffen der ProcessNet-Fachgruppen Mehrphasenströmungen, Partikelmesstechnik, Zerkleinern und Klassieren, Computational Fluid Dynamics, Mischvorgänge und dem TAK Aerosoltechnologie, 14.-17.03.2017, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-24956
Publ.-Id: 24956


Towards an analytic ion microscope

Hlawacek, G.

Review of the various analytical possibilities in the HIM

  • Lecture (others)
    HZB Seminar, 09.02.2016, Berlin, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-24955
Publ.-Id: 24955


Microbial diversity extreme saline environments

Franze, M.; Cherkouk, A.

Backgrounds: Rock salt formations are considered as potential host rock systems for the long-term storage of highly radioactive waste in a deep geological repository. To date, little is known about the habitat rock salt and the way of life of the microorganisms occurring there. Next to bacteria and fungi, extreme halophilic archaea are dominating this habitat. It is of interest to know what kind of microorganisms are living there, how active they are under repository relevant conditions and how these microorganisms can influence the safe storage of the waste. Objectives: A combination of culture-dependent and -independent methods was used to investigate the microbial diversity in rock salt from potential host rock for nuclear waste disposal as well as saline soil samples from Arava Desert, Israel. Methods: Culture-dependent: A specific portion of the two kinds of samples were incubated in three different sodium chloride concentrations of modified R2A resuscitation buffer and were spread on corresponding agar plates (37°C) to get isolates which were further characterized. Culture-independent: From two samples DNA was extracted, purified for PCR amplification of 16S rRNA genes and sequenced with Illumina MiSeq (RTL Genomics). Conclusions: Halophilic microorganisms could be isolated from both kinds of samples. The soil sample isolates can be assigned to different archaeal genera Natrinema, Halorubrum, and Halobacterium. Bacterial isolates could be related to Bacilli such as Halobacillus and Aquibacillus. From rock salt samples could be isolated different Halobacetrium species. The obtained isolates could be further used for investigations, regarding there activity under repository relevant conditions.

  • Poster
    FEMS 2017 7th congress of european microbiologists, 09.-13.07.2017, Valencia, Spanien

Permalink: https://www.hzdr.de/publications/Publ-24954
Publ.-Id: 24954


Development of highly affine and selective fluorinated cannabinoid receptor type 2 ligands

Moldovan, R.-P.; Hausmann, K.; Deuther-Conrad, W.; Brust, P.

CB2 receptors are involved in various pathological processes and the visualization of the expression level alteration with a non-invasive technique like PET is of high interest. In this work we focused on the introduction of the fluorine atom by modifying at various positions the structure of the highly affine and selective CB2 ligand N-(adamantan-1-yl)-5-ethyl-2-methyl-1-phenyl-1H-imidazole-4-carboxamide (5, Ki(CB2) = 1 nM, Ki(CB1) >10,000 nM). The highest CB2 binding affinity was obtained by derivatization of the imidazole 2-position. This study allowed the identification of compound 15 as one of the most potent (Ki(CB2) = 0.29 nM) and selective (CB1/CB2 > xx), CB2 ligand discovered so far, eligible for the development of an [18F]-PET radiotracer.

Keywords: Cannabinoid receptor type 2; Imidazole; Binding affinity; Fluorine; Positron emission tomography

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Permalink: https://www.hzdr.de/publications/Publ-24953
Publ.-Id: 24953


Development of Fluorinated Non-Peptidic Ghrelin Receptor Ligands for Potential Use in Molecular Imaging

Moldovan, R.-P.; Els-Heindl, S.; Worm, D. J.; Kniess, T.; Kluge, M.; Beck-Sickinger, A. G.; Deuther-Conrad, W.; Krügel, U.; Brust, P.

The ghrelin receptor (GhrR) is a widely investigated target in several diseases. However, the current knowledge of its role and distribution in the brain is limited. Recently, the small and non-peptidic compound (S)-6-(4-bromo-2-fluorophenoxy)-3-((1-isopropylpiperidin-3-yl)methyl)-2-methylpyrido[3,2-d]pyrimidin-4(3H)-one ((S)-9) has been described as a GhrR ligand with high binding affinity. Here, we describe the synthesis of fluorinated derivatives, the in vitro evaluation of their potency as partial agonists and selectivity at GhrRs, and their physicochemical properties. These results identified compounds (S)-9, (R)-9, and (S)-16 as suitable parent molecules for 18F-labeled positron emission tomography (PET) radiotracers to enable future investigation of GhrR in the brain.

Keywords: brain, ghrelin receptor; fluorine, positron emission tomography

Permalink: https://www.hzdr.de/publications/Publ-24952
Publ.-Id: 24952


Development of foam-based layered targets for laser-driven ion beam production

Prencipe, I.; Sgattoni, A.; Dellasega, D.; Fedeli, L.; Cialfi, L.; Choi, I. W.; Kim, I. J.; Janulewicz, K. A.; Kakolee, K. F.; Lee, H. W.; Sung, J. H.; Lee, S. K.; Nam, C. H.; Passoni, M.

We report on the development of foam-based double-layer targets (DLTs) for laser-driven ion acceleration. Foam layers with a density of a few mg cm−3 and controlled thickness in the 8–36 μm range were grown on μm-thick Al foils by pulsed laser deposition (PLD). The DLTs were experimentally investigated by varying the pulse intensity, laser polarisation and target properties. Comparing DLTs with simple Al foils, we observed a systematic enhancement of the maximum and average energies and number of accelerated ions. Maximum energies up to 30 MeV for protons and 130 MeV for C6+ ions were detected. Dedicated three-dimensional particle-in-cell (3D-PIC) simulations were performed considering both uniform and cluster-assembled foams to interpret the effect of the foam nanostructure on the acceleration process.

Keywords: ion acceleration; pulsed laser deposition; carbon foam; laser–plasma interaction; particle-in-cell

Permalink: https://www.hzdr.de/publications/Publ-24951
Publ.-Id: 24951


Inelastic scattering of fast neutrons from 56Fe

Beyer, R.; Birgersson, E.; Dietz, M.; Hannaske, R.; Junghans, A. R.; Kögler, T.; Massarczyk, R.; Matic, A.; Schwengner, R.; Wagner, A.

The inelastic scattering of fast neutrons on Fe-56 was investigated in different manners at the neutron time-of-flight facility nELBE.
The scattering cross section was determined via the measurement of the gamma-ray production and by means of a kinematically complete double time-of-flight method.
In a further measurement the gamma-ray angular distribution was determined to correct the measured cross sections for anisotropy.
The resulting inelastic scattering cross section determined from the photo production cross sections is in very good agreement with evaluations and previous measurements.
In contrast, the result of the double time-of-flight measurement is about 10 % lower than these data, giving a hint to neutron-gamma-ray angular correlations in the process of inelastic neutron scattering.

Keywords: nELBE; fast neutrons; neutron time-of-flight; inelastic scattering; iron

  • Invited lecture (Conferences)
    International Conference on Nuclear Data for Science and Technology - ND2016, 11.-16.09.2016, Bruges, Belgium
  • Open Access Logo European Physical Journal Web of Conferences 146(2017), 02017
    DOI: 10.1051/epjconf/201714602017

Permalink: https://www.hzdr.de/publications/Publ-24950
Publ.-Id: 24950


Electrical Characterization of DNA Origami Nanostructure

Bayrak, T.

DNA origami method provides programmable bottom up approach for developing of any desired shaped structures from nanoscale components and with the help of the hybridization formation of the functional nanostructures can be controllable by DNA origami nanostructure. One of the goals to design tailored bottom up based nanophotonics, optoelectronics and nanoelectronics devices based on DNA functionalization such as metallization or controlled attachment of nanoparticles to the DNA origami design. However, combination of bottom-up and top-down based methods is required for future nanoelectronic device applications. In this study, gold nanoparticles are decorated on DNA origami accumulated to SiO2 surface utilized for molecular electronics. For this novel approach, we combined bottom-up and top-down based methods, two metallic electrodes contacted on individual gold nanoparticles decorated DNA origami nanotube by electron beam lithography. The charge transport behavior of the molecule between gold nanoparticles occurs in temperature range from room temperature to 4.2 K.

  • Lecture (others)
    NanoNET PhD Seminar, 18.11.2016, IFW,Dresden, Germany

Permalink: https://www.hzdr.de/publications/Publ-24949
Publ.-Id: 24949


Hydrodynamics of descending gas-liquid flows in solid foams: Liquid holdup, multiphase pressure drop and radial dispersion

Zalucky, J.; Wagner, M.; Schubert, M.; Lange, R.; Hampel, U.

In this contribution we report on spatially resolved analysis of multiphase hydrodynamics in solid foam packed trickle bed reactors. For investigation we used ultrafast X-ray computed tomography and fast response pressure transducers. The SiSiC foams’ pore density, the liquid distribution system as well as gas and liquid flow rates were varied. The transient behavior of the liquid holdup at trickle and pulse flow as well as after drainage were examined and correlations for static and dynamic holdups were derived. The correlations are based on Eötvös, Reynolds and Galileo number, using porosity and specific area for the definition of the hydraulic diameter. The correlations are applicable to a wide range of foam morphologies, pore densities and operation conditions reported in the literature. The axial pressure gradients in the solid foams showed significantly lower pressure drop compared to particle packings of similar specific surface area. The evolution of liquid spreading was analyzed qualitatively and quantitatively with various irrigation patterns. In addition, an approach for the determination of radial liquid dispersion coefficients in solid foams is presented.

Keywords: Solid foams; Liquid holdup; Two-phase Flow; Ultrafast X-ray computed tomography; Radial dispersion; Liquid spreading

Permalink: https://www.hzdr.de/publications/Publ-24948
Publ.-Id: 24948


MHD turbulence in Keplerian disks with net azimuthal magnetic field - bypass concept of self-sustenance via interplay of linear transient growth and nonlinear transverse cascade

Mamatsashvili, G.

We studied the mechanism of the self-sustenance of MHD turbulence in spectrally stable stratified Keplerian disk flows threaded by nonzero net azimuthal magnetic field in the shearing box approximation. For this purpose, we performed direct numerical simulations of (homogeneous, subcritical) turbulence at different aspect ratios of the simulation boxes. Then, we analyzed the turbulence dynamics in Fourier/wavenumber/spectral space based on the simulation data to gain deeper insight into the self-sustaining dynamics of such subcritical MHD turbulence. Specifically, we examined the interplay of linear transient growth of Fourier harmonics and nonlinear processes. In the case of azimuthal field in the shearing box setup, the linear growth of (magnetic) perturbations has a transient nature and is strongly anisotropic in spectral space. This, in turn, leads to anisotropy of nonlinear processes in spectral space and, as a result, the main nonlinear process appears to be not a direct/inverse, but rather a transverse/angular redistribution of harmonics in Fourier space referred to as the nonlinear transverse cascade. It is demonstrated that the turbulence is sustained by interplay of the linear transient growth and the transverse cascade. The main scheme of this interplay in Keplerian disks was first proposed in Chagelishvili, Zahn, et al. (2003) and then its realization in real flows has been investigated in our recent papers Horton et al. (2010), Mamatsashvili et al. (2014, 2016). This scheme exemplifies the bypass concept of subcritical turbulence in spectrally stable shear flows. Both transient growth and transverse cascade mainly operate at large length scales, comparable to the box size. Consequently, the central, small wavenumber area of Fourier space is crucial in the self-sustenance of the turbulence and is labeled as the vital area. Outside the vital area both transient growth and transverse cascade are of secondary importance - Fourier harmonics are transferred to dissipative scales by the usual nonlinear direct cascade.

Keywords: Nonmodal approach; transient growth; turbulence; accretion disks; transverse cascade

  • Invited lecture (Conferences)
    Seminar at the Paris-Meudon Observatory, 16.-18.11.2016, Paris, France

Permalink: https://www.hzdr.de/publications/Publ-24947
Publ.-Id: 24947


Nonmodal analysis of helical magnetorotational instability

Mamatsashvili, G.; Stefani, F.

The helical and azimuthal magnetorotational instabilities (HMRI), relative of standard MRI, has recently received much attention in connection with liquid metal experiments. Its main advantage is that, being governed by Reynolds and Hartmann numbers, persists at very small magnetic Prandtl numbers (Pm) typical to liquid metals, in contrast to standard MRI. However, its applicability to Keplerian astrophysical discs with similarly small Pm is still not settled and controversial; the main hindering factor is the Liu’s criterion. Aiming to resolve this issue, we carry out nonmodal analysis of HMRI, which allows us to capture its growth at intermediate times due to the non-self-adjointness of shear flows, which can occur even when their modal growth would be absent according to Liu’s criterion.

Keywords: magnetorotationa instability; nonmodal growth; nonnormality; astrophysical Keplerian disks; liquid metals

  • Lecture (Conference)
    GdR dynamo meeting, 27.06.-01.07.2016, Barcelona, Spain

Permalink: https://www.hzdr.de/publications/Publ-24946
Publ.-Id: 24946


PET Imaging of α7 Nicotinic Acetylcholine Receptors: A comparative study of [18F]ASEM and [18F]DBT-10 in non-human primates, and further evaluation of [18F]ASEM in humans

Hillmer, A. T.; Li, S.; Zheng, M.-Q.; Scheunemann, M.; Lin, S.-F.; Nabulsi, N.; Holden, D.; Pracitto, R.; Labaree, D.; Ropchan, J.; Teodoro, R.; Deuther-Conrad, W.; Esterlis, I.; Cosgrove, K. P.; Carson, R. E.; Brust, P.; Huang, Y.

The α7 nicotinic acetylcholine receptor (nAChR) is implicated in many neuropsychiatric disorders, making it an important target for positron emission tomography (PET) imaging. The first aim of this work was to compare two PET radioligands specific to α7 nAChRs, [18F]ASEM (3-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-6-([18F]fluorodibenzo[b,d]thiophene 5,5-dioxide) and [18F]DBT-10 (7-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-2-([18F]fluorodibenzo[b,d]thiophene 5,5-dioxide), in nonhuman primates. The second aim was to further assess the quantification and test-retest variability of [18F]ASEM in humans.

Keywords: Nicotine; Nicotinic Acetylcholine Receptor; Alpha7; PET; ASEM

Permalink: https://www.hzdr.de/publications/Publ-24945
Publ.-Id: 24945


Linear stability analysis of magnetized relativistic rotating jets

Bodo, G.; Mamatsashvili, G.; Rossi, P.; Mignone, A.

We perform a linear stability analysis of magnetized rotating cylindrical jet flows in the approximation of zero thermal pressure. We focus our analysis on the effect of rotation on the current driven mode and on the unstable modes introduced by rotation. We find that rotation has a stabilizing effect on the current driven mode only for rotation velocities of the order of the Alfvén velocity. Rotation introduces also a new unstable centrifugal buoyancy mode and the `cold' magnetorotational instability. The first mode is analogous to the Parker instability with the centrifugal force playing the role of effective gravity. The magnetorotational instability can be present, but only in a very limited region of the parameter space and is never dominant. The current driven mode is characterized by large wavelengths and is dominant at small values of the rotational velocity, while the buoyancy mode becomes dominant as rotation is increased and is characterized by small wavelengths.

Keywords: instabilities; MHD; galaxies: jets

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Permalink: https://www.hzdr.de/publications/Publ-24944
Publ.-Id: 24944


Upgrade of the microprobe at Dresden-Rossendorf

Munnik, F.; Hanf, D.; Heller, R.

The nuclear microprobe that was in operation until 2104 at the Ion Beam Center of the Helmholtz-Zentrum Dresden-Rossendorf was installed in 1994 [1]. It has been in operation since then with only minor changes. This necessitated an upgrade to bring the setup up to current standards of technology and good working practice. This study presents the details of the upgrade and modernization process we have undertaken.
The major drawbacks of the old system were the poor resolution and low contrast and brightness of the optical microscope. However, a good optical image is essential to localise the areas of interest on, for example, large geological samples.
On the other hand, the main investigative tool is the focused beam of high-energy ions and the corresponding detectors. Any other system such as an optical microscope has to be designed around this equipment. A new custom-designed microscope has been installed for which the first light-collecting lens is mounted in the sample chamber at only a few centimetres from the sample. The light is then guided over large mirrors and focussed on a CCD camera outside the sample chamber. Also the illumination is fed in through the lenses instead of using a separate light source as in the old system. The lens system can also serve as a basis for a possible ionoluminescence detector.
Other improvements concern the beam deflection system, the control of the scanning system and the control and monitoring of all relevant parameters for the experiment. The control of the scanning system is done by custom-designed hardware to guarantee the real-time execution of the scanning without the need for a computer with a real-time operating system. This makes it possible to use a standard Windows based computer with commercial software for the data acquisition. A new channeltron has also been installed to detect secondary electrons that can be used to obtain a quick overview of an measurement area.
Technical details and first test measurements with the new system are presented.

[1] F. Herrmann, D. Grambole, Nucl. Instr. Meth. B 104 (1995) 26.

Keywords: PIXE; nuclear microprobe

  • Poster
    15th International Conference on Particle Induced X-ray Emission, 02.-07.04.2017, Split, Kroatien

Permalink: https://www.hzdr.de/publications/Publ-24943
Publ.-Id: 24943


Single Bubble Dynamics on a Vertical Wall in Sub-cooled Nucleate Boiling

Sarker, D.; Franz, R.; Hampel, U.

Nucleate boiling heat transfer is an efficient way of heat transfer for many engineering applications, like heat exchangers, boilers, electronic cooling systems etc. Therefore, heat transfer enhancement in the nucleate boiling region has received continuous interest for a long time. For the further enhancement of heat transfer performance, fundamental physics of bubble growth and departure process should be revealed clearly. In the current study, high resolution optical instrumentation and highly efficient parallel DNS (Direct Numerical Simulation) solver are used to investigate the single bubble dynamics. Numerical results are validated against experimental results of subcooled nucleate boiling in water at atmospheric pressure, where the heated surface was vertically oriented.

Keywords: subcooled nucleate boiling; single bubble; vertical heater

  • Poster
    Jahrestreffen der ProcessNet-Fachgruppe Wärme- und Stoffübertragung, 01.-02.03.2016, Kassel, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-24942
Publ.-Id: 24942


Nucleate Boiling on Vertical Heater Wall : Role of Heater Surface Properties

Sarker, D.; Franz, R.; Ding, W.; Hampel, U.

Nucleate Boiling is an efficient mode of heat transfer for different engineering applications.In real applications, heat transfer surfaces are always rough in different scale. The effects of surface roughness on single bubble dynamics; namely bubble growth and sliding has not been thoroughly investigated. Surface roughness noticeably affects bubble growth process with the interaction of bubble base or microlayer. In the current study, single bubble growth and sliding for different roughness are investigated and the high resolution imaging techniques are used. Experimental results show that intermediate roughness (Rq=90nm) enhances bubble growth process compare to Rq=4.48 nm and Rq=410 nm. As well as heat transfer to the bubble through microlayer is also noticeably higher for intermediate roughness.

Keywords: vertical heater; high speed video camera; roughness height; microlayer

  • Poster
    Kompetenzzentrum Ost für Kerntechnik, 08.12.2016, HZDR, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-24941
Publ.-Id: 24941


Ultrasound measurements in a physical model of Czochralski crystal growth in a horizontal magnetic field

Pal, J.; Grants, I.; Eckert, S.; Gerbeth, G.

A horizontal magnetic field (HMF) may improve conditions in the melt during large silicon single crystal growth by the Czochralski technique. This observation is counter-intuitive as the HMF evidently breaks the rotational symmetry. A previous study has shown that the HMF is not able to significantly delay the Rayleigh-Bénard instability in a rotating cylinder. It has been observed that an oscillating flow sets in soon after the linear onset. Can we expect a stabilizing effect of the HMF in the Czochralski growth? Why the symmetry breaking by the HMF is eventually not so relevant? These are two central questions for our primarily experimental study. Besides, it is also meant as a benchmark for comparison with the numerical codes. To serve the latter purpose the boundary conditions should be preferably well defined. Having this in mind the temperature boundary conditions are defined as follows. An isothermal heating is applied at the bottom of a cylindrical cell filled with GaInSn alloy. The side wall is thermally insulated. An optionally rotating isothermal cooler models the growing crystal. A water-cooled layer of an alkaline solution keeps the rest of the metal surface free from oxides and models the radiation heat loss. The maximum HMF strength is 0.3 T that corresponds to a Hartmann number of about 1200. Velocity profiles are measured by ultrasound Doppler velocimetry.

Keywords: Czochralski crystal growth; Horizontal magnetic field; Ultrasound Doppler velocimetry

  • Lecture (Conference)
    10th International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering, 28.-30.09.2016, Tokyo, Japan
  • Contribution to proceedings
    10th International Conference on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering, 28.-30.09.2016, Tokyo, Japan
    Proceedings of ISUD10, 9-12

Permalink: https://www.hzdr.de/publications/Publ-24940
Publ.-Id: 24940


Physical model of Czochralski crystal growth in a horizontal magnetic field

Pal, J.; Grants, I.; Eckert, S.; Gerbeth, G.

The horizontal magnetic field (HMF) may improve conditions in the melt during large silicon single crystal growth by the Czochralski technique. This observation is counter-intuitive as the HMF evidently breaks the rotational symmetry. A previous study has shown that the HMF is not able to significantly delay the Rayleigh-Bénard instability in a rotating cylinder [1]. It has been observed [2] that an oscillating flow sets in soon after the linear onset. Can we expect a stabilizing effect of the HMF in the Czochralski growth? Why the symmetry breaking by the HMF is eventually not so relevant? These are two central questions for our primarily experimental study. Besides, it is also meant as a benchmark for comparison with the numerical codes. To serve the latter purpose the boundary conditions should be preferably well defined. Having this in mind the temperature boundary conditions are defined as follows. An isothermal heating is applied at the bottom of a cylindrical cell filled with GaInSn alloy. The side wall is thermally insulated. An optionally rotating isothermal cooler models the growing crystal. A water-cooled layer of an alkaline solution keeps the rest of the metal surface free from oxides and models the radiation heat loss. The maximum HMF strength is 0.3 T that corresponds to a Hartmann number of about 1200. Velocity profiles are measured by ultrasound Doppler velocimetry. Temperatures are monitored in the vicinity of the triple point at the rim of the cooler, at the rim of the cell, inside of the cooler and of the heater. The Nusselt-Grashof number dependency is obtained by controlling the total heat flux injected at the bottom and measuring the temperature difference between the bottom plate and the cooler. The critical cooler rotation rate is determined at which the rotation introduces a significant variation of the velocity field dominated by the HMF-aligned convection rolls.


References
1. I. Grants, G. Gerbeth, J. Cryst. Growth, 358 (2012), 43-50
2. U. Burr, U. Müller, J. Fluid Mech., 453 (2002), 345-370

Keywords: Czochralski crystal growth; horizontal magnetic field

  • Lecture (Conference)
    1st German Czechoslovak Conference on Crystal Growth, 16.-18.03.2016, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-24939
Publ.-Id: 24939


The Clinical Target Volume in Lung, Head-and-neck, and Esophageal Cancer: Lessons from Pathological Measurement and Recurrence Analysis

Apolle, R.; Rehm, M.; Bortfeld, T.; Baumann, M.; Troost, E.

Radiotherapy research has achieved remarkable progress in target volume definition. Advances in medical imaging facilitate more precise localization of the gross tumor volume, alongside a more detailed understanding of the geometric uncertainties associated with treatment delivery that has enabled robust safety margins to be customized to the specific treatment scenario at hand. By contrast, the clinical target volume, meant to encompass gross tumor, as well as, adjacent sub-clinical disease, has evolved very little. It is more often defined by clinician experience and institutional convention than on a patient-specific basis. This disparity arises from the inherent invisibility of sub-clinical disease in current medical imaging. Its incidence and expanse can only be ascertained via indirect means. This article reviews two such strategies: histopathological measurements on resection specimen and analyses of locoregional recurrences after radiotherapy.

Keywords: microscopic tumor extension; clinical target volume; adaptive radiotherapy; particle beam irradiation

Permalink: https://www.hzdr.de/publications/Publ-24938
Publ.-Id: 24938


Single Bubble Dynamics during Subcooled Nucleate Boiling on a Vertical Heater Surface: An Experimental Analysis of the Effects of Surface Characteristics

Sarker, D.; Franz, R.; Ding, W.; Hampel, U.

In the present experimental study we investigated the effects of surface characteristics, such as wettability and roughness, on nucleate boiling in de-ionized water at a vertical heater. In the experiments, bubbles were generated from an artificial nucleation site on a stainless steel heater surface. High-resolution optical imaging has been used to capture the bubble life cycle, that is, departure, sliding, and lift-off. We found, that the lower wettability leads to larger departure diameter, longer sliding and larger lift-off diameter of bubbles. Also surface roughness effects have been analyzed and it was found that bubble departure and lift-off diameters are smaller and departure period is longer for a smooth surface. Bubble sliding velocity was found faster for a rough surface compared to a smooth surface. It was also found that the roughness is very influential to bubble growth and departure, which can be explained by considering its interaction with the microlayer underneath the bubble. An “optimal roughness”, which accelerates the bubble growth, was found. The knowledge gained from this study shall be particularly useful to improve nucleate boiling models for numerical simulations.

Keywords: vertical heater; nucleate boiling; bubble dynamics; wettability; optimal roughness

Permalink: https://www.hzdr.de/publications/Publ-24937
Publ.-Id: 24937


A few structural remarks on matrix PTQM and beyond

Günther, U.

In the first part of the talk, the historical and structural origin of PT-symmetric ix3 quantum models is briefly sketched: the Yang-Lee edge singularities for the distribution of the zeros of the partition function of the 2D Ising model in the complex plane, the close relation to criticality in the complex extended Landau-Ginzburg model for 2nd-order phase transitions, Fisher's infra-red (IR) approximation near criticality by a quantum field theory with ix3 coupling. Recent conceptually puzzling results from operator theoretic investigations of related quantum mechanical toy models with PT-symmetric ix3 couplings are reinterpreted in this phase transition context.
In the second part of the talk, the specific structural features of PT-symmetric matrix models are discussed: hidden group theoretical aspects, Lie triple systems following from Cartan decompositions of the corresponding Lie algebras, projectivization embeddings to resolve singularities at PT phase transitions. Starting from these structural findings for finite-dimensional PT-symmetric matrix setups, possible technically feasible extensions toward infinite-dimensional Hilbert-Schmidt Lie groups, Fredholm groups and PT-symmetry related Hilbert-Schmidt Grassmannians are sketched.

Keywords: PT symmetric quantum systems; phase transitions; 2D Ising model; Landau-Ginzburg model; infra-red limit; criticality; operator theory; group theory; Lie triple systems; projectivization embeddings; Hilbert-Schmidt Lie groups; Fredholm groups; Hilbert-Schmidt Grassmannians

  • Invited lecture (Conferences)
    PHHQP16: Progress in Quantum Physics with Non-Hermitian Operators, 08.-12.08.2016, Kyoto, Japan

Permalink: https://www.hzdr.de/publications/Publ-24936
Publ.-Id: 24936


From Landau-Ginzburg to PTQM: a few structural remarks

Günther, U.

The historical and structural origin of PT symmetric ix3 quantum models is briefly sketched. Open questions are discussed and related possible lines for future research are suggested.

Starting from the Yang-Lee result on the distribution of complex zeros of the partition function for the 2D Ising model the structure and origin of the so called Yang-Lee edge singularities is discussed. The close structural relation to complex extended criticality in the Landau-Ginzburg model of 2nd-order phase transitions is shown and Fisher's result is recalled on the field theoretical infra-red (IR) limit of the fluctuating fields as an effective quantum field theory (QFT) with ix3 coupling. Based on this specific phase-transition related conceptual background of the ix3 model recent operator-theoretic findings and still open puzzles of the corresponding quantum mechanical toy model approximations are interpreted as having their origin in such still operator-theoretically unexplored phase-transition related model peculiarities. Possible strategies for future research are briefly sketched to fill corresponding mathematical and technical gaps.

Keywords: PT quantum systems; 2D Ising model; Yang-Lee edge singularities; IR-limit; Landau-Ginzburg theory of phase transitions; operator theory

  • Invited lecture (Conferences)
    Analytic and algebraic methods in physics XIII, 06.-09.06.2016, Prague, Czech Republic

Permalink: https://www.hzdr.de/publications/Publ-24935
Publ.-Id: 24935


Full-field PIXE imaging: Multi-frame super-resolution to overcome optics pattern and imaging-based resolution limitations

Buchriegler, J.; Klingner, N.; Munnik, F.; Nowak, S. H.; von Borany, J.; Ziegenrücker, R.

The combination of a pnCCD-based detector (264 x 264 pixels) with a polycapillary X-ray optics was installed and examined at HZDR [1]. The set-up is intended for PIXE imaging with protons (2-4 MeV) to survey large, flat/polished geological samples with respect to their (trace) elemental composition. In the standard configuration a 1:1 polycapillary X-ray optics (78 mm length, 20 µm capillary diameter) is used to guide the emitted photons towards the pnCCD-chip divided into nearly 70000 pixels. Their dimensions of 48 x 48 µm² cause a native lateral resolution of about 100 µm. By applying dedicated sub-pixel algorithms to recalculate the footprint of the photon’s electron cloud in the chip [2], this limitation can be bypassed and the lateral resolution is then mainly determined by the capillary’s diameter.
Nevertheless, all images gathered with this kind of set-up from a single measurement are superimposed by the optics pattern. The optics’ capillaries are grouped in hexagonal bundles during the fabrication process and these bundles are grouped together again. This process results in a reduced transparency in the regions where the bundles are joint making the hexagonal pattern visible. This influence can be (largely) removed by combining several short measurements with slightly shifted positions. The optics pattern is averaged out and in addition the lateral information (shift-lengths) can be used to further improve the resolution limit beyond the pixels’/capillaries’ dimensions. The total measurement time can be kept almost similar by dividing the single measurement time by the number of “shots” without losing statistics/sensitivity.
Results from descriptive image-sets of first test-measurements will be shown to demonstrate the potential of this technique for full-field PIXE imaging.

[1] D. Hanf et al., NIM B, Vol. 377, pp. 17-24 (2016).
[2] S.H. Nowak et al., X-ray Spec., Vol. 44 (3), pp. 135-140 (2015).

Keywords: full-field imaging; capillary optics; super resolution

  • Lecture (Conference)
    15th International Conference on Particle Induced X-ray Emission, 02.-07.04.2017, Split, Croatia

Permalink: https://www.hzdr.de/publications/Publ-24934
Publ.-Id: 24934


Unravelling the structure of the ocean-continent transition from high resolution, photo-based 3D reconstructions of onshore dyke complexes

Kirsch, M.; Kaiser, A.; Eltner, A.

The temporal and spatial partitioning of strain between faulting and magmatism during continental breakup has important implications for the development of the crust- and upper mantle structure at rifted margins, but remains poorly understood. Late Neoproterozoic basaltic dyke complexes emplaced into continental basement and sedimentary cover units in the northern Scandinavian Caledonides represent an onshore-analogue of an ocean-continent transition. The dykes and their host-rocks are largely unaffected by Caledonian deformation and metamorphism, and are excellently exposed in three dimensions owing to a combination of glacial dissection and glacial retreat. Many of these outcrops of potentially high scientific value remain unmapped, mainly because they occur along steep, up to 300 m high ridges of glacier cirques in rugged mountain terrain that is largely inaccessible for traditional field mapping. Combined terrestrial and UAV-based Structure-from-Motion (SfM) photogrammetry provides an accurate and quick method of obtaining high resolution 3D information of such outcrops with minimal logistical effort. SfM-derived point clouds can be processed to identify structural discontinuities, such as faults and lithological contacts, and extract parameters such as strike, dip, thickness, density, and relative sequence of emplacement of the dykes. Based this information, the history of progressive intrusion and tilting can be reconstructed, and the amount of tectonic extension vs. magmatic dilation estimated. To demonstrate the effectiveness of this approach, we present a case study from a quarry in Lusatia, Germany. Here, like in northern Scandinavia, several generations of cross-cutting basalt dikes are exposed along a vertical, rocky cliff, but with the benefit of easy accessibility, permitting direct observation and verification of the digital data with field measurements. To improve accuracy, and to allow the extraction of oriented and scaled data as well as draping of independently acquired spectral data, ground control points are established in the scene using total station surveying. Multi-and hyperspectral data will potentially be used as complementary information to accurately distinguish composite dikes lacking intervening screens of host-rock.

Keywords: Structure-from-motion photogrammetry; structural geology; rifting

  • Poster
    2nd Virtual Geoscience Conference, 21.-23.09.2016, Bergen, Norway

Permalink: https://www.hzdr.de/publications/Publ-24933
Publ.-Id: 24933


Differential sorption behavior of U(VI) and Pu(VI) dependent on their redox chemistry

Hellebrandt, S.; Knope, K. E.; Lee, S. S.; Lussier, A. J.; Stubbs, J. E.; Eng, P. J.; Soderholm, L.; Fenter, P.; Schmidt, M.

In a recent paper (Schmidt et al. 2013) our group suggested the surface-catalyzed formation of Pu(IV)-oxo-nanoparticles due to an enhanced concentration of Pu(III) at the surface of muscovite mica in equilibrium with a small amount of Pu(IV). The study took three possible pathways for the reaction into account: (1) Pu(III) adsorbs on the muscovite surface, where the oxidation to Pu(IV) takes place. (2) The oxidation of Pu(III) to Pu(IV) happens in solution, whereupon Pu(IV) adsorbs on the surface. In both cases (1) and (2) the increased Pu(IV) concentration leads to oligomerization and afterwards the formation of Pu(IV)-oxo-nanoparticles. Another pathway (3) is the formation of Pu(IV)-oxo-nanoparticles in solution which subsequently adsorb at the mica surface. This pathway was considered less likely, due to a clear enhancement of the reaction in the presence of the interface, which cannot be explained by this process. Motivation of the current study was to test the viability of these mechanisms, but also to investigate the interfacial reactivity of Pu’s various oxidation states.
The mobility of radionuclides in the environment and thus their hazard potential will be controlled by their reactivity at the water/mineral interface. Thus, it is necessary to understand how Pu behave in contact with mineral surfaces on a molecular level, to make reliable long-term predictions about the safety of a nuclear waste repository. In order to understand these processes analytical methods shall ideally be both surface specific and sensitive. X-ray reflectivity techniques, particularly resonant anomalous X-ray reflectivity (RAXR) and crystal truncation rod (CTR) measurements have proven to be a successful combination to investigate geochemical interfacial regimes (Fenter 2002). Plutonium is one of the most important radionuclides in term of nuclear waste disposal due to its long half-life period and high radiotoxicity. That’s why it has been subject of different studies over the last decades. While these studies could show an enhancement of the mobility of plutonium in the presence of colloidal matter (Kersting et al., 1999 and Novikov et al., 2006), the formation of Pu(IV)-nanoparticles is still content of ongoing research (e.g. Kersting 2013, Walther & Deneke 2013).
In the current study a comparison of the interaction of UO22+ and PuO22+ ([Pu] = 0.1 mmol L-1, [U] = 1 mM mmol L-1, I(NaCl) = 0.1 mol L-1, pH 3.2 ± 0.2) with muscovite mica and the effect of the actinides’ different redox properties were investigated using a combination of surface X ray diffraction, alpha spectrometry and grazing-incidence X-ray adsorption near-edge structure (GI-XANES) spectroscopy. RAXR data of a Pu(VI) solution in contact with muscovite show a broad Pu distribution, which cannot be explained by simple ionic adsorption of PuO22+, indicating the formation of Pu(IV)-oxo-nanoparticles. Alpha spectrometry confirms these findings; the occupancy was determined to be ~ 8.3 Pu/AUC (where AUC = 46.72 Å2 is the unit cell area). This means the mechanism of the redox partner independent formation of Pu(IV)-nanoparticles previously observed for Pu(III) can be confirmed for Pu(VI) as well.
UO22+ shows clearly different performance. No RAXR signal was observable, indicating no adsorption of UO22+. The persistence of the hexavalent oxidation state of U was confirmed by GI-XANES spectroscopy. Furthermore, Alpha spectrometry and GI-XANES spectroscopy showed very weak signals or no signal at all, in agreement with the RAXR findings. Assuming that the sorption behavior of UO22+ and PuO22+ is equivalent excluding their redox chemistry, no Pu(VI) should be present at the surface. Therefore, the previously proposed mechanism (1) cannot contribute significantly to the observed formation of Pu(IV)-oxo-nanoparticles from Pu(VI) solution. To distinguish mechanisms (2) and (3) UV/Vis spectroscopy was performed similar to our previous study. No Pu(IV) was detectable, even if measured over a longer periode of time than available for the X-ray reflectivity experiment. Hence mechanism (3) also appears to be implausible. Apparently, the observed formation of Pu(IV)-nanoparticles follows mechanism (2). Because of the redox properties of Pu, an equilibrium of Pu(IV), Pu(V) and Pu(VI) will be present in solution. Thus available Pu(IV) will adsorb on the muscovite (001) basal plane. The tetravalent oxidation state of interfacial Pu was confirmed by GI XANES spectroscopy. Since a threshold is reached polymerization occurs as a consequence of hydrolysis, through an olation (Knope et al., 2015) or oxolation (Knope & Soderholm, 2013) mechanism.

Keywords: Plutonium; Pu; Uranium; U; Redox; Sorption; Muscovite; Nanoparticles

  • Lecture (Conference)
    Plutonium Futures - The Science 2016, 18.-22.09.2016, Baden-Baden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-24932
Publ.-Id: 24932


Self-Organized Compound Semiconductor Patterning by Polyatomic Ion Irradiation

Bischoff, L.; Böttger, R.; Pilz, W.; Facsko, S.; Heinig, K.-H.

Irradiation of solids by heavy polyatomic ions (e.g Au2, Bi3) can cause localized melting at the ion impact point due to the enhanced energy density in the collision cascade of a polyatomic ion impact [1]. Former studies demonstrated the formation of high aspect ratio, hexagonal dot patterns on Ge, Si and GaAs after high fluence, normal incidence irradiation choosing a suited combination of energy density deposition (i.e. poly- or monatomic ions) and substrate temperature, which facilitated transient melting of the ion collision cascade volume [2-5].
This study underscores the universality of this ion impact-melting-induced, self-organized pattern formation mechanism probing the compound semiconductors InSb and GaSb under polyatomic Au ion irradiation with various irradiation conditions.
Calculations of the needed melting energies per atom (Emelt) for different materials show, that among others InSb and GaSb are preferring candidates for a successful surface patterning by mon- and polyatomic heavy ions.
HRSEM, AFM and EDX analysis of irradiated surfaces reveal that for compound semiconductors, additional superstructures are evolving on top of the regular semiconductor dot patterns, indicating superposition of a second dominant driving force for pattern self-organization.
[1] C. Anders et al., Phys. Rev. B 87 (2013) 245434.
[2] L. Bischoff et al., Nucl. Instr. Meth. Phys. Res. B 272 (2012) 198.
[3] R. Böttger et al., J. Vac. Sci Technol. B 30 (2012) 06FF12.
[4] R. Böttger et al., Phys. Stat. Sol. RRL 7 (2013) 501.
[5] L. Bischoff et al., Appl. Surf. Sci. 310 (2014) 154.

Keywords: Surface pattern; polyatomic ions; temperature; angle of incidence

  • Lecture (Conference)
    Workshop für Ionenstrahlen und Nanostrukturen, 12.-15.02.2017, Göttingen, Germany

Permalink: https://www.hzdr.de/publications/Publ-24931
Publ.-Id: 24931


Time of Flight Spectrometry in the HIM

Klingner, N.; Heller, R.; Hlawacek, G.; von Borany, J.; Facsko, S.

Time of Flight Spectrometry in the HIM
N. Klingner1,2*, R. Heller1, G. Hlawacek1, J. von Borany1 and S. Facsko1
1 Ion Beam Center (IBC), Institute for Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Bautzner Landstraße 400, 01328 Dresden, Germany
2 Technical University Dresden, School of Science, Helmholtzstraße 10, 01069 Dresden, Germany
*n.klingner@hzdr.de
Helium Ion Microscopy (HIM) is well known for its high-resolution imaging and nano fabrication capabilities. However, in terms of analytic capabilities it lacks behind comparable techniques such scanning electron microscopy (SEM) or transmission electron microscopy (TEM). Although several primary and secondary particles are available, to date none of them has been exploited in a practical way to obtain analytic information.
Here, we present the first successful attempt to use time of flight backscattering spectrometry (TOF-BS) and secondary ion mass spectrometry (TOF-SIMS) in the HIM for materials characterization [1]. The successful use of sputtered particles for analytic purposes has also been demonstrated by adding a sophisticated SIMS spectrometer to the HIM [2].
For the TOF measurements the start pulse is generated by chopping the primary beam of the ion microscope using the build-in blanker and a custom made electronics that allows pulse length of 10 ns to 250 ns. The stop signal is given by the arrival of the backscattered particles at the counting micro channel plate. The setup provides high lateral resolution and a good time resolution. Moreover it is minimal invasive to the microscope and therefore the high-resolution capabilities of the device are not derogated when the TOF setup is not in use.
TOF-BS spectra of thin HfO2 films on Si are presented in fig. 1. The time resolution is limited by the physical length of the microscope blanker to approximately 17 ns or 5.4%. This value can be decreased to 2.7% by using a longer flight path. Thanks to a home built scan electronic to control the beam, TOF data can be recorded also in imaging mode. This allows an efficient post acquisition analysis by applying energy filters to extract the elemental distribution. A lateral resolution of 54 nm has been achieved so far. Although this is significantly worse than the native resolution of the tool, this value is close to the physical limit and can be overcome by using correlative approaches in connection with the high resolution SE data available in the HIM.
Modifying the sample holder slightly one can also perform TOF-SIMS. The sputtered particles are accelerated towards the stop detector of the TOF setup by means of a high voltage applied to the sample and a grounded grid. TOF-SIMS spectra obtained from different samples are presented in fig. 2. TOF-BS and TOF-SIMS performed in-situ complement each other and therefore deliver a maximum of compositional information on the sample.

References
[1] N. Klingner, R. Heller, G. Hlawacek, J. von Borany, J. A. Notte, J. Huang, S. Facsko: “Nanometer scale elemental analysis in the helium ion microscope using time of flight spectrometry”, Ultramicroscopy 162 (2016), 91-97. DOI:10.1016/j.ultramic.2015.12.005
[2] T. Wirtz, P. Philipp, J.-N. Audinot, D. Dowsett, S. Eswara. “High-resolution high- sensitivity elemental imaging by secondary ion mass spectrometry: from traditional 2D and 3D imaging to correlative microscopy”, Nanotechnology 26 (2015), 434001. DOI:10.1088/0957-4484/26/43/434001

  • Lecture (Conference)
    The 1st International Conference on Helium Ion Microscopy and Emerging Focused Ion Beam Technologies, 06.10.2016, Luxembourg City, Luxembourg
  • Poster
    Joint ICTP-IAEA Advanced Workshop on High Sensitivity 2D & 3D Characterisation and Imaging with Ion Beams, 28.09.2016, Trieste, Italien

Permalink: https://www.hzdr.de/publications/Publ-24930
Publ.-Id: 24930


Structural, Optical and Electrical Characteristics of Low Temperature Grown BaSrTiOx Thin Films

Bayrak, T.; Goldenberg, E.; Biyikli, N.

Among several perovskite oxides BaSrTiOx (BST) thin films have attracted a great interest for their potentials in oxide-based electronics. However, their reliability and efficiency depend strongly on the precise knowledge of microstructure, as well as optical and electrical constants. In the present work, BST films were deposited using radio frequency magnetron sputtering technique on UV fused silica and Si substrates at room temperature. The dependences of film microstructure, surface morphology, absorption edge, refractive index, and dielectric constants on deposition pressure, partial oxygen flow and the post-deposition annealing were examined by grazing-incidence X-ray diffraction, scanning electron microscopy, spectrophotometry, ellipsometry, as well as photoluminescence, capacitance-voltage and current-voltage measurements. Well-adhered, uniform and amorphous films were prepared at room temperature. For all as-deposited films, the average optical transmission was ~85% in the visible and near infrared spectrum. The refractive indices of BST films were in the range of 1.90 to 2.07 (λ = 550 nm) as a function of deposition conditions. Post-deposition annealing at 800 oC for 1 hr produced polycrystalline films, increased refractive indices and dielectric constants but considerably lowered film optical transmission. Frequency dependent dielectric constants were found to be 46-72, and the observed leakage current was very small ~1A. Initial results revealed that low-temperature-grown BST thin films have promising properties for device applications.

  • Poster
    IHRS NanoNET International Workshop, 30.08.-02.09.2016, Prague, Czech Republic

Permalink: https://www.hzdr.de/publications/Publ-24929
Publ.-Id: 24929


Coulomb breakup of neutron-rich 29,30Na isotopes near the island of inversion

Rahaman, A.; Datta, U.; Aumann, T.; Beceiro-Novo, S.; Boretzky, K.; Caesar, C.; Carlson, B. V.; Catford, W. N.; Chakraborty, S.; Chartier, M.; Cortina-Gil, D.; de Angelis, G.; Diaz-Fernandez, P.; Emling, H.; Ershova, O.; Fraile, L. M.; Geissel, H.; Gonzalez-Diaz, D.; Johansson, H.; Jonson, B.; Kalantar-Nayestanaki, N.; Kröll, T.; Krücken, R.; Kurcewicz, J.; Langer, C.; Le Bleis, T.; Leifels, Y.; Marganiec, J.; Münzenberg, G.; Najafi, M. A.; Nilsson, T.; Nociforo, C.; Panin, V.; Paschalis, S.; Plag, R.; Reifarth, R.; Ricciardi, V.; Rigollet, C.; Rossi, D.; Scheidenberger, C.; Scheit, H.; Simon, H.; Taylor, J. T.; Togano, Y.; Typel, S.; Volkov, V.; Wagner, A.; Wamers, F.; Weick, H.; Winfield, M.; Yakorev, D.; Zoric, M.; Weigand, J. S.

First results are reported on the ground state configurations of the neutron-rich 29,30Na isotopes, obtained via Coulomb dissociation (CD) measurements.
The invariant mass spectra of these nuclei have been obtained through measurement of the four-momenta of all decay products after Coulomb excitation of those nuclei on a 208Pb target at energies of 400-430 MeV/nucleon using the FRS-ALADIN-LAND setup at GSI, Darmstadt. Integrated inclusive Coulomb-dissociation cross-sections (CD) of 89 (7) mb and 167 (13) mb for one neutron removal from 29Na and 30Na, respectively, have been extracted up to an excitation energy of 10 MeV. The major part of one neutron removal, CD cross-sections of those nuclei populate the core, in its’ ground state. A comparison with the direct breakup model, suggests the predominant occupation of the valence neutron in the ground state of 29Na(3/2+) and 30Na(2+) is the d-orbital with a small contribution from the s-orbital, which are coupled with the ground state of the core. One of the major components of the ground state configurations of these nuclei are 28Nags(1+) ⊗ νs,d and 29Nags(3/2+) ⊗ νs,d,respectively. The ground state spin and parity of these nuclei obtained from this experiment are in agreement with earlier reported values. The spectroscopic factors for the valence neutron occupying the s and d orbitals for these nuclei in the ground state have been extracted and reported for the first time. A comparison of the experimental findings with shell model calculation using the MCSM suggests a lower limit of around 4.3 MeV of the sd − pf shell gap in 30Na.

Keywords: Coulomb-dissociation 29Na 30Na breakup spin parity

Permalink: https://www.hzdr.de/publications/Publ-24927
Publ.-Id: 24927


In-situ synchrotron studies of dendritic growth in solidifying Ga – In alloys

Grenzer, J.; Shevchenko, N.; Roshchupkina, O.; Baehtz, C.; Hoppe, D.; Cantelli, V.; Rack, A.

X-ray absorption contrast techniques are an important diagnostic tool to investigate solidification processes in metallic alloys. This work is devoted to an in situ visualization of the dendritic growth during the bottom-up solidification of a Ga-25wt%In alloy under natural convection. The coupling of X-ray imaging with X-ray diffraction techniques provides additionally information of the crystallographic orientation of the growing dendrites.
A main advantage of X-ray radiography is the possibility to study simultaneously solidification phenomena on different length scales delivering information on both: the dendrite structure and the flow patterns especially in the vicinity of solidification front. Melt flow, mainly the convective transport of the solute, induces various effects on the dendrite and grain morphology [5]. All these phenomena depend sensitively on the local conditions like the dendrite arm spacing and orientation, the detachment of side branches, the local direction and intensity of the flow it self [3-5]. A more detailed analysis of these particular processes requires a much better spatial resolution. Using synchrotron radiation the spatial resolution of the radiography experiment was improved by more than a factor of 10 reaching a value of below 1µm. Moreover, using synchrotron radiation, the time to record a tomogram was considerably reduced making it possible to visualize a stable dendrite structure in a melt flow.
An other not sufficiently investigated phenomenon is the orientation selection in dendrite evolution. It was demonstrated that the primary dendrite growth directions can vary continuously between different crystallographic directions as a function of the composition–dependent anisotropy parameters [6]. Therefore a challenging part of this experiment was the combination of two in situ techniques: X-ray radiography and synchrotron X-ray diffraction. X-ray diffraction measurements can help to reconstruct crystallographic orientations of growing dendrites.
The first radiography / diffraction experiments with solidifying Ga-25wt%In alloy were performed at BM20 and ID19 at a spatial resolution of < 0.5 µm. The radiography/ diffraction experiments performed at the beamline BM20 were carried out at an energy of 28.5keV, where as at ID 19 the radiography and tomographic experiments were performed at energies at about 65keV using a filtered undulator spectrum without any additional monochromator.
An existing solidification setup and the solidification cell were improved for synchrotron experiments guaranteeing a stable(~0.1°C) temperature gradient to control the convection inside the cell. The nominal composition of the Ga–25wt%In alloy was prepared from 99.99% Ga and 99.99% In. The alloy was melted and filled into the Plexiglas hele-shaw cell with an area of ~230 x 230 mm2 and inner cell thickness of 150μm. The tomography experiment was carried out using a capillary with an inner diameter of 400µm cell.

Keywords: In-Ga alloy solidification crystal growth

  • Poster
    XTOP 2016 – 13th Biennial Conference on High-Resolution X-Ray Diffraction and Imaging, 04.-08.09.2016, Brno, Czech Republic

Permalink: https://www.hzdr.de/publications/Publ-24926
Publ.-Id: 24926


Structural changes across the metal-insulator transition in thin epitaxial VO2 films

Grenzer, J.; Facsko, S.; Ou, X.; Ji, Y.

Vanadium dioxide (VO2) got much interest in the recent years not only from the fundamental point of view as a correlated electron system but as well as due to its intriguing electrical and optical properties, like the metal-insulator transition (MIT) close to room temperature. This makes VO2 favourable for optoelectronic, switching or even memory devices. The main challenge for device applications is the epitaxial growth of VO2 on suitable substrates. Sapphire seems to be one of the promising substrate candidates for the growth of high quality epitaxial VO2 phases.
Referring to literature, the MIT is directly connected with a change in the crystal structure, namely the transition from the low temperature monoclinic phase (P21/c) to the high temperature tetragonal (rutile) phase (P42/mnm). However, this symmetry change at the transition temperature should be strongly influenced by the epitaxy itself. Comparing our structural investigations and electrical measurements the results indicate that the MIT as observed by the resistance measurement in epitaxial VO2 thin films seems to be not necessary accompanied by a complete monoclinic to rutile phase transformation. A slight lattice distortion causing a possible change in the atomic positions without breaking the existing the epitaxial relationship appears to be sufficient.

Keywords: in-situ X-ray diffraction

  • Poster
    XTOP 2016 – 13th Biennial Conference on High-Resolution X-Ray Diffraction and Imaging, 04.-08.09.2016, Brno, Czech Republic

Permalink: https://www.hzdr.de/publications/Publ-24924
Publ.-Id: 24924


Tidally synchronized Tayler-Spruit dynamos

Stefani, F.; Galindo, V.; Giesecke, A.; Weber, N.; Weier, T.

The dynamo loop in Tayler-Spruit models for the generation of stellar magnetic fields can only be closed if the kink-type Tayler instability (TI) goes along with some alpha effect. While for large magnetic Prandtl numbers (Pm) some finite alpha can easily result from spontaneous symmetry breaking, low Pm systems show typically a vanishing or an oscillatory alpha effect. If the TI, with its typical m=1 azimuthal dependence, is exposed to an m=2 tidal forcing, we observe a sharp resonance if the tidal frequency equals the frequency of theintrinsic alpha oscillation. In the framework of a very simple alpha-Omega dynamo model we further show that this resonance can lead to synchronization of the dynamo. We also discuss the hypothetical possibility that this mechanism could link the 11.07 year periodicity of the tidally dominant Venus-Earth-Jupiter system with the Hale cycle of the solar magnetic field.

  • Lecture (Conference)
    17th MHD Days, 30.11.-02.12.2016, Göttingen, Germany

Permalink: https://www.hzdr.de/publications/Publ-24923
Publ.-Id: 24923


Liquid metal experiments on dynamo action and magnetically triggered flow instabilities

Stefani, F.

The magnetic fields of planets, stars and galaxies are generated by self-excitation in moving electrically conducting fluids. However, magnetic fields also play an active role in cosmic structure formation by destabilizing rotational flows that would be otherwise hydrodynamically stable. For a long time, both effects, i.e. hydromagnetic dynamo action and magnetically triggered flow instabilities, had been the subject of purely theoretical investigations. This situation changed in 1999 when the threshold of magnetic-field self-excitation was exceeded in the two liquid sodium experiments in Riga and Karlsruhe. Since 2006, the VKS dynamo experiment in Cadarache has successfully reproduced many features of geophysical interest such as reversals and excursions. In the same year, the helical version of the magnetorotational instability (MRI) was observed in the PROMISE experiment in Dresden-Rossendorf. More recently, the azimuthal MRI was found at the same facility. First evidence of the current-driven Tayler instability in a liquid metal was obtained, too. The lecture gives an overview about liquid metal experiments on dynamo action and magnetically triggered instabilities. New results from the enhanced PROMISE facility with a strongly symmetrized azimuthal magnetic fields are presented. An outlook on future experiments, including a precession driven dynamo and a large-scale Tayler-Couette experiment to be set-up in the framework of the DRESDYN project, is also given.

  • Invited lecture (Conferences)
    European GDR Meeting 2016, 27.06.-01.07.2016, Barcelona, Spain

Permalink: https://www.hzdr.de/publications/Publ-24922
Publ.-Id: 24922


The Tayler instability at low magnetic Prandtl numbers: Chiral symmetry breaking and synchronizable helicity oscillations

Stefani, F.; Galindo, V.; Giesecke, A.; Weber, N.; Weier, T.

The current-driven, kink-type Tayler instability (TI) is a key ingredient of the Tayler-Spruit dynamo model for the generation of stellar magnetic fields, but is also discussed as a mechanism that might limit the up-scaling of liquid metal batteries. Here, we focus on the chiral symmetry breaking and the related alpha-effect that would be needed to close the dynamo loop in the Tayler-Spruit model. For low magnetic Prandtl number, we observe intrinsic oscillations of the alpha-effect. These oscillations serve then as the basis for a synchronized Tayler-Spruit dynamo model, which could possibly link the periodic tidal forces of planets with the oscillation periods of stellar dynamos.

  • Contribution to proceedings
    10th PAMIR International Conference - Fundamental and Applied MHD, 20.-24.06.2016, Cagliari, Italy
    Proceedings of the 10th PAMIR International Conference - Fundamental and Applied MHD, 978-88-90551-93-2, 686-690
  • Open Access Logo Magnetohydrodynamics 53(2017)1, 169-178

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Permalink: https://www.hzdr.de/publications/Publ-24921
Publ.-Id: 24921


Flüssigmetallbatterien als stationäre Speicher - Strömungsmechanische Aspekte

Weier, T.; Barry, L.; Galindo, V.; Gerbeth, G.; Landgraf, S.; Seilmayer, M.; Starace, M.; Stefani, F.; Weber, N.

Flüssigmetallbatterien, d.h. elektrochemische Hochtemperaturbatterien mit vollständig flüssigem Inventar, werden derzeit als preiswerte Regelenergiespeicher diskutiert. Das ursprünglich auf kleinere Zellen als Bestandteil thermisch regenerierbarer Systeme gerichtete Konzept soll auf Zellen mit Grundflächen im Quadratmeterbereich übertragen werden.

Eine elektrochemische Zelle mit vollständig flüssigem Inventar hat eine Reihe von Vorteilen: bei gut abgestimmten Dichten von Elektrolyt und aktiven Materialien ist die Batterie selbstassemblierend, eine stabile Dichteschichtung bildet sich aus. Die strukturlosen (flüssigen) Elektroden sind für Alterungserscheinungen unanfällig, versprechen somit gute Zyklierbarkeit, die Kinetik an den flüssig-flüssig Phasengrenzen und die Diffusionsprozesse sind vergleichsweise schnell, was hohe Stromdichten ermöglicht. Als aktive Materialien können breit und ökonomisch verfügbare Ausgangsstoffe eingesetzt werden. Die Ausnutzung positiver Skaleneffekte ist ein wichtiges Mittel zur Kostensenkung und die einfache Skalierbarkeit eine der grundlegenden Annahmen bei der Entwicklung von Flüssigmetallbatterien.

Hohe Stromdichten und große Phasengrenzflächen resultieren jedoch in erheblichen Zellströmen. Diese generieren Magnetfelder und mithin beträchtliche elektromagnetische Kräfte. Das flüssige Inventar der Zelle kann auf diese elektromagnetischen Einwirkungen mit Instabilitäten und elektromagnetisch getriebener Konvektion reagieren. Die Konsequenzen solcher Strömungen und Maßnahmen zu ihrer Begrenzung werden diskutiert und realisierte Flüssigmetallbatterien vorgestellt.

Keywords: Flüssigmetallbatterie; Tayler Instabilität; Elektro-Wirbel-Strömung

  • Invited lecture (Conferences)
    Materialforum Rhein-Main, 22.02.2016, Gelnhausen, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-24920
Publ.-Id: 24920


Visual observations and charge/discharge behavior of liquid metal cells

Weier, T.; Landgraf, S.; Starace, M.; Stefani, F.; Weber, N.

Liquid metal batteries (LMBs) consist of a stable density stratification of a molten alkaline metal on the top, a liquid heavy metal on the bottom and a fused salt mixture sandwiched in between. Initially developed as part of energy conversion systems, today they are considered an inexpensive means for stationary large-scale storage of electrical energy. A special feature of LMBs is their very high current density enabled by the fast kinetics at liquid-liquid interfaces and the rapid mass transfer processes in fluids. Scale-up on the cell level will therefore result in large total currents per cell that might however trigger electromagnetic instabilities and/or generate electro-vortex flows.
To allow for an experimental verification of the numerical predictions, a number of low temperature liquid metal cells were tested in order to optimize operating conditions and material combinations. The final aim is the construction of a larger cell able to generate a considerable current.
Na and Li were tested as negative electrode materials versus a positive electrode made of Bi in both cases. Na has a relatively high solubility in its salts and forms a dark metal fog in the electrolyte. While this enables the visualization of an intense flow in the electrolyte, it also leads to an unwanted electronic conduction and accompanying capacity fading. Li solubility in its salts is much weaker and no metal fog formation was observed. Cycling behavior in the tethered drop cell is considerably more stable for Li than for Na. Estimated current densities based on the immersed surface of the drop exceed 2 A/cm2 by far, both for Na and Li. Electrolytes are varied as well since for the metal combinations mentioned above the melting point of the salt mixture determines the cell operating temperature.

Keywords: liquid metal battery; electrochemical energy storage

  • Poster
    67th Annual Meeting of the International Society of Electrochemistry, 21.-26.08.2016, Den Haag, Niederlande

Permalink: https://www.hzdr.de/publications/Publ-24919
Publ.-Id: 24919


The Need for Accurate Geometric and Radiometric Corrections of Drone-Borne Hyperspectral Data for Mineral Exploration: MEPHySTo—A Toolbox for Pre-Processing Drone-Borne Hyperspectral Data

Jakob, S.; Zimmermann, R.; Gloaguen, R.

Drone-borne hyperspectral imaging is a new and promising technique for fast and precise acquisition, as well as delivery of high-resolution hyperspectral data to a large variety of end-users. Drones can overcome the scale gap between field and air-borne remote sensing, thus providing high-resolution and multi-temporal data. They are easy to use, flexible and deliver data within cm-scale resolution. So far, however, drone-borne imagery has prominently and successfully been almost solely used in precision agriculture and photogrammetry. Drone technology currently mainly relies on structure-from-motion photogrammetry, aerial photography and agricultural monitoring. Recently, a few hyperspectral sensors became available for drones, but complex geometric and radiometric effects complicate their use for geology-related studies. Using two examples, we first show that precise corrections are required for any geological mapping. We then present a processing toolbox for frame-based hyperspectral imaging systems adapted for the complex correction of drone-borne hyperspectral imagery. The toolbox performs sensor- and platform-specific geometric distortion corrections. Furthermore, a topographic correction step is implemented to correct for rough terrain surfaces. We recommend the c-factor-algorithm for geological applications. To our knowledge, we demonstrate for the first time the applicability of the corrected dataset for lithological mapping and mineral exploration.

Keywords: UAV; UAS; drone; hyperspectral; exploration; processing; structure-from-motion; point matching; Minas de Riotinto

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Permalink: https://www.hzdr.de/publications/Publ-24918
Publ.-Id: 24918


Nonmodal analysis of helical and azimuthal magnetorotational instabilities

Mamatsashvili, G.; Stefani, F.

The helical and the azimuthal magnetorotational instabilities operate in rotating magnetized flows with relatively steep negative or extremely steep positive shear. The corresponding lower and upper Liu limits of the shear, which determine the threshold of modal growth of these instabilities, are continuously connected when some axial electrical current is allowed to pass through the rotating fluid. We investigate the nonmodal dynamics of these instabilities arising from the nonnormality of shear flow in the local approximation, generalizing the results of the modal approach. It is demonstrated that moderate transient/nonmodal amplification of both types of magnetorotational instability occurs within the Liu limits, where the system is stable according to modal analysis. We show that for the helical magnetorotational instability this magnetohydrodynamic behavior is closely connected with the nonmodal growth of the underlying purely hydrodynamic problem.

Keywords: Transient/nonmodal growth; nonormality; magnetorotational instability; liquid metals; astrophysical disks

  • Contribution to proceedings
    10th PAMIR International conference fundamental and applied MHD, 20.-24.06.2016, Cagliari, Italy
  • Open Access Logo Magnetohydrodynamics 53(2017), 107-117

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Permalink: https://www.hzdr.de/publications/Publ-24917
Publ.-Id: 24917


Dynamics of homogeneous shear turbulence: a key role of the nonlinear transverse cascade in the bypass concept

Mamatsashvili, G.; Khujadze, G.; Chagelishvili, G.; Dong, S.; Jiménez, J.; Foysi, H.

To understand the mechanism of the self-sustenance of subcritical turbulence in spectrally stable (constant) shear flows, we performed direct numerical simulations of homogeneous shear turbulence for different aspect ratios of the flow domain with subsequent analysis of the dynamical processes in spectral or Fourier space. There are no exponentially growing modes in such flows and the turbulence is energetically supported only by the linear growth of Fourier harmonics of perturbations due to the shear flow non-normality. This non-normality-induced growth, also known as nonmodal growth, is anisotropic in spectral space, which, in turn, leads to anisotropy of nonlinear processes in this space. As a result, a transverse (angular) redistribution of harmonics in Fourier space is the main nonlinear process in these flows, rather than direct or inverse cascades. We refer to this type of nonlinear redistribution as the nonlinear transverse cascade. It is demonstrated that the turbulence is sustained by a subtle interplay between the linear nonmodal growth and the nonlinear transverse cascade. This course of events reliably exemplifies a well-known bypass scenario of subcritical turbulence in spectrally stable shear flows. These two basic processes mainly operate at large length scales, comparable to the domain size. Therefore, this central, small wave number area of Fourier space is crucial in the self-sustenance; we defined its size and labeled it as the vital area of turbulence. Outside the vital area, the nonmodal growth and the transverse cascade are of secondary importance: Fourier harmonics are transferred to dissipative scales by the nonlinear direct cascade. Although the cascades and the self-sustaining process of turbulence are qualitatively the same at different aspect ratios, the number of harmonics actively participating in this process (i.e., the harmonics whose energies grow more than 10% of the maximum spectral energy at least once during evolution) varies, but always remains quite large (equal to 36, 86, and 209) in the considered here three aspect ratios. This implies that the self-sustenance of subcritical turbulence cannot be described by low-order models.

Keywords: Transient growth; turbulence; bypass concept

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Permalink: https://www.hzdr.de/publications/Publ-24916
Publ.-Id: 24916


RLumShiny - A graphical user interface for the R Package ’Luminescence’

Burow, C.; Kreutzer, S.; Dietze, M.; Fuchs, M.; Fischer, M.; Schmidt, C.; Brückner, H.

Since the release of the R package ’Luminescence’ in 2012 the functionality of the package has been greatly enhanced by implementing further functions for measurement data processing, statistical analysis and graphical output. Along with the accompanying increase in complexity of the package, working with the command-line interface of R can be tedious, especially for users without previous experience in programming languages. Here, we present a collection of interactive web applications that provide a user-friendly graphical user interface for the ’Luminescence’ package. These applications can be accessed over the internet or used on a local computer using the R package ’RLumShiny’. A short installation and usage guide is accompanied by the presentation of two exemplary applications.

Keywords: R; Software; GUI; Luminescence dating; Abanico Plot; Cosmic Dose Rate

  • Open Access Logo Ancient TL 34(2016)2, 22-32

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Permalink: https://www.hzdr.de/publications/Publ-24915
Publ.-Id: 24915


Collision cascades enhanced hydrogen redistribution in cobalt implanted hydrogenated diamond-like carbon films

Gupta, P.; Becker, H.-W.; Williams, G. V. M.; Hübner, R.; Heinig, K.-H.; Markwitz, A.

Hydrogenated diamond-like carbon films produced by C3H6 deposition at 5 kV and implanted at room temperature with 30 keV Co atoms to 12 at.% show not only a bimodal distribution of Co atoms but also a massive redistribution of hydrogen in the films. Resonant nuclear reaction analysis was used to measure the hydrogen depth profiles (15N-method). Depletion of hydrogen near the surface was measured to be as low as 7 at.% followed by hydrogen accumulation from 27 to 35 at.%. A model is proposed considering the thermal energy deposited by collision cascade for thermal insulators. In this model, sufficient energy is provided for dissociated hydrogen to diffuse out of the sample from the surface and diffuse into the sample towards the interface which is however limited by the range of the incoming Co ions. At a hydrogen concentration of ∼35 at.%, the concentration gradient of the mobile unbounded hydrogen atoms is neutralised effectively stopping diffusion towards the interface. The results point towards new routes of controlling the composition and distribution of elements at the nanoscale within a base matrix without using any heat treatment methods. Exploring these opportunities can lead to a new horizon of materials and device engineering needed for enabling advanced technologies and applications.

Permalink: https://www.hzdr.de/publications/Publ-24913
Publ.-Id: 24913


Compositional Calibration

van den Boogaart, K. G.; Tolosana-Delgado, R.; Renno, A. D.

An emerging field in the statistics of (geo)chemical data is compositional data analysis. In this field the ratio of concentrations between elements is seen as the essential information, while the absolute concentration is considered irrelevant. E.g. for physical age determination the ratio of concentration of various nuclides is very important, while the absolute concentrations change subject to dilution and depletion processes. In many applications chemical data is rescaled to 100%, or transformed with a log ratio transformation. In both cases the information about the absolute concentration is lost.

The aim of the contribution is to initiate a discussion on how the compositional approach might modify the tasks and methods of metrology. Asking a sligthly different question might lead to different answers. We will consider the calibration of an LA-ICP-MS (laser ablation inductive coupled plasma mass spectrometer), which measures multiple, but not all elements (quasi) simultaneously.

If log ratios are considered the relevant quantity, we should be able to measure them unbiasedly and provide a precision assessment of their measurement. We show that both aspects are problematic, when the log ratio is applied to measurements independently calibrated with respect to absolute standards. The same individual absolute errors can correspond to very different relative errors due to inherent correlation between the counts.

A supplementary concept for a traceable calibration of relative concentrations is introduced and discussed. In the example we use a method based on multivariate Poisson regression for a compositional calibration of LA-ICP-MS measurements. A compositionally calibrated measurement does not carry absolute information, but only relative information about a subset of elements. The concept of a compositional error compatible with the variance concepts in compositional data analysis is introduced. This error can not be computed from classical absolute calibration information of single elements and provides essential information for compositional methods.

In the case of the LA-ICP-MS the absolute amount of material turned into plasma is strongly matrix dependent, while the counting ratios for different elements are much less so. In such cases a relative calibration can guarantee better compositional precision and allows the useage of much loosely matrix-matched standards in a well-defined range of matrix compositions.

  • Lecture (Conference)
    MATHMET 2016 - International Workshop on Mathematics and Statistics for Metrology, 07.-09.11.2016, Berlin, Germany

Permalink: https://www.hzdr.de/publications/Publ-24912
Publ.-Id: 24912


Filtering lack of microhomogeneity in reference materials for microanalytical methods

van den Boogaart, K. G.; Renno, A. D.; Tolosana-Delgado, R.

Homogeneity is a relative property of a sample in relation to the measurement (analytical method), the analyte, and the intended purpose, like the usage as a reference material (RM). The verification of homogeneity is essential to define a RM as fit for purpose. In this context, there have been recent efforts to check the possible superiority of synthetic RMs over natural ones. The assessment of homogeneity is an integral part of these synthesis tests and of the following certification for use as RM. With regard to their spatial variability, five types of microheterogeneity of RMs can be found in the literature, depending on which is the source of heterogeneity that it presents: random, systematic, periodic, nugget and island.

This contribution presents a first attempt towards such tests of microhomogeneity for discussion. In a first step, we define a stochastic random function model that will describe each of the types of microheterogeneity mentioned before. Then, in a quite natural manner a particular sampling strategy for each of them is derived in the second step, with the goal to filter out the undesired source of variability. In the third step, we derive a strategy of characterization of the material, namely a strategy of estimation of the heterogeneity properties of the RM that should be used to certify the reference nature of the material. These the adequacy of these strategies is shown in this contribution by using simulations of the several heterogeneity structures and of the proposed sampling and characterization strategies.

For instance, for the case of a random heterogeneity we may assume that the concentration of the target element is described by a random function (RF). If the covariance function of this RF would be known, the sampling strategy would be to repeat measurements on random positions of a very fine regular grid in such a way that the variance of their average decreases as fast as possible, using as many locations as necessary to ensure that it falls below the method specifications. Finally, the way to characterise the RM would require calibrating the concentration of the target element on a coarser grid, on as many locations as necessary to appropriately set the covariance function, using classical concepts and models of Geostatistics. Similar strategies can be derived for the rest of the heterogeneity structures, like robust methods for nugget heterogeneity or geostatistical concepts related to intrinsic functions of order k for systematic heterogeneity

  • Lecture (Conference)
    MATHMET 2016 - International Workshop on Mathematics and Statistics for Metrology, 07.-09.11.2016, Berlin, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-24911
Publ.-Id: 24911


Resource Model Updating by Compositional Sequential Ensemble Filtering

Prior-Arce, A.; Tolosana-Delgado, R.; Verlaan, M.; Bendorf, J.

In mining engineering, resource and grade control models aim to characterize the spatial distribution generating by geostatistics methods of ore tonnage and grade in an deposit. A continuously self-updating resource model concept has recently been developed by Wambeke and others (2016) and aims to improve the raw material quality control and process efficiency of any type of mining operation. The proposed concept integrates sensor data measured with different support along the production line into the resource or grade control model and provides continuously locally more accurate estimates. Applications in underground mines include the identification of different components of the mineralogy and geochemistry.

This study aims to develop an efficient updating framework based on a sequential ensemble filtering on a compositional environment (Tolosana-Delgado (2013)). The importance of respecting as well physical conservation principles has long been recognized. During the data assimilation procedure, the mass of each component should be preserved within each ensemble member through the procedure used to update the model. Different approaches to constraining ensemble based Kalman Filters have been presented as solutions of a set of regularized least squares optimization problems. Some of these have been formulated by imposing non-negativity constrains or by using transform methods such as anamorphosis, at the price of violating mass preservation.

Compositional approaches supersede this problem by dealing with the positivity condition and the mass preservation implicitly through assimilating log-ratios instead of the original components. After a detailed literature review, a compositional sequential ensemble filter approach adapted to specific application in mining is presented. Method validation results are presented for a 2D case study in a fully controllable environment. After validation, a sensitivity analysis investigates the effects of different parameters and derived practical implementation aspects for an effective application.

This research is part of the European Union funded "Real Time Mining" project, which aims at developing a new framework to reduce uncertainties during the block extraction process. New sensor based technology will provide georeferenced information about mineralogy and grades by taking images of the mine face, during block extraction from the muckpile, or via sensors installed on the conveyor belt about the mean composition of the ores. Based on a discrete time event simulation, the project will test the capabilities of incorporating sensor imaging information as pointwise data and of mean compositions as block data.

  • Lecture (Conference)
    CoDaWork 2017, The 7th International Workshop on Compositional Data Analysis, 06.-09.06.2017, Abbadia San Salvatore, Italia

Permalink: https://www.hzdr.de/publications/Publ-24910
Publ.-Id: 24910


Applications of the atomic force microscope in flotation research

Babel, B.; Rudolph, M.

This abstract gives an overview of the applications of the Atomic Force Microscope (AFM) in flotation research. The AFM, which has a broad application in different disciplines, is a versatile tool to measure surface properties and particle-particle interactions. In the field of mineral processing the AFM can be utilized to gather information of mineral surfaces such as the surface roughness or particle-particle interactions related to separation processes like magnetic separation, triboelectric separation and flotation. In the context of flotation it is possible to measure the hydrophobic interaction between a colloidal probe (CP-AFM) and the mineral surface. One drawback of this technique is the missing link between the hydrophobicity and the chemical composition of the mineral surface, respectively the adsorbed layers. This limitation can be exceeded by the combined utilization of the AFM and Raman Spectroscopy named Tip-Enhanced Raman Spectroscopy (TERS). This surface sensitive technique enables the chemical analysis of thin films with nanometer resolution by the utilization of a plasmonic effect that occurs at the surfaces of noble metal nanoparticles. The aim is the detection of adsorbed flotation reagents on mineral surfaces. The combination of TERS and CP-AFM measurements enables the investigation of the adsorption of flotation reagents on mineral surfaces and their effect on hydrophobicity.

As outlined the application of the AFM in mineral processing can give a deeper understanding of occurring micro processes and thereby a better physical and chemical description of macro processes. Especially TERS offers a better understanding of the adsorption of flotation reagents.

The paper presents fundamental investigations of CP-AFM and TERS on actual mineral specimens (quartz, cassiterite, apatite, calcite, scheelite) which are prepared to achieve a surface roughness of less than 10 nm. The impact of collector and depressant adsorption on the measurement, i.e. hydrophobic interactions with CP-AFM and vibrational spectroscopy with TERS is shown.

Keywords: atomic force microscope; tip-enhanced Raman spectroscopy; flotation

  • Poster
    XXVIII International Mineral Processing Congress, 11.-15.09.2016, Québec city, Canada

Permalink: https://www.hzdr.de/publications/Publ-24909
Publ.-Id: 24909


Analysis of temporal changes of a mining site affected by AMD. UAV based hyperspectral monitoring of the Litov tailing (Sokolov, CZ)

Jackisch, R.; Jakob, S.; Zimmermann, R.; Gloaguen, R.

In this study we test and validate hyperspectral mineral mapping for acid mine drainage (AMD) detection using Unmanned Aerial Systems (UAS). The investigated area is a re-cultivated tailing and part of the Sokolov coal mine district in the Czech Republic. The dumped mining waste material bears pyrite and the consecutive weathering products. Mainly iron hydroxides and oxides affect the natural pH values of the Earth’s surface. While previous research done in this area relies on satellite and air-borne data, our approach focuses on lightweight drone systems providing ground readiness within hours. During April to September 2016, several field and flight campaigns were conducted. For validation, the waste heap was probed in-situ for pH, X-ray florescence (XRF), reflectance spectrometry and surveyed by GPS. Collected samples were measured for pH, X-ray diffraction (XRD) and XRF in laboratory conditions. A new in-house developed toolbox allowing the processing as well as the correction of topography and illumination effects, is applied on the UAS data. High-resolution point clouds and digital elevation models are build from UAS-borne data with Structure-from-Motion photogrammetric techniques. Changes of the canopy, topographic profiles and erosion values are derived from the 3D models. The classification of hyperspectral (HS) data detected the proposed minerals jarosite and goethite, which are associated with the acidic environmental conditions (mean pH = 2.9). The specific iron absorption bands are visible in the UAS-HS data, and can be confirmed with ground-truth spectroscopy. Evaluation of geochemical data shows a negative correlation for pH with sulfur content, which is in accordance with previous studies on AMD conditions. Interpolations of in-situ pH data support the UAS-based results. The evaluation of the applied methods highlights the UAS approach as a fast, non-invasive, inexpensive technique for multi-temporal monitoring.

Keywords: Hyperspectral; AMD; remote sensing; UAS; pH mapping; Absorption feature

  • Lecture (Conference)
    10th EARSeL SIG Imaging Spectroscopy Workshop, 19.-21.04.2017, Zürich, Schweiz

Permalink: https://www.hzdr.de/publications/Publ-24908
Publ.-Id: 24908


Drone-borne hyperspectral remote sensing of REE deposits in Namibia

Booysen, R.; Gloaguen, R.; Zimmermann, R.; Jakob, S.

Traditional exploration techniques are mainly based on extensive field work supported by geophysical surveying. These techniques can be restricted by field accessibility, financial status, area size and climate. Therefore, we suggest to increase the use of multi-scale hyperspectral remote sensing in order to decrease conventional restrictions in the exploration of minerals through the use of Unmanned Aerial Systems (UAS). We further argue that the addition of drone based hyperspectral data can vastly improve the accuracy of field mapping in future mineral exploration. Drone-borne measurements can supplement and direct geological observation immediately in the field and therefore allow better integration with in-situ ground investigations. In particular, in inaccessible and remote areas with little infra-structure, such systems are beneficial because it allows a systematic, dense and completely non-invasive surveying, which is often not possible using ground- based techniques. We use a hyperspectral camera attached to a hexacopter to acquire data from the visible (VIS) to the near-infrared (NIR) range of the electromagnetic spectrum. The hyperspectral data is then corrected of radiometric and geometric distortions using a new python-based in-house toolbox. In addition, high-resolution Digital Surface Models (DSM) and Orthomosaics are generated from drone data using Structure- from-Motion photogrammetry. The corrected data provide information on the spectral signatures of outcropping lithologies to the field geologists and the exploration teams. This is achieved by using end-member modelling and classification techniques such as non- linear machine learning algorithms, e.g., Neural Networks and tree based methods. The validation of the hyperspectral data is performed via field spectroscopy and portable XRF. The proposed method is currently tested on carbonatite-hosted REE occurrences in several locations in Namibia. These locations are characterised by difficult terrains and remote environments, that would impede or restrict traditional field surveys. Preliminary results indicate that UAS-based surveying has a very high potential in fundamentally lowering the acquisition costs and increasing the information potential of data captured in the field.

  • Lecture (Conference)
    10th EARSeL SIG Imaging Spectroscopy Workshop, 19.-21.04.2017, Zürich, Schweiz

Permalink: https://www.hzdr.de/publications/Publ-24907
Publ.-Id: 24907


Integrated drone-borne and ground-based hyperspectral imaging for exploration targeting

Herrmann, E.; Zimmermann, R.; Gloaguen, R.

The Iberian Pyrite Belt (IPB) in Southwest Spain and South Portugal is best known for its numerous massive sulphide deposits. With their 4000 years history of mining, the mines of Rio Tinto are of important economic importance in the IPB. Orebodies and related alteration zones were mined for copper, gold, silver and many other pre- cious metals. We selected this site as a case study for the exploration of massive sulphide deposits due to its excellent data basis, economic importance and excellent outcrops. Ground-based and drone-borne hyperspectral data were acquired during 3 field campaigns in 2017 and 2017. Drone-borne hyperspectral images are acquired by a RIKOLA Hyperspectral Imager in the wavelength range from 500 to 900 nm. Not only spectral information can be gathered that way, overlapping images can also be used to calculate 3D surface models using Structure-from-Motion photogrammetry. Resulting point cloud can be basis for 3D data integration and correlation of spatial orientation of surfaces to spectral characteristics. Ground-based data are acquired with a Specim AisaFenix hyperspectral camera with a wavelength range from 400 to 2500 nm.
A complex geological history lead to the appearance of numerous regional geochem- ical and structural characteristics correlated to the orebodies that comprise different types of syn- and post-mineralisation alteration zones. These features are hardly re- cognisable using classical geological fieldwork techniques. Until now, time consum- ing and expensive geological interpretations rely almost exclusively on geochemical data. Instead, we argue that by using in hyperspectral data, those alteration zones can easily be determined, due to characteristic features in the VNIR and SWIR range. This provides a powerful tool for a fast, low-cost and spatially precise altera- tion mapping to guide the exploration process. The result of this data integration is comparable to an early stage geological surface model. It can provide a very precise tool for exploration and mining at all stages.
We chose the Corta Atalaya in Riotinto/Spain, one of the most profitable mines in the Spanish part of the Iberian pyrite belt, to demonstrate this approach of modern, re- mote sensing based geological exploration methods. Methods include traditional al- gorithms like Spectral Feature Fitting and Mapping absorption wavelength, but also non-linear unsupervised classification algorithms. Different kind of alteration zones are discriminated and interpreted in terms of their spectral-spatial distribution.

  • Poster
    10th EARSeL SIG Imaging Spectroscopy Workshop, 19.-21.04.2017, Zürich, Schweiz

Permalink: https://www.hzdr.de/publications/Publ-24906
Publ.-Id: 24906


Hyperspectral imaging for structural geology: An example for more interdisciplinary data analysis in mineral exploration

Zimmermann, R.; Herrmann, E.; Rosa, D.; Kirsch, M.; Gloaguen, R.

Since the applications of hyperspectral imaging found their way into geological investigations, mineral mapping has reached a completely new level of spatial and spectral resolution. Thanks to increasing technological developments in hyperspectral imaging, system resolutions steadily became better at lower prices. Although these methods are already frequently applied in the field of economic geology and petrology, so far, these techniques have rarely been used for structural mapping and interpretation. The actual tool of choice, photogeology, makes just the use of true-color RGB images. However, detailed mineral and lithological maps from hyperspectral imaging can bring a new dimension. Hyperspectral imaging can highlight small mineralogical differences in rocks that cannot be picked-up in traditional RGB images and thus allow lithological contacts to be easily identified. Potential applications include, e.g., mapping dykes of different spectral signatures and their genetic relations or folds in (inaccessible) homogeneous rocks like marbles or quartzites. Furthermore, different types of alteration, associated with structural information can allow the detection of pathways for mineralising fluids and their structural control, one of the most important application of applied structural geology in mineral exploration. Potential methods include mapping absorption wavelength, spectral unmixing and non-linear classifications. All hyperspectral results are integrated into 3D point-clouds for interpretation. We exemplify the interdisciplinary analysis of hyperspectral data, independent of scale and source and their use for structural geology.
In the first case the barely accessible, 1000 m high marble cliffs of Maarmorilik/Greenland were scanned with a ground-based hyperspectral imaging (HSI) system. In certain structural positions a Pb-/Zn-mineralisation is hosted. A preliminary analysis with focus on carbonate mineralogy using absorption wavelengths identified fold structures of calcite-rich and dolomite-rich marbles, which are not visible in standard RGB images. Furthermore, some marble horizons are rich in evaporite minerals (e.g., anhydrite). The distribution of those horizons is mapped using spectral unmixing techniques and can be used to elucidate the structural relationship between deformation, mineralising fluids and ore emplacement.
At the inaccessible mine-pit of Peña de Hierro/Spain hyperspectral imaging is used to enhance the understanding of ore emplacement in relation with faults. Various mineral mapping methods, e.g., Spectral Feature Fitting, and non-linear unsupervised clustering, e.g., Self-Organising Maps, are applied. Faults and folds can easily recognised in RGB images here. However, in hyperspectral data we observe a degradation of hydrothermal activity around faults and orebodies that lead to a better understanding of the interaction between faulting and ore emplacement. Small scale structures in overlying meta- sedimentary rocks are highlighted in hyperspectral images too. They give indication of post-mineralisation deformation and, thus, pathways for secondary alteration and ore replacement.

  • Poster
    10th EARSeL SIG Imaging Spectroscopy Workshop, 19.-21.04.2017, Zürich, Schweiz

Permalink: https://www.hzdr.de/publications/Publ-24905
Publ.-Id: 24905


Modeling patterns of anatomical deformations in prostate patients undergoing radiation therapy with an endorectal balloon

Brion, E.; Richter, C.; Macq, B.; Stützer, K.; Exner, F.; Troost, E.; Hölscher, T.; Bondar, L.

External beam radiation therapy (EBRT) treats cancer by delivering daily fractions of radiation to a target volume. For prostate cancer, the target undergoes day-to-day variations in position, volume, and shape. For stereotactic photon and for proton EBRT, endorectal balloons (ERBs) can be used to limit variations. To date, patterns of non-rigid variations for patients with ERB have not been modeled. We extracted and modeled the patient-specific patterns of variations, using regularly acquired CT-images, non-rigid point cloud registration, and principal component analysis (PCA). For each patient, a non-rigid point-set registration method, called Coherent Point Drift, (CPD) was used to automatically generate landmark correspondences between all target shapes. To ensure accurate registrations, we tested and validated CPD by identifying parameter values leading to the smallest registration errors (surface matching error 0.13+-0.09 mm). PCA demonstrated that 88+-3.2% of the target motion could be explained using only 4 principal modes. The most dominant component of target motion is a squeezing and stretching in the anterior-posterior and superior-inferior directions. A PCA model of daily landmark displacements, generated using 6 to 10 CT-scans, could explain well the target motion for the CT-scans not included in the model (modeling error decreased from 1.83+-0.8 mm for 6 CT-scans to 1.6+-0.7 mm for 10 CT-scans). PCA modeling error was smaller than the naive approximation by the mean shape (approximation error 2.66+-0.59 mm). Future work will investigate the use of the PCA-model to improve the accuracy of EBRT techniques that are highly susceptible to anatomical variations such as, proton therapy.

Keywords: External beam radiation therapy; nonrigid registration; principal component analysis; mathematical modeling; motion management in radiotherapy; prostate cancer

  • Contribution to proceedings
    SPIE Medical Imaging 2017, 11.-16.02.2017, Orlando, USA
    Proc. SPIE 10135, Medical Imaging 2017: Image-Guided Procedures, Robotic Interventions, and Modeling: SPIE digital library, 1013506-1-1013506-9
    DOI: 10.1117/12.2251933

Permalink: https://www.hzdr.de/publications/Publ-24904
Publ.-Id: 24904


Understanding actinide reactivity at the water-mineral interface

Stumpf, T.

The behaviour of radionuclides in the environment is determined by interfacial reactions such as adsorption, ion exchange and incorporation processes. In literature such processes are often described by operational solid-liquid distribution ratios (Rd values). Distribution ratios are defined as the ratio of the quantity of a radionuclide sorbed per solid mass and the equilibrium concentration of the radionuclide. They are macroscopic parameters which are strictly valid only for the mineral and solute combination in the experimental system. For reliable and trustworthy long-term predictions of radionuclide transport behaviour, interaction mechanisms and processes occurring at the solid-water interface, they need to be understood at the molecular level. This can only be achieved by the application of spectroscopic methods.
In the talk a multi-spectroscopic approach will be presented. We used a combination of microscopy, laser and X-ray based techniques to gain process understanding on a molecular level of the interaction of actinides with mineral surfaces. The derived data are used to improve surface complexation modelling. Therefore, the advanced spectroscopic techniques used in Dresden-Rossendorf are an important contribution to the long-term performance assessment of a nuclear waste disposal.

Keywords: radionuclides; nuclear waste disposal

  • Invited lecture (Conferences)
    les Journées Nationales de Radiochimie et de Chimie Nucléaire (JNR) 2016, 08.-09.09.2016, Nice, Frankreich
  • Lecture (Conference)
    Frontiers in Environmental Radioactivity 2016, 06.-07.01.2016, London, United Kingdom

Permalink: https://www.hzdr.de/publications/Publ-24902
Publ.-Id: 24902


Electron effective mass enhancement in Ga(AsBi) alloys probed by cyclotron resonance spectroscopy

Pettinari, G.; Drachenko, O.; Lewis, R. B.; Tiedje, T.

The effect of Bi incorporation on the conduction band structure of Ga(AsBi) alloys is revealed by a direct estimation of the electron effective mass via cyclotron resonance absorption spectroscopy at THz frequencies in pulsed magnetic fields up to 65 T. A strong enhancement in the electron effective mass with increasing Bi content is reported, with a value of mass ∼40% higher than that in GaAs for ∼1.7% of Bi. This experimental evidence unambiguously indicates a Bi-induced perturbation of the host conduction band states and calls for a deep revision of the theoretical models describing dilute bismides currently proposed in the literature, the majority of which neglect or exclude that the incorporation of a small percentage of Bi may affect the conduction band states of the host material.

Permalink: https://www.hzdr.de/publications/Publ-24901
Publ.-Id: 24901


Organic synthesis, radiofluorination and in vitro evaluation of two novel fluorenones targeting the alpha 7 nicotinic acetylcholine receptor (α7 nAChR)

Scheunemann, M.; Teodoro, R.; Wenzel, B.; Deuther-Conrad, W.; Steinbach, J.; Brust, P.

Objectives: Derivatives of the antiviral drug tilorone have recently been discovered as novel α7 nAChR ligands. Based on a tricyclic heteroaromatic unit linked to one or two conformationally rigidified cyclic amines, highly affine compounds were obtained [1]. [18F]DBT10 [2] and [18F]ASEM [3] (Fig.) were developed as promising α7 nAChR PET tracers. This study investigates bioisosteric sulfur-free analogues of DBT10 and its ortho-isomer obtained by replacement of the functional group SO2 by CO.
Methods: The p-fluoro and o-fluoro derivatives 1 and 2 and the corresponding nitro precursors were prepared in three and five steps from the appropriate nitro-fluoren-9-ones. Affinities towards human α7, α4β2, and α3β4 nAChRs were determined. Radiosynthesis of [18F]1 and [18F]2 via nucleophilic aromatic radiofluorination was optimised and finally performed using an automated module. In vitro autoradiography of [18F]1 on pig brain slices was performed.
Results: The fluorenones 1 and 2 were prepared in 25% yield. Both bind with high affinity and selectivity towards nAChRs (1: Ki = 1.18 nM, 1500 nM, and 46.0 nM; 2: Ki = 1.12 nM, 1796 nM, and 33.2 nM for α7, α4β2, and α3β4 nAChR, respectively). Highest labeling efficiencies (≈ 80%) were obtained in DMF under microwave assisted heating, yielding [18F]1 and [18F]2 with radiochemical purities of ≥ 97% and molar activities of 30 ± 6 GBq/μmol and 44 ± 3 GBq/μmol, respectively. Binding of [18F]1 on pig brain slices was markedly reduced by α7 nAChR-specific ligands.
Conclusions: New highly affine α7 nAChR ligands were synthesized based on the tilorone scaffold by replacement of the SO2 by a CO group. The high nitro-to-[18F]fluoro conversion obtained for the fluorenones suggests a comparable electron withdrawing effect of the two functionalities. Further studies will investigate the potential of [18F]1 and [18F]2 as PET imaging agents.
References [1] M. R. Schrimpf, K. B. Sippy, C. A. Briggs, et al. Bioorg. Med. Chem. Lett. 2012, 22, 1633-1638. [2] R. Teodoro, M. Scheunemann, W. Deuther-Conrad, et al. Molecules 2015, 20, 18387-18421. [3] A. G. Horti, Y. Gao, H. Kuwabara, et al. J. Nucl. Med. 2014, 55, 672-677.

Keywords: α7 nAChR ligands; fluorenones; radiofluorination

  • Poster
    22nd International Symposium On Radiopharmaceutical Sciences (ISRS 2017), 14.-19.05.2017, Dresden, Deutschland
  • Open Access Logo Abstract in refereed journal
    Journal of Labelled Compounds and Radiopharmaceuticals 60(2017)1, 585
    DOI: 10.1002/jlcr.3508

Permalink: https://www.hzdr.de/publications/Publ-24900
Publ.-Id: 24900


Emission spectroscopy for the identification of rare earth elements using laser-induced photoluminescence

Fuchs, M. C.; Gloaguen, R.; Beyer, J.; Jakob, S.; Heitmann, J.

We present rare earth element (REE) emission spectra stimulated by lasers with wavelength of 325 nm, 442 nm and 532 nm. Measured spectra represent REE standards in phosphates and fluorides, one set embedded in epoxy resin and one set in form of free-standing single grains. The comparison of emission features from the different specimen allows to evaluate, which emission peaks are robust and unequivocal for REE identification. Results showed that photoluminescence spectroscopy allows successful identification of characteristic emission peaks for Dy3+, Er3+, Eu3+, Gd3+, Nd3+, Sc3+, Sm3+, Tb3+, and Yb3+, when using laser stimulation at 325 nm. However, strong matrix emissions such as from epoxy resin may mask weaker luminescence emissions. In such cases, the adequate choice of longer stimulation wavelength is crucial to suppress the strong matrix emission without loosing the REE luminescence. Using laser stimulation at 442 nm and 532 nm shows promising results, but the optimal laser wavelength seems to be REE-specific and consequently, requires comprehensive analyses.

Keywords: emission spectroscopy; photoluminescence; rare earth elements; exploration; raw materials

  • Lecture (Conference)
    WHISPERS - 8th workshop on hyperspectral image and signal processing, 21.-24.08.2016, Los Angeles, USA

Permalink: https://www.hzdr.de/publications/Publ-24898
Publ.-Id: 24898


Preclinical Applications of Brain PET/MRI

Deuther-Conrad, W.

The two imaging techniques positron emission tomography (PET) and magnetic resonance imaging (MRI) enable advanced in vivo imaging in different fields of biomedical research. The combination of these two imaging modalities into a single device merges functional and morphologic information. High-resolution MRI of morphology with good soft-tissue contrast and spectroscopic detection of endogenous metabolite distributions is completed by means of high-sensitive PET by functional parameters such as glucose metabolism, amino acid transport, proliferation, receptor density, or drug concentration.
In preclinical research, the acquisition of PET and MR images of the same animal has been realized using different strategies in recent years. While initially most studies have been performed on separate devices with subsequent co-registration of the images, new and more integrated systems are available. Accordingly, fused images are obtained by the use of (i) separate instruments, (ii) in-line/sequential PET/MRI, or (iii) fully integrated PET/MRI scanners. Preclinical PET/MRI benefits in particular brain imaging in various small-animal models of diseases, including genetically-engineered animals. By providing a powerful tool for identification as well as functional characterization of new drug targets or disease biomarkers, the obtained information supports e.g. the development and follow-up of the efficacy of accordingly targeting drugs significantly.
Our research group at the Helmholtz-Zentrum Dresden-Rossendorf has been focused on the development of radiopharmaceuticals for brain PET imaging for almost two decades. Four newly developed radioligands for imaging of depression and dementia have been translated into human application within the last eight years. Related to the programme-oriented research within the Helmholtz Association the main emphasis of the group has recently moved into cancer research trying to visualize and characterize molecular switches which are involved in brain tumor development and progression by means of preclinical PET/MRI.

  • Invited lecture (Conferences)
    BRAIN & BRAIN PET 2017, 01.-04.04.2017, Berlin, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-24897
Publ.-Id: 24897


3D Mapping of electric potentials and magnetic fields at the nanoscale using Electron Holographic Tomography

Wolf, D.; Lubk, A.; Lichte, H.

Off-axis electron holography (EH) is a TEM technique that records the phase information of an electron wave transmitted through a thin specimen in an electron hologram. By reconstructing this phase information, it enables electrostatic and magnetic potentials to be mapped quantitatively with high spatial resolution and, when combined with tomography to electron holographic tomography (EHT), in three dimensions (3D) [1,2]. Tomograms obtained by EHT provide the 3D mean inner potential (MIP) distribution of nanoscale materials from which the 3D morphology and the chemical composition can be inferred [3]. Moreover, functional potentials, e.g., introduced by doping of impurities in semiconductors, have been successfully revealed in 3D [4]. Recently, we succeeded in the 3D reconstruction of the axial component of the B-field prevailing in magnetic nanowires [5,6].

EHT as applied on magnetic samples proceeds as follows (see Fig. 1): (1) an electron hologram tilt series (ideally covering a range of 360°) is acquired, (2) the phase image tilt series is reconstructed from the holograms, (3) electric and magnetic phase shifts are separated by computing half of the sum/difference between opposite (180° tilted) projections, and (4) both the electric potential and the B-field component parallel to the tilt axis are reconstructed with tomographic techniques. Here, we report EHT studies achieved by means of tomography-dedicated TEM sample holders, in combination with advanced in-house developed software packages for acquisition, alignment and tomographic reconstruction.

Fig. 2 shows the 3D electric potential reconstruction of a GaAs/AlGaAs core-multishell nanowire (NW) grown by metalorganic vapour phase epitaxy (MOVPE) using an Au nanoparticle (NP) as catalyst. Such NWs may serve as novel coherent nanoscale light sources (lasers), because they provide an effective gain medium, low-loss optical waveguiding, and strong optical confinement for axially guided optical modes. The difference in the MIP allows discriminating between GaAs and AlGaAs within the NW. Longitudinal (Fig. 2b) and cross-sectional (Fig. 2e) 2D slices averaged over a well-defined thickness reveal not only the GaAs core and the AlGaAs shell, but also a 5nm thin GaAs shell within the AlGaAs, which acts as a quantum well.

Fig. 3 comprises two recent EHT studies revealing the B-field within a Co nanowire (NW) [5] and a Co2FeGa Heusler alloy NW [6] both with spatial resolution higher than 10 nm. The reconstructions of the dominant axial component of the magnetic induction exhibit a small inversion domain at the apex of the Co NW, whereas at the Co2FeGa NW, a magnetic dead layer of 10 nm width could be revealed.

The powerful approach presented here is widely applicable to a broad range of 3D electric and magnetic nanostructures and may trigger the progress of novel nanodevices.

[1] P A Midgley and R E Dunin-Borkowski, Nat. Mater. 8 (2009) p. 271.

[2] D Wolf, A Lubk, F Röder and H Lichte, Curr. Opin. Solid State and Mater. Sci. 17 (2013) p. 126.

[3] A Lubk, D Wolf, P Prete, N Lovergine, T Niermann, S Sturm and H Lichte, Phys. Rev. B 90 (2014) p. 125404.

[4] D Wolf, A Lubk, A Lenk, S Sturm and H Lichte, Appl. Phys. Lett. 103 (2013) p. 264104.

[5] D Wolf et al., Chem. Mater. 27 (2015) p. 6771.

[6] P Simon, D Wolf, C Wang, A A Levin, A Lubk, S Sturm, H Lichte, G H Fecher and C Felser, Nano letters 16 (2016) p. 114.

[7] We thank N Lovergine of University of Salento, Lecce for provision of the GaAs/AlGaAs core-multishell nanowire samples.

[8] This work was supported by the European Union under the Seventh Framework Program under a contract for an Integrated Infrastructure Initiative Reference 312483-ESTEEM2.

Keywords: 3D reconstruction; magnetic induction; mean inner potential; nanowires

  • Invited lecture (Conferences)
    The 16th European Microscopy Congress, Lyon, France, 28.08.-02.09.2016, Lyon, Frankreich

Permalink: https://www.hzdr.de/publications/Publ-24896
Publ.-Id: 24896


Optimization of the Aeration System in Municipal Wastewater Treatment Plant

Mohseni, E.; Herrmann-Heber, R.; Reinecke, S.; Hampel, U.

In Germany wastewater is treated in 10000 plants which use 4.400 GWh of energy per year. From this big figure, up to 70% is consumed in the biological treatment and the portion of only aeration of waste water in activated sludge tank is about 50% of the whole figure. Aeration is the essential part of the process since the microorganisms need to be provided by sufficient amount of oxygen to degrade ammonia. Aerators with flexible membranes located at the bottom of the aeration tank are currently state of the art for this process. However the process suffers from some limitations such as high pressure drop, insufficient mixing and underutilization of oxygen. These are mainly due to scarce knowledge about gas dispersion inside the tank.
Initial bubble size in gas dispersion is of a great importance since it determines the ascent and coalescence rate, macro mixing by turbulence, and oxygen transfer from the bubble to the fluid phase. The initial bubble size is drastically influenced by the type of aerator which is being used in the tank. The typical bubble size generated by flexible membrane aerators has been determined to be between 2-4 mm by Hasanen et al. (Hasanen, 2006). However, this is considerably higher than the optimal bubble size calculated by Motarjemi and Jameson (Motarjemi, 1978) who reported the range of 0.7-1.0 mm for the optimal value for the 95% oxygen transfer to water in 3-6 m tank depth.
A novel approach uses solid perforated stainless steel aerators with fine pores in micro scale for gas dispersion. In this work bubble formation of stainless steel aerators has been experimentally investigated with a high resolution optical measurement technique by means of videometry and the results are compared with membrane aerators. Preliminary results of the stainless steel membrane sparger showed a significant reduction in the bubble size up to 47 % and consequently an increase in bubble residence time in comparison with flexible membranes.
A comprehensive set of experiments have been conducted in which the bubble formation with respect to pore size, pore density, plate thickness, flow rate, membrane surface roughness, and bubble generation frequency have been investigated. Thanks to the state of the art manufacturing technique, very fine pores starting at 30 µm up to 200 µm have been processed which resulted in formation of bubbles in the range down to 1.4 mm diameter.
Current poster, presents the motivation of the project and the purposed approaches toward a solution and the initial results of the preliminary experiments done by means of videometry technique.

Keywords: Bubble Generation; Activated Sludge; Aeration; Biological Wastewater Treatment; Multiphase Flow

  • Poster
    11th HZDR PhD Seminar, 17.-19.10.2016, Oberwiesenthal, Germany

Permalink: https://www.hzdr.de/publications/Publ-24895
Publ.-Id: 24895


Development of concepts for high quality image reconstruction based on adaptive grid sizes

Pfahl, A.; Wagner, M.; Bieberle, A.; Buzug, T. M.

In this work an alternative data processing concept is investigated for the correct reconstruction of slice images acquired by an ultrafast electron beam X-ray tomography scanner mainly used for analyzing multiphase flows. Currently, image reconstruction is performed on regular pixel grids by filtered back projection to achieve rapid data processing performances but leading to non-optimal image qualities. To accomplish an improved image quality the usage of irregular reconstruction grids and iterative reconstruction methods is analyzed considering the geometric arrangement and, thus, the real spatial resolution of the ultrafast CT scanner. Finally, a two-stage reconstruction approach is proposed reducing the required amount of computer memory as well as the computational time and inserting prior knowledge about the object of interest. First simulations of different irregular grids provide a promising basis for further successful implementation of the proposed two-stage reconstruction concept.

Keywords: computed tomography; irregular grids

  • Contribution to proceedings
    Studierendentagung Lübeck, 07.-09.03.2017, Lübeck, Deutschland
    Student Conference Proceedings 2017
  • Poster
    Studierendentagung Lübeck, 07.-09.03.2017, Lübeck, Deutschland
  • Lecture (Conference)
    Studierendentagung Lübeck, 07.-09.03.2017, Lübeck, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-24894
Publ.-Id: 24894


Experimentelle Untersuchung zur Hydrodynamik der Partialoxidation von Isobutan in einem Mikroreaktor

Reichelt, C.

Die hydrodynamischen Eigenschaften eines zweiphasigen Gemisches aus Isobutan und Stickstoff wurden in einem Mikroreaktor experimentell untersucht. Dazu wurden eine Hochgeschwindigkeitskamera und eine spezielle Bildanalysesoftware verwendet, mit der Isobutan Gas Strömungen in einer Glaskapillare analysiert und ausgewertet werden konnten. Anhand der erhaltenen Ergebnisse konnten Rückschlüsse auf die Bedingungen der Isobutanoxidation in jenem Reaktor gezogen werden. Ziel war es, den Einfluss der betrachteten Parameter (Eduktströme und Betriebsdruck) auf die Zwei-Phasen-Strömung zu analysieren und zu verstehen. Spezielle Strömungsregime können dabei entscheidende Auswirkungen auf den Stofftransport und somit auf reaktionstechnische Kenngrößen wie Umsatz, Selektivität oder Ausbeute haben. Regelmäßige Taylor-Strömungen zeichnen sich in Mikrokanälen durch besonders gute Stoff- und Wärmetransporteigenschaften aus. Aus diesem Grund wurden die Parameter so gewählt, dass sich dieses Strömungsregime ausbildet. Durch diese Arbeit konnten die Strömungsregime der meisten der bisher durchgeführten Oxidationsexperimente als unregelmäßige Taylor-Strömungen charakterisiert werden. Darüber hinaus konnten die Einflüsse der jeweiligen Versuchsbedingungen auf die charakteristischen Kennzahlen der Taylor-Strömung ermittelt werden. Die Arbeit bietet aufgrund ihrer Ergebnisse eine wichtige Grundlage für die Durchführung der partiellen Oxidation von Isobutan im Mikroreaktor.

Keywords: Isobutan; Zweiphasenprozess; Taylor Flow; Charakteristik

  • Study thesis
    Hochschule für Technik und Wirtschaft Dresden, 2016
    Mentor: Thomas WIllms
    34 Seiten

Permalink: https://www.hzdr.de/publications/Publ-24893
Publ.-Id: 24893


In-situ spectroscopic identification of actinide(V/VI) sorption complexes at the mineral oxide water interface

Müller, K.

For the long-term safety assessment of nuclear waste repositories, neptunium and uranium are two of the most environmentally relevant components of nuclear waste to be considered. Hence, great attention is attracted to their geochemistry and migration behavior. Among the various geochemical processes, the migration of radioactive contaminants in the environment is strongly affected by molecular reactions in aqueous solution and at the solid-water interface, e.g. complexation with organic/inorganic ligands, sorption onto mineral phases, surface precipitation, and colloid formation. A detailed description of these interactions on a molecular level is required for a reliable modeling of the contaminants dissemination in the environment.
In the past decade, vibrational spectroscopy has been developed to a powerful tool for the study of dissolved complexes of heavy metal ions with a variety of inorganic and organic ligands and surface complexes on solid phases. In particular, a combined approach of in situ vibrational, time-resolved laser fluorescence and X-ray absorption spectroscopy potentially provides comprehensive molecular information. A survey of very recent spectroscopic results obtained from geochemical reactions of radionuclides, namely Np(V) and U(VI), is given.

Keywords: actinides; vibrational spectroscopy

  • Lecture (others)
    Institutskolloquium, 29.11.2016, Kyoto, Japan

Permalink: https://www.hzdr.de/publications/Publ-24892
Publ.-Id: 24892


Ferromagnetic resonance study of equiatomic FeRh thin films

Semisalova, A.; Stienen, S.; Barton, C. W.; Boettger, R.; Bali, R.; Thomson, T.; Farle, M.; Fassbender, J.; Potzger, K.; Lindner, J.

Chemically ordered FeRh alloy with nearly equiatomic composition is antiferromagnetic at room temperature and exhibits a first-order phase transition to the ferromagnetic (FM) state at 370 K. Here, we present the study of FM resonance (FMR) in non-capped and Pt-capped magnetron sputtered 40 nm FeRh films on a MgO(100) substrate and analyse the influence of ion irradiation and chemical disordering on their magnetic properties. The temperature dependent FMR study between 200-500 K allowed us to observe the hysteretic temperature behavior, accompanied by a clear transformation of the FMR line, and explore the complex magnetic structure of films. We distinguished and characterized the contribution of anomalous FM interfacial layers induced by atomic intermixing and lattice strain. Finally, we have revealed the formation of a magnetic phase with an out-of-plane easy axis of magnetization caused by low-fluence irradiation with Ne+ ions.

  • Lecture (Conference)
    80th Annual Conference of the DPG and DPG Spring Meeting, 06.-11.03.2016, Regensburg, Germany

Permalink: https://www.hzdr.de/publications/Publ-24891
Publ.-Id: 24891


Ferromagnetic resonance in Ferh thin films near the antiferromagnetic-ferromagnetic phase transition

Semisalova, A.; Stienen, S.; Barton, C. W.; Boettger, R.; Bali, R.; Thomson, T.; Farle, M.; Fassbender, J.; Potzger, K.; Lindner, J.

High-temperature FMR on FeRh thin films

Keywords: FeRh; phase transition; ferromagnetic resonance; ion irradiation

  • Lecture (Conference)
    Euro-Asian Symposium “Trends in Magnetism” (EASTMAG-2016), 15.-19.08.2016, Krasnoyarsk, Russia
  • Invited lecture (Conferences)
    AMP 2017, Interdisciplinary workshop “Acousto-magneto-plasmonics meets quantum optics”, 28.-30.06.2017, Versailles, France
  • Invited lecture (Conferences)
    International School-Conference Spinus 2018, 01.-06.04.2018, Saint Petersburg, Russia

Permalink: https://www.hzdr.de/publications/Publ-24890
Publ.-Id: 24890


Observation of Negative Magnetic Hysteresis Loop in ZnO Thin Films

Haseman, M.; Winarksi, D.; Saadatkia, P.; Hernandez, A.; Kusz, M.; Anwand, W.; Wagner, A.; Thapa, S.; Colosimo, A. M.; Selim, F. A.

We report on the observation of an unusual negative magnetic hysteresis loop in ZnO thin film co-doped with cobalt and aluminum (Co-Al:ZnO), while other transition-metal doped ZnO films such as Cu-doped ZnO and Mn-doped ZnO, exhibit normal hysteresis loops. The unusual magnetic behavior is ascribed to the presence of double magnetic layers with different magnetic moments due to the change of structural defects across the film layers. Positron annihilation measurements confirmed the presence of unique microstructural changes in the Co-Al:ZnO film. This study shows that defects in diluted magnetic semiconductors may induce not only ferromagnetism but also novel magnetic behaviors.

Keywords: magnetism ZnO hysteresis

Permalink: https://www.hzdr.de/publications/Publ-24889
Publ.-Id: 24889


Colloidal particle at a fluidic interface: a direct numerical simulation

Lecrivain, G.; Yamamoto, R.; Hampel, U.; Taniguchi, T.

Froth flotation is a separation process in which hydrophobic particles attach to the surface of rising air bubbles while the undesired hydrophilic particles settle down the bottom of the cell to eventually be discharged. Current numerical models developed for the simulation of the particle attachment process are still at an early stage of development. The “Smooth Profile Method”, a numerical method originally developed at the University of Kyoto for the direct numerical simulation of colloidal particles in monophasic fluids, was here combined with a newly-defined binary fluid model. The change in the trajectory as the particle approaches the fluid–fluid interface, the collision process, and the sliding down the bubble surface were all reproduced and compared remarkably well with on-site microscale experiments.

  • Lecture (Conference)
    NetFlot: Network of Infrastructure (NOI) - Modelling the Flotation Process, 19.-21.10.2016, Krakau, Gliwice, Polkowice, and Wroclaw, Poland

Permalink: https://www.hzdr.de/publications/Publ-24888
Publ.-Id: 24888


Beschleunigermassenspektrometrie in Dresden

Rugel, G.; Scharf, A.; Ziegenrücker, R.; Merchel, S.

Beschleunigermassenspektrometrie (AMS, accelerator mass spectrometry) ist eine höchstsensitive Methode um langlebige Radionuklide mit einer Halbwertszeit von einigen Jahren und länger zu messen. Seit Herbst 2011 werden an der AMS-Anlage DREAMS (DREsden AMS, siehe Abbildung), routinemäßig Radionuklide gemessen. Aus der zu analysierenden – bereits chemisch aufbereiteten – Probe werden in einer Cäsium-Sputterionenquelle negative Ionen (Moleküle oder Atome) extrahiert. Diese einfach negativ geladenen Ionen werden in einem Niederenergie-Massenspektrometer nach Energie und Impuls analysiert und gelangen nachfolgend in den Tandembeschleuniger, wo sie durch eine positive Hochspannung (bis zu 6 MV) beschleunigt werden. Beim Durchgang durch ein Argon-Stripper-Gas werden Elektronen abgestreift, dadurch Moleküle zerstört, und die nun positiven Ionen ein zweites Mal beschleunigt. Im Hochenergie-Massenspektrometer werden die Radionuklide dann mit einem geeigneten Detektionssystem (u.a. einer Ionisationskammer) identifiziert. Mit diesem Aufbau lassen sich Isobare sehr effizient, sowie molekularer Untergrund vollständig unterdrücken.
Momentan werden an DREAMS Routinemessungen der Nuklide 10Be, 26Al, 36Cl, 41Ca und 129I – meist in Kooperation mit externen Partnern - durchgeführt [1,2]. Die Nachweisgrenze liegt im Bereich von 10-15 – 10-16 Radionuklid zu stabilem Nuklid.
Um DREAMS weiterzuentwickeln wurde erfolgreich eine Ionenquelle für volatile Ionen - wie Chlor und Iod - entwickelt, die ein geringes Übersprechen (kurz- sowie langfristig) zeigt [3]. Eine Ionenquelle, die effizienter die wertvollen Proben ionisieren soll und damit neue Forschungsbereiche ermöglichen wird, ist gerade in Entwicklung [4].

Ref.: [1] S. Akhmadaliev et al., NIMB 294 (2013) 5. [2] G. Rugel et al., NIMB 370 (2016) 94. [3] S. Pavetich et al., NIMB 329 (2014) 22.
[4] BMBF-Verbundprojektforschung 05K16MG1 (H. Hofsäß, U Göttingen) & 05K16KTB (J. Feige, TU Berlin).

Keywords: AMS; accelerator mass spectrometry

  • Lecture (Conference)
    Ionentreffen, 13.-15.02.2017, Göttingen, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-24887
Publ.-Id: 24887


Upgrade of the nuclear microprobe at the Ion Beam Center at HZDR

Munnik, F.; Hanf., D.; Heller, R.

The nuclear microprobe that was installed in 1994 at the Ion Beam Center of the Helmholtz-Zentrum Dresden-Rossendorf [1], has been in operation up to 2014 with only minor changes. After 20 years it became necessary to make an upgrade to replace old components and bring the setup up to current standards of technology and good working practice.
The major drawback of the old system was the optical microscope, especially the poor resolution and low contrast and brightness. However, a good optical image is essential to localise the areas of interest on, for example, large geological samples. Since the focused beam of MeV ions and the corresponding detectors is the principal investigative tool, any other equipment such as an optical microscope has to be designed around this device. A new microscope has been installed for which the first light-collecting lens is mounted directly into the chamber at only a few centimetres from the sample. The light is then guided over large mirrors and focussed on a CCD camera outside the sample chamber. The illuminating light is fed in through the lenses instead of using an external light source as in the old system. In addition, there is also a light source opposite to the microscope, allowing transmission illumination including through a polarising filter making polarised imaging possible.
Other improvements concern the control of the scanning system as well as the control and monitoring of all relevant experimental parameters. The control of the scan magnets is done by custom-designed hardware to guarantee real-time execution of the scanning without the need for a computer running a real-time operating system. Controlling and monitoring of the experiment are implemented to facilitate easy and secure operation of the microprobe by the user with special emphasis on operation by un-experienced users, since the device is part of the user facility IBC. All standard IBA techniques like PIXE, PIGE, RBS, NRA, ERDA and STIM are available and, in addition, a Channeltron detector has been installed to detect secondary electrons that allow quick imaging of the measurement area.
Technical details and first test measurements with the new system are presented.

[1] F. Herrmann, D. Grambole, Nucl. Instr. Meth. B 104 (1995) 26.

  • Poster
    Ionentreffen (Teil der Serie Workshop Ionenstrahlphysik), 13.-15.02.2017, Göttingen, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-24886
Publ.-Id: 24886


GMR films with crossed anisotropies for perpendicular field measurements

Ganss, F.; Raberg, W.; Luber, S.; Arekapudi, S. S. P. K.; Hellwig, O.; Albrecht, M.

As 3-dimensional field sensors require one part being susceptible to perpendicular fields, thin film GMR stacks with a linear and reversible response to such fields are of interest for the development of monolithic sensor devices. Stacks with such a behavior can be achieved by combining a pinning layer with perpendicular easy axis and a free layer with perpendicular hard axis. Furthermore, the stray field of the pinning layer can be minimized and its coercivity enhanced if a ferrimagnet (or antiferromagnet) is used. For the present study, TbxCo100−x alloys have been chosen as the pinning layer material and Ni81Fe19 for the free layer. The single layers have been characterized by SQUID-VSM measurements, the complete layer stack by magneto-resistance measurements. Initial films confirm the expected behavior and exhibit a change in resistivity of 5% in a field range of 10 kOe. Further films will be optimized regarding their magnetic and electric properties. They shall be micro-structured then and equipped with bottom contacts through the substrate.

  • Lecture (Conference)
    Frühjahrstagung der Deutschen Physikalischen Gesellschaft, 19.-24.03.2017, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-24885
Publ.-Id: 24885


Magnetic properties of nanostructured Tb-Fe alloy thin films

Arekapudi, S. S. P. K.; Hellwig, O.; Albrecht, M.

Nanostructured ferrimagnetic Tb-Fe alloy thin films were prepared on pre-patterned substrates as underlying template, consisting of nanodot arrays with a dot diameter of 30 nm and a period of 60 nm. Two distinct magnetic configuration are possible, one where the magnetic material on the nanodot and in the trenches is decoupled from each other for film thicknesses below the nanodot height, and the other where full exchange coupling between magnetic material on the dots and in the trenches is effective for magnetic films thicker than the nanodot height. Regardless, of the magnetic configuration the reversal of the magnetic material on top of the nanodots is found to be nucleation dominated, while the magnetic material in the trenches reverses via domain wall propagation, as confirmed by in-field magnetic force microscopy. The distinct behavior in this system is attributed to the reduced exchange stiffness followed by relatively narrow domain walls (approx. 3-4 nm) present in these rare earth - transition metal alloys. A systematic study of magnetic properties on nanostructured Tb-Fe alloy films as a function of composition and film thickness will be presented.

  • Lecture (Conference)
    Frühjahrstagung der Deutschen Physikalischen Gesellschaft, 19.-24.03.2017, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-24884
Publ.-Id: 24884


A Comparative Study on Electrical Characteristics of Crystalline AlN Thin Films Deposited by ICP and HCPA-Sourced Atomic Layer Deposition

Altuntas, H.; Bayrak, T.

In this work, we aimed to investigate the effects of two different plasma sources on the electrical properties of low-temperature plasma-assisted atomic layer deposited (PA-ALD) AlN thin films. To compare the electrical properties, 50 nm thick AlN films were grown on p-type Si substrates at 200 °C by using an inductively coupled RF-plasma (ICP) and a stainless steel hollow cathode plasma-assisted (HCPA) ALD systems. Al/AlN/p-Si metal-insulatorsemiconductor (MIS) capacitor devices were fabricated and capacitance versus voltage (C-V) and current-voltage (I-V) measurements performed to assess the basic important electrical parameters such as dielectric constant, effective charge density, flat-band voltage, breakdown field, and threshold voltage. In addition, structural properties of the films were presented and compared. The results show that although HCPA-ALD deposited AlN thin films has structurally better and has a lower effective charge density (Neff) value than ICP-ALD deposited AlN films, those films have large leakage current, low dielectric constant, and low breakdown field. This situation was attributed to the involvement of Si atoms into the AlN layers during the HCPA-ALD processing leads to additional current path at AlN/Si interface and might impair the electrical properties.

Keywords: aluminum nitride; effective charge density; atomic layer deposition (ALD); hollow-cathode plasma; inductively coupled RF-plasma; dielectric

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Permalink: https://www.hzdr.de/publications/Publ-24883
Publ.-Id: 24883


Magnetic Reversal and Microstructure Phases in Antiferromagnetically Coupled Multilayer Systems with Perpendicular Anisotropy

Hellwig, O.

Strong perpendicular anisotropy systems consisting of Co/Pt or Co/Pd multilayer stacks that are antiferromagnetically coupled via thin Ru or Ir layers have been used as model systems to study the competition between interlayer exchange, perpendicular anisotropy and long-range dipolar interactions [1,2]. The typical layer structure for such systems is illustrated in Fig.1 for a [(Co/Pt)Co/Ru] multilayer system.
Magnetometry and Magnetic Force Microscopy (MFM) studies of such systems reveal complex magnetic phases with a mix of tilted versus non-tilted and antiferromagnetic (AF) versus ferromagnetic (FM) phases depending on the dominance of the various energy contributions. For thin Co/Pt multilayer blocks AF-exchange is dominating and the magnetic structure remains laterally uniform at any stage of the reversal process and is for even number of AF-coupled blocks characterized by bulk and surface spin-flop transitions as shown on the left in Fig. 2. For thicker Co/Pt multilayer blocks the perpendicular anisotropy term takes the lead and prevents any tilting of the spins away from the easy axis normal to the film plane. In this case the reversal into the AF-coupled remanent state occurs via FM/AFM domain formation as shown in the middle part of Fig. 2. For even thicker Co/Pt multilayer blocks the AF-exchange is also overcome by the demagnetization energy and the uniform AF remanent state is replaced by a laterally heterogeneous FM stripe domain state, as illustrated in Fig. 2 on the right.
Ion beam irradiation may be able to control the rich variety of phases in this system on a nanoscopic length scale, thus allowing for a lateral co-existance of all these phases within one and the same sample. This could allow fabricating an infrastructure for controlled spin-wave propagation, such as motivated by recent studies for domain walls and perpendicular anisotropy systems [3,4]. Examples of such initial studies will be discussed.

  • Lecture (Conference)
    Intermag 2017, 24.-28.04.2017, Dublin, Ireland

Permalink: https://www.hzdr.de/publications/Publ-24882
Publ.-Id: 24882


Far-Infrared Nonlinear Optics in Multilayer Epitaxial Graphene

Suess, R. J.; Winnerl, S.; Schneider, H.; Helm, M.; Berger, C.; de Heer, W. A.; Murphy, T. E.; Mittendorff, M.

We report terahertz pump-probe experiments on multilayer graphene that measure both reflected and transmitted radiation. The data reveal complex reflection and transmission dynamics which calculations attribute to a transition from interband to intraband processes.

Keywords: graphene; pump-probe spectroscopy; refelection; transmission

  • Lecture (Conference)
    Conference on Lasers and Electro-Optics (CLEO) 2016, 05.-10.06.2016, San Jose, USA

Permalink: https://www.hzdr.de/publications/Publ-24881
Publ.-Id: 24881


Nonlinear Plasmonic THz Absorption in Graphene Ribbons

Mittendorff, M.; Jadidi, M. M.; König-Otto, J. C.; Winnerl, S.; Sushkov, A. B.; Drew, H. D.; Murphy, T. E.

We investigate the nonlinear plasmonic absorption in graphene ribbons by THz pump-probe spectroscopy. The optical nonlinearity is increased by more than one order of magnitude, which is in excellent agreement with theoretical calculations.

Keywords: graphene ribbons; plasmonics; nonlinear optics

  • Lecture (Conference)
    Conference on Lasers and Electro-Optics (CLEO) 2016, 05.-10.06.2016, San Jose, USA

Permalink: https://www.hzdr.de/publications/Publ-24880
Publ.-Id: 24880


Three-dimensional Induction Mapping of Magnetic Nanoscale Materials by Electron Holographic Tomography

Wolf, D.; Lubk, A.; Lichte, H.

Off-axis electron holography (EH) is a powerful TEM technique that provides access to the 2D projections of both the electric potential, i.e., mainly the mean inner potential (MIP) contribution, and the components of the magnetic induction (B-field) that lie perpendicular to the electron beam path.
Currently, 2D magnetic induction maps may be obtained with a spatial resolution of a few nanometer [1,2] and a sensitivity of a few millitesla [2]. Frequently, however, nanomagnetic applications, such as spin valves or magnetic nanowires require a characterization of the 3D magnetic induction distribution.
In the following, we therefore elaborate on how electron holography is combined with electron tomography to electron holographic tomography (EHT) [1,3] in order to retrieve the 3D distribution of the magnetic induction.
As shown in Fig. 1, the tomographic reconstruction of one B-field component starts with acquiring a tilt series of electron holograms that is subsequently reconstructed to obtain a tilt series of phase images. In order to collect data for the crucial separation of electric and magnetic phase shifts, either a tilt series over 360° (Fig. 1), or two tilt series, one before and one after reversing the direction of magnetization in the specimen (e.g., using the TEM objective lens or flipping the sample up-side down in the holder), have to be acquired. Before separation of electric/magnetic phase shifts by computing half of the sum/difference of corresponding image pairs, relative displacements, rotations, and direction dependent magnification changes between the two phase images are corrected. Also, the alignment of the whole tilt series, i.e., an accurate tilt axis determination and subpixel displacements correction, is of crucial importance. Finally, from the aligned tilt series, tomograms of both the electric potential and the B-field component parallel to the tilt axis are reconstructed with tomographic reconstruction algorithms.
Fig. 2 shows two recent EHT studies revealing the B-field within a Co nanowire (NW) [4] and a Co2FeGa Heusler alloy NW [5] both with spatial resolution higher than 10 nm. The reconstructions of the dominant axial component of the magnetic induction exhibit two interesting features for each case: At the Co NW, a small inversion domain at the apex was observed, whereas at the Co2FeGa NW, a magnetic dead layer of 10 nm width was found. These results were achieved by means of dedicated TEM sample holders for acquisition, in combination with advanced in-house developed software packages for acquisition, alignment and tomographic reconstruction. The powerful approach presented here is widely applicable to a broad range of 3D magnetic nanostructures and may trigger the progress of novel spintronic nonplanar nanodevices. In a next step the reconstruction of all B-field components is envisaged.
References:
[1] P A Midgley and R E Dunin-Borkowski, Nat. Mater. 8 (2009) p. 271.
[2] M Körner, F Röder, K Lenz, M Fritzsche, J Lindner, H Lichte and J Fassbender, Small 10 (2014) p. 5161.
[3] D Wolf, A Lubk, F Röder and H Lichte, Current Opinion in Solid State and Materials Science 17(2013) p. 126.
[4] D Wolf et al., Chem. Mater. 27 (2015) p. 6771.

Keywords: magnetic induction; 3D reconstruction; nanostructure; electron holography; electron tomography

  • Invited lecture (Conferences)
    Microscopy & Microanalysis 2016 Meeting, 24.-28.07.2016, Columbus, Ohio, United States of America
  • Journal of Physics D: Applied Physics 49(2016)36, 364004-1-364004-9
    DOI: 10.1088/0022-3727/49/36/364004

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Permalink: https://www.hzdr.de/publications/Publ-24879
Publ.-Id: 24879


Unusual Coulomb scattering processes in graphene

Winnerl, S.

Graphene, the two-dimensional allotrope of carbon, is characterized by a gapless linear band structure. This unique band structure is directly related to some unusual phenomena regarding the ultrafast Coulomb dynamics in the material. The understanding of this dynamics is of vital interest, both from a fundamental as well as from an application oriented point of view. The elastic Coulomb scattering is the main mechanism for thermalization of a non-equilibrium carrier distribution.
We investigate the carrier dynamics in a regime, where scattering via optical phonons is strongly suppressed. To this end, time resolved spectroscopy was performed with radiation of photon energies around 75 meV, which is smaller than the optical phonon energy (~200 meV). In polarization resolved experiments using co- and cross-polarized linearly polarized pump and probe beams, respectively, the Coulomb scattering dynamics is investigated. We find a two-fold nature of this process, namely very fast (fs timescale) collinear Coulomb scattering but surprisingly slow (ps-timescale) non-collinear scattering [1].
In the presence of a magnetic field, the linear band structure of graphene splits up into a series of non-equidistant Landau levels. Studying the transitions between the lowest Landau levels in pump-probe and four-wave mixing experiments, we find evidence for strong Auger scattering [2, 3].

[1] J. C. König-Otto, M. Mittendorff, T. Winzer, F. Kadi, E. Malic, A. Knorr, C. Berger, W. A. de Heer, A. Pashkin, H. Schneider, M. Helm, and S. Winnerl, Phys. Rev. Lett. 117, 087401 (2016).

[2] M. Mittendorff, F. Wendler, E. Malic, A. Knorr, M. Orlita, M. Potemski, C. Berger, W. A. de Heer, H. Schneider, M. Helm, and S. Winnerl, Nature Phys. 11, 75 (2015).

[3] J. C. König-Otto, Y. Wang, A. Belyanin, C. Berger, W. A. de Heer, M. Orlita, A. Pashkin, H. Schneider, M. Helm, and S. Winnerl (submitted).

Keywords: graphene; carrier dynamics; Coulomb scattering; Auger scattering; ultrafast spectroscopy

  • Lecture (others)
    MML Workshop at DESY 2016, 14.-16.12.2016, Hamburg, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-24878
Publ.-Id: 24878


Plasmonic superlens based on doped GaAs

Winnerl, S.; Fehrenbacher, M.; Kuschewski, F.; von Ribbeck, H.-G.; Döring, J.; Kehr, S.; Eng, L. M.; Schneider, H.; Helm, M.

Infrared and THz free-electron lasers are interesting sources for near-field investigations as they are tunable in a range where suitable tabletop sources exist only at particular frequencies. The free-electron laser FELBE at Dresden covers the frequency range from 1.3–75 THz with narrowband (~ 1 % spectral width) radiation. We briefly show for the low-frequency region of FELBE (1.3–75 THz) that scattering near-field microscopy can be performed with a constant spatial resolution of 50 nm, which is determined by the diameter of the scattering tip. For the longest wavelength, this corresponds to /4600 [1]. Mainly, we present results on a superlens, which consists of a doped GaAs layer sandwiched between two intrinsic GaAs layers. Superlensing is expected when the condition 〖Re(ε〗_GaAs^doped)=〖-Re(ε〗_GaAs^intrinsic) is met in the spectral vicinity of the plasmonic resonance. Here, the Drude response in the doped layer induces resonant enhancement of evanescent waves accompanied by a significantly improved spatial resolution at radiation wavelengths around 15 THz (see Fig. 1) [2]. The resonance frequency is adjustable by changing the doping concentration. Compared to superlenses based on phononic resonances the plasmonic superlance features a somewhat broader range of the resonant response. Such a tunable superlense consisting of a single semiconductor material is
a versatile device to enhance signal and spatial resolution in near-field imaging of buried structures.

Keywords: near-field microscopy; superlens; plasmonics; free-electron laser

  • Invited lecture (Conferences)
    Samller and Faster: Infrared and Terahertz Spectral-Imaging at the Nanoscale with Synchrotron and Free Electron Laser Sources (SAFE), 01.-02.12.2016, Trieste, Italy

Permalink: https://www.hzdr.de/publications/Publ-24877
Publ.-Id: 24877


Carrier dynamics in Graphene

Winnerl, S.

We review our results ion the carrier dynamics in graphene and present an applacitation example, namely a high-speed ultra-broadband detector.

Keywords: graphene; carrier dynamics; detector

  • Lecture (others)
    4. Workshop of SPP 1459 "Graphene", 25.-29.09.2016, Chemnitz, Germany

Permalink: https://www.hzdr.de/publications/Publ-24876
Publ.-Id: 24876


Population and polarization dynamics in Landau-quantized graphene - evidence for strong Auger scattering

Winnerl, S.; Mittendorff, M.; König-Otto, J. C.; Wendler, F.; Malic, E.; Knorr, A.; Pashkin, A.; Schneider, H.; Helm, M.

The linear band structure of graphene results in a non-equidistant spectrum of Landau levels (LLs). While transport phenomena have been widely explored after the discovery of graphene in 2004, the ultrafast optical properties of Landau-quantized graphene have been investigated only recently. Predictions of very strong nonlinearities [1], carrier multiplication [2] and Landau-level gain [3] make the system very interesting both from a fundamental and an application-oriented point of view.
Here we present experiments complemented by microscopic theory on the population and polarization dynamics in the subsystem of Landau levels -1, 0 and 1. To this end, multilayer epitaxial graphene in a magnetic field of ~4 T is excited resonantly by ps-pulses of mid-infrared radiation (photon energy 75 meV). Applying circularly polarized radiation allows one to pump and probe the energetically degenerate LL-1 → LL0 and LL0 → LL1 transitions selectively. In pump-probe experiments using all four combinations of pumping and probing with right and left circularly polarized radiation, a surprising change in sign of the measured signal (i.e. induced transmission instead of induced absorption) is observed in one of these configurations. Our analysis shows that this can be associated with a depletion of the LL0 level, even though this level is optically pumped at the same time. Very efficient Auger scattering is responsible for this depletion [4].
Furthermore, we show a very recent study of the polarization dynamics in Landau quantized graphene by means of degenerate four-wave mixing (FWM) spectroscopy. It is carried out on the same subsystem of Landau levels and utilizes similar values of magnetic field and photon energy as in the pump-probe experiments. However, the two incoming beams are linearly polarized in the FWM experiment. A rapid dephasing of the microscopic polarization on a timescale shorter than the pulse duration (4 ps) is observed and attributed to Auger scattering. The FWM signals show the expected dependences on the power of two incoming beams and on the magnetic field.
The results, both for the population as well as the polarization dynamics, depend crucially on the doping level of the sample. The graphene used in the experiments is almost intrinsic, corresponding to a full LL-1 and empty LL1. The occupation of LL0 of about 0.64 is slightly larger than the half-filling corresponding to intrinsic graphene. We will evaluate the implications of this symmetry breaking on Auger scattering and on the FWM signals. Finally, the implications of our results on the feasibility of a graphene based Landau-level laser, which is tunable by the magnetic field, will be discussed.
The sample was grown by C. Berger and W.A. de Heer. We acknowledge valuable discussions with M. Orlita, M. Potemski, Y. Wang, and A. Belyanin. We are grateful to P. Michel and the FELBE team for their dedicated support.
References
[1] X. Yao and A. Belyanin, Phys. Rev. Lett. 108, 255503 (2012).
[2] F. Wendler, A. Knorr, and E. Malic, Nature Commun. 5, 3703 (2014).
[3] F. Wendler and E. Malic, Sci. Rep. 5, 12646 (2015).
[4] M. Mittendorff, F. Wendler, E. Malic, A. Knorr, M. Orlita, M. Potemski, C.

Keywords: Carrier; dynamics; graphene; Landau quantization; Auger scattering; polarization dynamics; four-wave mixing

  • Invited lecture (Conferences)
    22nd International Conference on High Magnetic Fields in Semiconductor Physics (HMF-22), 24.-29.07.2016, Sapporo, Japan

Permalink: https://www.hzdr.de/publications/Publ-24875
Publ.-Id: 24875


Zero-Offset Hall: A new concept for Antiferromagnetic Spintronics

Kosub, T.; Kopte, M.; Radu, F.; Schmidt, O. G.; Makarov, D.

Antiferromagnets have the potential to revolutionize spintronics due to their inherently magnetic-field stable magnetic order. The tiny uncompensated magnetic moment of the locally uncompensated antiferromagnetic lattice determines not only the physics of the materials, e.g. topology of antiferromagnetic domain walls [1], but also their application potential for novel magnetoelectric random access memory (MERAM) devices [2] or
antiferromagnetic analogues to racetrack devices. The rich physics of thin film antiferromagnets can be harnessed for prospective spintronic devices given that all-electric assessment of the tiny uncompensated magnetic moment is achieved.

We put forth a new method providing all-electric access to the field-invariant magnetization of antiferromagnetic thin films [3]. This technique – zero-offset Hall – is based on the combination of Anomalous Hall effect magnetometry with dynamic spinning-current offset compensation. We show that this technique opens the scope for magnetic phenomena that can go amiss in conventional Hall measurements with stationary current direction. The adoption of this technique will therefore substantially improve the reach of lab-based transport investigations in the thriving field of antiferromagnetic spintronics and
lead to new application concepts.

On the examples or metallic IrMn and insulating Cr 2 O 3 antiferromagnetic thin films, we demonstrate, that zero-offset Hall can probe thin film magnetism at unprecedented sensitivity, which allows us to reveal previously unknown peculiarities of the physical behavior. The access to insulating magnetic films is enabled by the magnetic proximity effect of certain conductors such as Pt.

The method is not exclusive to anomalous Hall measurements. Instead, all phenomena that alter the transversal resistance, such as the topological Hall effect and the quantum Hall effects, can be studied in greatly improved detail. At the same time, the measurements are technically easy and can be realized in most laboratories making it fast to adopt. Therefore, we believe that zero-offset Hall is of strong relevance for the community of topological and spin phenomena in nanostructures.

[1] M. Bode et al., Nature Materials 5, 477 (2006)
[2] X. He et al., Nature Materials 9, 579 (2010)
[3] T. Kosub et al., Phys. Rev. Lett. 115, 097201 (2015)

Keywords: Zero-Offset Hall; Antiferromagnetic Spintronics

  • Lecture (Conference)
    TOPSPIN 2, 18.-20.05.2016, Groningen, Niederlande

Permalink: https://www.hzdr.de/publications/Publ-24874
Publ.-Id: 24874


Long-lived Anisotropy of Photoexcited Graphene Electrons

Winnerl, S.; König-Otto, J. C.; Mittendorff, M.; Winzer, T.; Malic, E.; Knorr, A.; Pashkin, A.; Schneider, H.; Helm, M.

In polarization resolved pump-probe experiments complemented by microscopic modeling we show that an anisotropic carrier distribution persists on ps timescales. In particular, we evaluate the role of non-collinear Coulomb scattering.

Keywords: Carrier dynamics; graphene; ultrafast spectroscopy; Coulomb scattering

  • Lecture (Conference)
    Conference on Lasers and Electro-Optics (CLEO) 2016, 05.-11.06.2016, San Jose, USA

Permalink: https://www.hzdr.de/publications/Publ-24873
Publ.-Id: 24873


All-electric measurement of the field-invariant magnetization of antiferromagnets

Kosub, T.; Kopte, M.; Schmidt, O. G.; Makarov, D.

Antiferromagnetic materials combine magnetic properties in peculiar combination that is not found in their ferromagnetic counterparts. They are more robust against magnetic disturbances and show great promises in e.g. magnetoelectric applications [1,2]. Despite these fascinating fundamental properties, both the understanding of antiferromagnets and their commercial establishment have progressed slower than for ferromagnets. The tiny uncompensated magnetic moment of the locally uncompensated antiferromagnetic lattice is often the linking element to intrinsic processes that experimenters can exploit or devices can rely on.
One of the most sensitive techniques for thin film magnetometry, the anomalous Hall effect (AHE) [3], is routinely employed to study magnetization eversal in ferromagnets [4] by monitoring the magnetization-proportional anomalous Hall resistance.
However, this typically suffers from a sizable parasitic signal offset due to imperfect device geometry. But, the offset also contains real AHE signals generated by field-invariant magnetization components which are inaccessible in conventional Hall measurements.
We demonstrate, that AHE magnetometry using the spinning-current technique can measure such field-invariant magnetization on an absolute scale by dynamically compensating the parasitic resistance contribution (Fig.) [5]. Thus, we acquire previously unattainable figures about antiferromagnetic materials and refine existing measurements due to the great precision and time-efficiency of AHE magnetometry.
We establish that this technique is suitable to probe a wide range of materials by applying it to polycrystalline IrMn thin films as well insulating Cr 2 O 3 films. Moreover, our all-electric measurements also prove that it is feasible to entirely omit ferromagnets in antiferromagnet-based applications. Doing so paves the way to all-antiferromagnet spintronics with improved performance or improved reliability [6].

[1] X. He, Y. Wang, N. Wu, A. N. Caruso, E. Vescovo, K. D. Belashchenko, P. A. Dowben, and C. Binek, Nature Materials 9, 579 (2010).
[2] J. Heron, J. Bosse, Q. He, Y. Gao, M. Trassin, L. Ye, J. Clarkson, C. Wang, J. Liu, S. Salahuddin, and others, Nature 516, 370 (2014).
[3] N. Nagaosa, J. Sinova, S. Onoda, A. MacDonald, and N. Ong, Rev. Mod. Phys. 82, 1539 (2010).
[4] D. Bhowmik, L. You, and S. Salahuddin, Nature Nanotechnology 9, 59 (2014).
[5] T. Kosub, M. Kopte, F. Radu, O. G. Schmidt, and D. Makarov, Phys. Rev. Lett. 1, 1 (2015).
[6] T. Kosub, O. G. Schmidt, and M. Denys, Patent Applied For (n.d.).

Keywords: Zero-Offset-Hall; antiferromagnetic spintronics

  • Lecture (Conference)
    Joint MMM/Intermag San Diego, 11.-15.01.2016, San Diego, USA
  • Lecture (Conference)
    DPG Regensburg, 14.-18.03.2016, Regensburg, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-24872
Publ.-Id: 24872


Analytical Electron Microscopy Study to Resolve the Phase Morphology of Organic Solar Cell Blends

Sedighi, M.; Löffler, M.; Röder, F.; Zschech, E.

To increase the efficiency of bulk heterojunctions for organic photovoltaic devices, the complicated photon-to-electron conversion process has to be understood in detail. To this aim, one challenge is to resolve the correlation between processing parameters of organic solar cells (OSC), the resulting nanoscale morphology of the absorber layer, and efficiency of the completed device. Here, we present the effect of substrate heating on the morphology of the OSC where the active layer is a blend of two small molecules; ZnPc (ZnC32H18N8) as donor and C60 as acceptor.

Obtaining insights into the morphology of the active layer requires the spatial resolution and a contrast mechanism to discriminate two phases with similar average atomic number. To tackle this challenge, we combine electron microscopy imaging with different analytical techniques; energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) in TEM, as well as Energy selective Backscattered (EsB) imaging in SEM.

We imaged different phases of the donor and acceptor, forming ordered and non-ordered regions, depending on the way the heterojunction is fabricated. To this aim, we fabricated samples at substrate temperatures of 110°C and 150°C, each in two different configurations:

1) Focused ion beam prepared ultrathin lamella of a complete solar cell stack with glass substrate, Indium tine oxide (ITO) electrode, ZnPc:C60 blend of the active layer between electron and hole transport layers and aluminum top electrode.
2) Plane view sample as ZnPc:C60 blend deposited on a TEM grid coated with ITO layer.

It was shown that at a substrate temperature of 110°C, the solar cell device has high efficiency [1]., so we consider this as the optimum substrate temperature.

Since identification of the composition of each phase in the plane view sample is more straight forward, we use the plane view sample to attribute each structure to one component of the blend. SEM images recorded by using secondary electron detector show that, the high temperature sample consists of rod-like features and cube-shaped material in between the rods. By combination of analytical microscopy techniques, we can attribute the rod-like structures to ZnPc.

An energy-selective backscatter (EsB) electron detector in a SEM is used to obtain backscattered contrast of the plane view sample. Due to the atomic number difference between the donor phase and the acceptor phase, (the average atomic number of 8.6 for ZnPc and 6 for the C60) sufficient contrast can be achieved [2].

A clear confirmation for the attribution of the ZnPc phase to the rod-like features and the attribution of C60 to the granular structure in between can be drawn from the investigation of the sample in SEM using EDX and with even higher precision in TEM using an improved EDX system. The chemical mapping of zinc and carbon, proves the correct phase assignment, in both plane view and ultrathin samples, prepared at high temperature. For the sample produced at optimum temperature, a much smaller roughness was observed because of the absence of large ordered regions. Even in the plane view sample, the imaging contrast is low due to less separated phases and smaller domains.

To conclude, we clearly resolved the phase morphology of the interpenetrating network of ZnPc:C60 blends for high and optimum temperature samples in plane view and furthermore we were even able to reveal the morphology from TEM images of cross-section lamellas of the real solar cell stacks.

Since the ideal active layer should have domain sizes at the range of the exciton diffusion length (10-20nm), the sample produced at high temperature does not show the desired microstructure. Due to the domain sizes of ~100 nm there is no closed path for exciton dissociation. In addition to that, the sample shows a quite high roughness.

Acknowledgement:
Authors thank A. G. Cid for SEM images. This work was supported by the German Science Council Center of Advancing Electronics Dresden (cfaed). TEM-EDX results were achieved by funding (support code 03SF0451) through the Helmholtz Energy Materials Characterization Platform (HEMCP) initiated by the Helmholtz Association and the German Federal Ministry of Education and Research (BMBF).

References:
[1] S. Pfuetzner, C. Mickel, J. Jankowski, M. Hein, J. Meiss, C. Schuenemann, C. Elschner, A.A. Levin, B. Rellinghaus, K. Leo and M. Riede, “The influence of substrate heating on morphology and layer growth in C60:ZnPc bulk heterojunction solar cells” Org. Electron., 12 (2010), pp. 435–441
[2] A. G. Cid, M. Sedighi, M. Löffler, W. F. van Dorp and E. Zschech, “Energy-Filtered Backscattered Imaging Using Low-Voltage Scanning Electron Microscopy: Characterizing Blends of C60:ZnPc for Organic Solar Cells”. (DOI: 10.1002/adem.201600063)

  • Poster
    EMRS Fall Meeting 2016, 19.-22.09.2016, Warschau, Polen

Permalink: https://www.hzdr.de/publications/Publ-24871
Publ.-Id: 24871


Surprising Coulomb-scattering effects in graphene revealed by polarization-resolved THz spectroscopy

Winnerl, S.; König-Otto, J. C.; Mittendorff, M.; Pashkin, A.; Schneider, H.; Helm, M.; Winzer, T.; Wendler, F.; Malic, E.; Knorr, A.

We present experiments complemented by microscopic theory that highlight surprising Coulomb scattering effects. In particular, we find that non-collinear Coulomb scattering is compa-rably slow, namely on a scale of few ps. For the case of Landau-quantized graphene we find evidence for strong Auger scattering that can deplete a Landau level that is optically pumped at the same time. Both Coulomb effects are relevant for graphene based THz devices.

Keywords: graphene; carrier dynamcis; Coulomb scattering; Auger scattering; Landau quantization

  • Invited lecture (Conferences)
    5th EOS Topical Meeting on Terahertz Science & Technology, 08.-11.05.2016, Pecs, Hungary

Permalink: https://www.hzdr.de/publications/Publ-24870
Publ.-Id: 24870


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