Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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34121 Publications

Experiments and Simulations of the Magnetized Spherical Couette Problem

Garcia Gonzalez, F.; Stefani, F.

The magnetized spherical Couette system models experiments that are being carried out at Helmholtz-Zentrum Dresden-Rossendorf (HZDR). A liquid metal is confined within two differentially rotating spheres and immersed in a magnetic field parallel to the axis of rotation. Preliminary simulations of periodic and quasiperiodic flows, arising at the first bifurcations, will be presented. The aim is to study in detail the instabilities observed in the experiments and previous numerical studies. This study will reveal how the flow patterns depend on control parameters, reproducing thus different physical situations of the HZDR experiments.

  • Poster
    GDRI Dynamo meeting 2017, 27.-29.11.2017, Paris, France

Permalink: https://www.hzdr.de/publications/Publ-28773
Publ.-Id: 28773


New axisymmetric helical magnetorotational instability in dissipative rotating flows with positive shear

Mamatsashvili, G.; Stefani, F.; Hollerbach, R.; Rüdiger, G.

We present a new type of axisymmetric magnetorotational instability which is capable of destabilizing viscous and resistive magnetized flows with radially increasing angular velocity. Using short-wavelength WKB approach, supported by 1D linear stability calculations in Taylor-Couette flow, we show that this instability works only when a combination of axial and azimuthal magnetic fields is applied and when the magnetic Prandtl number is different from one. It might have grave consequences for the stability of the equator-near parts of the solar tachocline, and for the dynamo action in this region

  • Lecture (Conference)
    MHD Days and GdRI Dynamo Meeting, 26.-28.11.2018, Dresden, Germany

Permalink: https://www.hzdr.de/publications/Publ-28772
Publ.-Id: 28772


Experiments and Simulations of the Magnetized Spherical Couette Problem

Garcia Gonzalez, F.; Stefani, F.

The magnetized spherical Couette system models experiments that are being carried out at Helmholtz-Zentrum Dresden-Rossendorf (HZDR). A liquid metal is confined within two differentially rotating spheres and immersed in a magnetic field parallel to the axis of rotation. Preliminary simulations of periodic and quasiperiodic flows, arising at the first bifurcations, will be presented. The aim is to study in detail the instabilities observed in the experiments and previous numerical studies. This study will reveal how the flow patterns depend on control parameters, reproducing thus different physical situations of the HZDR experiments.

  • Poster
    Alexander von Humboldt Foundation Network Meeting 2018, 19.-21.02.2018, Potsdam, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-28771
Publ.-Id: 28771


A Tayler-Spruit type model of a tidally synchronized solar dynamo

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

We consider a solar dynamo model of Tayler-Spruit type whose Omega-effect is conventionally produced by a solar-like differential rotation but whose alpha-effect is assumed to be periodically modulated by planetary tidal forcing. This resonance-like effect relies on the tendency of the current-driven Tayler instability to produce intrinsic helicity oscillations which can be synchronized by periodic tidal perturbations. Specifically, we focus on the 11.07 years alignment periodicity of the tidally dominant planets Venus, Earth, and Jupiter, whose empirical synchronization with the solar dynamo is illustrated. The typically emerging dynamo modes are dipolar fields, oscillating with a 22.14 years period or pulsating with a 11.07 years period, but also quadrupolar fields with corresponding periodicities. In the absence of any constant part of alpha, we prove the subcritical nature of this periodic Tayler-Spruit type dynamo. Phase coherent transitions between dipoles and quadrupoles, which are reminiscent of the observed behaviour during the Maunder minimum, can be easily triggered by long-term variations of dynamo parameters. Further interesting features of the model are the typical second intensity peak and the intermittent appearance of reversed helicities in both hemispheres

  • Poster
    MHD Days and GdRI Dynamo Meeting, 26.-28.11.2018, Dresden, Germany

Permalink: https://www.hzdr.de/publications/Publ-28770
Publ.-Id: 28770


Rotating Waves in Spherical Geometry: Thermal Convection in Thin Rotating Shells and the Magnetized Spherical Couette System

Garcia Gonzalez, F.; Sánchez, J.; Net, M.; Chambers, F.; Watts, A.; Stefani, F.

Fluid dynamics plays an important role in many geophysical and astrophysical objects such as planets and stars. For instance, convection can occur in neutron stars' oceans formed by very thin layers of helium or hydrogen, which are subject to the influence of strong temperature gradients and rotation. In addition, instabilities observed in differentially rotating flows in the presence of a magnetic field (magnetized spherical Couette flows) were attributed to the magnetorotational instability (MRI), which is presently considered the most promising candidate to explain the transport mechanism of angular momentum in accretion disks around black holes and protostars. In this study, bifurcation diagrams of the first instabilities occurring in the two mentioned set-ups will be presented. They were obtained by means of continuation techniques. The arising flow patterns will be described. In both cases, pseudo-spectral high order methods as well as high order time integration methods are used for the time evolution of the Navier-Stokes equations.

  • Lecture (Conference)
    International Conference on Spectral and High-Order Methods ICOSAHOM'18., 09.-13.07.2018, London, United Kingdom

Permalink: https://www.hzdr.de/publications/Publ-28769
Publ.-Id: 28769


The DRESDYN project: Liquid metal experiments on dynamo action and magnetorotational instability

Stefani, F.

The dynamo effect in moving electrically conducting fluids is at the root of magnetic field generation in planets and stars. Yet, cosmic magnetic fields play also an active role in the formation of central objects, such as protostars and black holes, by destabilizing accretion disks that would be hydrodynamically stable. While often studied separately, dynamo action and magnetically triggered instabilities may also occur together in such highly non-linear processes as the MRI dynamo or the Tayler-Spruit dynamo.

The DRESDYN project at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) serves as a platform for continuing the liquid metal experiments of the last two decades which were related to dynamo action and magnetically triggered flow instabilities. After a short survey of the dynamo experiments in Riga, Karlsruhe and Cadarache, and the various MRI experiments at the PROMISE facility at HZDR, I discuss the preparatory status of a large-scale precession experiment and a Taylor-Couette experiment for investigating various forms of the MRI and their combinations with the Tayler instability. Special focus will be laid on the numerical predictions of both experiments, as well as on some recent findings concerning the relation of non-modal growth in rotating flows with dissipation-induced instabilities, such as helical and azimuthal MRI for negative and positive shear.

  • Invited lecture (Conferences)
    Waves, Turbulence, and Large-scale Structures in Rotating Magnetic Fluids: Above & Beyond Geophysical Fluid Dynamics, 10.-14.09.2018, Boulder, USA

Permalink: https://www.hzdr.de/publications/Publ-28768
Publ.-Id: 28768


A tidally synchronized Tayler-Spruid type model of the solar dynamo

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

We present a solar dynamo model of the Tayler-Spruit type whose Omega-effect is produced, as usual, by differential rotation but whose alpha-effect is assumed as being periodically modulated by planetary tidal forcing. This resonance-like effect has its rationale in the tendency of the current-driven Tayler instability to undergo intrinsic helicity oscillations which can be synchronized by periodic tidal perturbations. Specifically, we focus on the 11.07 years periodicity of the alignment of the tidally dominant planets Venus, Earth, and Jupiter. In the framework of a simple one-dimensional numerical model we prove the subcritical character of this Tayler-Spruit type dynamo. The typical dynamo modes are dipole fields, oscillating with a 22.14 year period, but also quadrupole fields pulsating with an 11.07 years period. Transitions between these field topologies are reminiscent of the observed behavior during the Maunder minimum. Further interesting features of the model are the emergence of mid-term fluctuations, and the intermittent appearance of reversed helicities in both hemispheres. With minor model modifications, the correct direction of the butterfly diagram comes out as a robust feature, too.

  • Lecture (Conference)
    Third Russian Conference on Magnetohydrodynamics, 18.-21.06.2018, Perm, Russia

Permalink: https://www.hzdr.de/publications/Publ-28767
Publ.-Id: 28767


Experiments and Simulations on the Magnetized Spherical Couette Problem

Garcia Gonzalez, F.; Stefani, F.

Experiments on the magnetized spherical Couette system are presently being carried out at Helmholtz-Zentrum Dresden-Rossendorf (HZDR). A liquid metal (GaInSn) is confined within two differentially rotating spheres and exposed to a magnetic field parallel to the axis of rotation. Bifurcation diagrams for rotating waves, obtained with continuation methods when only the magnetic field is increased, are presented. This allows us to carefully investigate the time-scales of the nonlinear saturation of the radial jet, return flow, and shear layer instabilities, as found in previous studies. In addition, modulated rotating waves, obtained at secondary bifurcations, are exhaustively studied by means of direct numerical simulations, with main focus on their spatio-temporal symmetries. We find that at moderate differential rotation the modulated rotating waves give rise to several types of chaotic flows, but only for the radial jet instability. With this study we reveal how the flow patterns and time-scales depend on the magnetic field, reproducing thus different physical situations of the HZDR experiments.

  • Lecture (Conference)
    MHD Days and GdRI Dynamo Meeting, 2018, 26.-28.11.2018, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-28766
Publ.-Id: 28766


A Tayler-Spruit model of the solar dynamo with tidal synchronization

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

We present a solar dynamo model of the Tayler-Spruit type whose Omega-effect is coventionally produced by differential rotation but whose alpha-effect is assumed as being periodically modulated by planetary tidal forcing. This resonance-like effect has its rationale in the tendency of the current-driven Tayler instability to undergo intrinsic helicity oscillations which can be synchronized by periodic tidal perturbations. Specifically, we focus on the 11.07 years periodicity of the alignment of the tidally dominant planets Venus, Earth, and Jupiter. In the framework of a simple one-dimensional numerical model we prove the subcritical character of this Tayler-Spruit type dynamo. The typical dynamo modes are dipole fields, oscillating with a 22.14 year period, but also quadrupole fields pulsating with an 11.07 years period. Transitions between these field topologies are reminiscent of the observed behavior during the Maunder minimum. Further interesting features of the model are the emergence of mid-term fluctuations, and the intermittent appearance of reversed helicities in both hemispheres. With minor model modifications, the correct direction of the butterfly diagram comes out as a robust feature, too.

  • Invited lecture (Conferences)
    Planetary-Stellar Connection: The Sun's Lesson, 07.-09.05.2018, Freiburg im Breisgau, Germany

Permalink: https://www.hzdr.de/publications/Publ-28765
Publ.-Id: 28765


Thermal convection in rotating spherical shells: Temperature-dependent internal heat generation using the example of triple-𝛼 burning in neutron stars

Garcia Gonzalez, F.; Chambers, F.; Watts, A.

We present an extensive study of Boussinesq thermal convection including a temperature-dependent internal heating source, based on numerical three-dimensional simulations. The temperature dependence mimics triple-α nuclear reactions and the fluid geometry is a rotating spherical shell. These are key ingredients for the study of convective accreting neutron star oceans. A dimensionless parameter Raₙ, measuring the relevance of nuclear heating, is defined. We explore how flow characteristics change with increasing Raₙ and give an astrophysical motivation. The onset of convection is investigated with respect to this parameter and periodic, quasiperiodic, chaotic flows with coherent structures, and fully turbulent flows are exhibited as Raₙ is varied. Several regime transitions are identified and compared with previous results on differentially heated convection. Finally, we explore (tentatively) the potential applicability of our results to the evolution of thermonuclear bursts in accreting neutron star oceans.

Downloads:

Permalink: https://www.hzdr.de/publications/Publ-28764
Publ.-Id: 28764


Self-excitation in a helical liquid metal flow: the Riga dynamo experiments

Gailitis, A.; Gerbeth, G.; Gundrum, T.; Lielausis, O.; Lipsbergs, G.; Platacis, E.; Stefani, F.

The homogeneous dynamo effect is at the root of magnetic field generation in cosmic bodies, including planets, stars and galaxies. While the underlying theory had increasingly flourished since the middle of the 20th century, hydromagnetic dynamos were not realized in the laboratory until 1999. On 11 November 1999, this situation changed with the first observation of a kinematic dynamo in the Riga experiment. Since that time, a series of experimental campaigns has provided a wealth of data on the kinematic and the saturated regime. This paper is intended to give a comprehensive survey about these experiments, to summarize their main results and to compare them with numerical simulations.

Downloads:

Permalink: https://www.hzdr.de/publications/Publ-28763
Publ.-Id: 28763


Continuation and stability of rotating waves in the magnetized spherical Couette system: secondary transitions and multistability

Garcia Gonzalez, F.; Stefani, F.

Rotating waves (RW) bifurcating from the axisymmetric basic magnetized spherical Couette (MSC) flow are computed by means of Newton–Krylov continuation techniques for periodic orbits. In addition, their stability is analysed in the framework of Floquet theory. The inner sphere rotates while the outer is kept at rest and the fluid is subjected to an axial magnetic field. For a moderate Reynolds number Re = 10^3 (measuring inner rotation), the effect of increasing the magnetic field strength (measured by the Hartmann number Ha) is addressed in the range Ha ∈ (0, 80) corresponding to the working conditions of the HEDGEHOG experiment at Helmholtz-Zentrum Dresden-Rossendorf. The study reveals several regions of multistability of waves with azimuthal wavenumber m = 2, 3, 4, and several transitions to quasi-periodic flows, i.e modulated rotating waves. These nonlinear flows can be classified as the three different instabilities of the radial jet, the return flow and the shear layer, as found in the previous studies. These two flows are continuously linked, and part of the same branch, as the magnetic forcing is increased. Midway between the two instabilities, at a certain critical Ha, the non-axisymmetric component of the flow is maximum.

Downloads:

Permalink: https://www.hzdr.de/publications/Publ-28762
Publ.-Id: 28762


The DRESDYN project: liquid metal experiments on dynamo action and magnetorotational instability

Stefani, F.; Gailitis, A.; Gerbeth, G.; Giesecke, A.; Gundrum, T.; Rüdiger, G.; Seilmayer, M.; Vogt, T.

Magnetic fields of planets, stars and galaxies are generated by self-excitation in moving electrically conducting fluids. Once produced, magnetic fields can play an active role in cosmic structure formation by destabilising rotational flows that would be otherwise hydro-dynamically stable. For a long time, both hydromagnetic dynamo action as well as magnetically triggered flow instabilities had been the subject of purely theoretical research. Meanwhile, however, the dynamo effect has been observed in large-scale liquid sodium experiments in Riga, Karlsruhe and Cadarache. In this paper, we summarise the results of liquid metal experiments devoted to the dynamo effect and various magnetic instabilities such as the helical and the azimuthal magnetorotational instability and the Tayler instability. We discuss in detail our plans for a precession-driven dynamo
experiment and a large-scale Tayler–Couette experiment using liquid sodium, and on the prospects to observe magnetically triggered instabilities of flows with positive shear.

Downloads:

  • Secondary publication expected

Permalink: https://www.hzdr.de/publications/Publ-28761
Publ.-Id: 28761


Numerical simulation of multicomponent flows with the presence of density gradients for the upgrading of advanced turbulence models

Huang, M.; Höhne, T.

The turbulence effects during the buoyancy-driven mixing was investigated at a vertical mixing (VeMix) test facility, which was developed to investigate the mixing of high borated and low borated coolant in nuclear reactor. Additional buoyancy terms are included in buoyancy-modified turbulence models, which have been implemented in the CFD code ANSYS CFX and validated with experimental data captured by optical methods and conductivity measurement technology. The physicality of the flow phenomena and the vortical oscillations analyzed by Fourier tranformation in both the experiments and simulations show good agreement under different flow conditions. The influence of different buoyancy models were investigated in detail and optimal models for simulations at similar flow conditions have been selected.

Keywords: Multicomponent flow; CFD; turbulence models; SBES; RANS; LES

Downloads:

Permalink: https://www.hzdr.de/publications/Publ-28760
Publ.-Id: 28760


Simulation der Strömungsverhältnisse in einem DWR (Vor-Konvoi)

Höhne, T.; Kliem, S.

Ziel der Untersuchung war der Nachweis möglicher Wirbel zwischen der 8.-9. Abstandshalterebene eines Vorkonvoi-Kerns, die zu Temperatur-Wechselbelastungen führen könnten. Es wurde eine 3D-CFD Modell genutzt, die instationäre Rechnung wurde mit einem geeignetem hybriden RANS-LES SBES Turbulenzmodell durchgeführt. Die CFD-Rechnung war off-line gekoppelt mit einem Neutronenkinetikprogramm (DYN3D). Es erfolgte eine Nutzung bekannter Druckverluste über der Kernhöhe zur Festlegung von Permeabilität und Widerstandskoeffizienten. Hierbei erfolgte eine Absicherung durch Vergleichsrechnungen mit einem Modell aus der Literatur. Die neutronenkinetische Rechnung wurde unter Normalbetriebsbedingungen mit einer prototypischen Leistungsverteilung im Kern bei EOC durchgeführt. Die Ergebnisse der Berechnung zeigen keine großflächigen dominanten Wirbel im oberen Kernbereich. Der Kern wirkt als Gleichrichter und dämpft großräumige Wirbel. Die Analysen umfassten mehrere Höhenebenen im Kern und zeigen, dass in einigen Bereichen des Kernquerschnitts eine nach oben zunehmend gerichtete laterale Strömung zu den Austrittsstutzen auftritt. In anderen Bereichen des Kernquerschnitts tritt dagegen kaum Querströmung auf. Die Geschwindigkeiten sind Leerrohrgeschwindigkeiten. Um diese in Strömungsgeschwindigkeiten umzurechnen, müssen diese durch die Porosität (ca. 0.4 in vertikaler und ca. 0.25 in horizontaler Richtung) geteilt werden. Die berechnete Leerrohrgeschwindigkeit von 2 m/s entspricht einer Strömungsgeschwindigkeit im Kern von ca. 5 m/s. Analog ist die Transversalgeschwindigkeit von 0.05 m/s (Leerrohr) einer Strömungsgeschwindigkeit von 0.2 m/s gleichzusetzen.
Folgende Einschränkungen des Modells gelten jedoch: Im Modell sind alle Brennelemente gleich behandelt und Querströmungen auf Grund unterschiedlicher axialer Druckverluste bei verschiedenen BE-Typen können nicht dargestellt werden. Die komplexe Struktur der BEs (bspw. Strömungsfahnen in AHs) könnte einen Einfluss auch auf die Bildung großräumiger Wirbel haben. Dieser Effekt könnte nur mit einer sehr hohen Anzahl Gitterelemente aufgelöst werden (mehrere Milliarden). Dies ist momentan rechentechnisch noch nicht möglich. Auch der mögliche Einfluss von Zweiphasenströmungen wurde nicht betrachtet.

Keywords: Vor-Konvoi; CFX; DYN3D; RPV

  • Open Access Logo Wissenschaftlich-Technische Berichte / Helmholtz-Zentrum Dresden-Rossendorf; HZDR-099 2019
    ISSN: 2191-8708, eISSN: 2191-8716
  • Lecture (Conference)
    CFD-Verbund, 12.-13.03.2019, Garching, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-28758
Publ.-Id: 28758


Example of computations for boron dilution (system code, CFD and neutronic assessment)

Höhne, T.; Kliem, S.

Analysis of core responses during boron dilution scenarios requires use of different codes
Realistic boundary conditions especially for the coolant mixing can mitigate the consequences of higher slug volumes
Coupled neutron kinetic/thermal hydraulic core calculations needed for assessment of consequences
According to the results no violation of safety criteria observed

Keywords: DYN3D; ROCOM; Konvoi; PWR

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Permalink: https://www.hzdr.de/publications/Publ-28757
Publ.-Id: 28757


Numerical simulation of density driven flows (PTS)

Höhne, T.

Buoyancy driven mixing was investigated under simulated natural circulation conditions at the test facility ROCOM.
A transition matrix from momentum to buoyancy-driven flow experiments was selected for validation of the CFD software ANSYS CFX.
Buoyancy generated source and dissipation terms were proposed and introduced into the balance equations for the turbulent kinetic energy .
The results of the experiments and of the numerical calculations show that mixing strongly depends on buoyancy effects: At higher Froude numbers (Fr>1.5) the injected slug propagates in the circumferential direction around the core barrel. With lower Froude numbers (Fr<0.85) buoyancy effects reduce this circumferential propagation at lower mass flow rates and/or higher density differences. The ECC water falls in an almost vertical path and reaches the lower downcomer sensor directly below the inlet nozzle.

Keywords: buoyancy-driven flow; PTS; ROCOM; ECC

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Permalink: https://www.hzdr.de/publications/Publ-28756
Publ.-Id: 28756


Numerical simulation of boron dilution and cold-water transients

Höhne, T.

Boron Dilution scenarios were indentified for the use of 3D flow and mixing tools
Buoyancy driven mixing was investigated under simulated natural circulation conditions at the test facilities ROCOM and GMTF.
Hybrid meshes consisting at least of 2-8 million nodes were used.
The experiment and CFD calculations show in both cases significant mixing effects due to the density differences.
The ANSYS CFX calculations show a good qualitative agreement with the data.
CFD simulation of fibre material transport in a PWR core under loss of coolant conditions
Implementation of strainer model for the spacer grid

Keywords: CFX; PWR; ROCOM; Mixing

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Permalink: https://www.hzdr.de/publications/Publ-28755
Publ.-Id: 28755


ROCOM Experiments – 2

Höhne, T.; Kliem, S.

Goal: Investigation of coolant mixing at constant flow rates in the primary circuit

Variation:
Mass flow rate in the loop (10 - 160 %)
Number of operating loops
Status of non-operating loops (reverse flow or closed)
Friction losses at core inlet (lower, nominal and enhanced)

Keywords: ROCOM; Mixing Scalar; RPV; Konvoi

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Permalink: https://www.hzdr.de/publications/Publ-28754
Publ.-Id: 28754


ROCOM Experiments – 1

Höhne, T.; Kliem, S.

Goal: Investigation of coolant mixing at constant flow rates in the primary circuit

Variation:
Mass flow rate in the loop (10 - 160 %)
Number of operating loops
Status of non-operating loops (reverse flow or closed)
Friction losses at core inlet (lower, nominal and enhanced)

Keywords: ROCOM; Mixing Scalar; RPV; Konvoi

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Permalink: https://www.hzdr.de/publications/Publ-28753
Publ.-Id: 28753


Overview of the Mixing Test Facility ROCOM

Höhne, T.; Kliem, S.; Prasser, H.-M.

PWR is equipped with 2 or more loops (German KONVOI: 4)
Boron dissolved in the coolant acts as neutron absorber
Hypothetical accidents with creation of lower borated slugs in single loops
Importance of mixing of coolant with different boron content

Keywords: ROCOM; Konvoi; PWR; RPV; Wire Mesh Sensor

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Permalink: https://www.hzdr.de/publications/Publ-28752
Publ.-Id: 28752


Theory and Practice PTS

Höhne, T.; Lucas, D.

PTS is an important issue of Nuclear Safety Research
Thermohydraulic part: more reliable simulations on mixing needed  3D  CFD
Two-phase PTS is one of the most challenging problems for CFD simulations.
In principle simulations are possible, but with too large uncertainties (even for single regions and phenomena).
There are still open question regarding the most appropriate models to be used for a simulation of two-phase PTS.
Step by step improvement of single effect models is ongoing.
New experimental data with high resolution in space and time are needed for model development and validation.

Keywords: PTS; RPV; NPP

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Permalink: https://www.hzdr.de/publications/Publ-28751
Publ.-Id: 28751


Theory and Practice Boron Dilution Transients

Höhne, T.; Kliem, S.

Development of calculation tools for the modeling of hypothetical accidents in NPPs
Conduction of calculations to show that safety criteria are fulfilled (safety goals)
For existing reactors
For future reactors
Two examples
Overcooling transient
Boron dilution accident

Keywords: Overcooling transient; PWR; Boron dilution; RPV; NPP

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Permalink: https://www.hzdr.de/publications/Publ-28750
Publ.-Id: 28750


PWR Design Attributes Relevant to PTS and Boron Dilution

Höhne, T.

The primary system (also called the Reactor Coolant System) consists of the reactor vessel, the steam generators, the reactor coolant pumps, a pressurizer, and the connecting piping. A reactor coolant loop is a reactor coolant pump, a steam generator, and the piping that connects these components to the reactor vessel. The primary function of the reactor coolant system is to transfer the heat from the fuel to the steam generators. A second function is to contain any fission products that escape the fuel.
The following drawings show the layout of the reactor coolant systems for three pressurized water reactor vendors. All of the systems consist of the same major components, but they are arranged in slightly different ways. For example, Westinghouse has built plant with two, three, or four loops, depending upon the power output of the plant. The Combustion Engineering plants and the Babcock & Wilcox plants only have two steam generators, but they have four reactor coolant pumps.

Keywords: PWR; NPP; Primary System

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Permalink: https://www.hzdr.de/publications/Publ-28749
Publ.-Id: 28749


FDG-PET/MRI in patients with pelvic recurrence of rectal cancer: first clinical experiences

Plodeck, V.; Rahbari, N.; Weitz, J.; Radosa, C.; Laniado, M.; Hoffmann, R.; Zoephel, K.; Beuthien-Baumann, B.; Kotzerke, J.; van den Hoff, J.; Platzek, I.

Objectives
To determine the value of 18F-FDG-PET/MRI in the diagnosis and management of patients with pelvic recurrence of rectal cancer.

Methods
Forty-four patients (16 women, 28 men) with a history of rectal cancer who received FDG-PET/MRI between June 2011 and February 2017 at our institution were retrospectively enrolled. Three patients received two FDG-PET/MRIs; thus a total of 47 examinations were included. Pelvic recurrence was confirmed either with histology (n = 27) or imaging follow-up (n = 17) (> 4 months). Two readers (one radiologist, one nuclear medicine physician) interpreted the images in consensus. Pelvic lesions were assessed regarding FDG uptake and morphology. Sensitivity, specificity, positive and negative predictive values as well as accuracy of PET/MRI in detecting recurrence were determined.

Results
In 47 FDG-PET/MRIs 30 suspicious pelvic lesions were identified, 29 of which were malignant. Two patients underwent resection and had histologically proven pelvic recurrence without showing suspicious findings on FDG-PET/MRI. Changes in management due to FDG-PET/MRI findings had been implemented in eight patients. Eighty per cent (16/20) of resected patients had histologically negative resection margins (R0), one patient had uncertain resection margins. Sensitivity of FDG-PET/MRI in detecting recurrence was 94%, specificity 94%, positive/negative predictive value and accuracy were 97%, 90% and 94%, respectively.

Conclusions
FDG-PET/MRI is a valuable tool in the diagnosis and staging of pelvic recurrence in patients with rectal cancer.

Keywords: Positron-emission tomography; Magnetic resonance imaging; Rectal cancer; local Neoplasm Recurrence

Permalink: https://www.hzdr.de/publications/Publ-28748
Publ.-Id: 28748


Investigation of the ion induced magnetization in FeRh

Semisalova, A.; Barton, C.; Bali, R.; Böttger, R.; Thomson, T.; Potzger, K.; Lenz, K.; Lindner, J.; Fassbender, J.

Structurally B2-ordered equiatomic FeRh thin films are known for unique properties such as a temperature, magnetic field, and spin polarized current driven phase transition from the antiferro- to the ferromagnetic state. The strain and structural disorder also influences the magnetic properties of FeRh, which opens a new way for controllable modification of properties at the micro- and nanoscale. Namely, structural modification by ion beam irradiation was shown to be an effective tool for tuning the phase transition temperature in FeRh as well as the saturation magnetization [1-3]. Here, we present a detailed study of magnetic properties of ion irradiated 40 nm thick FeRh films using magnetometry and broadband ferromagnetic resonance technique. The structurally ordered films were deposited epitaxially on MgO(001) substrates using magnetron sputtering. The irradiation was performed with 25 keV Ne ions at fluences of 0.1 – 4 ions/nm2 leading to a controllable reduction of the order parameter. The ion beam induced magnetization of FeRh at room temperature was shown to be as high as 1300 kA/m. Ferromagnetic resonance measurements performed at frequencies up to 40 GHz show that the Gilbert damping in structurally disordered ferromagnetic FeRh films is comparable to Py films. Such a relatively low damping in combination with the highly tunable saturation magnetization appears promising for further experiments on magnetization dynamics and spin wave propagation in FeRh thin films and nanostructures fabricated using ion beam irradiation.
[1] N. Fujita et al., J. Appl. Phys. 107 (2010) 09E302
[2] A. Heidarian et al., Nucl. Instr. Meth. B 358 (2015) 251-254
[3] S.P. Bennett et al., Mater. Res. Lett. 6 (2018) 106-112

Keywords: FeRh; disorder; FMR; Gilbert damping; ion irradiation

  • Lecture (Conference)
    21st International Conference on Ion Beam Modification of Materials IBMM 2018, 24.-29.06.2018, San Antonio, Texas, USA

Permalink: https://www.hzdr.de/publications/Publ-28747
Publ.-Id: 28747


Probing the exchange coupling in the complex modified Ho-Fe-B compounds by high-field magnetization measurements

Tereshina, I. S.; Pyatakov, A. P.; Tereshina-Chitrova, E. A.; Gorbunov, D. I.; Skourski, Y.; Law, J. M.; Paukov, M. A.; Havela, L.; Doerr, M.; Zvezdin, A. K.; Andreev, A. V.

By examining the Ho2Fe14B case, we explored the influence of substitution and absorption atoms on the high-field behavior of magnetization of rare-earth (R)-Fe intermetallics. The value of the first critical field shows that the inter-sublattice exchange interactions remain practically unchanged when the substitution takes place in the R sublattice (replacement of up to 50 % of Ho by Nd). On the contrary, hydrogen absorption by Ho2Fe14B and Ho1Nd1Fe14B of the maximum possible hydrogen concentration 5.5 at./f.u. decreases the strength of the R-Fe Exchange by 30%. Remarkably, the influence of hydrogenation is stronger in the compound modified by substitution.

Permalink: https://www.hzdr.de/publications/Publ-28746
Publ.-Id: 28746


2D Materials Under Ion Irradiation: In-situ Experiments and the Role of the Substrate

Hlawacek, G.; Kretschmer, S.; Maslov, M.; Ghaderzadeh, S.; Ghorbani-Asl, M.; Krasheninnikov, A. V.

Helium ion Microscopy (HIM) is frequently used for the fabrication of 2D nanostructures in graphene, MoS 2 and other materials. While some of the experiments are carried out with freestanding materials most of the work is done on supported material. While the defect production is understood for the former case, it is not fully understood in the latter setup. We used a combination of analytical potential molecular dynamics and Monte Carlo simulations to elucidate the role of the different damage channels, namely primary ions, backscattered atoms and sputtered substrate atoms.
Using this approach we looked at the defect production by helium and neon ions in MoS 2 and graphene supported by SiO 2 at typical energies used in HIM. We show that depending on ion species and energy defect production for supported 2D materials can be dominated by sputtered atoms from the support, rather than direct damage induced by the primary ion beam. We also evaluated the consequences of these additional damage mechanisms on the achievable lateral resolution for HIM based defect engineering and nano-fabrication in 2D materials. The obtained results agree well with experimental results obtained by in-situ and ex-situ characterization of defects in graphene and MoS 2 .

Keywords: him; 2D; Simulations

  • Lecture (Conference)
    AVS International Symposium & Exhibition, 21.-26.10.2018, Long Beach, USA

Permalink: https://www.hzdr.de/publications/Publ-28744
Publ.-Id: 28744


Organized Single Si Quantum Dots in tiny SiO2 volumes: Self-alignment for Single Electron Transistors

Heinig, K.-H.; von Borany, J.; Prüfer, T.; Xu, X.; Möller, W.; Ahmed, G.; Tiron, R.; Gregor, H.; Bischoff, L.; Engelmann, H.-J.; Facsko, S.

Room temperature (RT) operation of Single Electron Transistors (SETs) is based on two conditions: (i) The Coulomb blockade energy of charging a dot must be smaller than kT, i.e. a Si quantum dot must be <5nm. (ii) The electron tunneling distance from the Si dot embedded in SiO2 to an electrode must be <1.5nm. Such dimensions are beyond the limits of top-down processes like Electron Beam Lithography (EBL) and Reactive Ion Etching (RIE).
As we demonstrate by atomistic computer simulations, a functional nanostructure for RT-SETs can be achieved by bottom-up processes, self-organization and self-alignment: Phase separation in a tiny volume ~(10nm)3 of metastable SiOx results in the formation of a single Si precipitate in SiO2 . And, if this SiOx volume is bordered at two sides by an Si/SiOx interface, the Si dot becomes self-aligned (isolated) by an SiO2 layer (SiOx denuted by excess Si).
The tiny SiOx volume has been formed by top-down processes: From a bulk Si/7nm SiO2/a-Si layer stack nanopillars of <20nm diameter have been fabricated by EBL and RIE. Then, the SiO2 layer embedded in the nanopillar was transformed into SiOx by 50keV Si+ ion irradiation. During subsequent annealing the single Si dot is expected to form. Two nanoscale phenomena not observed so far have to be overcome to get a reliable RT-SET fabrication.
This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 688072.

Keywords: HIM; nanostructures

  • Lecture (Conference)
    E-MRS Fall meeting, 16.-20.09.2018, Warsaw, Poland

Permalink: https://www.hzdr.de/publications/Publ-28742
Publ.-Id: 28742


Manufacturability of Single Si Quantum Dots for Single Electron Transistors operating at Room Temperature

Heinig, K.-H.; von Borany, J.; Prüfer, T.; Xu, X.; Möller, W.; Gharbi, A.; Tiron, R.; Hlawacek, G.; Bischoff, L.; Engelmann, H.-J.; Facsko, S.

Single Electron Transistors (SETs) are an extremely low power consuming alternative to Field Effect Transistors (FETs). Their room temperature operation is based on two conditions:
(i) The Coulomb energy of charging the dot with an electron must exceed kT. That requires dot sizes <5 nm. (ii) The tunneling distance between dot and electrodes through SiO2 must be <1.5nm.
These requirements are beyond top-down approaches. Thus, we follow a bottom-up approach: (i) A single Si dot forms by self-organization during phase separation of a tiny metastable SiOx volume into a Si precipitate and a SiO2 matrix. (ii) If the tiny SiOx volume is sandwiched between Si, then the single dot becomes self-aligned, i.e. two tunnel barriers form due to condensation of excess Si of SiOx onto the Si/SiO2 interfaces.
Here, a CMOS compatible manufacturabilty of vertical-nanowire-based SETs will be presented. Regular arrays of Si nanowires with diameters down to 20nm are fabricated by top-down processes. A SiO2 layer of 7nm thickness is sandwiched between Si of the wire. This SiO2 is transformed to SiOx(x<2) by ion beam mixing. During subsequent thermal activation (RTA) the dot structure should evolve as described above. Experimental and computer simulation results will be presented, critical fundamental issues of the nanofabrication will be discussed.
This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 688072.

Keywords: HIM; nanostructures

  • Lecture (Conference)
    E-MRS Fall meeting, 16.-20.09.2018, Warsaw, Poland

Permalink: https://www.hzdr.de/publications/Publ-28741
Publ.-Id: 28741


Extraction separation of rare-earth elements using an acidic diamide-type ligand and structural analysis of the extracted complex

Shimojo, K.; Sasanuki, T.; Schöne, S.; Sugita, T.; Okamura, H.; Ikeda-Ohno, A.

An acidic diamine type ligand was synthesised and its capability for extraction and separation of rare-earth elements were studied. The results showed that the ligand has a remarkably high selectivity for Sc amongst the rare-earth elements. Possible extraction and separation mechanisms are further discussed based on structure information derived from single-crystal X-ray diffraction.

Keywords: Separation; solvent extraction; rare-earth elements; single-crystal X-ray diffraction; coordination

  • Lecture (Conference)
    79th meeting of the Japan Society for Analytical Chemistry, 18.05.2019, Kita-Kyushu, Japan

Permalink: https://www.hzdr.de/publications/Publ-28740
Publ.-Id: 28740


HeFIB 2018: Helium and emerging focused ion beams

Hlawacek, G.; Facsko, S.; Bischoff, L.; Klingner, N.; Xu, X.; Serralta, E.; Ghaderzadeh, S.

Gas field ion sources (GFIS) using helium and neon as ion species are new and rapidly growing ion beam techniques.
However, GFIS based focused ion beams (FIB) are not the only new ion beam techniques offering new capabilities that go
beyond what classic Ga based FIB can do. Based on the contributions to the recently held meeting on Helium and
Emerging Focused Ion Beams (HeFIB) I will report on the newest developments in this field.
I will try to highlight new technological developments in the field of GFIS based FIBs, but also present new and emerging
alternative FIB source techniques such as Laser cooled sources, liquid metal alloy source, or Xe plasma FIBs. However,
such new techniques also open up many new application fields. I will present selected examples of in which focused ion
beams have been used for imaging, localized materials modification as well as classical FIB based fabrication of nano-
structures.

Keywords: HIM

  • Invited lecture (Conferences)
    CAARI 2018 - The Conference on Application of Accelerators in Research and Industry, 13.-17.08.2018, Fort Worth, USA

Permalink: https://www.hzdr.de/publications/Publ-28739
Publ.-Id: 28739


Molecular Dynamics simulations of 30 keV He impacts on gold nano-clusters

Ghaderzadeh, S.; Hlawacek, G.; Krasheninnikov, A.

At the Helmholtz-Zentrum Dresden-Rossendorf, molecular dynamics computer simulations are employed to study the sputtering yield and channeling effects in Gold nano-clusters of different sizes. Primary ion energy and crystal orientation are varied to obtain a holistic image of the possible effects relevant for scanning transmission ion microscopy. Our results show that ion-channeling occurs not only in the principal low-index, but also in other directions in between. The strengths of different channels are specifed, and their correlations with sputtering yield and damage production is addressed. The figure below shows sputtering under 30 keV He ion irradiation of 5 nm Gold nano-clusters.

Keywords: HIM; nanostructures; simulations; channeling

Permalink: https://www.hzdr.de/publications/Publ-28738
Publ.-Id: 28738


Imaging and analytic possibilities in the Helium Ion Microscope

Hlawacek, G.; Klingner, N.; Heller, R.; Veligura, V.; van Gastel, R.; Poelsema, B.; von Borany, J.; Facsko, S.

Helium Ion Microscopy (HIM) utilizes a Gas Field Ion Source (GFIS) to create a Helium or Neon ion beam with a diameter better than 0.5 nm and 1.8 nm, respectively. The method is well known for its high resolution imaging and nano-fabrication capabilities which it is able to provide not only for conducting but also insulating samples without the need for a conductive coating. The latter specimens are typically found in the fields of biosciences, MEMS/NEMS technology, catalyst research and many others. The availability of He and Ne ions with either low or moderate sputter yields, allow direct write nano-structuring with a precision below 10 nm in the HIM [1, 2]. However, the existing GFIS based focused ion beam (FIB) tools suffer from the lack of a well integrated analytic method that can enrich the highly detailed morphological images with materials contrast. While HIM technology is relatively young several efforts have been made to add such an analytic capability to the technique. So far, ionoluminescence [1, 3], backscattering spectrometry (BS) [1, 4], and secondary ion mass spectrometry (SIMS) using a magnetic sector [5] or time of flight (TOF) setup have been demonstrated [4].
I will present results obtained using the above mentioned methods beginning with iono-luminescence and its application to various materials systems. The method is in particular suited for the analysis of various defects present in the sample and the behaviour of defects under ion beam irradiation. In the second part of the talk I will present our newly developed TOF-BS and TOF-SIMS setup which allow to obtain information on the composition of the sample. They both utilize the same cost efficient and minimal invasive pulsing scheme for the primary ion beam. The lateral resolution reached for TOF-BS is approximately 50 nm while for TOF-SIMS a value of 8 nm could be reached. First images will be presented and the performance of the TOF-SIMS spectrometer will be discussed.

Keywords: HIM; nanostructures; analytic; SIMS

  • Invited lecture (Conferences)
    Imaging 2020 Workshop, 06.04.2018, Sønderborg, Denmark
  • Lecture (others)
    PNNL Seminar, 19.10.2018, Richland, USA

Permalink: https://www.hzdr.de/publications/Publ-28737
Publ.-Id: 28737


Dispersion relation of interlayer exchange coupled tailored ferrimagnets

Sorokin, S.; Gallardo, R.; Fowley, C.; Atcheson, G.; Dennehy, G. S.; Rode, K.; Stamenov, P.; Lindner, J.; Fassbender, J.; Deac, A. M.

Different ferromagnetic resonance (FMR) modes in micron-sized antiferromagnetically interlayer exchange coupled SiO2/Ta(5nm)/Py(t)/Ru(0.85 nm)/Py(3nm)/Ru(3nm) structures, with t = 3, 6 and 9 nm, were studied by means of the electrically detected ferromagnetic resonance (ED-FMR).
The main magnetoresistance effect used in ED-FMR was anisotropic magnetoresistance (AMR). Bilinear and biquadratic coupling strengths for each sample were determined by fitting SQUID-VSM measurements on 4×4 mm² thin films, using equilibrium total energy minimization. The existence of two different resonance modes (in-phase (acoustic) and out-of-phase (optic)) is shown for asymmetric samples. For the symmetric sample only the acoustic mode was observed, due to the compensation of AMR response from Py layers for the out-of-phase mode. The obtained dispersion relations show a clear dependence of the acoustic mode frequency minimum on the bilinear coupling strength.

Keywords: FMR; ED-FMR; Synthetic Antiferromagnets; Dispersion relation

  • Poster
    The European School on Magnetism, 17.09.2018, Krakow, Poland

Permalink: https://www.hzdr.de/publications/Publ-28736
Publ.-Id: 28736


Dispersion relation of the interlayer exchange coupled tailored ferrimagnets

Sorokin, S.; Fowley, C.; Atcheson, G.; Dennehy, G.; Duan, J.; Khudorozhkov, A.; Rode, K.; Lindner, J.; Fassbender, J.; Deac, A.; Stamenov, P.

Here we present a study of different ferromagnetic resonance (FMR) modes in micron-sized antiferromagnetically interlayer exchange coupled SiO2/Ta(5nm)/Py(t)/Ru(0.85 nm)/Py(3nm)/Ru(3nm) structures, with t = 3, 6 and 9 nm, by means of the electrically detected ferromagnetic resonance (ED-FMR).
The main magnetoresistance effect used in ED-FMR was anisotropic magnetoresistance (AMR). Bilinear and biquadratic coupling strengths for each sample were determined by fitting SQUID-VSM measurements on 4×4 mm² thin films, using equilibrium total energy minimization. The existence of two different resonance modes (in-phase (acoustic) and out-of-phase (optic)) is shown for asymmetric samples. For the symmetric sample only the acoustic mode was observed, due to the compensation of AMR response from Py layers for the out-of-phase mode. The obtained dispersion relations show a clear dependence of the acoustic mode frequency minimum on the bilinear coupling strength. For asymmetric samples, mode intermixing occurs for certain resonance fields, accompanied by abrupt jumps in both mode frequencies. Such behavior is not observed for symmetric samples, in accordance with predictions based on VNA-FMR experiments and simulations performed elsewhere.

Keywords: FMR; ED-FMR; Synthetic antiferromagnets; Dispersion relation

  • Lecture (Conference)
    JEMS 2018 - The Joint European Magnetic Symposia, 03.09.2018, Mainz, Germany

Permalink: https://www.hzdr.de/publications/Publ-28735
Publ.-Id: 28735


Magneto-transport measurements in para- and ferromagnetic Fe60Al40 wires

Liersch, V.; Schmeink, A.; Eggert, B.; Warnatz, T.; Wintz, S.; Ehrler, J.; Böttger, R.; Hlawacek, G.; Potzger, K.; Lindner, J.; Faßbender, J.; Wende, H.; Bali, R.

Certain alloys (Fe60Al40, Fe50Rh50, Fe65V35) have stable, chemically ordered, B2 and metastable, chemically dis-ordered, A2 crystal structures with different magnetic properties.
B2 Fe60Al40 is paramagnetic (PM), A2 Fe60Al40 is ferromagnetic (FM).
Possible applications are in phase change memories and sensors.
We investigate change of resistivity 𝜌 during A2 ↔ B2 transitions in Fe60Al40 wires.

Keywords: HIM; magnetic structures; nanopatterning

  • Poster
    DPG-Frühjahrstagung 2018, 11.-16.03.2018, Berlin, Germany

Permalink: https://www.hzdr.de/publications/Publ-28734
Publ.-Id: 28734


Synthesis, Structural Characterization and Photodecarbonylation Study of a Dicarbonyl Ruthenium(II)-Bisquinoline Complex

Kubeil, M.; Joshi, T.; Wood, B. R.; Stephan, H.

A photoactivatable ruthenium(II) carbonyl complex [Ru(II)(BisQ)Cl(CO)2]PF6 2 was prepared using a tridentate bisquinoline ligand (BisQ = (2,6-diquinolin-2-yl)pyridin). Compound 2 was thoroughly characterized by standard analytical methods and single crystal X-ray diffraction. The crystal structure of the complex cation reveals a distorted octahedral geometry. The CO release upon exposure to UV light was monitored by UV/VIS absorbance and Fourier transform infrared spectroscopies in acetonitrile and 1% (v/v) DMSO in water, respectively. The photodecarbonylation follows a stepwise CO release. The first CO release occurs very quickly whereas the second decarbonylation step proceeds more slowly. Moreover, the photoreaction in acetonitrile is more distinguished and faster than in 1% aq. (v/v) DMSO.

Keywords: photoCORM; ruthenium; tridentate ligand; UV/VIS and FTIR; photodecarbonylation

Permalink: https://www.hzdr.de/publications/Publ-28733
Publ.-Id: 28733


Luminescence spectroscopy of uranium in environmental systems

Steudtner, R.; Drobot, B.; Zabelt, D.; Bader, M.; Hilpmann, S.; Großmann, K.

Luminescence spectroscopy is a powerful tool to study the chemistry of uranium in trace concentration. Manifold operating mode, e.g. steady state, time-resolved, laser-induced, site-selective, cryogenic, etc. were used to investigate the environmental behavior of uranium in various geological and biological systems.
Hydrolysis is the basis for more complex aquatic systems and thus a deep understanding of those systems is indispensable. In case of U(VI) we demonstrated that a combination of luminescence spectroscopic methods together with state of the art data analysis (parallel factor analysis – PARAFAC) and quantum chemical calculations is a powerful setup to gain information on that system. We were able to extract thermodynamic constants for the mononuclear hydrolysis species using optimized data processing. Furthermore, advanced deconvolution of individual luminescence spectra demonstrates the correlation of luminescence spectroscopy and vibrational spectroscopy.
For kinetic studies of geological or biological sorption phenomena, different microscopic or flow-through cell techniques are useable. For online monitoring and characterization of U(VI) sorption species we develop a new technical in situ luminescence spectroscopy setup in comparability to the well established in situ time resolved ATR FT-IR spectroscopy. For biological systems, we combined microscopy with luminescence spectroscopic measurements for localization, visualization and chemical characterization of uranium complexes. This approach enables us to distinguish between biosorption, intracellular uptake or biomineralization as dominant retention process for uranium in biological samples.
Under reducing conditions expected in the near field of nuclear waste repository, the tetravalent uranium should be the major oxidation state. We studied the U(IV) luminescence characteristics in presence of various inorganic ligands (ClO4–, Cl–, SO42–). By using cryo-TRLFS at 77 K the speciation analysis limit for U(IV) was determined with 5·10–6 M and this corresponds to uranium concentrations occurring in the environment.

  • Lecture (Conference)
    8. RCA-Workshop, 12.-14.06.2018, Dresden, Germany

Permalink: https://www.hzdr.de/publications/Publ-28732
Publ.-Id: 28732


Towards an understanding of U(VI) interaction with Boom Clay dissolved organic matter by TRLFS

Buchatskaya, Y.; Salah, S.; Durce, D.; Steudtner, R.; Devillers, M.

Uranium represents the most abundant radionuclide in nuclear waste and its behavior is a major concern for geological waste disposal. The speciation of Uranium in solution affects its mobility in porous media and must in consequence be well understood. The dissolved organic matter (DOM) present in pore waters of geological formations can change the speciation and the geochemical behavior of uranium [1]. DOM present in the potential host rock formation Boom Clay in Belgium differs in size (from hundreds to thousands of Da), solubility at different pH (humic and fulvic acids) and functionality. Due to DOM polydispersity and polyfunctionality, its interaction with U(VI) could involve various mechanism which need to be studied in details. Time-resolved laser-induced fluorescence spectroscopy (TRLFS) is a well-known technique used to characterize U(VI) speciation and complex formation with inorganic and organic ligands. This method was already successfully applied to derive complex stability constants of U(VI) and humic acids [2]. The objective of the present work is to perform a systematic and multi-parametric study on the U(VI) complexation with BC DOM fractions using TRLFS.
Two DOM fractions different in molecular size were separated, concentrated and purified from natural Boom Clay organic matter: one “colloidal” (100 kDa – 0.45 µm) and one “small” (< 1 kDa). The separation and concentration were performed using ultrafiltration and solid phase extraction methods. The size distribution and reactivity of final fractions were characterized using size-exclusion chromatography, X-ray photoelectron spectroscopy and potentiometric titrations. Complex formation of U(VI) with different DOM fractions was studied using TRLFS in presence and absence of carbonates in 0.1 M NaClO4 for a pH range 4-12.5.

  • Lecture (Conference)
    International conference Uranium biogeochemistry, 21.-26.10.2018, Ascona, Switzerland

Permalink: https://www.hzdr.de/publications/Publ-28731
Publ.-Id: 28731


Mechanistic understanding for biochemical and biological processes of uranium(VI) by time-resolved laser-induced fluorescence spectroscopy (TRLFS)

Steudtner, R.; Hilpmann, S.; Bader, M.; Jessat, J.; Sachs, S.; Cherkouk, A.

The transfer of radionuclides into the food chain is of central concern for the safety assessment of both nuclear waste repositories and radioactive contaminated areas, such as legacies of the former uranium mining. The interaction of radionuclides, here in this particular case uranium(VI), with microorganism or plants is mostly described by transfer factors without knowing the underlying processes. In two examples we want present that luminescence spectroscopy is a powerful tool to study these unknowing processes of uranium(VI) on a molecular level.
Rock salt formations are considered as potential host rocks for the long-term storage of highly radioactive waste in a deep geological repository. Extremely halophilic archaea, e.g. Halobacterium species, dominate this habitat. We studied and compared the interactions of different extremely halophilic Halobacterium species with uranium(VI) by classical chemical and biological, by multi-spectroscopic and microscopic and by molecular biological methods. Depending on the used initial uranium(VI) concentration the different Halobacterium species showed a different bioassociation behaviour of uranium(VI). By using TRLFS the formation of uranium(VI) phosphate minerals, such as meta-autunite, as well as the complexation with carboxylate groups was observed as a function of the uranium(VI) concentration and the Halobacterium species.
In a second example, we studied the interaction of uranium(VI) with canola cells (Brassica napus) focusing on the concentration dependent impact of uranium(VI) on the cell metabolism. Previous studies showed, for instance, a speciation dependent influence of radionuclide uptake and translocation in plants [1]. Heavy metal stress induces the synthesis of metal-binding metabolites, storage of metal chelates in vacuoles or the secretion into the rhizosphere [2], which changes the plant cell metabolism. To study the interaction of lanthanides with Brassica napus on a cellular level, callus and suspension cells were exposed to uranium(VI). Besides the kinetics of the bio-association, the amount of associated uranium(VI) and its effect on cell growth and viability was determined. TRLFS was used as direct speciation technique to determine the uranium(VI) species on callus cells and the supernatant. In combination with high performance liquid chromatography (HPLC) experiments the metabolic answer of the callus cells during the presence of uranium(VI) will be investigated.

  • Lecture (Conference)
    4th International Workshop on Advanced Techniques in Actinide Spectroscopy (ATAS), 06.-09.11.2018, Nice, France

Permalink: https://www.hzdr.de/publications/Publ-28730
Publ.-Id: 28730


Estimating mean residence times and flow velocities to quantify recharge to the Western Dead Sea aquifer system using multiple environmental tracers

Wilske, C.; Suckow, A.; Roediger, T.; Geyer, S.; Merchel, S.; Rugel, G.; Pavetich, S.; Merkel, B.; Siebert, C.

The geologically complex, partly karstic limestone aquifers located west of the Dead Sea provide vital freshwater sources for all populated areas from Ramallah to Hebron. Discharge from these aquifers also sustains sensitive ecosystems located along the western shore of the lake. Recharge to these aquifers is restricted to winter precipitation events, which only occur in Hebron and Jerusalem uplands. To inform future sustainable water management strategies, a detailed characterization of these aquifers, including the volume of sustainable extraction is needed. In this study, the time scales of groundwater recharge to the two main aquifers were estimated through the sampling and interpretation of a suite of young age environmental tracers including tritium (³H), chlorine-36 (³⁶Cl), chlorofluorocarbons (CFC-11, CFC-12 and CFC-113) and sulfur hexafluoride (SF₆).

Keywords: water management; AMS; tritium; groundwater

  • Lecture (Conference)
    International Symposium on Isotope Hydrology: Advancing the Understanding of Water Cycle Processes, 20.-24.05.2019, Wien, Österreich

Permalink: https://www.hzdr.de/publications/Publ-28729
Publ.-Id: 28729


¹⁰Be exposure dating on moraines in the Sayan Mountains, Siberia

Rauh, P.; Schweri, L.; Garcia Morabito, E.; Merchel, S.; Rugel, G.; Zech, R.

Mountain glaciers are very sensitive to (past) climate changes. Paleoglaciation of the Tibetan Plateau and the Himalaya received plenty of attention for the reconstruction of past climate and environmental conditions, whereas few studies have been conducted so far in the adjacent Altai and Sayan Mountains in Siberia, although they promise a complex history.
The investigated Sayan Mountains in the Russian Altai indicate extensive glacial activity during the Pleistocene and a shifting of the local Last Glacial Maximum (LGM). Rich glacial residues in terms of moraine ridges and erratic boulders in the region provide excellent opportunities to establish a detailed age chronology using in-situ cosmogenic ¹⁰Be surface exposure dating and derive information about paleoclimatic conditions.
We present 28 ¹⁰Be surface exposure ages from the Ergaki Range and Tuva Uplands in the western Sayan Mountains. Distinct lateral moraines document the extents of former glaciation during a last glacial maximum. Several recessional moraines in the Ergaki Range additionally record stand stills or readvances during the last glacial termination.
The prominent lateral moraine in the Ergaki Range yields three exposure ages of ~19 ka, indicating a MIS 2 glaciation. The maximum advances correlate with the end of the global LGM. Concluding, glaciers in the Ergaki Range existed continuously during the late Pleistocene only varying in their extent. The well-clustered ages of the Ergaki Range provide a robust deglaciation chronology. The readvances/ stand stills consistently date to ~18 ka (three boulders), ~17.5 ka (two boulders), ~17 ka (three boulders), and ~16 ka (two boulders), with a deglaciation starting after 16 ka. The valley is not elevated enough to record younger, less extensive glaciation, e.g. during the Younger Dryas and the Holocene. Exposure ages from the Tuva Uplands are more difficult to interpret: The lateral moraine documenting the most extensive glaciation yields two exposure ages of ~22 ka. The immediately adjacent inner lateral moraine has two slightly younger and stratigraphically consistent ages of ~21 ka, yet three more boulders from that moraine are ~27 and 43 ka. At this point, we suspect these older boulders to have inheritance, but we cannot confidently exclude a much older deposition age for both moraines. Planned are analyzes of other cosmogenic nuclides, such as ²⁶Al and/or in situ ¹⁴C, which might help to more robustly identify inheritance and complex exposure histories.

Keywords: AMS; glaciation; moraine; LGM

  • Poster
    INQUA 2019 (International Union for Quaternary Research), 25.-31.07.2019, Dublin, Ireland

Permalink: https://www.hzdr.de/publications/Publ-28728
Publ.-Id: 28728


Perforating Freestanding Molybdenum Disulfide Monolayers with Highly Charged Ions

Kozubek, R.; Tripathi, M.; Ghorbani-Asl, M.; Kretschmer, S.; Madauß, L.; Pollmann, E.; O'Brien, M.; Mcevoy, N.; Ludacka, U.; Susi, T.; Duesberg, G. S.; Wilhelm, R. A.; Krasheninnikov, A. V.; Kotakoski, J.; Schleberger, M. Y.

Porous single-layer molybdenum disulfide (MoS2) is a promising material for applications such as DNA sequencing and water desalination. In this work, we introduce irradiation with highly charged ions (HCIs) as a new technique to fabricate well-defined pores in MoS2. Surprisingly, we find a linear increase of the pore creation efficiency over a broad range of potential energies. Comparison to atomistic simulations reveals the critical role of energy deposition from the ion to the material through electronic excitation in the defect creation process, and suggests an enrichment in molybdenum in the vicinity of the pore edges at least for ions with low potential energies. Analysis of the irradiated samples with atomic resolution scanning transmission electron microscopy reveals a clear dependence of the pore size on the potential energy of the projectiles, establishing irradiation with highly charged ions as an effective method to create pores with narrow size distributions and radii between ca. 0.3 and 3 nm.

Keywords: ion irradiation; highly charged ions; molybdenum disulfide; 2D material; STEM; MD simulation; perforation

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Permalink: https://www.hzdr.de/publications/Publ-28727
Publ.-Id: 28727


Study of the influence of water gaps between fuel assemblies on the activation of an aeroball measurement system (AMS)

Konheiser, J.; Mueller, S. E.; Seidl, M.

The aeroball measuring system (AMS) at Siemens/KWU built pressurized water reactors (PWR) is an important part of the in-core instrumentation to determine in detail the local power distribution. Simulations were carried out with the help of the MCNP6 Monte Carlo program to determine the possible impact of an additional the water gap between the fuel assemblies with regard to the 51V(n,γ)52V reaction rate in the AMS. A simplified geometric model in a 3x3 matrix of identical fuel assemblies was used and four AMS lances were simulated in the central fuel assembly. By shifting the outer 8 fuel assemblies, different water gaps were created and the effects on the reaction rate of the AMS were calculated for different burn-up values and boron contents in the cooling water. It was found that the change of reaction rates can reach up to 10% for an assumed gap maximum of 1cm. The changes are largest for burn-up values at 30 and 45 GWd/t and slowly increase with decreasing boron concentration. The results are an important piece of information to assess the possibility of detecting non-nominal water gaps during reactor operation.

Keywords: AMS; PWR; MCNP6; in-core instrumentation

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  • Secondary publication expected

Permalink: https://www.hzdr.de/publications/Publ-28726
Publ.-Id: 28726


Site-controlled Si Nanodot Formation for a RT-SET via Ion Beam Mixing and Phase Separation

Xu, X.; Prüfer, T.; Wolf, D.; Hübner, R.; Bischoff, L.; Engelmann, H.-J.; Gharbi, A.; Heinig, K.-H.; Hlawacek, G.; von Borany, J.

CMOS-compatible formation of Si nanodots (NDs) as Coulomb islands is a prerequisite for an RT Single Electron Transistor operation. In this work, Si NDs are formed via ion beam mixing and thermally stimulated phase separation. Broad-beam Si+ and Ne+ beams followed by a rapid thermal annealing treatment were utilized to create a layer of NDs and visualized by Energy-Filtered Transmission Electron Microscopy (EFTEM). The conditions for ND formation are optimized based on an extensive survey of the parameter space. The work is guided by TRIDYN simulations during the ion beam mixing and 3D Kinetic Monte-Carlo simulation for the phase separation during the thermal treatment. To tailor towards a single Si ND, the focused Ne+ beam from the Helium Ion Microscope (HIM) is utilized to create patterns of NDs in planar layer stacks. The formation of site-controlled single NDs with a diameter of 2.2 nm is confirmed by comparing the EFTEM Si plasmon-loss intensity with simulated intensity.

  • Lecture (Conference)
    HeFIB2018 Helium and emerging Focused Ion Beams, 11.06.2018, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-28724
Publ.-Id: 28724


Avoiding Amorphization Related Shape Changes of Nanostructures during Medium Fluence Ion Beam Irradiation of Semiconductor Materials

Xu, X.; Hlawacek, G.; Engelmann, H.-J.; Heinig, K.-H.; Möller, W.; Gharbi, A.; Tiron, R.; Bischoff, L.; Prüfer, T.; Hübner, R.; Facsko, S.; von Borany, J.

We present an approach to mitigate the ion beam induced damage inflicted on semiconductor nano-structures during ion beam irradiation. Nanopillars (with diameter a of 35 nm and height of 70 nm) have been irradiated with both, a 50 keV Si+ broad beam and a 25 keV focused Ne+ beam from a helium ion microscope (HIM). Upon irradiation of the nanopillars at room temperature with a medium fluence (2e16 ions/cm2), strong plastic deformation has been observed which hinders further device integration. This differs from predictions made by the Monte-Carlo based simulations using the TRI3DYN. However, irradiation at elevated temperatures with the same fluence would preserve the shape of the nanopillars.
It is well known that a critical temperature exists for silicon above which it will recrystallize during ion beam irradiation. This prevents the amorphization of the target material independent of the applied fluence. At high enough temperatures and not for too high flux this prevents the ion beam hammering and viscous flow of the nano-structures. These two effects are responsible for the shape change observed at low temperature. This has been observed previously mainly for swift heavy ions and energies higher than 100 keV. We used HIM and transmission electron microscopy to follow the morphological evolution of the pillars and their crystallinity. While irradiation at room temperature results in amorphization and the related destruction of the nanopillars, irradiation above 650 K preserves the crystalline nature of the pillars and prevents viscous flow. This effect has been observed previously mainly for swift heavy ions and energies higher than 100 keV. Such high-temperature irradiation, when carried out on a nanopillar with Si/SiO2/Si layer stack, would induce ion beam mixing without suffering from the plastic deformation of the nanostructure. Due to a limited mixing volume, single Si-NCs would form in a subsequent rapid thermal annealing process via Oswald ripening and serve as a basic structure of a gate-all-around single electron transistor device.
This work is supported by the European Union’s H-2020 research project ‘IONS4SET’ under Grant Agreement No. 688072.

  • Lecture (Conference)
    AVS 65th International Symposium & Exhibition, 22.10.2018, Long Beach, USA

Permalink: https://www.hzdr.de/publications/Publ-28723
Publ.-Id: 28723


Tuneable vortex dynamics with ion implantation

Ramasubramanian, L.; Kákay, A.; Fowley, C.; Yildirim, O.; Matthes, P.; Lindner, J.; Deac, A. M.

The fundamental oscillation mode of magnetic vortices in thin-film elements has been proposed as working principle for spin-torque-driven nano-oscillators [Nat. Phys., 3:498, 2007].
Commercial applications require tuning of the output frequency by external parameters, such as spin-polarized currents. The tunability of vortex-based devices is limited, since the gyrotropic frequency is specific to the individual sample design. The fundamental frequency is determined by the saturation magnetisation, Ms, as well as the geometrical confinement of the magnetisation i.e. the diameter and height of a magnetic disk. Our micromagnetic simulations have shown that if regions with different Ms can be induced in a magnetic disk, multiple precession frequencies can be generated. Here, we show that ion implantation [Phys. Rev. B 73, 184410, 2006] is a novel route to fabricate such devices.
Permalloy (Py) disks of various diameters and thicknesses were patterned and contacted to study the interaction of an applied AC current with the magnetic vortex. Using a conventional lock-in technique, the resonance frequencies are measured based on the anisotropic magnetoresistance (AMR) effect. Regions of different Ms are induced in single disks by ion implantation, yielding different resonance frequencies corresponding to the specific area where the core is precessing. The work represents a novel way to obtain multiple oscillation frequencies from a single disk.

Keywords: magnetic vortex; ion implantation; frequency tunability

  • Lecture (Conference)
    9th JEMS Conference 2018, 03.-07.09.2018, Mainz, Germany

Permalink: https://www.hzdr.de/publications/Publ-28722
Publ.-Id: 28722


Phase transition lowering in dynamically compressed silicon

Mcbride, E. E.; Krygier, A.; Ehnes, A.; Galtier, E.; Harmand, M.; Konôpková, Z.; Lee, H. J.; Liermann, H.-P.; Nagler, B.; Pelka, A.; Rödel, M.; Schropp, A.; Smith, R. F.; Spindloe, C.; Swift, D.; Tavella, F.; Toleikis, S.; Tschentscher, T.; Wark, J. S.; Higginbotham, A.

Silicon, being one of the most abundant elements in nature, attracts wide-ranging scientific and technological interest. Specifically, in its elemental form, crystals of remarkable purity can be produced. One may assume that this would lead to silicon being well understood, and indeed, this is the case for many ambient properties, as well as for higher-pressure behaviour under quasi-static loading. However, despite many decades of study, a detailed understanding of the response of silicon to rapid compression—such as that experienced under shock impact—remains elusive. Here, we combine a novel free-electron laser-based X-ray diffraction geometry with laser-driven compression to elucidate the importance of shear generated during shock compression on the occurrence of phase transitions. We observe lowering of the hydrostatic phase boundary in elemental silicon, an ideal model system for investigating high-strength materials, analogous to planetary constituents. Moreover, we unambiguously determine the onset of melting above 14 GPa, previously ascribed to a solid–solid phase transition, undetectable in the now conventional shocked diffraction geometry; transitions to the liquid state are expected to be ubiquitous in all systems at sufficiently high pressures and temperatures.

Keywords: compressed silicon; shock; LCLS; x-ray diffraction; XFEL; phase transition; MEC; high energy density

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Permalink: https://www.hzdr.de/publications/Publ-28721
Publ.-Id: 28721


Magnetic vortex dynamics and frequency tunability in Cr-implanted permalloy disks

Ramasubramanian, L.; Kákay, A.; Fowley, C.; Yildirim, O.; Matthes, P.; Böttger, R.; Lindner, J.; Fassbender, J.; Gemming, S.; Schulz, S. E.; Deac, A. M.

The fundamental oscillation mode of magnetic vortices in thin-film elements has recently been proposed for designing spin-torque-driven nano-oscillators [1]. Commercial applications require tuning of the output frequency by external parameters, such as applied fields or spin-polarized currents. However, the tunability of vortex-based devices is limited, since the gyrotropic frequency is specific to the individual sample design. Indeed, the fundamental frequency is known to be determined by the saturation magnetisation, M_s , as well as the geometrical confinement of the magnetisation, i.e. the diameter and height of the magnetic disk [2, 3]. Micromagnetic simulations [4] have shown that if regions with different saturation magnetisation can be induced in a magnetic disk, multiple precession frequencies can be generated. We show that ion implantation [5] is a novel route to fabricate such devices.
Permalloy (Py) disks of various diameters and thicknesses were prepared using electron beam lithography followed by electron beam evaporation. Individual disks were contacted by gold leads to study the interaction of spin-polarized current with the magnetic vortex. The presence of vortex is verified by magneto optic Kerr effect (MOKE), X-ray magnetic circular dichroism (XMCD) and magnetotransport measurements. The magnetic field dependence of the vortex position can be tuned by the disk size as shown by XMCD (Figure 1 (a)). Higher magnetic stability due to larger annihilation fields can be achieved by smaller disk diameters, whereas larger field sensitivity is present in larger disks (Figure 1 (b)). Magnetotransport measurements on electrically contacted disks show the presence of anisotropic magnetoresistance (AMR) in different disks with varying thickness (Figure 1 (c)).
Using a conventional lock-in technique, the resonance frequencies are measured for disks with different radii as shown in Figure 2 (a), with the inset showing the scanning electron microscope image of an electrically contacted disk. In order to modify the magnetisation within a single disk and to achieve two different oscillation frequencies, we implant chromium in different regions of the disk (inner and outer). Cr-implantation leads to a decrease in the Curie temperature and thus a reduction in the magnetic moment [6]. The reduction of M_s as a function of Cr fluence was optimised on extended Py films using a vibrating sample magnetometer – superconducting quantum interference device (VSM-SQUID), see figure 2 (b). A clear drop in M_s with increasing the chromium ion fluence is observed. Concentric donut-like structures were then implanted with Cr and the modification of dynamics as a function of magnetic field was investigated. An example of Cr implantation in a 3 µm radius disk at 30 keV with a fluence of 1.2×〖10〗^16 ions/cm^2 is shown in Figure 2 (c). The vortex core is shifted between the two different magnetisation regions by applying an external in-plane field. The vortex nucleates in the irradiated region at – 2.281 mT, leading to a resonance frequency of 30.2 MHz (shown in orange in Figure 2 (c)). Further increasing the external field pushes the vortex core to the non-irradiated region where the resonance frequency is 42.3 MHz (shown in green), corresponding to a field of + 1.597 mT. The results show that ion implantation is a novel way to obtain multiple frequencies from a single disk.

Keywords: magnetic vortex; ion implantation; frequency tunability

  • Lecture (Conference)
    INTERMAG 2018 - The IEEE International Magnetics Conference (INTERMAG, 23.-27.04.2018, Singapore, Singapore

Permalink: https://www.hzdr.de/publications/Publ-28720
Publ.-Id: 28720


Effect of insertion layer on the properties of Mn₂RuₓGa in magnetic tunnel junctions

Titova, A.; Fowley, C.; Clifford, E.; Lau, Y.-C.; Borisov, K.; Betto, D.; Atcheson, G.; Hübner, R.; Xu, C.; Stamenov, P.; Coey, M.; Rode, K.; Lindner, J.; Fassbender, J.; Deac, A. M.

The choice of proper materials for magnetic tunnel junctions (MTJs) for storage and communication applications (like MRAM or spin-transfer-torque nano-oscillators (STNOs)) is always an issue. On the one hand, the magnetic layers should exhibit as little stray field as possible and be mostly insensitive to the external magnetic field. On another hand, in order to maximize the tunneling magnetoresistance (TMR) ratio, these materials should provide high spin polarization, or even ideally possess half-metallic properties. An option which satisfies both criteria are the compensated half metallic ferrimagnets (CHFMs) — a class of materials predicted in 1995 by van Leuken and de Groot. CHFMs are materials which behave as antiferromagnets (AFMs) with respect to external magnetic fields, since the magnetic moments of the two sublattices compensate, while simultaneously exhibiting half-metal behavior from the point of view of magnetotransport. Experimentally, the first identified zero-moment half-metal was Mn₂RuₓGa (MRG) in 2014. It was already known that Mn-based Heusler compounds possess huge uniaxial anisotropy fields (exceeding tens of teslas); this, together with their vanishing magnetization, lead to resonance frequencies of several hundred GHz in such materials, making them very attractive candidates for STNOs in the sub-THz range. Such devices, due to the much higher bandwidth accessible, are expected to open the way for remote hospitals, 3-D remote meetings and much more.
Earlier MRG studies have already shown that these materials exhibit tunable magnetic properties. Indeed, the compensation temperature varies between 2 and 450K, depending on the Ru concentration. They also yield giant spontaneous Hall angle (7.7%). MRG has also been successfully integrated into perpendicular MRG/MgO/CoFeB MTJs, with low-bias TMR reaching up to 40 % at 10 K and 7 % at 300 K.
As the low value of TMR was attributed to diffusion of Mn atoms inside the MgO barrier, here, we investigate the effect of different insertion layers introduced between MRG and MgO on the magnetic properties and transport of MTJs. Mn₂Ruₓ Ga (23)/insertion layer(t)/MgO(1.7)/CoFeB(1)/Ta(0.3)/CoFeB(0.9)MgO(0.7)/Ta(3)/Ru(4) multilayers were deposited using a “Shamrock” fully automated sputter deposition tool (thickness given in nm). Mn₂RuₓGa was grown by co-sputtering from a Mn₂Ga and a Ru target. Different MRG compositions (Mn₂Ru1.1Ga, Mn₂Ru0.9Ga, Mn₂Ru0.75Ga, and Mn₂Ru0.65Ga) have been obtained by varying the sputtering power of Mn₂Ga while keeping the sputtering power of Ru constant. Changing Ru concentration in MRG allows adjusting the compensation temperature Tcomp from 2 to 450 K.
We fabricated MTJs without insertion layers, as well as stacks with Ta (0.3 nm, 0.6 nm, 0.9 nm) and Al (0.3 nm, 0.6 nm, 0.9 nm) insertion layers. The switching properties of MTJs were analyzed through magnetotransport measurements as a function of applied bias voltage at room temperature. Al 0.6 nm acts as the best diffusion barrier. Magnetic properties of the multi-layers were characterized by the quantum design superconducting quantum interference device (SQUID) with a maximal applied field of 7n T at the range of temperatures from 60 K to 300 T.
The magnetometry data was extracted from the typical out-of-plane hysteresis loop of the investigated MTJs (Fig.1). As the magnetic field is swept from +7 T to – 7 T, the magnetic moment of CoFeB starts to rotate first and switches close to 0 T. The sharp jump observed at -0.4 T is attributed to the reversal of MRG magnetization. With conducting the same measurements at different temperatures, it is possible to detect the compensation temperature of MRG, which will lead to a decrease of its magnetic moment and a divergence of the coercive field. The temperature with zero magnetic moment and extremely high Hc corresponds to the compensation point of MRG. In Fig.2 the temperature dependence of magnetic properties of MTJs with the same MRG composition, but different diffusion barriers, is presented. For different insertion layers, Tcomp can shift over a large range, showing that the choice of insertion layer can have a dramatic effect on the properties of MRG. For instance, in MTJs with no insertion layer 100 K < Tcomp < 160 K; the shift to the higher temperatures is observed for Ta 0.3 nm insertion (140 K < Tcomp < 200 K), and to the lower temperatures with Al 0.6 nm insertion (Tcomp < 120 K). Moreover, we demonstrate that Tcomp can also be altered by post-annealing, as a 20 K shift is observed after annealing at 325°C for 1 hour.
Mn₂RuₓGa integrated into MTJs demonstrates a low magnetic moment, high coercivity, and thereby high immunity to the applied magnetic field over a broad temperature range (60 K – 300 K). At the same time, these MTJs show TMR even at the compensation temperature, highlighting a fundamental difference between an AFM and a CHFM. All these make MRG extremely attractive for spintronics applications, and for the excitation of magnetic resonances in STNOs.

Keywords: Ferrimagnetism; Half-metals; Magnetic Tunnel Junctions; Heusler Alloy

  • Lecture (Conference)
    The IEEE International Magnetics Conference (INTERMAG), 23.-27.04.2018, Singapore, Singapore

Permalink: https://www.hzdr.de/publications/Publ-28719
Publ.-Id: 28719


Running Status of SRF Gun-II at ELBE Center

Xiang, R.; Arnold, A.; Lu, P.; Murcek, P.; Schaber, J.; Teichert, J.; Vennekate, H.; Zwartek, P.

As a new electron source with higher brilliance, the second version of the superconducting RF photoinjector (SRF Gun-II) has been successfully commissioned at the ELBE Center for high power radiation sources since 2014. SRF Gun-II features an improved 3.5-cell niobium cavity as well as a superconducting solenoid in the same cryomodule. With Mg photocathode SRF Gun-II can provide high current beam with bunch charge up to 300 pC at 100 kHz repetition rate. For user operation the SRF Gun-II successfully generated stable beam with 200 pC in CW mode and sub-ps bunch length. In this presentation the gun's status and beam parameters will be presented.

Keywords: photoinjector; SRF Gun; Mg photocathode; CW mode; sub-ps

  • Open Access Logo Contribution to proceedings
    29th Linear Accelerator Conference, 16.-21.09.2018, Beijing, China
    Proceedings of the 29th Linear Accelerator Conference, ISBN 978-3-95450-194-6

Permalink: https://www.hzdr.de/publications/Publ-28718
Publ.-Id: 28718


Robust Cs2Te and Mg photocathodes in SRF gun at ELBE center successful for CW IR FEL and THz radiation

Xiang, R.; Teichert, J.

Quality of photocathodes is one of the critical issues for the stability and reliability of the light source facility. In 2014, SRF gun-I with Cs2Te provided stable electron beams successfully for IR FEL at HZDR. Cs2Te worked in SRF gun for more than one year without degradation. Currently, Mg photocathodes with QE up to 0.5% are applied in SRF Gun II, which is able to generate e- beam with bunch charges up to 200 pC in CW mode with sub-ps bunch length for the high power THz radiation facility for the ELBE users.

Keywords: Photocathode; SRF gun

  • Lecture (Conference)
    Photocathode Physics for Photoinjectors 2018, 15.-17.10.2018, Santa Fe, NM, USA

Permalink: https://www.hzdr.de/publications/Publ-28717
Publ.-Id: 28717


Development and characterization of novel anti-GD2 target modules for retargeting of Universal CAR T cells toward GD2 expressing tumors

Mitwasi, N.; Feldmann, A.; Bergmann, R.; Berndt, N.; Rössig, C.; Bachmann, M.

Although chimeric antigen receptor (CAR) engineered T cells demonstrated promising therapeutic effect against cancer, they are still associated with adverse side effects which could be life threatening in some cases. Therefore, in our group we have developed a switchable universal CAR T cell platform “UniCAR”, which can be repeatedly switched on and off. This system consists of CAR T cells that cannot bind tumor antigens directly but instead they are redirected with a target module (TM). Such TMs are mainly composed of an epitope on one side, which is recognized by the UniCAR T cells, and on the other side a tumor antigen-binding domain. Once the TM is eliminated, the UniCAR T cells are no more activated. Disialoganglioside GD2 was shown previously to be a very promising target for several tumors such as neuroblastoma and Ewing’s sarcomas. Therefore, anti-GD2 TMs were developed and evaluated regarding their functionality. They were shown to be functional in activating the UniCARs to secrete important pro-inflammatory cytokines and to kill GD2+ tumor cells both in vivo and in vitro. To further characterize the anti-GD2 TM with PET imaging, it was labeled with radioactive Cu64 . The TM showed a specific enrichment at the site of the GD2+ growing tumor, and it was mainly eliminated through the kidneys within half an hour due to its small size. Such short half-life, provides the UniCAR system with the fast safety switch in case any complications occurred in patients treated with the UniCAR T cells.

  • Poster
    The International Conference on Lymphocyte Engineering, 13.09.2018, Madrid, Spain
  • Open Access Logo Abstract in refereed journal
    Human Gene Therapy 29(2018)11, A9
    DOI: 10.1089/hum.2018.29071.abstracts

Permalink: https://www.hzdr.de/publications/Publ-28716
Publ.-Id: 28716


Automated Target Model Determination from MEIS Spectra Utilizing an Evolutionary Algorithm

Heller, R.

To extract chemical compositions and layer thicknesses of layered samples from back scattering spectra experimentalists usually have to take the following approach: Simulation of a theoretical spectrum for an initial target configuration and comparison to the measured data followed by the successive re-adjustment of the target model iteratively, until simulation result and experimental spectrum fit together. For multi-layer samples this procedure can get rather time consuming, especially when a series of similar samples with varying layer thickness and/or stoichiometry has to be analyzed.

Although modern IBA spectrum simulation software like SimNRA[1] or WINDF[2] have become quite powerful and handy tools, the analysis of the spectra consumes still a significant fraction of an IBA scientist’s working time. SimNRA offers therefore the opportunity to partially fit layer thicknesses and/or elemental ratios for a given layer within a certain region of a spectrum. WINDF goes a step further and implements an automated spectrum fitting based on a simulated annealing algorithm. However, it takes the user quite some time to set up the boundary conditions and fit parameters until the actual fit procedure can be initiated. Furthermore, the outcome of the fit procedure in some cases contains non-physical artifacts and requires multiple re-adjustments of the boundary conditions / fit parameters.

An approach that came up in the past (and is still being applied for particular tasks) is the application of artificial neural networks (ANN) to derive sample information from IBA spectra [3,4]. In a nut-shell this method basically trains an algorithm how the shape of a spectrum is correlated to the sample’s target model without introducing any physics (numerical calculations) to the code. Therefore, the ANN is fed with many (typically several 10 thousand) training spectra with a known target model. After this training procedure (which can be quite time consuming) the ANN spits out the target model of any unknown spectra in almost zero time. However, the spectra must be of the same type as all the training spectra since an ANN can only interpolate and not extrapolate, which is for sure one of the mayor drawbacks of this approach. However, all these efforts are justified in some special scenarios e.g. if a large series of spectra of similar type has to be evaluated.

In this contribution, we present a new approach of automated IBA spectra fitting applying an evolutionary algorithm (EA). We show that EA is well suited and robust for complete and fast IBA spectrum fitting with minimum input of boundary conditions. The benefits of this algorithm and the particular differences to simulated annealing and ANN are pointed out. Special emphasis is put on the adoption of this algorithm to the analysis of MEIS spectra, since there is a couple of differences to classical IBA methods that needs to be considered.

Based on this algorithm a platform independent software package has been developed that comprises a clean and easy-to-use graphical user interface. We will introduce this software in a basic overview.

Keywords: Ion beam analysis; evaluation software; evolutionary algorithm

  • Invited lecture (Conferences)
    9th International Workshop on High-Resolution Depth Profiling (HRDP-9), 25.-29.06.2018, Uppsala, Schweden

Permalink: https://www.hzdr.de/publications/Publ-28715
Publ.-Id: 28715


Structural and magnetic properties of epitaxial Mn–Ge films grown on Ir/Cr buffered MgO(0 0 1)

Dash, S.; Schleicher, B.; Schwabe, S.; Reichel, L.; Heller, R.; Fähler, S.; Neu, V.; Patra, A. K.

Epitaxial Mn–Ge films with varying composition have been prepared on Ir/Cr buffered MgO(0 0 1) substrates using DC magnetron sputtering. The effect of composition on phase formation, texture and magnetic properties of Mn–Ge films has been investigated. These films grow epitaxially on Ir/Cr buffered MgO(0 0 1) with a tetragonal D022 type structure. From the pole figure analysis the epitaxial relationship is determined to be: D022 Mn–Ge [1 0 0] (0 0 1)||Ir [1 0 0] (0 0 1)||Cr [1 1 0] (0 0 1)||MgO [1 0 0] (0 0 1). Mn–Ge films close to stoichiometric composition (Mn77.5Ge22.5) exhibit perpendicular magnetic anisotropy with crystallographic c-axis being the easy axis of magnetization. The room temperature measured values of coercivity µ 0 H c, saturation magnetization M S and anisotropy field µ 0 H A for Mn77.5Ge22.5 are 2.86 T, 90 kA m−1 and 9.6 T, respectively. Mn–Ge films with low and high Mn concentration possess high coercivity but extremely low magnetization and that can be ascribed to the presence of secondary phases of non-magnetic/low magnetic nature, possible interdiffusion, and especially partial substitution of excess Mn atoms into the Ge site.

Keywords: epitaxial growth; magnetron sputtering; ion beam analysis; Mn; Ge; magnetic propoerties

Permalink: https://www.hzdr.de/publications/Publ-28714
Publ.-Id: 28714


Hydrogen storage in Mg2FeSi alloy thin films depending on the Fe-to-Si ratio measured by conversion electron Mössbauer spectroscopy

Trinh, T. T.; Asano, K.; Heller, R.; Reuther, H.; Grenzer, J.; Schreuders, H.; Dam, B.; Potzger, K.

Sputter deposited Mg2FexSi1-x films of 250 nm thickness have been investigated by means of Mössbauer spectroscopy. While non-hydrogenated films are characterized by a quadrupole split doublet, hydrogenated regions show a singlet with reduced isomer shift. The relative areas of the spectra reflect the relation between loaded and unloaded regions prepared at the same loading conditions.

Keywords: Hydrogen storage; MgFe; Mössbauer

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  • Secondary publication expected

Permalink: https://www.hzdr.de/publications/Publ-28713
Publ.-Id: 28713


Interaction of hydrogen with hafnium dioxide grown on silicon dioxide by the atomic layer deposition technique

Kolkovsky, V.; Scholz, S.; Kolkovsky, V.; Schmidt, J. U.; Heller, R.

The electrical and structural properties of thin hafnia films grown by the atomic layer deposition technique were investigated before and after different annealing steps as well as after a dc H plasma treatment. By using the nuclear reaction analysis, the authors demonstrated that high concentrations of hydrogen (about 1–2 at. %) could be observed even in as-grown hafnia layers. An additional hydrogenation of the samples with atomic H led to a significant shift of the flatband voltage. This shift could be explained by the introduction of positively charged H-related defects which were found to be stable at room temperature. By comparing the experimental findings with the theory and the data from muon spin spectroscopy, they tentatively ascribed these defects to interstitial H in HfO2.

Keywords: hafnium oxide; ion beam analysis; nuclear reaction analysis; hydrogen depth profiling

Permalink: https://www.hzdr.de/publications/Publ-28712
Publ.-Id: 28712


Untersuchung an Meteoriten mittels INAA am FRM II

Li, X.; Lierse Von Gostomski, C.; Merchel, S.; Leister, N.

Im Rahmen eines Schülerprogrammes (TUM-Kolleg) der TU München wurden drei gewöhnliche Chondrite aus Deutschland (Cloppenburg (Fund 2017) [1,2], Oldenburg (Bissel), Benthullen) und drei Achondrite der Gruppe der HEDs (Howardite-Eukrite-Diogenite) aus dem Oman und aus Nord-West-Afrika (Dhofar 1675, NWA 2690, NWA 2698) sowie ein Mond-, ein Marsmeteorit (NWA 7986, NWA 4925) und zwei potentielle Mikrometeorite mit Probengewichten von je 38 µg und 22 µg mittels der INAA im radiochemischen Labor der Radiochemie analysiert. Die Mikrometeoriten wurden einmal 24 h lang in der Hochflussposition (>1E14 cm-2s-1) und die anderen größeren Proben mit einem Gewicht von je ca. 20 mg jeweils für 3 min bzw. 5 min und 45 min in der Rohrpostanlage bestrahlt. Durch den hohen und reinen thermischen Neutronenfluss am FRM II konnten insgesamt bis zu 45 Elemente in den Proben bestimmt werden.
Anhand der Elementzusammensetzung konnten die beiden Meteoritengruppen eindeutig unterschieden werden, und damit die Klassifizierung der Meteoriten bestimmt bzw. bestätigt werden [Abb. 1 & 2].
Der Marsmeteorit konnte mit seinem hohen Eisenanteil von 17,2 % von den anderen Achondriten unter-schieden werden. Das Ergebnis vom Mondmeteorit zeigte hingegen zwar einen höheren Gehalt von Elementen seltener Erden, aber keine deutliche KREEP-Signatur. Eine negative Europium-Anomalie konnte aber auch nicht zweifelfrei festgestellt werden [Tab. 1].
Bei den beiden potentiellen Mikrometeoriten konnten trotz geringer Probenmassen bis zu 14 Elemente detektiert werden. Beide haben einen hohen Fe-Gehalt von 65 Gew.% bzw. 55 Gew.%, aber für Ni wurde nur eine Nachweisgrenze von 0,3 Gew.% bzw. 0,45 Gew.% fest-stellt, so dass ein extraterrestrischer Ursprung kaum möglich ist.
Danksagung: Wir danken A. Muszynski und M. Szyszko (Poznan, PL), A. Bischoff (U Münster), D. Heinlein, J. Feige (TU Berlin) und A. Gärtner (Senckenberg Dresden) für die Bereitstellung der Meteoritenproben.

[1] www.dlr.de/dlr/desktopdefault.aspx/tabid-10081/151_read-24638/#/gallery/28819 (Jan. 2019)
[2] J. Storz et al., (Jan. 2019) www.paneth.eu/PanethKolloquium/2017/0075.pdf.
[3] A. Palme et al., GCA 55 (1991) 3105-3122.
[4] H. Binder, Lexikon der chemischen Elemente, ISBN 3-7776-0736-3 (1999).
[5] J.A. Barrat et al., Geochim. Cosmochim. Acta 83 (2012) 79–92.

Keywords: INAA; meteorite; cosmic rays

  • Lecture (Conference)
    27th Seminar on Activation Analysis and Gamma Spectrometry (SAAGAS 27), 24.-27.02.2019, München, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-28711
Publ.-Id: 28711


Immune Monitoring of Cancer Patients Prior to and During CTLA-4 or PD-1/PD-L1 Inhibitor Treatment

Tunger, A.; Kießler, M.; Wehner, R.; Temme, A.; Meier, F.; Bachmann, M.; Schmitz, M.

Targeting the immune checkpoint receptors cytotoxic T lymphocyte antigen 4 (CTLA-4), programmed cell death protein 1 (PD-1), or programmed cell death 1 ligand 1 (PD-L1) represents a very attractive treatment modality for tumor patients. The administration of antibodies against these receptors can promote efficient antitumor effects and can induce objective clinical responses in about 20–40% patients with various tumor types, accompanied by improved survival. Based on their therapeutic efficiency, several antibodies have been approved for the treatment of tumor patients. However, many patients do not respond to checkpoint inhibitor therapy. Therefore, the identification of biomarkers is required to guide patient selection for this treatment modality. Here, we summarize recent studies investigating the PD-L1 expression or mutational load of tumor tissues as well as the frequency and phenotype of immune cells in tumor patients prior to and during CTLA-4 or PD-1/PD-L1 inhibitor treatment.

Permalink: https://www.hzdr.de/publications/Publ-28710
Publ.-Id: 28710


Multimodal PET/MRI Imaging Results Enable Monitoring the Side Effects of Radiation Therapy.

Kovács, N.; Szigeti, K.; Hegedűs, N.; Horváth, I.; Veres, D. S.; Bachmann, M.; Bergmann, R.; Máthé, D.

Radiotherapy is one of the most frequently applied treatments in oncology. Tissue-absorbed ionizing radiation damages not only targeted cells but the surrounding cells too. The consequent long-term induced oxidative stress, irreversible tissue damage, or second malignancies draw attention to the urgent need of a follow-up medical method by which personalized treatment could be attained and the actually dose-limiting organ could be monitored in the clinical practice. We worked out a special hemisphere irradiation technique for mice which mimics the radiation exposure during radiotherapy. We followed up the changes of possible brain imaging biomarkers of side effects, such as cerebral blood flow, vascular endothelial function, and cellular metabolic processes for 60 days. BALB/c mice were divided into two groups (n=6 per group) based on the irradiation doses (5 and 20 Gy). After the irradiation procedure arterial spin labeling (ASL), diffusion-weighted imaging (DWI) in magnetic resonance modality and [18F]fluoro-deoxy-D-glucose positron emission tomography (FDG-PET) scans of the brain were obtained at several time points (3, 7, 30, and 60 days after the irradiation). Significant physiological changes were registered in the brain of animals following the irradiation by both applied doses. Elevated standard uptake values were detected all over the brain by FDG-PET studies 2 months after the irradiation. The apparent diffusion coefficients from DWI scans significantly decreased one month after the irradiation procedure, while ASL studies did not show any significant perfusion changes in the brain. Altogether, our sensitive multimodal imaging protocol seems to be an appropriate method for follow-up of the health status after radiation therapy. The presented approach makes possible parallel screening of healthy tissues and the effectiveness of tumor therapy without any additional radiation exposure.

Permalink: https://www.hzdr.de/publications/Publ-28709
Publ.-Id: 28709


Late-Stage Preclinical Characterization of Switchable CD123-Specific CAR-T for Treatment of Acute Leukemia

Loff, S.; Meyer, J.-E.; Dietrich, J.; Spehr, J.; Riewaldt, J.; von Bonin, M.; Gründer, C.; Franke, K.; Feldmann, A.; Bachmann, M.; Ehninger, G.; Ehninger, A.; Cartellieri, M.

Application of autologous T cells genetically engineered to express CD19-specific chimeric antigen receptors (CAR-T) is highly effective in the treatment of B cell malignancies. To this date, application of CAR-T therapy beyond CD19 remains challenging due to the inability to control CAR-T reactivity in patients and the lack of tumor-associated antigens exclusively expressed by malignant cells. The interleukin-3 receptor alpha chain (CD123) is a promising immunotherapeutic target and associated with leukemia-initiating compartments in myeloid- or lymphoid derived diseases. However, in contrast to CD19, CD123 is a precarious target due to its prevalent expression on healthy hematopoietic stem and progenitor cells (HSPC) as well as endothelial cells. Thus, CAR-T lacking any fine-tuned control mechanisms are at risk to cause life threatening toxicities or can only act as bridging therapy to an allogeneic stem cell transplantation. To extend application of CAR-T therapy and safely redirect CAR engineered T cells to challenging targets such as CD123, a switch-controllable universal CAR T platform (UniCAR) was recently introduced. The UniCAR system consists of two components: (1) a non-reactive inducible second generation CAR with CD28/CD3ζ stimulation for an inert manipulation of T cells (UniCAR-T) and (2) soluble targeting modules (TM) enabling UniCAR-T reactivity in an antigen-specific manner.
Here we provide late stage pre-clinical data for UniCAR-T in combination with a CD123 specific TM (TM123) for treatment of acute leukemia. Primary patient-derived CD123-positive leukemic blasts were efficiently eradicated by TM123-redirected clinical grade manufactured UniCAR-T in vitro and in vivo. Activation, cytolytic responses and cytokine release were proven to be strictly switch-controlled. Moreover, anti-leukemic responses of UniCAR-T were demonstrated to be comparable to conventional CD123 specific CAR-T in vitro. In contrast to conventional CD123 CAR-T, TM123-redirected UniCAR-T discriminate between CD123high malignant cells and CD123lowhealthy cells with negligible toxicity towards HSPC in vivo. As 4-1BB mediated co-stimulation is known to enhance CAR-T activity in vivo, a novel CD123-specific targeting module bearing a covalently bound trimeric 4-1BB ligand (4-1BBL) was developed and characterized for co-stimulation at the leukemic site in trans. Specific binding of TM123 4 1BBL was demonstrated against native 4-1BB as well as CD123-positive leukemic blasts. In long-term tumor eradication models, TM123 4 1BBL ameliorated the killing capability of UniCAR-T in vitro. Additionally, the increased hydrodynamic radius of trimeric 4-1BBL-coupled TM123 prolonged plasma half-life and enhanced bioavailability in vivo. In conclusion, UniCAR-T maintain high anti-leukemic efficacy, while adding a sophisticated mechanism for immediate control to improve safety and versatility of CD123-directed CAR-T therapy. Moreover, switching between several TMs from short to moderate plasma half-life allows for an individualized treatment of various leukemic settings while minimizing potential adverse effects.

Permalink: https://www.hzdr.de/publications/Publ-28708
Publ.-Id: 28708


Low-energy carrier dynamics in graphene and other 2D materials

Winnerl, S.; König-Otto, J. C.; Mittendorff, M.; Pashkin, A.; Venanzi, T.; Schneider, H.; Helm, M.

Phonons in graphene and interexcitonic transition in transition metal dichalcogenides are examples for low-energy excitations in 2D materials. Free-electron lasers such as FELBE deliver tunable short mid-infrared pulses that are ideally suited to study the carrier dynamics in 2D materials in the energy range of these low-energy excitations. We present results on the carrier dynamics in graphene and MoSe2.

Keywords: free-electron laser; ultrafast dynamics; low energy excitations; 2D materials

  • Invited lecture (Conferences)
    18th International Conference on Laser Optics (ICLO 2018), 04.-08.06.2018, St. Petersburg, Russland

Permalink: https://www.hzdr.de/publications/Publ-28707
Publ.-Id: 28707


Ultrafast Carrier Dynamics in Hybrid Plasmonic Nanostructured metal/graphene

Huang, X.; Winnerl, S.; Schneider, H.

Graphene plasmonic circuits critically depend on convertingincident light into propagating graphene plasmons (GPs), and on controlling their propagation and focusing to enhance light-matter interactions. Here, the theoretical analysis and experimental studies are mainly focused on the GP induced hot-carrier generation and injection on graphene when energy transferred at different SPP interference states. We characterize the ultrafast carrier dynamics in the hybrid metal/graphene nanostructures using ultrafast pump-probe spectroscopy in the mid-inferred range. The renormalized plasmon dispersions in the interface of the metal/graphene nanostructures are investigated. And, the characterization of nonlinearity phase of the high order harmonic generation signals of the hybrid nanostructures are also demonstrated.

Keywords: graphene; plasmonics; hybride graphene-metal nanostructure

  • Poster
    Graphene 2018, 26.-29.06.2018, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-28706
Publ.-Id: 28706


ESUO - The European Synchrotron and FEL User Organisation: Aims and activities

Arčon, I.; Arikan, P.; Bittencourt, C.; Boscherini, F.; Braz Fernandes, F. M.; Brooks, N.; Buljan, M.; Casu, B.; D'Angelo, M.; D'Astuto, M.; Feiters, M.; Froideval, A.; Gross, S.; Gutt, C.; Hase, T.; Huotari, S.; Jablonska, K.; Jergel, M.; Kajander, T.; Khan, A.; Kirm, M.; Kokkinidis, M.; Kövér, L.; Lamba, D.; Larsen, H. B.; Lechner, R. T.; Logan, D. T.; López, O.; Lorentz, K.; Lüning, J.; Mariani, C.; Marinkovic, B.; Mc Guinness, C.; Meedom Nielsen, M.; Mickevicius, S.; Mikulík, P.; Petukhov, A.; Pietsch, U.; Pokroy, B.; Purans, J.; Renault, L.; Santoro, G.; Shivachev, B.; Stangl, J.; Tromp, M.; Vankó, G. A.; Blasetti, C.; Górkiewicz, A.; Grobosch, M.; Helm, M.; Schramm, B.; Seidlhofer, B. K.; van Daalen, M.; Vollmer, A.

The European Synchrotron and free-electron laser User Organisation (ESUO) established in 2010 today represents about 30.000 users. We aim at representing the users from all European countries. Each country is represented within ESUO by one up to four national delegate(s), depending on the size of the user community in the respective country. The ESUO aims and activities are shown in this poster.

Keywords: European synchrotron and FEL user organisation (ESUO); European projects; scientific collaborations; European synchrotron and FEL radiation facilities

  • Poster
    10th Joint BER II and BESSY II User Meeting, 05.-07.12.2018, Berlin, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-28705
Publ.-Id: 28705


Rotation-Free Scattered-Radiation Imaging with a Radiotherapy X-Ray Linac

Simoes, H.; Ferreira Marques, R.; Rachinhas, P. J. B. M.; Wagner, A.; Crespo, P.

The OrthoCT (acronym for orthogonal computed tomography) concept, based on orthogonal ray imaging, is a new low-dose imaging technique under investigation to potentially assist external-beam radiation therapy treatments. It consists in detecting radiation scattered in the patient and emitted at approximately 90 degrees with respect to the direction of the incoming beam. Such radiation can be collected by a 1D-detector system with a multi-sliced collimator positioned parallel to the incident beam axis. This system can be potentially useful for on-board imaging with the patient positioned and ready for treatment, or for real-time treatment monitoring. In this work, a multi-pixel, small OrthoCT detector prototype was developed and tested experimentally. This system is based on gadolinium orthosilicate crystals coupled to photomultiplier tubes and a collimator made of lead slices. The experimental measurements were performed with a heterogeneous phantom of acrylic with an air cavity inside, using a TrueBeam linac operated at 6MV in the flattening-filter-free modality. The results allow concluding that this new imaging technique is capable to provide, in 1.3 s, morphological images of the phantom without the need to rotate the X-ray source around the object to be irradiated, showing the feasibility of such system.

Keywords: Low-dose imaging for radiotherapy; Rotationfree megavoltage tomography; X-ray detection; Image-guided radiotherapy (IGRT)

  • Lecture (Conference)
    IEEE Nuclear Science Symposium and Medical Imaging Conference, 10.-17.11.2018, Sydney, Australien
  • Contribution to proceedings
    IEEE Nuclear Science Symposium and Medical Imaging Conference, 10.-17.11.2018, Sydney, Australien
    IEEE Nuclear Science Symposium and Medical Imaging Conference Proceedings (NSS/MIC): IEEE, 1-3
    DOI: 10.1109/NSSMIC.2018.8824227

Permalink: https://www.hzdr.de/publications/Publ-28704
Publ.-Id: 28704


Double-bended saturation of optically induced bleaching in graphene

Winnerl, S.; Winzer, T.; Mittendorff, M.; Mittenzwey, H.; Jago, R.; Schneider, H.; Helm, M.; Malic, E.; Knorr, A.

Saturable absorption due to Pauli blocking is a fundamental optical phenomenon that can be described fully analytically for a two-level system. In solids, the related carrier dynamics is typically much more complex. Nevertheless, the fluence dependence of the induced bleaching is typically qualitatively similar to the behaviour of a two-level system. Saturable absorbers are important photonic devices for realizing short laser pulses.
We present a joint theory-experiment study, where the bleaching of graphene is studied in a wide range of fluences. In pump-probe experiments utilizing 30 fs near-infrared (wavelength 800 nm) pulses the pump-induced transmission is measured. The study reveals an unusual double-bended saturation behaviour. For fluences in the mJ/cm2 range the induced transmission saturates due to Pauli blocking. Interestingly, a qualitatively similar behaviour is found at fluences that are 1000 times smaller. In this range one would expect a linear fluence dependence of the induced transmission. Microscopic theory based on the density matrix formalism shows that the unexpected saturation at low fluences is related to intensity dependent many-particle scattering. The crucial point is the balance between in- and out-scattering of electrons from the optically excited k-space regions. The occupation of this region determines the observed transmission [1].
Full understanding of the saturation behaviour in graphene is of relevance for graphene-based saturable absorbers. Graphene is an interesting material for this purpose as it can be applied in a very broad spectral range from THz to UV [2,3]. Also the high damage threshold, which is verified in our experiments, is an attractive feature.

[1] T. Winzer et al., Nature Commun. 8 (2017) 15042
[2] V. Bianchi et al., Nature Commun. 8 (2017) 15763
[3] D. G. Purdie et al., Appl. Phys. Lett. 106 (2015) 253101

Keywords: graphene; ultrafast dynamics; saturable absorption

  • Poster
    Graphene 2018, 26.-29.06.2018, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-28703
Publ.-Id: 28703


Unusual Coulomb effects in graphene

Winnerl, S.

In many semiconductors Coulomb scattering plays an essential role in the thermalization process of a non-equilibrium carrier distribution. Here we discuss three surprising and fascinating manifestations of Coulomb scattering in graphene. All phenomena are explored both experimentally and by manybody theory. The experimental techniques for time-resolved studies at rather low photon energy (mi-infrared and terahertz range) are introduced and we explain why epitaxial graphene samples are particularly well suited for our experiments.
The first observation concerns a double-bended saturation behavior of bleaching induced by near-infrared radiation. The second phenomenon is the optically induced anisotropy in k-space for excitation with linearly polarized radiation and its relaxation to a Fermi-Dirac distribution. The third set of experiments tackles the dynamics of graphene in a magnetic field perpendicular to the graphene layer. Here evidence for strong Auger scattering is found. We discuss the possibility to apply Landau quantized graphene as a gain medium in a tunable laser and as a tunable nonlinear optical material.

Keywords: graphene; ultrafast dynamics; Coulomb scattering

  • Invited lecture (Conferences)
    Tianjin International Symposium on Epigraphene (TISEG), 22.-27.07.2018, Tianjin, China

Permalink: https://www.hzdr.de/publications/Publ-28702
Publ.-Id: 28702


Low energy relaxation dynamics in graphene and MoSe2

Winnerl, S.

Time-resolved investigations of 2D materials such as graphene and transition metal dichalcogenides (TMDs) in the energetic vicinity of their low-energy excitations provide deep insights into the physical processes involved in the carrier relaxation dynamics. We show results of pump-probe experiments in the mid-infrared and terahertz rage using a free-electron laser as a source. They allow one to disentangle the role of carrier-carrier and carrier-phonon scattering. As an application, an ultra-broadband fast photodetector is demonstrated. Furthermore we present very recent results on monolayer MoSe2. Here, fairly slow carrier cooling is observed. At the overlap of the terahertz pump pulse with the near-infrared probe pulse we see a red-shift of both the exciton and trion peak. This signal may stem from either the Franz-Keldysh or the AC-Stark effect.

  • Lecture (others)
    Seminar of Physics Departmern, Chalmers University, 14.11.2018, Göteborg, Schweden

Permalink: https://www.hzdr.de/publications/Publ-28701
Publ.-Id: 28701


Pump-probe spectroscopy in the infrared

Winnerl, S.

We discuss infrared pump-probe spectroscopy experiments, where both tabletop lasers and a free-electron laser are applied as radiation sources covering the inire infrared range from the near infrared to terahertz.The focus of the talk is on the basic principle of the experiment and related techniques like four-wave mixing, and on the experimental capabilities in our labs at HZDR. Electronic transitions in semiconductor quantum wells, interband transitions in graphene, plasmonic excitations in graphene ribbons will serve as examples for low-energy excitations that can be investigated in a time-resolved manner. In particular, examples for excitations with different polarization states will be discussed. Finally we will show the strength of probing with terahertz time-domain spectroscopy.

Keywords: Pump-Probe spectroscopy

  • Lecture (others)
    Leibniz-Institut für Polymerforschung, 18.12.2018, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-28700
Publ.-Id: 28700


Application of electromagnetic fields in material processing, metallurgy, casting and crystal growth

Eckert, S.

Basic and applied research on Liquid Metal Technologies and Magnetohydrodynamics represents a surprising bandwidth ranging from high-temperature energy conversion, new kinds of liquid metal batteries, the production of solar-grade silicon, carbon dioxide free production of hydrogen, liquid metal targets in modern neutron sources and transmutation systems, casting of steel and light metals, welding and soldering processes, to basic laboratory experiments with relevance to liquid metal cooled systems, materials processing as well as to geo- and astrophysics.
Most of the metallurgical and crystal growth processes comprise phases with liquid metals (or liquid semiconductors with very similar properties). The application of diverse electromagnetic fields has proven to be a very effective tool of influencing and controlling such liquid metal flows and the corresponding heat and mass transport. For instance, most of the problems in casting of metal alloys affecting the product quality are associated with an improperly conditioned fluid flow during the process. Small improvements in the flow pattern can achieve therefore large effects in terms of quality assurance and energy savings.
This presentation gives an overview of the research activities at the HZDR with respect to the use of magnetic fields in metallurgy, casting and crystal growth. The experimental work is based on model experiments for the detailed investigation of flow processes under the influence of magnetic fields. This presentation presents a number of examples and discusses corresponding results in the light of the respective technologies considered.

Keywords: Magnetohydrodynamics; crystal growth; metallurgy; metal casting

  • Invited lecture (Conferences)
    The Third Russian Conference on Magnetohydrodynamics, 18.-21.06.2018, Perm, Russia

Permalink: https://www.hzdr.de/publications/Publ-28699
Publ.-Id: 28699


Flow structures in liquid metal Rayleigh-Benard convection under the influence of DC magnetic fields

Vogt, T.; Schindler, F.; Zürner, T.; Schumacher, J.; Tasaka, Y.; Yanagisawa, T.; Eckert, S.

One of the classical problems in fluid dynamics is the Rayleigh-Bénard convection (RBC) where a fluid layer is exposed to a temperature difference ΔT between a colder lid and a warmer bottom. RBC under the influence of a steady magnetic field is of particular interest in geo- and astrophysics, but has also some relevance for technical applications such as the use of liquid metals for new type of batteries or heat removals in fusion blankets. In this paper we present flow measurements conducted in various geometries and magnetic field configurations. Systematic flow measurements were performed by means of the ultrasound Doppler velocimetry (UDV). Experiments were carried out in GaInSn using a cylindrical fluid vessel in a vertical magnetic field and a rectangular container exposed to a horizontal magnetic field. Various flow regimes have been identified by the velocity measurements, in particular, we studied the transition from a quasi-two-dimensional state towards a three-dimensional flow occurring with decreasing magnetic field strength.

Keywords: Rayleigh; Benard convection; liquid metals; magnetoconvection; flow measurements; ultrasound Doppler velocimetry

  • Lecture (Conference)
    9th International Symposium on Electromagnetic Processing of Materials (EPM2018), 14.-18.10.2018, Awaji Island, Japan

Permalink: https://www.hzdr.de/publications/Publ-28698
Publ.-Id: 28698


A material experiment for small satellites to characterise the behaviour of carbon nanotubes in space – development and ground validation

Abbe, E.; Renger, T.; Sznajder, M.; Klemmed, B.; Sachse, E.; Hübner, R.; Schüler, T.; Bärtling, Y.; Muchow, B.; Tajmar, M.; Schmiel, T.

Over the last years, Carbon Nanotubes (CNT) drew interdisciplinary attention. Regarding space technologies a variety of potential applications were proposed and investigated. However, no complex data on the behaviour and degradation process of carbon nanotubes under space environment exist. Therefore, it is necessary to investigate the performance of these new materials in space environment and to revaluate the application potential of CNTs in space technologies. Hence, CiREX (Carbon Nanotubes – Resistance Experiment) was developed as a part of a student project. It is a small and compact experiment, which is designed for CubeSat class space satellites. These are a class of nanosatellites with a standardized size and shape. The CiREX design, electrical measurements and the satellites interfaces will be discussed in detail. CiREX is the first in-situ space material experiment for CNTs. To evaluate the data obtained from CiREX, ground validation tests are mandatory. As part of an extensive test series the behaviour of CNTs under solar ultra violet light (UV) and vacuum ultraviolet light (VUV) was examined. Single-walled carbon nanotubes (SWNT), multi-walled carbon nanotubes (MWNT) and MWNT/resin composite (ME) were exposed to different light sources. After the exposure, the defect density was investigated with Raman spectroscopy. There is a clear indication that UV and VUV light can increase the defect density of untreated CNTs and influence the electrical behaviour.

Keywords: Carbon nanotubes; CubeSat; Electrical behavior; Material experiment; Solar light; Space environment

Permalink: https://www.hzdr.de/publications/Publ-28697
Publ.-Id: 28697


Nanocomposite thin film research using ion beams and in situ techniques

Krause, M.

Lecture about research activities on "Nanocomposite thin film research using ion beams and in situ techniques"

  • Invited lecture (Conferences)
    Institutsseminar in der Veranstaltungsreihe - Doctorado en Nanociencia y Tecnologías de Materiales Facultad de Ciencias, 19.10.2018, Puerto Real, Cadiz, Spanien

Permalink: https://www.hzdr.de/publications/Publ-28696
Publ.-Id: 28696


Zr and Mo thin films with reduced residual impurities’ uptake under high vacuum conditions at room temperature

Meško, M.; Bohovičová, J.; Munnik, F.; Grenzer, J.; Hübner, R.; Čaplovič, Ľ.; Čaplovičová, M.; Vančo, Ľ.; Vretenár, V.; Krause, M.

In recent years, transition (refractory) metals such molybdenum (Mo) and zirconium (Zr) have been studied as infrared (IR) reflector in solar absorber applications. The sputter process parameters are very important for depositing a high quality thin film achieving the necessary low emittance. IR reflectance of the metal film is influenced by the film microstructure, presence of residual impurities and surface roughness. The main objective of the present study is to prepare Mo and Zr metallic thin films with improved optical properties by high power impulse magnetron sputtering at room temperature under high vacuum conditions. In comparison to the Mo and Zr thin films deposited by direct current magnetron at the same average power, thin films deposited by HiPIMS exhibits dense microstructure without voids, grown preferentially along c-axis, have smooth surface and are free of residual contaminants. Compared to the dcMS films we observed an element specific reduction of impurities measured by elastic recoil detection analysis (ERDA) by a factor 4/8 for N, 3/4 for H and 9/14 for O for Mo/Zr thin films respectively. The compositional effects are correlated with differences in the film morphology microstructure revealed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis.

  • Poster
    16th International Conference on Plasma Surface Engineering, 16.-21.09.2018, Garmisch-Partenkirchen, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-28695
Publ.-Id: 28695


Vacuum and in-air thermal stability studies of SnO2-based TCO for concentrated solar power applications

Mendez, A.; Lungwitz, F.; Schumann, E.; Janke, D.; Guillén, E.; Escobar Galindo, R.; Gemming, S.; Fernández-Martínez, I.; Krause, M.

In concentrated solar power plants, the receiver tubes are one of the key components to increase the solar-thermal conversion efficiency. Absorber materials of those tubes have to exhibit high-temperature and air stability, high optical absorption in the solar region and low thermal emittance. In state of the art central tower plants black paints (i.e. Pyromark 2500) are used as absorber material. However, the high emissivity of those paints leads to high radiative energy losses. Moreover, these paints undergo a temporal degradation and performance loss during the lifetime of the plant.
Here, an alternative concept for high-temperature stable solar-selective coatings is presented. It consists of a transparent conductive oxide (TCO) deposited as solar-selective transmitter on a black body absorber. For this purpose, SnO2:Ta thin films were reactively sputtered on fused quartz substrates. Their vacuum and in-air stability up to 800°C were studied by in situ Rutherford backscattering spectrometry (RBS), Raman spectroscopy and spectroscopic ellipsometry (SE). The correlation between structural, optical and electric transport properties was analyzed by RBS, SE, UV-Vis spectrometry, and Hall effect measurements. Solar selective properties are demonstrated as proof of concept for the TCO deposited on a silicon substrate.
Financial support by the EU, grant No. 645725, project FRIENDS2, and the HGF via the W3 program (S.G.) is gratefully acknowledged.

  • Lecture (Conference)
    16th International Conference on Plasma Surface Engineering, 16.-21.09.2018, Garmisch-Partenkirchen, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-28694
Publ.-Id: 28694


Design and high-temperature durability tests of solar-selective coatings based on aluminium titanium oxynitrides AlyTi1-yOxN1-x

Escobar-Galindo, R.; Guillén, E.; Heras, I.; Lungwitz, F.; Munnik, F.; Rincon-Llorente, G.; Alcon-Camas, M.; Schumann, E.; Azcona, I.; Krause, M.

Aluminium titanium oxynitrides were studied as candidate materials for high temperature absorbers in solar-selective coatings (SSC) due to their excellent stability and their tuneable optical behaviour. A set of individual AlyTi1-y(OxN1-x) layers with different oxygen content was prepared by cathodic vacuum arc (CVA) deposition. A comprehensive analysis of the sample microstructure and morphology allowed an accurate modelling of the optical constants in the whole wavelength range of solar interest (190 nm - 25 μm). The optical properties of these films can be controlled from metallic to dielectric character by adjusting the oxygen content. Complete multilayer SSC, including a TiN layer as IR reflector, were designed by applying optical simulations, obtaining excellent optical selective properties (α = 94.0% and εRT = 4.8%). The design concepts were validated by an excellent agreement between simulated and experimental stacking order, composition and optical properties. The durability of two multilayers was studied under conditions simulating realistic operation of central receiver power plants. Both SSC stacks were stable in single stage tests of 12 h at 650°C in air. During cyclic tests, the coarser microstructure of multilayer 1, was found to be more resistant against oxidation than multilayer 2 constituted of four oxynitride layers with a graded oxygen content. Multilayer 1 fulfilled the performance criterion of PC ≤ 5% for 300 symmetric, 3 h long cycles at 600°C in air confirming that the designed SSCs are exciting candidate material for concentrated solar power applications at high temperature. Financial support by the EU, grant No. 645725, project FRIENDS22, is gratefully acknowledged.

Keywords: Solar-selective coatings; CSP; Optical simulation; Thermal test

  • Lecture (Conference)
    16th International Conference on Plasma Surface Engineering, 16.-21.09.2018, Garmisch-Partenkirchen, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-28693
Publ.-Id: 28693


On the Effect of Thin Film Growth Mechanisms on the Specular Reflectance of Aluminium Thin Films Deposited via Filtered Cathodic Vacuum Arc

Rincón-Llorente, G.; Heras, I.; Guillén Rodríguez, E.; Schumann, E.; Krause, M.; Escobar-Galindo, R.

The optimisation of the specular reflectance of solar collectors is a key parameter to increase the global yield of concentrated solar power (CSP) plants. In this work, the influence of filtered cathodic vacuum arc deposition parameters, particularly working pressure and deposition time, on the specular and diffuse reflectance of aluminium thin films, was studied. Changes in specular reflectance, measured by ultraviolet–visible and near-infrared spectroscopy (UV-vis-NIR) spectrophotometry, were directly correlated with thin film elemental concentration depth profiles, obtained by Rutherford backscattering spectrometry (RBS), and surface and cross-sectional morphologies as measured by scanning electron microscopy (SEM) and profilometry. Finally, atomic force microscopy (AFM) provided information on the roughness and growth mechanism of the films. The two contributions to the total reflectance of the films, namely diffuse and specular reflectance, were found to be deeply influenced by deposition conditions. It was proven that working pressure and deposition time directly determine the predominant factor. Specular reflectance varied from 12 to 99.8% of the total reflectance for films grown at the same working pressure of 0.1 Pa and with different deposition times. This transformation could not be attributed to an oxidation of the films as stated by RBS, but was correlated with a progressive modification of the roughness, surface, and bulk morphology of the samples over the deposition time. Hence, the evolution in the final optical properties of the films is driven by different growth mechanisms and the resulting microstructures. In addition to the originally addressed CSP applications the potential of the developed aluminium films for other application rather than CSP, such as, for example, reference material for spectroscopic diffuse reflectance measurements, is also discussed.

Keywords: filtered cathodic vacuum arc; total and specular reflectance; thin film deposition conditions; structural characterisation

Permalink: https://www.hzdr.de/publications/Publ-28692
Publ.-Id: 28692


Statistical entropy analysis as tool for circular economy: Proof of concept by optimizing a lithium-ion battery waste sieving system

Reuter, M. A.; van den Boogaart, K. G.; Lundströma, M.; Santasalo-Aarnioa, A.; Velázquez Martíneza, O.; Serna-Guerreroa, R.

With the concept of circular economy gaining strength as an alternative for the sustainable production of raw materials, there is an inherent need to develop methods capable of quantifying the efficiency of recycling systems, provide guidelines for optimization of existing technologies, and support the design of new products based on sound, scientific and engineering principles. The work hereby presented proposes the use of statistical entropy coupled with material flow analysis as a basis for the optimization of separation and purification processes. Unlike other efficiency parameters, this approach provides an analysis of component concentration or dilution from a systemic perspective, taking into consideration products, by-products and waste streams. As a proof-of-concept, a sieving process for waste lithium-ion batteries (LIB) was chosen. It is demonstrated that using this approach it is possible to determine the stages that do not contribute to the concentration of components thus offering guidelines for process optimization. In the present case, the total number of sieving stages can be decreased with a minimum impact on the concentration of the products. In comparison, it is also shown that the widely accepted exergy analysis is not able to identify the opportunities for optimization due to the particular characteristics of this exemplary system, i.e., negligible change in energy consumption as a function of sieving stages and absence of chemical changes. Finally, the experimental results suggest that Al and Cu can be concentrated using a simple sieving pre-processing step, perhaps in preparation for a subsequent refining stage.

Keywords: Circular economy; Material flow analysis; Relative statistical entropy; Lithium-ion batteries; Process simulation; Process optimization

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Permalink: https://www.hzdr.de/publications/Publ-28691
Publ.-Id: 28691


FPGAs and the Cloud – An Endless Tale of Virtualization, Elasticity and Efficiency

Knodel, O.

The flexible use of reconfigurable devices within a cloud context requires abstraction from the actual hardware through virtualization to offer these resources to service providers. In this talk, we present our Reconfigurable Common Computing Frame (RC2F) approach – inspired by system virtual machines – for the profound virtualization of reconfigurable hardware in cloud services. Using partial reconfiguration, our framework abstracts a single physical FPGA into multiple independent virtual FPGAs (vFPGAs). A user can request vFPGAs of different size for optimal resource utilization and energy efficiency of the whole cloud system. To enable such flexibility, we create homogeneous partitions on top of an inhomogeneous FPGA fabric abstracting from physical locations and static areas. On the host side our Reconfigurable Common Cloud Computing Environment (RC3E) offers different service models and manages the allocation of the dynamic vFPGAs.

Keywords: Reconfigurable Hardware; FPGA; Cloud; Virtualization

  • Invited lecture (Conferences)
    13th HiPEAC Workshop on Reconfigurable Computing (WRC'2019), 21.01.2019, Valencia, Spain

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Permalink: https://www.hzdr.de/publications/Publ-28690
Publ.-Id: 28690


Incorporating cell-wise errors in compositional exploratory analysis

Pospiech, S.; Kronz, A.; Tolosana-Delgado, R.

Geochemical data sets often include values with varying uncertainties. The uncertainties are small if samples had been measured under optimal analytical conditions, but are typically high if trace elements values are close to the detection limit or if the data set includes samples of very different characteristics or with inhomogeneity. From the data itself it is not necessarily possible to distinguish geochemical valuable signals from data noise without prior knowledge about the sample set and the analytical method. Especially for a small geochemical data set not knowing or ignoring the uncertainties might lead to misinterpretations. One method to circumvent this problem is to incorporate cell-wise errors which describe the uncertainty for each value and can serve as weights in statistical analysis. However, incorporating cell-wise errors into statistical analysis of geochemical data sets is rarely applied, especially when it comes to multi-variate analysis. Geochemical data sets are mostly composed of compositional data. Accordingly, the characteristics of constrained values should be taken into account for incorporating errors.
Principal component analysis (PCA) helps to explore the variance–covariance structure with the objective of data reduction and interpretation of a multivariate setting. In combination with biplots it is a powerful tool for explorative data analysis. In our contribution we will propose a method to include cell-wise errors as weights into the PCA with the aim to use the information provided by uncertainties in an early stage of data exploration. This is another approach than the weighted PCA based on the Tucker-3 method presented by Gallo and Buccianti (2013) or the spectral map analysis (SMA) presented by Lewi (2005). We will use a geochemical data set of archaeological glasses with standard deviations for each value (based on 3 - 5 measurements) as a case study for incorporating analytical uncertainties in PCA of compositional data.

Keywords: compositional data; analytical error; cell-wise error; weighted PCA

  • Lecture (Conference)
    The 8th International Workshop on Compositional Data Analysis, 03.-08.06.2019, Terrassa, Spain

Permalink: https://www.hzdr.de/publications/Publ-28689
Publ.-Id: 28689


Magnetically Induced Cavitation for the Dispersion of Particles in Liquid Metals

Sarma, M.; Grants, I.; Bojarevics, A.; Gerbeth, G.

A contactless excitation of cavitation is possible by superposition of induction heating with a static axial magnetic field. This creates an alternating electromagnetic body force in a liquid metal which in turn produces pressure oscillations. Using this method, the onset of cavitation has been clearly observed in various liquid metals (tin, zinc, aluminum, steel SAE 304) at pressure oscillations in the range of 28…50 kPa. The present study aims to extend the previous work by producing steel metal matrix composites (MMC) and assessing the feasibility of the proposed method for particle dispersion in steel. Stainless steel (SAE 316L) samples with different ceramic inclusions, e.g. TiN, Al2O3, TiB2 as well as others, have been created. It has been demonstrated that the cavitation onset in the liquid steel varies extensively and depends on the cavitation nuclei rather than the strength of acoustic pressure. The microstructure of the produced samples has been analyzed using SEM and EDS.

Keywords: MMC production; Steel composites; Cavitation treatment

  • Contribution to proceedings
    TMS 2018, 11.-15.03.2018, Phoenix, Arizona, United States of America
    Magnetically Induced Cavitation for the Dispersion of Particles in Liquid Metals: Springer, Cham, 978-3-319-72852-0
    DOI: 10.1007/978-3-319-72853-7_12

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Permalink: https://www.hzdr.de/publications/Publ-28687
Publ.-Id: 28687


Dendritic structure formation of magnesium alloys for the manipulation of corrosion properties: Part 1 – microstructure

Lakoma, P.; Ditze, A.; Scharf, C.

Besides impurities in magnesium alloys containing aluminum, the microstructure also plays a role in the corrosion properties of the alloy. By targeted manipulation of grain size, secondary dendrite arm spacings and segregation in terms of amount and position, the corrosion properties are expected to be improved. For this, experiments were carried out by casting alloys with 0, 3, 6, 9, and 12%aluminum into a mold with different applied cooling rates. The samples were analyzed regarding microstructure and composition, and the grain size and secondary dendrite arm spacings, as a function of aluminum content and cooling rate, were modeled. The results show a decrease in grain size and secondary dendrite arm spacings with an increased cooling rate. The segregated b phase was predominantly situated at the grain boundaries as divorced eutectic and in lamellar form. The assumed influences on the corrosion properties will be examined in Part 2.

Keywords: Magnesium alloy; Microstructure; Corrosion behavior; SDAS

  • International Journal of Materials Research 109(2018)12, 1081-1091
    DOI: 10.3139/146.111708

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  • Secondary publication expected

Permalink: https://www.hzdr.de/publications/Publ-28686
Publ.-Id: 28686


Novel magnetite-biocomposites for clean-up of highly diluted Ga and As containing industrial wastewaters

Vogel, M.; Matys, S.; Hübner, R.; Pollmann, K.

Gallium is an essential element in many semi-conducting products, electronic devices and opto-electronic components. It is mostly applied as GaAs and GaN. Despite it’s growing demand it is currently not recycled. The development of novel biocomposites based on magnetite and biopolymers for the selective binding of Ga and As from diluted flushing solutions of the semi-conducting industry offers a promising concept for the efficient, economical and sustainable recycling of these metal(loids).

In this study several magnetite based-biocomposites for an efficient and easy separation in magnetic field were developed and optimized regarding an effective sorption behavior for Ga and As from complex but highly diluted industrial wastewaters (each 4 mg/L, pH 8.4). Combinations of magnetite nanoparticles and S-layer proteins removed up to 100% of both elements depending on the used amount of material. Additionally, the S-layer-magnetite-composites showed a better stability compared to pure magnetite during the sorption process. The materials showed a preferred sorption of Ga compared to As. An optimization of (selective) sorption and stability of the materials was achieved by addition of a second biomolecule (bovine serum albumin, phosvitin, chitosan) to S-layer-magnetite-composites. Raman spectroscopy and transmission electron microscopy with electron diffraction of the biocomposites revealed pure magnetite particles with size between 5-10 nm surrounded by the biocomponent. The zeta-potential of the biocomposites is negative which favors the binding of Ga3+ compared to negatively charged As species. Sustainability of the biocomposites is given as bound Ga and As could be completely removed from the composites with EDTA, so that the biocomposites could be reused for several sorption-desorption cycles.

In conclusion, these regenerative materials enable an efficient and selective removal of gallium and arsenic from highly diluted industrial wastewaters. The combination of magnetite with biomolecules provides a promising approach to improve metal recycling and contribute to environmentally friendly and sustainable processes.

Keywords: S-layer; magnetite; biosorption; gallium; arsenic

  • Lecture (Conference)
    4th Green & Sustainable Chemistry Conference, 05.-08.05.2019, Dresden, Germany

Permalink: https://www.hzdr.de/publications/Publ-28685
Publ.-Id: 28685


Factor Analysis, Monte Carlo Modeling and Landweber Iteration: advanced approaches to EXAFS analysis

Rossberg, A.; Scheinost, A. C.

Factor analysis, monte carlo modeling and Landweber iteration are advanced EXAFS data analysis methods used for structurally complicated chemical systems for which common EXAFS shell fit analysis fails. Here we present the mathematical background of the advanced methods and their application.

Keywords: EXAFS; monte carlo simulation; Landweber iteration

  • Invited lecture (Conferences)
    Demi-journée de l'Atelier de l'OSUG sur le Traitement du Signal pour la Spectroscopie et l'imagerie Hyperspectrale, 25.01.2019, Grenoble, France

Permalink: https://www.hzdr.de/publications/Publ-28684
Publ.-Id: 28684


Spectral random masking: a novel dynamic masking technique for PIV in multiphase flows

Anders, S.; Noto, D.; Seilmayer, M.; Eckert, S.

A novel masking technique for PIV-analysis of multiphase flows is presented. With this new approach, the velocity-field of an unmasked particle fraction (e. g. PIV-tracers) can be determined without the influence of a second (masked) particle fraction (e. g. bubbles or solid particles). Starting from a series of segmented grayscale images in which different particle fractions are determined for each frame, different masking techniques are compared. The problems caused by state of the art masking in case of dynamic masks (individual masks for each frame) are discussed. As a solution the novel spectral random masking algorithm is presented which replaces masked regions in the images by randomized intensity patterns in order to hide them from subsequent PIV-analysis. The advantages over conventional masking techniques are demonstrated for a model experiment of crystallizing ammonium-chloride solution.

Keywords: PIV; Dynamic Image Masking; Multiphase Flow; Particle Laden Flow; Optical Measurement Techniques; Image Processing; Double Diffusive Convection

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Permalink: https://www.hzdr.de/publications/Publ-28683
Publ.-Id: 28683


Significance of the electrode widths in GaAs based Photoconductive Terahertz Emitters

Abhishek, S.; Winnerl, S.; Helm, M.; Schneider, H.

Significance of the electrode widths in GaAs based Photoconductive Terahertz Emitters

Keywords: Terahertz emitter; stripline antenna; GaAs photoconductive THz emitter

  • Poster
    8th International Workshop on Terahertz Technology and Applications, 20.-21.03.2018, Kaiserslautern, Germany

Permalink: https://www.hzdr.de/publications/Publ-28682
Publ.-Id: 28682


Broadband THz emission from Ge photoconductive antenna

Singh, A.; Pashkin, A.; Winnerl, S.; Helm, M.; Schneider, H.

We demonstrate THz emission from a Ge-based photoconductive antenna and compare its bandwidth with the GaAs-based analog. Due to its non-polar character, the Ge emitter demonstrates a broader THz spectrum reaching up to ~ 7.5 THz.

Keywords: Broadband terahertz emitter; Ge photoconductive emitter; Terahertz spectroscopy

  • Poster
    EOS Topical Meeting on Terahertz Science & Technology (TST 2018), 06.-09.05.2018, Berlin, Germany

Permalink: https://www.hzdr.de/publications/Publ-28681
Publ.-Id: 28681


Broadband Spectrum from a Photoconductive Emitter Spanning up to 13 THz

Singh, A.; Pashkin, A.; Winnerl, S.; Helm, M.; Schneider, H.

We demonstrate here ultra-broadband THz emission from a Ge based photoconductive emitter with the gapless spectrum extending up to ~ 13 THz, twice as far as what has been previously achieved with other materials. THz emission properties of Ge based emitter are compared with that of GaAs based emitter.

Keywords: Terahertz emitter; Germanium (Ge) photoconductive switch; Broadband terahertz

  • Contribution to proceedings
    2018 43rd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), 09.-14.09.2018, Nagoya, Japan
    DOI: 10.1109/IRMMW-THz.2018.8510016

Permalink: https://www.hzdr.de/publications/Publ-28680
Publ.-Id: 28680


Impact of flash-lamp-annealing on the structure of TiO2-based films

Gago, R.; Prucnal, S.; Hübner, R.; Jiménez, I.; Palomares, J.

Many applications of titanium dioxide (TiO2) partially rely on its good performance as solvent for impurities [1]. For example, metal (cation) dopants can functionalize or enhance TiO2 as catalyst [2], diluted magnetic semiconductor [3] or transparent conductor [4]. Special attention has been devoted to TiO2 photoactivity where doping has been extensively studied towards band-gap narrowing to achieve visible-light (VISL) response [2]. Here, the most common approach has relied on anion (B,C,N,F,S,…) dopants triggered by the work of Asahi et al. [5]. However, cation (Cr,V,Fe,Ni,Mo,…) doping can also effectively increase VISL absorption but introducing severe structural distortions that additionally result in carrier recombination centers [4].
Our interest is focused on improving the structural quality of metal (co-)doped TiO2 films by post-deposition rapid and non-contact thermal treatments such as flash-lamp-annealing (FLA). An additional objective is to do so with (single and mixed) phase selectivity. For example, the promotion of anatase would be preferable due to the superior photoactivity of this phase or phase mixtures with high anatase content [6]. In particular, in this paper we address the impact of FLA on pure and (co-)doped (Cr,N) TiO2 films produced by magnetron sputtering as relevant systems for VISL photoactivity. The results on monolithic films [7] will be presented as well as promising alternatives to promote anatase by the use of modulated film architectures. The interest of FLA processing can also be extended to other dopants in TiO2 for improving the photoactivity or any other functionality. Therefore, the reported methodology can be attractive for many industrial applications dealing with the synthesis of band-gap engineered TiO2-based materials.
[1] Sacerdoti et al., J. Solid State Chem. 177, 1781 (2004); [2] Henderson, Surf. Sci. Rep. 66, 185 (2011); [3] Matsumoto et al. Science 291, 854 (2001); [4] Serpone et al., J. Phys. Chem. B 110, 24287 (2006); [5] Asahi et al. Science 293, 269 (2001) [6] Scanlon et al., Nat. Mater. 12, 798 (2013); [7] R. Gago, S. Prucnal et al., J. Alloys & Compounds 729 (2017) 438.

  • Lecture (Conference)
    XV Congreso Nacional de Materiales / 1st Iberian Meeting on Materials Science, 04.-06.07.2018, Salamanca, Spain

Permalink: https://www.hzdr.de/publications/Publ-28679
Publ.-Id: 28679


An Algorithm for Maximum-Likelihood Estimation of the Timing Resolution in TOF-PET

Nikulin, P.; Lougovski, A.; Hofheinz, F.; Maus, J.; van den Hoff, J.

INTRODUCTION
As is well known, utilization of time-of-flight (TOF) information can reduce noise and improve convergence rate in PET image reconstruction since it allows to incorporate the (Gaussian) probability density function (the ”TOF-kernel”) of the annihilation event position along the respective LOR into the image reconstruction process. In doing so, it is crucial to use the best possible estimate of the actually given timing resolution in order to achieve a realistic contrast recovery and minimize noise, see [1]. Moreover, it was recently recognized that using the correct timing resolution is necessary to avoid artifacts in Maximum-Likelihood reconstruction of Attenuation and Activity (MLAA), which is in agreement with our own experience. In this context, it is relevant to recognize that the timing resolution of a time-of-flight PET system is count rate dependent, see e.g. [2]. However, count rate dependent TOF-resolution calibration is usually not provided by the vendors. We, therefore, developed a procedure which is compatible with clinical routine and is also applicable retrospectively to existing data.

METHODS
We propose a novel Maximum-Likelihood Timing Resolution Estimation (MLTRE) algorithm that maximizes likelihood by updating activity image and TOF-kernel width alternately. For activity update, TOF-MLEM was used and quadratic surrogate based maximization of the likelihood was performed
to timing resolution update. The listmode version of the algorithm was used to exclude a possible influence of the TOF-binning procedure on the timing resolution estimation process. Ordered subsets optimization was applied in order to reduce computation time. The algorithm was integrated into our
previously developed Tube of response High resolution OSEM Reconstruction (THOR) [3]. THOR incorporates normalization, attenuation, dead-time, randoms corrections, as well as different options for TOF scatter correction including TOF-Single Scatter Simulation algorithm (TOF-SSS), see [4], and
two accelerated in-house algorithms. MLTRE was evaluated using phantom and patient studies covering a large range of count rates that were acquired with a Philips Ingenuity TF PET/MR scanner. Studies were grouped by imaging protocol (”brain” and ”whole-body”, covering different transaxial fields
of view). The iteration scheme of five timing resolution updates per single MLEM update was chosen to achieve convergence of the algorithm in all studies. Iteration was initialized with a timing resolution estimate of 700 ps. Performance of the algorithm was also investigated in the extreme high contrast
conditions that are typical for pelvis region investigations. A dedicated phantom was used for this purpose. Global convergence of MLTRE is not guaranteed. Therefore it is recommended to use a reasonable TOF-kernel width as the start value for the algorithm. In order to assess the stability of the algorithm regarding this start value, MLTRE reconstructions of the same phantom study with varying initial values for the TOF-kernel width in the range [500 − 1000] ps were performed.

RESULTS
Our evaluation yields a linear dependency of timing resolution on count rate which is in correspondence with the reports by other groups, see [2]. According to our results the scanner reaches 551 ps timing resolution at “zero” count rate. Resolution degrades by 146 ps for each ten millions of singles counts per second. Timing resolution degradation can exceed 150 ps ( 25% of initial TOF-kernel width) for clinical relevant count rates. No difference in estimated timing resolution between brain and whole-body protocal was encountered. However, scatter and attenuation correction errors can adversely affect timing resolution estimation. Therefore, using patient data for calibration purposes in PET/MR is not optimal. Dedicated phantom measurements are preferable. However, the procedure is still suitable for retrospective application to existing clinical data if TOF-scatter and attenuation correction
are reliable. Although global convergence of MLTRE algorithm is not guaranteed, no significant influence of the chosen start value for TOF-kernel width on estimated timing resolution was encountered in our data. The results are deviating from the mean by ∼ 4 ps (≈ 0.5%) at maximum whilst the initial values were varied between 500 and 1000 ps.

CONCLUSION
Our preliminary results indicate that the proposed algorithm is capable of realistic timing resolution estimation, while being convenient and easy to use in clinics. To the best of our knowledge, the dependency of timing resolution on singles rate of the Philips Ingenuity TF PET/MR scanner was never published before. According to our findings, the timing resolution of this scanner degrades rapidly with increasing count rate. This fact should be accounted for during image reconstruction.

REFERENCES
[1] S. Vandenberghe, S. Matej, M. E. Daube-Witherspoon, M. Guerchaft, J. Verhaeghe, A. Bol, L. Van Elmbt, I. Lemahieu, and J. S. Karp, “Determining timing resolution from TOF-PET emission data,” IEEE Nuclear Science Symposium Conference Record, vol. 4, pp. 2727–2731, 2007.
[2] S. Surti, A. Kuhn, M. E. Werner, A. E. Perkins, J. Kolthammer, and J. S. Karp, “Performance of Philips Gemini TF PET/CT scanner with special consideration for its time-of-flight imaging capabilities.” Journal of nuclear medicine : official publication, Society of Nuclear Medicine, vol. 48, no. 3, pp. 471–480, 2007.
[3] A. Lougovski, F. Hofheinz, J. Maus, G. Schramm, E. Will, J. van den Hoff, and J. van den Hoff, “A volume of intersection approach for on-the-fly system matrix calculation in 3D PET image reconstruction,” Physics in Medicine and Biology, vol. 59, no. 3, pp. 561–577, feb 2014.
[4] C. C. Watson, “Extension of Single Scatter Simulation to Scatter Correction of Time-of-Flight PET,” IEEE Transations on Nuclear Science, vol. 54, no. 5, pp. 1679–1686, 2007.

Keywords: PET; PET/MR; TOF; time resolution; calibration

  • Poster
    PSMR 2018 - 7th Conference on PET-MRI and SPECT-MRI, 21.-23.05.2018, Isola d'Elba, Italy

Permalink: https://www.hzdr.de/publications/Publ-28678
Publ.-Id: 28678


Sorption of Eu(III) on Eibenstock granite studied by µTRLFS: A novel spatially-resolved luminescence-spectroscopic technique

Molodtsov, K.; Schymura, S.; Rothe, J.; Dardenne, K.; Schmidt, M.

Granite is one of the possible host rocks for a high-level radioactive waste repository, as such it is important to understand its interactions with relevant radionuclides like Cm(III) and Am(III). Because granite is a highly heterogeneous mixture of quartz, feldspar, and mica, as well as minor components, it is difficult to study sorption mechanisms on the molecular level with conventional techniques. Consequently, most studies use isolated constituents rather than the whole system. In this study a novel technique, micro-focus time-resolved laser-induced luminescence spectroscopy (µTRLFS) is presented to overcome the problem of spatial heterogeneity. µTRLFS is a spatially-resolved upgrade of conventional TRLFS, which separates the many-phase problem of granite into many single-phase problems by reducing the beam size of the analytic laser beam to below the size of mineral grains within the natural material. This allows a point-by-point mapping of sorption capacity as well as speciation of a luminescent probe, here Eu3+. A thin-section of granitic rock from Eibenstock, Saxony, Germany was analyzed regarding its mineralogy with microprobe X-ray fluorescence (µXRF) and electron probe microanalysis (EPMA). Afterwards, it was reacted with 5.0 × 10-5 mol/L Eu3+ at pH 8.0 and uptake was quantified by autoradiography. Finally, the speciation of adsorbed Eu3+ and its uptake were studied by µTRLFS. Despite the extremely low concentration of Eu3+ on the sample, results from µTRLFS clearly show that the materials interact differently with Eu3+, and often even on one mineral grain different speciations can be found. Alkali-feldspar shows very high uptake, with an inhomogeneous distribution, and intermediate sorption strength. On quartz uptake is almost 10-fold lower, while the complexation strength is higher than on feldspar. This may be indicative of adsorption only at surface defect sites, in accordance with long lifetimes and low hydration of the observed species.

Keywords: TRLFS; granite; µTRLFS; Europium; sorption

Permalink: https://www.hzdr.de/publications/Publ-28677
Publ.-Id: 28677


Coherent control of spin qubit modes associated with defects in silicon carbide

Astakhov, G. V.

Quantum bit or qubit is a two-level system, which builds the foundation for quantum computation, simulation, communication and sensing. Quantum states of higher dimension, i.e., qutrits (D = 3) and especially qudits (D = 4 or higher), offer significant advantages. Particularly, they can provide noise-resistant quantum cryptography, simplify quantum logic and improve quantum metrology. Flying and solid-state qudits have been implemented on the basis of photonic chips and superconducting circuits, respectively. However, there is still a lack of room-temperature qudits with long coherence time and high spectral resolution. The silicon vacancy centers in silicon carbide (SiC) with spin S = 3/2 are quite promising in this respect. Here, Dr. Astakhov reports a two-frequency protocol to excite and image multiple qudit modes in a SiC spin ensemble under ambient conditions. Strikingly, their spectral width is about one order of magnitude narrower than the inhomogeneous broadening of the corresponding spin resonance. By applying Ramsey interferometry to these spin qudits, a spectral selectivity of 600 kHz and a spectral resolution of 30 kHz are achieved. As a practical consequence, we demonstrate absolute DC magnetometry insensitive to thermal noise and strain fluctuations.

Keywords: Spin defects; quantum applications; coherent control; silicon carbide

  • Invited lecture (Conferences)
    Theo Murphy international scientific meeting. SiC quantum spintronics: towards quantum devices in a technological material, The Royal Society at Chicherley Hall, 05.-06.11.2018, Buckinghamshire, United Kingdom

Permalink: https://www.hzdr.de/publications/Publ-28676
Publ.-Id: 28676


Millisecond Flash Lamp Annealing and Application for SiGe-HBT

Scheit, A.; Lenke, T.; Schumann, T.; Rebohle, L.; Skorupa, W.; Häberlein, S.

A 200 mm millisecond flash lamp annealing (FLA) prototype was developed beside the EU project DOTSEVEN, named after the target for the maximum oscillation frequency (fmax) of 0.7 THz of a SiGe-HBT (Hetero Bipolar Transistor) [1]. The substitution of the final spike annealing (SPA) by FLA reduces the thermal budget despite higher peak temperatures. The development of the FLA process has to focus on the best dopant activation of the implants for high fmax and low deactivation of the intrinsic HBT base for high transit frequency (fT). For FLA the wafers are heated by halogen lamps to an intermediate temperature between 650°C and 850°C. The following Xenon flash lamp pulse increases the front surface temperature with an energy density between 10 J/cm² and 25 J/cm² within 1,0 ms (105 K/s). The pulse energy is limited by the occurrence of wafer breakage.
Among different experiments n-type Si (100) wafers (8-12 Ωcm) were pre amorphized with Germanium (5*1014/cm²; 15 keV) followed by Boron (B) (2*1015/cm²; 1 keV) [2]. FLA (780°C, 5s & 16 J/cm²) results in a suppressed B diffusion (Fig.1a) with concurrent higher activation (Rs = 202 Ω/sq) compared to spike annealing (SPA) at 1020°C with 250 K/s (Rs = 348 Ω/sq). The dopant loss trough the surface is about 30% for both annealings. A higher ratio of flash energy versus pre heat temperature (730°C, 5s & 23 J/cm²) enables a higher activation (Rs = 164 Ω/sq) with a steeper profile (5 nm/dec) and a negligible dopant loss. The slope can be adjusted from 5 nm/dec to 10 nm/dec. The second experiment is based on a model SiGe-HBT with a B base of 8 nm width at a concentration of 1018/cm³ to investigate the influence of the thermal treatment on the shape of the profile. SPA at 1020°C with 250 K/s results in a 40 nm broad profile. The reduction of this broadening down to 20 nm was defined as an internal project goal. FLA (780°C, 30s & 14 J/cm²) reduces the profile broadening by 10 nm. A stronger flash pulse with a colder pre heat (730°C, 5s & 21 J/cm²) achieve our requirements. Based on this FLA process an experimental SiGe HBT technology featuring fT / fmax / BVCEO = 505 GHz / 720 GHz / 1.6 V was developed.

Keywords: flash lamp annealing; heterojunction bipolar transistor; spike annealing; Germanium

  • Lecture (Conference)
    22nd International Conference on Ion Implantation Technology 2018, 16.-21.09.2018, Würzburg, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-28675
Publ.-Id: 28675


Engineering of highly coherent spin centers in silicon carbide

Astakhov, G. V.

Building quantum devices based on silicon carbide (SiC) is highly desirable, facilitated by established SiC CMOS technology. Optoelectronic SiC devices have already been demonstrated, however, the signal- mediating quantum defects are usually introduced in a semi-random manner, by bulk electron or neutron irradiation. We present the controlled generation of quantum centers in silicon carbide (SiC) by focused proton beam in a noncomplex manner without need for pre- or postirradiation treatment. The generation depth and resolution can be predicted by matching the proton energy to the material’s stopping power, and the amount of quantum centers at one specific sample volume is tunable from ensembles of millions to discernible single photon emitters. We identify the generated centers as silicon vacancies through their characteristic magnetic resonance signatures and demonstrate that they possess highy coherent spin properties even at room temperature.

Keywords: Spin defects; quantum applications; silicon carbide

  • Invited lecture (Conferences)
    3rd International Conference on Metamaterials and Nanophotonics METANANO 2018, 16.-21.09.2018, Sochi, Russian

Permalink: https://www.hzdr.de/publications/Publ-28674
Publ.-Id: 28674


Magnetic imaging with high spatial and temporal resolution at the PolLux endstation of the Swiss Light Source

Finizio, S.; Wintz, S.; Witte, K.; Watts, B.; Raabe, J.

Scanning transmission x-ray microscopy (STXM) is an x-ray microscopy technique that relies on the use of diffractive optics (Fresnel zone plates) to focus a monochromatic x-ray beam onto a spot of an x-ray transparent sample, and record the transmitted x-ray intensity with a suitable detector. By raster scanning the sample with a piezoelectric stage and recording the transmitted intensity at each point of the scan, a STXM image is then formed. STXM imaging combines the elemental sensitivity of monochromatic x-rays with the x-ray magnetic circular dichroism (XMCD) effect, allowing for the imaging of e.g. micro- and nanostructured magnetic materials, and multilayered magnetic systems. Depending on the employed zone plate, spatial resolutions on the order of 10 to 30 nm can be achieved with STXM imaging in the soft x-ray region. Time-resolved STXM imaging in the pump-probe regime is also possible. This is made possible by the use of avalanche photodiodes to detect the x-rays, and allows the achievement of temporal resolutions on the order of 50-100 ps. In this contribution, we present the current status and operation performances of the X07DA PolLux STXM endstation of the Swiss Light Source [1]. In particular, the perfor- mance of the endstation in imaging magnetic systems at high spatial and temporal resolutions will be presented. Furthermore, thanks to its modular design, the PolLux endstation allows for the implementation of specific setups aimed at the imaging of magnetic systems under special configurations. An example of such specific setups, which will be presented in this contribution, is the combination of STXM imaging with electrical transport measurements (e.g. aimed at the measurement of topological contributions of magnetic skyrmions to the anomalous Hall effect [2]).

  • Poster
    International Conference on Magnetism, 15.-20.07.2018, San Francsisco, USA

Permalink: https://www.hzdr.de/publications/Publ-28673
Publ.-Id: 28673


Evaluation of Nanoparticle Inks on Flexible and Stretchable Substrates for Biocompatible Application

Schubert, M.; Wang, Y.; Vinnichenko, M.; Rebohle, L.; Fritsch, M.; Schumann, T.; Bock, K.

The flexible and stretchable electronic market is increasing particularly in the field of biomedical electronics. Widely used printed silver conductive tracks today are only eligible for on-skin applications. However, for biomedical applications fully biocompatible, flexible and even stretchable materials for device fabrication are needed. This paper presents an additive printing approach to fabricate flexible and stretchable electronics by using a biocompatible platinum material. Usually, in order to realize electrically conducting Pt-interconnects by inkjet printing, it requires a furnace sintering at prohibitively high temperatures, which are not compatible with thermal sensitive polymeric substrates. This paper describes a high-power diode laser sintering (HPDL) and a flash lamp annealing (FLA) as promising alternative sintering methods. Both processes are eligible whereas laser sintering showed slightly better results. Bending tests and adhesive strength tests of platinum printed inks on polyimide with up to 180 000 cycles, show that printed platinum is a suitable biocompatible material for flexible electronics.

Keywords: flexible; stretchable; biocompatible; photonic sintering; platinum; silver; nanoparticle ink; flash lamp; laser sintering

  • Poster
    7th Electronic System-Integration Technology Conference, 18.-21.09.2018, Dresden, Deutschland
  • Open Access Logo Contribution to proceedings
    7th Electronic System-Integration Technology Conference, 18.-21.09.2018, Dresden, Deutschland
    DOI: 10.1109/ESTC.2018.8546494

Permalink: https://www.hzdr.de/publications/Publ-28672
Publ.-Id: 28672


Time-resolved imaging of the gyration dynamics of nπ states in weak PMA systems

Finizio, S.; Wintz, S.; Gliga, S.; Mruczkiewicz, M.; Kirk, E.; Wessels, T.; Zeissler, K.; Weigand, M.; Raabe, J.

Despite the numerous predictions from micromagnetic simulations [1], only limited experimental investigations of the dynamical processes of perpendicularly magnetized spin configurations, such as e.g. magnetic skyrmions [2,3] exist. One of the causes behind this is the relatively high Gilbert damping (e.g. for Pt/Co stacks on the order of 0.2 [4]), and to the relatively high density of pinning sites of the typical materials employed for the stabilization of such magnetic states. Such high values of both pinning and Gilbert damping strongly influence the behavior of the perpendicularly magnetized spin configurations both statically [5] and dynamically [3]. In this contribution, we present an alternative solution to the use of multilayer stacks exhibiting perpendicular magnetic anisotropy (PMA) for the time-resolved investigation of the dynamical processes of magnetic skyrmions and the more complex nπ states. This solution relies on the use of a Permalloy-based PMA system exhibiting a weak PMA [6]. By judiciously optimizing the sample design, we were able to stabilize magnetic configurations ranging from magnetic skyrmions to more complex nπ states (see Fig. 1) in nano- structured elements. The nπ states stabilized in the nanostructured elements were excited by RF and pulsed magnetic fields generated through the injection of electrical currents across a tailored antenna fabricated close to the magnetic nanostructures. Thanks to the combination of a low Gilbert damping and a relatively low density of pinning sites of this weakly-PMA system, the gyration dynamics in magnetic states ranging from magnetic skyrmions to the more complex nπ states could be imaged by time-resolved scanning transmission x-ray microscopy, proving the feasibility of this material for the study of the dynamical processes in magnetic skyrmions and in nπ states.

  • Lecture (Conference)
    International Conference on Magnetism, 15.-20.07.2018, San Francisco, USA

Permalink: https://www.hzdr.de/publications/Publ-28671
Publ.-Id: 28671


When polymers meet carbon nanostructures: expanding horizons in cancer therapy

Cirillo, G.; Peitzsch, C.; Vittorio, O.; Curcio, M.; Farfalla, A.; Voli, F.; Dubrovska, A.; Iemma, F.; Kavallaris, M.; Hampel, S.

The development of hybrid materials, which combines inorganic with organic materials, is receiving increasing attention by researchers. As a consequence of carbon nanostructures high chemical versatility, they exhibit enormous potential for new highly engineered multifunctional nanotherapeutic agents for cancer therapy. Whereas many groups are working on drug delivery systems for chemotherapy, the use of carbon nanohybrids for radiotherapy is rarely applied. Thus, nanotechnology offers a wide range of solutions to overcome the current obstacles of conventional chemo- and/or radiotherapies. Within this review, the structure and properties of carbon nanostructures (carbon nanotubes, nanographene oxide) functionalized preferentially with different types of polymers (synthetic, natural) are discussed. In short, synthesis approaches, toxicity investigations and anticancer efficacy of different carbon nanohybrids are described.

Keywords: Carbon Nanotubes; Graphene Oxide; Functional Polymers; Nanohybrids; Cancer Therapy; Chemosensitization; Radiosensitization

Permalink: https://www.hzdr.de/publications/Publ-28670
Publ.-Id: 28670


Flash lamp annealing of memristive BiFeO3 thin films simulated with COMSOL Multiphysics

Bürger, D.; Baitule, S.; Rebohle, L.; Schulz, S.; Schmidt, H.

Annealing techniques are key technologies in semiconductor industry. Removing implantation damage, activating dopants, and crystallizing sputtered amorphous layers are only some applications of annealing technologies. Pulsed laser annealing on the nanosecond timescale and longterm annealing in the range of hours are on the short and on the long end of the time scale, respectively. On an intermediate millisecond time scale, flashlamp annealing (FLA) plays an important role as an homogenous annealing technique with a large throughput for industrial applications [1]. Adjustable FLA parameters are energy density and the duration of the pulse. Pulse repetition frequency and shape of the pulse are technical aspects of FLA setups with lower energy density which are commonly used for materials on flexible substrates. So far, less attention has been paid to the geometry of the chamber and to the substrate holder influencing the temperature of the annealed wafer. The former one is influencing the irradiance distribution, the latter one is influencing the heat conduction and the cooling rate of the wafer. The irradiance distribution has a large influence on the temperature close to inhomogeneities on the surface or the edge of the sample [2]. The cooling rate after the FLA pulse determines the effect of thermal treatment on the sample properties due to diffusion processes within the sample. Our studies on BiFeO3 thin films on Si/SiO2/Ti/Pt substrates, which we develop for resistive switching applications, show that highly energetic pulses may lead to a phase separation in BiFeO3 and can cause formation of metallic-like conduction paths whereas multiple flashes with lower energy density keep the insulating and memristive properties of BiFeO3 (BFO).
To understand these aspects in detail, we simulated the FLA of BFO using COMSOL. In a first step, we have constructed the geometry of a typical flashlamp annealing chamber (Fig. 1). Within this geometry, the Raytracing module of COMSOL has been used to study the distribution of the rays within the chamber. As a result after MATLAB-treatment of the raw data, we have obtained an polar information of the irradiance distribution depending on the angle of incidence (Fig. 2). This information can help to understand the incorporated energy on the sample surface/wafer edges in a better way. For example, the intensity of the light at an angle of 30° with respect to the wafer-normal is still 89% of the intensity compared with the intensity directly from the top (0°). At 45° and at 65° the intensity is reduced to 75% and 38%, respectively. This irradiance distribution causes overheating of wafers at positions with non-planar structure.
In a second step, the heat conduction through a wafer has been simulated with different underlying materials of different thermal conductivity. The motivation for these simulation is caused by the fact that BFO thin films on Si/SiO2/Ti/Pt substrates annealed by one highly energetic pulse do not show the expected memristive switching. Instead, BFO thin films on Si/SiO2/Ti/Pt substrates which have been annealed with multiple low energy pulses show memristive switching. In that case the sample lies on a thermally insulating SiO2 wafer which should prevent the flow of the energy in the underlying metallic plate. The strongly reduced cooling rate of the annealed wafer after each FLA pulse allows a step-by-step increase in temperature during multiple FLA pulses. The complete annealing process with 10 3 ms-pulses takes 6.6 seconds, but it is still more effective for BFO than alternative thermal annealing techniques. Fig. 3 presents the simulated surface temperature after 10 ms. Due to the transmission of the light, the surface of the SiO2 wafer remains quite cold. In contrast to that the surface temperature of the BFO thin film on Si/SiO2/Ti/Pt is enhanced. After 100 ms (Fig. 4), the temperature of the surface of the SiO2 wafer is increasing because the hot steel plate transfer a part of the energy to the SiO2 wafer. In general, the temperature of the BFO on Si/SiO2/Ti/Pt is enhanced with respect to the SiO2 wafer. Multiple pulses lead to a step-by-step increase in temperature which might be one reason for the success of the annealing of BFO.

Keywords: flash lamp annealing; BiFeO3; COMSOL Multiphysics

  • Lecture (Conference)
    Materials for Advanced Metallization, 18.-21.03.2018, Milano, Italy

Permalink: https://www.hzdr.de/publications/Publ-28669
Publ.-Id: 28669


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