Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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31863 Publications
Plasma immersion ion implantation of fluorine for oxidation-resistant TiAl alloys
Yankov, R. A.; Kolitsch, A.; Munnik, F.; Donchev, A.; Schütze, M.;
Titanium aluminides are being used in an expansive variety of advanced applications requiring lightness in weight and retention of strength at elevated temperatures. To date, the upper temperature limit has been at about 700°C because of the poor oxidation resistance of the TiAl alloys at higher temperatures. It has recently been established that the TiAl alloys can be rendered highly resistant to environmental degradation by ion implanting halogens, notably fluorine. In modifying the oxidation properties of the TiAl alloys, strong preference has been given to the technique of plasma immersion ion implantation (PIII) because of the possibility to shorten substantially the implant times as well as to avoid the line-of-sight limitation inherent to the standard beamline implantation process. In this work, various TiAl alloys in the form of either rectangular coupons or actual components have been treated by PIII of fluorine. A cheap, easy-to-handle and environmentally-friendly hydrofluorocarbon (difluoromethane) mixed with argon has been used as the precursor gas to implant fluorine into the TiAl alloys. Energy Dispersive X-ray Analysis, Scanning Electron Microscopy, Rutherford Backscattering Spectrometry and Elastic Recoil Detection Analysis have been undertaken for sample characterization. Optimum processing windows and conditions have been identified under which the modified TiAl alloys acquire a stable, adherent and highly protective alumina scale against high-temperature oxidation. The degree of oxidation protection has been evaluated by testing F-implanted samples under conditions of both isothermal oxidation and thermal cyclic oxidation in air at temperatures up to 1050°C and for times as long as 6000 hours.
  • Poster
    16th International Conference on Ion Beam Modification of Materials (IBMM’08), 31.08.-05.09.2008, Dresden, Germany

Publ.-Id: 12089 - Permalink


Plasma immersion ion implantation of fluorine: an efficient technique for enhancing the oxidation resistance of TiAl alloys
Yankov, R.; Kolitsch, A.; Munnik, F.; Wolfhard, M.; Donchev, A.; Schütze, M.;
Titanium aluminides are promising lightweight materials for novel aerospace, automobile and power generation applications. However, because of their insufficient oxidation resistance at temperatures above 700°C, they cannot yet find broader use. The oxidation behaviour of TiAl-alloys can be improved considerably by adding small amounts of fluorine into the subsurface zone of the components (microalloying). The use of TiAl-components after fluorine treatment is feasible up to 1050°C in oxidising environments. One possibility to introduce fluorine into the near-surface of complex TiAl-components is the plasma immersion ion implantation (PI³) technique. The use of an Ar/CH2F2-plasma for the F-PI³ processing leads to a positive halogen effect. The main characterization techniques used in this study have been ERDA and RBS. The oxidation behavior of TiAl samples treated by PI³ of F has been examined using thermogravimetric analysis, and oxidation kinetics curves have been derived. Post-oxidation analyses by SEM reveal a thin protective alumina scale at the surface of the implanted samples as opposed to the thick mixed oxide scale (TiO2/Al2O3) that unavoidably grows upon high temperature oxidation in air.
  • Invited lecture (Conferences)
    11th International Conference on Plasma Surface Engineering, 15.-19.09.2008, Garmisch-Partenkirchen, Germany

Publ.-Id: 12088 - Permalink


Ion implantation of fluorine: towards high-temperature oxidation-resistant TiAl alloys
Yankov, R. A.; Kolitsch, A.; Munnik, F.; Donchev, A.; Schütze, M.;
Due to their light weight and high strength, TiAl alloys are of great interest for advanced automobile, aerospace and power generation applications. However, destructive oxidation occurring in these materials in air at temperatures above 700°C has to date hindered their widespread use. In the present study, the high-temperature oxidation behaviour of TiAl has been examined with consideration of the role of ion-implanted fluorine in providing practically useful oxidation protection. Samples of technical TiAl alloys have been treated by either beamline ion implantation of F or plasma immersion ion implantation (PIII) of F using various F-containing precursor gases. The degree of oxidation protection has been evaluated by testing ion implanted samples under conditions of isothermal and thermal cyclic oxidation at temperatures as high as 1050°C. Optimized ion implantation processing has been found to produce marked improvement in the oxidation behaviour of the TiAl alloys. After PIII of F at a bias voltage of 30 kV, broad fluorine implant profiles extending to depths well beyond those predicted by the theory have been identified by elastic recoil detection analysis (ERDA) in the as-implanted samples. The alloys modified in this way acquire a stable, adherent and highly protective alumina scale against environmental oxidation while retaining the bulk mechanical properties of the starting material. Some of the most important process parameters that enable the formation of an efficient protective scale are considered and assessed. PIII of F using suitable F-containing precursor gases has been successfully applied in protecting machine components made of gamma-TiAl and TNB such as turbine blades and turbochargers.
Keywords: Composition and Microstructure, Fluorine, TiAl alloys, Corrosion.
  • Lecture (Conference)
    2008 MRS Fall Meeting, 01.-05.12.2008, Boston, USA

Publ.-Id: 12087 - Permalink


Laser-driven Electron Acceleration in Plasmas with Few-Cycle Pulses
Veisz, L.; Schmid, K.; Tavella, F.; Benavides, S.; Tautz, R.; Herrmann, D.; Buck, A.; Hidding, B.; Marcinkevicius, A.; Schramm, U.; Geissler, M.; Meyer-Ter-Vehn, J.; Habs, D.; Krausz, F.;
We report on laser-driven electron acceleration with 8 fs -three optical cycle- long pulses and only 40 mJ energy. This situation constitutes an ideal parameter range for laser wake-¯eld acceleration as prophesied by theory and numerical simulations, which was experimentally not explored before. The produced electron spectra are monoenergetic with a peak in the tens-of-MeV range -up to 50 MeV- and free of low-energy electrons with thermal spectrum. The electron beam has a typical divergence of 5 - 10 mrad. The accelerator is routinely operated at 10 Hz and correspondingly it is a promising source for several applications.
Keywords: bubble regime ; bubble ; monoenergetic ; electron acceleration ; ultrashort ; few-cycle ; laser ; laser plasma
  • Comptes Rendus Physique 10(2009)2-3, 140-147

Publ.-Id: 12086 - Permalink


Penning Trap Progress in Munich
Szerypo, J.; Kolhinen, V. S.; Bussmann, M.; Habs, D.; Neumayr, J. B.; Schuermann, C.; Sewtz, M.; Thirolf, P. G.; Schramm, U.;
The MLLTRAP, presently under construction at the Maier---Leibnitz Laboratory (Garching), is a Penning trap system designed to decelerate, purify, charge-breed and cool the radioactive ions with the aim to perform the high-accuracy nuclear mass measurements. It involves novel techniques, like sympathetic cooling of highly-charged ions of interest with laser-cooled Mg+ ions. The goal is to reach an accuracy of 10-10, which is required for high precision fundamental physics studies like the determination of fundamental constants and measurement of electron binding energies for QED at strong fields.
Keywords: Penning trap, sympathetic laser cooling, mass measurement
  • Open Access LogoActa Physica Polonica B 39(2008), 471-476

Publ.-Id: 12085 - Permalink


X-ray investigations off nano structures manufactured by focused ion beam techniques
Grenzer, J.;
Nanowires and chains of nanoparticles are of emerging interest in nanoelectronics, nanophotonics and plasmonics. One possible method is the use of a focused ion beam (FIB) that allows to write any pattern directly into the sample to create a particular nano structure. FIB implantation opens the way to manipulate the device structure locally on a length scale of a few 10 nm only and therefore as well as its electronic and optical properties. FIB combines implantation and lithography. The big advantage over other techniques is the reduction of technological steps necessary to make a particular device structure.
We report on strain and defect analysis of lateral nanostructures created in GaAs and Si substrates. The investigations were performed utilizing the method of high resolution X-ray diffraction using grazing incidence and micro focusing techniques at ESRF beamlines.
We have used two different ways to create lateral nanostructures in Si and GaAs, either by creating one or two dimensional patterns by FIB implantation using a liquid metal ion source of Ga or AuGeSi or by focused ion beam synthesis that allows the fabrication of epitaxial CoSi2 layers embedded in silicon. The spot size of the focused ion beam was in the order of 50nm; an ion beam energy in the order of 25keV was used. The whole implanted area had a size of less than 0.2 mm2.
Keywords: ion beam synthesis, X-ray diffraction, nano focussing
  • Lecture (others)
    Seminar on structure analysis, 06.01.2009, Prag, Tschechische Republik

Publ.-Id: 12084 - Permalink


Tuning exchange spring magnets by ion irradiation and annealing: X-ray investigations
Roshchupkina, O.; Grenzer, J.; Fassbender, J.ORC; Choi, Y.; Jiang, J. S.; Bader, S. D.
The coupling effectiveness of the exchange spring effect can be influenced by the interface structure between hard and soft magnetic films. We have investigated the structural and magnetic properties of an Fe/Sm2Co7 exchange spring bilayer system after annealing and after low-energy ion irradiation. To study the interface properties X-ray reflectivity (XRR) measurements were carried out. From the XRR measurements it was shown that annealing influences the roughness of the layer surfaces causing an almost symmetrical broadening of the interfacial layers. Irradiation induces changes in the top three layers and the most pronounced effect upon irradiation is a change in electron density of the first two top layers and an interface broadening between Fe and Sm2Co7. In contrast annealing after irradiation triggers a material flow influencing the whole irradiated layers. The annealing afterwards therefore influences mainly the Fe layer by creating an Fe/Sm2Co7 intermixed region.
Keywords: X-ray reflectivity, magnetic nano structures
  • Lecture (Conference)
    DPG Frühjahrstagung, 22.-27.03.2009, Dresden, Germany

Publ.-Id: 12083 - Permalink


MLLTRAP: A Penning trap facility for high-accuracy mass measurements
Kolhinen, V.; Bussmann, M.; Habs, D.; Neumayr, J.; Schramm, U.; Schurmann, C.; Sewtz, M.; Szerypo, J.; Thirolf, P.;
The MLLTRAP, presently under construction at the Maier-Leibnitz-Laboratory (Garching), is a Penning trap facility designed to
combine several novel techniques to decelerate, purify, charge breed and cool the reaction products and perform high-accuracy nuclear mass measurements of highly charged, laser-cooled ions.
Keywords: Gas cell; Ion cooling; Mass measurement; Penning trap; RFQ; Sympathetic laser-cooling

Publ.-Id: 12082 - Permalink


Xe+ ion beam induced rippled structures on Si miscut wafers
Hanisch, A.; Grenzer, J.; Biermanns, A.; Pietsch, U.;
We report on the influence of the initial roughness and crystallography of the substrate on the formation of self-organized ripple structures on semiconductors surfaces by noble gas ion bombardment. The Bradley-Harper theory predicts that an initial roughness is most important for starting the sputtering process which in the ends leads to the evolution of regular patterns. We produced regular patterns with intermediate Xe+ ion energies (5-70 keV) at different incidence and azimuthal angles which lead to the assumption that also crystallography plays a role at the beginning of ripple evolution. Most of the previous investigations started from the original roughness of a polished silicon wafer. We used (001) silicon wafers with a miscut angle of 1°, 5° and 10° towards [110]. We studied the ripple formation keeping the ion beam parallel to the [111], [-1-11] or [-111] direction, i.e. parallel, antiparallel or prependicular to the miscut direction [110]. The parallel and antiparallel case implies a variation of the incidence angle with increased roughness over the surface step terraces. The perpendicular orientation means almost no roughness. The results were compared to normal Si(001) and Si(111) wafers.
Keywords: rippled structures nanostructures miscut self-organization
  • Lecture (Conference)
    DPG Frühjahrstagung der Sektion Kondensierte Materie (SKM) 2009, 22.-27.03.2009, Dresden, Germany

Publ.-Id: 12081 - Permalink


Transition from smoothing to roughening of ion-eroded GaSb surfaces
Keller, A.; Biermanns, A.; Carbone, G.; Grenzer, J.; Facsko, S.; Plantevin, O.; Gago, R.; Metzger, T. H.;
During ion sputtering of GaSb(100) surfaces a transient behavior from initial smoothing to roughening accompanied by self-organized pattern formation has been observed using in-situ x-ray reflectivity and grazing incidence small angle scattering. A characteristic hexagonal dot pattern with a spatial periodicity of 30 nm is observed and the correlation length increases with ion fluence. In the framework of the Bradley-Harper model, where the dot pattern formation results from an interplay of surface roughening due to sputtering and surface smoothing due to diffusion, the initial smoothing behavior results from the same diffusion processes as the pattern formation.
Keywords: ion beam erosion, X-ray scattering, nano strucutures

Publ.-Id: 12080 - Permalink


Ripple structures on surfaces and underlying crystalline layers in ion beam irradiated Si wafers
Grenzer, J.; Biermanns, A.; Mücklich, A.; Grigorian, S. A.; Pietsch, U.;
We report on the formation of ion beam induced ripples in Si(001) wafers when bombarded with Ar+ ions at an energy of 60keV. A set of samples varying incidence and azimuthal angles of the ion beam with respect to the crystalline surface orientation was studied by two complementary near surface sensitive techniques, namely atomic force microscopy and depth-resolved X-ray grazing incidence diffraction. Additionally cross-section TEM investigations were carried out. The ripple-like structures are formed at the sample surface as well as at the buried amorphous-crystalline interface. Best quality of the ripple pattern was found when the irradiating ion beam was aligned parallel to the <111> planes. The quality decreases rapidly if the direction of ion beam deviates from <111>.
Keywords: ion beam erosion, nano structures, X-ray diffraction, TEM, AFM
  • Physica Status Solidi (A) 206(2009)8, 1731-1735

Publ.-Id: 12079 - Permalink


Submicron resolution X-ray diffraction from periodically patterned GaAs nanorods grown onto Ge[111]
Davydok, A.; Biermanns, A.; Pietsch, U.; Grenzer, J.; Paetzelt, H.; Gottschalch, V.; Bauer, J.;
We present spatially resolved X-ray diffraction pattern of individual GaAs nanorods grown catalyst-free throughout a pre-patterned amorphous SiNx mask onto germanium [111] surfaces. The experiment has been performed using synchrotron radiation using a micro-sized beam prepared by compound refractive lenses. Due to the non-polar character of the substrate the shapes of NR´s appear not uniform and vary between deformed hexagonal and trigonal in symmetry. Because the average diameter of NR´s equals the experimental resolution certain cuts through slightly inclined edges or corners of individual nanorods with lateral size of about 225 nm could be selected using spatially resolved reciprocal space mapping.
Keywords: X-ray diffraction; nano structures
  • Physica Status Solidi (A) 206(2009)8, 1704-1708

Publ.-Id: 12078 - Permalink


Polyedrisierung dreidimensionaler digitaler Objekte mit Mitteln der konvexen Hülle
Schulz, H.;
  • Open Access LogoWissenschaftlich-Technische Berichte / Forschungszentrum Dresden-Rossendorf; FZD-506 2008

Downloads:

Publ.-Id: 12077 - Permalink


Annual Report 2007 - Institute of Safety Research
Weiss, F.-P.; Rindelhardt, U.; (Editors)
The Institute of Safety Research (ISR) is one of the six Research Institutes of Forschungszentrum Dresden-Rossendorf e.V. (FZD e.V.), which is a member institution of the Wissenschaftsgemeinschaft Gottfried Wilhelm Leibniz (Leibniz Association). Together with the Institute of Radiochemistry, ISR implements the research programme „Safety and Environment“, which is one of the three scientific programmes of FZD. In the framework of this research programme, the institute is responsible for the programme areas “Plant and Reactor Safety” and “Thermal Fluid Dynamics”, respectively (see Table 1). By participating in the development and operation of a pulsed photo-neutron source at the radiation source ELBE (Electron linear accelerator for beams of high brilliance and low emittance), we also contribute to the project “Neutron Induced Processes”, which is part of the FZD programme dedicated to the structure of matter.
  • Open Access LogoWissenschaftlich-Technische Berichte / Forschungszentrum Dresden-Rossendorf; FZD-501 2008

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Publ.-Id: 12076 - Permalink


Large-area Terahertz Emitters based on GaInAsN
Peter, F.; Winnerl, S.; Schneider, H.; Helm, M.; Köhler, K.;
The high price and the complexity of commonly used titanium-sapphire lasers hinder a more widespread use of pulsed THz systems. Er-doped fiber lasers can be a promising alternative. Since the band gap of GaAs is larger than the photon energy at 1.55 µm, this material – while being the standard material for photoconductive THz antennas excited with titanium-sapphire lasers – is not suitable for systems driven by fiber lasers. Dipole antennas have been successfully demonstrated on low-temperature grown InGaAs [1] and ion-irradiated InGaAs [2]. However, the resistivity of these small-gap materials is too low for microstructured large-area emitters. Such emitters consist of an interdigitated finger structure on the semiconductor substrate and a second metallization preventing destructive interference from THz wavelets excited in regions with opposite direction of the bias field [3]. Since these microstructured large-area emitters can withstand large excitation power and high bias fields, they show improved efficiency and higher THz power as compared to conventional antennas.
A 1000 nm thick Ga0.89In0.11As0.96N0.04 layer was grown lattice matched on semi-insolating GaAs by molecular beam epitaxy. On top, additional layers of 60 nm AlGaAs and 5 nm GaAs were grown, resulting in a higher resistivity of the substrate. A THz emitter with an active area of 1 mm  1 mm with a microstructure similar to the one described in ref. 3 was produced using standard optical lithography. The emitter was excited with radiation from an optical parametric oscillator (OPO; tuning range: 1.1 - 1.5 µm). The THz radiation was detected by electro-optic sampling in a ZnTe crystal gated with an 800 nm beam from a titanium sapphire laser. Strong THz emission is observed for excitation wavelength below 1.35 µm [4]. No saturation effects occur within the available range of average power (up to 50 mW) of the OPO. Furthermore the GaInAsN emitter was compared with an emitter based on semi-insolating GaAs at an excitation wavelength of 800 nm. The THz field amplitude of the GaAs emitter is eight times higher as compared to the GaInAsN emitter. Again, in the available excitation power range (up to 0.5 W), no saturation effects are observed.
In summary an efficient and easy-to-align microstructured THz emitters based on GaInAsN has been demonstrated. While further improvement of the material is necessary for excitation at 1.55 µm, the material studied here is suitable for fiber lasers operating at 1.1 µm.

References

[1] M. Suzuki and M. Tounouchi, Appl. Phys. Lett. 86, 163504, (2005).
[2] A. Takazato, et al., Appl. Phys. Lett. 91, 011102, (2007).
[3] A. Dreyhaupt, et al., Appl. Phys. Lett. 86, 121114 (2005).
[4] F. Peter et al., Appl. Phys. Lett. 93, 101102 (2008).
  • Lecture (Conference)
    SPIE Europe Michrotechnologies for the New Millenium, 04.-06.05.2009, Dresden, Deutschland
  • Contribution to proceedings
    SPIE Europe Michrotechnologies for the New Millenium, 04.-06.05.2009, Dresden, Deutschland, 04.-06.05.2009, Dresden, Deutschland
    Proceedings of SPIE, 73661R-73661R-6

Publ.-Id: 12075 - Permalink


Terahertz Emitters and Detectors for Radially and Azimuthally Polarized Beams
Winnerl, S.; Zimmermann, B.; Peter, F.; Schneider, H.; Helm, M.;
Typical pulsed terahertz (THz) systems operate with freely propagating THz waves of linear polarization and Gaussian beam profile. In the range of visible light and near-infrared radiation, Bessel-Gauss modes have stimulated great interest over last years. The lowest order Bessel-Gauss modes have a donut-like intensity distribution and are either radially or azimuthally polarized. Radially polarized beams have interesting fundamental properties such as smaller beam waists in the focus as compared to Gaussian modes and strong longitudinal field components in the focus. In the THz range, superior coupling properties for radially polarized light to plasmonic guided modes on wires, so called Sommerfeld modes are predicted. While there are many investigations on these guided THz modes, only little work is done on freely propagating modes of radial polarization [1] and no publications exist for azimuthally polarized THz beams. Here we present a concept that allows creating emitters and detectors for any desired mode. The principle is demonstrated by showing results on radially and azimuthally polarized THz beams.
The photoconductive emitter and detector structures are based on a microstructured electrode pattern on a semiconductor substrate. The electrode pattern is inverse to the desired polarization pattern, i.e. emitters for radially polarized modes consist of equidistant concentric ring electrodes, while emitters for azimuthally polarized modes are a circular structure of metallized sectors. To avoid electric fields pointing in the opposite direction, a second metallization layer is used in a similar way as previously described for emitter of linearly polarized radiation [2]. While the emitters were fabricated on semi-insulating GaAs, detectors were prepared on GaAs substrates implanted with N+. A titanium sapphire laser was used for excitation of the emitters and gating of the detectors. Beam profiles were measured for the divergent beam 25 mm behind the emitters, and for the refocused beam after a path of ~0.5 m. In this experiment the detector was sensitive for a linearly polarized component of the radiation. The observed beam profiles agree well with the ones expected for Bessel-Gauss beams. Furthermore radiation with radial polarization was detected with an antenna optimized for radial polarization and one for azimuthal polarization. With the detector for azimuthal polarization no signal is expected. However, due to imperfections of the beam profile of the near-infrared laser, a residual signal of about 10 % compared to the signal measured with the detector for radial polarization is observed. This experiment serves as a proof of principle that the emitter-detector combination well suited for studying changes of the mode structure.

References:

[1] G. Chang, Ch.J. Divin, C.-H. Liu, S.L. Williamson, A. Galvanauskas, and T.B. Norris, Opt. Lett. 32, 433 (2006).
[2] A. Dreyhaupt, S. Winnerl, T. Dekorsy, and M. Helm, Appl. Phys. Lett. 86, 121114 (2005).
  • Lecture (Conference)
    SPIE Europe Microtechnologies for the New Millenium, 04.-06.05.2009, Dresden, Deutschland
  • Contribution to proceedings
    SPIE Europe Microtechnologies for the New Millenium, 04.-06.05.2009, Dresden, Deutschland
    Proceedings of SPIE, 73661N-73661N-7

Publ.-Id: 12074 - Permalink


Terahertz Vector Beams
Winnerl, S.; Zimmermann, B.; Peter, F.; Schneider, H.; Helm, M.;
We present a concept for the design of photoconductive antennas which allows the generation and detection of terahertz radiation pulses of freely selectable modes. We demonstrate the principle by showing electric field distributions for vector beams, namely the lowest order Gaussian-Bessel beams. They have a donut-like intensity distribution and either radial or azimuthal polarization. For comparison, Gaussian beams are investigated.
  • Poster
    33rd International Conference on Infrared, Millimeter, and Terahertz Waves, 14.-19.09.2008, Pasadena, USA
  • Contribution to proceedings
    33rd International Conference on Infrared, Millimeter, and Terahertz Waves, 14.-19.09.2008, Pasadena, USA
    Conference Digest of the 33rd International Conference on Infrared, Millimeter, and Terahertz Waves

Publ.-Id: 12073 - Permalink


Terahertz Bessel-Gauss beams of radial and azimuthal polarization from microstructured photoconductive antennas
Winnerl, S.; Zimmermann, B.; Peter, F.; Schneider, H.; Helm, M.;
We report on emission and detection of pulsed terahertz radiation of radial and azimuthal polarization by microstructured photoconductive antennas. To this end the electrode geometry of the emitter is inverse to the desired THz field pattern and a second periodic structure prevents destructive interference effects. Beam profiles of freely propagating THz waves are studied for divergent and refocused beams. They can be well described as the lowest order Bessel-Gauss modes with a divergence comparable to linearly polarized Gaussian beams. Additionally, polarization sensitive detection is demonstrated for radially polarized radiation.
  • Open Access LogoOptics Express 17(2009), 1571-1576

Publ.-Id: 12072 - Permalink


Nanocrystals formation in a –Si/SiO2 layer by ion mixing and plasma immersion ion implantation in different energy ranges
Mello, C. B.; Ueda, M.; Beloto, A. F.; Mücklich, A.; Reuther, H.;
Nanocrystals formation in a –Si/SiO2 layer by ion mixing and plasma immersion ion implantation in different energy ranges
  • Poster
    16th International Conference on Ion Beam Modification of Materials, 31.08.-05.09.2008, Dresden, Deutschland

Publ.-Id: 12071 - Permalink


Nitrogen PIII in SS304 stainless steel using different temperature time-histories
Ueda, M.; Diaz, C.; Lepienski, C. M.; Reuther, H.;
  • Poster
    16th International Conference on Ion Beam Modification of Materials, 31.08.-05.09.2008, Dresden, Deutschland

Publ.-Id: 12070 - Permalink


Tribological Effects of Plasma Immersion Ion Implantation Heating Treatments on SS304 Stainless Steel
Mello, C. B.; Ueda, M.; Geraldo, A. S.; Nascimento, D. P.; Lepienski, C. M.; Reuther, H.;
  • Poster
    VII Encontro da SBPMat, Symposium C – Mechanical properties of surfaces, thin films and coatings, 28.09.-02.10.2008, Guarujá, Brasilien

Publ.-Id: 12069 - Permalink


Terahertz Si:B blocked-impurity-band detectors defined by nonepitaxial methods
Rauter, P.; Fromherz, T.; Winnerl, S.; Zier, M.; Kolitsch, A.; Helm, M.; Bauer, G.;
The molecular beam epitaxial (MBE) fabrication of blocked-impurity-band detectors (BIB) has been a technologically complex and delicate matter ever since its demonstration in silicon, and has not been adapted for other material systems offering detection onsets at lower terahertz frequencies. We report the fabrication and characterization of a vertical Si:B BIB, circumventing the intrinsically troublesome MBE growth of an ultrapure blocking layer by employing ion implantation. We present a thorough characterization of our device, which exhibits highly competitive figures of merits. Our results not only increase the accessibility of BIB fabrication tools for ultrasensitive terahertz detection but also open a road to other material systems.
Keywords: Infrared detector

Publ.-Id: 12068 - Permalink


Contribution of the surface dipole to deformation of superconductors
Lipavský, P.; Morawetz, K.; Kolácek, J.; Brandt, E. H.; Schreiber, M.;
The interaction of the ionic lattice with the superconducting condensate is treated in terms of the electrostatic force in superconductors. It is shown that the surface dipole supplies the force responsible for the volume difference of the normal and superconducting states. Assuming this mechanism, we argue that the usual parametrization of the theory of deformable superconductors should be revisited.
  • Physical Review B 77(2008), 014506-1-014506-6

Publ.-Id: 12067 - Permalink


Synthesis report on work package 2.1: Pressureized Thermal Shock (PTS)
Lucas, D.; Bestion, D.; Coste, P.; Pouvreau, J.; Morel, C.; Martin, A.; Boucker, M.; Bodele, E.; Schmidtke, M.; Scheuerer, M.; Smith, B.; Dhotre, M. T.; Niceno, B.; Lakehal, D.; Galassi, M. C.; Mazzini, D.; D’Auria, F.; Bartosiewicz, Y.; Seynhaeve, J.-M.; Tiselj, I.; ŠTrubelj, L.; ProšEk, A.; Ilvonen, M.; Kyrki-Rajamäki, R.; Tanskanen, V.; Laine, M.; Puustinen, J.;
This report summarizes the results of the NURESIM project for the work package 2.1 “Pressurized Thermal Shock (PTS)”. It includes summaries of the single tasks done by the partners involved in this work package. In the Introduction chapter some more general information on the PTS issue is given, which should help to clarify the integration of the single activities. Since the PTS scenario involves different flow situations, for which also different modelling approaches are necessary, the tasks are sorted according to these flow situations. The relation of the work done to the general aim of the NURESIM project, which is to establish a new code platform, is indicated by assigning the activities to 6 different types. The results achieved in the PTS work package are in agreement with the expectations to the NURESIM project. The conclusion drawn from the single investigations and recommendations for future work are discussed in a separate chapter. It was shown, that for further improvement of the CFD-code capabilities for the two-phase PTS case new well-instrumented experimental data are needed especially for condensation at the surface of a sub-cooled liquid jet in a steam environment as well as on free surfaces, turbulence production and bubble entrainment below the jet and mixing in a stratified flow. Integral experiments, which reflect the PTS flow situations, are important to test the interplay between all the sub-models. Some of the local flow situations can be already captured quite well by presently available CFD codes, for other still many open questions exist. In general more flexible models are required which allow switching between different approaches within one flow domain but for the different local flow situations. Examples for such model approaches are the Large Scale Simulation (LSS) which should allow the application of a two-fluid model for dispersed flows and Interface Tracking Methods for large surfaces and the Scale Adaptive Simulations (SAS) which allow the simulation of large eddies while modelling the turbulence at the unresolved scales. The work done leads to a clear improvement of the simulation capabilities regarding a two-phase PTS situation, but caused by the complexity of the issue it will still be a long way to enable predictive simulations for all the different phenomena that occur in this application. In the near term, one may envisage a simplified treatment of two-phase PTS transients by neglecting some effects which are not yet controlled.
Keywords: Pressurized Thermal Shock, CFD, two-phase flow
  • Article, self-published (no contribution to HZDR-Annual report)
    Forschungszentrum Rossendorf 2008
    Institutsbericht FZD\FWS\2008\11
    40 Seiten

Publ.-Id: 12065 - Permalink


Be-10 and Cl-36 interlaboratory comparisons
Merchel, S.; Bremser, W.; Alfimov, V.; Arnold, M.; Aumaître, G.; Benedetti, L.; Bourlès, D. L.; Braucher, R.; Caffee, M.; Christl, M.; Fifield, L. K.; Finkel, R. C.; Freeman, S. P. H. T.; Ruiz-Gómez, A.; Kubik, P. W.; Rood, D. H.; Sasa, K.; Steier, P.; Tims, S. G.; Wallner, A.; Wilcken, K. M.; Xu, S.;
Driven by the progress in AMS and its spreading application within geosciences, measurements of increasing numbers of samples with low isotopic ratios will be required in the future. Therefore, we have examined the linearity of 10Be/9Be as a function of isotope ratio by distributing 3 secondary standards (dilutions of NIST4325: 10-12-10-14) to 9 AMS labs. The problem of low ratio samples is even more crucial for 36Cl mainly due to the high volatility of chlorine. Thus, we have prepared large quantities of 3 36Cl/Cl solutions from a certified 36Cl activity (NIST4943) by dilution with NaCl. AgCl precipitated from these solutions (10-11-10-13) has been distributed to 9 AMS labs. Some measurements are still ongoing. First results from 6 labs for each nuclide show that these interlaboratory exercises are very valuable.
Keywords: accelrator mass spectrometry, AMS, round-robin, standards
  • Lecture (Conference)
    73. Jahrestagung der DPG und DPG Frühjahrstagung der Sektion AMOP (SAMOP), 02.-06.03.2009, Hamburg, Deutschland

Publ.-Id: 12064 - Permalink


Intersubband relaxation and dephasing in narrow InGaAs/AlAsSb quantum well structures
Grimm, C. V.-B.; Pfau, C.; Ohser, S.; Winnerl, S.; Schneider, H.; Helm, M.; Biermann, K.; Künzel, H.;
The InGaAs/AlAsSb heterostructure has been attractive in recent years due to its high conduction band offsets, which is necessary for intersubband transition (ISBT) based devices at short wavelength (< 3 microns) [1]. To achieve this goal, very thin QWs are required, leading to a particular band configuration where a X or L cross-over takes place. Nevertheless it has been shown that intervalley transfer is rather inefficient for ISBT wavelengths as short as 2.3 m [2], indicating that quantum cascade lasers (QCL) at such short wavelength are feasible. Indeed, QCL with wavelength around 3.0 μm [3], using the same material, were recently demonstrated. Besides the intersubband relaxation time, providing the nonradiative lifetime of the QCL, another important parameter for the design of QCL is the homogeneous linewidth, since it would affect the gain profile of the laser. Therefore the study of the dephasing time, being directly relevant for the homogeneous linewidth, is as important as the intersubband relaxation time.
In this work we present a study of the intersubband relaxation dynamics and dephasing time of QW doped InGaAs /AlAsSb samples grown on InP substrate, through, respectively, degenerate pump-probe and four-wave-mixing (FWM) measurements, at wavelengths around 2 m. The measured intersubband relaxation time constants, between 1 and 1.8 ps, increase with the decrease of the temperature due to the reduced LO phonon scattering. The measured FWM time constant increases by a factor of 4, inferring from the increase of the FWM signal intensity with the decrease of the temperature from 300K to13K.
[1] C. V-B. Grimm, M. Priegnitz, S. Winnerl, H. Schneider, M. Helm, Appl. Phys. Lett. 91, 191121(2007).
[2] C.V-B. Tribuzy, S. Ohser, S. Winnerl, J. Grenzer, H. Schneider, M. Helm, J. Neuhaus, T. Dekorsy, K. Biermann, H. Künzel, Appl. Phys. Lett. 89, 171104 (2006).
[3] D. G. Revin, J. W. Cockburn, M. J. Steer, R. J. Airey, M. Hopkinson, A. B. Krysa, L. R. Wilson, and S. Menzel, Appl. Phys. Lett. 90, 021108 (2007).
Keywords: multiple quantum well, dephasing time, intersubband relaxation
  • Lecture (Conference)
    35th International Symposium on Compound Semiconductors, 21.-24.09.2008, Europa-Park, Rust, Deutschland

Publ.-Id: 12063 - Permalink


Laboratory Underground for Nuclear Astrophysics (LUNA)
Bemmerer, D.;
Deep underground in the Gran Sasso national laboratory in Italy, the 400 kV LUNA accelerator supplying up to 0.5 mA H+ and He+ beams has been installed. Due to the 3400 meters (water equivalent) overburden present at Gran Sasso, the cosmic-ray induced laboratory background in gamma-ray detectors is strongly reduced at LUNA. The accelerator is being used for the study of radiative-capture reactions of astrophysical interest, both by in-beam gamma-ray spectrometry and by activation. The features of the site and accelerator will be shown on the poster; some applications and a future outlook will also be presented.
  • Poster
    IEEE-NSS 2008 Nuclear Science Symposium, 19.-25.10.2008, Dresden, Deutschland

Publ.-Id: 12062 - Permalink


Metal-oxide semiconductor light-emitting devices based on Ge nanocrystals co-implanted with Er ions
Kanjilal, A.; Rebohle, L.; Voelskow, M.; Skorupa, W.; Helm, M.;
Si nanocrystals (NCs) have widely been used as sensitizers in Er-doped SiO2 layers where following the recombination of excitons (electron-hole pairs) in optically excited Si NCs energy is transferred to the higher energy levels of the 4f shell of Er3+ ions and subsequently decay radiatively via 4I13/2 → 4I15/2 transition. This as a result emits light at ~1.53 m, which coincides well the silica-based optical fibers. However, the temperature and concentration quenching of Er3+ ions in Si-rich SiO2 matrices are the basic obstacles for practical application. In the quest for an alternative host material, Ge-rich SiO2 layers have presently attracted a considerable interest due to strong quantum confinement effect and a better control of the surface oxidation of Ge NCs with respect to Si-NCs. In fact, the growth of Er-doped Ge nanoperticles embedded in a SiO2 layer and the photoluminescence (PL) properties have recently been studied. Despite progress in PL response, electrically pumped light-emitting devices (LEDs) are highly required from the optoelectronic standpoints.
We have investigated Ge NCs embedded in a metal-oxide semiconductor (MOS) structure co-doped with Er3+ ions and the corresponding electroluminescence (EL). Here we follow the two step implantation approach: (i) First, 130 keV Ge ions have been implanted (dose of 2–6E16 ions/cm2) into a thermally grown 200 nm thick SiO2 layer followed by furnace annealing (FA) at 950 C for 60 min, which is followed by (ii) 250 keV Er implantation (dose of 1-5E15 ions/cm2) combined with FA in the range of 800-1100oC for 30 min in nitrogen ambience. The MOS structures have been fabricated by depositing indium-tin-oxide (ITO) and aluminium in the front and rare sides of the samples, respectively, and patterning the ITO layer using photolithography. Interestingly, instead of electrically driven pumping of Er3+ ions by Ge NCs, we found that an EL band ~407 nm associated to the Ge-related oxygen-deficiency centres (GeODCs) have been pumped by Er3+ ions and discuss the observed phenomenon on the ground of an inverse energy transfer process. Concentration and temperature dependent modification of microstructure and the subsequent impact on EL response will also be discussed.
Keywords: GeODC, EL, nanocrystals, Er
  • Lecture (Conference)
    SPIE Europe - Microtechnologies for the New Millennium, 04.-06.05.2009, Dresden, Germany

Publ.-Id: 12061 - Permalink


Strong Er luminescence at 1.53 um in rapid thermal annealed Si-rich SiO2 layers co-implanted with Er
Kanjilal, A.; Rebohle, L.; Voelskow, M.; Skorupa, W.; Helm, M.;
The Er-doped SiO2 layers containing Si nanocrystals (Si-ncs) have attracted considerable interest for more than a decade in realizing efficient light sources at 1.54 um, owing to the coincidence of the luminescence wavelength to the absorption minimum of the silicon based optical fibres.1 Although about two orders of magnitude Er luminescence has been noticed in long time annealed sputtered deposited samples,2 observation of such high efficiency in ion implantation processed samples is scarce. Recently, we have succeeded in producing such a system by a combination of sequential Si and Er implantations and rapid thermal annealing (RTA). The processing conditions have been optimized for achieving maximum Er photoluminescence (PL) at 1533 nm at the expense of a broad luminescence band peaking at ~580 nm. Spectral analyses suggest that the appearance of such visible range PL band can be explained in the light of the interfacial state mediated recombination of carriers in the Si-ncs according to the model proposed by Wolkin et al.3. The energy migration from Si-ncs to the nearby Er ions has further been manifested using time-resolved PL measurements.
[1] A. Polman, J. Appl. Phys. 82, 1 (1997).
[2] M. Fujii, M. Yoshida, Y. Kanzawa, S. Hayashi, and K. Yamamoto, Appl. Phys. Lett. 71, 1198 (1997).
[3] M. V. Wolkin, J. Jorne, P. M. Fauchet, G. Allan, and Delerue, Phys. Rev. Lett. 82, 197 (1999).
Keywords: nanocrystals, Er, RTA, PL
  • Poster
    72. Jahrestagung der DPG und DPG Frühjahrstagung des Arbeitskreises Festkörperphysik mit anderen Fachverbänden und den Arbeitskreisen der DPG, 25.-29.02.2008, Berlin, Germany

Publ.-Id: 12060 - Permalink


Strong Er luminescence in rapid thermal annealed Er-doped SiO2 layers containing Si nanoclusters
Kanjilal, A.; Rebohle, L.; Voelskow, M.; Skorupa, W.; Helm, M.;
The Er-doped SiO2 layers containing Si nanocrystals (Si-ncs) have attracted considerable interest for more than a decade in realizing efficient light sources at 1.54 um [1,2], owing to the coincidence of the luminescence wavelength to the absorption minimum of the silicon based optical fibers. Although about two orders of magnitude Er luminescence has been noticed in presence of Si-ncs in a long time annealed sputtering deposited samples [3], observation of such high efficiency in ion implantation processed samples is scarce. Recently, we have succeeded in producing such a system by a combination of sequential Si and Er implantations and rapid thermal annealing (RTA). The processing conditions have been optimized for achieving maximum Er photoluminescence (PL) at 1533 nm at the expense of a broad luminescence band peaking at ~580 nm. Spectral analyses suggest that the appearance of such visible range PL band can be explained in the light of the interfacial state mediated recombination of carriers in the Si-ncs according to the quantum-confinement model [4]. The energy migration from Si-ncs to the nearby Er ions has further been manifested using time-resolved PL measurements.

[1] A. Polman, J. Appl. Phys. 82, 1 (1997).
[2] G. Franzò, S. Boninelli, D. Pacifici, F. Priolo, F. Iacona, and C. Bongiorno, Appl. Phys. Lett. 82, 3871 (2003).
[3] M. Fujii, M. Yoshida, Y. Kanzawa, S. Hayashi, and K. Yamamoto, Appl. Phys. Lett. 71, 1198 (1997).
[4] M. V. Wolkin, J. Jorne, P. M. Fauchet, G. Allan, and Delerue, Phys. Rev. Lett. 82, 197 (1999).
Keywords: Nanocrystals, PL, Er
  • Poster
    E-MRS 2008 Spring Meeting, 26.-30.05.2008, Strasbourg, France

Publ.-Id: 12058 - Permalink


Electrically driven reverse energy transfer process in Er-doped SiO2 layers containing Ge nanocrystals
Kanjilal, A.; Rebohle, L.; Voelskow, M.; Skorupa, W.; Helm, M.;
Optically active Er3+ ions dispersed in a SiO2 layer are often used for amplifying signals at the wavelength of ~1.5 m that coincides with the maximum transparency window of silica-based optical fibers. In order to enhance the 1.5 m Er luminescence, a sufficient number of Er ions must be in the excited state, and it is only possible by employing a high-power pumped laser. Si nanocrystals (NCs) embedded in an Er-doped SiO2 layer show a new avenue to excite Er3+ ions indirectly, where Si-NC behaves like a sensitizer [1]. The basic mechanism of the above mentioned system relies on three facts: (i) optical excitation of Si-NC, (ii) nonradiative transfer of energy from Si-NC to the neighbouring Er3+, and (iii) subsequent relaxation from the first excited state to the ground state of Er3+ by emitting light at 1.5 m. However, to integrate these photonic systems into current microelectronics, it is essential to fabricate metal-oxide-semiconductor based light-emitting devices. Based on electroluminescence (EL) experiments, some groups have shown the importance of Si-NCs in enhancing the 1.5 µm Er luminescence.
Here we use an experimental approach for evaluating the influence of Ge NCs on the Er EL in Er-doped SiO2 layers. In particular, we demonstrate an increase in intensity of the 400 nm band, characteristic of the Ge-NC related oxygen-deficiency centres (GeODC), at the expense of the Er-related signals with maxima at 522, 550, 660 and 1532 nm during electrical pumping at room-temperature  indicating an energy transfer process from the excited Er ions to the confined carriers in the GeODC, which is just opposite to the concept commonly used for the Er-doped SiO2 layers containing Si-NCs. Formation of Ge-NCs was verified by transmission electron microscopy.

[1] Polman A, Nature Materials 2002;1:10.
Keywords: nanocrystals, Er, EL
  • Lecture (Conference)
    VIIth International Conference: Ion Implantation and Other Applications of Ions and Electrons, 16.-19.06.2008, Kazimierz Dolny, Poland

Publ.-Id: 12056 - Permalink


Energy Transfer from the Er3+ to Ge Nanocrystals During Electroluminescence in MOSLEDs.
Kanjilal, A.; Rebohle, L.; Voelskow, M.; Skorupa, W.; Helm, M.;
It is well established that Si nanocrystals (NCs) can act as sensitizers in Er-doped SiO2 during optical pumping [1-4]. In fact, following the recombination of excitons (electron-hole pairs) in optically excited Si NCs transfer their energy to the higher energy levels of the nearest Er3+ ions and subsequently decay to the ground state by intra-4f transitions. Among those, the 4I13/2 → 4I15/2 radiative transition has attracted substantial interest since the respective luminescence at ~ 1.53 m corresponds to the maximum transparency of silica-based optical fibers [2]. Ge is another group-IV element with similar electronic properties to that of Si. We have examined the impact of Ge NCs in Er-doped SiO2 layers by investigating electroluminescence (EL) of the metal-oxide semiconductor (MOS) structures, where the Er-doped Ge-rich SiO2 layers have been prepared by ion implantation technique combined with rapid thermal annealing (RTA). The samples have been prepared by two steps: (i) 130 keV Ge ions have been implanted with a dose of 2  1016 ions/cm2 in a 200 nm thick thermally grown SiO2 layers followed by RTA at 1050 oC for 180 s, and subsequently (ii) 250 keV Er ions have been implanted with a dose of 1  1015 ions/cm2 followed by RTA in the range of 850-1050 oC for 6-150 s in nitrogen ambience. Transmission electron microscopy experiments reveal formation of randomly oriented Ge NCs with average size ~4 nm. The MOS structures have been fabricated by depositing indium-tin-oxide (ITO) and aluminium in the front and rare sides of the samples, respectively, and patterning the ITO layer using photolithography. During EL measurements, in absence of the visible range band correlated to the quantum confinement in Ge NCs a band appears at ~400 nm in Ge-rich SiO2 layer as a consequence of hot electron mediated impact excitation in Ge-related oxygen-deficiency centres (GeODCs) during electrical pumping [5]. We find an increase of the 400 nm EL intensity with a concomitant reduction of the Er-related emission, and discuss the observed phenomenon on the ground of an inverse energy transfer process [4] from excited Er3+ to the GeODCs.

[1] A. Polman, J. Appl. Phys. 82, 1 (1997).
[2] O. Savchyn, F. R. Ruhge, P. G. Kik, R. M. Todi, K. R. Coffey, H. Nukala, and H. Heinrich, Phys. Rev. B 76, 195419 (2007).
[3] M. Fujii, K. Imakita, K. Watanabe, and S. Hayashi, J. Appl. Phys. 95, 272 (2004).
[4] I. Izeddin, A. S. Moskalenko, I. N. Yassievich, M. Fujii, and T. Gregorkiewicz, Phys. Rev. Lett. 97, 207401 (2006).
[5] L. Rebohle, J. von Borany, R. A. Yankov, W. Skorupa, I. E. Tyschenko, H. Fröb, and K. Leo, Appl. Phys. Lett. 71, 2809 (1997).
Keywords: nanocrystals, Er, energy transfer
  • Poster
    2008 MRS Fall Meeting, 01.-05.12.2008, Boston, MA, USA

Publ.-Id: 12055 - Permalink


Stabilization of extra dimensions in higher-dimensional cosmological models
Günther, U.;
We present an overview over research on higher-dimensional cosmological models carried out from 1997 till 2004. The basic starting point is the problem of stabilizing the sizes of the extra-dimensional factor spaces so that they are kept unobservable at low energies --- as indicated by current data from high-energy collider experiments and astrophysical observations. This problem is well known as the moduli stabilization problem in string theory and M-theory. Stabilizing the scale factors of the extra-dimensional factor spaces by some effective potentials (which are naturally induced via dimensional reduction of the higher-dimensional setups) fixes only the average sizes (moduli) but still allows for small (quantum) fluctuations around the minimum position (radion fluctuations). Once these fluctuations will depend on the coordinates of our external (uncompactified) space they will act as multicomponent scalar fields. Due to their Planck-scale-suppressed coupling to usual standard matter they may be considered as possible candidates for dark matter or UHECR (ultra-high energy cosmic rays).
In the talk, we comment on various dimensional reduction schemes, large extra dimensions (ADD-setups) as well as on orbifold compactification schemes of Randall-Sundrum type. We provide a brief summary about the obtained results published during the 8 years of active research in this field --- including discussions of effective static and dynamic perfect fluid scenarios, relations to reheating, transitions from radiation dominance to matter dominance, quintessence, UHECR, reaction channels with standard matter and possible creation of moduli excitations in ultra-strong background magnetic fields of magnetars. Finally, we give a brief overview about results on higher-dimensional f(R) theories as they are used in 4D as possible explanation for dynamically induced dark energy.
Keywords: string theory, M-theory, phenomenology, cosmology, extra dimensions, moduli stabilization, radion fluctuations, perfect fluids, UHECR, magnetars, f(R) gravity, dark matter, dark energy
  • Invited lecture (Conferences)
    Institutsseminar, 10.12.2008, Jena, Deutschland

Publ.-Id: 12054 - Permalink


Two models of Krein-space related physics: the MHD α2-dynamo and the PT-symmetric Bose-Hubbard model
Günther, U.; Kirillov, O.; Graefe, E.-M.; Korsch, H.-J.; Niederle, A.;
In the first part of the talk, the eigenvalue behavior λ(α,β) of the 2×2 matrix differential operator of the spherically symmetric α2-dynamo of magnetohydrodynamics is considered for constant α-profiles and boundary conditions which depend on a parameter β. Specifically, β∈[0,1] acts as parameter in the homotopic interpolation between idealized (Dirichlet) and physically realistic (Robin) boundary conditions (BCs). For the quasi-exactly solvable monopole setup (with spherical mode number l=0) the characteristic equation is derived explicitly. It is shown that the β-homotopy describes an interpolation between spectra of mesh type (idealized BCs) and a countably infinite set of parabolas (physically realistic Robin Bcs). Interestingly, the mesh nodes (semisimple twofold degenerate eigenvalues) are fixed points of the β-homotopy. An underlying ruled-surface structure of the spectrum is uncovered.

In the second part of the talk, we provide a brief summary of recent results on the spectral behavior of the PT-symmetric
Bose-Hubbard system as it is used for the description of quantum Bose-Einstein condensates with balanced gain-loss interactions. For an N-particle system the corresponding Fock-space Hamiltonian reduces to an N×N-matrix which is selfadjoint in an N-dimensional Pontryagin space. The unfolding of higher-order branch-points of the spectrum is considered under parameter perturbations. Numerical as well as analytical results are presented which demonstrate the relevance of the Hessenberg type of the Hamiltonian as defining matrix structure for the occurrence of specific Galois cycles in the eigenvalue rings of the unfolding branch points.
partially based on: J. Phys. A 41 (2008) 255206; arXiv:0802.3164 [math-ph].
Keywords: Krein space, operator theory, boundary values, homotopy, spectral mesh, ruled surface, Bose-Hubbard model, Bose-Einstein condensate, Hessenberg type, spectral degeneration, Jordan block structure
  • Lecture (Conference)
    8th Workshop "Operator Theory in Krein Spaces and Inverse Problems", 18.-21.12.2008, Berlin, Deutschland

Publ.-Id: 12053 - Permalink


J-self-adjoint operators with C-symmetries: extension theory approach
Kuzhel, S.; Günther, U.; Albeverio, S.;
A linear densely defined operator A acting in a Krein space with fundamental symmetry J and indefinite metric [.,.]J =(J.,.) is called J-selfadjoint if A*J = JA.  In contrast to self-adjoint operators in Hilbert spaces (which necessarily have a purely real spectrum), J-selfadjoint operators, in general, have a spectrum which is only symmetric with respect to the real axis. However, one can ensure the reality of the spectrum by imposing an extra condition of symmetry. In particular, a J-selfadjoint operator A has the property of C-symmetry if there exists a bounded linear operator C in H such that: (i) C2 =I; (ii) JC > 0; (iii) AC = CA. 
The properties of C are nearly identical to those of the charge conjugation operator in quantum field theory and the existence of C provides an inner product (.,.)C=[C.,.]J whose associated norm is positive definite and the dynamics generated by A is therefore governed by a unitary time evolution. However, the operator C depends on the choice of A and its finding is a nontrivial problem. 
The report deals with the construction of C-symmetries for J-selfadjoint extensions of a symmetric operator Asym with finite deficiency indices . We present a general method allowing us: (i) to describe the set of J-selfadjoint extensions A of Asym with C-symmetries; (ii) to construct the corresponding C-symmetries in a simple explicit form which is closely related to Clifford algebra operator structures; (iii) to establish a Krein-type resolvent formula for J-self adjoint extensions A with C-symmetries. 
The results are exemplified on 1D pseudo-Hermitian Schrödinger and Dirac Hamiltonians with complex point-interaction potentials. 
Keywords: J-selfadjoint operators, Krein space, extension theory, deficiency index, C-symmetry, Clifford algebra
  • Lecture (Conference)
    8th Workshop "Operator Theory in Krein Spaces and Inverse Problems", 18.-21.12.2008, Berlin, Deutschland

Publ.-Id: 12052 - Permalink


Determining role of Krein signature for 3D Arnold tongues of oscillatory dynamos
Kirillov, O.; Günther, U.; Stefani, F.;
Using a homotopic family of boundary eigenvalue problems for the meanfield α2-dynamo with helical turbulence parameter α(r)= α0 + γΔα(r) and homotopy parameter β∈[0,1], we show that the underlying network of diabolical points for Dirichlet (idealized, β=0) boundary conditions substantially determines the choreography of eigenvalues and thus the character of the dynamo instability for Robin (physically realistic, β = 1) boundary conditions. In the (α0,β,γ)-space the Arnold tongues of oscillatory solutions at β =1 end up at the diabolical points for β = 0. In the vicinity of the diabolical points the space orientation of the 3D tongues, which are cones in first-order approximation, is determined by the Krein signature of the modes involved in the diabolical crossings at the apexes of the cones. The Krein space induced geometry of the resonance zones explains the subtleties in finding α-profiles leading to spectral exceptional points, which are important ingredients in recent theories of polarity reversals of the geomagnetic field.
Keywords: MHD dynamos, oscillatory dynamos, Krein space, Krein signature, diabolical points, perturbation theory, homotopy, boundary conditions, Arnold tongues, resonance zones
  • Lecture (Conference)
    8th Workshop "Operator Theory in Krein Spaces and Inverse Problems", 18.-21.12.2008, Berlin, Deutschland

Publ.-Id: 12051 - Permalink


The Structure of Polynuclear Uranyl Sorption Complexes at the Gibbsite/Water Interface
Rossberg, A.; Tsushima, S.; Scheinost, A. C.;
Up to now only occasionally polynuclear sorption complexes of uranyl have been reported. This is undoubtedly surprising, since polynuclear hydrolysis species dominate aqueous uranyl speciation across a wide pH range and at solution concentrations typically used for sorption studies (10-6 - 10-4 M). Using U-LIII EXAFS spectroscopy at 15 K, we demonstrate here that polynuclear sorption complexes occur consistently at the gibbsite surface. They were observed across a pH range of 5.5 to 7.5 in equilibrium with atmospheric CO2, and at pH 8.5 in absence of CO2.
At these conditions, thermodynamic calculations predict the trimeric (UO2)3(OH)5+ complex to be the predominant hydrolysis species in the aqueous phase. This complex has been investigated recently by EXAFS spectroscopy and DFT calculations [1]. Both methods showed a stoichiometry of (UO2)3O(OH)3+ for this complex with a central oxo bridging, and provided a relatively short U-U distance of 3.8 Å. In contrast, coordination numbers and radial distances found for the sorption complex are 2xOaxial @ 1.8 Å, 6xOequatorial @ 2.4 Å and 1xU @ 4.2 Å. Both EXAFS shell fitting and a newly developed approach for the calculation of the radial pair distribution function achieved the same result. The U-U distance of 4.2 Å together with the coordination number of six for equatorial oxygen are in line with formation of uranyl dimers, where the two uranyl units are linked in edge-sharing configuration. This unusual structure is currently further investigated by DFT calculations.

REFERENCES
1. S. Tsushima, A. Rossberg, A. Ikeda, K. Mueller, A. C. Scheinost, Inorganic Chemistry 46, 10819-10826 (2007).
  • Lecture (Conference)
    5th Workshop on Speciation, Techniques and Facilities for Radioactive Materials at Synchrotron Light Sources, Actinide-XAS-2008, 15.-17.07.2008, Saint-Aubin, France

Publ.-Id: 12050 - Permalink


Pair distribution function, Monte Carlo modelling and Factor Analysis: New approaches to EXAFS analysis.
Rossberg, A.; Funke, H.; Scheinost, A. C.;
X-ray absorption fine-structure (EXAFS) spectroscopy allows the study of a large number of elements and compounds in different physical and chemical states. The most prominent structural parameters derived from EXAFS are coordination numbers, interatomic distances, static and vibrational disorder expressed by the Debye-Waller factor. These parameters are used in the conventional shell fitting approach to approximate the true radial pair distribution function (PDF). This shell fitting approach has three critical limitations for which we present possible solutions.

a) Due to the approximation of the PDF with Gaussian peak shapes, the conventional shell fitting approaches fails for all systems which show anharmonic contributions to the vibrational or static displacement of atoms, caused for instance by temperature effects or week bonds in adsorbate-adsorbent interactions [1]. To overcome this limitation we present two techniques: (1) a Monte Carlo simulation technique for EXAFS spectra which takes not only the first order scattering, but also higher order scattering into account [2] and (2) a novel application designed for the direct calculation of the PDF from the EXAFS spectrum.

b) Due to the limited resolution in R, the shell fitting approach commonly fails, if the X-ray absorbing atom is present in different chemical environments at the same time, i.e. in case of chemical mixtures. For this case we present a way to isolate the pure spectral components by Iterative Factor Analysis [3] and/or by a mathematical combination of Factor Analysis and Monte Carlo simulation [2].

c) In order to find a unique structural solution, shell fitting tends to be a soft modelling approach, because many structural arrangements may lead to a similar description of the EXAFS signal and have to be tested. Especially for the investigation of the unknown structures of sorption complexes, a hard modelling approach, which can take the complete substrate structure into account, would be much better suited. We show that the Monte Carlo simulation technique can be used as a hard modelling approach to solve the structure of sorption complexes [4].


[1] Crozier, E. D. Impact of the Asymmetric Pair Distribution Function in the Analysis of Xafs. Physica B-Condensed Matter 209, 330-333 (1995).
[2] Rossberg, A. & Scheinost, A. C. Three-dimensional modeling of EXAFS spectral mixtures by combining Monte Carlo Simulations and Target Transformation Factor Analysis. Analytical and Bioanalytical Chemistry 383(1), 56-66 (2005).
[3] Rossberg, A., Reich, T. & Bernhard, G. Complexation of uranium(VI) with protocatechuic acid - application of iterative transformation factor analysis to EXAFS spectroscopy. Analytical and Bioanalytical Chemistry 376, 631-638 (2003).
[4] Ulrich, K.-U., Rossberg, A., Foerstendorf, H., Zänker, H. & Scheinost, A. C. Molecular characterization of uranium(VI) sorption complexes on iron(III)-rich acid mine water colloids. Geochimica Et Cosmochimica Acta 70, 5469-5487 (2006).
  • Invited lecture (Conferences)
    2008 SSRL/LCLS Users' Meeting and Workshops, 15.-18.10.2008, Stanford, USA

Publ.-Id: 12049 - Permalink


Neuartige kapazitive Sensoren für die Visualisierung von Mehrphasenströmungen
Da Silva, M. J.; Thiele, S.; Hampel, U.;
In diesem Beitrag werden zwei neuartige, bildgebende Sensoren zur Untersuchung von Mehrphasenströmungen beschrieben - der kapazitive Gittersensor und der kapazitive Flächensensor. Beide Sensoren basieren auf einer matrixförmigen Anordnung von Messelementen, mit denen die elektrische Kapazität eines umgebenden Fluides sehr schnell abgetastet wird. Dadurch sind diese Sensoren in der Lage, zeitlich und räumlich hoch aufgelöste Bilder der Phasenverteilung einer Mehrphasenströmung zu erzeugen. Die Sensoren und die zugehörige Messelektronik werden präsentiert. Darüber hinaus werden ausgewählte Ergebnisse von Strömungsvisualisierungen dargestellt und diskutiert.

In this article, two novel imaging sensors for the investigation of multiphase flows are introduced – the capacitive wire-mesh sensor and the capacitive planar array sensor. Both sensor systems are based on a matrix-type arrangement of sensing elements by which the electrical capacitance of a surrounding fluid is very fast scanned. Thus these sensors are able to produce high temporal and spatial resolution images of the phase distribution in a multiphase flow. The sensors and associated measuring electronics are presented. Furthermore some selected flow visualization results are represented and discussed.
Keywords: flow visualization, multiphase flow, electrical capacitance, electric permittivity, wire-mesh sensor, interdigital sensor

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Publ.-Id: 12048 - Permalink


Coupled neutronic thermo-hydraulic analysis of full PWR core with BGCore system
Kotlyar, D.; Fridman, E.; Shwageraus, E.;
BGCore reactor analysis system, recently developed at Ben-Gurion University for calculating in-core fuel composition and spent fuel emissions following discharge, couples the Monte Carlo neutronic code (MCNP4C) with an independently developed burnup and decay module SARAF. The BGCore utilizes multi-group approach for generation of one group cross-sections. According to this approach, only multi-group neutron spectrum is calculated by MCNP, while reaction rates are calculated in a separate subroutine using pre-generated multi-group cross-section set and the fine group neutron spectrum obtained from MCNP.
BGCore code system offers a number of advantages over similar MCNP-depletion codes. These include:
- Multi-group coupling approach significantly reduces the code execution time without compromising the accuracy of the results.
- Use of the most recent data based on JEFF-3.1 data files,
- Careful choice of about 1700 isotopes to cover all potentially significant aspects of fuel irradiation and decay. All nuclides are included in the calculation matrix with no asymptotic approximation,
- The fact that all of the isotopes, and not just the most neutronically important, are tracked in the BGCore throughout all depletion steps, allows calculations of post-irradiation fuel characteristics such as, activity, radiotoxicity, and decay heat with high degree of accuracy.
Substantial reduction in the BGCore code execution time allows consideration of problems with much higher degree of complexity, such as introduction of thermal hydraulic (T-H) feedback into the calculation scheme. Recently, a new T-H feedback module (THERMO) was developed and integrated into the BGCore system. At each computation point, the THERMO module receives as an input the power and the fuel burnup distributions in the core from the neutronic solver (MCNP) and calculates the temperatures distribution in core components and the coolant flow distribution in the core channels. The results of the THERMO calculations can be used for updating the relevant parameters in the MCNP input, such as fuel and moderator temperatures, and moderator density.
This study presents the results of coupled neutronic T-H analysis of full PWR core performed with BGCore system. The verification of BGCore system results against alternative state of the art computer code is also presented.
Keywords: Monte-Carlo burnup, coupled neutronic thermal-hydraulic analysis
  • Contribution to proceedings
    Jahrestagung Kerntechnik 2009, 12.-14.05.2009, Dresden, Deutschland
  • Lecture (Conference)
    Jahrestagung Kerntechnik 2009, 12.-14.05.2009, Dresden, Deutschland

Publ.-Id: 12047 - Permalink


Fracture mechanics characterisation of the WWER-440 reactor pressure vessel beltline welding seam of Greifswald unit 8
Viehrig, H.-W.; Schuhknecht, J.;
WWER-400 second generation (V-213) reactor pressure vessels (RPV) were produced by IZHORA in Russia by and SKODA the former Czechoslovakia. The surveillance Charpy-V and SE(B) specimens of both producers have different orientation. The main difference is the crack extension direction which is through the RPV thickness and circumferential for ISHORA and SKODA RPV, respectively. In particular for the investigation of weld metal from multilayer submerged welding seams is the crack extension direction of importance. Depending on the crack extension direction in the specimen there are different welding beads or a uniform structure along the crack front. This is especially important for the fracture toughness determined according to the Master Curve (MC) approach as standardised in the ASTM Standard Test Method E1921-05. This approach was applied on weld metal of the RPV beltline welding seam of Greifswald Unit 8 RPV. Charpy size SE(B) specimens from 13 locations equally spaced over the thickness of the welding seam were tested. The specimens are TL and TS orientation.
The fracture toughness values measured on the SE(B) specimens with both orientations follow the course of the MC. Nearly all values lie within the fracture toughness curves for 5% and 95% fracture probability. There is a strong variation of the reference temperature T0 though the thickness of the welding seam, which can be explained with structural differences. The scatter is more pronounced for the TS SE(B) specimens. It can be shown that specimens with TL and TS orientation in the welding seam have a differentiating and integrating behaviour, respectively. The statistical assumptions behind the MC approach are valid for both specimen orientation even if the structure is not uniform along the crack front.
Keywords: reactor pressure vessel, multilayer welding seam, specimen orientation, structure, fracture toughness, Master Curve approach, reference temperature, integrity assessment of base metal from the Greifswald Unit 8 RPV show large scatter
  • Lecture (Conference)
    Jahrestagung Kerntechnik 2009, 12.-14.05.2009, Dresden, Deutschland
  • Contribution to proceedings
    Jahrestagung Kerntechnik 2009, 12.-14.05.2008, Dresden, Deutschland
    Proceedings Jahrestagung Kerntechnik 2009: INFORUM GmbH

Publ.-Id: 12046 - Permalink


Intense white photoluminescence emission of V-implanted ZnO oxide thin films
Müller, S.; Lorenz, M.; Czekalla, C.; Benndorf, G.; Hochmuth, H.; Grundmann, M.; Schmidt, H.; Ronning, C.;
Pulsed laser deposited ZnO films were imlpanted with vanadium ions using ion energies between 30 and 250 keV with different fluences yielding vanadium concentrations in the range between 0.8 and 5 at. %. After annealing under oxygen ambient at 800°C, a broad luminescence band observed by photoluminescence covers nearly the total visible spectral region. This luminescence is a superposition of different bands triggered by the incoporated V and remaining implantation defects. The visual impression of the bright whitish emission of the implanted ZnO has been quantified using the color space map of the Commission internationale de l'Eclairage. Furthermore, the intensity of the white emission strongly increases with increasing V concentration, whereas Ar-implanted reference sample shows only weak white emission.
Keywords: ZnO, white luminescence

Publ.-Id: 12045 - Permalink


Hochdynamische Prozesse im Röntgenblick
Hampel, U.; Fischer, F.;
Ein ultraschneller Elektronenstrahl-Tomograph visualisiert strömende Stoffgemische berührungsfrei mit Bildraten von bis zu 7000 Bildern pro Sekunde und einer räumlichen Auflösung von einem Millimeter. Das Gerät kann so Prozesse in Blasensäulenreaktoren sichtbar machen.
Keywords: Elektronenstrahltomographie
  • Nachrichten aus der Chemie (2009)1, 47-49

Publ.-Id: 12044 - Permalink


Influence of air entrainment on the liquid flow field caused by a plunging jet
Krepper, E.; Alt, S.; Renger, S.;
During a jet gaseous bubbles are entrained. These bubbles influence the liquid flow field. The liquid flow field transports fibers in a sump geometry. Dependent on the flow situation different flow characteristics might occur. In the paper results of CFD calculations are presented and compared to experiments.
Keywords: plunging jet, two phase flow, gas entrainment, fiber transport
  • Contribution to proceedings
    Jahrestagung Kerntechnik 2009, 12.-14.05.2009, Dresden, Germany
    Proceedings of the Annual Meeting of Nuclear Technology (CD-ROM), paper No. 220
  • Lecture (Conference)
    Jahrestagung Kerntechnik 2009, 12.-14.05.2009, Dresden, Germany

Publ.-Id: 12043 - Permalink


Porovnani experimentalnich technik v oblasti dvoufazoveho proudeni (Comparison of experimental techniques in two-phase flow)
Otahal, J.; Hampel, U.; Schleicher, E.;
We performed wire-mesh and X-ray micro-tomography studies of the two-phase flow in the mixing chamber of an effervescent atomizer. Flow patterns in the mixing chamber can be revealed by the miniature wire mesh sensor with a temporal resolution of 10 kHz whereas microtomography provides accurate and high-resolution axial-radial gas fraction profiles. The paper describes both measurement techniques and first results of experimental investigation.
Keywords: effervescent atomizer, two-phase flow, wire mesh sensor
  • Contribution to proceedings
    27th International Conference of Fluid Dynamics and Thermodynamics Departments, 24.-27.06.2008, Plzen, Czech Republic
    Proceedings of 27th International Conference of Fluid Dynamics and Thermodynamics Departments, Paper No. SK06

Publ.-Id: 12042 - Permalink


Two-phase flow characteristics in the mixing chamber of the effervescent atomizer
Otahal, J.; Hampel, U.; Schleicher, E.; Jicha, M.;
The article deals with the description of two-phase flow in the mixing chamber of an effervescent atomizer. The first observation has been carried out with the use of high-speed records of the flow inside the mixing tube. The flow in the mixing chamber is very fast and inhomogeneous thus the need to use a high-sampling frequency device has arisen in order to describe changes in the flow. Therefore, an experimental technique has been found which is able to describe the liquid-air distributions in small channels. As a two-phase flow measurement instrument, a miniature wire-mesh conductivity sensor to deal with cross-sections of 8 mm in diameter was designed and built. The frame rate of this sensor is 10 000 images per second. In this study, a special model of a transparent nozzle spraying deionized water that makes use of air as the atomizing medium was used. The effervescent nozzle is of "inside-out gas injection" configuration with the internal diameter of the mixing channel 8 mm. During the experiment, the effervescent atomizer was operated at different air pressure levels ranging from 0.1 to 0.5 MPa and mass GLR (Gas-to-liquid- ratio) from 0.1 to 25%. Mass flow rates of water ranged from 5 up to 65 g·s-1. The results reveal the unstable behavior of two-phase flow in the mixing chamber.
Keywords: effervescent atomizer, two-phase flow, wire mesh sensor
  • Contribution to proceedings
    ILASS Europe 2008 - 22nd European Conference on Liquid Atomization and Spray Systems, 08.-10.09.2008, Como Lake, Italy

Publ.-Id: 12041 - Permalink


Empirical and numerical studies on gas entrainment by impinging jets
Schmidtke, M.; Danciu, D.;
This work concerns experiments as well as CFD simulations on the gas entrainment for the impinging jet configuration. Impinging jets may occur in different situations related to reactor safety analyses. Many experiments have been carried out on impinging jets. A comprehensive overview by Bin [1993] reveals that the results depend critically on the individual setup (e.g. the nozzle geometry) and it is difficult to draw general conclusions. For the qualification of Computational Fluid Dynamics (CFD) models for such flow situations data with high resolution in space and time are required. Therefore a new experiment with a very simple setup was designed and first CFD simulations were done for these experiments.
Keywords: impinging jet, bubble entrainment
  • Contribution to proceedings
    Jahrestagung Kerntechnik 2009, 12.-14.05.2009, Dresden, Deutschland
    Beitrag 215
  • Lecture (Conference)
    Jahrestagung Kerntechnik 2009, Beitrag 215, 12.-14.05.2009, Dresden, Deutschland

Publ.-Id: 12038 - Permalink


Huge performance increase of Tb-implanted MOS light emitting devices with SiOxNy layers moderating hot carrier effects
Rebohle, L.; Sun, J.; Prucnal, S.; Nazarov, A.; Tyagulskii, I.; Helm, M.; Skorupa, W.;
The electrical stability of Tb-implanted SiO2 light emitting devices was drastically improved by using a SiON dielectric buffer layer. For fabrication thermally grown oxide layers on Si were implanted with Tb followed by a thermal treatment and the deposition of a SiON protection layer by plasma-enhanced chemical vapor deposition. The structures were finally provided with an indium tin oxide front contact and an aluminum rear contact. The incorporation of the SiON layer increases the breakdown electric field from 7.5 to 10.5 MV/cm and enhances the operation time of the light emitters up to three orders of magnitude under constant injection currents. By varying the SiO2 and SiON layer thickness it was found that the largest stability enhancements can be achieved if the SiON layer thickness is more than twice the thickness of the SiO2 layer.
The beneficial role of the SiON layer is mainly explained by reducing the chance of destructive avalanche breakdowns in the oxide layer and by an efficient cooling process of hot electrons moving in the conduction band of SiO2. The latter effect is based on the lower electric fields in SiON as compared to SiO2 and the lower band offset of SiON relative to the gate electrode. In addition, the SiON layer acts as a diffusion barrier against moisture from the working ambient and broadens the operation range of the light emitters on the voltage scale.
Keywords: electroluminescence, Terbium implanted MOS device, Si based light emission, silicon oxynitride
  • Lecture (Conference)
    2008 MRS Fall Meeting, 01.-05.12.2008, Boston, USA

Publ.-Id: 12037 - Permalink


Electroluminescence, Charge Trapping and Clustering in Rare-Earth Implanted SiO2-Si Light-Emitting Diodes
Nazarov, A.; Tyagulskii, I.; Tyagulskiy, S.; Rebohle, L.; Prucnal, S.; Lehmann, J.; Biskupek, J.; Kaiser, U.; Skorupa, W.;
In this work a comparative study of charge trapping, electroluminescence intensity (ELI) and clustering in SiO2 implanted by different rare-earth (RE) impurities (Eu, Tb, Gd, Er, Tm) with following high-temperature annealing is performed to clarify the connection between the electrical properties, the structure of the luminescent centers, the ELI and the EL spectra. RE impurities were implanted into the bulk of thermally grown SiO2 on n-type Si. The implanted doses were chosen in such a way that the maximum concentration corresponded to 0.1, 0.5, 1.5 and 3.0 at %. To activate the RE implanted impurities a post implantation furnace anneal in the temperature range of 800-1100 °C for 30 min and flash lamp annealing (FLA) for 20 ms at 1000 °C in a nitrogen ambient have been carried out. The ITO layer was used as a transparent electrode. Charge trapping was studied by the shifting of the high-frequency CV characteristics and the changing of the applied voltage during constant current electron injection from Si into SiO2. The EL signal was recorded at the same injection regime at room temperature in the wavelength range of 300 to 750 nm. Some control structures were studies by transmission electron microscopy with high resolution (XTEM).
It was shown that RE impurities such as Tb, Gd, Er and Tm implanted into SiO2 cause mainly net positive charge trapping in the range of the injected charge from 1x1015 to 2x1017 e/cm2 and stable ELI of their main luminescence lines in green, UV, IR and blue spectral region of the EL spectra, respectively. Above 1x1018 e/cm2 of the injected charge an electron trapping in the bulk of the oxide and a hole trapping at the SiO2-Si interface is observed for all types of the RE impurities. The electron trapping correlates with the EL quenching of the main EL lines for all studied RE implanted structures with the exception of the Eu implanted one. The Eu implanted oxide demonstrates effective electron trapping up to 1x1020 e/cm2 without EL quenching of the main studied EL lines: in red spectral region with a maximum at 618 nm (5D0-7F2 transition for Eu3+ ions); in the blue-green spectral region around 460-470 nm and in blue-violet one at 410 nm (corresponding to a 4f6d-4f7 transition of the Eu2+ ion). The XTEM measurements discovered that the clustering in the Eu-implanted SiO2 is enhanced considerably in comparison with the Tb-implanted one. It is suggested that the enhanced electron trapping in the Eu implanted structures is associated with enhanced clustering, which is partly caused by low valency (2+) oxides existing for the Eu impurity such as EuO and Eu3O4. The use of FLA for the Eu implanted SiO2 results in a decrease of the nanocluster size and an increase of the ELI in the red region of the spectrum.
Keywords: electroluminescence, rare earth implanted MOS devices, charge trapping, clustering
  • Poster
    2008 MRS Fall Meeting, 01.-05.12.2008, Boston, USA

Publ.-Id: 12036 - Permalink


Correlation between electroluminescence and charge trapping in multi-color Eu-implanted Si-based light-emitting diodes
Nazarov, A. N.; Tyagulskii, I. P.; Tyagulskii, S. I.; Rebohle, L.; Skorupa, W.; Biskupek, J.; Kaiser, U.;
Electroluminescence (EL) spectra, charge trapping during operation of EL devices and clustering of rare earth oxides in SiO2 have been investigated in Eu-implanted SiO2-Si structures which demonstrate luminescence associated with the light-emitting transitions in Eu2+ and Eu3+. Strong electron trapping in all studied regions of the injected charge (from 1x10E14 to 1x10E18 e/cm2) during operation of the light-emitting devices has been found that considerably differed from the oxides implanted by other rare earth impurities (Ce, Tb, Gd, Er, Tm). It has been shown that the observed strong electron trapping and the low EL intensity in the Eu implanted structures were associated with enhanced clustering of the Eu oxides. The mechanisms of electron trapping in the SiO2 containing a large cluster concentration is discussed, and flash lamp annealing is proposed to decrease the nanocluster size and to enhance the EL intensity.
Keywords: MOSLEDs, rare earth implanted oxide, electroluminescence, charge trapping, clustering
  • Lecture (Conference)
    EMRS 2008 Spring Meeting, 26.-30.05.2008, Strasbourg, France
  • Physica E 41(2009), 902

Downloads:

Publ.-Id: 12035 - Permalink


Entwicklung neuer Chelatsysteme für nuklearmedizinische Anwendungen
Stephan, H.;
kein Abstract verfügbar
  • Invited lecture (Conferences)
    Gastvorlesung Fachhochschule Zittau/Görlitz, 11.12.2008, Zittau, Deutschland

Publ.-Id: 12034 - Permalink


Cis-Trans-Isomerie in Bispidinkomplexen
Kraus, W.; Emmerling, F.; Juran, S.; Matterna, M.; Walther, M.; Stephan, H.;
Radioaktive Kupferisotope nehmen sowohl für den Einsatz in der nuklearmedizinischen Diagnostik als auch in der Therapie einen besonderen Stellenwert ein. 64Cu und 67Cu sind dabei aufgrund günstiger kernphysikalischer Eigenschaften (Halbwertzeit, Art der Strahlung) von besonderem Interesse. Derivate des Bispidins (3,7-Diazabicyclo[3.3.1]nonan) bilden mit Übergangsmetall-kationen wie Cu2+, Co2+ oder Fe2+, Komplexe hoher Stabilität [1,2].
Für Bispidine ergeben sich drei unterschiedliche Konformationsisomere. Die abgeflachte Doppel-Sessel-Form repräsentiert die thermodynamisch stabilste Konformation. Darin sind die beiden Aminstickstoff-Donoratome N3 und N7 für die Koordination mit Metallionen optimal vororganisiert. Die Stellung von Substitutenten in den Positionen C2 und C4 bezüglich der Ringebene kann zur Bildung von Isomeren einer cis-trans-Konfigurationsisomerie führen. Sterisch anspruchsvolle Reste erzwingen die Bildung von trans-Isomeren. Das gilt in gleicher Weise für die Bindung voluminöser Donoreinheiten – wie Benzimidazolyl-Substituenten – in N3-Position des Bispidin-Gerüsts. Das entsprechende Piperidon (Fig.1) sowie das Bispidon (Fig. 2) zeigen die erwartete trans-Isomerie [3]. Dieser Befund wurde mit Hilfe der Kristallstrukturen nachgewiesen. Der Precursor (C27H25N5O5), a=8.1752(6) Å, b=11.1511(8) Å, c=15.8128(14) Å, V=1260.89(17) Å3, Z=2, R1=0.043 kristallisiert in der Raumgruppe P-1. In der Kristallstruktur des Kupferkomplexes [Cu(C35H33N7O5)(NO2)2] • H2O, P21/n, a=13.242(5) Å, b=17.971(7) Å, c=15.650(6) Å, V=3722.1(2) Å3, Z=4, R1=0.0451 2) ist das Kupferatom verzerrt quadratisch pyramidal koordiniert (dCu−N = 1.947(5) - 2.308(7) Å) und durch den Bispidin-Liganden nahezu vollständig von der Umgebung abgeschirmt.
Erste radiopharmakologische Untersuchungen zeigen, dass die Entwicklung kopplungsfähiger Liganden und deren Anknüpfung an ausgewählte Biomoleküle einen weiteren Schritt in Richtung einer besseren Bioverfügbarkeit von radioaktiv markierten Kupferverbindungen darstellt.
  • Poster
    17. Jahrestagung der Deutschen Gesellschaft für Kristallographie, 09.-12.03.2009, Hannover, Deutschland

Publ.-Id: 12033 - Permalink


Nanopatterning of semiconductor surfaces by sputtering from an inductively coupled plasma
Zhou, J.; Facsko, S.; Keller, A.; Möller, W.;
Self-organized dot patterns have been fabricated on GaSb and Si surfaces by sputtering from an inductively coupled plasma. The dependence of the patterns on the ion energy has been investigated. In agreement with previous studies using plasmas or Kaufman sources, the dot wavelength on GaSb increases with energy. On Si surfaces, however, the dot wavelength increases with energy only in the range E < 800 eV. For 800 eV < E < 1500 eV, the dot wavelength is constant.
  • Lecture (Conference)
    Workshop Complex Nanostructures, 06.-07.10.2008, Dresden, Germany

Publ.-Id: 12032 - Permalink


Digital positron lifetime spectroscopy at EPOS
Krille, A.; Krause-Rehberg, R.; Jungmann, M.; Becvar, F.; Brauer, G.;
As the digital equipment to measure positron lifetimes gets cheaper and more widely used, it is decided that EPOS, the ELBE positron source will sample the signals from the photomultipliers directly and evaluate it online or offline by digital means. Still using isotope sources, the EPOS lifetime spectrometer results in a timing resolution of around 170 ps (with Co-60), which compares good to analog equipment. A distinct improvement is expected when a coincidence setup will be used at ELBE. However, also the software needs further improvement: while one of the goals is of course to achieve the best time resolution, there is also the aspect of runtime and expandability. Results of evaluations will be presented and compared with results from other groups.

Publ.-Id: 12031 - Permalink


Efforts to verify neutron fluence calculations at pressure vessels of decommissioned reactors by means of retrospective niobium dosimetry
Konheiser, J.; Noack, K.; Mittag, S.; Rindelhardt, U.; Borodkin, G.; Borodkin, P.; Gleisberg, B.;
For the first time, trepans from a decommissioned VVER-440 (Greifswald-1) reactor pressure vessel (RPV) have been examined. Activities of a trepan, taken at the RPV weld with the highest fast-neutron load, were measured and estimated on the basis of fluence calculations by the codes TRAMO and DORT. A maximum fluence of 4.05*1019 n/cm2 (E>0.5 MeV) was calculated. The average deviation between the two codes is 2.6 %. Activities resulting from the reaction 93Nb(n,n’)93mNb were measured, niobium being a trace element in the RPV steel. Unfortunately, 93mNb is also produced by neutron capture in the alloy component 92Mo, the built-up 93Mo decaying by electron capture. The ratios of calculated to measured (C/E) 93mNb gamma activities for several trepan samples are between 0.42 and 0.97. The fact that all C/E ratios are below unity suggests that the measured values may have been additionally heightened by activities from other nuclides.
  • Lecture (others)
    8.AAA Meeting Garching(München), 08.12.2008, München, Germany

Publ.-Id: 12030 - Permalink


A biogenic catalyst for hydrogenation, reduction and selective dehalogenation in non-aqueous solvents
Creamer, N. J.; Deplanche, K.; Snape, T. J.; Mikheenko, I. P.; Yong, P.; Samyahumbi, D.; Wood, J.; Pollmann, K.; Selenska-Pobell, S.; Macaskie, L. E.;
We report the activity of a new palladium catalyst supported on fundamentally different Gram negative (Desulfovibrio) and Gram positive (Bacillus) bacterial surfaces (bio-Pd). Under H-2 (electron donor), cells of both strains reduced Pd(II) to Pd(0) as discrete nanoparticles located in the periplasmic space of D. desulfuricans or between the peptidoglycan and the proteinaceous surface layer (S-layer) of B. sphaericus. The catalytic activity of the preparations was similar in their ability to reduce Cr(VI) to Cr(III) (aq), and in the hydrogenation of itaconic acid (aq.) Bio-Pd on D. desulfuricans was also an effective catalyst in a range of reactions in methanol, performing comparably to a commercially available catalyst (10% Pd/C) in the conversion of 4-azidoaniline to 1,4-phenytenediamine. In the hydrogenation of 3-nitrostyrene, the bio-Pd showed selectivity for the partly reduced (dehalogenated) product (1-ethyl-3-nitrobenzene -74%, 1-ethyl-3-aminobenzene -7%) whereas the ! commercial catalyst produced only the fully reduced product (1-ethyl-3-aminobenzene -73%). In the case of 1-bromo-2-nitrobenzene, again bio-Pd was selective for the dehalogenated product, nitrobenzene, whereas the commercial catalyst produced the salt aniline hydrobromide.

Publ.-Id: 12029 - Permalink


Activities towards p-type doping of ZnO
Brauer, G.;
  • Lecture (Conference)
    Seminar an der University of Hong Kong (Physics Department), 26.11.2008, Hong Kong, China

Publ.-Id: 12028 - Permalink


Potential of mirror-image oligonucleotides as molecular probes for pretargeting
Schlesinger, J.; Közle, I.; Bergmann, R.; Klussmann, S.; Vonhoff, S.; Pietzsch, H.-J.;
To explore the potential of complementary mirror-image oligonucleotides as recognition systems for pretargeting, the present work deals with radiopharmacological evaluation of an 86Y-labeled 17mer l-DNA and of a hybrid consisting of the radiolabeled l-DNA and a complementary l-RNA (Figure 1). The 17mer l-DNA was functionalized with DOTA-Nhydroxysuccinimide ester and radiolabeled with 86Y (t1/2 = 14.7 h) resulting in the HPLCpurified [86Y-DOTA]-17mer (31 MBq; 2.0 GBq/μmol). Biodistribution studies with [86Y-DOTA]-17mer in Wistar rats showed high renal excretion and moderate kidney uptake (SUV: 18 ± 2.4, 18 h p. i.). More than 87% of [86Y-DOTA]-17mer were found intact in rat urine 60 min after injection. In thermal denaturation studies, a high melting point of 68°C (32 mM [Na+]) was determined for the hybrid. In conclusion, the high metabolic stability of the l-DNA and the high thermal stability of the l-DNA/l-RNA hybrid suggest the potential of this class of compounds as molecular probes for pretargeting.
  • Poster
    XI Turku PET Symposium New Targets in Molecular Imaging, 24.-27.05.2008, Turku, Finland
  • Contribution to proceedings
    XI Turku PET Symposium New Targets in Molecular Imaging, 24.-27.05.2008, Turku, Finland
    Abstracts of the XI Turku PET Symposium, Kaarina, Finland: Painola, 978-951-29-3563-5, 38

Publ.-Id: 12027 - Permalink


The structure of monomeric and dimeric uranyl adsorption complexes on gibbsite: A combined DFT and EXAFS study
Hattori, T.; Saito, T.; Ishida, K.; Scheinost, A. C.; Tsuneda, T.; Nagasaki, S.; Tanaka, S.;
We investigated the structure of uranyl sorption complexes on gibbsite (pH 5.6 - 9.7) by two independent methods, density functional theory (DFT) calculations and extended X-ray absorption fine structure (EXAFS) spectroscopy at the U-LIII edge. To model the gibbsite surface with DFT, we tested two Al (hydr)oxide clusters, a dimer and a hexamer. Based on polarization, structure, and relaxation energies during geometry optimization, the hexamer cluster was found to be the more appropriate model. An additional advantage of the hexamer model is that it represents both edges and basal faces of gibbsite. The DFT calculations of (monomeric) uranyl sorption complexes show an energetic preference for the corner-sharing versus the edge-sharing configuration on gibbsite edges. The energy difference is so small, however, that possibly both surface species may coexist. In contrast to the edge sites, sorption to basal sites was energetically not favorable. EXAFS spectroscopy revealed in all investigated samples the same interatomic distances of the uranyl coordination environment (RU-Oax ≈ 1.80 Å, RU-Oeq ≈ 2.40 Å), and towards the gibbsite surface (RU-O ≈ 2.87 Å, RU-Al ≈ 3.38 Å). In addition, two U-U distances were observed, 3.92 Å at pH 9.7 and 4.30 Å at pH 5.6, both with coordination numbers of ~ 1. The short U-U distance is close to that of the aqueous uranyl hydroxo dimer, UO2(OH)2, reported as 3.875 Å in the literature, but significantly longer than that of aqueous trimers (3.81-3.82 Å), suggesting sorption of uranyl dimers at alkaline pH. The longer U-U distance (4.30 Å) at acidic pH, however, is not in line with known aqueous uranyl polymer complexes. Based on the EXAFS findings we further refined dimeric surface complexes with DFT. We propose two structural models: in the acidic region, the observed long U-U distance can be explained with a distortion of the uranyl dimer to form both a corner-sharing and an edge-sharing linkage to neighboring Al octahedra, leading to RU-U = 4.150 Å. In the alkaline region, a corner-sharing uranyl dimer complex is the most favorable. The U-O path at ~2.87 Å in the EXAFS spectra arises from the oxygen atom linking two Al cations in corner-sharing arrangement. The adsorption structures obtained by DFT calculations are in good agreement with the structural parameters from EXAFS analysis: U-Al (3.394 Å), U-U (3.949 Å), and U-O (2.823 Å) for the alkaline pH model, and U-Al (3.279 Å), U-U (4.150 Å), and U-O (2.743 Å) for the acidic pH model.
Keywords: EXAFS DFT uranyl sorption gibbsite dimer trimer
  • Geochimica et Cosmochimica Acta 73(2009)20, 5975-5988

Publ.-Id: 12025 - Permalink


Surface diffusion assisted phase separation during the growth of carbon : transition metal thin films
Abrasonis, G.;
Encapsulated nanostructures formed by surface diffusion assisted phase separation during thin film growth are promising candidates for the multifunctional devices, high density magnetic storage media, multifunctional coatings, as large scale templates for nanowire fabrication. In this talk our activities concerning the investigation of the growth mechanisms, structure of the metallic ‘inclusion’ and carbon ‘tissue’ phase as well as their interface structure of carbon: transition metal (TM=V,Co,Ni,Cu) thin films will be summarized. It will be demonstrated that a combined use of laboratory and element selective synchrotron radiation based analytical tools allows identification of the morphology, nearest neighbour coordination as well as the electronic structure of the metallic nanoparticles and the carbon matrix individually and tracking of their changes as a function of the growth temperature, metal type and metal content.
  • Lecture (others)
    Invited seminar, 28.11.2008, Melbourne, Victoria, Australia

Publ.-Id: 12024 - Permalink


A High Resolution Time-of-Flight Spectrometer for Fission fragments and Ion beams
Kosev, K. M.;
Thesis For the purpose of fission-fragment detection a double time-of-flight (TOF) spectrometer has been developed. The key component of the spectrometer is a TOF detector consisting of multichannel-plate (MCP) detectors with a position-sensitive readout, a foil for secondary electron production and an electrostatic mirror. The spectrometer performance is tested at tandem and superconducting linear accelerator.
  • Other report
    Dissertation: Technische Universität Dresden, 2009
    119 Seiten

Publ.-Id: 12023 - Permalink


An improved experiment for the investigation of the helical magnetorotational instability
Stefani, F.; Gerbeth, G.; Gundrum, T.; Szklarski, J.; Rüdiger, G.; Hollerbach, R.;
The magnetorotational instability (MRI) is widely believed to play an essential role in the formation of stars and black holes. Destabilizing hydrodynamically stable Keplerian flows, the MRI triggers turbulence and enables outward transport of angular momentum in accretion discs which is necessary to explain the mass accumulation rates of central objects. The Potsdam Rossendorf Magnetic InStability Experiment (PROMISE) is intended to study the helical version of MRI which works already at Reynolds numbers of the order of 1000 and Hartmann numbers of the order 10. We focus on the results of an improved experiment in which split end caps are used to minimize the effect of Ekman pumping.
  • Lecture (Conference)
    11th MHD days, 01.-03.12.2008, Ilmenau, Germany

Publ.-Id: 12022 - Permalink


Semiconductor-based spintronics
Schmidt, H.;
In the talk the advantages of semiconductor-based spintronics are presented.
  • Lecture (others)
    FZD Lecture, 05.05.2008, Dresden, Deutschland

Publ.-Id: 12021 - Permalink


Room temperature ferromagnetism in Nd- and Mn-codoped ZnO films
Xu, Q.; Schmidt, H.; Hochmuth, H.; Lorenz, M.; Setzer, A.; Meinecke, C.; Grundmann, M.;
Nd and Mn were codoped into ZnO films which have been grown on a-plane sapphire substrates by pulsed laser deposition with the thickness ranging between 46 and 971 nm. The room temperature resistivity of the codoped films is independent of the film thickness. Large positive magnetoresistance and clear anomalous Hall effect were observed at 5 K. Ferromagnetism with clear hysteresis up to 290 K was observed. Codoping is suggested to be an efficient method to introduce energy levels in the ZnO band gap to mediate electron spins of the magnetic doping ions.
Keywords: ZnO, ferromagnetism

Publ.-Id: 12020 - Permalink


Vacuum ultraviolet dielectric function and band structure of ZnO
Schmidt-Grund, R.; Rheinländer, B.; Kaidashev, E. M.; Lorenz, M.; Fritsch, D.; Schubert, M.; Schmidt, H.; Herzinger, C. M.;
For ZnO, the optical dielectric functions for polarizations parallel and perpendicular to the optical axis were determined in the photon energy range from 4.0 to 9.5 eV by using generalized spectroscopic ellipsometry and the band structure was calculated by means of the empirical pseudopotential method. From the band structure, a theoretical dielectric function was derived. The dielectric functions reveal features that were identified as band-to-band transitions. The energies of those transitions were assigned to band-to-band transition energies of critical points of the calculated band structure.
Keywords: ZnO, empirical pseudopotential, generalized spectroscopic ellipsometry, dielectric function
  • Journal of the Korean Physical Society 53(2008)1, 88-93

Publ.-Id: 12019 - Permalink


Intensity of optical absorption close to the band edge in strained ZnO films
Fritsch, D.; Schmidt, H.; Schmidt-Grund, R.; Grundmann, M.;
Besides other one of the remarkable properties making wurtzite ZnO such an interesting material is its large exciton binding energy of about 60 meV, leading to stable excitons at room-temperature. Also, the Curie temperature of this wide-gap material has been predicted to lie above room temperature, making ZnO alloyed with magnetic ions a possible material for spintronics applications. One big challenge in the fabrication of the ZnO-based heterostructure devices is the lattice mismatch between the ZnO films and the substrates and the different thermal expansion coefficient inducing biaxial strain. This work reports on the electronic band structure of biaxially strained ZnO for strains along the a- or c-axis ranging from -1% to 1 %, as calculated by means of the empirical pseudopotential method. Thereby, we also account for relativistic effects in the form of the spin-orbit interaction, as well as for the energy dependence of the crystal potential through the use of nonlocal model potentials. Moreover, the application of a variable plane wave basis set allows us to directly obtain the strain-induced variations of the electronic and the optical properties of wurtzite ZnO.
Keywords: ZnO, emipirical pseudopotential method, dielectric function
  • Journal of the Korean Physical Society 53(2008)1, 123-126

Publ.-Id: 12018 - Permalink


Homoepitaxial ZnO thin films by PLD: Structural properties
Lorenz, M.; Wagner, G.; Rahm, A.; Schmidt, H.; Hochmuth, H.; Schmid, H.; Mader, W.; Brandt, M.; von Wenckstern, H.; Grundmann, M.;
Homoepitaxial ZnO films deposited on annealed hydrothermal O-face ZnO single crystals show superior structural quality. This is demonstrated by narrow ZnO(00.2) rocking curves with FWHM of typically 23 to 35 arcsec, and nearly dislocation-free TEM cross sections. Nominally undoped ZnO films indicate a minor in-plane strain of about 250 ppm and no out-of-plane strain. Target doping by 0.01% P2O5 or 0.5% Li3N results in pseudomorphic film growth without in-plane strain. Increasing doping concentration of 0.1 and 1% P2O5 results in both in-plane and out-of-plane strain up to 0.9 % indicating relaxed films. The O-face polarity of the homoepitaxial ZnO films is confirmed by convergent beam electron diffraction.
Keywords: ZnO, pulsed laser deposition

Publ.-Id: 12017 - Permalink


THz sideband generation in multi quantum wells
Wagner, M.; Schneider, H.; Helm, M.; Schartner, S.; Andrews, A. M.; Roch, T.; Strasser, G.;
Ac THz electric fields which couple strongly with intraband excitations in semiconductors can lead to spectral sidebands when an interband excitation is present. In this nonlinear mixing process a near-infrared (NIR) laser beam is mixed with a THz beam to generate sidebands around the NIR frequency with a frequency spacing equal to the THz frequency or multiples of it. In the last years this effect has been investigated in various semiconductor systems (i.e. in bulk GaAs or in multi quantum wells).
We investigated the third-order nonlinear mixing process between a near-infrared laser and a free-electron laser in an undoped symmetric AlGaAs/GaAs multi quantum well. Differently from the literature where electronic intersubband transitions were used, we are using the transition between the heavy-hole and light-hole states. This transition around 73 µm is pumped close to normal incidence by FELBE, the free-electron laser (FEL) of the Forschungszentrum Dresden-Rossendorf. The picosecond NIR laser is transmitted through the sample where the GaAs substrate has been etched away. It is focused on the entrance slit of the spectrometer of a Streak-camera system. With the Streak-camera temporal and spectral measurements are possible.
The n=+2 sideband conversion efficiency is of the order of 0,005% with respect to the incoming NIR intensity. Among other things we present the power dependency on NIR and FEL intensity and the resonance behavior with respect to the NIR and FEL wavelengths.
Keywords: THz sidebands, free-electron laser, FEL
  • Poster
    GDRE-THz workshop 2008 Paris, 25.-26.09.08, Paris, Frankreich
  • Poster
    EOS Annual Meeting 2008, 29.09.-02.10.2008, Paris, Frankreich
  • Poster
    Nonna 2008 - Workshop on Nonequilibrium Nanostructures, 01.-06.12.2008, Dresden, Deutschland

Publ.-Id: 12016 - Permalink


High electron mobility of phosphorous-doped homoepitaxial ZnO thin films grown by pulsed-laser deposition
Brandt, M.; von Wenckstern, H.; Schmidt, H.; Rahm, A.; Biehne, G.; Benndorf, G.; Hochmuth, H.; Lorenz, M.; Meinecke, C.; Butz, T.; Grundmann, M.;
The transport properties of phosphorous-doped ZnO thin films, grown by pulsed-laser deposition on thermally pretreated hydrothermally grown ZnO single-crystal substrates, are reported. The ZnO:P thin films show very good morphological and structural properties as confirmed by atomic force microscopy (AFM), high resolution x-ray diffraction, and Rutherford backscattering (RBS) channeling. Steps of height c/2 are visible in AFM investigations for all samples. For an oxygen partial pressure of 0.1 mbar, two-dimensional growth was found. RBS channeling of a ZnO:P film shows a minimum yield of 0.034 which is comparable to that of an annealed substrate (0.033). Hall effect measurements revealed that all films are n-type for the present growth conditions. Peak mobilities of 800 cm^2/Vs have been observed around 70 K, in line with the high structural quality of the samples. Room-temperature mobility in ZnO:P is up to 170 cm^2/Vs.
Keywords: ZnO, phosphorous-doping

Publ.-Id: 12015 - Permalink


X-ray scattering and diffraction from Xe-induced ripples in crystalline (001) silicon
Biermanns, A.; Pietsch, U.; Grenzer, J.; Hanisch, A.; Facsko, S.; Carbone, G.; Metzger, H.;
The formation of surface-nanostructures with a characteristic size ranging from several nanometer up to microns has attracted significant interest in the last decades in the context of fabrication of novel opto-electronic and storage devices. One kind of those nanostructures are wave-like patterns (ripples) produced by an interplay between a roughening process caused by ion beam erosion (sputtering) of the surface and smoothening processes caused by surface diffusion. In this contribution we report on investigations of patterned Si (001) surfaces after irradiation with Xe-ions using ion-energies up to 70keV. During the sputtering, an amorphous surface-layer is formed followed by a rather sharp interface towards crystalline material, showing the same morphology as the surface. The structures of the amorphous layer and the amorphous-crystalline interface were studied by means of grazing incidence- small angle scattering (GISAXS) and diffraction (GID) using synchrotron-radiation. We found that the crystal structure at the interface is expanded along the ripples, caused by the creation of defects inside the surface region, whereas this expansion is strongly reduced across the ripples, which can be explained by an anisotropic defect distribution close to the amorphous-to-crystalline interface.
Keywords: nanostructures ion-beam patterning GISAXS GID
  • Poster
    16th International Conference on Ion Beam Modification of Materials - IBMM 08, 31.07.-05.09.2008, Dresden, Germany

Publ.-Id: 12014 - Permalink


Paramagnetism in Co-doped ZnO films
Xu, Q.; Zhou, S.; Markó, D.; Potzger, K.; Fassbender, J.ORC; Vinnichenko, M.; Helm, M.; Hochmuth, H.; Lorenz, M.; Grundmann, M.; Schmidt, H.
Co-doped ZnO films with various electron concentrations up to 4.61×1019 cm-3 at room temperature were prepared by pulsed laser deposition on a-plane sapphire substrates. Only paramagnetism was observed down to 2 K for all the samples, which was also confirmed by x-ray magnetic circular dichroism (XMCD) measurements at 30 K. The average magnetic moment per Co2+ ion is significantly smaller than the expected moment for Co2+ ions (L=1.07, S=3/2), mainly due to the antiferromagnetic exchange interaction between the neighbouring Co2+ ions in the ZnO matrix. Also clustering instead of a uniform distribution of Co2+ ions may play a role. The formation of Co clusters is hindered at higher substrate temperature during the thin film growth. Clear anomalous Hall effect was observed in the highly conducting Co-doped ZnO films at low temperatures up to 100 K.
Keywords: diluted magnetic semiconductor, paramagnetism, anomalous Hall effect

Publ.-Id: 12013 - Permalink


X-ray absorption and photoelectron spectroscopy investigation of selenite reduction by FeII-bearing minerals
Funke, H.; Scheinost, A.;
Wavelet transform (WT) has been proven as a valuable tool for EXAFS data analysis for structures, where two types of backscattering atoms are at the same distance from the central atom [1-3]. WT provides not only radial distance resolution of the spectra like the Fourier transform, but resolves also the wave vector space permitting to probe the discrimination of atoms by their elemental nature.
A short introduction to the continuous WT will be given. The variation of the parameters of the Morlet mother wavelet will be used to build an adapted wavelet for the analysis of specific EXAFS spectra. Thereby the analysis of the (Heisenberg) uncertainty boxes of the Morlet wavelet is a central point to understand the resolution limitations of the WT as well as the form of the wavelet ridges.
Two examples for the use of WT for EXAFS data analysis will be given.
First, the method is applied to a structural problem of Zn-Al layered double hydroxides, demonstrating the homogeneity of the metal cation distribution in the hydroxide layers. Depending on the specific problem, either the well-known Morlet wavelet was used, or a newly developed FEFF-Morlet wavelet, based on theoretical EXAFS back scattering functions.
Second, the reduction from soluble selenium species to elemental Se and iron selenides by Fe containing minerals is examined in detail [4]. Here, the wavelet analysis is used to answer the question: Is Se coordinated to Se atoms/elemental Se or to the Fe (and Se) atoms? This analysis is performed by comparison to well–known reference spectra.
In the outlook two open questions will be discussed: the k weighting and the destructive interference by phase shifts.

References:
[1] H. Funke, A. C. Scheinost, and M. Chukalina, Phys. Rev B, 71, 094110, 2005.
[2] H. Funke, M. Chukalina, and A. C. Scheinost, J. Synchrotron Rad. 14, 426-432, 2007.
[3] M. Muñoz, P. Argoul, and F. Farges, Am. Mineral. 88, 694, 2003.
[4] A. C. Scheinost, L. Charlet , Environ. Sci. Technol. 42, 1984, 2008.
  • Lecture (Conference)
    Workshop on X-ray absorption spectroscopy and advanced XAS techniques, 06.-10.10.2008, Villigen, Switzerland

Publ.-Id: 12012 - Permalink


S-Layer für neue Nanomaterialien
Pollmann, K.; Raff, J.;
Im Rahmen des Vortrags werden die Arbeiten zur Herstellung von Nanopartikeln und Nanomaterialien mit Hilfe von bakteriellen Hüllproteinen vorgestellt und Möglichkeiten einer zukünftigen Zusammerarbeit diskutiert.
  • Lecture (others)
    Koordinierungstreffen mit dem Leibniz-Institut für Katalyse e.V. der Universität Rostock, Außenstelle Berlin, 17.04.2008, Berlin, Deutschland

Publ.-Id: 12009 - Permalink


Tailoring the FePt orientation on amorphous substrates by magnetron sputtering, structural and magnetic investigations
Cantelli, V.; Grenzer, J.; von Borany, J.; Fassbender, J.ORC
Towards an increase of the magnetic recording density, materials with a high magnetic anisotropy are strongly recommended to overcome the physical limits due to superparamagnetism. For this reason, FePt alloys are widely studied because of the excellent magnetocrystalline anisotropy (KU ~ 5-8 x 10-7 erg/cm3) and large magnetic moments at 300K. [1] But, for perpendicular recording media, a (001) preferential orientation, perpendicular to the layer surface, is required.
10 and 15 nm Fe55Pt45 layers were deposited by dual magnetron sputtering on amorphous SiO2 / Si (001) substrate, varying the deposition methods, form co-deposition to monolayers sequence deposition, and working gas, from Ar to Xe, at 0.3 Pa.
The Ar plasma is more energetic than the Xe one: it is characterized by sputtered atoms with a mean energy of about 12 eV and Ar reflected neutrals, reaching a mean energy of about 100 eV when backscattered from the Pt target. This energy budget that enhance the surface adatom mobility during deposition (few eV are required), and create vacancies (E > 40 eV), that decrease the phase transition temperature to 450°C for thin layers. [2]
But, the impact of energetic atoms supports a vertical layers intermixing resulting in a randomly oriented FePt A1 structure at RT. Subsequent Rapid Thermal Annealing at 750°C is completely transforming the layer into the L10 phase, with a coercivity field HC = 1 T, but introducing a weak perpendicular magnetic anisotropy, not evidencing any difference between the deposition techniques.
The reduction of the plasma energy, by using Xe as working gas, is not decreasing the layer magnetic properties (HC = 1 T) after annealing, but strongly enhance the (001) preferential orientation, with a dispersion of the (001) direction around the surface normal of 6°, already in the co-deposition experiment.
The layer by layer technique, associated with Xe plasma, supports the (001) layer orientation after RTA at 750°C, underlining the importance to reduce the elements diffusion path to an atomistic scale. [3]

[1] H. Kanazawa, G. Lanhoff, T. Suzuki, J. Appl. Phys. 87 (2000) 6143
[2] V. Cantelli, J. von Borany, A. Mücklich, Shengqiang Zhou, J. Grenzer,
Nucl. Instr. and Meth. B, 257, 1-2 (2007) 406-410.
[3] M. L. Yan, N. Powers, D. J. Sellmyer, J. Appl. Phys. 93, 8292 (2003)
Keywords: FePt, sputtering gas, (001) orientation
  • Lecture (Conference)
    53th Conference on Magnetism and Magnetic Materials, 10.-14.11.2008, Austin Texas, USA
  • Lecture (Conference)
    DPG Frühjahrstagung der Sektion Kondensierte Materie (SKM) 2009, 22.-27.03.2009, Dresden, Germany

Publ.-Id: 12007 - Permalink


Experiments on nuclear astrophysics in Dresden
Bemmerer, D.;
Recent experimental work on nuclear astrophysics questions at Research Center Dresden-Rossendorf (FZD) is discussed. Several large scale experimental facilities hosted in the city of Dresden/Germany have been used. The ELBE accelerator provides a 40 MeV intensive electron beam driving secondary radiation sources, for example bremsstrahlung and neutron time-of-flight. At the FZD ion beam center, a study on hydrogen burning is underway. The Felsenkeller shallow-underground counting facility in Dresden is used for a study of the astrophysical p-process.
  • Invited lecture (Conferences)
    Seminar, INFN sezione di Padova, 27.11.2008, Padua, Italy
  • Invited lecture (Conferences)
    Nuclear Physics Seminar, 14.05.2009, Legnaro (PD), Italien

Publ.-Id: 12006 - Permalink


MRPC prototype developments for NeuLAND/R3B at FZ Dresden Rossendorf
Bemmerer, D.; Yakorev, D.; Stach, D.; Wagner, A.;
Multigap resistive plate chamber detector (MRPC) prototypes for the future NeuLAND detector at R3B/FAIR have been developed and built at Forschungszentrum Dresden-Rossendorf. The prototypes have been submitted to a variety of tests, including highly precise timing measurements at the ELBE electron beam in Dresden. Results and future perspectives of MRPC prototyping for NeuLAND by the Dresden group are discussed.
  • Lecture (Conference)
    Joint R3B/EXL/ELISE Collaboration Meeting, 11.-14.10.2008, Göteborg, Sweden

Publ.-Id: 12005 - Permalink


Präzise Kernphysik für die Sonne
Bemmerer, D.;
Neue astronomische Beobachtungen ermöglichen es, ein bisher unerreicht genaues Bild unserer Sonne zu zeichnen. Solch ein Bild ist erforderlich, um den langfristigen Einfluss der Sonne auf das Erdklima zu verstehen, und um aus der Sonne einen kalibrierten Referenzstern zu machen.

Allerdings gibt es beim Verständnis der für die Energieproduktion im Sonneninnern verantwortlichen Kernfusionsreaktionen noch Diskrepanzen. Um sie aufzuklären, sind präzise kernphysikalische Messungen vonnöten. In dem Vortrag werden in Dresden und am Gran Sasso (Italien) untersuchte und für die Sonne relevante Kernreaktionen diskutiert und gezeigt, wie die neu gewonnenen Daten bisher bestehende Genauigkeitslücken schließen.

Abschließend wird auch auf die Auswirkungen der neuen Daten auf das Verständnis der Urknall-Nukleosynthese und der Entstehung der chemischen Elemente in Sternen, die schwerer als die Sonne sind, eingegangen.
  • Invited lecture (Conferences)
    Physikalisches Kolloquium, 02.12.2008, Dresden, Deutschland

Publ.-Id: 12004 - Permalink


The 3He(alpha,gamma)7Be S-factor at solar energies: The prompt gamma experiment at LUNA
Costantini, H.; Bemmerer, D.; Confortola, F.; Formicola, A.; Gyürky, G.; Bezzon, P.; Bonetti, R.; Broggini, C.; Corvisiero, P.; Elekes, Z.; Fülöp, Z.; Gervino, G.; Guglielmetti, A.; Gustavino, C.; Imbriani, G.; Junker, M.; Laubenstein, M.; Lemut, A.; Limata, B.; Lozza, V.; Marta, M.; Menegazzo, R.; Prati, P.; Roca, V.; Rolfs, C.; Rossi Alvarez, C.; Somorjai, E.; Straniero, O.; Strieder, F.; Terrasi, F.; Trautvetter, H. P.;
The 3He(alpha,gamma)7Be process is a key reaction in both Big-Bang nucleosynthesis and p–p chain of Hydrogen Burning in Stars. A new measurement of the 3He(alpha,gamma)7Be cross section has been performed at the INFN Gran Sasso underground laboratory by both the activation and the prompt gamma detection methods. The present work reports full details of the prompt gamma detection experiment, focusing on the determination of the systematic uncertainty. The final data, including activation measurements at LUNA, are compared with the results of the last generation experiments, and two different theoretical models are used to obtain the S -factor at solar energies.

Publ.-Id: 12003 - Permalink


The effect of the sputtering gas (Ar, Xe) on FePt clusters formation. Structural and magnetic properties
Cantelli, V.; Grenzer, J.; von Borany, J.; Fassbender, J.ORC
In this study it will be reported about the effect of the sputtering gases, Ar and Xe, on FePt clusters formation using magnetron sputtering deposition at high working pressures. Two different deposition techniques were investigated, a sequential layer by layer deposition and a co-deposition of Fe and Pt. All layers with bulk equivalent thicknesses between 3 to 5 nm were realized at RT and subsequently rapid thermal annealed in order to induce the A1-L10 ordering transformation at 550°C.
The highest L10 fraction in the annealed samples was found using Xe as sputtering gas: Xe decreases the transformation activation energy and therefore, reduces the critical thickness necessary to obtain the strong ferromagnetic phase.
The Ar assisted depositions give FePt clusters with the L10 phase only if the layer-by-layer growth is used, whereas for Xe no differences in the deposition techniques were observed.
Keywords: FePt, Magnetron sputtering, sputtering gases, clusters
  • Lecture (Conference)
    53th Conference on Magnetism and Magnetic Materials, 10.-14.11.2008, Austin Texas, USA
  • Contribution to proceedings
    53th Conference on Magnetism and Magnetic Materials, 10.11.-14.12.2008, Austin Texas, USA
    Journal of Applied Physics 105(2009), 07B529-07B529-3
  • Poster
    DPG Frühjahrstagung der Sektion Kondensierte Materie (SKM) 2009, 22.-27.03.2009, Dresden, Germany

Downloads:

Publ.-Id: 12002 - Permalink


Cavity layer introduction in SIMOX technology
Ou, X.; Kögler, R.; Mücklich, A.; Skorupa, W.; Möller, W.; Wang, X.;
The oral conference contribution reports about latest results of the improvement of the SIMOX process for fabrication of silicon-on-insulator (SOI) materials by defect engineering.
Keywords: SOI, SIMOX, ion implantation, defect engineering
  • Lecture (Conference)
    72. Jahrestagung der DPG und DPG Frühjahrstagung des Arbeitskreises Festkörperphysik, 25.-29.02.2008, Berlin, Germany

Publ.-Id: 12001 - Permalink


Kelvin probe force microscopy imaging of cross-sections of Si multilayer structures
Baumgart, C.; Müller, A.-D.; Müller, F.; Helm, M.; Möller, A.; Schmidt, H.;
Kelvin probe force microscopy (KPFM) is a standard technique for the investigation of surface potentials. We present its applicability to cross-sectionally prepared p-p+ Si multilayer structures. The contact potential difference (CPD) image between tip and sample has been recorded by means of an Anfatec Level-AFM with a 2nd amplifier and NSC15 probes from MikroMash. Using an active mixer, the excitation amplitude of the NSC15 probes is almost independent on the working frequency. The probed CPD signal difference between the layers ranges between 60 meV and 850 meV and can be correlated to the variation of the diffusion potential in the Si multilayer structure. The p-type of majority charge carriers and the corresponding acceptor dopant profile have been pinpointed by scanning capacitance measurements. Starting from the known donor dopant concentration in the NSC15 probe, we simulated the CPD and determined the acceptor concentration in the whole p-p+ Si multilayer structure. From the frequency dependence of the CPD we can clearly distinguish between surface and bulk effects.
  • Lecture (Conference)
    DPG-Frühjahrstagung der Sektion Kondensierte Materie (SKM) 2009, 22.-27.03.2009, Dresden, Germany

Publ.-Id: 12000 - Permalink


Influence of the fluid properties on co-current two-phase flows in a horizontal channel connected to a riser
Vallee, C.; Seidel, T.; Lucas, D.; Beyer, M.; Prasser, H.-M.; Pietruske, H.; Schütz, P.; Carl, H.;
0In the event of a loss-of-coolant-accident (LOCA) in a pressurised water reactor (PWR), emergency strategies have to be mapped out in order to guarantee the reliable removal of the decay heat from the reactor core. During a hypothetical small break LOCA with failure of the high pressure emergency core cooling system, the decay heat has to be released to the secondary circuit over the steam generators. Therefore, the primary circuit is designed to forward a natural circulation if the main coolant pumps are not available. Furthermore, if steam is generated in the primary circuit due to its depressurisation, stratified two-phase flow regimes can occur in the main cooling lines, which could be relevant for the reactor safety. It is intended that a computational fluid dynamics (CFD) approach could increase the simulation accuracy of such transient accident scenarios compared to the state of the art system codes.

In order to investigate the two-phase flow behaviour in a complex reactor-typical geometry and to supply suitable data for CFD code validation, a model of the hot leg of a pressurised water reactor was built at Forschungszentrum Dresden-Rossendorf (FZD). The hot leg is the line connecting the reactor pressure vessel (RPV) to the steam generator (SG) and is composed of a horizontal pipe, a 50° upward bend and an inclined riser (Figure 1). The hot leg model is operated in the pressure chamber of the TOPFLOW facility of FZD (Figure 1), which is used to perform high-pressure experiments under pressure equilibrium with the inside atmosphere of the chamber. Therefore, the test section does not have to support overpressures and can be designed with thin materials. Consequently, parts of the test section could be equipped with big size windows for the application of optical observation techniques, also at reactor typical boundary conditions. In order to provide optimal observation possibilities, a flat test-section design was chosen with a width of 50 mm.

Co-current flow experiments were performed in the hot leg model, simulating a two-phase natural circulation in the primary circuit of a PWR. The experiments were done with air and water at 3.0 bar and room temperature as well as with steam and water at pressures up to 50 bar and the corresponding saturation temperature (i.e. up to 264°C). Over this range of boundary conditions, the main fluid properties vary significantly. The frequency distribution of the water level measured in the RPV simulator was used to characterise the flow in the hot leg (Figure 2). It was found that the form of the distribution informs about the stationarity of the water flow to the steam generator: the flatter the distribution, the more discontinuous the transport of water over time. This tendency was confirmed by the high-speed video observations (Figure 3), which were also used to identify the flow regime. Furthermore, Figure 2 shows a comparison between the frequency distributions obtained from the air/water and the steam/water experiments. Generally, the distributions are flatter for the cold experiments than for the hot ones. This shows that, due to the lower surface tension and viscosity, the transport of water induced by the gas is more constant in time for the steam/water flow.
Keywords: two-phase flow, co-current flow, fluid properties, hot leg, pressurised water reactor, PWR
  • Contribution to proceedings
    ExHFT-7 - 7th World Conference on Experimental Heat Transfer, Fluid Mechanics and Thermodynamics, 28.06.-03.07.2009, Krakow, Poland, Krakow: AGH University of Science and Technology Press, 978-83-7464-235-4, 443-452
  • Lecture (Conference)
    ExHFT-7 - 7th World Conference on Experimental Heat Transfer, Fluid Mechanics and Thermodynamics, 28.06.-03.07.2009, Krakow, Poland

Publ.-Id: 11999 - Permalink


Comparison of air/water and steam/water flooding experiments in a model of the hot leg of a pressurised water reactor
Vallee, C.; Seidel, T.; Lucas, D.; Beyer, M.; Prasser, H.-M.; Pietruske, H.; Schütz, P.; Carl, H.;
In order to investigate the two-phase flow behaviour in a complex reactor-typical geometry and to supply suitable data for CFD code validation, a model of the hot leg of a pressurised water reactor (PWR) was built at Forschungszentrum Dresden-Rossendorf (FZD). Counter-current flow limitation (CCFL) experiments were performed with air and water at room temperature and pressures up to 3.0 bar as well as with steam and water at pressures up to 50 bar and the corresponding saturation temperature of 264°C.

One selected 50 bar experiment is presented, analysed and high-speed camera images are shown. Furthermore, the flooding curves obtained from the different experimental runs are presented in terms of the Wallis parameter and Kutateladze number, which are commonly used in the literature. However, both parameters fail to correlate properly the data: a discrepancy is observed between the air/water and steam/water series. Therefore, a modified Wallis parameter is proposed, which takes into account the effect of the fluid viscosities on the CCFL.
Keywords: two-phase flow, air water flow, steam water flow, flooding, counter-current flow limitation, CCFL, hot leg, pressurised water reactor, PWR, Wallis parameter, viscosity
  • Contribution to proceedings
    Jahrestagung Kerntechnik 2009, 12.-14.05.2009, Dresden, Germany, Berlin: INFORUM Verlags- und Verwaltungsgesellschaft mbH, Compact221
  • Lecture (Conference)
    Jahrestagung Kerntechnik 2009, 12.-14.05.2009, Dresden, Germany

Publ.-Id: 11998 - Permalink


Library Information Services - not only books in shelves
Reschke, E.;
The presentation shows the Library information services, focused on the elctronic services within the intranet and the internet. The main point was the link resolver for linking services.
Keywords: online catalogue, document deliver, Interlibrary loan, elctronic databases, virtual library, publishing, copyright, document managment systems, open access
  • Lecture (others)
    FZD Lecture, 01.12.2008, Dresden, Deutschland

Publ.-Id: 11997 - Permalink


On the influence of spatial discretization on cross-sction preparation during burnup calculations with HELIOS 1.9
Merk, B.;
Cell- and burnup calculations are the fundament for all deterministic static and transient 3D full core calculations for different operational states of the reactor. The spatial discretization used for the c burnup calculations influences the results for the used integral transport solutions significantly. The arising differences in the infinite multiplication factor and in the localized burnup ditribution in the fuel rod for identical averaged burnup are shown and analyzed. Special emphasis is given to the influence of different discretization strategies on the calculation of homogenized two group cross sections which are forwarded to the 3D full core calculations .
Keywords: Cell- and Lattice calculation, cross section preparation, HELIOS, Discretization, Burnup calculation, Depletion
  • Contribution to proceedings
    Jahrestagung Kerntechnik 2009, 12.-14.05.2009, Dresden, Deutschland
  • Lecture (Conference)
    Jahrestagung Kerntechnik 2009, 12.-14.05.2009, Dresden, Deutschland

Publ.-Id: 11995 - Permalink


Sicherheitsorientierte Regelungsstrategien für Batch- und Semibatch-Prozesse in der Feinchemie
Kryk, H.; Hessel, G.;
Insbesondere in der Feinchemie und der pharmazeutischen Industrie erfolgt die Produktion von Spezialchemikalien meist im Batch- oder Semibatch-Betrieb in sog. Mehrzweckanlagen. Als Reaktionsgefäße kommen Rührkesselreaktoren zum Einsatz. Das instationäre und nichtlineare Prozessverhalten ist dabei durch eine Reihe von Einflussfaktoren gekennzeichnet, welche sich sowohl auf die Prozesssicherheit als auch auf die Effizienz des Gesamtverfahrens auswirken können. Demgegenüber werden jedoch gerade in Mehrzweckanlagen meist konventionelle Prozessgrößen für die Steuerung komplexer chemischer Prozesse verwendet. Da PAT bei Mehrzweckanlagen nur in begrenztem Umfang einsetzbar ist, stehen Echtzeit-Informationen über Konzentrationsverläufe, die für den Prozesszustand und das Gefahrenpotenzial des Prozesses kennzeichnend sind, oftmals nicht zur Verfügung. Somit besteht für den Operator das Problem, tolerierbare von unerwünschten Prozessabweichungen zu unterscheiden und die Ursachen bestimmter Prozess-Trends zu identifizieren. Von besonderer Bedeutung ist die Entwicklung und Etablierung industriell anwendbarer Methoden für eine objektive Detektion des aktuellen Prozesszustandes in Echtzeit sowie für die frühzeitige Identifikation unerwünschter Betriebszustände. Aus diesen Gründen entwickelt das Institut für Sicherheitsforschung des FZD bereits seit Jahren Methoden zum Online-Monitoring sicherheitsrelevanter Batch- und Semibatch-Prozesse. Zur Vermeidung des Einsatzes von teuren und empfindlichen Online-Analysesystemen, wurden Methoden entwickelt, welche auf Echtzeit-Stoff- und Energiebilanzen basieren. Diese können den Anlagenfahrern als Hilfe zur Objektivierung der Prozessführung und, im Falle unerwünschter Prozesszustände, als Werkzeug zur Handlungsempfehlung für geeignete Gegenmaßnahmen dienen.
Die Idee ist nun, aufbauend auf die bisherigen Entwicklungen, derartige Verfahren als Komponente einer sicherheitsbasierten Regelungsstrategie zu nutzen. Die Echtzeit-Resultate des Monitoring-Systems könnten dann als Führungsgrößen für eine entsprechende Prozessregelung, bis hin zur inhärent sicheren, vollautomatischen Betriebsweise von Batch-Prozessen in Mehrzweckanlagen, genutzt werden.
  • Invited lecture (Conferences)
    ProcessNet-Arbeitsausschuss "Sicherheitsgerechtes Auslegen von Chemieanlagen", 19.-20.11.2008, Frankfurt a.M., Deutschland

Publ.-Id: 11994 - Permalink


The influence of different thermal hydraulic models on the results of a DYN3D boron dilution transient calculation
Kliem, S.; Mittag, S.; Rohde, U.; Weiß, F.-P.; Kozmenkov, Y.;
The availability of three different thermal hydraulic models for the neutron kinetic core model DYN3D gave the possibility to investigate their influence on the results of a boron dilution transient. Quantitative differences were found in the single solutions. So the different degree of numerical diffusion in the boron transport models affects the height and time of the first power peak. Differences were also found in the heat transfer and the drift flux models. Concerning assessed safety criteria like maximum fuel and cladding temperature as well the DNB ratio the same conclusions can be drawn from all calculations: Safety-relevant margins are not reached. A spreading of the calculated data is present but there are no qualitative differences in the consid-ered transient.
  • Contribution to proceedings
    Jahrestagung Kerntechnik 2009, 12.-14.05.2009, Dresden, Deutschland
    Tagungsband der Jahrestagung Kerntechnik 2009, paper 114, Berlin: INFORUM GmbH
  • Lecture (Conference)
    Jahrestagung Kerntechnik 2009, 12.-14.05.2009, Dresden, Deutschland

Publ.-Id: 11993 - Permalink


Complexation of uranium(IV) with fluoride and phosphate using TRLFS
Lehmann, S.;
A novel TRLFS system was set up for investigation of short-lived fluorescence emitting metal ions like uranium(IV) or americium(III). This laser system was applied to study the complexation of uranium(IV) with fluoride and phosphate and to determine their respective complex formation constants.
Keywords: uranium(IV) fluoride, uranium(IV) phosphate, time-resolved laser fluorescence spectroscopy
  • Lecture (others)
    Workshop of the Institute of Radiochemistry (IRC) and the Paul Scherrer Institute (PSI), Villigen , Switzerland, 03.-04.11.2008, Dresden, Germany

Publ.-Id: 11992 - Permalink


Computational Fluid Dynamics (CFD) for gas-liquid flows
Lucas, D.; Tiselj, I.; Hassan, Y.; Moretti, F.;
Computational Fluid Dynamics (CFD) codes are widely used in industrial applications for single phase flows, e.g., in the automotive or aircraft industries. On the other hand the application of CFD for multiphase systems is not yet mature. Safety analyses related to nuclear light water reactors require reliable simulations for different scenarios including two-phase flow situations. Prominent examples for Pressurized Water Reactor (PWR) analyses are the prevention from Departure from Nucleate Boiling (DNB) which is related to Critical Heat Flux (CHF) or the Pressurized Thermal Shock (PTS) problem which has to be considered in connection with some hypothetical Loss of Coolant Accident (LOCA) scenarios and may also lead to two-phase flow situations in the cold leg and in the downcomer. In case of Boiling Water Reactors (BWR) analyses e.g. the prevention from Dryout is an important issue. The special issue of the on Science and Technology of Nuclear Installations Journal on "Computational Fluid Dynamics (CFD) for Gas-Liquid Flows" discusses the state of the art and the progress regarding CFD simulations on such two-phase flows.
Keywords: CFD, two-phase

Publ.-Id: 11991 - Permalink


A novel time-resolved laser fluorescence spectroscopy system for research on complexation of uranium(IV)
Lehmann, S.; Geipel, G.; Grambole, G.; Bernhard, G.;
Research on the chemical speciation of complexes by determining the fluorescence properties of metal ions whose emitted fluorescence lifetime is in the range of only few nanoseconds, has been very limited to date due to a lack of the technical possibilities necessary to conduct respective measurements. We were able to overcome the technical problems and set up a new time-resolved laser fluorescence spectroscopy system that meets the requirements to carry out research on the fluorescence properties of metal ions with very short fluorescence lifetimes such as uranium(IV) and its compounds. We investigated the fluorescence of uranium(IV) in perchloric acid and determined the detection limit of uranium(IV) to be 1 ± 10-6M. Additionally, we found the fluorescence decay time of uranium(IV) to be 2.73 ns ± 0.40 ns. Further application of the novel laser system addressed the complexation of uranium(IV) with fluoride by studying the fluorescence properties during reaction. Evaluation of the data recorded resulted in the finding of a 1 : 1 complex (uranium(IV) : fluoride). We determined the corresponding complex formation constant of uranium(IV) fluoride (UF)3+ with logβº = 9.43 ± 1.94. The application of our novel time-resolved laser fluorescence spectroscopy system demonstrated that speciation measurements of metal ions and their compounds with very short-lived fluorescence lifetimes can be conducted successfully. Using this laser system, analytical investigation of such elements and compounds is possible in environmentally relevant concentration ranges.
Keywords: time-resolved laser fluorescence spectroscopy, uranium(IV) fluoride, detection limit, fluorescence lifetime, complex formation constant
  • Spectrochimica Acta Part A 73(2009)5, 902-908

Publ.-Id: 11990 - Permalink


Main results of the European project NURESIM on the CFD-modelling of two-phase Pressurized Thermal Shock (PTS)
Lucas, D.; Bestion, D.; Coste, P.; Pouvreau, J.; Morel, C.; Martin, A.; Boucker, M.; Bodele, E.; Schmidtke, M.; Scheuerer, M.; Smith, B.; Dhotre, M. T.; Niceno, B.; Lakehal, D.; Galassi, M. C.; Mazzini, D.; D’Auria, F.; Bartosiewicz, Y.; Seynhaeve, J.-M.; Tiselj, I.; ŠTrubelj, L.; Ilvonen, M.; Kyrki-Rajamäki, R.; Tanskanen, V.; Laine, M.; Puustinen, J.;
Pressurized Thermal Shock (PTS) and Direct Contact Condensation (DCC) were identified by the European project EUROFASTNET as two of the most important industrial needs related to nuclear reactor safety where CFD may bring a real benefit. One typical PTS scenario limiting the Reactor Pressure Vessel (RPV) lifetime is cold water Emergency Core Cooling (ECC) injection into the cold leg during a hypothetical SB-LOCA. The injected water mixes with the hot fluid present in the cold leg and the mixture flows towards the downcomer where further mixing with the ambient fluid takes place. Such a scenario may lead to high thermal gradients in the structural components and consequently to thermal stresses. Therefore, the loads upon the RPV must be reliably assessed. The NURESIM sub-project 2 (Thermohydraulics) Work Package 2.1 focuses on a two-phase flow configuration resulting from a partially or fully uncovered cold leg. In the case of a partially uncovered cold leg, a stratification of cold water on the bottom of the cold leg with counter-current flow of hot water and steam on top of this cold-water layer may occur. There is mixing between hot and cold water. Condensation takes place at the free surfaces between steam and water, e.g. at the cooling water jet. Mixing and condensation are strongly dependent on the turbulence in the fluids. Reliable numerical simulations are required. Two-phase PTS constitutes one of the most challenging exercises for a Computational Fluid Dynamics (CFD) simulation. Presently available CFD tools are not yet able to reproduce all the separate phenomena taking place in the cold leg and the downcomer during the ECC injection, let alone an accurate simulation of the whole process. Improvements of the two-phase modelling capabilities have to be undertaken to qualify the codes for the simulation of such flows. A really accurate simulation of all the phenomena that occur in the scenario will only be possible in the far future and a step-by-step improvement of the quality of the forecasts is necessary. However, a reasonable prediction of the most important phenomena may be reached in a short or medium term and the use of CFD in industrial studies related to PTS is already possible in the frame of some limitations.
Keywords: Pressurized Thermal Shock, CFD, two-phase flow
  • Kerntechnik 74(2009), 238-242

Publ.-Id: 11989 - Permalink


Experimental and numerical analysis of coolant mixing in VVER-1000 in the framework of TACIS project R2.02/02
Höhne, T.; Rohde, U.; Melideo, D.; Moretti, F.; D'Auria, F.; Shishov, A.; Lisenkov, E.;
A pump start-up experiment with the presence of a tracer slug, conducted on a Gidropress mixing facility in the framework of TACIS Project R2.02/02, was simulated with the CFD code ANSYS CFX. The numerical results were compared against the experimental data, which consist in tracer concentration measurements at several locations at the core inlet. The results showed an good agreement with the experiment from the qualitative point of view: in particular, the morphology of the tracer concentration distribution at the core inlet was correctly described. This qualitative agreement is quite an important achievement, since the addressed scenario is featured by a complex, highly three-dimensional, flow distribution in the downcomer. From a quantitative point of view, the results in terms of maximum perturbation (and related timing), core-averaged perturbation are also satisfactory. The perturbation peak is over-predicted by 5%, which is comparable with the experimental uncertainty. The predicted time history of the core-averaged perturbation shows a less smooth trend than the experiment, which seems to indicate a less effective mixing.
Keywords: CFD, pump start up, VVER-1000
  • Contribution to proceedings
    Jahrestagung Kerntechnik 2009, 12.-14.05.2009, Dresden, Deutschland
    CD-ROM
  • Lecture (Conference)
    Jahrestagung Kerntechnik 2009, 12.-14.05.2009, Dresden, Deutschland

Publ.-Id: 11986 - Permalink


Superconducting Ge:Ga layers produced by ion implantation and flash lamp annealing
Heera, V.; Herrmannsdörfer, T.; Ignatchik, O.; Mücklich, A.; Posselt, M.; Reuther, H.; Schmidt, B.; Skorupa, W.; Voelskow, M.; Wündisch, C.; Wosnitza, J.; Helm, M.;
Recently, superconductivity has been discovered in heavily boron-doped group IV semiconductors like diamond [1] and silicon [2]. Because theoretical studies predict only a weak tendency to superconductivity in heavy p-type doped Ge [3] investigations of the low-temperature transport behaviour in Ge are still lacking.
In order to obtain superconductivity in group IV semiconductors, heavy p-type doping above the metal-insulator-transition and low lattice damage is required. The combination of both conditions make it difficult to apply ion implantation as doping technique. The challenge is to reconstruct the damaged or even amorphized crystal lattice and to activate the acceptor atoms after implantation by annealing, avoiding at the same time long range diffusion and precipitation of the acceptors in the supersaturated semiconductor. So far only in-situ doping during growth (high-temperature-high-pressure synthesis [1] and chemical vapour deposition) for boron-doped diamond and ultra-short-time laser melting of the Si surface in BCl3 atmosphere (gas immersion laser doping [2]) have met these conditions.
Here an alternative process compatible with semiconductor technology is presented. Ga implantation and flash lamp annealing in the ms range enables the production of Ga supersaturated (up to 15 at%) crystalline Ge layers which become superconducting below 0.5 K.
The layer structure investigated by AES, XTEM, RBS/C and the electrical transport properties at low temperatures are reported.

[1] E. A. Ekimov, V. A. Sidorov, E. D. Bauer, et al. , Nature 428 (2004) 542
[2] E. Bustarret, C. Marcenat, P. Achatz, et al., Nature 444 (2006) 465
[3] L. Boeri, J. Kortus, O. K. Anderson, J. Phys. Chem. Solids 67 (2006) 552
Keywords: Germanium, Ga-Implantation, Flash lamp annealing, superconductivity
  • Poster
    IBMM 2008 - 16th International Conference on Ion Beam Modification of Materials, 31.08.-05.09.2008, Dresden, Deutschland

Publ.-Id: 11985 - Permalink


Structural identification of thioarsenates by EXAFS
Suess, E.; Scheinost, A. C.; Bostick, B. C.; Merkel, B. J.; Wallschlaeger, D.; Planer-Friedrich, B.;
short-range structure of thioarsenite and mono-, di-, tri-, and tetra-arsenate determined by XAS
Keywords: arsenic EXAFS XANES XAS thioarsenate thioarsenite
  • Poster
    Synchrotron Environmental Science IV, 11.-13.12.2008, San Francisco, USA
  • Poster
    AGU Fall Meeting 2008, 15.-19.12.2008, San Francisco, USA

Publ.-Id: 11984 - Permalink


NanoFoto - Neue Wege zur verwertungsorientierten Netzwerkbildung in der Nanobiotechnologie
Pollmann, K.; Raff, J.;
Ziel des Vortrags ist es, einen Überblick über die laufenden und geplanten Aktivitäten im Rahmen des NanoFoto-Projekts zu geben. Wissenschaftlich steht die Nutzung bakterieller Hüllproteine zur Entwicklung fotokatalytisch aktiver Schichten zur Eliminierung von Arzneimittelrückständen aus Wasser im Mittelpunkt. Ergänzt werden die wissenschaftlichen Arbeiten durch zahlreiche Maßnahmen zur Verwertung erhaltener Ergebnisse und zur verwertungsorientierten Netzwerkbildung.
  • Lecture (others)
    Kick-off Meeting / 1. Workshop „Entwicklung und Erprobung neuer Instrumente zur Bildung von Verwertungs- und Transfernetzen in der Leibniz-Gemeinschaft“, 11.03.2008, Berlin, Deutschland
  • Lecture (others)
    2. Projekt-Workshop zum Thema „Verwertungsorientierte Netzwerke“, 16.12.2008, Berlin, Deutschland

Publ.-Id: 11982 - Permalink


Granular magnetic ZnO: structure, magnetism and transport properties
Zhou, S.; Potzger, K.; Xu, Q.; Shalimov, A.; Schmidt, H.; Helm, M.; Fassbender, J.ORC
Transition metal (TM) doped ZnO has been extensively investigated due to its potential application as a diluted magnetic semiconductor with Curie temperature above room temperature (RT). After one decade effort, however the research community realized that (i) ZnO diluted with TM ions only shows paramagnetism [1], and (ii) the observed ferromagnetic signal mostly originates from secondary phases [2,3]. The aim of our research is now to investigate the application potential of granular structures which are created by TM ion implantation into ZnO single crystals. By varying implantation and post-annealing temperatures, we can control the chemical state of TM ions. ZnO with dispersed TM ions can be obtained by ion implantation at temperatures below RT or by using defective ZnO substrates. In this case, TM ions are in ionic states, and only show paramagnetism. Concerning the nature of phase separation, three regimes have been established. (I) ZnO embedded with TM nanocrystals can be obtained by ion implantation at elevated temperatures (e.g. 350oC) and by post-annealing at mild temperatures (below 350oC). In this regime, TM ions are mostly in metallic states (i.e. Fe, Co, Ni). Co and Ni nanocrystals have crystallographic orientation relationship with the ZnO matrix [2]. (II) ZnO embedded with nanocrystalline spinel ferrites AFe2O4 (A=Zn, Co, Ni) can be obtained by co-implantation pulsing post-annealing at 800 oC [4,5]. (III) ZnO embedded with disordered nanosized regions can be obtained by ion implantation with very large ion-fluences. The heavily disordered nanosized regions consists of large Co concentration [6]. Although ferromagnetism has been observed in all the three regimes, magneto-transport properties are drastically different. Only ordinary magneto-resistance (MR) has been observed in regimes I and II, while the samples in regime III reveal negative MR and anomalous Hall effect simultaneously. The anomalous Hall resistivity is saturated at low field giving hope for applicability in spintronics.

[1] A. Ney, , et al., Phys. Rev. Lett. 100, 157201 (2008)
[2] S. Zhou, et al., Phys. Rev. B 77, 035209 (2008).
[3] K. Potzger, and S. Zhou, phys. stat. sol. (a), submitted (2008).
[4] S. Zhou, et al., J. Phy. D-Appl. Phys., 40, 964 (2007).
[5] S. Zhou, et al., Phys. Rev. B, submitted (2008).
[6] K. Potzger, et al., Appl. Phys. Lett., submitted (2008).
  • Poster
    Magnetism meets Semiconductors: Spin Phenomena in Heterostructures and Novel Materials, 05.-07.01.2009, Bad Honnef, Germany
  • Poster
    5th International School and Conference on Spintronics and Quantum Information Technology, 04.-11.07.2009, Krakow, Poland

Publ.-Id: 11981 - Permalink


Magneto-transport properties of nanocomposite cobalt/carbon systems
Zhou, S.; Berndt, M.; Bürger, D.; Abrasonis, G.; Heera, V.; Fassbender, J.ORC; Helm, M.; Schmidt, H.
In producing diluted magnetic semiconductors (DMS), it remains challenging to obtain a uniform distribution of magnetic ions. Composite ferromagnet/semiconductor systems are easily formed when the concentration of magnetic ions is larger than its solubility at given preparation conditions. This is usually considered as a failure for spintronics applications. However, as long as the embedded (nano)ferromagnets can polarize charge-carriers in the semiconducting matrix, the composite systems can also fullfill spintronics requirements. For example, nanocomposite systems with ferromagnetic MnAs nanocrystals being epitaxially embedded inside GaAs matrix reveal a giant magnetoresistance [1]. Recently, the phase separation in Mn doped Ge [2] and Si [3] has been indentified by high resolution characterization techniques. Therefore, the research interest in composite ferromagnet/semiconductor systems arises in parallel with that in DMS materials. Here we present the magnetic and magneto-transport properties of cobalt nanocrystals embedded inside carbon. Co(40%)/C nanocomposite films were prepared by ion beam co-sputtering method using silicon substrates with a 500 nm thick oxide layer. The phase separation was controlled by varing substrate temperatures from room temperature to 500 °C. We have measured their magnetic and magneto-transport properties. Two significant observations will be discussed: (i) a giant anomalous Hall effect (AHE) amounting to 2 μohm cm compared with pure Co metal [4], and (ii) a negative magnetoresistance. This encourages future applications in Hall sensors and spintronic-devices.
  • Poster
    Magnetism meets Semiconductors: Spin Phenomena in Heterostructures and Novel Materials, 05.-07.01.2009, Bad Honnef, Germany
  • Lecture (Conference)
    SPIE Europe Microtechnologies for the New Millennium, 04.-06.05.2009, Dresden, Germany
  • Poster
    International Conference on Magnetism, 26.-31.07.2009, Karlsruhe, Germany

Publ.-Id: 11980 - Permalink


ZnO-based magnetic tunnel junctions
Zhou, S.; Xu, Q.; Hartmann, L.; Mücklich, A.; Helm, M.; Biehne, G.; Hochmuth, H.; Lorenz, M.; Grundmann, M.; Schmidt, H.;
Spin-polarized tunnel magnetoresistance (TMR) effects occur when two ferromagnets are separated by a thin insulator. The resistance of the tunneling current changes with the relative magnetization orientation of the magnetic bottom and top electrode. The research in this field is fuelled by the demanding of magnetoresistive random access memory (MRAM) devices. Novel MRAM cells are based on magnetic tunnel junctions with current-induced switching. It has been shown that semiconductors need a current pulse for switching which is two orders of magnitude smaller in comparison to metals. Using wide-gap magnetic semiconductors, e.g. ZnO, the magnetic tunnel structure may be transparent and may possess a Curie temperature above room temperature. In this talk, we report the clearly observed tunneling magnetoresistance at 5 K in magnetic tunnel junctions with Co-doped ZnO as the bottom electrode and Co as the top electrode prepared by pulsed laser deposition and thermal evaporation [1], respectively. Spin-polarized electrons were injected from Co-doped ZnO to the crystallized Al2O3 separation layer and tunnelled through the amorphous part of the Al2O3 barrier. Our studies demonstrate the spin polarization in Co-doped ZnO and its possible application in future ZnO-based spintronics devices.
[1] Q. Xu et al., Phys. Rev. Lett. 101, 076601 (2008)
  • Lecture (Conference)
    DPG Frühjahrstagung der Sektion Kondensierte Materie (SKM) 2009, 22.-27.03.2009, Dresden, Germany

Publ.-Id: 11979 - Permalink


Semiconductor-based tunnel structures: preparation and application
Zhou, S.; Schmidt, H.;
Spin-polarized tunnel magnetoresistance (TMR) effects occur when two ferromagnets are separated by a thin insulator. The resistance of the tunneling current changes with the relative magnetization orientation of the magnetic bottom and top electrode [1,2]. The research in this field is fuelled by the demanding of magnetoresistive random access memory (MRAM) devices. In 2004, Parkin et al. were able to make Fe/MgO/Fe junctions with 200% TMR at room temperature [3]. However, the development of high-density metal-based MRAM scale devices is hampered by large switching fields and multidomain structures. Theoretically, diluted magnetic semiconductors (DMS) reveal a larger degree of spin polarization and therefore also bigger spin transport effects. Mn-doped GaAs is a successful DMS and magnetic tunnel junctions based on epitaxially grown GaMnAs/AlAs/GaMnAs have shown promising TMR values up to 75% at 8 K [5]. Novel MRAM cells are based on magnetic tunnel junctions with current-induced switching. It has been shown that semiconductors [6] need a current pulse for switching which is two orders of magnitude smaller in comparison to metals [7]. Using wide-gap magnetic semiconductors, e.g. ZnO, the magnetic tunnel structure may be transparent and may possess a Curie temperature above room temperature [9,10].

In this talk, we report the clearly observed tunneling magnetoresistance at 5 K in magnetic tunnel junctions with Co-doped ZnO as the bottom electrode and Co as the top electrode prepared by pulsed laser deposition and thermal evaporation [11], respectively. Spin-polarized electrons were injected from Co-doped ZnO to the crystallized Al2O3 separation layer and tunnelled through the amorphous part of the Al2O3 barrier. Our studies demonstrate the spin polarization in Co-doped ZnO and its possible application in future ZnO-based spintronics devices. Additionally, we will show preliminary results of Si:Mn based tunnelling structures. In this system, SiO2 is the barrier layer while ferromagnetic granular Si:Mn obtained by Mn ion implantation into Si and Co are the bottom and top electrode, respectively.

[1] J. S. Moodera et al., Phys. Rev. Lett. 74, 3273 (1995).
[2] T. Miyazaki et al., J. Magn. Magn. Mater. 139, L231 (1995).
[3] S. S. P. Parkin et al., Nat. Mat. 3, 862 (2004).
[4] S. D. Sarma et al., Solid State Commun. 119, 207 (2001).
[5] M.Tanaka et al., Phys.Rev. Lett. 87, 026602 (2001).
[6] M. Yamanouchi et al., Nature 428, 539 (2004).
[7] J.A. Katine et al., Phys. Rev. Lett. 84, 3149 (2000).
[8] T. Jungwirth et al., Phys. Rev. B 72, 165204 (2005).
[9] T. Dietl et al., Science 287, 1019 (2000).
[10] K. Sato et al., Jpn. J. Appl. Phys. 39, L555 (2000).
[11] Q. Xu et al., Phys. Rev. Lett. 101, 076601 (2008)
  • Invited lecture (Conferences)
    International Workshop on Advances in Spintronic Materials: Theory and Experiment, 26.-28.11.2008, Duisburg, Germany

Publ.-Id: 11978 - Permalink


n-type diamond produced by MeV lithium implantation in channeling direction
Chernyshev, V.; Meijer, J.; Grambole, D.; Herrmann, F.; Dagkaldiran, U.; Wieck, A.;
Natural diamond implanted with lithium ions at high energy (E = 2MeV) in axial channeling direction is investigated. Hall measurements show n-type conductivity caused by the lithium implantation. These measurements show two regions in the Arrhenius plot for both resistance and charge carrier concentration. The resistance measurement in the low and high temperature range revealed the activation energies of E-Rlow = 406meV and E-Rhigh = 105meV. The slope for the charge carrier concentration shows a more complex behaviour. After annealing, the n-type conductivity caused by implanted lithium disappears.
  • Diamond and Related Materials 17(2008)11, 1933-1935

Publ.-Id: 11977 - Permalink


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