Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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35810 Publications

Target poisoning during magnetron sputtering: Real-time in-situ analysis and collisional computer simulation

Güttler, D.; Abendroth, B.; Grötzschel, R.; Depla, D.; Möller, W.

The evolution of reactive gas uptake at the target surface has been investigated by real-time in-situ diagnostics during magnetron sputtering. Using a planar circular DC magnetron for reactive sputter deposition of TiN from a Ti target in an argon/nitrogen gas mixture, the target uptake of nitrogen was determined at varying gas flow of nitrogen using the 14N(d,a)12C nuclear reaction, directly demonstrating the target "poisoning" effect. The expected hysteresis behaviour at increasing/decreasing nitrogen gas flow is confirmed. In the centre of the racetrack, the nitrogen uptake is significantly smaller than on the remaining target surface. Within the precision of the measurement, the nitrogen content remains unaltered after switching off the magnetron, indicating the absence of a significant mobile fraction of nitrogen in the target. The maximum amount of retained nitrogen significantly exceeds one adsorbed monolayer, which is attributed to nitrogen ion implantation and recoil implantation of adsorbed nitrogen. This is quantitatively reproduced by TRIDYN collisional computer simulations.

  • Poster
    VEIT 2005, 14th International Summer School on Vacuum, Electron and Ion Technologies, 12.-16.09.2005, Sunny Beach, Bulgaria

Publ.-Id: 8680

Relaxation dynamics of interminiband transitions and electron cooling in doped GaAs/AlGaAs superlattices

Stehr, D.; Winnerl, S.; Helm, M.; Andrews, A. M.; Roch, T.; Strasser, G.

Semiconductor superlattices represent a key model system for artificial crystals and are an essential component of novel infrared devices such as detectors and quantum cascade lasers. Hence their optical and transport properties have been investigated extensively during the past two decades. However, unlike for quantum well structures where considerable knowledge on the intersubband relaxation dynamics has been obtained, so far no experimental work has been published on the interminiband relaxation dynamics in superlattices.

We have studied the transient transmission of a doped GaAs/Al0.3Ga0.7As superlattice in pump-probe experiments [1]. The superlattice with thickness of 9.0 nm and 2.5 nm of the wells and barriers, respectively, was n-doped in the center of the wells, resulting in a doping density of 1.51016 cm-3 averaged over one superlattice period. Picosecond infrared pulses in the range from 4 µm to 22 µm were generated at 13 MHz repetition rate by the free-electron laser FELBE at the Forschungszentrum Rossendorf. In particular, the experiments were performed at the absorption maxima of the superlattice at 9.0 µm and 15.8 µm. These wavelengths are the spectral positions of the van Hove singularities of the joint density of states in the center and at the edge of the mini-Brillouin zone, respectively.

The measured pump-probe signals consist of a fast component due to the bleaching of the interminiband transition and subsequent relaxation and thermalization, and a slower component due to cooling of the heated electron system. The fast component decays typically around 2-2.5 ps, in reasonable agreement with published theoretical values [2]. The slower component due to cooling is positive for excitation at 9.0 µm and negative at 15.8 µm and shows a strong temperature and excitation density dependence with cooling times ranging from 5 to 50 ps. This behavior is consistent with the temperature dependence of the linear absorption spectrum, i.e. yielding higher or lower transmission for increasing electron temperature. The effect provides an internal thermometer for the miniband electrons on a picosecond timescale.

We will also report on data from superlattices with higher doping density, which shows the influence of electron-electron scattering.

[1] D. Stehr et al., submitted to Appl. Phys. Lett.
[2] F. Compagnone, A. Di Carlo, and P. Lugli, Appl. Phys. Lett. 80, 920 (2002)

  • Lecture (Conference)
    28th International Conference on the Physics of Semiconductors, 24.-28.07.2006, Wien, Austria
  • Contribution to proceedings
    28th International Conference on the Physics of Semiconductors, 24.-28.07.2006, Wien, Austria
    AIP Conference Proceedings 893, 485-486

Publ.-Id: 8679

Theory of impurity states in coupled quantum wells and superlattices and their infrared absorption spectra

Stehr, D.; Helm, M.; Metzner, C.; Wanke, M. C.

The problem of shallow impurities in confined semiconductor systems has been extensively investigated over the past two decades. It is well known that in two dimensions the binding energy is increased and the degeneracy of the states is lifted due to the symmetry breaking. Yet virtually all calculations were based on variational approaches for the impurity levels; some more sophisticated calculations, on the other hand, were mostly done solely for the ground state and other low-lying states.

Here we present an essentially exact calculation, treating the quantum well (QW) and impurity potential on the same footing in a unified framework [1], by diagonalizing the fully three-dimensional Schrödinger equation exactly on a 100 x 100 nm grid (however neglecting electron-electron interaction). This results in some thousand states. To facilitate comparison with existing experiments we calculate the infrared (IR) absorption spectra by properly evaluating the matrix elements between all relevant states, taking into account their proper thermal occupation.

These calculations have lead to some remarkable insights:

In a superlattice (SL; we use 20 QWs for the calculation) we show that there is not only a an excited 2pz state slightly below the second miniband [2], but rather an excited, resonant impurity band which partly overlaps the second miniband [1]. Consequently the low-temperature IR absorption spectrum in not too highly doped SLs exhibits a transition from the 1s ground state to this resonant impurity band, showing two maxima related to the band edges. This implies that there are impurity states not only attached to the lower miniband edge, but also to the higher miniband edge. The localized nature of the high-energy peak can only be identified by analyzing the final-state wavefunction in the xy plane, since it occurs at nearly the same spectral position as the kz=0 interminiband peak. This observation requires re-interpretation of previous interminiband absorption experiments and is important for the experimental diagnostics of the magnetic-field induced metal-insulator transition. Note that the excited impurity band does not result from impurity levels overlapping in the xy plane, but from the state coupling in z-direction, just like the miniband.

By studying the energy levels and IR spectra for systems with a varying number of quantum wells, we can map out the transition from coupled QWs to a SL, i.e. from sharp intersubband and/or impurity lines to band-like spectra including their van Hove singularities. In a coupled double QW, where the 1-3 intersubband transition is symmetry forbidden, the related impurity transition can nevertheless be observed.

A particularly spectacular example is provided by the analysis of previously not understood experimental IR absorption spectra of a quadruple QW system. The complex temperature dependent absorption pattern is reproduced by our theory with a remarkable degree of accuracy.

[1] D. Stehr et al., Phys. Rev. Lett. 95, 257401 (2005)
[2] M. Helm et al., Phys. Rev. B 48, 1601 (1993)

  • Poster
    28th International Conference on the Physics of Semiconductors, 24.-28.07.2006, Wien, Austria
  • Contribution to proceedings
    28th International Conference on the Physics of Semiconductors, 24.-28.07.2006, Wien, Austria
    AIP Conference Proceedings 893, 243-244

Publ.-Id: 8678

Use of models for lift, wall and turbulent dispersion forces acting on bubbles for poly-disperse flows

Lucas, D.; Krepper, E.; Prasser, H.-M.

Closure laws are needed for the qualification of CFD codes for two-phase flows. In case of bubbly and slug flow, forces acting on the bubbles usually model the momentum transfer between the phases. These forces depend on the liquid flow field as well as on the size and the shape of the bubbles. The validation of these forces base on a detailed experimental database for vertical pipe flows, which contains data on the radial distribution of bubbles of different size and local bubble size distributions. A one-dimensional model resolving the variables in radial direction and considering a large number of bubble size classes, is used for an analysis of several bubble force models. Results of this model were compared to results of 3D simulations done by the commercial code CFX-5.7 for simplified cases with only one bubble class. The effects of the number of bubbles classes as well as the effect of the lateral extension of the bubbles were analysed. For the validation of bubble force models measured bubble size distributions were taken as an input for the model. On basis of the assumption of equilibrium of the lateral bubble forces, radial volume fraction profiles were calculated separately for each bubble class. In the result of the validation of different models for the bubble forces, a set of Tomiyama lift and wall force, deformation force and Favre averaged turbulent dispersion force was found to reflect the experimental data with best agreement. Some discrepancies remain at high liquid superficial velocities.

Keywords: bubbly flow; bubble forces; CFD; pipe flow

Publ.-Id: 8677

Off-normal ion erosion of silicon – simultaneous observation of two ripple modes

Keller, A.; Roßbach, S.; Facsko, S.; Möller, W.

It is well known that oblique low and mediate energy (typically 0.1 – 100 keV) ion erosion of solid surfaces can lead to the formation of periodic ripple patterns with wavelength ranging from 10 to 1000 nm. These structures were found on a large variety of materials, such as semiconductors, metals, and insulating surfaces [1]. The first attempt to describe the formation of surface ripples was made by Bradley and Harper [2]. In their framework the ripples are either parallel or perpendicular to the direction of the ion beam, depending on the angle of incidence. This was also shown experimentally on HOPG [3].
In the presented work we studied the formation of ripples on Si(100) surface under sub-keV Ar+ bombardment using ex-situ AFM. In addition to previous experimental observations two simultaneous ripple modes are found with direction perpendicular and parallel to the ion beam, respectively. The dependence of each mode on ion energy, fluence, flux, and incidence angle is investigated. The two modes have wavelengths of some ten and a few hundred nanometres and show different temporal behaviour. Finally, our observations are compared to simulations based on the damped Kuramoto-Sivashinsky equation [4].

[1] M. A. Makeev et al., Nucl. Inst. Meth. Phys. Res. B 197 (2002), 185
[2] R. Bradley and J. Harper, J. Vac. Sci. Technol. A 6 (1988), 2390
[3] S. Habenicht et al., Phys. Rev. B 60 (1999), R2200
[4] S. Facsko et al., Phys. Rev. B 69 (2004), 153412

  • Lecture (Conference)
    22nd International Conference on Atomic Collisions in Solids, 21.-26.07.2006, Berlin, Deutschland

Publ.-Id: 8676

Computational studies on the design of an electromagnetic pump for the process of aluminum investment casting

Galindo, V.; Eckert, S.; Gerbeth, G.; Willers, B.

The mould filling process of aluminum investment casting consists basically of the liquid metal flow in a U-bend. It shows a high pouring velocity at the beginning and decreasing flow velocity values during the course of the process. The main problem is the occurrence of large velocity values at the beginning of the casting process, leading to an accumulated generation of vortices inside the pouring channel. A high rate of turbulence in the flow is supposed to entail the transported impurities, oxides or gas bubbles from the walls and the free surface, respectively, into the bulk of the casting patterns. As a result, the mechanical properties of the casting products are deteriorated.
We present results on the design and application of electromagnetic pumps to control the velocity of the aluminum melt during the pouring process. The induced electromagnetic force in the melt caused by the coil system were calculated using the commercial finite-element code OPERA-3d (Vector Fields Ltd.). The free surface problem which occurs in the riser of the casting unit was taken into account by a Volume-of-Fluids Method. Three-dimensional transient calculations using the commercial finite-element code FIDAP (FLUENT Inc.) were carried out for a simplified model system. Parallel to the simulations model experiments have been performed using the low melting eutectic GaInSn.
The casting unit was modeled by a perspex model [1], which also allows the monitoring of the filling process. Ultrasonic Doppler velocimetry measurements were applied in the model in order to validate the numerical calculations [2]. A comparison between numerical and experimental results showed an excellent agreement.
The main goal of this study is the optimization of the pump design depending on the geometry of the coil system in oder to achieve the best possible homogeneity in the electromagnetic force distribution with an integral flow rate of sufficient strength. Inhomogeneities in the force distribution can lead to undesirable fluid vortices in the melt, which on this part can induce a detriment of the material properties after solidification.


[1] A. Cramer, S. Eckert, V. Galindo, G. Gerbeth, W. Willers, W. Witke,
Liquid metal model experiments on casting and solidification processes
Journal of Materials Science 39 (2004) 7285-7294
[2] S. Eckert, G. Gerbeth, Th. Gundrum, F. Stefani, W. Witke,
New approaches to determine the velocity field in metallic melts,
Proceedings of the 4th Int. Conference on Electromagnetic Processing of
Materials (EPM 2003), Lyon (France), October 14-17, 2003, pp. 601-608

  • Lecture (Conference)
    7th World Congress on Computational Mechanics, 16.-22.07.2006, Los Angeles, California, United States

Publ.-Id: 8675

Nanostructures by ion beams

Schmidt, B.

Ion beam techniques, including conventional broad beam ion implantation, ion beam synthesis and ion irradiation of thin layers as well as local ion implantation with fine-focused ion beams (FIB) have been applied in different fields of micro- and nanotechnology. The ion beam synthesis of nanoparticels in high dose ion implanted solids is explained as phase separation of nanostructures from a super-saturated solid state through precipitation and Ostwald–ripening during subsequent thermal treatment of the ion implanted samples. A special topic will be addressed to self-organization processes of nanoparticles during ion irradiation of flat and curved solid state interfaces. As an example of silicon nanocrystal application the fabrication of silicon nanocrystal non-volatile memories will be described. Finally, the fabrication possibilities of nanostructures, like nanowires and chains of nanoparticles (e.g. CoSi2), by ion beam synthesis using a focused Co+ ion beam will be demonstrated and possible applications will be mentioned.

Keywords: Ion implantation; Ion beam synthesis; Ion irradiation; nanoclusters; nanowires

  • Radiation Effects and Defects in Solids 162(2007)3-4, 171-184
    DOI: 10.1080/10420150601132743
  • Invited lecture (Conferences)
    Workshop on Ion Beam Studies of Nanomaterials: Synthesis, Modification and Characterization, 26.06.-01.07.2006, Trieste, Italy

Publ.-Id: 8674

In-beam PET measurements of biological half-lives of {12}C irradiation induced beta+-activity

Fiedler, F.; Sellesk, M.; Crespo, P.; Jülich, R.; Parodi, K.; Pawelke, J.; Pönisch, F.; Enghardt, W.

One of the long-standing problems in carbon-ion therapy is the monitoring of the treatment, i.e. of the delivered dose to a given tissue volume within the patient. Over the last 8 years, in-beam positron emission tomography (PET) has been used at the experimental carbon ion treatment facility at the Gesellschaft für Schwerionenforschung (GSI) Darmstadt and has become a valuable quality assurance tool. In order to determine and evaluate the correct delivery of the patient dose, a simulation of the positron emitter distribution has been compared to the measurement. One particular effect is the blurring as well as the reduction of the measured activity distribution via washout. The objective of this study is the investigation of tissue dependent effective half lives from patient data. We find no significant dependence of the effective half life on the Hounsfield unit but on the local dose. The biological half-live within the high dose region is longer than in the low dose region. Furthermore, the influence of the overall treatment time on the kinetics of the positron emitter is reported. There are indications for a metabolic response of the tissue on the irradiation. Taking into account the biological half-life in the simulation leads to an improvement of the quality of the PET-images in some cases.

Keywords: in-beam PET; washout

Publ.-Id: 8673

Excited and ground state properties of LaSrMnO4: A combined x-ray spectroscopic study

Kuepper, K.; Klingeler, R.; Reutler, P.; Büchner, B.; Neumann, M.

The electronic properties of the parent compound of the single layered manganites La1−xSr1+xMnO4, namely LaSrMnO4 has been investigated in a detailed spectroscopic study.
We apply different complementary x-ray spectroscopic techniques in the soft x-ray regime. X-ray photoelectron spectroscopy and normal like x-ray emission spectroscopy were used to reveal a detailed picture of the total and partial densities of states in this compound. Furthermore we apply resonant x-ray emission spectroscopy to the Mn L2,3 edges. The spectra exhibit a rich multiplet structure. Resonant x-ray emission spectroscopy at the O K edge is used to study the local partial densities of states of the in plane and out of plane oxygen atoms. The results are discussed along available band structure calculations as well as charge transfer multiplet calculations.

Keywords: PACS numbers: 71.20.-b; 75.47.Lx; 78.70.En; 79.60.-i

Publ.-Id: 8672

One-step reductive etherification of 4-[18F]fluoro-benzaldehyde with decaborane

Funke, U.; Jia, H.; Fischer, S.; Scheunemann, M.; Steinbach, J.

Reductive coulling reactions between 4-[18F]fluoro-benzaldehyde ([18F]19 and different alcoholsby use of decaborane (B10H14)as reducing agent have the potential to synthesize 4-[18F]fluoro-benzylethers in one step. [18F]1 was sythesized from 4-trimethylammonium benzaldehyde (triflate salt) via a standard fluorination procedure (K[18F]F/Kryprofix 222) in dimethylformamide at 90°C for 25 min and purified by solid-phase extraction. Subsequently, reductive etherifications of [18F]1 were performed as one-step reactions with primary and secondary alcohols, mediated by B10H14 in acetonitrile at 60°C. Various 4-[18F]fluorobenzyl ethers (6 examples are shown) were obtained within 1-2h reaction time in decay-corrected radiochemical yields of 12-45%.

Keywords: 4[18F]fluoro-benzylether; fluorine-18; reductive etherification; decarborane; labelling

  • Journal of Labelled Compounds and Radiopharmaceuticals 49(2006), 745-755
    DOI: 10.1002/jlcr.1087

Publ.-Id: 8671

On the efficiency of MHD drag reduction

Gerbeth, G.; Shatrov, V.

Permanent magnets and high electric current densities are often used to achieve reasonable Lorentz forces for a magnetohydrodynamic flow control. This choice, however, usually leads to a low energetic efficiency for the flow control of seawater. We present results of direct numerical simulations of turbulent channel flow [1] drag reduction using electromagnetic forces. The Lorentz force is created by a permanent magnetic field and an electric current from electrodes placed on the bottom wall. The Reynolds number Re, the magnetic interaction parameter N and the load factor k are the control parameters of the problem.
In the first case we used a spanwise oscillating force. The efficiency is the ratio between saved and used power [2]. We found that the efficiency at the load factor k = 4 is about 100 times larger than at k = 1000.
In the second case a streamwise Lorentz force [3] was used. The flow is accelerated near the bottom and the thrust force applied to the bottom wall is directed against the mean flow. The energetic efficiency is the ratio between the mechanical power necessary to move the flat plate with a given velocity and the used electric power. The efficiency is more than 0.9 at k = 0.8.
The main result is that a significant efficiency improvement is possible if load factors k in the order of 1 are used.
1. R.D. Moser, J. Kim and Nagi N. Mansour, "Direct numerical simulation of turbulent channel flow up to Ret = 590," Phys. Fluids, v. 11, no. 4, p. 943-945, 1999.
2. T.W. Berger, J. Kim, C. Lee and L. Lim, "Turbulent boundary layer control utilizing the Lorentz force," Phys. Fluids, v. 12, no. 3, p. 631-649, 2000.
3. A. Gailitis and O. Lielausis, "On the possibility of drag reduction of a flat plate in an electrolyte," Applied Magnetohydrodynamics, Rep. of the Phys. Inst., AN Latv. SSR, v. 12, p. 143-146, (in Russ.), 1961.

Keywords: MHD; drag reduction

  • Lecture (Conference)
    7th World Congress on Computational Mechanics, 17.-22.07.2006, Los Angeles, United States

Publ.-Id: 8670

The GDT as neutron source in a sub-critical system for transmutation?

Noack, K.; Rogov, A.; Ivanov, A. A.; Kruglyakov, E. P.

In the last decade, a great progress was made in developing projects of sub-critical fission systems dedicated to transmutation of nuclear waste. In contrast to a fission reactor, such a device is fed with neutrons from an outer source in order to sustain a steady-state power generation. The Budker Institute of Nuclear Physics has made the proposal of a powerful 14 MeV neutron source based on a gas dynamic trap (GDT). This neutron source is primarily thought as irradiation facility for fusion material research. So, the question raises, whether the GDT based neutron source could be a candidate to efficiently drive such a sub-critical system too. The contribution pursues this question using results of first neutron transport calculations. The calculations were made for a simplified model of an actinides burner, which has been developed for an international benchmark exercise performed under the auspices of the Nuclear Energy Agency (NEA) of the Organization for Economic Cooperation and Development (OECD). Important parameters of the burner are compared for two cases - when driven by a spallation or by the GDT neutron source. From this comparison some advices for further improvements of the GDT neutron source are concluded.

Keywords: transmutation of nuclear waste; sub-critical system; minor actinides burner; plasma neutron source; gas dynamic trap

  • Invited lecture (Conferences)
    Open Magnetic Systems for Plasma Confinement, 17.-21.07.2006, Tsukuba, Japan
  • Fusion Science and Technology 51(2007), 65-68

Publ.-Id: 8668

Spin-up and spin-down dynamics driven by a single rotating magnetic field pulse

Nikrityuk, P. A.; Eckert, S.; Eckert, K.

This paper presents a study concerning the transient dynamics of the flow field inside a liquid metal column created by an application of a rotating magnetic field (RMF) in form of a single pulse. The flow structure is governde by an impulsive spin-up form the rest state which is followed by a spin-down phase under the inertia of the fluid. A distinctive influence of pulse length the transient fluid flow has been found. Our results reveal that the recirculating flow in the radial-meridional plane shows periodic reversals. This phenomena is especially pronounced if the pulse length of the electromagnetic forcing corresponds to the so-called initial adjustment phase.

Keywords: electromagnetic stirring; rotating magnetic field; UDV flow measurment

Publ.-Id: 8667

Synthesis and evaluation of novel multimeric neurotensin(8-13) analogs

Hultsch, C.; Pawelke, B.; Bergmann, R.; Wüst, F.

Neurotensin(8–13) is a hexapeptide with subnanomolar affinity to the neurotensin receptor 1 which is expressed with high incidence in several human tumor entities. Thus, radiolabeled neurotensin(8–13) might be used for tumor targeting. However, its application is limited by insufficient metabolic stability. The present study aims at improving metabolic stability by the synthesis of multimeric neurotensin(8–13) derivatives rather than commonly employed chemical modifications of the peptide itself. Thus, different dimeric and tetrameric peptides carrying C- or N-terminal attached neurotensin(8–13) moieties have been synthesized and their binding affinity toward the neurotensin receptor has been determined. The results demonstrate that branched compounds containing neurotensin(8–13) attached via its C-terminus only show low receptor affinities, whilst derivatives with neurotensin(8–13) attached via the N-terminus show IC50 values in the nanomolar range. Moreover, within the multimeric neurotensin(8–13) derivatives with neurotensin(8–13) attached via the N-terminus an increasing number of branching units lead to higher binding affinities toward the neurotensin receptor.

Keywords: Neurotensin; Multimer; Peptides; Tumor targeting

  • Bioorganic & Medicinal Chemistry 14(2006)17, 5913-5920

Publ.-Id: 8666

Flow control in bubble-driven liquid metal flows by means of external magnetic fields

Zhang, C.; Eckert, S.; Gerbeth, G.

In our experiments we investigated the consequences of an application of both DC and AC magnetic fields on the velocity field of bubble plumes. A restructuring of the entire flow field can be observed if a bubble plume is exposed to a DC magnetic field. The application of a transverse field leads not only to a general damping of the flow, but also favours the occurrence of vortices aligned parallel to the magnetic field direction. AC magnetic fields can be applied to generate flow structures being different from the recirculating flow known from classical bubble plumes. A tailoring of the flow using magnetic fields obviously allows a control of the heat and mass transfer in bubble plumes.

Keywords: bubble plume; DC magnetic field; AC travelling magnetic field; electromagnetic stirring; velocity measurements; ultrasound Doppler technique

  • Contribution to proceedings
    EPM2006, 5th International. Conference on Electromagnetic Processing of Materials, 23.-27.10.2006, Sendai, Japan
    Proceedings: The Iron and Steel Institue of Japan (ISIJ), 4-930980-55-0 C3057, 242-247
  • Lecture (Conference)
    EPM2006, 5th International. Conference on Electromagnetic Processing of Materials, 23.-27.10.2006, Sendai, Japan

Publ.-Id: 8665

Velocity measurements in metallic melts driven by AC magnetic fields in a square vessel

Pal, J.; Eckert, S.; Zhang, C.; Gerbeth, G.

Model experiments are an important tool to understand the details of the flow structure and the transport properties of flows occurring in real-scale metallurgical facilities as well as to validate the multitude of numerical codes for flow simulation. The present study considers the electromagnetic stirring of a liquid metal by a rotating magnetic field (RMF). Measurements of the velocity field in a square column filled with the eutectic GaInSn have been carried out utilising the ultrasound Doppler velocimetry (UDV). In this paper the spatiotemporal properties of both the primary, swirling and the secondary flow arising from a sinusoidal modulation of the RMF will be presented.

Keywords: electromagnetic stirring; rotating magnetic field; velocity measurements; ultrasound Doppler technique

  • Contribution to proceedings
    5th International Conference on Electromagnetic Processing of Materials, 23.-27.10.2006, Sendai, Japan
    Proceedings: The Iron and Steel Institute of Japan (ISIJ), 4-930980-55-0 C3057, 725-730
  • Lecture (Conference)
    EPM 2006, 23.-27.10.2006, Sendai, Japan

Publ.-Id: 8664

The influence of electromagnetically-driven convection on temperature field and solute distribution during directional solidification

Willers, B.; Nikrityuk, P. A.; Eckert, S.; Eckert, K.

Solidification experiments as well as numerical simulations were carried out considering the directional solidification of Pb Sn alloys from a water cooled copper chill. A rotating magnetic field (RMF) was applied for melt agitation. The comparison between numerical simulations and solidification experiments delivered a good agreement. Our results disclose that the forced convection causes distinct modifications of the temperature and concentration field. The electromagnetic stirring promotes the columnar-to-equiaxed transition and causes a considerable grain refinement. Indications for a flow-induced fragmentation have been found in the microstructure of the stirred samples.

Keywords: solidification; fluid flow; electromagnetic stirring; rotating magnetic field

  • Contribution to proceedings
    EPM2006, 5th Int. Conference on Electromagnetic Processing of Materials, 23.-27.10.2006, Sendai, Japan
    Proceedings: The Iron and Steel Institute of Japan (ISIJ), 4-930980-55-0 C3057, 407-412
  • Lecture (Conference)
    EPM2006, 5th International Symposium on Electromagnetic Processing of Materials, 23.-27.10.2006, Sendai, Japan

Publ.-Id: 8663

Buoyancy driven coolant mixing studies of natural circulation flows at the ROCOM test facility using ANSYS CFX

Höhne, T.; Kliem, S.; Rohde, U.; Weiss, F.-P.

Coolant mixing in the cold leg, downcomer and the lower plenum of pressurized water reactors is an important phenomenon mitigating the reactivity insertion into the core. Therefore, mixing of the de-borated slugs with the ambient coolant in the reactor pressure vessel was investigated at the four loop 1:5 scaled ROCOM mixing test facility. Thermal hydraulics analyses showed, that weakly borated condensate can accumulate in particular in the pump loop seal of those loops, which do not receive safety injection. After refilling of the primary circuit, natural circulation in the stagnant loops can re-establish simultaneously and the de-borated slugs are shifted towards the reactor pressure vessel (RPV).
In the ROCOM experiments, the length of the flow ramp and the initial density difference between the slugs and the ambient coolant was varied. From the test matrix experiments with 0 resp. 2% density difference between the de-borated slugs and the ambient coolant were used to validate the CFD software ANSYS CFX. To model the effects of turbulence on the mean flow a higher order Reynolds stress turbulence model was employed and a mesh consisting of 6.4 million hybrid elements was utilized. Only the experiments and CFD calculations with modeled density differences show a stratification in the downcomer. Depending on the degree of density differences the less dense slugs flow around the core barrel at the top of the downcomer. At the opposite side the lower borated coolant is entrained by the colder safety injection water and transported to the core. The va¬li¬¬dation proves that ANSYS CFX is able to simulate appropriately the flow field and mixing effects of coolant with different densities.

Keywords: PWR; Boron Dilution; ROCOM

  • Contribution to proceedings
    14th International Conference on Nuclear Engineering, 16.-20.07.2006, Miami, United States
    CD-ROM, paper ICONE 14- 89120
  • Lecture (Conference)
    14th International Conference on Nuclear Engineering, 16.07.2006, Miami, United States

Publ.-Id: 8662

Competition between damage buildup and dynamic annealing in ion implantation into Ge

Posselt, M.; Bischoff, L.; Grambole, D.; Herrmann, F.

Channeling implantation of Ga into Ge is performed at two very different ion fluxes (1011 and 1019 cm-2 s-1), at two temperatures (room temperature and 250 0C), and at five different fluences. The fluence dependence of the range profiles and of the implantation damage is strongly influenced by defect accumulation and dynamic annealing. At 250 0C a significant reduction of the ion-beam-induced defects is already observed 1 s after an ion impact, and the maximum lifetime of the defects is less than 10 s. On the other hand, at room temperature no significant annealing is found within the first 10 s after ion impact. The measured Ga depth profiles are reproduced very well by atomistic computer simulations.

Keywords: Germanium; ion implantation; defects; computer simulation

  • Applied Physics Letters 89(2006), 151918

Publ.-Id: 8661

Alkaline earth elements in the uranium phosphate system - a TRLFS study

Geipel, G.

wird nachgereicht

  • Lecture (others)
    Kolloquium, 01.08.2006, Tokyo, Japan

Publ.-Id: 8659

Uranium speciation in water - lectures learned from the nature

Geipel, G.

wird nachgereicht

  • Lecture (others)
    Seminar, 31.07.2006, Tokyo, Japan

Publ.-Id: 8658

Interaction of Cu(II) with dendritic ligands studied by TRLFSand some remarks on luminescence of uranium(V)

Geipel, G.

wird nachgereicht

  • Lecture (others)
    Seminar, 01.08.2006, Tokyo, Japan

Publ.-Id: 8657

Evaluation of Research Institutes in Germany - Basics, Procedures, Results

Joehnk, P.

  • Invited lecture (Conferences)
    Workshop "Mechanism and criteria for efficiency of evaluation of research institutions", 14.07.2006, Moskva, Russia

Publ.-Id: 8656

Systematics of pion emission in heavy ion collisions in the 1A GeV regime

Reisdorf, W.; Stockmeier, M.; Andronic, A.; Benabderrahmane, M. L.; Hartmann, O. N.; Herrmann, N.; Hildenbrand, K. D.; Kim, Y. J.; Kis, M.; Koczon, P.; Kress, T.; Leifels, Y.; Lopez, X.; Merschmeyer, M.; Schüttauf, A.; Alard, J. P.; Barret, V.; Basrak, Z.; Bastid, N.; Caplar, R.; Crochet, P.; Dupieux, P.; Dzelalija, M.; Fodor, Z.; Grishkin, Y.; Hong, B.; Kang, T. I.; Kecskemeti, J.; Kirejczyk, M.; Korolija, M.; Kotte, R.; Lebedev, A.; Matulewicz, T.; Neubert, W.; Petrovici, M.; Rami, F.; Ryu, M. S.; Seres, Z.; Sikora, B.; Sim, K. S.; Simion, V.; Siwek-Wilczynska, K.; Smolyankin, V.; Stoicea, G.; Tyminski, Z.; Wisniewski, K.; Wohlfarth, D.; Xiao, Z.; Xu, H. S.; Yushmanov, I.; Zhilin, A.

Using the large acceptance apparatus FOPI, we study pion emission in the reactions (energies in A GeV are given in parentheses): 40Ca+40Ca (0.4, 0.6, 0.8, 1.0, 1.5, 1.93), 96Ru+96Ru (0.4, 1.0, 1.5), 96Zr+96Zr (0.4, 1.0, 1.5), 197Au+197Au (0.4, 0.6, 0.8, 1.0, 1.2, 1.5). The observables include longitudinal and transverse rapidity distributions and stopping, polar anisotropies, pion multiplicities, transverse momentum spectra, ratios (pi+/pi-) of average transverse momenta and of yields, directed flow, elliptic flow. The data are compared to earlier data where possible and to transport model simulations.

Publ.-Id: 8654

Synthesis of metal nanoparticles using bacterial surface layers (S-layers) as biological templates

Pollmann, K.; Merroun, M.; Raff, J.; Fahmy, K.; Hennig, C.; Selenska-Pobell, S.; Herrmannsdörfer, T.

The regular structured paracrystalline surface layers (S-layers), which are one of the most common surface structures found in bacteria and archaea, are promising biological templates for the production of metal nanoclusters. Most of them are composed of protein monomers with the ability to self assemble in two-dimensional arrays. They possess regularly arranged pores of identical size that enable the exchange of ions and small molecules between the living cells and their environment.
In the present work the purified S-layer sheets of the uranium mining waste pile isolate B. sphaericus JG-A12 were used as biological template to produce Pd, Au, and Pt nanoparticles of a size of 0.8-1 nm. Such metal nanoparticles may have strongly altered properties compared to their bulk counterparts making them interesting for the development of new materials and applications in the field of catalysis. The deposition of metal nanoclusters was performed via a two step process, consisting of biosorption by exposition of the S-layer proteins to a hydrolyzed solution of metal salt (I) and of metal reduction (II) by the addition of a strong reducing agent. The interaction of Pd(II) with the S-layer of B. sphaericus JG-A12 and the formed Pd-nanoclusters were analyzed using EXAFS-spectroscopy and ATR-FT-IR-spectroscopy. The analyses demonstrated the complexation of Pd(II) to carboxyl groups. The metal nanoclusters were investigated using EXAFS spectroscopy and SQUID magnetometry. The Pd and Pt nanoclusters reveal interesting magnetic properties in a wide temperature and magnetic field range.

  • Poster
    E-MRS 2006 Spring Meeting, 29.05.-02.06.2006, Nice, Frankreich

Publ.-Id: 8653

Defects in N+ ion-implanted ZnO single crystals studied by positron annihilation and Hall effect

Brauer, G.; Anwand, W.; Skorupa, W.; Kuriplach, J.; Melikhova, O.; Cizek, J.; Prochazka, I.; von Wenckstern, H.; Brandt, M.; Lorenz, M.; Grundmann, M.

High quality ZnO single crystals of dimensions 10 x 10 x 0.5 mm3, grown by a hydrothermal approach, have been implanted by 40 keV N+ ions to a fluence of 1 x 1015 cm-2 at room temperature. Their proper-ties revealed by positron annihilation and Hall effect measurements are given in the as-grown and as-irradiated states, and after post-implantation annealing in an oxygen ambient at 200 °C and 500 °C.

Keywords: positron annihilation; zinc oxide; ion implantation; Hall effect; defects; annealing

  • Physica Status Solidi (C) 4(2007), 3642-3645
  • Lecture (Conference)
    14th International Conference on Positron Annihilation (ICPA-14), 23.-28.07.2006, Hamilton/Ontario, Canada
  • Lecture (Conference)
    The 4th International Wokshop on ZnO and Related Materials, 03.-06.10.2006, Giessen, Deutschland
  • Lecture (Conference)
    19th International Conference on the Application of Accelerators in Research and Industry (CAARI2006), 20.-25.08.2006, Ft. Worth/Texas, USA

Publ.-Id: 8652

Electrical characterization of deep acceptor states in nitrogen implanted ZnO single crystals

von Wenckstern, H.; Schmidt, H.; Pickenhain, R.; Biehne, G.; Brandt, M.; Brauer, G.; Lorenz, M.; Grundmann, M.

nicht vorhanden

  • Lecture (Conference)
    E-MRS Spring Meeting 2006, 29.06.-02.07.2006, Nice, France

Publ.-Id: 8651

Optical and microelectrical characterization of ZnO single crystals implanted with group V elements

Brandt, M.; von Wenckstern, H.; Benndorf, G.; Lenzner, J.; Schmidt, H.; Lorenz, M.; Grundmann, M.; Braunstein, G.; Brauer, G.

nicht vorhanden

  • Lecture (Conference)
    Frühjahrstagung der Deutschen Physikalischen Gesellschaft, 04.-09.03.2006, Dresden, Deutschland

Publ.-Id: 8650

Electrical spectroscopy of acceptor states in N+ implanted ZnO single crystals

von Wenckstern, H.; Schmidt, H.; Pickenhain, R.; Biehne, G.; Brandt, M.; Brauer, G.; Lorenz, M.

nicht vorhanden

  • Lecture (Conference)
    DPG Spring Meeting, 04.-09.03.2006, Dresden, Deutschland

Publ.-Id: 8649

Recent results of slow positron implantation spectroscopy to characterize vacancy-type damage in ion-implanted 6H-SiC

Brauer, G.

nicht vorhanden

  • Lecture (Conference)
    Seminar an der University of Hong Kong, Physics Department, 13.01.2006, Hong Kong, China
  • Lecture (Conference)
    Seminar an der Chinese University of Hong Kong, Department of Electronic Engineering, 17.01.2006, Hong Kong, China

Publ.-Id: 8648

Die intensive Positronenquelle EPOS am Forschungszentrum Rossendorf

Krause-Rehberg, R.; Brauer, G.

nicht vorhanden

  • Lecture (others)
    Kolloquium der Friedrich-Schiller-Universität Jena, 02.12.2005, Jena, Deutschland

Publ.-Id: 8647

Verwendung von Mikroorganismen zur Synthese von Nanopartikeln

Pollmann, K.

Mikroorganismen können in verschiedenster Weise für die Synthese von Nanopartikeln genutzt werden. Hierzu gehören die enzymatische Reduktion von Metallionen durch Bakterien und die Verwendung von Biomolekülen wie DNA, Aktinfilamente, S-Layer-Proteinen als Matrizes für die Synthese. Der Vortrag gibt einen Überblick über diese Mechanismen, die Verwendungsmöglichkeiten und unsere bisherigen Arbeiten auf diesem Gebiet.

  • Lecture (others)
    Mikrobiologisches Kolloquium, 03.07.2006, Mainz, Deutschland

Publ.-Id: 8646

Influence of hydrogen on the toughness of irradiated reactor pressure vessel steels

Müller, G.; Uhlemann, M.; Ulbricht, A.; Böhmert, J.

The influence of hydrogen on the mechanical behaviour of different reactor pressure vessel (RPV) steels was investigated by tensile tests in correla-tion to the chemical composition, the neutron fluence, the hydrogen charg-ing condition, the strain rate, and the temperature. Small-angle neutron scattering (SANS) experiments, hydrogen analyses and thermal desorp-tion investigations were performed to prove the evidence of hydrogen trapping at irradiation defects. An increasing susceptibility to hydrogen embrittlement indicated by reduction of area was observed at room tem-perature with in-situ hydrogen-charged specimens when loaded by low strain rates or with specimens, which had been irradiated at low tempera-ture. Generally, the susceptibility increases with increasing strength of the steels. At 250°C hydrogen embrittlement was not evident. The results do neither prove that irradiation defects are favoured traps for hydrogen nor give evidence that hydrogen affects the RPV integrity under normal oper-ating conditions.

Keywords: hydrogen embrittlement; reactor pressure vessel steel; irradiation; fluence; tensile test; toughness; hydrogen charging

Publ.-Id: 8645

Preparation and Biological Characterization of Isomeric 188Re(V) Oxocomplexes with Tetradentate S4 Ligands Derived from DMSA for Labeling of Biomolecules

Seifert, S.; Heinrich, T.; Jentschel, C.; Smuda, C.; Bergmann, R.; Pietzsch, H.-J.

A new type of tetradentate S4 ligand has been synthesized by bridging two molecules of meso 2,3-dimercapto succinic acid for stable binding and easy conjugation of rhenium-188 to biologically interesting structures. The stereoisomeric tetrathiolato S4 ligands form very robust anionic five-coordinated oxorhenium(V) and oxotechnetium(V) complexes. Two routes for the preparation of the 188Re(V) oxocomplexes with (iBu)2N(O)C-C(SH)-C(SH)-C(O)NH-(CH2)3–NH-(CH2)3–NHC(O)-C(SH)-C(SH)-C(O)N(iBu)2 (ligand 1) and its hydrophilic crown ether derivative (ligand 2) were tested and optimized. Several isomers were separated by HPLC from the preparation solutions and characterized in vitro and in vivo. The identity of the species obtained was determined by comparison with the HPLC profiles of reference 185/187Re analogues which were characterized by ESI-MS. All of them were absolutely stable in rat and human plasma solutions. Challenge experiments with cysteine corroborated the high inertness of the isomers towards ligand exchange reactions. Various in vivo samples, taken off at different times from blood, intestine and urine of rats, confirmed the high in vivo stability of the 188Re-S4 complexes. Biodistribution studies using male Wistar rats were performed and resulted in a high uptake and fast clearance from the liver of the more lipophilic cis and trans isomers of complex I (log Po/w between 1.5 and 1.7), whereas the isomers of the hydrophilic complex II (log Po/w about -1.75) were rapidly excreted via the renal and the hepatobiliary pathway. The low level of radioactivity in the stomach confirms good in vivo stability. Thus, these new 188Re-S4 complexes fulfill the requirements for a stable and high specific activity labeling of biomolecules with rhenium-188.

Publ.-Id: 8644

Safety aspects of the process control of Grignard reactions

Kryk, H.; Hessel, G.; Schmitt, W.; Tefera, N.

Grignard reactions comprise considerable hazard potentials due to the spontaneous heat release during the initiation of the exothermic reactions and the high reactivity of the Grignard compounds. To establish industrially applicable methods for an objective detection of the reaction start-up and for the accumulation of the organic halide during the process, calorimetric studies of a special Grignard reaction in a pressurised vessel were carried out using several on-line monitoring methods. In general, the process signal profiles, FTIR measurements and balance-based on-line monitoring systems are applicable to provide the operator with additional information on the process state. Further experiments at adiabatic and isothermal conditions show significant influences of impurities (i.e. water) on the thermal process behaviour.

Keywords: Chemical processes; Grignard reaction; On-line monitoring; Process control; Safety; Chemical analysis

  • Poster
    ISCRE 19, 19th International Symposium on Chemical Reaction Engineering, 03.-06.09.2006, Potsdam, Germany
  • Contribution to proceedings
    ISCRE 19, 19th International Symposium on Chemical Reaction Engineering, 03.-06.09.2006, Potsdam, Germany
    Book of Abstracts, 362-363
  • Chemical Engineering Science 62(2007), 5198-5200

Publ.-Id: 8643

Colloid Generation and Stability in Simulated Reduced Groundwaters

Zänker, H.; Weiß, S.; Sachs, S.

The focus was on the investigation of U(IV) colloids during this phase of the project. First, a technique to reduce U(VI) to U(IV) was developed. Colloid formation and solubility of U(IV) in acidic HClO4/NaClO4 solutions were investigated by coulometric titration. Quantification of traces of U(VI) by laser fluorescence spectroscopy proved that the tetravalent state of uranium had been maintained. Laser-induced breakdown detection (LIBD) was applied for the detection of traces of uranium colloids as the pH was increased. The pH values at the onset of colloid formation were used for thermodynamic calculations aimed at determining the solubility products of crystalline and amorphous uranium dioxides. In acidic solutions, the UO2·xH2O(am) colloids showed colloidal stability over a period of at least 10 months during which they did not aggregate and precipitate. Relatively high zeta potentials were found for the UO2 colloids in the acidic and in the alkaline pH regions; the point of zero charge was at 6.9. No indications of the formation of mixed U(IV)-Al(III) complexes were found. There was also no influence of Al(III) on the UO2(cr) solubility.

Keywords: Uranium(IV); intrinsic colloids; pseudocolloids

  • Lecture (others)
    2nd FUNMIG RTDC-2 Progress Meeting Karlsruhe, August 2, 2006, 02.08.2006, Karlsruhe, Germany

Publ.-Id: 8641

Synthesis and biological evaluation of tricarbonyl Re(I) and Tc(I) complexes anchored by poly(azolyl)borates: application on the design of radiopharmaceuticals for the targeting of 5-HT1A receptors

Garcia, R.; Gano, L.; Maria, L.; Paulo, A.; Santos, I.; Spies, H.

The building blocks fac-[99mTc{kappa3-HB(timMe)3}(CO)3] and fac-[99mTc{kappa3-R(mu-H)B(timMe)2}(CO)3] [R is H (4a), Ph (5a); timMe is 2-mercapto-1-methylimidazolyl] were obtained almost quantitatively by reacting fac-[99mTc(CO)3(H2O)3]+ with the corresponding scorpionate. These compounds cross the intact blood-brain barrier in mice, with significant retention in the case of 4a and 5a. Using 4a as the lead structure, we have synthesized the functionalized complexes fac-[M{kappa3-H(mu-H)B(timBu-pip)2}(CO)3] [M is Re (8), 99mTc (8a); timBu-pip is methyl[4-((2-methoxyphenyl)-1-piperazinyl)butyl](2-mercapto-1-methylimidazol-5-yl)methanamide] and fac-[M{kappa3-H(mu-H)B(timMe)(timBu-pip)}(CO)3] [M is Re (9), 99mTc (9a)] and evaluated their potential as radioactive probes for the targeting of brain 5-HT1A serotonergic receptors. The Re complexes exhibit excellent affinity [IC50=0.172 +/- 0.003 nM (8); IC50=0.65 +/- 0.01 nM (9)] for the 5-HT1A receptor. The radioactive congeners (99mTc) have shown an initial brain uptake of 1.38 +/- 0.46%ID g-1 (8a) and 0.43 +/- 0.12%ID g-1 (9a), but suffer from a relatively fast washout.

Keywords: Technetium; Rhenium; Poly(azolyl)borates; 5HT1A receptor; Bivalent approach

Publ.-Id: 8640

Properties of p- and w-mesons in dense and hot nuclear matter near the critical pion mode softening

Bunatian, G.; Kämpfer, B.

  • Open Access Logo Forschungszentrum Rossendorf; FZR 08 1992
    ISSN: 1436-3976


Publ.-Id: 8639

Velocity measurements and concentration field visualizations in copper electrolysis under the influence of Lorentz forces and buoyancy

Weier, T.; Cierpka, C.; Hüller, J.; Gerbeth, G.

Velocity measurements and shadowgraph visualizations for copper electrolysis under the influence of a magnetic field are reported. Experiments in a rectangular cell show the expected strong correlation between flow features and limiting current density. The flow can be understood as driven by the interplay of Lorentz forces and buoyancy. For a cylindrical cell with only slightly non-parallel electric and magnetic field lines, the presence and importance of the Lorentz force is demonstrated by velocity measurements.

Keywords: electrolysis; Lorentz force; copper deposition

  • Magnetohydrodynamics 42(2006)4, 379-387

Publ.-Id: 8638

Development of dendritic ligands for binding metallic radionuclides

Röhrich, A.; Noll, S.; Stephan, H.; Steinbach, J.; Bergamini, G.; Balzani, V.

Two star-like cyclam derivatives 1 and 2 possessing four sugar units on the periphery have been prepared. UV-vis measurements point to slow formation of stable 1:1 complexes of Cu(II) with 1 and 2. Preliminary studies show that the dendritic ligands also complex 99mTc.

  • Poster
    7th International Symposium on Technetium in Chemistry and Nuclear Medicine, 06.-09.09.2006, Bressanone, Italy
  • Contribution to external collection
    U. Mazzi: Technetium, Rhenium and Other Metals in Chemistry and Nuclear Medicine, Padova: SGEditoriali, 2006, 88-89884-04-5, 233-234

Publ.-Id: 8637

Synthesis, characterization and evaluation of novel chelating agents for copper radionuclides

Stephan, H.; Juran, S.; Walther, M.; Steinbach, J.; Born, K.; Comba, P.

Three different hexadentate bispidine ligands L4 – L6 have been prepared. Studies which have been performed to label these ligands with 67Cu indicate a rapid formation of stable complexes under mild conditions (room temperature, aqueous solution). Challenge experiments of these complexes in the presence of a high excess of competing ligands such as glutathione, histidine and cyclam gave no evidence of radiocopper exchange. The 67Cu complexes of the bispidine ligands investigated are also stable in rat plasma at least for 24 h.

  • Poster
    7th International Symposium on Technetium in Chemistry and Nuclear Medicine, 06.-09.09.2006, Bressanone, Italy
  • Contribution to external collection
    U. Mazzi: Technetium, Rhenium and Other Metals in Chemistry and Nuclear Medicine, Padova: SGEditoriali, 2006, 88-89884-04-5, 219-222

Publ.-Id: 8636

Stability aspects of higher dimensional gravitational models with nonlinear scalar curvature coupling R4

Günther, U.

Subject of the talk is a multidimensional gravitational model with scalar curvature nonlinearity R4. It is assumed that the higher dimensional spacetime manifold of this model undergoes a spontaneous compactification to a manifold with warped product structure. The main attention is paid to the stability of the extra-dimensional factor spaces and it is shown that for certain parameter regions the system allows for a freezing stabilization of these spaces. The most interesting fact, which is demonstrated, is a dependence of the stability region (in parameter space) on the total dimension D=dim(M) of the higher dimensional spacetime M. For D>8 the stability region consists of a single (absolutely stable) sector which is shielded from a conformal singularity (and an antigravity sector beyond it) by a potential barrier of infinite height and width. This sector is smoothly connected with the stability region of a curvature-linear model. For D<8 an additional (metastable) sector exists which is separated from the conformal singularity by a potential barrier of finite height and width so that systems in this sector are prone to collapse into the conformal singularity. This second sector is not smoothly connected with the first (absolutely stable) one. Several limiting cases and the possibility for inflation are discussed. The talk is mainly based on:
Class. Quantum Grav. 22, (2005), 3135-3167, hep-th/0409112

Keywords: cosmology; higher dimensional gravity; stabilization of extra dimensions; conformal singularity; gravitational collapse

  • Invited lecture (Conferences)
    Eleventh Marcel Grossmann Meeting on recent developments in theoretical and experimental general relativity, gravitation and relativistic field theories., 23.-29.07.2006, Berlin, Germany

Publ.-Id: 8635

Mn-silicide nanoparticles: the origin of ferromagnetism in Mn-implanted Si?

Zhou, S.; Potzger, K.; Zhang, G.; Mücklich, A.; Eichhorn, F.; Grötzschel, R.; Schmidt, B.; Skorupa, W.; Helm, M.; Fassbender, J.

Recently Si and Ge semiconductors doped with Mn are reported to be ferromagnetic [1, 2]. The ferromagnetism is attributed to the coupling between Mn ions and carriers. However some MnxSiy and MnxGey compounds are ferromagnetic. A careful structural analysis is crucial to clarify the ferromagnetism in Mn doped Si or Ge. In this work, Mn ions were introduced to p-Si by ion implantation. Structural and ferromagnetic properties of Mn-implanted Si were investigated by Rutherford backscattering, x-ray diffraction (XRD), transmission electron microscopy (TEM) and superconducting quantum interference device magnetometer (SQUID). Synchrotron irradiation XRD and TEM revealed the formation of Mn4Si7 nanoparticles already in the as implanted samples (Fig. 1). Depending on the Mn-fluence they grew up from 5 nm to 20 nm upon rapid thermal annealing. No significant evidence is found for Mn substituting Si sites either in the as-implanted or annealed samples. By SQUID, hysteretic loops were observed, while the coecivity and remanence are drastically decreased with increasing temperature. The magnetization upon zero-field cooled and field cooled procedure shows a characteristic of a magnetic nanoparticle system. Together with structural analysis, we attributed the observed ferromagnetism to Mn-silicides nanoparticles.

(1) Y. D. Park, et al., Science 295, 651 (2002).
(2) M. Bolduc, et al., Phys. Rev. B 71, 033302 (2005).

  • Poster
    IBMM 2006, 18.-22.09.2006, Taormina, Italy

Publ.-Id: 8634

A brane model, its AdS-dS states and their agitated extra dimensions

Günther, U.; Vargas Moniz, P.; Zhuk, A.

We consider multidimensional gravitational models with a nonlinear scalar curvature term and form fields. It is assumed that the higher dimensional spacetime undergoes a spontaneous compactification to a warped product manifold. Particular attention is paid to models with quadratic scalar curvature terms and a Freund-Rubin-like ansatz for solitonic form fields. It is shown that for certain parameter ranges the extra dimensions are stabilized for any sign of the internal space curvature, the bulk cosmological constant and of the effective four-dimensional cosmological constant. Moreover, the effective cosmological constant can satisfy the observable limit on the dark energy density.

Keywords: cosmology; higher dimensional gravity; stabilization of extra dimensions

Publ.-Id: 8633

Remarks on dimensional reduction of multidimensional cosmological models

Günther, U.; Zhuk, A.

Multidimensional cosmological models with factorizable geometry and their dimensional reduction to effective four-dimensional theories are analyzed on sensitivity to different scalings. It is shown that a non-correct gauging of the effective four-dimensional gravitational constant within the dimensional reduction results in a non-correct rescaling of the cosmological constant and the gravexciton/radion masses. The relationship between the effective gravitational constants of theories with different dimensions is discussed for setups where the lower dimensional theory results via dimensional reduction from the higher dimensional one and where the compactified space components vary dynamically.

Keywords: cosmology; higher dimensional gravity; stabilization of extra dimensions

Publ.-Id: 8632

Boron Dilution Transients during natural circulation flow in PWR – experiments and CFD simulations

Höhne, T.; Kliem, S.; Rohde, U.; Weiss, F.-P.

Partial depletion of the primary circuit during a postulated small break loss of coolant accident can lead to the interruption of one-phase flow natural circulation. In this case, the decay heat is removed from the core in the reflux-condenser mode. In case of the partial non-availability of the safety injection system, weakly borated condensate can accumulate in particular in the pump loop seal of those loops, which do not receive this safety injection. After refilling of the primary circuit, natural circulation re-establishes and the lower-borated slugs are shifted towards the reactor pressure vessel (RPV). Mixing in the downcomer and the lower plenum is an important phenomenon mitigating the reactivity insertion into the core in this postulated scenario. Therefore, mixing of the lower-borated slugs with the ambient coolant in the RPV was investigated at the four loop 1:5 scaled ROCOM mixing test facility. In these experiments, the length of the flow ramp and the initial density difference between the slugs and the ambient coolant was varied. From the test matrix an experiment with 2% density difference between the de-borated slugs and the ambient coolant was used to validate the CFD software ANSYS CFX. To model the effects of turbulence on the mean flow a Reynolds stress turbulence model was employed and a hybrid mesh consisting of 3.6 million nodes and 6.4 million elements was used. The experiment and CFD calculation show a stratification in the downcomer. The less dense slugs flow around the core barrel at the top of the downcomer. At the opposite side the lower borated coolant is entrained by the colder safety injection water and transported to the core. The validation proves that ANSYS CFX is able to simulate appropriately the flow field and mixing effects of coolant with different densities.

Keywords: PWR; CFD; ROCOM; Boron Dilution

Publ.-Id: 8631

Einfluss von Uran auf die Archae-Diversität in Bodenproben aus der Haberlandhalde

Reitz, T.

Der Einfluss von Uranylnitrat und Natriumnitrat auf eine in einer Uranabfallhalde natürlich vorkommende archaeale Gemeinschaft wurde mit Hilfe molekularbiologischer Methoden auf der Grundlage der 16S rRNA-Genanalyse untersucht. Es zeigte sich, dass sich die archaeale Diversität der Originalprobe auf einige wenige Abstammungslinien mesophiler Crenarchaeota begrenzte.
In allen Versuchsansätzen, unabhängig von der zugegebenen Menge des Uranyl- bzw. Natriumnitrats, der Inkubationsdauer und der Sauerstoffverfügbarkeit, konnte eine Verschiebung der archaealen Populationen zu einer Gruppe eng verwandter, mikrodiverser Crenarchaeota nachgewiesen werden.
Unterschiedliche Nährmedien wurden hergestellt um Vertreter dieser Gruppe anzureichern und zu isolieren. Dabei gelang es mesophile Crenarchaeota der oben genannten Gruppe in zwei Nährmedien zu kultivieren. Die bakterielle Begleitflora beider Archaea-Kulturen war geprägt durch Vertreter der Klassen Bacilli und Clostridia.
In einem weiteren Teil der Arbeit wurde die Biosorptionskinetik von Uran(VI) an den Zellen des hyperthermophilen Crenarchaeons Pyrobaculum islandicum (DSM 4184) sowie des bakteriellen Haldenisolats Paenibacillus sp. JG35+U4-B1 untersucht. Es konnte gezeigt werden, dass bei beiden Stämmen eine schnelle Sättigung (< 30 min) der Zelloberflächen mit Uran bei Raumtemperatur eintritt. Die Uranbindungskapazität von P. islandicum lag mit etwa 28 mg pro g Trockenmasse deutlich unter der des bakteriellen Isolats Paenibacillus sp. JG35+U4-B1 mit 65 mg Uran/g TM. Ein Grund dafür könnte sein, dass die Versuchsbedingungen nicht dem physiologischen Temperaturoptimum des hypothermophilen Archaeons entsprechen.

  • Other report
    Technische Universität Dresden: Diplomarbeit (Studiengang Biologie), 2006
    83 Seiten

Publ.-Id: 8630

Winzlinge im Kampf gegen Uran

Bittner, D.; Flemming, K.; Raff, J.

Im Rahmen der Praktikumsarbeit "Winzlinge im Kampf gegen Uran" wurde ein Internetbeitrag zu „Forschung populär – Von Schülern für Schüler“ über die Arbeiten zur Identifizierung von Bakterien, der Untersuchung ihrer Wechselwirkungen mit Schwermetallen und Radionukliden sowie deren Nutzung zur Reinigung belasteter Abwässer erstellt. Ziel ist es, einzelne Arbeitsgebiete einem breiten Publikum vorzustellen.

Publ.-Id: 8629

FZR activities on in-vessel melt retention

Altstadt, E.; Willschütz, H.-G.

An overview of the modelling activities of FZR in the field of in-vessel corium melt retention (IVR) is given. The thermal and mechanical models and their application to various IVR scenarios are described. The thermo-chemical interaction between molten corium and reactor vessel steel is considered in the calculations.

Keywords: In-vessel melt retention; Finite element model; corrosion; VVER-1000; FOREVER experiments

  • Lecture (others)
    2nd meeting of the ISTC-project "Reactor Core Melt", 05.07.2006, Moskva, Russia

Publ.-Id: 8628

Monocrystalline spinel nanotube fabrication based on the Kirkendall effect

Fan, H. J.; Knez, M.; Scholz, R.; Nielsch, K.; Pippel, E.; Hesse, D.; Zacharias, M.; Gösele, U.

There is a deep interest in methods to fabricate hollow anocrystals for potential application as high-efficiency catalysts or drug-delivery agents. Tubular one-dimensional nanocrystals have been prepared for a wide variety of materials, including semiconductors1,2, metals3,4, ferroelectrics5,6 and magnetite7. They can be produced by rolling up layered materials or via an axial growth in a rolled-up form8–10, coating pores in templates11 or by eliminating the core of a core–shell nanowire1,7. The Kirkendall effect, a classical phenomenon in metallurgy12, was recently applied to explain the formation of hollow spherical nanocrystals13–17. Although the experimental demonstration and theoretical treatment mainly concern binary compounds and planar interfaces or nanoscale spherical interfaces, the fabrication route provided by the Kirkendall effect should be generic, and should also work for high-aspect-ratio hollow cylinders (that is, nanotubes) or even more complex superstructures. In this letter, we report, for the first time, on ultra-long single-crystal ZnAl2O4 spinel nanotubes (total diameter: ∼40 nm, wall thickness: ∼10 nm) fabricated through a spinel-forming interfacial solid-state reaction of core–shell ZnO–Al2O3 nanowires involving the Kirkendall effect. Our results simultaneously represent an extension of applying the Kirkendall effect in fabricating hollow nano-objects from zero-dimensional to multidimensional, and from binary to ternary systems.

Keywords: nanowires; nanotubes; kirkendall effect

Publ.-Id: 8627

Synthesis and radiopharmacological characterization of 2β-carbo-2’-[18F]fluoroethoxy-3β-(4-bromo-phenyl)tropane ([18F]MCL-322) as a potential PET radiotracer for imaging the dopamine transporter (DAT)

Wüst, F.; Berndt, M.; Strobel, K.; van den Hoff, J.; Peng, X.; Neumeyer, J. L.; Bergmann, R.

The fluoroalkyl-containing tropane derivative 2β-carbo-2’-fluoroethoxy-3β-(4-bromo-phenyl)tropane (MCL-322) is a highly potent and selective ligand for the dopamine transporter (DAT). The compound was labeled with the short-lived positron emitter 18F in a single step by nucleophilic displacement of the corresponding tosylate precursor MCL-323 with n.c.a. [18F]fluoride. The positron emission tomography (PET) radiotracer 2βcarbo-2’-[18F]fluoroethoxy-3β-(4-bromo-phenyl)tropane [18F]MCL-322 was obtained in decay-corrected radiochemical yields of 30-40 % at a specific radioactivity of 1.6-2.4 Ci/µmol (60-90 GBq/µmol) at the end-of-synthesis (EOS). Small animal PET, ex vivo and in vivo biodistribution experiments in rats demonstrated a high uptake in the striatum (3.2 % ID/g) 5 min after injection, which increased to 4.2 % ID/g after 60 min. The uptake in the cere¬bellum was 1.8 % ID/g and 0.6 % ID/g after 5 min and 60 min post injection, respectively. Specific binding to DAT of [18F]MCL-322 was confirmed by blocking ex¬periments using the high affinity DAT ligand GBR 12909. The radiopharmacological characterization was completed with metabolite and autoradiographic studies confirming the highly selective uptake of [18F]MCL-322 in the striatum.
It is concluded that the simple single-step radiosynthesis of [18F]MCL-322 and the promising radiopharmacological data make [18F]MCL-322 an attractive candidate for the further development of a PET radiotracer potentially suitable for clinical DAT imaging in the human brain.

Publ.-Id: 8626

Aktuelle Themen der Reaktorsicherheitsforschung in Deutschland - KTG-Fachtagung 2006

Grauf, E.; Stoll, U.; Teschendorff, V.; Weiß, F.-P.

Der Fachtag fand am 03. und 04. April 2006 im Forschungszentrum Rossendorf (FZR) e.V. bei Dresden statt.
Die Veranstaltung griff mit der Borverdünnung in Druckwasserreaktoren bzw. mit der Verstopfung der Sumpfansaugsiebe durch freigesetztes Isolationsmaterial zwei Themen der Reaktorsicherheit auf, die auch in aktuellen Aufsichtsverfahren eine Rolle spielen. Eingebettet in den internationalen Kontext beleuchteten die Veranstalter die sicherheitstechnische Bedeutung dieser Themen für die deutschen Anlagen und stellten die Auswirkungen auf die zu erbringenden Sicherheitsnachweise und den Anlagenbetrieb dar. Dabei kamen Gutachter, Vertreter der Forschung, Hersteller und Betreiber gleichermaßen zu Wort.
Der Fachtag vermittelte den Teilnehmern aber insbesondere, welche Beiträge die privat und öffentlich finanzierte Reaktorsicherheitsforschung zur Aufklärung der jeweiligen Ereignisabläufe und ihrer sicherheitstechnischen Bedeutung geleistet hat.
In diesem Forschungskontext spielen, auch international, die Methoden der so genannten Computational Fluid Dynamics (CFD) eine zunehmende Rolle. Deshalb war eine Sitzung den Grundlagen, Möglichkeiten und Grenzen von CFD-Methoden gewidmet. Dabei wurden u. a. Anwendungen zur Borvermischung und zum Verhalten von Mineralwolle im Sumpf präsentiert.

  • atw - International Journal for Nuclear Power (2006)6, 405-409

Publ.-Id: 8625

Strong improvement of the electroluminescence stability of SiO2:Gd layers by potassium co-implantation.

Prucnal, S.; Sun, J. M.; Reuther, H.; Skorupa, W.; Buchal, C.

Metal-Oxide-Silicon-Based light emitting diodes with Gd implanted SiO2 layers exhibit strong ultraviolet electroluminescence at 316 nm coming from Gd3+ ions. Co-implantation of potassium ions into the SiO2:Gd layer leads to elimination of the luminescent peak from point defects in SiO2 layers. The electropositive potassium ions compensate the negative charge trapping in the gate oxide and thus improve the electrical stability of the ultraviolet emission from Gd3+ ions. The operating time of the devices was increased more than 30 times by using potassium co-implantation and millisecond flash lamp annealing.

Keywords: ion implantation; MOSLEDs; electroluminescence; silicon dioxide

  • Electrochemical and Solid State Letters 10(2007), 330

Publ.-Id: 8624

Dipole-strength distributions up to the particle-separation energies and photodissociation of Mo isotopes

Schwengner, R.; Benouaret, N.; Beyer, R.; Dönau, F.; Erhard, M.; Frauendorf, S.; Grosse, E.; Junghans, A. R.; Kosev, K.; Klug, J.; Nair, C.; Nankov, N.; Rusev, G.; Schilling, K. D.; Wagner, A.

Dipole-strength distributions in the nuclides 92$Mo, 98Mo and 100Mo have been investigated in photon-scattering experiments at the superconducting electron accelerator ELBE of the Forschungszentrum Rossendorf.
A simulation of gamma cascades was performed in order to estimate the distribution of inelastic transitions to low-lying states and thus to deduce the correct dipole-strength distributions up to the neutron-separation energies. The obtained absorption cross sections connect smoothly to (gamma,n) cross sections and give novel information about the low-energy tail of the Giant Dipole Resonance below the neutron-separation energies. The experimental cross sections are compared with predictions of a Quasiparticle-Random-Phase Approximation in a deformed basis.

Photoactivation experiments were performed at various electron energies to study the 92Mo(gamma,n), 92Mo(gamma,p) and 92Mo(gamma,alpha) reactions. The deduced activation yields are compared with theoretical predictions.

Keywords: Photon scattering; photoabsorption cross section; dipole strength; Random-Phase-Approximation; photoactivation; p-nuclei; reaction yields

  • Contribution to proceedings
    International Conference on Collective Motion in Nuclei under Extreme Conditions, 20.-23.6. 2006, Sankt Goar, Deutschland
  • Nuclear Physics A 788(2007), 331c-336c

Publ.-Id: 8623

Excess vacancy generation in SiGe

Kögler, R.

The excess vacancy generation in SiGe by ion implantation was investigated. In contrast to theory the excess vacancy generation in SiGe was observed to be higher than in Si. The reason is the incomplete vacancy-interstitial recombination of defects in SiGe.

Keywords: Ion implantation; implantation defects; excess defects; vacancies; Si; SiGe

  • Lecture (Conference)
    Europien Materials Research Society Conference 2006 - E-MRS2006 -, 29.05.-02.06.2006, Nizza, France

Publ.-Id: 8622

Ion beam Engineering in the initial stage of SIMOX processing

Kögler, R.

Different modes of defect engineering were applied for synthesis of SiO2 nanoclusters in Si. In particular He ion pre-implantation and simultaneous dual-implantation of Si and O ions were performed. It was demonstrated that He pre-implantation is especially suitable for creation of a well defined narrow oxide layer.

Keywords: Silicon-on-insulator; SOI; SiO2 nanocluster; defect engineering; ion beam synthesis

  • Lecture (Conference)
    Ion Implantation - Jahrestagung, 16.-17.05.06, Berlin, Deutschland

Publ.-Id: 8621

Kinetics of precipitation in neutron irradiated steel

Gokhman, A.; Bergner, F.; Ulbricht, A.

Gegenstand der Vortrags ist die ratentheoretische Modellierung der Entwicklung der Größenverteilung von Cu-reichen Defekt-Fremdatom-Clustern in Cu-angereicherten Fe-Basis-Modelllegierungen unter Neutronenbestrahlung. Die berechneten Clustergrößenverteilungen werden mit Ergebnissen von Neutronenkleinwinkelstreumessungen verglichen.

  • Lecture (Conference)
    Mathematical problems to describe the phase transformation of the 1-st kind, 02.-08.07.2006, Bugas, Ukraina

Publ.-Id: 8620

A novel needle probe based on high-speed complex permittivity measurements for investigation of dynamic fluid flows

Da Silva, M. J.; Schleicher, E.; Hampel, U.

For the investigation of multiphase or multicomponent flows, which are of interest, for instance, in oil extraction and processing or in chemical engineering, there are only few suitable measuring techniques. For this reason, we have developed a high-speed complex permittivity needle probe. Such probes are able to distinguish the different phases or components of a flow by measuring the complex value of the electrical permittivity at a high data rate (up to 20000 samples/s). The performance of the system, as well as its ability to differentiate organic substances, has been analyzed. A time-resolved experiment in an oil–water–gas flow, as well as a two-substance mixing experiment in a stirred tank, is presented.

Keywords: complex permittivity measurement; heterogeneous mixture; multiphase flow; multicomponent flow; needle probe


Publ.-Id: 8619

EXAFS investigation of U(VI), U(IV) and Th(IV) sulfato complexes in aqueous solution

Hennig, C.; Schmeide, K.; Brendler, V.; Moll, H.; Tsushima, S.; Scheinost, A.

The structures of U(VI), U(IV) and Th(IV) sulfato complexes were investigated by LIII edge EXAFS spectroscopy in aqueous solutions with total sulfate concentrations, [SO42-]total, ranged from 0.05 to 3 M in the pH range 1.0-2.7. In equimolar U(VI)/[SO42-]total solutions, the species distribution is dominated by monodentate sulfate coordination with a U-Smon distance of 3.57±0.02 Å. With increasing U(VI)/[SO42-]total ratio bidentate coordination becomes dominant with a U-Sbid distance of 3.11±0.02 Å. The aqueous Th(IV) sulfate comprises both, monodentate and bidentate coordination with Th-S distances of 3.14±0.02 and 3.81±0.02 Å, respectively. A similar coordination is obtained for U(IV) sulfato complexes at pH 1 with U-S distances of 3.08±0.02 Å and 3.67±0.02 Å. By increasing the pH value to 2, a U(IV) precipitate occurs, where sulfate act exclusively as bridging ligand with a U-S distance 3.67±0.02 Å.

Keywords: EXAFS; uranium; thorium; sulfate; coordination

  • Inorganic Chemistry 46(2007)15, 5882-5892
  • Poster
    Actinide XAS 2006, 18.-20.09.2006, Karlsruhe, Germany
  • Contribution to proceedings
    Actinide XAS 2006, 18.-20.09.2006, Karlsruhe, Germany
    Speciation Techniques and Facilities for Radioactive Materials at Synchrotron Light Sources: OECD, 978-92-64-99006-7, 219-224

Publ.-Id: 8618

Evidence for double-electron excitations in X-ray absorption spectra of actinides

Hennig, C.

A systematic investigation of double-electron resonances in L3 edge X-ray absorption spectra of actinides in aqueous solution is presented. Actinide species in valence states IV and III were investigated by using experimental data of Th4+, U4+, Np4+, Pu3+, and Am3+ hydrates. The double-electron excitation was identified as a L3N6,7 shake-up effect. Energy positions of the double-electron features were found in good agreement with the Z+1 approximation. The double-electron resonances undergo a larger chemical shift as the L3 edge.

Keywords: double-electron excitation; shake-up; Z+1 approximation; actinides

Publ.-Id: 8617

Enhanced local void and temperature measurements for highly transient two-phase flows

Schleicher, E.; Da Silva, M. J.; Hampel, U.

Abstract: Local void fractions measurements in two-phase flow phenomena are commonly carried out by the use of needle probes. The measuring principle of these probes is based on conductivity or optical measurements. In the past advanced needle probes with integrated micro-thermocouples have been introduced by Prasser et al., making possible to measure local temperatures at the same position where the void fractions are determined because the sheath of the micro-thermocouple serves as the measuring electrode for the conductivity measurement. Thereby - in principle - the temperatures of the two different phases (e. g. steam and water) can be distinguished. The big disadvantage of this technique is the relative long time constant (~20 ms) of mineral-insulated sheathed thermocouples. The usage of this type of thermocouples was necessary because the electronic was not able to separate the two signals (temperature and conductivity) from each other. Measuring of high-transient two phase flows were impossible due to the slow time response. Additionally the two signals had to be sampled sequentially because of influence of the rectangular excitation signal into the small temperature voltage. Investigations of temperature changing in the interfacial area between gas and liquid were therefore very difficult. To solve this problem we have developed a new combined temperature and conductivity needle probe measuring system, which is able to handle grounded or direct sheathed thermocouples (where the thermocouple wires are electrically-joined to the protective sheath) as well as open thermocouples (exposed junction).

Keywords: needle probe; temperature measurement; two-phase flows; micro-thermocouple

Publ.-Id: 8614

Improving Resolution in k and r Space: A FEFF-based Wavelet for EXAFS Data Analysis

Funke, H.; Chukalina, M.; Voegelin, A.; Scheinost, A.

Applying the wavelet analysis of Extended X-Ray Absorption Fine Structure (EXAFS) spectra to a Zn-Al layered double hydroxide, we could unequivocally identify the presence of Al and Zn atoms at the same distance of r ≈ 3 Å from the central Zn atom. The focused paths (including MS) at r ≈ 5 - 6 Å (2nd and 3th metal shell) show also both Al and Zn for all LDH spectra. With the newly developed FEFF-Morlet wavelet it is possible to resolve the two shells individually. The result is, that the 2nd shell not contains Al. This confirms directly the homogeneity of the Al - Zn distribution. No significant differences were found by shell fit- and wavelet analysis between the EXAFS spectra of pure Zn-,Co- or Ni LDH on the one hand and the mixed Zn/Co and Zn/Ni LDH on the other hand.

Keywords: EXAFS; wavelet analysis; layered double hydroxides

  • Lecture (Conference)
    13th International Conference on X-ray Absorption Fine Structure (XAFS13), 09.-14.07.2006, Stanford, United States
  • Contribution to proceedings
    13th International Conference on X-ray Absorption Fine Structure (XAFS13), 09.02.-14.07.2006, Stanford, United States
    AIP Conference Proceedings, Vol. 882, Melville, N.Y., 72-74

Publ.-Id: 8613

Influence of carbonate on U(VI) aqueous speciation and adsorption to ferrihydrite investigated by advanced EXAFS data analysis methods

Rossberg, A.; Ulrich, K.-U.; Brendler, V.; Scheinost, A.

At alkaline pH conditions, formation of soluble uranyl carbonato complexes commonly prevents U(VI) sorption to mineral surfaces, thereby promoting uranium migration in the liquid phase. Microbial activity may increase the partial CO2 pressure by several orders of magnitude in relation to the atmosphere, hence negatively charged carbonato complexes may play a significant role even at acidic pH values, where sorption to positively charged surfaces may compete with formation of binary inner-sphere sorption complexes. Therefore, we studied by EXAFS the sorption of UO22+ on ferrihydrite in aqueous solution as a function of pH and at elevated carbonate concentrations. We used iterative target transformation factor analysis (ITFA) [1] to derive the spectra of the pure species from the EXAFS spectral mixtures. Then we applied Monte Carlo simulations (MCTFA) [2] to determine the three-dimensional structure of the pure species and the radial pair distribution functions of the single and multiple scattering paths. The spectra of the mixtures can be described by a linear combination of two species. Species 1 is an inner-sphere binary U(VI)-ferrihydrite complex, with U(VI) coordinating to the edge of an Fe octahedron. In species 2, U(VI) is coordinated to two or three carbonato ligands, suggesting formation of [(UO2)(CO2)2]2- or [(UO2)(CO2)3]4- complexes associated with the ferrihydrite solid.

[1] 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).
[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).

  • Poster
    13th International Conference on X-ray Absorption Fine Structure (XAFS13), 09.-14.07.2006, Stanford, United States

Publ.-Id: 8612

Modified spectro-electrochemical cell for experiments with ionic liquids

Hennig, C.

The presentation concludes the technical development of a spectro-electrochemical cell for actinide research. Electrolysis can be applied with an electrode of second order or by using a diaphragm to seperate catodic and anodic space. The results of chloro and sulfato coordination in presence of U(VI) and U(IV) will be discussed.

Keywords: EXAFS; spectro-electrochemistry; uranium

  • Invited lecture (Conferences)
    Actinet Meeting "Actinides and lanthanides solution chemistry in water stable Room Temperature Ionic Liquids", 08.-09.06.2006, Strasbourg, France

Publ.-Id: 8611

The Structure Of Uranyl Sulfate In Aqueous Solution - Monodentate versus Bidentate Coordination

Hennig, C.; Schmeide, K.; Brendler, V.; Moll, H.; Tsushima, S.; Scheinost, A. C.

The structure of U(VI) aquo sulfato complexes has been investigated by LIII edge EXAFS spectroscopy. A monodentate coordination with a U-Smon distance of 3.57±0.02 Å prevail in equimolar [SO42-]total/U(VI) solutions. With increasing [SO42-]total/U(VI) ratio, bidentate coordination with a U-Sbid distance of 3.11±0.02 Å becomes dominant.

Keywords: EXAFS; U(VI); aquo sulfato complex

  • Contribution to proceedings
    13th International Conference on X-ray Absorption Fine Structure (XAFS13), 09.-14.07.2006, Stanford, United States
    API - Special Edition

Publ.-Id: 8610

Coordination of uranium aquo chloro and sulfato complexes under different redox conditions

Hennig, C.; Schmeide, K.; Brendler, V.; Moll, H.; Tsushima, S.; Scheinost, A. C.

Natural aquatic systems may exert large variations in actinide oxidation states because of limited oxygen diffusion, and microbially and inorganic (surface catalyzed) redox processes. Redox state and coordination hence is a key variable controlling uranium mobility at former uranium mining areas. We have developed a spectro-electrochemical cell, which allows to study the structure speciation of aqueous U(VI) and U(IV) complexes in situ by X-ray absorption spectroscopy, while applying and maintaining a constant potential [1]. Here we present data on the influence of redox state on the uranium coordination by chloride and sulfate. The reduction of U(VI) to U(IV) involves an electron transfer and a chemical reaction transforming the trans-dioxo cation to the spherically coordinated U4+ cation. With [Cl] increasing from 0 to 9 M, we observed for U(VI) the complexes UO2(H2O)52+, UO2(H2O)4Cl+, UO2(H2O)3Cl20 and UO2(H2O)2Cl3, and for U(IV) the complexes U(H2O)94+, U(H2O)8Cl3+, U(H2O)6-7Cl22+ and U(H2O)5Cl3+ [1]. Elevated sulfate concentrations have been monitored at the former uranium mine Königstein/Germany, where uranium was leached by sulfuric acid. We observed that tetra- and hexavalent uranium forms both monodentate and bidentate complexes with sulfate. However, with increasing [SO42-] concentration the bidentate coordination prevails. The observed formation of soluble U(IV) sulfato complexes may prevent uraninite precipitation even at low pH, thus increasing the mobility of uranium even under reducing conditions.

Keywords: EXAFS; U(VI); U(IV); electrochemistry

  • Poster
    XAFS-13 Conference, 09.-14.07.2006, Stanford, USA

Publ.-Id: 8609

The problem to account for processes on different time scales in the investigation of neutron embrittlement

Gokhman, A.; Bergner, F.; Ulbricht, A.

Neutrons with energy of about 1 МeV or higher, to which reactor pressure vessel steels are exposed under service conditions of nuclear power plants, can stimulate the phenomenon of so-called neutron embrittlement. The reason is the irradiation-induced formation and evolution of precipitates and solute/defect clusters. The set of experimental methods to investigate the nanostructure, including X- ray and neutron small angle scattering, atom probe field ion microscopy method, TEM as well as direct determination of the mechanical properties of the reactor vessel is usually carried out after irradiation intervals of the order of one year.
Therefore a project called virtual test reactor has been started recently within the framework of the European program “PERFECT”. Experimental data obtained for neutron irradiated pressure vessel steels are used as the reference for working out and study theoretical models of the kinetics of neutron embrittlement.
In the cascade stage, following the interaction of a neutron with a so-called primary knock-on atom, the recoil energy transferred to the crystal lattice causes a local increase of the temperature to beyond the melting temperature and the formation of defects. A typical time scale for these processes is picoseconds. Such processes are studied by the molecular dynamics and Monte-Carlo methods.
Unlike the case of electron irradiation, where only Frenkel pairs are formed, point defect clusters and Cu-precipitates are produced in the case of neutron irradiation. The evolution of these clusters can be studied in the rate theory approach, in particular by cluster dynamics method. This method allows for the determination of the time dependence of the number density of clusters (precipitates) via integration of the system of ordinary differential equations with the initial data as the final data of the cluster number density after cascade stage. The characteristic relaxation time depends on the kind of the irradiated feature. For example for point defect clusters this time is about 1 day but for Cu-precipitates the relaxation time is about one month. Hence the problem to integrate the master equation of cluster dynamics is the problem of integration of so-called stiff ordinary differential equations.
The objective of our present investigation is the comparative analysis of software tools applied to the cluster dynamics modeling. The most effective one, namely the code D02EJF from the licensed Fortran NAG Library has been used to investigate the kinetics of the following three kinds of neutron irradiated features in reactor pressure vessel steels:

  • The system of free and clustering vacancies (V) and interstitials (I), denoted (V+I)-system,
  • The system of the Cu-precipitates favoured because of irradiation-enhanced Cu diffusivity, denoted (V+I)=>Cu system,
  • The (V+I)óCu system, when the effect of Cu-precipitates on the sink strength in the first and second model is additionally considered.
The comparison of the experimental and cluster dynamics data has been carried out on the basis of low-Cu and Cu-enriched mock-up alloys after neutron irradiation.
  • Lecture (Conference)
    4-th Russian Symposium "Physics Problems of Ultrashort Phenomena in Strongly Non-Equilibrium Media", 22.07.-01.08.2006, New Athos, Russia

Publ.-Id: 8608

CFD-simulation of OECD V1000CT2 with advanced turbulence models

Höhne, T.

CFD calculations have been performed for the themalhydraulic benchmark V1000CT-2.
The numerical grid model was generated with the grid generator IC4C (ICEM-CFD) and the preprocessor ANSYS CFX and contains 4.7 Mio. tetrahedral elements Different advanced turbulence models were used in the numerical simulation The best agreement with the Kozloduy experiment at the core inlet shows the DES simulation. Strong fluctuations occur in the downcomer of the RPV.
The results show a clear sector formation of the affected loop at the downcomer, lower plenum and core inlet. The maximum local values of the relative temperature rise in the experiment amount 97.7% and in the calculation 97.3%
Deficits: Estimation of experimental values at the core outlet to the core inlet, interpolation

Keywords: OECD; VVER-1000; CFD

  • Lecture (Conference)
    Annual meeting of Working Group C and G of AER, 22.-23.05.2006, Balatonfüred, Hungary

Publ.-Id: 8607

Fluid mixing and flow distribution ín a primary circuit of a nuclear pressurized water reactor – Validation of CFD codes

Rohde, U.; Höhne, T.; Kliem, S.; Hemström, B.; Scheuerer, M.; Toppila, T.; Aszodi, A.; Boros, I.; Farkas, I.; Muehlbauer, P.; Vyskocil, V.; Klepac, J.; Remis, J.; Dury, T.

The EU project FLOMIX-R was aimed at describing the mixing phenomena relevant for both safety analysis, particularly in steam line break and boron dilution scenarios, and mixing phenomena of interest for economical operation and the structural integrity.
This report will focus on the Computational Fluid Dynamics (CFD) code validation. Slug mixing tests simulating the start-up of the first main circulation pump have been performed with two 1:5 scaled facilities: The Rossendorf coolant mixing model ROCOM and the VATTENFALL test facility. Additional data on slug mixing in a VVER-1000 type reactor gained at a 1:5 scaled metal mock-up at EDO Gidropress are provided. Experimental results on mixing of fluids with density differences obtained at ROCOM and the FORTUM PTS test facility were made available. Concerning mixing phenomena of interest for operational issues and thermal fatigue, flow distribution data available from commissioning tests have been used together with the data from the ROCOM facility as a basis for the flow distribution studies. CFD calculations have been accomplished for selected experiments with two different CFD codes (CFX-5, FLUENT). Best practice guidelines (BPG) were applied in all CFD work when choosing computational grid, time step, turbulence models, modelling of internal geometry, boundary conditions, numerical schemes and convergence criteria. The strategy of code validation based on the BPG and a matrix of CFD code validation calculations have been elaborated. Based on the “best practice solutions”, conclusions on the applicability of CFD for turbulent mixing problems in PWR were drawn and recommendations on CFD modelling were given.


Publ.-Id: 8606

Surface speciation of uranyl(VI) on gibbsite: A combined spectroscopic approach

Arnold, T.; Scheinost, A.; Baumann, N.; Brendler, V.

Sorption phenomena are important immobilization processes to be considered in the design of nuclear waste repositories. Confidence in respective performance assessments requires a mechanistic understanding of the dominant surface reactions. Highly sensitive and non-invasive techniques such as Time-Resolved Laser-induced Fluorescence Spectroscopy (TRLFS) and Extended X-ray Absorption Fine-Structure (EXAFS) spectroscopy are valuable tools to reveal the true surface speciation.
Here TRLFS and EXAFS were applied to study the species of uranyl(VI) adsorbed onto gibbsite particles. This system is important as a model for the characterization of the edge sites of aluminosilicates, relevant as potential host rock and as part of engineered barrier systems.
The experiments were performed with both normal and CO2-free atmosphere, in the pH range 4 to 9, with a total uranium concentration of 10 µM, an ionic strength of 0.01 M (NaClO4), a solid concentration of 4.15 and 12.5 g/L, and using a grain size of 0.2 – 12 µm.
TRLFS at room temperature provided evidence for two adsorbed uranium(VI) surface species. The two species showed similar positions of the fluorescence emission bands and different fluorescence lifetimes indicating a different coordination environment for the two species. The first surface species with the shorter fluorescence lifetimes was assigned to a mononuclear surface complex, where EXAFS indicated (AlO)2UO2 distance of 3.6 Å. The second species with the longer fluorescence lifetimes was attributed to polynuclear surface species supported by a U – U distance of 4.2 Å obtained by EXAFS at cryogenic temperature. Cryogenic TRLFS experiments at 10 K implied the presence of a third surface species. The significant shift of the fluorescence emission bands to shorter wavelength (approximately 16 nm) points to a ternary uranyl carbonato surface species, supported by an EXAFS-derived U – C distance of 2.9 Å. These results will help to formulate more realistic surface reactions on aluminosilicates.

  • Lecture (Conference)
    2006 Fall Meeting, 27.11.-01.12.2006, Boston, United States
  • Contribution to proceedings
    2006 Fall Meeting, 27.11.-01.12.2006, Boston, United States

Publ.-Id: 8605

Analytische und experimentelle Untersuchungen zur Modellierung der Fluid-Struktur-Wechselwirkung in einem 2D-Ringspalt

Grunwald, G.; Altstadt, E.

  • Open Access Logo Forschungszentrum Rossendorf; FZR 1 1993
    ISSN: 1436-3976


Publ.-Id: 8604

Complexation of uranium(VI) with aromatic acids in aqueous solution – A comparison of hydroxamic acids and benzoic acid

Glorius, M.; Moll, H.; Bernhard, G.

The complex formation of uranium(VI) with salicylhydroxamic, benzohydroxamic, and benzoic acid in 0.1 M NaClO4 was studied by UV-vis spectroscopy at pH 3 and 4. Uranium(VI) species of the type MpLqHr were identified from the UV-vis spectra in all three systems. An increase in the absorption combined with a blue shift of the absorption maxima in comparison to the bands of the free uranyl ion of 22.5 ± 2 nm was observed in the uranium (VI)-salicylhydroxamic acid-system. Besides indications for a 1:2 complex, the formation of a 1:1 complex with a stability constant of log β111= 17.12 ± 0.10 could be demonstrated by its individual absorption spectrum and molar absorption coefficient. Also in the uranium(VI)-benzohydroxamic acid-system a blue shift of the absorption maxima in comparison to the bands of the free uranyl ion of 27 ± 1.4 nm indicate the complex formation. The stability constants are log β110 = 7.96 ± 0.05 for UO2[C6H4CONHO]+ and log β120 = 15.25 ± 0.11 for UO2[C6H4CONHO]2. In contrast to the hydroxamic acids, benzoic acid shows a red shift of the absorption maxima of 2.5 ± 2 nm. Only the 1:1 complex UO2[C6H4COO]+ with a stability constant of log β110 = 3.37 ± 0.14 is existent. An estimate is made in order to discuss the dependencies observed in the absorption spectra in relation to possible coordination modes of uranium(VI). The strength of the complex formation between uranyl and the three aromatic acids is discussed.

Keywords: Uranyl; Hydroxamic acid; Spectroscopy; UV-vis spectra; Complexation

Publ.-Id: 8603

ROSY - Rossendorfer Synchrotronstrahlungsquelle, Vorschlag für die Errichtung einer Synchrotronstrahlungsquelle für die Materialforschung im Forschungszentrum Rossendorf

Einfeld, D.; Matz, W.

Der Projektvorschlag für eine kompakte Synchrotronstrahlungsquelle der 3. Generation ROSY wird beschrieben. Die Quelle sol1 für die Materialforschung dediziert sein, die für die Forschung im regionalen Umfeld von Rossendorf prägend ist.
Bei einer Speicherringenergie von 3 GeV wird Synchrotronstrahlung im harten Röntgenbereich mit einer kritischen Energie des Spektrums von Ec = 8,4 keV (hc=0,14 nm) emittiert. Mit einer naturlichen Emittanz von 28 n nm rad wird eine sehr brillante Strahlung zur Verfügung gestellt. Neben der Strahlung aus Ablenkmagneten kann Strahlung aus Wigglern und Undulatoren genutzt werden. Für diese insertion devices sind 8 Einbaumöglichkeiten vorhanden, von denen 4 in nichtdispersionsfreien geraden Strecken liegen. Der Speicherring hat eine vierfache Symmmetrie, einen Umfang von 148 m und ist als modifizierte FODO-Struktur konzipiert. Ein nachträglicher Einbau von supraleitenden Ablenkmagneten zur Erzeugung eines harteren Spektrums und
damit zur Ausweitung des Anwendungsbereiches ist möglich.
Der Teil I enthält die wissenschaftliche Begründung fur ROSY und eine Darstellung der Nutzungsmöglichkeiten. Im Teil II werden das Speicherringkonzept und die technischen Details der einzelnen Komponenten beschrieben.

  • Open Access Logo Forschungszentrum Rossendorf; FZR 10 1992
    ISSN: 1436-3976


Publ.-Id: 8602

Electronegativity and point defect formation in ion implanted SiO2 layers

Prucnal, S.; Sun, J.; Reuther, H.; Skorupa, W.; Buchal, C.

The Metal-Oxide-Silicon (MOS) diode structure containing ion implanted electropositive (M+) and electronegative (M-) ions is one of the most promising candidates for a new type of high-efficiency electroluminescence (EL) devices which can be integrated with standard silicon CMOS technology. The implantation process creates defects in the SiO2 layer. After implantation an annealing process leads to the diffusion of the implanted elements and a broadening of the SiO2/Si interface. The influence of the different implanted ions (Gd, F, K) was investigated by electroluminescence measurements and correlated to different defects in the oxide layer. Implanted electronegative ions (such as F) lead defects comprising O2 molecules and peroxy radicals (POR). On the other hand, the electropositive ions (Gd and K) increase the number of the oxygen vacancy defects.

Keywords: ion implantation; electronegativity; electroluminescence

  • Vacuum 81(2007)10, 1296-1300

Publ.-Id: 8601

Annual Report 1992, Institute of Nuclear and Hadronic Physics

Dönau, F.; Prade, H.; (Editors)

  • Open Access Logo Forschungszentrum Rossendorf; FZR 10 1993
    ISSN: 1436-3976


Publ.-Id: 8600

Evolution of interfacial area concentration in a vertical air-water flow measured by wire-mesh sensors

Prasser, H.-M.

An application of wire-mesh sensors to obtain the interfacial area concentration in vertical pipes is presented as an alternative to widely used multiple-tip electrical or optical fibre probes. The measuring data of a mesh sensor consists in a three-dimensional matrix of local instantaneous gas fractions measured at each crossing point of the wires and recorded as a time sequence. Bubbles are clearly reflected in this data matrix, since they represent regions of interconnected elements containing the gaseous phase. The method to deduce the interfacial area concentration from this data is based on a full reconstruction of the gas-liquid interface, where the interfacial area of each bubble is recovered as the sum of the surface area of all surface elements belonging to the given bubble. The new method can be applied to large bubbles with an arbitrary shape. To study the change of the interfacial area concentration along the pipe the distance between sensor and gas injection was varied. Obtained results were compared to the findings reported in literature.

Keywords: two-phase flow; flow pattern; interfacial area concentration; vertical upwards flow; wire-mesh sensors

  • Nuclear Engineering and Design 237(2007), 1608-1617

Publ.-Id: 8599

CFD modelling of subcooled boiling – concept, validation and application to fuel assembly design

Krepper, E.; Koncar, B.; Egorov, Y.

The paper describes actual CFD (Computational Fluid Dynamics) approaches to subcooled boiling and investigates their capability to contribute to fuel assembly design. In a prototype version of the CFD code CFX a wall boiling model is implemented based on a wall heat flux partition algorithm. It can be shown, that the wall boiling model is able to calculate the cross sectional averaged vapour volume fraction of vertical heated tubes tests with good agreement to published experimental data. The most sensitive parameters of the model are identified. Needs for more detailed experiments are established which are necessary to support further model development. The model is applied for investigation of the phenomena inside a hot channel of a fuel assembly. Here the essential phenomenon is the critical heat flux. Although subcooled boiling represents only a preliminary state towards the critical heat flux occurrence, essential parameters like swirl, cross flow between adjacent channels and concentration regions of bubbles can be determined. By calculating the temperature of the rod surface the critical regions can be identified which may later on lead to departure from nucleate boiling and possible damage of the fuel pin. The application of up-to-date CFD with a subcooled boiling model for the simulation of a hot channel enables the comparison and the evaluation of different geometrical designs of the spacer grids of a fuel rod bundle.

Keywords: CFD; bubbly flow; boiling; code validation; fuel rod bundle

Publ.-Id: 8597

Picosecond dynamics of interminiband transitions in doped GaAs/AlGaAs superlattices

Stehr, D.; Winnerl, S.; Helm, M.; Andrews, A. M.; Roch, T.; Strasser, G.

Semiconductor superlattices are an essential component of novel infrared devices such as detectors and quantum cascade lasers and their optical and transport properties have been investigated extensively during the past two decades. However, unlike for quantum well structures where considerable knowledge on the intersubband relaxation dynamics has been obtained, so far no experimental work has been published on the interminiband relaxation dynamics in superlattices.

In this work we have studied the transient transmission of a doped GaAs/Al0.3Ga0.7As superlattice in pump-probe experiments [1]. The superlattice with thickness of 9.0 nm and 2.5 nm of the wells and barriers, respectively, was n-doped in the center of the wells, resulting in a doping density of 1.51016 cm-3 averaged over one superlattice period. Picosecond infrared pulses with energies up to 100 nJ in the range from 4 µm to 22 µm were generated at 13 MHz repetition rate by the free-electron laser FELBE at the Forschungszentrum Rossendorf. In particular, the experiments were performed at the absorption maxima of the superlattice at 9.0 µm and 15.8 µm (compare Fig. 1). These wavelengths are the spectral positions of the van Hove singularities of the joint density of states in the center and at the edge of the mini-Brillouin zone, respectively.

The measured pump-probe signals shown in Fig 2 consist of a fast component due to the bleaching of the interminiband transition and subsequent relaxation and thermalization, and a slower component due to cooling of the heated electron system. The fast component decays typically around 2-2.5 ps, in reasonable agreement with published theoretical values [2]. The slower component due to cooling is positive for excitation at 9.0 µm and negative at 15.8 µm and shows a strong temperature and excitation density dependence with cooling times ranging from 5 to 50 ps. This behavior is consistent with the temperature dependence of the linear absorption spectrum, i.e. yielding higher or lower transmission for increasing electron temperature. The effect provides an internal thermometer for the miniband electrons on a picosecond timescale.

[1] D. Stehr et al., Appl. Phys. Lett. (in print)
[2] F. Compagnone, A. Di Carlo, and P. Lugli, Appl. Phys. Lett. 80, 920 (2002)

  • Lecture (Conference)
    POISE Summer School, 25.-30.06.2006, Cortona, Italia

Publ.-Id: 8596

Nanocluster formation in Fe implanted GaN

Talut, G.; Reuther, H.; Mücklich, A.; Eichhorn, F.; Potzger, K.

The formation of Fe-clusters in wurtzite GaN implanted with 200 keV 57Fe ions at 350 ±C was investigated. Cluster sizes from few nanometers up to several 100 nm depending on ion fluence, implantation and annealing temperature have been observed for ion fluences between 4·1016 cm−2 and 1.6·1017 cm−2. A clear epitaxial relation between Fe and GaN was determined. X-ray diffraction, conversion electron Mössbauer spectroscopy, transmission electron microscopy and Auger electron spectroscopy were used for the characterization of the implanted samples. Mössbauer spectroscopy shows that precipitation of Fe occurs already during implantation.

Keywords: Fe; clusters; GaN; DMS; Mössbauer

Publ.-Id: 8595

Magnetohydrodynamic drag reduction and its efficiency

Shatrov, V.; Gerbeth, G.

We present results of direct numerical simulations of a turbulent channel flow influenced by electromagnetic forces. The magnetohydrodynamic Lorentz force is created by the interaction of a steady magnetic field and electric currents fed to the fluid via electrodes placed at the wall surface. Two different cases are considered. At first, a time-oscillating electric current and a steady magnetic field create a spanwise time-oscillating Lorentz force. In the second case, a stationary electric current and a steady magnetic field create a steady, mainly streamwise Lorentz force. Besides the viscous drag, the importance of the electromagnetic force acting on the wall is figured out. Regarding the energetic efficiency it is demonstrated that in all cases a balance between applied and flow induced electric currents improves the efficiency significantly. But even then, the case of a spanwise oscillating Lorentz force remains with a very low efficiency, whereas for the self-moved regime in case of a steady streamwise force much higher efficiencies are found. Still, no set of parameters has yet been found for which an

energetic breakthrough, i.e. a saved power exceeding the used power, is reached.

  • Physics of Fluids 19, 035109(2007)

Publ.-Id: 8594

Low-energy cross section of the 7Be(p , γ)8B solar fusion reaction from the Coulomb dissociation of 8B

Schümann, F.; Typel, S.; Hammache, F.; Sümmerer, K.; Uhlig, F.; Böttcher, I.; Cortina, D.; Förster, A.; Gai, M.; Geissel, H.; Greife, U.; Grosse, E.; Iwasa, N.; Koczon, P.; Kohlmeyer, B.; Kulessa, R.; Kumagai, H.; Kurz, N.; Menzel, M.; Motobayashi, T.; Oeschler, H.; Ozawa, A.; Ploskon, M.; Prokopowicz, W.; Schwab, E.; Senger, P.; Strieder, F.; Sturm, C.; Zhi-Yu, S.; Surowka, G.; Wagner, A.; Walus, W.

An exclusive measurement of the Coulomb breakup of 8B into 7Be + p at 254A MeV was used to infer the low-energy 7Be(p , γ)8B cross section. The radioactive 8B beam was produced by projectile fragmentation of 350A MeV 12C and separated with the FRagment Separator (FRS) at Gesellschaft für Schwerionenforschung in Darmstadt, Germany. The Coulomb-breakup products were momentum-analyzed in the KaoS magnetic spectrometer; particular emphasis was placed on the angular correlations of the breakup particles. These correlations demonstrate clearly that E1 multipolarity dominates within the angular cuts selected for the analysis. The deduced astrophysical S 17 factors exhibit good agreement with the most recent direct 7Be(p , γ)8B measurements. By using the energy dependence of S 17 according to the recently refined cluster model for 8B of P. Descouvemont [Phys. Rev. C 70, 065802 (2004)], we extract a zero-energy S factor of S 17(0)= 20.6 ± 0.8(stat) ± 1.2(syst) eV b. These errors do not include the uncertainty of the theoretical model to extrapolate to zero relative energy, estimated by Descouvemont to be about 5%.

  • Physical Review C 73(2006), 15806-1-15806-13
    DOI: 10.1103

Publ.-Id: 8593

Study of crystal damage by ion implantation using micro RBS/Channeling

Grambole, D.; Herrmann, F.; Heera, V.; Meijer, J.

The combination of microbeam implantation and in-situ micro RBS/Channeling analysis in the Rossendorf nuclear microprobe facility enables crystal damage studies with high current densities not achievable in standard ion implantation experiments. Si(100) samples were implanted with 600 keV Si+ ions and a fluence of 1 x 1016 Si/cm². Using a beam spot of 200 µm x 200 µm current densities from 4 to 120 µA/cm² were obtained. The substrate temperature was varied between RT and 265 °C. The implanted regions were subsequently analysed by micro RBS/Channeling with a 3 MeV He+ beam having a spot size of 50 µm x 50 µm.
Crystal damage up to amorphisation was obtained in dependence on the substrate temperature. Above a critical temperature TC no amorphisation occurs. TC was determined for each series of samples implanted with the same ion current density j. It was found that the empirical Arrhenius relation j ~ exp(- Em/ kTC), known from standard implantation experiments, is also valid at high current densities. The observed Arrhenius law can be derived from a model of epitaxial crystallisation stimulated by defect diffusion.

Keywords: Nuclear microprobe; Micro RBS/Channeling; Experimental setup; Ion implantation; Si ions; Silicon; Ion current density; Radiation damage; Amorphisation; Critical temperature

  • Invited lecture (Conferences)
    10th International Conference on Nuclear Microprobe Technology and Applications, 10.-14.07.2006, Singapore, Singapore
  • Nuclear Instruments and Methods in Physics Research B 260(2007), 276-280

Publ.-Id: 8592

Ion beam analysis by nuclear microprobes

Grambole, D.; Herrmann, F.

The nuclear microprobe is extensively applied to a wide research field, due to the combination of the good ability of the ion beam analysis and the spatial resolution in the range of a few micrometer.
The nuclear microprobe at the 3MV Tandetron accelerator of the Forschungs-zentrum Rossendorf is one of very few probes about the world, at which all ion beam analytical methods are availably:

  • micro Rutherford Backscattering Spectrometry (µRBS) for detection of mainly heavier elements,
  • micro RBS – Channeling (µRBS/Channeling) to investigate the crystal structure and damage in small regions,
  • micro Elastic Recoil Detection Analysis (µERDA) for profiling of light elements,
  • micro Particle Induced X-ray Emission (µPIXE) for mapping of especially trace elements and
  • micro Nuclear Reaction Analysis (µNRA) for detection of special light isotopes.
In order to demonstrate the generally analytical performance of nuclear microprobes a selection of various uses of the Rossendorf microprobe for ion beam analysis will be presented and discussed.
  • Invited lecture (Conferences)
    6th International Conference "Ion Implantation and other Applications of Ions and Electrons" ION 2006, 26.-29.06.2006, Kazimierz Dolny, Poland

Publ.-Id: 8591

Steam bubble condensation in sub-cooled vertical pipe flow

Lucas, D.; Prasser, H.-M.

Detailed experimental data on the evolution of a two-phase flow along a vertical pipe with an inner diameter of 195 mm and a length of 8 m were obtained for adiabatic air-water mixture as well as for steam-water mixture under adiabatic and non-adiabatic conditions at different pressure. The experiments were performed at the TOPFLOW facility of our institute. The measurements were carried out using the wire-mesh sensor technology. The data include local gas volume fraction values (lateral resolution: 3 mm) as well as bubble size distributions and local volume fraction distributions decomposed according to bubble size classes. A slight sub-cooling of the water (max. 4 K) was achieved by throttling the globe valve at the upper end of the vertical test section.

Since the wire-mesh sensor disturbs the flow in downflow direction (but still measures the undisturbed flow!) it doesn’t make sense to place a number of wire-mesh sensors behind each other in order to investigate the evolution of the flow along the pipe. Because of the large dimensions of the facility it is also not applicably to shift a single sensor to different height positions by dismantling the facility each time. For this reason the measuring plane was fixed at the upper end of the test section and a variable gas injection was used. The gas was injected by rings of holes at the pipe wall, which are placed at different height positions 1 mm and 4 mm holes can be used for the injection.

The previously developed Multi-Bubble-Size-Group Test Solver was extended to consider the phase transfer. The large number of bubble classes (50) in the simulation allows the investigation of the influence of the bubble size distribution. The interfacial area density is calculated according to the bubble size distributions and the assumption of ellipsoidal bubbles with a deformation according to the well-known Wellek-correlation. Simulations show, that there are clear differences in the condensation process along the pipe for poly-dispersed flow in comparison with the assumption of mono-dispersed flow is discussed.

The results of the simulations show a good agreement with the experimental data. The condensation process is clearly slower, if large bubbles are injected (4 mm holes). Also bubble break-up has a strong influence on the condensation process because of the change of the interfacial area. Some unsureness arises from the unknown interfacial area for large bubbles and possible uncertainties of the heat transfer coefficient.

Keywords: Condensation; Bubbly Flow; Phase Transfer

  • Lecture (others)
    44rd European Two-Phase Flow Group Meeting, 07.-09.06.2006, Lausanne, Schwizerland

Publ.-Id: 8590

The traveling wave MRI in cylindrical Taylor-Couette flow: comparing wavelengths and speeds in theory and experiment

Rüdiger, G.; Hollerbach, R.; Stefani, F.; Gundrum, T.; Gerbeth, G.; Rosner, R.

We study experimentally the flow of a liquid metal confined between differentially rotating cylinders, in the presence of externally imposed axial and azimuthal magnetic fields. For increasingly large azimuthal fields a wave-like disturbance arises, traveling along the axis of the cylinders. The wavelengths and speeds of these structures, as well as the field strengths and rotation rates at which they arise, are broadly consistent with theoretical predictions of such a traveling wave magnetorotational instability.

  • The Astrophysical Journal Letters 649(2006)2, L145-L147
    DOI: 10.1086/508422

Publ.-Id: 8589

QWIP Camera Development and Applications

Schneider, H.

Quantum well infrared photodetector (QWIP) technology opens up new opportunities to realize focal plane arrays (FPA) for high-performance thermal imaging. Large infrared detector arrays with excellent thermal and spatial resolution, low fixed-pattern noise, low 1/f noise, and high pixel yield can thus be realized at moderate cost.
In this tutorial, I will report on typical QWIP structures optimized for thermal imaging applications, the development of focal plane arrays, and the performance of state-of-the-art QWIP cameras. Dual-band QWIP FPAs detecting simultaneously in different spectral bands will also be addressed.
In most cases, a successful novel technology is not only suitable for existing applications; it will also create its new applications. Talking about applications of QWIP FPAs, I will show that QWIP technology has in fact lead to new and interesting opportunities, in particular in the medical market.

Keywords: quantum well infrared photodetector; QWIP; dual-band QWIP; photovoltaic QWIP; thermal imaging; focal plane array; noise-equivalent temperature difference; GaAs/AlGaAs

  • Invited lecture (Conferences)
    International Workshop on Quantum Well Infrared Photodetectors (QWIP2006), 18.-24.06.2006, Kandy, Sri Lanka

Publ.-Id: 8588

Autocorrelation measurements of free-electron laser radiation using a two-photon QWIP

Schneider, H.; Drachenko, O.; Winnerl, S.; Helm, M.; Maier, T.; Walther, M.

The two-photon QWIP approach involves three equidistant subbands, namely two bound states localized in the quantum well and an extended state in the continuum. Here the intermediate state generates a huge optical nonlinearity, six orders of magnitude larger than in usual bulk semiconductors, which makes this device very promising for quadratic autocorrelation measurements of pulsed mid-infrared lasers. In addition, temporal resolution is only limited by the sub-ps intrinsic time constants of the quantum wells, namely the intersubband relaxation time and the dephasing time of the intersubband polarization [1]. We will report here on autocorrelation measurements of ps optical pulses from a free-electron laser (FEL). Due to the intense nonlinear signal, two-photon QWIP operation is possible at room temperature. The device is thus ideally suited for standard diagnostics of the FEL pulse shape.
[1] H. Schneider, T. Maier, H. C. Liu, M. Walther, P. Koidl, Ultra-sensitive femtosecond two-photon detector with resonantly enhanced nonlinear absorption, Optics Letters 30, 287 (2005).

Keywords: quantum-well infrared photodetector; QWIP; quadratic autocorrelation; GaAs/AlGaAs; free-electron laser

  • Lecture (Conference)
    International Workshop on Quantum Well Infrared Photodetectors (QWIP2006), 18.-24.06.2006, Kandy, Sri Lanka
  • Infrared Physics and Technology 50(2007), 95-99
    DOI: 10.1016/j.infrared.2006.10.028

Publ.-Id: 8587

Consideration of steel corrosion rates from the MASCA / METCOR-tests to the RPV strength under IVR conditions

Willschuetz, H.-G.; Altstadt, E.

Physicochemical interaction of prototypic UO2-ZrO2-Zr corium melt and VVER vessel steel was examined during the 2nd Phase of the ISTC METCOR Project. Rasplav-3 test facility was used for conducting four tests, in which the Zr oxidation degree and interaction front temperature were varied; in one of the tests, stainless steel was added to the melt.
Direct experimental measurements and posttest analyses were used for determining corrosion kinetics and maximum corrosion depth, as well as the steel temperature conditions during the interaction, and finally the structure and composition of crystallized ingots, including the interaction zone. The minimum temperature on the interaction front boundary, which determined its final position and maximum corrosion depth was ~1090°С. An empirical correlation for calculation of corrosion kinetics has been derived.
In this presentation the impact of the so called "corrosion" is compared to calculations without a consideration of the corrosion. The evaluated scenario considers the In-Vessel-Retention for a VVER-1000 with a stratified melt pool and with a homogeneous melt pool.

Keywords: In-Vessel-Retention; VVER-1000; vessel wall corrosion; homogeneous and stratified melt pool

  • Invited lecture (Conferences)
    Seminar at the Division of Nuclear Power Safety, 26.06.2006, Stockholm, Sveden

Publ.-Id: 8586

Innovationspool Natur: Bakterien und Bionik

Raff, J.

Ein Ziel der heutigen Forschung ist es, neue und innovative Wege zur Lösung technischer Fragestellungen zu erschließen. In der Bionik wird dazu versucht, von der Natur zu lernen. Höchst erfolgreiche Beispiele dafür sind der Lotuseffekt, der Klettverschluss oder die Nutzung der Struktur der Haifischschuppe zur Reduktion des Luftwiderstandes bei Flugzeugen. Aber nur wenige wissen, dass auch die kleinsten Lebewesen unseres Planten, die Bakterien, hier einen großen Beitrag leisten können.

  • Lecture (others)
    Vortrag anlässlich des Besuch der Berufschulzentrum Technik, Bautzen, 26.01.2006, Dresden, Germany
  • Lecture (others)
    Vortrag anlässlich des Besuch des Leistungskurses Physik des Vizthum-Gymnasiums Dresden, 19.06.2006, Dresden, Germany

Publ.-Id: 8585

Measurement of porosity in polyolefin particles using X-ray microtomography

Boden, S.; Bieberle, M.; Hampel, U.

The morphology of polymer macroparticles is directly linked to the various parameters of the polymerisation process. X-ray cone beam microtomography is a tool to provide key information on internal structure like morphology, porosity and interfacial area of such particles to the process engineers. By further application of image processing tools to the reconstructed tomograms, we were able to identify different characteristic morphologies within different polyolefin samples and to detect enclosed voids, which will ultimately not contribute to the active surface area.

Keywords: microstructure; polymer; polyolefin; macroparticle; microtomography; X-ray

  • Contribution to proceedings
    4th International Symposium on Process Tomography in Poland, 14.-15.09.2006, Warsaw, Poland
    Proceedings, 83-60660-01-8, 102-105

Publ.-Id: 8584

Isospin dependence of relative yields of K+ and K0 mesons at 1.528 AGeV

Lopez, X.; Kim, Y. J.; Herrmann, N.; Andronic, A.; Barret, V.; Basrak, Z.; Bastid, N.; Benabderrahmane, M. L.; Caplar, R.; Cordier, E.; Crochet, P.; Dupieux, P.; Dzelalija, M.; Fodor, Z.; Gasparic, I.; Gobbi, A.; Grishkin, Y.; Hartmann, O. N.; Hildenbrand, K. D.; Hong, B.; Kang, T. I.; Kecskemeti, J.; Kirejczyk, M.; Kis, M.; Koczon, P.; Korolija, M.; Kotte, R.; Lebedev, A.; Leifels, Y.; Merschmeyer, M.; Neubert, W.; Pelte, D.; Petrovici, M.; Rami, F.; Reisdorf, W.; Ryu, M. S.; Schüttauf, A.; Seres, Z.; Sikora, B.; Sim, K. S.; Simion, V.; Siwek-Wilczynska, K.; Smolyankin, V.; Stoicea, G.; Tyminski, Z.; Wagner, P.; Wisniewski, K.; Wohlfarth, D.; Xiao, Z.; Yushmanov, I.; Zhilin, A.; Ferini, G.; Gaitanos, T.

New results of K+ and K0 meson production in Ru+Ru and Zr+Zr collisions at beam energy of 1.528 AGeV, measured with the FOPI detector at GSI Darmstadt, are presented as a possible probe of isospin effects in high density nuclear matter. The measured double ratio (K+/K0)Ru/K+/K0)Zr is compared with a thermal model and a Relativistic Mean Field transport model using two different collision scenarios and under different assumptions on the stiffness of the symmetry energy. We found a good agreement with the thermal model predictions and the assumption of a soft symmetry energy for infinite nuclear matter.

Publ.-Id: 8583

Si Nanocrystals for nanoelectronic applications

Zacharias, M.; Gösele, U.

First experimental results more than one decade ago demonstrated the visible room temperature luminescence of Si nanocrystals in implanted SiO2 or porous Si which triggered the strong interest in Si nanocrystal based systems and their properties. Basic fundamental questions concerning quantum confinement effects in indirect semiconductors as well as potential applications such as light emission from electrically excited Si nanocrystals, energy transfer to Er 3+ ions, and non volatile Si NC based memories also stimulated the broad scientific interest in this material system. For clarifying the origin of the observed luminescence signal as well as for applications, tight control over the size of the nanocrystals is essential. The talk will give an overview about ways for size controlled Si nanocrystals. Basic properties as well as various applications including memory applications will be discussed in details.

Keywords: Si nanocrystals; optical properties; electronic properties

  • Invited lecture (Conferences)
    E-MRS Spring Meeting 2006, Symposium C, 29.05.-02.06.2006, Nice, France

Publ.-Id: 8582

Ordered Arrays of ZnO Nanowires: Piezoelectric properties and stimulated emission from single wires

Fan, H. J.; Alexe, M.; Hauschild, R.; Kalt, H.; Zacharias, M.

The current interest in ZnO nanostructures is based on its possible applications in UV optoelectronics. In particular, ZnO nanowires (NWs) appear to be promising candidates for the realization of dense arrays of nanosized lasers.
ZnO NW arrays were grown by a modified vapor liquid solid process which results in the Au catalyst remaining at the base of the nanowires. The fabrication process is template assisted involving a gold nanodisk array. This allows a site-specific growth of the ZnO NWs in an ordered hexagonal arrangement. The typical separation of the wires is around 500 nm, the NWs have a narrow size distribution centered at 300 nm and a typical height of 1.5 mm. The effective piezoelectric coefficient of individual ZnO NWs will be shown for various sizes and length. High-excitation phenomena, in particular excitonic nonlinearities and lasing will be reported. Under low excitation the NWs show bulk-like PL spectra. However, there is still a one-dimensional confinement of exciton-polaritons resulting in the absence of the P-band emission resulting from polariton-polariton scattering in thin wires. Stimulated emission is observed in single NWs up to 150 K. Upon raising the excitation fluency a spectrally and temporally narrow emission peak shows up while the P-band emission is quenched. We will show that free carriers are involved in the laser emission. We observe, that the lasing is influenced by details of the wire geometry. Calculations show the increasing leakage of the guided modes into the vacuum when decreasing the wire diameter. There is also a significant leakage of the resonator modes into the substrate. This loss can be reduced significantly by introducing thin layers of gold at the resonator facets as done here.

Keywords: nanowire; stimulated emission; piezoelectric effect

  • Lecture (Conference)
    E-MRS, Symposium E, 29.05.-02.06.2006, Nice, France

Publ.-Id: 8581

Computer Simulations of Epitaxial Recrystallization and Amorphous-to-Crystalline Transition in 4H-SiC

Gao, F.; Zhang, Y.; Devanathan, R.; Posselt, M.; Weber, W. J.

Molecular dynamics (MD) methods have been employed to study the epitaxail recrystallization and amorphous-to-crystalline (a-c) transition in 4H-SiC, with simulation times of up to a few hundred ns and at temperatures ranging from 1000 to 2000 K. Three nano-sized amorphous layers with the normal of a-c interfaces along the [-12-10], [-1010] and [0001] directions, respectively, were created within a crystalline cell to investigate the anisotropies of recrystallization processes. The amorphous structures were analyzed using a topological method, which indicates the complete loss of long-rang order, and the existing short-range disorder is quantified by the fraction of homonuclear bonds. The recovery of bond defects at the interfaces is an important process driving the initial epitaxial recrystallization of the amorphous layers. The amorphous layer with the a-c interfaces normal along the [-12-10] direction can be completely recrystallized at the temperatures of 1500 and 2000 K, but the recrystallized region is defected with dislocations and stacking faults. The temperatures required for complete recrystallization are in good agreement with those observed at experiments. On the other hand, the recrystallization processes for the a-c interfaces normal along [-1010] and [0001] directions are hindered by the nucleation of polycrystalline phases. These secondary ordered phases have been identified as 4H- and 3C-SiC with different crystallographic orientations to the original 4H-SiC. The bond mismatches at the interfaces between different microcrystals result in the formation of number stacking faults. The temperature is an important parameter to control the nucleation of secondary ordered phase, whereas the size of amorphous region has a significant effect on their growth. These results are in good agreement with the previous experimental observations. One of the most important results is that the epitaxial recrystallization of amorphous layer with a-c interface along the c axis is much slower than those long the basal plane, which provides atomic-level insights into the anisotropies in the different activation energies for recrystallization.
Based on a model developed in the previous annealing simulations of 3C-SiC, the activation energy spectra for recrystallization along the three directions have been determined. In general, the activation spectra show that there is a number of activation energy peaks associated with different recrystallization processes. These activation energy values for full recrystallization are in the range of from 1.2 to 1.7 eV for the amorphous layers with the a-c interfaces along [-12-10] and [-1010] directions, and 1.1 to 2.3 eV for the amorphous layer with the a-c interfaces along [0001] direction. However, the highest activation energy of 2.3 eV is consistent with the experimental value of 2.1±0.5 eV reported for 6H-SiC. The internal energy distribution provides a detailed analysis of energy paths to recrystallization, and the nucleation and growth of the secondary ordered phases.

Keywords: SiC; atomistic simulation; recrystallization

  • Poster
    8th Int. Conf. on Computer Simulation of Radiation Effects in Solids (COSIRES 2006), 18.-23.06.2006, Richland, WA, USA

Publ.-Id: 8580

An improved interatomic potential for Ge

Geßner, H.; Posselt, M.

Two classes of interatomic potentials for germanium are presently available: (i) a Stillinger-Weber-type potential with different parametrizations, and (ii) a Tersoff-type potential. The evaluation shows that one group of potentials overestimates the melting temperature considerably, whereas the other group does not give the correct values for the lattice parameter and the cohesive energy. In the present work an improved Stillinger-Weber-type potential is developed by adjusting the two three-body parameters in such a manner, that the potential yields the correct lattice constant and the correct cohesive energy, and the melting point and other properties are reproduced satisfactorily. The influence of the three-body parameters on the structural and thermal properties is investigated in detail. Besides the lattice constant, the cohesive energy, and the melting point, the elastic constants and the thermal expansion coefficient of the diamond structure are considered. Furthermore, the properties of the liquid and the amorphous state as well as different crystalline structures of Ge are studied. Particular attention is paid to point defects and self-diffusion in the diamond structure. The influence of the three-body parameters on the formation and migration energies of vacancies and self-interstitials is investigated.

Keywords: Ge; atomistic simulations; interatomic potential

  • Lecture (Conference)
    8th International Conference on Computer Simulation of Radiation Effects in Solids (COSIRES 2006), 18.-23.06.2006, Richland, WA, USA

Publ.-Id: 8579

Interstitial contribution to self-diffusion in Si: Dependence of the correlation factor on the migration mechanism

Posselt, M.

Self-diffusion in Si is determined by the concentration and the mobility of both vacancies and self-interstitials. The self-diffusion coefficient is usually given by Dsd = fV CV DV + fI CI DI, where CV and CI are the relative concentrations of vacancies and self-interstitials, respectively; DV and DI denote the diffusivities. The quantities fV and fI describe the correlation between the migration of Si atoms and the migration of vacancies and self-interstitials; fV and fI are therefore called correlation factors. The statistical theory of diffusion allows the determination of these factors if certain atomic mechanisms for vacancy and self-interstitial migration are assumed. On the other hand, the self-diffusion coefficient per point defect as well as the point defect diffusivity can be calculated by molecular dynamics (MD) simulations. The ratio of both quantities yields the correlation factors fV and fI. In this manner, they can be determined without any assumption about the atomic migration mechanisms. In the present work, point defect migration and the related atomic mobility are investigated by MD simulations using the interatomic potentials of Stillinger-Weber and Tersoff. It is shown that the value for fV obtained by MD simulations is identical with that determined by the statistical diffusion theory, since the simple atomic mechanism assumed in the theory is also found by the simulations. The mechanisms of self-interstitial migration are more complex. The detailed study, including a visual analysis and investigations with the nudged elastic band method, reveals a variety of transformations between different self-interstitial configurations. MD simulations with the Stillinger-Weber potential show, that the self-interstitial migration is dominated by the dumbbell mechanism, whereas in the case of the Tersoff potential the interstitialcy mechanism prevails. The corresponding values of the correlation factor fI are different.

Keywords: diffusion Si defects atomistic simulations

  • Lecture (Conference)
    8th International Conference on Computer Simulation of Radiation Effects in Solids (COSIRES 2006), 18.-23.06.2006, Richland, WA, USA

Publ.-Id: 8578

Effect of electron-Beam Treatment on Subsequent Titanium Alloy Nitriding

Markov, A. B.; Gunzel, R.; Reuther, H.; Shevchenko, N.; Rotshtein, V. P.; Akhmadeev, Y. K.; Schanin, P. M.

  • Electrotehnica, Elektronica 5-6(2006), 230-233
  • Lecture (Conference)
    8th International conference on Electron Beam Technologies, 05.-10.06.2006, Varna, Bulgaria

Publ.-Id: 8577

Atomic-level simulation of epitaxial recrystallization and phase transformation in SiC

Gao, F.; Devanathan, R.; Zhang, Y.; Posselt, M.; Weber, W. J.

A nano-sized amorphous layer embedded in an atomic simulation cell was used to study the amorphous-to-crystalline (a-c) transition and subsequent phase transformation by molecular-dynamics computer simulations in 3C–SiC. The recovery of bond defects at the interfaces is an important process driving the initial epitaxial recrystallization of the amorphous layer, which is hindered by the nucleation of a polycrystalline 2H–SiC phase. The kink sites and triple junctions formed at the interfaces between 2H– and 3C–SiC provide low-energy paths for 2H–SiC atoms to transform to 3C–SiC atoms. The spectrum of activation energies associated with these processes ranges from below 0.8 eV to about 1.9 eV.

Keywords: SiC; atomistic simulation; recrystallization

Publ.-Id: 8576

Thermohydraulische Modellierung der Kondensation von Dampf in einer unterkühlten Flüssigkeitsströmung

Gregor, S.; Prasser, H.-M.; Beyer, M.

Nach einer kurzen technischen Beschreibung der Mehrzweck-Thermohydraulikversuchsanlage TOPFLOW und der verwendeten Messtechnik werden die theoretischen Grundlagen zur Modellierung der Kondensation von Dampf in einer Wasserströmung erläutert. Dabei gehen die Autoren besonders auf die Auswahl geeigneter Modelle zur Beschreibung des Wärmeübergangs und der Zwischenphasengrenzfläche im Druckbereich zwischen 10 und 65 bar detailliert ein. Außerdem werden verschiedene Drift-Flux-Modelle auf ihre Tauglichkeit anhand von experimentellen Daten geprüft.
Da Veränderungen thermodynamischer und strömungstechnischer Parameter hauptsächlich in axialer Richtung stattfinden, wurden diese Modelle in einen eindimensionalen Code eingebettet, mit dem der Strömungsverlauf entlang einer vertikalen Rohrleitung mit einer Länge von 8 m und einem Nenndurchmesser von 200 mm berechnet werden kann. Anschließend werden Aufbau und Funktion dieses Programms vorgestellt.
Nachfolgend vergleichen die Autoren experimentelle und berechnete Strömungsverläufe bei der Kondensation von Dampf sowohl in einer unterkühlten Wasserströmung als auch nahe der Siedetemperatur. Dabei wird der Einfluss wichtiger Randbedingungen, wie z.B. Druck oder Primärblasengröße, auf die Kondensationsintensität analysiert.
Eine Einschätzung der Fehlerbanden für die experimentellen Daten, die verwendeten Gittersensoren und die numerische Simulation schließen den Bericht ab.

Keywords: multi-phase flow; condensation; wire-mesh sensor; interfacial area; heat transfer; TOPFLOW

  • Open Access Logo Wissenschaftlich-Technische Berichte / Helmholtz-Zentrum Dresden-Rossendorf; FZR-440 2006
    ISSN: 1437-322X


Publ.-Id: 8575

Melt extraction of short metallic filaments: fibre formation process revisited

Cramer, A.; Gerbeth, G.

This article presents experimental results of the extraction of metallic fibres from the surface of a melt pool. A low-melting alloy was used in a model facility to produce filaments of equal length. Operational parameters such as e.g. wheel speed and feed rate were varied to yield thin fibres. The analysis of the fibre cross-section showed, if ever, only a small decrease with increasing wheel speed. Highspeed video recording of the meniscus region was employed to investigate this falling short of expectation with respect to the commonly accepted tendencies. The recordings revealed that short fibres may only be produced during duty cycles. As available literature does not provide explanation of the observations, also a simple model was developed.

  • Journal of Materials Processing Technology 204(2008), 103-110

Publ.-Id: 8573

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