Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

Carbon nanotubes (CNTs) are attracting the attention of many scientists due to their unique structural, mechanical, electronic, chemical and biological properties [1]. Large efforts have been attended to improving their synthesis, determining their structure, measuring their properties and finding applications.
The oxidation process is of great importance for CNT application, since it is necessary for CNT purification and functionalization [2]. Acid treatment was recommended as a method of purification [2]: it removes the non-fullerene components and converts the metal catalyst to a more soluble form. Additionally the oxidation causes chemical and/or structural changes on CNTs, which modify their properties.
Due to their hollow and nanosized structure the use of CNTs as adsorbent for pollutants such as dioxin, Cd2+, Pb2+, Am3+ [3, 4, 5] for environmental remediation purposes has been considered. Li et al. [2003] reported that HNO3 oxidized CNTs show a higher sorption capacity for heavy metal ions compared with other adsorbents such as activated carbon. These studies suggested that CNTs may be a promising adsorbent for use in environmental protection. The high cost of CNTs still limits their practical use.
This study is aimed at gaining information on both the properties of the CNTs as a potential adsorbent material in water purification and the behavior of the CNTs as potential carriers of pollutants in the case of their accidental release to the environment. Two aspects are considered: the changes in surface properties after acid treatment and the influence of acid treatment on the sorption of U(VI).
Different methods (FT-IR, SEM, BET, potentiometric titration) were used to study surface properties of Multi Wall CNTs after acid treatment with HNO3/H2SO4. We observed that the acid treatment has a significant effect on the surface properties but not on the morphology of the CNTs.
In this work we report the first data on CNTs as an adsorbent for U(VI) removal. The acid treatment has a great impact on the adsorption capacity of CNTs for U(VI) at different pH values. The sorption capacity of the modified CNTs increases by almost one order of magnitude due to the modification. Under the given conditions, precipitation of U(VI) from solution could be excluded, i.e. the major adsorption mechanism is obviously surface complexation of the uranyl ion. It is well known that U(VI) forms strong complexes with carboxylic groups [6] which have been identified as the major functional surface groups of our modified CNTs.
Investigations into the effect of acid treatment of CNTs on their dispersibility in aqueous solutions simulating natural ground and surface water as well as the transport behavior of CNTs in porous media are underway.

Reference: [1] Tasis et al. 2006, Chem. Rev., 106, 1105; [2] Chiang et al. 2001, J. Phys. Chem. B, 105, 1157; [3] Long and Yang 2001, J. Am. Chem. Soc., 123, 2058; [4] Li et al. 2003, Carbon, 41, 1057; [5] Wang et al. 2005, Environ. Sci. Technol., 39, 2856; [6] Moll et al. 2003, Radiochim. Acta, 91, 11.

Vanadium and silicon form several binary compounds; the most well characterized structures have the compositions V:Si= 3:1, 5:3, 6:5, 1:2. Spin-polarized density functional band-structure calculations with the PAW-method have been carried out for the stable binary compounds in the system Si-V. As many rare earth and early transition metals belong to the class of refractory materials, the study focuses on the ground state structures and their stabilities determining the elastic properties. The magnetic properties were investigated, as well, because structurally related Si-Mn compounds exhibit magnetism. As result, all investigated silicides are refractory, hard, metallic materials and no magnetism can be found in the ground state
structures.

We show that metallic secondary phase formation inside ZnO(0001) single crystals implant-doped with Ni at an atomic concentration of 5 % can be suppressed. All the Ni ions are in 2+ valence state after mild post-annealing. The suppression is achieved by means of annealing of the crystals in high vacuum prior to implantation and is correlated with structural defects of the ZnO single crystals. The observed ferromagnetic properties of the pre-annealed crystals degrade at ambient temperature within several days.

Ion implantation-induced nanoclusters were synthesized in reactive sputtered Ta2O5 films by Ge+ implantation and subsequent annealing. The effects of ion fluence and post-implantation thermal treatment on the kinetics of the nanoclustering were investigated. Ge+ ions with energy of 40 keV and fluences of 5 × 1015, 1 × 1016 and 5 × 1016 cm− 2 were implanted in the Ta2O5 layers at room temperature. The samples were thermally treated by rapid thermal annealing in vacuum at 700 °C and 1000 °C for 30, 60 and 180 s. Structural studies of all samples were done by Cross-sectional Transmission Electron Microscopy in diffraction and phase contrast mode. Under optimized conditions (high implantation fluence, subsequent annealing) nanoclusters are formed around the projected ion range of the implanted Ge+ ions. The structure of the implanted Ta2O5 matrix changes from amorphous to orthorhombic when the annealing was performed at 1000 °C. Although the Ta2O5 matrix crystallizes, no evidence is obtained for crystallization of the embedded nanoclusters even after annealing at 1000 °C.

An overview on recent results regarding ion beam processing for silicon-based light emission using rare earths is presented

We have developed a new casting technique basing on old recipes from the 17. and 18. century to restaurate old lead pipes as well as to produce new pipes approaching the quality of the old ones. Materials studies of old and new metal probes were performed by metallography, scanning electron microscopy, and x-ray diffraction analysis. Different casting variants as well as the influence of hammering the casted metal plates onto the microstructure were investigated. Moreover we investigated by PIXE (Proton induced X-ray Emission) the residuals of old glue on the old pipes which remaining from tin foil adornment. In this manner we positioned the pipe directly in front of an external ion beam and analysed it without cutting pieces away from the metal pipe.

Reactive Sputtering is a widely used technique in processing of thin compound films. Such films can be sputtered from metal targets, which are comparatively cost efficient. Also the fact that sputtering from metal targets can ccur in the dc mode reduces the cost of the sputtering equipment. To keep the deposition process stable, its necessary to know the effects of target poisoning including its hyteresis behavior. The aim of this work was to nvestigate the evolution of reactive gas coverage on a titanium magnetron target surface, by real time, in-situ ion beam analysis during magnetron sputtering. A cylindrical 2 inch magnetron was used for reactive sputtering of TiN. It was operated in an Ar/N₂ gas mixture at achamber pressure of about 3∙10^-3 mbar. The argon/nitrogen flux ratio was variated between 0 and 20%. The nitrogen concentration on the target was determinated using the ¹⁴N(d, α)¹²C, nuclear reaction at a deuterium beam energy of 1.8 MeV. Depending on the adjusted nitrogen flow the target incorporation varies between 0 and about 1∙10¹⁶ N∙cm^-2. Further the expected hysteresis behaviour ofnitrogen partial pressure, target voltage and nitrogen concentration at increasing/decreasing nitrogen gas flow is confirmed. The lateral distribution of nitrogen was measured across the diameter of target surface. In the zone of higher erosion (the "race track") the nitrogen concentration is 50% lower than in the middle or the edge of the target. A deposition zone in the center of the target could not be detected. By increasing the nitrogen flow into the chamber a saturation in nitrogen content in the target was found at an Ar/N₂ flow ratio of about 10%. Assuming nitrogen implantation with a depth of 2.5 nm under the influence of typical target voltage during magnetron sputtering, this saturation is at a concentration value where stoichiomtric TiN is formed. Within the precision of the measurements, a mobile fraction of nitrogen could not determined. The concentration in the target remains unchanged after switching off the magnetron.

The intermixing and near-interface cluster formation of Pt and FePt thin flms deposited on different oxide surfaces by means of Pt+ ion irradiation and subsequent annealing was investigated. Irradiated as well as post-annealed samples were investigated using high resolution transmission electron microscopy (HRTEM). In MgO and Y:ZrO2 covered with Pt, crystalline clusters with mean size of 2 nm and 3.5 nm were found after the Pt+ irradiation with 8x10^15 cm^-2 and 2x10^16 cm^-2 and subsequent annealing, respectively. In MgO samples covered with FePt, clusters with mean size of 1 nm and 2 nm were found after the Pt+ irradiation with 8x10^15 and 2x10^16 cm^-2 and subsequent annealing, respectively. In Y:ZrO2 samples covered with FePt, clusters up to 5 nm in size were found after the Pt+ irradiation with 2x10^16 cm^-2 and subsequent annealing. In LaAlO3 the irradiation was accompanied by a full amorphization of the host matrix and appearance of embedded clusters of different sizes. The determination of the lattice constant and thus the kind of the clusters in samples covered by FePt was hindered due to strong deviation of the electron beam by the ferromagnetic FePt.

A study investigating the WWER-440/213 reactor pressure vessel (RPV) cladding properties and their degradation due to neutron irradiation has been recently completed under an Hungarian National Government funded project "Evaluation of the lifetime of NPP structural materials". This has been carried out in co-operation with European partners and the key results from this project are presented in this paper. The RPV cladding, designed primarily to protect the RPV ferritic steel from primary water corrosion, also has structural integrity functions. The clad is made of a thin layer of austenitic steel with good ductility and toughness that offers additional robustness to the vessel. The degradation of the properties of the cladding material needs to be carefully assessed in order to properly consider the cladding as a relevant element in the global RPV integrity assessment. By taking into account the cladding it is possible to gain safety margins. This is an important result since it has a direct impact on the global lifetime evaluation of the RPV as well as avoiding the adoption of other very expensive and less effective mitigation methods.

Es existiert kein Abstract.

Using the free-electron laser facility FELBE, we have studied the influence of the intensity on the quadratic autocorrelation measured with two-photon quantum well infrared photodetectors (QWIP). At high illumination powers, the shape of the autocorrelation trace is affected by photocurrent saturation of the two-photon QWIP. We describe the saturation mechanism by different analytical models taking account of the photocurrent nonlinearity in analogy to linear QWIPs and give conditions where true quadratic behavior can be observed.

Particle pulses generated by laser-plasma interaction are characterized by ultrashort duration, high particle density, and sometimes a very strong accompanying electromagnetic pulse (EMP). Therefore, beam diagnostics different from those known from classical particle accelerators such as synchrotrons or linacs are required. Easy to use single-shot techniques are favored, which must be insensitive towards the EMP and associated stray light of all frequencies, taking into account the comparably low repetition rates and which, at the same time, allow for usage in very space-limited environments. Various measurement techniques are discussed here, and a space-saving method to determine several important properties of laser-generated electron bunches simultaneously is presented. The method is based on experimental results of electron-sensitive imaging plate stacks and combines these with Monte Carlo-type ray-tracing calculations, yielding a comprehensive picture of the properties of particle beams. The total charge, the energy spectrum, and the divergence can be derived simultaneously for a single bunch.

Truly table-top sized radiation sources based on compact laser-plasma accelerators require compact and strong focusing devices and efficient short-period undulators. Complementing our recent theoretical work on the feasibility of a table-top FEL, we here present the design and successful experimental characterizations of a 5 mm period length undulator and miniature quadrupole magnets with field gradients of the order of 500  T/m.

Flash-lamp annealing (FLA) oil a millisecond time scale has been shown to be a promising tool in the preparation of high-quality semiconducting materials. The process imposes time varying through-thickness temperature profiles on the substrates being processed, and consequently thermal stresses. A combined thermal and optical model has been developed to predict the substrate temperature distribution and this model has been linked to a structural Model to compute stresses and deflections The paper shows how these models can be used to explore process conditions in flash lamp annealing; with particular regard to the annealing of ion implants in silicon and the crystallization of amorphous silicon layers on glass substrates.

Abstract A recent breakthrough in laser-plasma accelerators, based upon ultrashort high-intensity lasers, demonstrated the generation of quasi-monoenergetic GeV-electrons. With future Petawatt lasers ultra-high beam currents of ∼100 kA in ∼10 fs can be expected, allowing for drastic reduction in the undulator length of free-electron-lasers (FELs). We present a discussion of the key aspects of a table-top FEL design, including energy loss and chirps induced by space-charge and wakefields. These effects become important for an optimized table-top FEL operation. A first proof-of-principle VUV case is considered as well as a table-top X-ray-FEL which may also open a brilliant light source for new methods in clinical diagnostics.

A phenomenological quasiparticle model is surveyed for 2+1 quark flavors and compared with recent lattice QCD results. Emphasis is devoted to the effects of plasmons, plasminos and Landau damping. It is shown that thermodynamic bulk quantities, known at zero chemical potential, can uniquely be mapped towards nonzero chemical potential by means of a thermodynamic consistency condition and a stationarity condition.

We have realized a p-type ZnO surface layer by N+ ion implantation of a high quality ZnO wafer and subsequent annealing. The conduction type of this surface layer was revealed by scanning capacitance microscopy. Rectifying current–voltage characteristics for processed devices were coherent with the existence of an internal pn junction. Deep donor- and acceptor-like defects were investigated by junction deep level transient spectroscopy. The donor-like levels correspond to those commonly observed for E1 and E3 defects. The acceptor states resolved have thermal activation energies of about 150 meV and 280 meV, respectively.

Uranium(IV) sulfate in an aqueous solution and the solid state has been investigated with extended X-ray absorption fine structure (EXAFS) and X-ray diffraction (XRD). The coordination polyhedron comprises monodentate sulfate, bidentate sulfate, and water molecules. The coordination modes of sulfate in solution have been determined from the U-S distances with EXAFS. The U-S distance of 3.67 +/- 0.02 angstrom indicates monodentate sulfate, and the U-S distance of 3.08 +/- 0.02 A indicates bidentate c oordination. The obtained sulfur coordination numbers of a solution with a SO4 (2-)]/[U4+] ratio of 40 suggest species with compositions of, [U(SO4,bid)(2)(SO4,(mon))(2)center dot nH(2)O](4-) and [U(SO4,bid)(3) (SO4,(mon))(2)center dot mH(2)O](6-). Charge-compensating countercations or ion pairing with Na+ and NH4+ could not be detected with EXAFS. One of the solution species, [U(SO4)(5)H2O!
(4.5)[U(SO4)(5 center dot)H2O]center dot H2O [space group P (1) over bar, a = 9.4995(16) angstrom, b = 9.8903(16) angstrom, c = 12.744(2) angstrom, alpha = 93,669(2)degrees, beta = 103.846(2)degrees,gamma = 109.339(2)degrees] has been determined by single-crystal XRD. Two monomeric uranium(IV) sulfate complexes and three sodium units are linked in alternating rows and form a one-dimensoinal ribbon structure parallel to the a axis.

UO2, UO3, Nd2O3, Eu2O3 and Pr6O11 powders were successfully dissolved in the ionic liquid
1-methyl-3-butyl-imidazolium bistriflimide (BumimTf2N) with the help of small amounts of HNO3.
The uranyl species have been characterised through UV-vis and EXAFS spectroscopies. The uranium
dissolution leads to the formation of UO2(NO3)3−, even in the presence of water. The kinetics of UO2
dissolution/oxidation is intricate, due to the transient existence of NO2−, which is absent in the case of
the UO3 dissolution, occurring with no change in the oxidation state of U.

The main objective for the quantification of the fluid mixing in the downcomer and the lower plenum is the demonstration of the safety of the nuclear plant during non-symmetrical transients.

This concerns two main topics: The risk of fragile brittle of the Reactor Pressure Vessel (RPV) during Pressurized Thermal Shock (PTS) transients and the risk of core reactivity excursion during non-symmetrical transient such as Main Steam Line Breaks (MSLB) or Boron Dilution Transients (BDT).

These scenarios are studied in the 1:5 scaled VVER-1000 reactor model at OKB “Gidropress” in the framework of a TACIS project: “Development of safety analysis capabilities for VVER-1000 transients involving spatial variations of coolant properties (temperature or boron concentration) at core inlet”.

Cell calculations are the fundament for all deterministic static and transient 3D full core calculations. The spatial discretization used for the cell calculations influences the results for these transport solutions significantly. The arising differences in the neutron flux distribution due to different spatial discretization are demonstrated. These differences in the flux distribution cause significant changes in the kinf value. Different optimized schemes for economic cell calculations are developed.

We present a hybrid algorithm for the simulation of vicinal surface growth which combines a lattice gas Ising model with a phase-field model. The molecular behavior of individual adatoms is resolved by the kinetic Monte-Carlo (KMC) generated dynamics of the anisotropic Ising model. The microstructure dynamics on the vicinal surface are calculated using the phase-field method. Therefore adsorption processes on two different scales are described. In addition to step-flow growth the hybrid algorithm describes nucleation processes on the terraces which lead to an epitaxial layer-by-layer growth controlled by temperature and by deposition rate. The method is faster than pure KMC simulation and can take into account the stochastic processes in a comparable way.

Star-shaped mesogens with a phloroglucinol or a trimesic acid core and oligobenzoate arms with up to five repaeting units have been synthesised. These non-conventional mesogens possess various columnar mesophases over a broad temperature range. The liquid crystal phases are characterised by optical microscopy, differential scanning calorimetry, X-ray diffraction, dilatometry and Solid State NMR spectroscopy. Beside a high temperature hexagonal columnar phase, the columnar structures transform to higher ordered, undulated columns in a body-centered orthorhombic unit cell at low temperatures. A model of E-shaped folded conformers helically displaced along the columns is proposed. Helical preorganisation in the hexagonal phase precedes the transition to the low temperature phases. Space-filling and nano-segregation compete in the self-organisation process, thus aliphatic chains and polar oligobenzoate scaffold are not perfectly separated for these star-shaped mesogens.

For the ELBE superconducting linear accelerator at FZD a radiofrequency photoelectron injector with a superconducting cavity (SRF gun) is under development. The SRF gun combines the excellent beam quality which can be delivered by RF photoinjectors with the possibility of continuous wave operation. The superconducting niobium cavity of the injector consists of 3½ cells and contains a Cs2Te photocathode which is normal-conducting and cooled by liquid nitrogen. The RF frequency of the cavity is 1.3 GHz. The final electron energy will be about 9.5 MeV and the average electron current will be 1 mA. In the past years the SRF photo injector has been designed and fabricated. Several critical subsystems have been tested. For the cavity, the results of the RF measurements will be shown. An UV driver laser system has been developed which fulfils the different requirements (77 pC @ 13 MHz, 1 nC @ 500 kHz) for the future operation at ELBE. A photo cathode preparation system was developed and installed. The equipment is now in operation and the first series of Cs2Te photo cathodes have been produced.

The Burton-Cabrera-Frank (BCF) model describes the structural evolution of vicinal surfaces in terms of an incoming particle flux and concentration-dependent desorption and surface diffusion terms. A continuum formulation of the BCF scheme given by a phase-field implementation for the moving-boundary problem yields the long-term evolution of the step structure during a step-flow growth mode. Phenomena like step bunching or meandering are well covered by such an approach, but the nucleation of additional structures is not. A particle-based Ising-type approach with a Metropolis-Monte-Carlo kinetics provides such nucleation processes in a temperature-controlled manner and on a shorter time and length scale.

We have integrated both approaches in a hybrid algorithm, which describes adsorption, nucleation, and structure evolution processes at solid-liquid and solid-gas interfaces on both time and length scales. The short-term nucleation from individual adatoms or molecules is resolved by the Monte-Carlo generated dynamics of an anisotropic Ising model, whose interaction parameters stem from first-principles calculations. The long-term microstructure dynamics on the vicinal surface is calculated using the phase-field method. Several growth modes are distinguished by the present scheme: In addition to step-flow growth the nucleation processes on the terraces can lead to roughening or an epitaxial layer-by-layer growth controlled by temperature and by flux. This hybrid algorithm has been applied to the decoration of structured crystalline surfaces with optically active molecules and to the coverage of glass surfaces with inhibitor molecules, which suppress glass corrosion.

The dependence of morphology and composition in molybdenum sulfide nanostructures was investigated by density-functional-based calculations. Sulfur-rich compounds preferentially form two-dimensional platelet structures, which can accomodate a large extent of excess sulfur atoms along the platelet edges. Compounds which are understoichiometric in sulfur, i.e., with a negative sulfur extent, contain a metalloid molybdenum core structure, which is stabilised by Mo-Mo metal bonds and decorated by sulfur atoms. Small species of this type are polyhedral clusters, which grow into nanowires with increasing atom number. Close to the bulk stoichiometry of Mo:S = 1:2 larger polyhedral and partially hollow clusters are obtained, which exhibit an octahedral shape at system sizes of up to a few thousands of atoms and finally grow into spherical fullerenes at even larger sizes.

Nanoscaled materials comprised of organic and inorganic components are becoming more and more important in nanotechnology due to the diversity of applications. The use of self-assembling organic systems as part of such a hybrid material, serving as template for the fabrication of arrays of inorganic nanoparticles, is an attractive approach for the development of new materials. Especially the proteinaceous bacterial surface layers (S-layers) that envelop bacterial cells are attractive for fabricating and patterning of nanostructures. These proteins are composed of protein monomers with the ability to self-assemble into two-dimensional arrays. The regular distributed pores of these paracrystalline arrays work as binding sites for various metals and offer ideal structures for the formation of regular distributed metallic nanoclusters of a defined size (15). Such arrays are very attractive for technical applications ranging from the development of novel catalysts to biomedical applications, development of innovative filter materials, the programmed assembly of nanometre scale electronic devices, and optical industry (7, 9). Another approach is the embedding of S-layer proteins into ceramics thus producing metal binding functionalized nanocomposites (12).
Our current work focuses on the synthesis and characterization of different S-layer templated inorganic nanoclusters. Due to quantum size effects nanosized grains often exhibit changed physical and chemical properties in comparison to bulk material. We demonstrate the fabrication of catalytic active nanoparticles such as ZnO. The photocatalytic properties of ZnO-particles are interesting for their application as nanoscaled catalytic material, i.e. for the degradation of pharmaceutical residues that are released in environment.

Understanding the process of defect formation due to impurity incorporation is crucial for the control of electrical and optical properties of ZnO. The influence of As doping of ZnO on the film structure and properties is much less understood than the effect of N or Al incorporation. Therefore, this study is aimed at investigation of the ZnO film structure and dielectric function modification by doping with As. Two approaches to As incorporation have been used. In the first, the film has been deposited onto Al2O3 (0001) substrate by reactive pulsed magnetron sputtering of a Zn-As target at a temperature of 450 °C, thereby providing an As concentration of 2.0 at % in the layer. In the second approach, epitaxial films of ZnO deposited by the same method on Al2O3 (0001) substrate at 550 °C have been implanted with As+ ions (energy 300 keV) to provide the same As concentration. The film thickness has been adjusted to rule out any ion-beam intermixing effects at the ZnO/Al2O3 (0001) interface. The As concentration has been determined by particle induced X-ray emission, whereas film structure has been characterized by X-ray diffraction (XRD). Spectrscopic ellipsometry measurements have yielded the dielectric function of the films, as well as their thickness and roughness. The results have been compared with the structure and the optical properties of undoped ZnO.
Reactive pulsed magnetron sputtering of Zn target produces ZnO epitaxial layers with single-domain in-plane ordering and strong c-axis texture (rocking curve width of 0.37°). In contrast, ZnO:As layers are in nanostructured state as indicated by the broad peak at 34.2° on the XRD patterns. The doped layers also show much lower degree of in-plane ordering than the undoped films. XRD measurements suggest stronger ZnO structure modification due to the As+ ion implantation than to As incorporation during magnetron sputtering, although the impurity concentration is the same in both cases. The pole figure measurements show that the recrystallization of ZnO films heavily damaged by ion bombardment leads to a much more pronounced in-plane and off-plane textures of the samples annealed at 750-950 °C. The annealing also decreases film thickness which points to evaporation of the damaged layer [1]. The crystalline quality of the annealed ZnO is only slightly lower than that of the epitaxial undoped ZnO. Spectroscopic ellipsometry shows that the dielectric function in the spectral range around the band gap is sensitive to the crystalline quality of the films. The increased defect density in the doped layers broadens the oscillator used for the dielectric function parameterization. The band gap of the doped layers increases compared with that of the undoped ones.

[1] V.A. Coleman, H.H. Tan, C. Jagadish, S.O. Kucheyev, and J. Zou, Appl. Phys. Lett. 87, 231912 (2005).

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With respect to their dynamics semiconductor superlattices (SL) have been much less investigated than other heterostructures like quantum wells. We have already reported on single-color pump-probe measurements on doped superlattices where we could observe the interminiband relaxation and subsequent cooling of the heated electron distribution in the lower miniband of a SL after excitation to the upper one [1]. However, superlattices have a broad absorption range so that excitation at a special k-value in the mini-Brillouin zone influences the electron distribution over the entire zone. Therefore we have performed two-color pump-probe measurements on three doped superlattices to investigate the intraminiband dynamics.
We employed infrared pulses from the free-electron laser FELBE at the Forschungszentrum Dresden-Rossendorf as the pump beam and synchronized broadband THz pulses generated by optical rectification in GaSe as the probe beam. The three GaAs/AlGaAs superlattices had nearly the same doping concentration but different miniband widths of 10 meV, 22 meV and 45 meV, which is smaller or larger, respectively, than the optical phonon energy of 36 meV.
We have first analyzed the cooling behavior. At low temperature, electrons were excited to the upper miniband by the FEL pulse at the zone center of the superlattice mini-Brillouin zone and the interminiband transition was probed at the zone edge. The excited electrons relax back to the ground miniband where they heat up the electronic distribution. A higher temperature leads to more absorption at the zone edge low energy transition. Therefore we found an induced absorption signal after excitation, which decays when the electronic distribution cools down. In the superlattice structures with miniband widths below the optical phonon energy we measured cooling times of 40-50 ps for pump intensities higher than 20 MW/cm². For smaller pump intensities the time constants rose up to 200 ps. The sample with the miniband width of 45 meV showed a much shorter cooling time of 3.5 ps, nearly independent of the pump intensity. This can be explained by the new relaxation channel through polar optical phonons.
Additionally we performed room temperature measurements where the lower miniband is already occupied at the zone edge so that no induced absorption should be observed. In fact we measured a positive transmission change which decays in a few picoseconds in all samples. This can be attributed to a relaxation process in the lower miniband where the electrons at the zone edge relax to the miniband center, which is depleted of electrons by the pump pulse.
[1] D. Stehr et al., Appl. Phys. Lett. 88, 151108 (2006)

A finite-time thermodynamics (FTT) formalism (Andresen, Salamon \& Berry 1977) is proposed to compute the mean flow and fluctuation levels of unsteady, incompressible, shear flows. That formalism yields a definition for a thermodynamic degree of freedom of the velocity fluctuation as well as conditions for local thermal equilibrium. In general, the dynamics of unsteady flow is shown to be in partial thermal equilibrium, a state governed by finite time scales of energy transfer. The FTT model has been successfully applied to shear flows with simple to complex dynamics, e.g. vortex shedding and homogenous shear turbulence.

Turbulent fluid has often been conceptualized as a transient thermodynamic phase. Here, a finite-time thermodynamics (FTT) formalism (Andresen, Salamon & Berry 1977) is proposed to compute mean flow and fluctuation levels of unsteady incompressible flows. The proposed formalism builds upon the Galerkin model framework which simplifies a continuum 3D fluid motion into a finite-dimensional phase-space dynamics and subsequently, into a thermodynamics energy problem. The Galerkin model consists of a velocity field expansion in terms of flow configuration dependent modes and of a dynamical system describing the temporal evolution of the mode coefficients. Each mode may be considered as a wave, parameterized by a wave number and frequency. In our FTT formalism, the mode is treated as one thermodynamic degree of freedom, characterized by an energy level. The dynamical system approaches local thermal equilibrium where each mode has the same energy if it is governed only by internal (triadic) mode interactions. However, in the generic case of unsteady flows, the full system approaches only partial thermal equilibrium with unequal energy levels due to strongly mode-dependent external interactions. In these interactions, large-scale modes typically gain energy from the mean flow while small-scale modes loose energy to the heat bath. The energy flow cascade from large to small scales is thus a finite-time transition phenomenon. The FTT model is first illustrated by a traveling wave governed by a 1D Burgers equation. It is then applied to two flow benchmarks: the relatively simple laminar vortex shedding which is dominated by 2 eigenmodes, and the homogeneous shear turbulence which has been modeled with 1459 modes.

A combined approach of laser fluorescence spectroscopy and confocal laser scanning microscopy was used to study in vivo fluorescent particles in living multispecies biofilms which were exposed to 10-5 M and 5×10-6 M uranium, respectively. These particles ranged between 1 and 7 µm in width and up to 20 µm in length and were located at the bottom and at the edges of biofilms colonies.
Analysis of the amplified 16S rRNA gene fragments have shown that the biofilm consisted of members of the alpha-, beta-, and, gamma-subgroup of Proteobacteria, as well as of representatives of Actinobacteria, Firmicutes, and Bacteroidetes. Restriction fragment length polymorphism (RFLP) screening and cloning of selected clones indicated a similar composition of the microbial communities in biofilm reactors with and without U addition. Both clone libraries were dominated by sequences affiliated to the alpha-subgroup of Proteobacteria. Fluorescence in situ hybridization (FISH) with group specific probes indicated a slightly higher coverage of the uranium contaminated biofilm by cells affiliated with the beta-Proteobacteria. Beta-Proteobacteria are recognized for their ability to reduce uranium in the presence of nitrate. However, an enrichment of additional bacteria known for their ability to reduce U(VI) did not become apparent, neither by 16S rRNA gene retrieval nor by the FISH technique. From both techniques it can be concluded that the multispecies biofilms grown without and with uranium addition were very similar regarding their microbial composition.
Laser fluorescence spectroscopy was used to identify these particles. The particles showed either a characteristic fluorescence spectrum in the wavelength range of 415-475 nm, indicative for uranium-(V), or in the range of 480-560 nm, which is typical for uranium(VI). Particles of uranium(V) as well as uranium-(VI) were simultaneously observed in the biofilms. These uranium particles were attributed for uranium(VI) to biologically mediated precipitation and for uranium(V) to redox processes taking place within the biofilm. The detection of uranium(V) in a multispecies biofilm was interpreted as a short-lived intermediate of the uranium(VI) to uranium-(IV) redox reaction. Its presence clearly documents that the uranium(VI) reduction is not a two electron step but that only one electron is involved.

To simulate dispersed two-phase flows CFD tools for predicting the local particle number density and the size distribution are required. These quantities do not only have a significant effect on rates of mixing, heterogeneous chemical reaction rates or interfacial heat and mass transfers, but also a direct relevance to the hydrodynamics of the total system, such as the flow pattern and flow regime. The Multiple Size Group (MUSIG) model available in the commercial codes CFX-4 and CFX-5 was developed for this purpose. Mathematically, this model is based on the population balance method and the two-fluid modeling approach. The dispersed phase is divided into N size classes. In order to reduce the computational cost, all size groups are assumed to share the same velocity field. This model allows to use a sufficient number of particle size groups required for the coalescence and breakup calculation. Nevertheless, the assumption also restricts its applicability to homogeneous dispersed flows. We refer to the CFX MUSIG model mentioned above as the homogeneous model, which fails to predict the correct phase distribution when heterogeneous particle motion becomes important. In many flows the non-drag forces play an essential role with respect to the bubble motion. Especially, the lift force acting on large deformed bubbles, which is dominated by the asymmetrical wake, has a direction opposite to the shear induced lift force on a small bubble. This bubble separation cannot be predicted by the homogeneous MUSIG model. In order to overcome this shortcoming we developed an efficient inhomogeneous MUSIG model in cooperation with ANSYS CFX. A novel multiple velocity multiple size group model, which incorporates the population balance equation into the multi-fluid modeling framework, was proposed. The validation of this new model is discussed in this report.

Turbulence dispersion is a phenomenon of practical importance in many multiphase flow systems. It has a strong effect on the distribution of the dispersed phase. Physically, this phenomenon is a result of interactions between individual particles of the dispersed phase and the continuous phase turbulence eddies. In a Lagrangian simulation, a particle-eddy interaction sub-model can be introduced and the effect of turbulence dispersion is automatically accounted for during particle tracking. Nevertheless, tracking of particleturbulence interaction is extremely expensive for the small time steps required. For this reason, the Lagrangian method is restricted to small-scale dilute flow problems. In contrast, the Eulerian approach based on the continuum modeling of the dispersed phase is more efficient for densely laden flows. In the Eulerian frame, the effect of turbulence dispersion appears as a turbulent diffusion term in the scalar transport equations and the so-called turbulent dispersion force in the momentum equations. The former vanishes if the Favre (mass-weighted) averaged velocity is adopted for the transport equation system. The latter is actually the total account of the turbulence effect on the interfacial forces. In many cases, only the fluctuating effect of the drag force is important. Therefore, many models available in the literature only consider the drag contribution. A new, more general derivation of the FAD (Favre Averaged Drag) model in the multi-fluid modeling framework is presented and validated in this report.

Many flow regimes in Nuclear Reactor Safety Research are characterized by multiphase flows, with one phase being a continuous liquid and the other phase consisting of gas or vapour of the liquid phase. In dependence on the void fraction of the gaseous phase the flow regimes e.g. in vertical pipes are varying from bubbly flows with low and higher volume fraction of bubbles to slug flow, churn turbulent flow, annular flow and finally to droplet flow. In the regime of bubbly and slug flow the multiphase flow shows a spectrum of different bubble sizes. While disperse bubbly flows with low gas volume fraction are mostly mono-disperse, an increase of the gas volume fraction leads to a broader bubble size distribution due to breakup and coalescence of bubbles. Bubbles of different sizes are subject to lateral migration due to forces acting in lateral direction different from the main drag force direction. The bubble lift force was found to change the sign dependent on the bubble size. Consequently this lateral migration leads to a de-mixing of small and large bubbles and to further coalescence of large bubbles migrating towards the pipe center into even larger Taylor bubbles or slugs. An adequate modeling has to consider all these phenomena. A Multi Bubble Size Class Test Solver has been developed to investigate these effects and test the influence of different model approaches. Basing on the results of these investigations a generalized inhomogeneous Multiple Size Group (MUSIG) Model based on the Eulerian modeling framework has been proposed and was finally implemented into the CFD code CFX. Within this model the dispersed gaseous phase is divided into N inhomogeneous velocity groups (phases) and each of these groups is subdivided into Mj bubble size classes. Bubble breakup and coalescence processes between all bubble size classes Mj are taken into account by appropriate models. The inhomogeneous MUSIG model has been validated against experimental data from the TOPFLOW test facility.

For the investigation of stratified two-phase flow, two horizontal channels with rectangular cross-section were built at Forschungszentrum Dresden-Rossendorf (FZD). The channels allow the investigation of air/water co-current flows, especially the slug behaviour, at atmospheric pressure and room temperature. The test-sections are made of acrylic glass, so that optical techniques, like high-speed video observation or particle image velocimetry (PIV), can be applied for measurements. The rectangular cross-section was chosen to provide better observation possibilities. Moreover, dynamic pressure measurements were performed and synchronised with the high-speed camera system. CFD post-test simulations of stratified flows were performed using the code ANSYS CFX. The Euler-Euler two fluid model with the free surface option was applied on grids of minimum 4∙105 control volumes. The turbulence was modelled separately for each phase using the k-ω based shear stress transport (SST) turbulence model. The results compare well in terms of slug formation, velocity, and breaking. The qualitative agreement between calculation and experiment is encouraging and shows that CFD can be a useful tool in studying horizontal two-phase flow.
Furthermore, CFD pre-test calculations were done to show the possibility of slug flow generation in a real geometry and at relevant parameters for nuclear reactor safety. The simulation was performed on a flat model representing the hot-leg of the German Konvoi-reactor, with water and saturated steam at 50 bar and 263.9°C. The results of the CFD-calculation show wave generation in the horizontal part of the hot-leg which grow to slugs in the region of the bend.

Für die Untersuchung von Luft/Wasser-Strömungen wurde ein horizontaler Acrylglas-Kanal mit rechteckigem Querschnitt gebaut. Der Kanal ermöglicht Gleich- und Gegenstrom-Versuche bei Atmosphärendruck, insbesondere die Untersuchung der Schwallströmung.
Es wurden optische Messungen mit einer Hochgeschwindigkeits-Kamera durchgeführt, die durch synchronisierte dynamische Druckmessungen ergänzt wurden. Für die Analyse der Bilder wurde eine Methode zur Erfassung der Phasengrenze entwickelt und diese anhand möglicher Anwendungen getestet. Die Druckmessungen zeigten, dass der Druck bei Schwallströmungen um einige Kilopascal ansteigt und wieder abfällt, sobald der Schwall aus dem Kanal austritt. Zudem wurden Geschwindigkeiten in der flüssigen Phase mittels nicht invasiver Verfahren gemessen. Das durchschnittliche Geschwindigkeits-Profil am Kanaleintritt wurde mit Ultraschall-Köpfen bestimmt. Die Ermittlung des Geschwindigkeitsfeldes in einem Schwall erfolgte mit PIV (Particle Image Velocimetry).

A novel technique to study the two-phase flow field around an asymmetric diaphragm in a vertical pipe is presented, that enables producing data for CFD code validation in complex geometries. Main feature is a translocation of the diaphragm to scan the 3D void field with a stationary wire-mesh sensor. Besides the measurement of time-averaged void fraction fields, a novel data evaluation method was developed to extract estimated liquid velocity profiles from the wire-mesh sensor data. The flow around an obstacle of the chosen geometry has many topological similarities with complex flow situations in bends, T-junctions, valves, safety valves and other components of power plant equipment and flow phenomena like curved stream lines, which form significant angles with the gravity vector, flow separation at sharp edges and recirculation zones in their wake are present. In order to assess the quality of the CFD code and their underlying multiphase flow and turbulence models pre-test calculations by ANSYS CFX 10.0 were carried out. A comparison between the calculation results and the experimental data shows a good agreement in term of all significant qualitative details of the void fraction and liquid velocity distributions. Furthermore, the report contains a method to assess the lateral components of bubble velocities in the form of a basic theoretical description and visualisation examples. The plots show the deviation of the flow around the obstacle in term of vectors represented the average velocities of the instantaneous cross-sections of all bubbles in the time interval when they pass the measuring plane. A detailed uncertainty analyse of the velocity assessments concludes the presented report. It includes remarks about the comparison with a second method for calculating bubble velocity profiles – the cross-correlation. In addition, this chapter gives an overview about the influence of acceleration and deceleration effects on the velocity estimation.

Two-phase flow experiments at vertical pipes are much suitable for studying the action of different constitutive relations characterizing the momentum exchange at the gas/liquid interface as well as the dynamic behaviour of the gas/liquid interface itself. The flow can be observed in its movement along the pipe and, in particular, within the shear field close to the pipe wall over a considerable vertical distance and, consequently, over a comparatively long time without the immediate separation of gas and liquid characteristic for horizontal flows.
Wire-mesh sensors, which were the working horse in the described experiments, supplied sequences of instantaneous two-dimensional gas fraction distributions with a high-resolution in space and time. This allows to derive from the data not only void fraction and bubble velocity profiles, but also bubble size distributions, bubble-size resolved radial gas fraction profiles as well as the axial evolution of these distributions. An interfacial surface reconstruction algorithm was developed in order to extract the extension of interfacial area from the wire-mesh sensor data. The sensors were upgraded to withstand parameters that are close to nuclear reactor conditions. Most of the experiments were performed for both air/water flow at ambient pressure and steam/water flow of up to 6.5 MPa at identical combinations of the gas and liquid superficial velocities. This offers excellent conditions for studying the influence of the fluid properties.

The works aimed at the further development and validation of models for CFD codes. For this reason, the new thermal-hydraulic test facility TOPFLOW was erected and equipped with wire-mesh sensors with high spatial and time resolution. Vertical test sections with nominal diameters of DN50 and DN200 operating with air-water as well as steam-water two-phase flows provided results on the evaluation of flow patterns, on the be¬haviour of the interfacial area as well as on interfacial momentum and heat transfer. The validation of the CFD-code for complex geometries was carried out using 3D void fraction and velocity distributions obtained in an experiment with an asymmetric obstacle in the large DN200 test section. With respect to free surface flows, stratified co- and counter-current flows as well as slug flows were studied in two horizontal test channels made from acrylic glass. Post-test calculations of these experiments succeeded in predicting the slug formation process.
Corresponding to the main goal of the project, the experimental data was used for the model development. For vertical flows, the emphasis was put on lateral bubble forces (e.g. lift force). Different constitutive laws were tested using a Multi Bubble Size Class Test Solver that has been developed for this purpose. Basing on the results a generalized inhomogeneous Multiple Size Group (MUSIG) Model has been proposed and implemented into the CFD code CFX (ANSYS). Validation calculations with the new code resulted in the conclusion that particularly the models for bubble coalescence and fragmentation need further optimisation.
Studies of single effects, like the assessment of turbulent dissipation in a bubbly flow and the analysis of trajectories of single bubbles near the wall, supplied other important results of the project.

Ziel der Arbeiten war die Weiterentwicklung und Validierung von Modellen in CFD-Codes. Hierzu wurde am FZD die thermohydraulische Versuchsanlage TOPFLOW aufgebaut und mit räumlich und zeitlich hochauflösenden Gittersensoren ausgestattet. Vertikale Teststrecken mit Nenndurchmessern von DN50 bzw. DN200 für Luft/Wasser- sowie Dampf/Wasser-Strömungen lieferten Ergebnisse zur Entwicklung von Strömungsformen, zum Verhalten der Zwischenphasengrenzfläche sowie zum Wärme- und Impulsaustausch zwischen den Phasen. Die Validierung des CFD-Codes in komplexen Geometrien erfolgte anhand von 3D Gasgehalts- und Geschwindigkeitsfeldern, die bei Umströmung eines asymmetrischen Hindernisses auftreten, das in der Teststrecke DN200 eingebaut war. Im Hinblick auf Strömungen mit freier Oberfläche untersuchte das FZD in zwei horizontalen Acrylglas-Kanälen geschichtete Zweiphasenströmungen im Gleich- bzw. Gegenstrom sowie Schwallströmungen. Bei den Nachrechnungen dieser Versuche gelang die Simulation der Schwallentstehung.
Entsprechend des Projektziels wurden die experimentellen Ergebnisse zur Modellentwicklung genutzt. Bei vertikalen Strömungen stand die Wirkung der lateralen Blasenkräfte (z. B. Liftkraft) im Vordergrund. Zum Test unterschiedlicher Modellansätze wurde hierzu ein Mehrblasenklassen-Testsolver entwickelt und genutzt. Darauf aufbauend wurde ein neues Konzept für ein Mehrblasenklassenmodell, das Inhomogene MUSIG Modell erarbeitet und in den kommerziellen CFD Code CFX (ANSYS) implementiert. Bei Validierungsrechnungen zeigte sich, dass vor allem die Blasenkoaleszenz- und –zerfallsmodelle weiter optimiert werden müssen. Untersuchungen zu Einzeleffekten, wie z. B. die Abschätzung von Turbulenzkoeffizienten und die Analyse der Trajektoren von Einzelblasen in unmittelbarer Wandnähe, lieferten weitere wichtige Ergebnisse des Projekts.

We describe a localized proton magnetic resonance spectroscopy (1H-MRS) method for in vivo measurement of lipid composition in very small voxels
(1.5 mm x 1.5 mm x 1.5 mm) in adipose tissue in mice. The method uses localized point resolved spectroscopy (PRESS) to collect 1H-spectra from voxels in intra-abdominal white adipose tissue (WAT) and brown adipose tissue (BAT) deposits. Nonlinear least squares fits of the spectra in the frequency domain allow for accurate calculation of the relative amount of saturated, monounsaturated, and polyunsaturated fatty acids.
All spectral data are corrected for spin-spin relaxation. The data show BAT of NMRI mice to be significantly different from BAT of NMRI nu/nu mice in all aspects except for the fraction of monounsaturated fatty acids; for WAT only the fraction of monounsaturated fatty acids is different. BAT and WAT of NMRI mice differ in the amount of saturated and diunsaturated fatty acids. This method provides a potential tool for studying lipid metabolism in small animal models of disease during initiation, progression and manifestation of obesity-related disorders in vivo.

Our results clearly demonstrate that localized 1H-MRS of adipose tissue in vivo is possible at high spatial resolution with voxel sizes down to 3.4 ml.

Defect studies of Nb irradiated with 10 MeV electrons were performed in the present work by means of positron annihilation spectroscopy. The lattice defects were characterized by positron lifetime spectroscopy. Moreover, defect depth profiles were studied by slow positron implantation spectroscopy. The experimental investigations were accompanied by first principles theoretical calculations of positron parameters. It was found that irradiation-induced vacancies in Nb specimens are surrounded by H, which causes a shortening of the lifetime of trapped positrons. The influence of a Pd and Cr over-layer on the H concentration in the Nb specimens was examined.

Multispecies biofilms were cultured in annular rotating biofilm reactors and subsequently exposed to U(VI) in ecological relevant concentration (5×10-5 M and 5×10-6 M). Such concentrations are comparable with uranium concentrations typically found in seepage waters of uranium tailings, e.g. in Saxony/Germany. The resulting response of the microbial biofilm community to the added U(VI) was then studied by electrochemical oxygen microsensors with tip diameters of 10 µm and by staining methods using the fluorogenic redox indicator 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) and the DNA-binding fluochrome 4,6-diamidino-2-phenylindole (DAPI) in combination with confocal laser scanning microscopy (CLSM). The visualized ratio of CTC-formazan to DAPI intensity was used as an indication of the specific respiratory activity within the biofilms. In addition, 16S rDNA analysis and fluorescence in situ hybridization (FISH) investigations were carried out to study the effect of added uranium on the bacterial diversity.

The microsensor measurements revealed that the oxygen concentration in the multispecies biofilms exposed to uranium decreased faster with increasing biofilm depths in comparison to the uranium free biofilms. Analyses of the amplified 16S rDNA gene fragments showed that the addition of uranium induced no changes to the bacterial diversity in the multispecies biofilms. However, the analyses clearly indicated that a stable multispecies biofilms had developed. The metabolic activity, determined by CTC measurements increased in the upper layers of the biofilms by the addition of the uranium shown by faster oxygen consumptions. This indicates that the bacteria in the biofilms battle the toxic effects of aqueous uranium with an increased metabolic activity proven by the increased CTC activity and in particular by faster oxygen consumption in the biofilm profiles.

... Die Untersuchungsergebnisse sind von grundlegender Bedeutung für Aussagen zur Mobilität dieser nuklide in der Geo- und Biosphäre, denn in wässrigen Sysstemen sind besonders die Actinide in verschiedenen Oxidationsstufen stabil und sie bilden eine Reihe von Komplexen mit anorganischen und organischen Liganden. Durch den sehr niedrigen Konzentrationsbereich, der teilweise durch die geringe Löslichkeit der Actiniden verursacht wird, weisen Datenbanken oft große Lücken und Unsicherheiten auf. Diese gelten vor allem für die unter natürlichen reduzierenden Bedingungen stabilen, niedrigen Oxidationsstufen dieser Elemente.
...
Die meisten erforschten Reduktionsvorgänge verschiedener Actinide liegen im sauren pH-Bereich, die neutralen beziehungsweise basischen pH-Bereiche wurden selten betrachtet. Dabei spielen auch diese pH-Werte eine wichtige Rolle für die Weiterentwicklung thermodynamischer Datenbanken, da diese Datensätze eine entscheidende Voraussetzung zu Aussagen für die Langzeitsicherheit von nuklearen Endlager und Sanierungsvorhaben der Altlasten des Uranerzbergbaus bilden.
Ziel dieser Arbeit war es, die Reduktion von U(VI) mit dem Bioliganden Glucose in dem pH-Bereich vier bis neun zu erforschen.

The presentation gives an overview about the air/water experiments performed in the hot leg model built in the pressure vessel of the TOPFLOW facility.
These experiments were conducted in a flat test-section representing a model of the hot leg of a pressurised water reactor. The region of the elbow and of the steam generator inlet chamber are equipped with glass side walls in order to allow high-speed video observations of the two-phase flow in this region. Three types of experiments were performed: co-current flow experiments, steady counter-current flow experiments and counter-current flow limitation (CCFL) experiments. The test procedures, detailed test matrices and selected examples of the measured data are presented. Furthermore, an analysis of the CCFL experiments is shown, including a comparison with similar experiments and empirical correlations available in the literature.
In the next future, the acquired data will be treated to allow the validation of CFD codes.

The investigation of reactor pressure vessel (RPV) materials from decommissioned NPPs offers the unique opportunity to scrutinize the irradiation behaviour under real conditions. Trepans taken from the whole RPV wall enable a comprehensive material characterisation. The paper describes the trepanning technology applied to the decommissioned WWER-440/230 RPVs of the Greifswald NPP. The Greifswald RPVs represent different material conditions such as irradiated, irradiated and recovery annealed and irradiated, recovery annealed and re-irradiated. The working program is focussed on the characterisation of the RPV steels (base and weld metal) through the RPV wall. The key part of the testing is aimed at the determination of the reference temperature T0 following the ASTM Test Standard E1921-05 to determine the fracture toughness of the RPV steel in different thickness locations. In a first step the material of the core welding seam was investigated. It could be shown that the Master Curve approach as adopted in E1921 is applicable to the investigated original RPV weld metal. The weld metal located in a distance of about 22 mm from the inner surface of the RPV wall yielded a T0 of 50°C which is about 40K higher than T0 close to the inner surface. This outcome is important for the assessment of results retrieved from so called boat samples taken directly from the RPV surface after the recovery annealing. It shows that boat samples do not represent the material with the lowest toughness.

The roughness evolution of carbon films deposited from hyperthermal species was investigated by AFM. 10 eV C deposition at normal incidence angle starts with formation of 10 nm high islands followed by continuous, sp² rich films at larger doses with essentially the same feature height and film roughness. 40 eV C deposition at normal incidence angle (0°) forms sp³ rich, atomically smooth films, which become sp² rich and rough at oblique angles (≥60°). The limitations of currently available molecular dynamic simulations prevent their use to describe the island formation during 10 eV C bombardment. Dedicated calculations probing the effect of incidence angle on 40 eV C deposition exhibit similar trends to the experimental data i.e. decrease of the sp³ fraction and increase of the roughness with increasing incidence angle. The results are in accord with the “subplantation” scheme, linking roughness and sp² bonding to surface entrapment. Implications on recent works discussing growth mechanisms or surface smoothening are given.

We discuss an asymmetry of the decay omega -> e+ e- in nuclear matter with respect to the electron and positron energies. This asymmetry is sensitive to the properties of the omega meson self-energy and, in particular, it has a non-trivial dependence on the omega energy and momentum. Therefore, this asymmetry may serve as a powerful tool in studying the properties of the omega meson in the nuclear medium.

We consider two models of spherically-symmetric MHD α²-dynamos; one with idealized boundary conditions (BCs); and one with physically realistic BCs. As it has been shown in our previous work, the eigenvalues λ of a model with idealized BCs and constant α-profile α₀ are linear functions of α₀ and form a mesh in the (α₀, λ)-plane. The nodes of the spectral mesh correspond to double-degenerate eigenvalues of algebraic and geometric multiplicity 2 (diabolical points). It was found that perturbations of the constant α-profile lead to a resonant unfolding of the diabolical points with selection rules of the resonant unfolding defined by the Fourier coefficients of the perturbations. In the present contribution we present new exact results on the spectrum of the model with physically realistic BCs and constant α. For non-degenerate (simple) eigenvalues perturbation gradients are found at any particular α₀. We briefly discuss the spectral behavior of the α²-dynamo operator over a family of homotopic deformations of the BCs between idealized ones and physically realistic ones. Furthermore, we demonstrate that although the spectral singularities are lifted, a memory about their locations remains deeply imprinted in the homotopic family of spectral deformations due to a hidden underlying invariance.

A new procedure for (U,Pu)O2 nuclear fuel manufacturing based on the oxalic coprecipitation of U(IV) and Pu(III) followed by the thermal conversion of the coprecipitate into oxide is under development. In order to fully investigate the ideality of solid solution with Pu content equal to 50, 30, 15 and 7 at.%, X-ray powder diffraction (XRD) and X-ray absorption spectroscopy (XAS) characterizations at uranium and plutonium LIII edges have been undertaken. Using XRD, a face centred cubic structure was observed in each case, and the cell parameter deduced follows satisfactorily the Vegard’s law. However, extended X-ray absorption fine structure (EXAFS) measurements moderate these results; only the (U0.5Pu0.5)O2 sample leads to the same conclusion as XRD. For the lower plutonium concentration, a disordered hyperstoichiometric structure (U1−yPuy)O2+x has been revealed. In those compounds, cuboctahedral oxygen defects are only located around uranium atoms and not in the Pu environment. A much more complex structure than that suggested by the XRD is thus observed with a non-random distribution of plutonium atoms within the uranium sites of the (U1−yPuy)O2+x structure.

A multi-technique approach using UV-visible absorption spectroscopy, EXAFS spectroscopy combined with the factor analysis based on iterative target testing (ITT), and DFT calculations has been performed in order to investigate the speciation of uranium(VI) nitrate species in acetonitrile and to identify the complex structure of each fundamental species in the system. UV-visible spectral titration suggests that there are four different species in the system, that is, pure solvated species, mono-, di-, and trinitrate species. ITT-based factor analysis extracts the pure EXAFS spectra of fundamental species from the measured spectral mixtures on the basis of the speciation diagram calculated from the UV-visible data. Data analysis of the extracted EXAFS spectra and DFT calculations provide the most probable complex structures of fundamental species. That is, the pure solvated species corresponds to a 5-fold uranyl hydrate complex of [UO2(H2O)5]2+. Nitrate ions tend to coordinate to the uranyl(VI) ions with a bidentate fashion rather than a unidentate one in acetonitrile. The mononitrate species forms a 5-fold complex of [UO2(H2O)3NO3]+, while the di- and trinitrate species show a 6-fold complex arrangement, corresponding to [UO2(H2O)2(NO3)2]0 (D2h) and [UO2(NO3)3]- (D3h), respectively. This study demonstrates that the combination of UV-visible absorption and EXAFS spectroscopies with DFT calculations is very powerful for the structural determination of individual species in the mixed system, in which several different species coexist together.

We briefly review the principles of the Doppler Broadening of the positron annihilation radiation line, the most common technique used in defect depth profiling of solids relevant to dc-beams. We focus on some specific examples of Slow Positron Implantation Spectroscopy (SPIS) investigations related to technological issues such as, for instance,
i) phase transitions in metal coatings possibly induced by internal stresses,
ii) substrate pre-treatment or annealing dependence of defect profiles at metal/polymer interfaces or in the deposited layers, and
iii) near-surface structural modification by ion implantation in ceramics and multilayers.
In each case we elaborate on the possibility of using SPIS results and possible depth profile features as criteria for on- or off-line quality control in industrial processes. We finally conclude with an overall picture of the operating characteristics of positron annihilation techniques.

Some intermetallic alloys exhibit a transition from paramagnetic to ferromagnetic states when they are atomically disordered. This is the case, for example, of the atomically-ordered Fe60Al40 (at. %) alloy, which is paramagnetic at room temperature but becomes ferromagnetic when subject to structural disordered. This effect has been used to generate arrays of ferromagnetic dots embedded in a paramagnetic matrix. This can be accomplished by two different routes: (i) selective mechanical deformation by means of nanoindentation and (ii) local irradiation procedures, either through lithographed masks or using focused ion beam. Concerning route (i), two types of geometries were designed: periodic arrays of triangular dots (resulting from the pyramidal shape of the Berkovich indenter) and arrays of straight lines produced by scratch [1]. Route (ii) has the advantage over nanoindentation that, due to the low doses used, it does not induce any roughening of the surface, avoiding tribological problems. The fabricated entities exhibit a range of magnetic properties depending on the size and shape, which were investigated by means of a magneto-optical Kerr effect setup, while the local character of the induced ferromagnetism was examined by magnetic force microscopy. Furthermore, when the patterned sheets are annealed at sufficiently high temperatures (i.e., around 800 K), the magnetic properties are lost due to the annealing-induced atomic reordering. This method may be easily extrapolated to a variety of other intermetallic systems such as Fe2AlMn, Ni3Sn2, CoAl or CoGa and the obtained magnetic structures may have interesting applications, such as patterned recording media (free tribological problems and detrimental exchange interactions) or magnetic sensors.

[1] J. Sort, A. Concustell, E. Menéndez, S. Suriñach, K.V. Rao, S.C. Deevi, M.D. Baró and J. Nogués. Periodic arrays of micrometer and sub-micrometer magnetic structures prepared by nanoindentation of a nonmagnetic intermetallic compound. Adv. Mater. 18, 1717-1720 (2006).

The PT-symmetric (PTS) quantum brachistochrone problem is re-analyzed as a composite quantum system consisting of a non-Hermitian PTS component and a purely Hermitian component simultaneously. A general approach is proposed for the construction of partially PTS systems which are not reducible to purely Hermitian ones. A natural ingredient of these systems are non-unitary operator equivalence classes (conjugacy orbits) with at least one Hermitian representative. With the help of a geometric analysis the compatibility of the vanishing passage time solution of a PTS brachistochrone with the Anandan-Aharonov lower bound for passage times of Hermitian brachistochrones is demonstrated. Via embedding of the PTS Hamiltonian into a Dirac Hamiltonian the vanishing passage time solution is related to an ultra-relativistic regime.

There is a clear and increasing interest in short time annealing processing far below one second, i.e. the lower limit of Rapid Thermal Processing (RTP) called spike annealing. This was driven by the need of suppressing the so-called Transient Enhanced Diffusion in advanced boron-implanted shallow pn-junctions in silicon technology. Meanwhile the interest in flash lamp annealing (FLA) in the millisecond range spread out into other fields related to silicon technology and beyond. This paper reports on recent experiments regarding shallow junction engineering in germanium, annealing of ITO layers on glass and plastic foil to form an conductive layer as well as investigations which we did during the last years in the field of wide band gap semiconductor materials (SiC, ZnO). A more common feature evolving from our work was related to the modeling of wafer stress during millisecond thermal processing with flash lamps. Finally recent achievements in the field of silicon-based light emission basing on Metal-Oxide-Semiconductor Light Emitting Devices will be reported.

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The main objective for the quantification of the fluid mixing in the downcomer and the lower plenum is the demonstration of the safety of the nuclear plant during non-symmetrical transients. This concerns two main topics: The risk of fragile brittle of the Reactor Pressure Vessel (RPV) during Pressurized Thermal Shock (PTS) transients and the risk of core reactivity excursion during non-symmetrical transient such as Main Steam Line Breaks (MSLB) or Boron Dilution Transients (BDT). These scenarios are studied in the 1:5 scaled VVER-1000 reactor model at OKB “Gidropress” in the framework of a TACIS project: ““Development of safety analysis capabilities for VVER-1000 transients involving spatial variations of coolant properties (temperature or boron concentration) at core inlet”.

The 3-D computational fluid dynamics (CFD) codes provide an effective tool for mixing calculations. In recent years, the rapid development of both the software and the computers has made it feasible to study the coolant mixing in sufficient detail and to perform the calculations for transient conditions. The CFD-Code used was ANSYS CFX. The geometric details of the construction internals inside the RPV have a strong influence on the flow field and on the mixing. Therefore, a detailed representation of the inlet region, the spacer in the downcomer, the elliptical perforated plate and the complicated structures in the lower plenum was necessary. All parts of the lower plenum structures were modeled in detail. The computational grid contained 4.3 Million nodes. In the VVER-1000 reactor, similar characteristic flow and mixing pattern are observed in the case of nominal flow conditions like for Western type PWR. Sensitivity analyses were performed following recommendations included in the ECORA Best Practice Guidelines.

Regarding the flow field and mixing in the downcomer during four loop operation at nominal flow rates, it has been shown that a sharp sector formation like in western 4-loop reactors appears, the flow field is inhomogeneous; in fact high velocity values occur beside the loop positions, and not below the inlet nozzles, which indicates the presence of recirculation areas or stagnant zones. Regarding the flow field and mixing at the core inlet, it has been shown that the mass flow rate distribution is more or less homogenous over the core diameter due to the lower plenum internals, the perturbed sector covers more or less one fourth of the core; a sharp sector formation like in western 4-loop reactors appears, weak mixing zones appear (around 97% of the unperturbed concentration). In most cases, the sensitivity analyses performed did not show any appreciable dependence of the results with respect to the addressed parameters. A three loop operation was chosen to show the differences of the flow and mixing behavior compared to the four loop operation.

An extensive experimental program is now running, studying different flow conditions in the reactor mock up, like the start of the 1st coolant pump or natural circulation conditions with density differences of the primary coolant. Pre and post test CFD simulations are carried out for code validation and for a deeper understanding of the flow and mixing behavior in the VVER-1000 reactor also in the future of the project.

New materials based on metal-oxygen clusters are promising stages of development in nano/micro electronic applications that can lead to the emergence of a new technology. Molecules containing several transition metal ions can exhibit properties similar to nanoscale magnetic particles. Our approach in this field is the study of the electronic structure of these materials by means of spectroscopic investigations in combination with theoretical calculations. The electronic structure of magnetic molecules of the type [Fe(FeL2)3]*4CHCl3 where L=CH3N(CH2CH2O)^2-_2 NO2 has been studied using X-ray photoelectron spectroscopy, soft X-ray emission spectroscopy as well as theoretical density-functional-based methods. Experimental XPS results are in a good agreement to the calculated tDOS, also the partial DOS agrees very well with the XES results. The Fe core level spectra indicate that Fe is in the Fe 2+ state, which is in a good agreement with the calculated magnetic moment of 4 muB/f.u. The neighbouring atoms primarily oxygen and nitrogen exhibit a magnetic polarisation yielding effective spin S=5/2 per iron atom. Due to antiparallel orientation of the magnetic moments of the central Fe ion relatively to three peripheric Fe, the total resulting spin per molecule in the ground state amounts to S=5.

Magnetic micro-objects and their dynamics have attracted considerable attention recently. When excited by either a short (~100ps) magnetic field pulse [1,2], or by a high frequency (>100MHz) sine wave [3] it is possible to investigate various excitations of the magnetization in such thin film structures[4]. Depending on the size and geometry these objects contain either only a single magnetic domain, or a pattern consisting of several domains separated by domain walls and by vortices or antivortices.
We combine time resolved x-ray microscopy experiments with micro-magnetic simulations to gain insight into the dynamics of such objects and into the coupling between the various excitations.
We start with simple discs or squares, where three modes connected to the homogenously magnetized domains, the domain walls and the vortex exist. From there we move to more complex objects. For instance, in a rectangular platelet a configuration containing a stable combination of vortices and an antivortex can be created. Such a single cross-tie wall can be understood as being a coupled micromagnetic system with three static solitons. We report on its magnetization dynamics including the vortex-antivortex interactions. The spectrum of eigenmodes is investigated as well as the effect of different vortex core orientations. We show that the vortex dynamics can be used to identify the core configuration which is not directly accessible to x-ray microscopy because of its limited spatial resolution.

[1] J. Raabe et al., Phys. Rev. Lett. 94, 217204 (2005)
[2] M. Buess et al., Phys Rev. B 74, 100404 (2006)
[3] B. Van Waeyenberge et al., Nature 444, 461 (2006)
[4] R. Hertel et al., Phys. Rev. Lett. 97, 177202 (2006)

The influence of density differences on the mixing of the primary loop inventory and the Emergency Core Cooling (ECC) water in the downcomer of a Pressurised Water Reactor (PWR) was analyzed at the ROssendorf COolant Mixing (ROCOM) test facility. ROCOM is a 1:5 scaled model of a German PWR, and has been designed for coolant mixing studies. It is equipped with advanced instrumentation, which delivers high-resolution information for temperature or Boron concentration fields. This paper presents ROCOM experiments in which water with higher density was injected into a cold leg of the reactor model. Wire-mesh sensors measuring the concentration of a tracer in the injected water were installed in the cold leg, in the upper and lower part of the downcomer. An experiment with 15 % of the design flow rate in one loop and 10 % density difference between the ECC and loop water was selected for validation of the CFD software ANSYS CFX. A mesh with two million control volumes was used for the calculations. The effects of turbulence on the mean flow were modelled with a Reynolds stress turbulence model, Shear stress transport model including gravity terms in turbulence production and dissipation. The results of previous numerical studies showed allready that mixing is influenced by buoyancy effects. In this recent study the numerical grid was improved and extendend.
Therefore ANSYS CFX could predict the observed flow patterns and mixing phenomena a lot better than in previous studies.

Buoyancy driven flow is often presented in many engineering application such as mixing process of fluids with different density. The aim of our study is to simulate mixing processes that are relevant in safety analyses of nuclear reactors. The boron dilution problem is one such safety analysis issue.
Borated and unborated water are of different densities and may be of different temperatures, what causes additional density differences. The degree of mixing of low and high borated coolant is a critical issue with respect to reactivity insertion into the reactor core.
As nuclear reactor geometries are complex, the mixing process not widely understood and the influence of the buoyancy is small, a VeMix (Vertical Mixing) test facility was chosen for validation of numerical models of the mixing process. The experimental data was taken from the optical method and surface wire mesh method.

Magnetic objects of nano- or micrometer dimensions have fascinating properties. They contain regions with long range order between the atomic moments (domains) which are separated by domain walls. These domain patterns can be controlled by the size, the shape and the material. We use x-ray microscopy to study such magnetic micro-objects and investigate their dynamics. Using a stroboscopic pump-probe technique we determine the eigen-modes, their frequencies, damping and the coupling between them.
Starting from simple objects like squares containing only domains, domain walls and a single vortex, we move to more complex objects like rectangles with a cross-tie configuration. The latter contain two vortices and one anti-vortex. Because these three vortices are coupled their dynamics is very complex and depends on the relative vortex orientation despite of a degenerate ground state.
Magnetic microstructures imaged using a photoemission electron microscope (PEEM) and showing homogenous domains separated by domain walls. The square (left) contains a single vortex, the rectangle (right) contains two vortices and an anti-vortex in the center.

Übersichtsvortrag zu neueren Arbeiten zur Funktionalisierung von Oberflächen für Magnetsensor-Anwendungen.

In recent years the tailoring of magnetic properties by means of ion irradiation techniques has become fashionable. Since the magnetic properties of multilayers depend sensitively on the mutual interfaces a modification of these interfaces by light ion irradiation leads to a local modification of the magnetic anisotropy, the exchange bias or the interlayer exchange coupling [1]. As an example it will be demonstrated that ion irradiation in an applied magnetic field allows to set the uniaxial anisotropy direction on a micrometer scale in the case of soft magnetic alloys [2]. However, in order to modify the structural and magnetic properties not only light ion irradiation but also ion implantation doping can be used. If, for example, Cr is implanted in thin Permalloy films the Curie temperature and the saturation mag¬netization can be reduced, which consecutively leads to a decrease of the magnetic anisotropy and an increase of the magnetic damping behavior [3]. The formation of magnetically dead layers at the interfaces to buffer and cap layers can be investigated using Ni implantation [4].
In an alternative route to design magnetic properties periodically modulated substrates are employed [5]. These modulated substrates are created by means of low energy ion erosion. A ripple structure with a typical periodicity of 30 – 50 nm and a ripple height of about 2 nm is created on a Si substrate. Subsequently deposited Permalloy films exhibit a uniaxial anisotropy which is about a factor of 20 larger than conventionally prepared films. If exchange bias bi¬layers are deposited the interplay between the unidirectional and the ripple-induced uniaxial anisotropy contributions can be investigated.

Refs.:

1. J. Fassbender, D. Ravelosona, Y. Samson, J. Phys. D 37, R179 (2004).
2. J. McCord, T. Gemming, L. Schultz, J. Fassbender, M. O. Liedke, M. Frommberger,
E. Quandt, Appl. Phys. Lett. 86, 162502 (2005).
3. J. Fassbender, J. von Borany, A. Mücklich, K. Potzger, W. Möller, J. McCord,
L. Schultz, R. Mattheis, Phys. Rev. B 73, 184410 (2006).
4. J. Fassbender, J. McCord, Appl. Phys. Lett. 88, 252501 (2006).
5. M. O. Liedke, B. Liedke, A. Keller, B. Hillebrands, A. Mücklich, S. Facsko,
J. Fassbender, Phys. Rev. B 75, 220407(R) (2007).

A fascinating property of micromagnetism comes from the possibility to control the domain and vortex configuration through the sample shape and size. For instance, in a rectangular platelet a configuration containing a stable combination of two vortices and an antivortex can be created. Such a single cross-tie wall can be understood as being a coupled micromagnetic system with three static solitons. Here we report on its magnetization dynamics including the vortex-antivortex interactions [1]. The spectrum of eigenmodes is investigated as well as the effect of different vortex core orientations. These are important for the magnetization dynamics since they determine the sense of rotation for the gyrotropic motion. Since three cores are present in total 23 = 8 configurations are possible. On the left side of the figure micromagnetic simulations of the vortex (left, right) and antivortex displacements upon field pulse excitation are shown for the first 16 ns. It is clearly observed that different types of configurations lead to completely different dynamic behaviors. The origin is the dynamic coupling of the cores which is mediated by the exchange coupling through the adjacent domain walls. This coupling is significant and introduces unexpected effects, such as the quenching of gyrotropic motion for the antivortex in certain core configurations. Another consequence is the absence of simple eigen modes describing the vortex gyration. The experimental investigation of the vortex core dynamics by means of time-resolved photoemission electron microscopy using x-ray magnetic dichroism as a contrast mechanism allows to determine the actual core configuration although the lateral core size is below the spatial resolution of the microscope. This is done by comparing the experimentally determined core displacements with the micromagnetically simulated ones as shown on the right side of the figure.

Ion irradiation of Fe60Al40 alloys results in the phase transformation from the paramagnetic, chemically ordered B2-phase to the ferromagnetic, chemically disordered A2-phase. The magnetic phase transformation is related to the number of displacements per atom (dpa) during the irradiation. For heavy ions (Ar+, Kr+, Xe+) a universal curve is observed with a steep increase in the fraction of ferromagnetic phase reaching saturation, i. e., a complete phase transformation, at about 0.5 dpa. However, already less than 0.05 dpa are sufficient to create significant ferromagnetism without a change of surface topography. If light ions are used (He+, Ne+) a pronounced deviation from the universal curve is observed. This is attributed to bulk vacancy diffusion from dilute collision cascades, which leads to a partial recovery of the thermodynamically favored B2-phase. In any case the initial paramagnetic state can be restored by annealing the samples to 800 K. In addition, the potential to create local ferromagnetic areas embedded in a paramagnetic matrix is demonstrated.

Sub-50 nm ferromagnetic dots embedded in a non-magnetic matrix have been controllably generated by selective, low fluence, ion irradiation of paramagnetic Fe60Al40 (atomic %) intermetallic sheets. The process is demonstrated by sequential focused ion beam irradiation and by broad beam irradiation through lithographically defined masks. Due to the low fluences required, this method does not induce any corrugation of the surface. The dots exhibit a range of magnetic properties depending on the size and shape of the structures, with the smallest ones (<50 nm) having square hysteresis loops with coercivities in excess of µ0HC = 50 mT. This presents a promising novel type of patterned recording media free from tribological and exchange coupling effects.

Diluted magnetic semiconductors (DMS) are based on common semiconducting material like GaAs, Si, Ge, GaN or ZnO doped with a few percent of a transition or rare earth metal. While early work has been performed mainly by polish groups in the 1970ies and 80ies, DMS have attracted worldwide scientific attention during the last 7 years due to their application potential in spintronics. This was triggered by the discovery of ferromagnetic GaMnAs and the theoretical prediction of room temperature ferromagnetism for ZnO:Mn and GaN:Mn by T. Dietl and H. Ohno [1]. One of the main obstacles while creating a DMS is secondary phase formation. Since solubility limits are rather low, non-equilibrium doping techniques like low temperature film growth are commonly used. On the other hand, ion implantation offers superb possibilities for low temperature doping but is always connected with lattice damage of the target material. Combining ion implantation with another non-equilibrium technique, i.e. rapid thermal annealing, leads to a diluted state while the crystallinity of the target material is restored. This has been shown for Si:Mn [2].
Recently, we investigated the secondary phase formation for Fe,Co and Ni as well as Gd, Tb implanted in ZnO single crystals. We found, that at elevated temperatures tiny superparamagnetic nanoparticles are formed in all transition metal (TM) doped samples. These phases can hardly be identified using lab X-ray diffraction (XRD), e.g. in the case of Fe. Only application of high resolution methods like synchrotron XRD, susceptometry, Mössbauer spectroscopy and transmission electron microscopy allows their identification [3]. On the other hand, low temperature implantation leads to ferromagnetic properties not originating from conventional superparamagnetism. The applicability of both the granular as well as the diluted magnetic materials will be discussed.

Reference
[1] T. Dietl, et al., Science 287, 1019 (2000).
[2] M. Bolduc, et al., Phys. Rev. B 71, 033302 (2005).
[3] K. Potzger, et al., Appl. Phys. Lett. 88, 052508 (2006).

Atomically ordered Fe60Al40 (at. %) alloys are paramagnetic at room temperature, while disordered Fe60Al40 becomes ferromagnetic. This effect can be used to generate arrays of ferromagnetic dots embedded in a paramagnetic matrix. This can be accomplished by two different routes: (i) selective mechanical deformation by means of nanoindentation and (ii) local irradiation procedures, either through lithographed masks or using focused ion beam. Concerning route (i), two types of geometries were designed: periodic arrays of triangular dots (resulting from the pyramidal shape of the Berkovich indenter) and arrays of straight lines produced by scratch [1]. Route (ii) has the advantage over nanoindentation that due to the low doses used it does not induce any roughening of the surface, avoiding tribological problems. The fabricated entities exhibit a range of magnetic properties depending on the size and shape, which were investigated by means of a magneto-optical Kerr effect setup, while the local character of the induced ferromagnetism was examined by magnetic force microscopy. Furthermore, when the patterned sheets are annealed at sufficiently high temperatures (i.e., around 900 K), the magnetic properties are lost due to the annealing-induced atomic reordering. Hence, these methods may lead to a novel type of patterned recording media free from exchange coupling effects. Moreover, they can be easily extrapolated to a variety of other systems, such as CoZr, CoAl, CoGa, CoV, NiSn, FeGe, FePt3, FeV or even austenitic stainless steel.

[1] J. Sort, A. Concustell, E. Menéndez, S. Suriñach, K.V. Rao, S.C. Deevi, M.D. Baró and J. Nogués. Periodic arrays of micrometer and sub-micrometer magnetic structures prepared by nanoindentation of a nonmagnetic intermetallic compound. Adv. Mater. 18, 1717-1720 (2006).

Experimental and numerical studies of a gas-liquid two-phase flow were applied to a non-baffled laboratory-scale stirred tank reactor, mechanically agitated by a gas-inducing turbine. The dispersion of air as gas phase into isopropanol as liquid phase at room temperature under different stirrer speeds was investigated. The X-ray cone beam computed tomography (CBCT) measurements were taken at five different stirrer speeds starting from 1000 rpm at which the gas inducement occurs for the given operating conditions. The considerable difficulties in acquiring the phase distribution due to beam hardening and radiation scattering effects were overcome by developing a suitable measurement setup as well as by calibration and software correction methods to achieve high accuracy. The computational fluid dynamics analyses of the stirred tank reactor were performed in 3D with CFX 10.0 numerical software.

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From today´s point of view, the so called Accelerator Driven Systems (ADS) seem to have a good chance for playing an important role in a long-term utilization of fission reactor technology. The special task which could be solved by means of these facilities is the incineration of minor actinides, which represent the component of nuclear waste causing the greatest concern. Current concepts of such ADS use an intense spallation neutron source for driving the sub-critical fission system in which the minor actinides are to be burned. The Budker Institute Novosibirsk (Russia) is developing a project of a 14 MeV fusion neutron source, which is primarily destined as irradiation facility for fusion material research. The potential of this neutron source as driver of a minor actinides burner was studied and compared with the spallation driven burner. The lecture presents the main results of the study.

We present a focused ion beam-based approach for the synthesis of an nanofiber network.
The nanofibers, with a homogeneous distribution of diameters of about 25 nm and lengths up to several microns, are synthesized in a self-assembling process without any additional material source at room temperature. It is possible to recrystallize the as-grown amorphous nanofibers by moderate rapid thermal annealing at 473 K. These results have been verified by means of scanning electron microscopy, Auger electron spectroscopy, high-resolution transmission electron microscopy, selected area electron diffraction, and energy dispersive x-ray analysis. As this approach is not limited solely to the material discussed here, other substrates (e.g., GaSb and Ge ) and ion sources should extend this method to other materials, which offers a great potential for future nanoscale devices and applications.

A quasi-particle model is employed to derive from available lattice QCD calculations an equation of state useable in hydrodynamical simulations of the expansion stage of strongly interacting matter created in ultra-relativistic heavy-ion collisions. Various lattice results give an astonishing agreement of the pressure as a function of energy density at large energy densities supposed the pseudo-critical temperature is in the range $170 \pm 15$ MeV, while in the transition region the equation of state is not yet well constrained. Therefore, one can construct a family of equations of state by bridging the uncertain region from the uniquely given high-energy density region part to a hadronic equation of state by suitable interpolation together with the extrapolation to non-zero baryon density by means of the quasi-particle model. We present a series of tests of the model, discuss the chiral extrapolation and the role of Landau damping. We also briefly sketch the path of cosmic matter in the early universe in the phase diagram.

A novel planar array sensor based on electrical conductivity measurements is presented which may be applied to visualize surface fluid distributions. The sensor is manufactured using printed-circuit board fabrication technology and comprises of 64 x 64 interdigital sensing structures. An associated electronics measures the electrical conductivity of the fluid over each individual sensing structure in a multiplexed manner by applying a bipolar excitation voltage and by measuring the electrical current flowing from a driver electrode to a sensing electrode. After interrogating all sensing structures, a two-dimensional image of the conductivity distribution over a surface is obtained which in turn represents fluid distributions over sensor’s surface. The employed electronics can acquire up to 2500 frames per second thus being able to monitor fast transient phenomena. The system has been evaluated regarding measurement accuracy and depth sensitivity. Furthermore, the application of the sensor in the investigation of two different flow applications is presented.

The lattice parameters and magnetic properties of the C15 Laves phase Fe2Zr vary systematically within the homogeneity range of the compound from 66.7 at.% to 74.5 at.% Fe. In the non-stoichiometric compounds Zr is partly substituted by the excess Fe. Themagnetic moment per iron atom increases with the Fe content. Electronic structure calculations show an enhanced magnetic moment of the excess Fe at the Zr site, which explains well the composition dependence.

We present an ultra-fast electron beam X-ray computed tomography technique usable for imaging of fast processes, such as multi phase flows or moving parts in technical or biological objects. The setup consists of an electron beam unit with fast deflection capability and an ultra fast multi-element X-ray detector and achieves 10,000 frames per second image rate. Since full sampling of the Radon space requires an angular overlap of source path and detector which strongly decreases axial resolution we devised a limited-angle type tomography. As a demonstration we visualised the movement of particles and gas bubbles rising in a stagnant liquid.

In vielen Bereichen der Industrie bestimmen Mehrphasenströmungen die Effizienz und Sicherheit von technischen Verfahren und Prozessen. Ob in Chemiereaktoren, bei der Erdölförderung und -verarbeitung, in Wasseraufbereitungsanlagen, Biogasanlagen, Kraftwerkskühlkreisläufen oder bei Strömungsmaschinen wie Pumpen und Turbokupplungen komplexe mehrphasige Strömungsformen von Stoffgemischen sind im industriellen Umfeld immer schwierig messtechnisch zu erfassen. Im Forschungszentrum Dresden-Rossendorf wurden Sensoren entwickelt, die mehrphasige Strömungen mit sehr hoher räumlicher und zeitlicher Auflösung vermessen. Erstmalig können damit komplexe Strömungen in beliebigen Stoffgemischen sichtbar gemacht werden.

A high voltage (HV) pulse generator based on Blumlein technology was used to implant nitrogen ions into silicon substrates by immersion in a plasma generated by the HV pulse itself. Working pressures, applied HVs and treatment times were varied. Elemental depth profiles determined by Auger electron spectroscopy showed deeper penetration for higher voltages and broader profiles for increased treatment times indicating higher diffusion, as expected. Penetration depths, however, were about half of the values calculated by the SRIM code, probably due to the short duration of the HV pulse. The high-resolution x-ray diffraction ω/2θ scans measured around the (0 0 4) Si Bragg reflection of implanted samples had shoulders in the lower ω side, indicating a lattice expansion in the direction normal to the surface. Dynamical diffraction theory of Takagi–Taupin was used to fit the measured spectra, thus finding the strain profiles in the implanted samples. Both maximum strain values and integrated strains increased for samples implanted with higher voltages and treatment times and were almost independent of pressure.

To determine the influence of humic acid (HA), pH, and presence of atmospheric CO2 on the sorption of U(VI) onto kaolinite, the structure of the surface complexes was studied by U LIII edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The best fits to the experimental EXAFS data were obtained by including two uranium coordination shells with two axial and five equatorial oxygen atoms at 1.77 ± 0.02 and 2.34 ± 0.02 Å, respectively, and two coordination shells with one Al/Si atom each at 3.1 and 3.3 Å. As in case of the binary system U(VI)-kaolinite, uranium forms inner-sphere surface complexes by edge-sharing with aluminum octahedra and/or silicon tetrahedra. HA and atmospheric CO2 as well as pH had no influence on the EXAFS structural parameters in the pH range of 5 – 8. In spite of the presence of HA, U(VI) prefers to sorb directly onto kaolinite and not to HA that is bound to the clay surface. X ray photoelectron spectroscopy (XPS) measurements of kaolinite particles that had been exposed to HA suspensions showed that significant parts of the kaolinite surface are not covered by HA.

We discuss coherent and incoherent phi meson photoproduction off the deuteron at low energy and small momentum transfer with the aim to check whether the recent experimental data need for their interpretation an inclusion of exotic channels. Our analysis of the differential cross section and spin-density matrix elements shows that new data on the gamma D phi X reaction at E_gamma 2~GeV may be understood on the basis of conventional dynamics. However, a certain ambiguity of the deviation between the model predictions and the LEPS data on gamma p phi p reaction still remains. For a firm conclusion about a possible manifestation of exotic channels one has to improve the resolution of the data with providing additional information on channels with spin- and double-spin flip transitions being sensitive to the properties of the photoproduction amplitude in gamma p and gamma D reactions which may be used as an additional independent test of the phi meson photoproduction mechanism.

Metal plasma immersion ion implantation and deposition (MePIIID) has been proved to be an effective approach to enhance surface properties of various types of materials. In this work structure, phase composition, microhardness and surface properties, such as wettability and surface energy of layers of the ternary system Ti-N-O produced by MePIIID were investigated. To study the correlation between structure of coating and hemocompatibility the thrombocyte adhesion as well as the fibrinogen adsorption on the surface were measured. The blood compatibility of Ti oxide can be improved by the addition of nitrogen into the layer. The thrombocyte adhesion and fibrinogen adsorption were lower for TiNxOy than for TiO2. This correlates with a lower hydrophobicity and higher polar component of the surface energy for TiNxOy. The best hemocompatibility as well as the maximal microhardness have been found for the coating TiN0.4O1.6.

Cyclam macrocycles tetrasubstituted with amino-, thiourea-, and sugar-terminated side chains are ionized by electrospray ionization mass spectrometry (ESI-MS) as singly or doubly protonated species or as transition-metal complexes. Their fragmentation behavior is examined in a Fourier-transform ion-cyclotron-resonance (FT-ICR) mass spectrometer by collision-induced dissociation (CID) experiments. Typically, fragmentation occurs within the side chains through a number of different 1,2-elimination reactions irrespective of the absence or presence of a transition metal ion such as Co²⁺, Ni²⁺, or Zn²⁺. A remarkable exception is Cu²⁺ which induces ring cleavage reactions. This is traced back to an electron transfer from the cyclam nitrogen atoms to the Cu²⁺ ion. The electron transfer creates a cation-radical within the macrocycle which induces typical fragmentation reactions such as -cleavages that lead to fragmentation within the macrocycle. This interpretation is in line with fragmentation experiments on unsubstituted cyclam and its complexes

Derivatives of 1,10-phenanthroline and their metal complexes are of considerable interest in bioinorganic chemistry, biology and medicine.¹ In this nexus, dendritic modification gain in importance as they open the way for tailoring nano dimension, and solubility or complexation behaviour.² We described hydrophobic oxybathophenanthroline ligands with attached Frechet-type dendrons capable of forming stable copper(II) complexes in organic media.³ Currently, effort is also to be devoted to the synthesis of hydrophilic 4,7-diphenyl-1,10-phenanthroline ligands LG1 – LG3 containing short methoxypolyethoxy (‘PEG’) groups on the surface. Radiotracer experiments using ⁶⁴Cu and time-resolved laser-induced spectroscopy point to a spontaneous formation of stable 1:2 complexes (metal:ligand) with copper(II) in aqueous solution.

Ziel ist die Herstellung mehrfunktionaler Liganden mit peripheren Biomolekülen sowie einem metallbindenden Zentrum. Als Kernbaustein wird Cyclam ausgewählt, das zur stabilen Bindung von diagnostisch und therapeutisch relevanten Radiometallnukliden, wie ^64/67Cu, ^99mTc und ¹⁸⁸Re, geeignet ist. Die Anknüpfung von Biomolekülen, wie Zucker oder Peptide, an die Oberfläche des Ligandgerüstes soll die Einstellung definierter Löslichkeits- und selektiver Bindungseigenschaften erlauben.

Two novel glycocluster ligands with cyclam core bearing thiourea-linked D-glucose and 2-acetamido-2-deoxy-D-glucose at the periphery have been synthesized. The interaction with concanavalin A has been studied by isothermal titration microcalorimetry for characterizing protein-ligand interactions. The sugar-containing multivalent ligands showed higher association affinity compared to the sugar monomers which is attributed to an entropy driven glycoside clustering effect.

Metalloradiopharmaceuticals of the metallic radionuclides ^64/67Cu, ^99mTc, ^186/188Re and ⁹⁰Y are often used for diagnostic and therapeutic purposes.¹ Ligand multimers derived from radionuclide chelating macrocycles are attractive candidates for applications in oncologic diagnostics and endoradiotherapy. In this perspective, cyclam and its derivatives form very stable complexes in particular with transition and rare earth metal ions as the radionuclides mentioned above.² Recently, we could show that a star-like cyclam ligand appended with four PEG-arms rapidly forms stable copper(II) complexes.³

Currently, we are focusing our attention on the development of dendritic ligands having both enhanced complex stability and improved bio-availability. Branched ligands possessing biomolecules such as sugar and peptide moieties may show both unique cell uptake behaviour and specific ligand-receptor interaction. In this nexus, we built up ligands I and II with a cyclam core. D-glucose, 2-acetamido-2-deoxy-D-glucose and the hexapeptide (Arg-Arg-Pro-Tyr-Ile-Leu-OH) have been chosen as biomolecules.

The interaction of the sugar-containing ligands I with concanavalin A has been studied using isothermal titration microcalorimetry in order to characterize protein-ligand interaction. The results clearly indicate a cluster glycoside effect.

The preparation of asymmetric branched ligands possessing peptide moieties is currently in progress. Using this synthetic concept, it appears possible to adjust the solubility properties, the binding behaviour, and the biodistribution of these bioinspired ligands. The corresponding radiocopper complexes would allow the non-invasive in vivo imaging and therapeutic application.

1 X. Liang and P. Sadler, Chem. Soc. Rev., 2004, 33, 246.
2 R. M. Izatt, K. Pawlak, J. S. Bradshaw and R. L. Bruening, Chem. Rev., 1991, 91, 1721.
3 H. Stephan, G. Geipel, D. Appelhans, G. Bernhard, D. Tabuani, H. Komber and B. Voit, Tetrahedron Lett. 2005, 46, 3209.

In our effort to develop imaging agents for brain glucocorticoid receptors, we have prepared several novel glucocorticoids possessing a 2-methylsulfanyl-acetyl side chain. The synthesis was accomplished via a Mitsunobu reaction with thiobenzoic acid starting from cortisol, prednisolone, dexamethasone and triamcinolone acetonide to give the corresponding S-thiobenzoates in 75-82% yield. Subsequent saponification and reaction with methyl iodide afforded C-21 methylthioethers in 68-82% yield. All compounds were tested in an in vitro glucocorticoid receptor binding assay. Triamcinolone acetonide-based compound 12 showed promising binding affinity of 144% relative to dexamethasone (100%).
Compound 12 was selected for radiolabeling with the short-lived positron emitter carbon-11. The radiolabeling was carried out starting from S-thiobenzoate 8 and in situ formation of the corresponding sodium thiolate, which was further reacted with [¹¹C]methyl iodide. The obtained radiochemical yield was 20-30%. The specific activity was determined to be 20-40 GBq/µmol at the end-of-synthesis, and the radiochemical purity exceeded 98%.

We report on a measurement of electron pair production in ¹²C+¹²C collisions at an incident energy of 2 GeV per nucleon with the HADES spectrometer. The measured pair production probabilities span over five orders of magnitude from the pi⁰-Dalitz to the rho/omega invariant-mass region. Dalitz decays of pi⁰ and account for all the yield up to 0.15 GeV/c², but for only about 50% above this mass. The excess yield is in agreement with the former DLS result if one assumes that it scales with beam energy like pion production. A preliminary analysis of ¹²C+¹²C collisions at an incident energy of 1 A GeV, measured with the HADES detector, supports this scenario.

The presentation gives an overview over tomographic and imaging techniques applied to multiphase flow diagnostic, including wire-mesh sensors, gamma and X-ray tomography. Industrial and scientific applications are being discussed.

The experimental investigation of flow phenomena plays an important role in many fields of research as well as in industrial applications. Wire-mesh sensors allow the study of flows with high spatial and temporal resolution. This type of sensor was introduced about ten years ago and since then it has been employed to investigate a number of single phase and two-phase flow phenomena. This paper gives an overview of the principles of operation and the obtaining of physical flow parameters from the electrical measured signals of wire-mesh sensors. Besides describing the state of the art of wire-mesh sensor technology, the latest development of a temperature wire-mesh sensor is depicted in detail.

Motiviert durch die in der Literatur bisher unvollständige Beschreibung der Relaxation hochgeladener Ionen vor Festkörperoberflächen, besonders in Bezug auf den Eintrag potenzieller Energie in Oberflächen und der Aufstellung einer vollständigen Energiebilanz, werden in dieser Arbeit komplement äre Studien präsentiert, die sowohl die Ermittlung des Anteils der deponierten potenziellen Energie als auch die Ermittlung der emittierten potenziellen Energie ermöglichen. Zum Einen wird
zur Bestimmung des eingetragenen Anteils der potenziellen Energie eine kalorimetrische Messanordnung verwendet, zum Anderen gelingt die Bestimmung der emittierten potenziellen Energie mittels doppeldifferenzieller Elektronenspektroskopie. Für vertiefende Studien werden Materialien unterschiedlicher elektronischer Strukturen (Cu, n-Si, p-Si und SiO2) verwendet. Im Falle der Kalorimetrie wird festgestellt, dass die eingetragene potenzielle Energie linear mit der inneren potenziellen Energie der Ionen wächst. Dabei bleibt das Verhältnis zwischen der eingetragenen potenziellen Energie und der inneren potenziellen Energie nahezu konstant bei etwa (80±10)%. Der Vergleich von Cu, n-Si und p-Si zeigt im Rahmen der Messfehler keine signifikanten Unterschiede in diesem Verhältnis. Es liegen jedoch deutlich unter jenem von SiO2. Die Elektronenspektroskopie liefert ein dazu komplementäres Ergebnis. Für Cu und Si konnte ebenfalls eine lineare Abhängigkeit zwischen emittierter Energie und innerer potenzieller Energie festgestellt werden. Das Verhältnis wurde hierfür bis zum Ladungszustand bis Ar7+ zu etwa (10±5)% unabhängig vom Ladungszustand bestimmt. Im Gegensatz dazu liefert SiO2 eine nahezu verschwindende Elektronenausbeute. Für Ar8+ und Ar9+ steigt die Elektronenausbeute wegen der Beiträge der LMM-Augerelektronen f¨ur alle untersuchten Materialien leicht an. Der Anteil der emittierten Energie eines Ar9+-Ions wird f¨ur Cu und Si zu etwa 20% und f¨ur SiO2 zu etwa 10% angegeben. Diese Ergebnisse sind in guter Übereinstimmung mit den Kalorimetrieexperimenten und erfüllen die Energiebilanz.
Zusätzlich werden die experimentellen Ergebnisse mit einer Computersimulation modelliert, welche auf dem erweiterten dynamischen klassischen Barrierenmodell basiert. Aus diesen Rechnungen kann zudem jener Anteil der deponierten potenziellen Energie erhalten werden, welcher durch Bildladungsbeschleunigung vor der Oberfläche in kinetische Energie umgewandelt wurde.

The present study considers the solidification of an Al-7wt%Si alloy under the influence of electromagnetic melt stirring using a rotating magnetic field (RMF). The effect of a continuously applied RMF is compared with an RMF pulse sequence of alternating direction (RMF-PSAD). The resulting flow structure in a cylindrical liquid metal column has been measured by isothermal experiments using the ternary alloy GaInSn. The solidification experiments performed with the Al-7wt%Si alloy confirm our numerical predictions concerning the temperature field during solidification and the distribution of primary crystals and eutectic phase in the solidified samples. The application of the RMF-PSAD regime at suitable frequencies of the reversals of the magnetic field direction fP delivers an equiaxed microstructure without macrosegregation.

At the present time, great efforts are undertaken in order to improve the properties of photovoltaic elements. In case of Si-based solar cells the interest is focussed on a higher electrical efficiency connected with a more effective technology and with economies of scale. One of the possible methods for the realisation of this ambitious goal represents the Flash Lamp Annealing (FLA) technology. FLA allows a fast heating up of solid surfaces with a single light flash between some hundred microseconds and some milliseconds. Thereby, the achievable final temperature of the surface layer could be higher than the melting point depending on the intensity of the light flash. This method was applied to the modification of thin amorphous Si layers on SiO2 and glass. Slow Positron Implantation Spectroscopy (SPIS) was used for the characterisation of the microstructure before and after FLA. Changes in the structure down to a depth of some micrometers below the surface observed with SPIS will be presented and discussed.

A major limitation for future nanotechnology, particularly for bottom-up manufacturing is the non-availability of 2-dimensional massively parallel probe arrays. Scanning proximity probes are uniquely powerful tools for analysis, manipulation and bottom-up synthesis: they are capable of addressing and engineering surfaces at the atomic level and are the key to unlocking the full potential of Nanotechnology. Generic massively parallel intelligent cantilever-probe platforms is demonstrated through a number of existing and ground-breaking techniques. A packaged VLSI NEMS-chip (Very Large Scale Integrated Nano Electro Mechanical System) incorporating 128 proximal probes, fully addressable with control and readout interconnects and advanced software will be presented.