Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

Auf der Jahrestagung Kerntechnik 1999 in Karlsruhe wurden in der Sektion 2 vier technische Sitzungen zu den Themenschwerpunkten: Stabilitätsuntersuchungen in Zweiphasenströmungen, DWR Integralexperimente, Experimente und Code-Validierung für WWER-Reaktoren, 3D Phänomene, Methoden und Rechnungen sowie 1 Postersession durchgeführt. Die einzelnen Beiträge werden in dem vorliegenden Artikel zusammengefaßt.

We show that coalescence of nucleons emitted prior to thermalization in highly excited nuclei can explain the anomaly of kinetic energies of helium fragments. This mechanism accomplishes the statistical approach to nuclear rections formerly used to describe intermediate mass fragment production.

Fragment yields and energy spectra of He, Li and Be isotopes have been measured in p+⁹Be interactions at 1 GeV incident energy at 30 deg. and 126 deg. using a two-arm
spectrometer based on Bragg chambers. The obtained yield ratio of the low abundant
⁸He to ⁸Li in the spectator momentum range is compared with published data.

Depth profiling of light elements in the surface layer of solids by means of ERDA is limited to depths smaller than about 1 mu m. In order to measure the distributions
of hydrogen isotopes in larger depth regions such as needed for the divertor tiles from the tokamak fusion experiment ASDEX-Upgrade in the Max-Planck-Institut
fur Plasmaphysik, samples cut from the divertor tiles (about 0.6 mm tungsten on carbon) were bevelled at different angles by mechanical cutting and polishing or by
ion beam slope cutting. The mechanically cut slopes were additionally cleaned and polished by oblique incidence ion beam sputtering with target rotation to avoid
anisotropic etching.
The surface composition along the slope was measured by ERDA with the Rossendorf Nuclear Microprobe, A 10 MeV Si ion beam was focused to about 3 mu m
and scanned linearly across the slope. The hydrogen and deuterium recoils were detected by a surface barrier detector at a recoil angle of 30 degrees after passing
through a 10 mu m thick Al foil for stopping scattered primary ions. Deuterium was found at depths up to about 25 mu m and hydrogen up to 750 mu m on the
samples analysed.

Isotopic yield ratios were derived from double-differential cross sections measured in p+⁹Be and p+¹²C collisions at 1 GeV. Enhanced ⁹He yields were found compared to heavier target nuclei.

Aufgrund der nur unvollkommen erfaßbaren, keineswegs stabilen chemischen Struktur und Funktionalität von Huminsäuren aus natürlichen Recourcen haben operationell mit HS übereinstimmende, synthetische Polymere als Modellhuminsäuren in vielen Bereichen der experimentellen Wissenschaften Eingang gefunden. Sie sind vor dem Hintergrund der Umweltkontaminationen durch toxische und radiotoxische Schwermetalle besonders für Bindungsstudien mit Metallionen interessant, da ihre Funktionalität durch Variation der Precursoren und der Synthesebedingungen in weiten Grenzen "eingestellt" werden kann.

Wir haben FTIR-spektrometrisch die festen Komplexe von Uran(VI) mit unterschiedlichen, gut charaktersierten Huminsäuren, zwei natürlichen Huminsäuren (Aldrich-Huminsäure und eine aus Moorwasser des "Kleinen Kranichsees" isolierte Huminsäure) sowie drei synthetischen Huminsäuren (zwei Melanoidinfraktionen sowie das Oxidationsprodukt von 3,4,5 Trihydroxybenzoesäure), verglichen. Trotz unterschiedlicher Herkunft der Huminsäuren, die in ihren FTIR-Spektren deutlich wird, scheint die Urananbindung an die Huminsäuren sehr ähnlich zu sein, was durch EXAFS-Untersuchungen an Uranylkomplexen natürlicher Huminsäuren und synthetischer Huminsäuren vom Melanoidintyp gestützt wird. Sowohl die asymmetrischen UO22+-Streckschwingungen im mittleren Infrarot, die wir zwischen 933.5 cm-1 und 923.1 cm-1 registrieren, als auch die UO22+-Deformationsschwingungen im fernen Infrarot zwischen 264.8 cm-1 und 260.6 cm-1 sind gut miteinander vergleichbar.
Die Einflußnahmen der strukturellen und funktionellen Unterschiede der Huminsäuren spiegeln sich in unterschiedlichen Verschiebungen der Uranylbanden wieder.

For better understanding of the influence of the deleterious elements on the neutron embrittlement of reactor pressure vessel steels, the microstructural evolution due to neutron irradiation was investigated by SANS experiments at ternary iron alloys with different contents of copper and phosphorus. In every case irradiation produced nanoscaled inhomogeneities. The volume fraction of the inhomogeneities incrases with the copper content but not with the phosphorus content. Surprisingly, the high-pure alloy shows a relatively high irradiation effect. The irradiation defects vary in type and kinetic of evolution for the different alloys.

Al_xGa_1-xAs epitaxial layers grown on semi-insulating GaAs (SI-GaAs) substrates with x=0,25 and on in-doped GaAs with x= 0,45 were implanted at room temperature with 1.5 MeV Se ions to fluences ranging from 0,6x10¹⁴ to 4x10¹⁴ at/cm² . Implanted crystals were analyzed with 1.8 MeV ⁴He ions using the RBS/channeling technique. In order to determine the defect structure and their depth profiles measurements were carried out at two different temperatures: 295 and 105 K. XRD with synchrotron beams was used for structural analysis. White beam topography was used to detect lattice deformation and strain relaxation. The lattice parameters were determined by rocking curve measurements with a monochromatic beam. For all samples a signifcant tetragonalization of the implanted region was observed. For the layers grown on SI-GaAs the lattice distortion increases with increasing ion dose until a critical lattice strain is attained. At that point the complete relaxation of the epitaxial layer occurs. In contrast, no relaxation was observed for layers grown on practically defect free indoped substrates. The mechanism of this transformation is attributed to the much higher dislocation density in the SI-GaAs substrate. Nucleation of new dislocations at defects produced by ion implantation and their interaction with threading dislocations leads to the strain relaxation by plastic
deformation of epitaxial layers.

A computer program was designed for the conversion of element-resolved energy spectra obtained from elastic recoil
detection analysis into composition depth profles. The algorithm is described in detail. An important feature of the
program is that the contribution of the analyzed elements to the stopping power is accounted for self-consistently. Extensions of the algorithm address the problem of the simultaneous analysis of recoil and forward scattering events, respectively, and of distortions in the recoil spectra due to the presence of target constituents with isotopic distributions that cannot be resolved when using standard ionization chambers for recoil detection. Two examples are given to demonstrate how one may benefit from the capabilities of the conversion program in the context of specifc thin film problems. The first example addresses compositional changes in Si3N4/C bilayers due to N-ion implantation, while the second one is dealing with the problem of impurity accumulation at the hexagonal-to-cubic phase boundary in BN films.

This paper describes investigations using the Particle Induced X-ray Emission method (PIXE) to evaluate the con-
centration of Ca together with that of other elements (S, Cl, and K) across the articular cartilage of domestic pigs in
order to verify a putative influence of calcium on the structural changes in early arthrotic stage, derived from NMR
microscopic imaging. The measurements were carried out with focused ion beams of a lateral resolution down to
20 lm. The main matrix components (C, N, O) were evaluated from proton backscattering spectra.

The photoluminescence (PL) and electroluminescence (EL) properties of Ge-implanted SiO₂ films thermally-grown on a Si substrate have been investigated and compared to those of Si-implanted SiO₂ films. It is found that the blue-violet PL from both Si and Ge-rich layers reaches a maximum after annealing at 500°C for 30 min. The PL and EL from Ge-implanted SiO₂ are distinctly higher than that from Si-implanted layers and well visible for the naked eye. The EL spectrum from the Ge-implanted oxide annealed at 1000 °C correlates very well with the PL one and shows a linear dependence on the injected current. The neutral oxygen vacancy is assumed to be responsible for the observed luminescence. In the case of Ge the microstructure after high temperature annealing is studied.

We implanted H3+ at 300 eV into Cu-chalcopyrite semiconductors at temperatures between 50 °C and 300 °C. The surface chemistry is similar to the previously reported behavior of CuInS2 implanted with a H2+, H+ low energy ion beam with respect to secondary phase etching. We also found an increase of radiative recombination (photoluminescence), which had been attributed to defect passivation and, hence, as an indicator of hydrogen incorporation. Under the 300 eV H3+ implantation conditions, however, we observed neither a hydrogen concentration in a few hundred nm surface range exceeding the NRA detection limit of about l x exp(19) cm-3 nor a pronounced stoichiometry variation in the ternary material, as proved by Raman measurements.
We conclude, therefore, that a 300 eV H3+ implantation introduces significantly less atomic hydrogen into the volume of the sample than previously reported for other beam compositions under similar temperature and current density conditions. This could be a result of the very low energy of less than 100 eV which can be expected for atomic H produced by dissociation of 300 eV H3+ at the surface, making the instant out-diffusion into the high vacuum of the implantation chamber a favored process.

For measuring sputtering yields with MeV heavy ions only low current densities can be used in order to avoid excessive target heating. Consequently self-cleaning of the target surface during the irradiation cannot be achieved and a contamination free surface has to be maintained by means of UHV conditions. A UHV-setup including a differential pumping system has been designed and operated at a vacuum of 10^-10 mbar during experiments. Sputtered particles were collected on Si catchers and subsequently measured by two different surface analysis methods: Total Reflection X-Ray Fluorescence (TXRF), and Heavy Ion Rutherford Backscattering Spectrometry (HI-RBS). In-situ surface cleaning by 8 keV Xe ions as well as online monitoring by ERDA were found to be essential. With sufficient precautions on the surface conditions reproducible sputtering yields were obtained. First measurements for 230 MeV Au ions on Au, Zr, and Ti targets are presented.

Thin films of Ba3Ca1.18Nb1.82O8.73, prepared in a sol-gel process by multiple dip-coating on silicon wafers were charged with hydrogen by dissociative water
absorption at definite values of water vapour pressures and temperatures. The hydrogen content was determined using nuclear resonance reaction analysis. From the resulting water vapour pressure/hydrogen composition isotherms the absorption enthalpies and the absorption entropy were calculated in the framework of a two site
model, based on Fermi-Dirac statistics. On Ba3Ca1.18Nb1.82O8.73 also impedance spectroscopy was performed yielding the bulk conductivity. From these data
in combination with proton transport numbers and the thermodynamic results as noted above the proton diffusion coefficient could be evaluated.

The enrichment of F on Antarctic meteorites is the result of their exposure to the atmosphere, and its measurement allows a subdivision of the terrestrial age into a
duration of exposure on the ice and the time a meteorite was enclosed by the ice. In many cases, the periods of surface exposure are only small fractions of the
terrestrial ages of meteorites collected in Antarctica. The enrichment of F on the surfaces of Antarctic achondrites was investigated by means of nuclear reaction
analysis (NRA): scanning proton beams with an energy of 2.7 and 3.4 MeV were used to induce the reactions F-19(p,alpha gamma)160 and
F-19(p,p'gamma)F-19, respectively. Gamma signals proportional to the F content were measured. The following Antarctic achondrites were investigated: Martian
meteorite ALH 84001; diogenite ALHA77256; the eucrites ALHA81011 and ALHA78132; and in addition, the H5 chondrite ALHA79025. For ALH 84001, our
data indicate a period of exposure on the ice of <500 years. Thus, this specimen was enclosed in the ice >95% of its terrestrial age of 13 000 years.

Due to their unique topology and unusual guest-binding behaviour, dendrimers are promising reagents for use in diagnostic imaging and therapy. We are especially interested in efficient binding and transport of medicinal and biological relevant anions as pertechnetate and the nucleotides AMP, ADP and ATP. Poly(propylenamine)-Dendrimers (POPAM) of different generations have been functionalized at the periphery with the goal to obtain a hydrophobic shell. The dendrimers synthesized (cf. Fig.) were characterized by solvent extraction experiments at the system pertechnetate,(nucleotide)-buffer-H₂O/dendrimer-CHCl₃. A dendritic effect is clearly visible of all dendrimers investigated. The influence of pH and temperature on the extraction of pertechnetate and nucleotides with dendrimers is reported.

Urea-functionalized dendrimers are prepared, which show very efficient phase transfer in particular of the diagnostically relevant anions pertechnetate, perrhenate and ATP. The extractability rates are evaluated quantitavely by tracer methods. Their pH dependancy allows to steer the release of guest molecules from the dendrimer host.

Für die im Forschungszentrum Rossendorf im Bau befindliche Strahlungsquelle ELBE werden die Möglichkeiten zur Erzeugung kohärenter Strahlung mit Hilfe eines Freie Elektronen Lasers im Bereich des mittleren und fernen Infrarot vorgestellt. Mit Hilfe einfacher Modelle werden die Vorgänge bei der Erzeugung und Verstärkung kohärenter Strahlung erläutert und ihre Eigenschaften in Abhängigkeit von den Parametern des Elektronenstrahls und des verwendeten Undulators berechnet. Insgesamt werden 4 mögliche Varianten eines Undulators diskutiert. Anwendbarkeit und Genauigkeit der verwendeten Näherungsformeln werden mit Hilfe eindimensionaler Simulationsrechnungen getestet.

We have conducted systematic experimental studies of K⁺ and K^- production and propagation in nucleus-nucleus collisions at beam energies close to and below the production threshold for free nucleon-nucleon (NN) collisions. In noncentral Au+Au collision, the K⁺ mesons are preferentially emitted perpendicular to the reaction plane and the excitation functions for K⁺ and K^- mesons nearly coincide when correcting for the threshold energy. In contrast, for NN collisions the K⁺ yield exceeds the K^- yield by two orders of magnitude near the respective NN threshold. Both effects are considered to be experimental signatures of a modification of kaon properties in the nuclear medium.

Surface modification of austenitic stainless steel by high current ion nitriding at 400°C has been studied by Slow Positron Implantation Spectroscopy (SPIS). In addition, Nuclear Reaction Analysis (NRA) has been applied to obtain information about the nitrogen depth profile. The results obtained from both methods are compared and discussed with respect to the nature of defects produced by high current ion nitriding and their influence on the formation of a surface layer, called expanded austenite.

The report gives an overview on the scientific work of the Institute of Safety Research in 1998.

Surface water has been studied from the mountain bog 'Kleiner Kranichsee'. This bog is situated in the Johanngeorgenstadt area (Saxony/Germany) close to an abandoned uranium mine and mine tailing piles. The bog water contains about 130 mg/L of organics (primarily humic and fulvic acid).
First, bog water colloids were determined for their particle size and size distribution using photon correlation spectroscopy (PCS) and scanning electron microscopy (SEM) in combination with size fractionation by filtration. Second, about 400 L of the bog water were processed in order to isolate aquatic humic substances. SupeliteTM DAX-8 (Supelco) was used as adsorption resin. The humic material was separated into humic and fulvic acids. A total of 14 g of humic acid and 10 g of fulvic acid were isolated. The humic substances were characterized in terms of their elemental composition, functional properties including proton exchange capacity, charge/size ratios and spectroscopic characteristics. The results were compared with data of a commercial humic acid from Aldrich.

The migration behavior of uranium in a sandy aquifer system rich in humic substances was studied in laboratory column experiments. For the investigations we used a Pleistocene aeolian quartz sand and groundwater GoHy-532 from the Gorleben site (Germany). The column experiments were performed in a glove box under anaerobic conditions (Ar + 1 % CO₂). ²³²U(VI) was used as a tracer. The migration behavior of uranium was investigated as a function of the uranium/groundwater equilibration time before injection into the column, the groundwater flow velocity and the column length.
From the breakthrough curves one can conclude that a part of the injected uranium migrates slightly faster than groundwater. The observed migration behavior is attributed to the association of a part of uranium with humic colloids, which move faster due to size exclusion processes. Depending on the experimental conditions the recovery of humic colloid-bound transported uranium amounts to 0.4 up to 7.6 %. The recovery of non-retarded colloid-borne uranium increases with increasing uranium/groundwater equilibration time before injection into the column. Beyond it, the recovery of humic colloid-borne uranium decreases with decreasing groundwater flow velocity and increasing column length, which corresponds to an increasing residence time in the column.
The results refer to the fact that the migration behavior of uranium is strongly influenced by kinetically controlled interaction processes of uranium with humic colloids.

Die Untersuchung des Ausbreitungsverhaltens von Actinidionen in wäßrigen Systemen ist unabdingbar zur Erstellung von Risikoabschätzungen zur Langzeitsicherheit von ober- und untertägigen Einrichtungen des ehemaligen Uranerzbergbaus sowie von Endlagern für radioaktiven Abfall. Das Migrationsverhalten von Actinidionen in einem natürlichen Aquifer wird stark von den vorherrschenden Bedingungen beeinflußt. Dabei können Huminstoffe als ubiquitäre, organische Komplexbildner einen entscheidenden Einfluß auf die Immobilisierung bzw. Mobilisierung der Actinidionen ausüben.
Die vorliegende Arbeit zeigt Ergebnisse von Säulenexperimenten zur Untersuchung des Einflusses aquatischer Huminstoffe auf das Migrationsverhalten von Uran(VI) in einem sandigen Grundwasserleiter. Für die Untersuchungen wurden ein Grundwasser (GoHy-532, DOC: 30 mg C/l) sowie pleistozäner Flugsand aus dem Gebiet Gorleben verwendet. Der in Säulen (Länge 25 cm und 50 cm; Durchmesser 5 cm) eingebaute Sand wurde unter Inertgasbedingungen (Ar + 1 % CO₂) mit dem Grundwasser konditioniert. Zur Bestimmung der hydraulischen Parameter der Säulen wurde tritiertes Wasser als Tracer verwendet. Die Migrationsuntersuchungen erfolgten in Abhängigkeit von der Kontaktzeit des Grundwassers mit Uran(VI) (²³²UO₂Cl₂) bis zur Injektion auf die Säule, der Grundwasserfließgeschwindigkeit sowie der Säulenlänge.
Die Uran-Durchbruchskurven der Säulenexperimente zeigen, daß ein Teil des Urans ungehindert, an Huminstoffe gebunden und dabei bis zu 5 % schneller als das Wasser durch das Sediment transportiert wird. Der Wiedererhalt an Uran liegt für die durchgeführten Experimente im Bereich zwischen 2 % und 9 %.
Der ungehinderte, huminstoffgetragene Urantransport ist von der Kontaktzeit des Grundwassers mit Uran(VI) abhängig und steigt mit Erhöhung der Konditionierungszeit. Darüber hinaus nimmt der Wiedererhalt mit abnehmender Grundwasserfließgeschwindigkeit sowie zunehmender Säulenlänge, d.h. mit Erhöhung der Kontaktzeit mit dem Sediment, ab. Dieses Ergebnis deutet auf eine kinetisch kontrollierte Wechselwirkung des huminstoffgebundenen Urans mit der Sandoberfläche hin.

As a contribution to Task 1 (Sampling and Characterization), we have characterized Derwent Reservoir fulvic acid by means of FTIR spectroscopy and capillary zone electrophoresis and compared to Kranichsee humic and fulvic acid and Aldrich humic acid.
Our contribution to Task 2 (Complexation) consists of two parts. First, we studied the complexation of Kranichsee humic acid (HA) and fulvic acid (FA) with uranyl(VI) ions by laser-induced fluorescence spectroscopy at pH 4 and an ionic strength of 0.1 M (NaClO4). The loading capacities were determined to be 14 ± 1 % and 13 ± 1 % for humic acid and fulvic acid, respectively. The complexation constants for Kranichsee humic acid and fulvic acid were found to be log ß = 6.35 ± 0.22 and log ß = 6.21 ± 0.20, respectively. These results were compared to the uranyl complexation behavior of other natural humic acids such as Aldrich HA and GoHy-573 HA. Second, we investigated original bog water of the mountain bog 'Kleiner Kranichsee' by ultrafiltration to determine the hydrodynamic particle size distribution, i.e., the 'apparent molecular weight' distribution of the organic substances in the bog water. Furthermore, the influence of uranyl complexation on this particle size distribution of humic colloids was investigated. No significant influence of the uranyl ions on the molecular weight distribution of the humic substances was found.

The effect of humic acid (HA) on the uranium(VI) sorption onto phyllite and onto its individual main mineralogical constituents, muscovite, albite, and quartz was studied in air-equilibrated batch experiments in the pH range of 3.5 to 9.5. The uranyl(VI) and HA concentration was 1×10-6 M and 5 mg/L, respectively. The ionic strength was held constant at 0.1 M (NaClO4 solution). A size fraction of 63 to 200 µm of the solids was used, the mass loading was 12.5 g/L, and the experimental volume was 40 mL.
The rock material phyllite shows both the highest HA sorption over the entire pH range and the highest uranyl sorption in the neutral pH range compared to the pure mineral phases muscovite, albite and quartz. In contrast to the minerals the maximum of the uranium sorption on phyllite is not shifted to lower pH values but remains largely unchanged in the pH range from 6 to 7.8 when HA is present at 5 mg/L. This is attributed to ferrihydrite a secondary mineral phase formed during the sorption experiments.
The humic acid and uranyl sorption on the mineral surfaces of the constituents of phyllite decreased in the sequence: muscovite > albite > quartz. Below pH 5, the uranium uptake is generally increased in the presence of HA. This is attributed to the fact that the HA is sorbed on the mineral surface thereby providing additional sorption sites due to their complexing ability and/or due to adsorption of uranyl humate complexes on the mineral surface. In the neutral pH range, the HA sorption decreases with increasing pH. The dissolved HA forms aqueous uranyl humate complexes thereby decreasing the uranyl uptake on the solids. At alkaline pH values, the HA has little or no effect on the amount of uranium sorbed by muscovite, albite and quartz. Inorganic carbonates with its high complexing ability towards uranyl ions predominate the influence of HA under the given experimental conditions. We conclude that the uranium sorption is determined by the kind of rock material or mineral and is strongly affected by both the pH and the presence of organic material.
In addition, the uranium adsorption on phyllite was studied as a function of the HA concentration (5 and 60 mg HA/L). Below pH 5.7, the uranium sorption is enhanced in the presence of 60 mg HA/L compared to experiments carried out with 5 mg HA/L. From pH 5.4 to 9.4, the uranium uptake, on a percentage basis, decreased as the humic acid concentration increased. Thus, a strong relationship between the initial humic acid concentration in solution and the amount of uranium adsorbed at a given pH was found.

Die Bewertung des Einflusses von Huminstoffen auf die Mobilität von Radionukliden in natürlichen Aquifersystemen ist von großem Interesse und ermöglicht beispielsweise eine Einschätzung zur Langzeitsicherheit stillgelegter Uranbergwerke bzw. Endlagerstätten für radioaktive Stoffe. Infolge der Komplexierung von Radionukliden durch Huminstoffe kann deren Löslichkeit erhöht und somit ihre Mobilität in der Geosphäre erhöht werden. Zur Quantifizierung der Wechselwirkung zwischen Huminstoffen und Radionukliden kann die Komplexbildungskonstante genutzt werden.
Aquatische Huminstoffe wurden aus dem Oberflächenwasser des Hochmoores 'Kleiner Kranichsee' isoliert, welches im ehemaligen Uranbergbaugebiet bei Johanngeorgenstadt (Sachsen, D) liegt. Die Huminstoffe wurden in Humin- und Fulvinsäurefraktionen getrennt und hinsichtlich ihrer Elementarzusammensetzung, funktionellen Eigenschaften einschließlich der Protonenaustauschkapazität, Ladungs-Größenverhältnisse und spektroskopischer Eigenschaften charakterisiert.
Die Komplexierung von Uranyl(VI) durch Huminsäure (HS) und Fulvinsäure (FS) wurde bei pH 4 und einer Ionenstärke von 0.1 M mittels zeitaufgelöster Laserfluoreszenzspektroskopie (TRLFS) untersucht. Mit Hilfe des Ladungsneutralisationsmodells wurden Komplexbildungskonstanten (log ß) und Beladungskapazitäten (LC) der Huminstoffe mit UO₂ ²⁺ bestimmt. Folgende Ergebnisse wurden erhalten:
HS: log ß = 6.35±0.22, LC = 14%; FS: log ß = 6.21±0.20, LC = 13%.
Weiterhin wurde der Einfluß von SO₄ ^2- als anorganischer, konkurrierender Ligand auf die Uranyl-Huminstoff-Komplexierung untersucht, da Sickerwässer sächsischer Uranbergbauhalden neben anderen Ionen relativ hohe Konzentrationen an Uran (10^-5 M) und Sulfat (3x10^-2 M) enthalten. Dabei wurde festgestellt, daß das Sulfat-Ion auf die Komplexierung zwischen Uranyl und Huminstoff keinen Einfluß hat, da die Uranyl-Huminstoff-Komplexbildungskonstanten trotz steigender Sulfat-Konzentrationen (0, 10^-3, 10^-2 M) innerhalb des experimentellen Fehlers unverändert bleiben. Dagegen sinkt die Beladungskapazität der Huminstoffe gegenüber Uranyl-Ionen mit steigender Sulfat-Konzentration. D.h., das Sulfat-Ion muß einen Einfluß auf die komplexierenden Gruppen der Huminstoffe ausüben.

Die Auslegung von Apparaten der chemischen Verfahrenstechnik erfolgt in der Praxis oft noch auf der Grundlage experimenteller Untersuchungen oder mit groben Abschätzungen. Mit der Verbesserung der Rechentechnik nimmt jedoch die Bedeutung der numerischen Simulation der gekoppelten strömungsmechanischen, thermodynamischen und chemischen Prozesse in diesen Apparaten zu. Ziel der vorliegenden Modellentwicklung war es, einen weitgehend allgemeingültigen Programmkern, der die Transportgleichungen löst, zur Verfügung zu stellen. Spezielle Modelle für einzelne Phänomene oder Apparate sollen leicht über Interfaces ankoppelbar sein. Das ermöglicht insbesondere Untersuchungen zum Einfluß spezieller Phänomene und Modelle auf das Simulationsergebnis. Dreidimensionale Rechnungen erfordern i.a. einen sehr hohen Aufwand und sind meist nur für spezielle Probleme praktisch anwendbar. Bei dem vorgestellten Simulationstool für Mehrphasenströmungen in einem Behälter erfolgte daher eine Beschränkung auf die axiale Behälterkoordinate. Ausgangspunkt war die Simulation von Druckentlastungsvorgängen. Das Behältermodell muß das Aufwallen des Behälterinhalts wiedergeben und Aussagen zur Zusammensetzung des in die Abblaseleitung eintretenden Fluids liefern. Diese hängt von der Phasenseparation einschließlich des Schaumverhaltens des Fluids ab. Schäume treten in einer Vielzahl technischer Apparate wie Rührreaktoren, Blasensäulen, Trennkolonnen, Elektrolysezellen oder Bioreaktoren auf. Die Beschreibung von Schaumentstehung und Schaumzerfall am Gemischspiegel war ein spezielles Ziel der Modellentwicklung. Durch die Anwendung einer speziellen Partikelmethode für die Modellierung der Transportvorgänge wird zum einen die numerische Diffusion vollständig unterdrückt, zum anderen das Transportproblem weitgehend von den anderen Prozessen entkoppelt. Das Fehlen der numerischen Diffusion bringt Vorteile bei der Beschreibung von Diskontinuitäten, wie dem Übergang zwischen dem Mehrphasengemisch und dem Gasraum und damit bei der Modellierung der Schaumzone. Um das Modell einfach zu halten, wurden statt der Impulserhaltung nur ein zeitabhängiger Behälterdruck (ggf. mit Beachtung des geostatischen Drucks) und die Relativbewegungen der Phasen auf der Grundlage empirischer oder halbempirischer Ansätze (Driftbeziehungen) berücksichtigt.

Boron was implanted into 6H-SiC layers to form a 500 nm thick layer with plateau concentrations up to 1.5E21 cm-3. The electrical and microstructural effects were investigated using temperature dependent Hall measurements, XTEM and SIMS. After annealing at temperatures between 1550 and 1750 C for B concentrations above the solubility extraordinary B diffusion and Ostwald ripening is observed forming boron clusters. For B concentrations near the solubility the outdiffusion is reduced. These effects are confirmed by Hall Effect measurements showing a stagnation of hole concentration with increasing B concentration above 1.5E19 cm-3. The electrical activation is limited due to the solubility of boron in 6H-SiC at about 1600 C. Higher B concentrations as well as higher annealing temperatures, including short time annealing at about 2000 C, do not lead to higher activation.