Preparation of Nb3Ge films by chemical transport reaction and their critical properties

Niobium-germanium films have been deposited on sapphire substrates at 900 C by a chemical transport reaction method. The highest superconducting transition onset temperature T _C,on of 22.4 K is observed for a nearly stoichiometric Nb₃Ge film with the A15-type structure (thickness ~ 93.5 m). Lattice constants for the Nb₃Ge phase formed in the Nb-Ge films with both T _C,on above 22 K and T _C,midpoint above 21 K are found to extend from 5.143 to 5.153 ». Deposition rates for the obtained films are in the range of 2–10 m/min. Critical current densities for the Nb₃Ge film with the highest T _C,on value are observed to be relatively low (~ 10³ A/cm² at 19 K at self-field). This is due to the coarse grain structure of the film or the low density of effectual pinning centers in the film. Field variations of the pinning forces operating in this film in magnetic fields both parallel to the film surface and perpendicular to the film surface are found to follow closely b ^1/2(1\s-b)², to which the pinning force for flux pinning at the surface of normal regions, such as grain boundaries, film surfaces, etc., is proportional, and where b is the reduced magnetic induction (B/B _C2).A small increase in J _C at low fields is caused by the presence of a small amount of the Nb₅Ge₃ phase in a Nb₃Ge film, and seems attributable to additional flux pinning on Nb₅Ge₃-phase particles in the film.Supported by Deutsche Forschungsgemeinschaft.     

Cu diffusion in α-Fe: determination of solute diffusivities using atomic-scale simulations

The nucleation and growth of radiation-induced Cu-rich clusters have proved an important topic in the study of microstructural and chemical evolution responsible for property changes in pressure vessel steels subjected to neutron irradiation. Small Cu clusters can act as precipitate precursors and contribute to the hardening of the material by impeding dislocation glide. Reliable Cu diffusion coefficients provide critical input to Monte Carlo and rate theory approaches to study the kinetics of diffusion and clustering that lead to the formation of such Cu-rich features. In this paper we report the results of a molecular dynamics study of Cu solute-atom diffusion in a Fe–0.9at%Cu alloy, using high-temperatures and high-vacancy concentrations to hasten the convergence of the calculation. We find a value of 0.61 eV for the migration energy of Cu in the b.c.c Fe matrix, in good agreement with available experimental results. The main diffusion mechanisms under the conditions simulated are identified and discussed, and the effect of vacancy and solute clustering on Cu migration is assessed. In addition, a rigorous computation of the solute enhancement factor, correlation factor and solute diffusivity is performed within the five-frequency theoretical framework and the harmonic approximation. The results obtained are compared with those yielded by the direct MD simulations and the observed differences discussed.     

Protection Without Protectionism

"The key is to develop a local capacity … for computer power, to confine data flow within borders, and to provide protection for those local development efforts ."     

Production of toxic glucosinolate derivatives from rapeseed meal by intestinal microflora of rat and chicken

This work was based on a comparison between rats and chickens with different bacteriological status and fed rapeseed meal diets (DARMOR 00,39·0 % and 32·8%, respectively). Results obtained with conventional vs axenic animals show that the rat intestinal microflora is responsible for the dramatic growth depression and for the slight hypertrophy of glucosinolate target organs (thyroid, liver, kidneys). Only a strong goitrogenic effect is seen with young conventional birds. Chimera animal models (axenic animals inoculated with the whole faecal flora of another animal species) prove that the nature of the recipient host (rat) does not influence the expression of chicken intestinal microjlora in our experimental conditions. The rat whole faecal flora does not interfere with chicken growth rate, and thyroid hypertrophy is very moderate compared with conventional birds. Finally, of the two molecules responsible for the goitrogenicity, one is destroyed by the feed irradiation required for axenic experiments.     

The bending performance of T&G joints subjected to concentrated loads

The stiffness, considered to be the slope of the linear region of the load-displacement chart has been calculated. Linear regression lines between failure load and stiffness were sought. Relatively good correlation (R²=0.88) was found for the unjointed samples, however, less good correlation (R²=0.63) was found for the glued jointed samples and even worse correlation (R²=0.53) was found for the unglued jointed ones. Generally, the stiffness can be considered as a useful indicator for the failure load and failure mode of the particleboard samples. For increased stiffness values the boards tend to fail at higher loads and in a more brittle mode, providing a sudden vertical drop in the load/displacement curves. At values over 456 N/mm `catastrophic' failure modes (instant total failures) were observed (span size of 450 mm and thickness of the boards of 18 mm). However, at values less than 420 N/mm the failure mode was more ductile, described by `smooth' post failure curves. By gluing the joints the stiffness of the structure increases significantly. The failure load also increases but not by an amount that is statistically significant. The failure mode of glued jointed structures seems to be more ductile when compared to unglued ones. This means that after the maximum failure load has been achieved, the boards continue to sustain the load for a longer period of time up to ultimate failure occurs.

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2 =0,88), schwache für verleimte Proben (R²=0,63) und noch schw?chere für unverleimte Verbindungen (R²=0,53). Allgemein erwies sich die Steifigkeit der Spanplatten als guter Indikator für die zu erwartende Bruchlast und die Art des Bruchs. Bei erh?hter Steifigkeit erfolgte der Bruch bei h?herer Belastung, war zunehmend spr?der und zeigte als ein pl?tzlicher senkrechter Abfall in der Verformungskurve. Bei Werten oberhalb 456 N/mm erfolgte ein unmittelbares totales Versagen (Katastrophenfall). Bei Werten unterhalb 420 N/mm war der Bruchverlauf z?her mit “sanfterem” Kurvenverlauf nach dem Bruch.

     

Embrittlement recovery due to annealing of reactor pressure vessel steels

The irradiation embrittlement of nuclear reactor pressure vessels (RPV) can be reduced by thermal annealing at temperatures higher than the normal operating conditions. The objective of this work was to analyze the pertinent data and develop quantitative models for estimating the recovery in 41 J (30 ft-lb) Charpy transition temperature (TT) and Charpy upper shelf energy (USE) due to annealing. An analysis data base was developed, reviewed for completeness and accuracy, and documented as part of this work. Models were developed based on a combination of statistical techniques, including pattern recognition and transformation analysis, and the current understanding of the mechanisms governing embrittlement and recovery. The quality of models fitted in this project was evaluated by considering both the Charpy annealing data used for fitting and a surrogate hardness data base. This work demonstrates that microhardness recovery is a good surrogate for shift recovery and that there is a high level of consistency between the observed annealing trends and fundamental models of embrittlement and recovery processes.     

Critical data of type II superconductors under hydrostatic pressure

Thin films of Nb₃Sn, Nb₃Ga, and NbN deposited on sapphire substrates by different preparation techniques and diffusion wires were measured under hydrostatic pressures up to 15 kbar. The results on critical current and upper critical induction under pressure can be described by scaling laws similar to those already known for temperature as parameter. The anisotropies of the critical currents and fields depend on the microstructure of the samples. The pinning mechanisms are not changed under hydrostatic pressure, so that the pinning forces in these superconductors can be compared to existing pinning theories.Supported by the Deutsche Forschungsgemeinschaft.