The effect of boron excess in the structure and superconducting properties of NbB 2 is reported. Rietveld refinements of the x-ray diffraction patterns indicate that boron excess induces significant changes in the Nb-B bond length, increasing the c-axis. In contrast, the B-B bond length remains essentially constant. Magnetization behaviour was studied in the temperature range from 2 to 15 K. We found that for (B/Nb) exp 2.20(2) of boron excess samples display superconductivity with a maximum T C of about 9.8 K at (B/Nb) exp = 2.34(1). High pressure measurements in samples with two different boron contents reveal that T C decreases at different ratios, dT C /d P. Superconducting parameters were determined, indicating that NbB 2+x is a type II superconductor. We correlated the change of T C with the evolution of the structural parameters and found that it coincides with theoretical predictions of band structure.

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In the process of magnetizing cylindrical specimens of a typical high field superconductor Nb-50 at.%Ti, flux jumps were induced by magnetic disturbances. The stability limit field H fj increased steadily with increasing temperature, and no magnetic instability occurred for temperatures in excess of about 6.5 K. The calculation of H fj was performed taking into account the cylindrical sample geometry and the critical state equation J c B 1-γ=α. According to the relative magnitudes of the magnetic diffusivity D m and the thermal diffusivity D t, the expression of H fj was derived for two cases; (1) D m>D t, and (2) D m ≪D t. Good agreement between experiment and theory was obtained on the stability limit field H fj and the temperature above which magnetic instabilities do not take place.

The specific heats of the pl(DO,) and Pi(18R) phases in a Cu + 16.1 wt% Zn + 8.0 wt% A1 alloy are measured at temperatures close to its martensitic transformation temperature by two different calorimetric techniques. I n the measured temperature range no appreciable difference in the specific heat values of the phases is detected. Some relevant features of the kinetics of the martensitic transformation are deduced from the measurements.Nous avous mesure, par deux diffkrentes methodes calorimktriques, les chaleurs spkcifiques des phases P1(DO,) et f&(lSR) de l'alliage Cu + 16,l %poids Zn + 8,0 %poids A1 Q des temperatures proches de sa temperature de transformation martensitique.Nous n'avons pas dktectk de diffkrence dans la chaleur spkcifique des deux phases dans l'intervalle de mesure. De plus, nous dkcrivons certaines caractkristiques inbressantes de la cinbtique de la transformation, qui peuvent se dkduire des mesures rkalisbes.