National Institute Of Materials Physics - Romania

High T-c bulk superconductors Y(Tm)Ba2Cu3O7-x and YSryBa2-yCu3O7-x were pre pared following new synthesis routes by the method of thermal decomposition of polynuclear coordination complexes. The best superconducting samples were obtained using oxalic and malic acids as ligands. The effect of substituting of Pa by Sr nas a decrease of T-c with increasing the Sr:Ba ratio. Electrical resistance, magnetic susceptibility and structural measurements have been performed on the superconducting samples.

The thermally activated dissipation in highly anisotropic Bi2Sr2CaCu2O8+delta superconducting thin films was investigated by conventional resistive measurements, with the magnetic field B=mu0H applied perpendicular to the (a, b) plane and ranging between 5 mT and 3 T. We observed a significant change in the form of the ''irreversibility line'' (determined with a constant-resistivity criterion) and of the magnetic-field dependence of the activation energy at an applied field value B(cr) almost-equal-to 30 mT. This change is discussed in connection to the predicted ''dimensional crossover'' in the vortex system, in the case of highly anisotropic superconductors. For the investigated specimen, we found that the model of thermally activated plastic flux motion with double vortex kink formation describes the irreversibility line at intermediate field values. This indicates that the ''pancake'' vortices in the (CuO2)2 blocks are not completely decoupled, at least up to 3 T. In the low field region, due to the predicted increase of the tilt modulus in the vortex system, the above model becomes inappropriate, and the form of the irreversibility line can be explained in terms of thermally assisted flux flow in a three-dimensional vortex ensemble, with a coherence length along the flux line of the order of 50 nm.

Due to fast relaxation of magnetization and high critical current densities it is difficult in HTSC to reach the critical state experimentally. In order to obtain complete information about it from subcritical state measurements an extrapolation scheme based on the sweep rate dependence of the hysteresis widths has been developed.

CO conversion to hydrocarbons at various reaction temperatures and atmospheric pressure has been investigated on alumina-supported Fe, Ni and Fe-Ni catalysts. Maxima of activity and selectivity were obtained in hydrocarbon formation as a function of Fe content in the samples, which corresponds to a certain stoichiometry of metallic components.

The surface of alumina supported catalysts containing 10 wt.% metal (Fe and/or Ni) has been spectroscopically investigated by CO adsorption. The stretching vibration range characteristic for CO bonded to metallic species shows very large absorptions, indicating a high surface heterogeneity. A certain Fe:Ni stoichiometry seems to impose the reduction extent and the catalytic properties.

The icosahedral metastable Al-Mn phase was obtained in thin films prepared by the high temperature sequential vapor deposition method. Analysis of X-ray diffraction line profiles allows an estimate of the random phason strain, revealing the presence of a highly disordered icosahedral fraction. Its concentration is correlated with the degree of crystallographic perfection of the Al substrate. Thereby, the crucial role of Al diffusion in the formation of the icosahedral phase is evidenced.