Search

The crystalline phases in the superconducting ceramic Bi:Sr:Ca:Cu2:Ox exhibiting either low superconducting transition (≈ 80 K) or both, low and high superconducting transitions (≈ 110 K) are indentified by X-ray and electron diffraction. Two types of modulated structures are observed and their relation to Tc is discussed within the frame of the oxygen-displacive mode theory. Der keramische Werkstoff Bi:Sr:Ca:Cu2:Ox zeigt entweder nur einen einzigen niedrigen supraleitenden Ubergang (≈ 80 K) oder zwei Ubergange, einen niedrigen und einen hohen (≈ 110 K). Die in diesem Werkstoff vorliegenden kristallinen Phasen werden mittels Rontgenbeugung und Elektronenmikroskopie untersucht. Zwei verschieden modulierte Strukturen werden betrachtet und ihr Einflus auf die Ubergangstemperatur im Rahmen der „oxygen-displacive mode”-Theorie diskutiert.

Real regenerated supported palladium (0.5wt.%Pd-LiAl5O8) catalysts used in the hydrogenation of alkenes or alkynes and alkadienes were sintered at high temperatures (300, 500 or 600 °C) in a hydrogen atmosphere, and subjected to alternative treatments in either reducing and/or oxidizing atmospheres. X-ray diffraction analysis indicated hydride formation in the reducing stage, with intermediate compositions ranging between α-PdH and β-PdH, while on using the extended X-ray absorption fine structure technique palladium oxide was observed in catalysts subjected to oxidizing conditions. The statistics of the palladium-crystallite size distributions, as obtained by transmission electron microscopy, showed evidence of sintering and/or redispersion (splitting), as a result of the alternative reducing or oxidizing treatments. The catalytic activity in the alkadiene hydrogenation reaction is influenced by the treatment sequence applied to the regenerated catalysts.

The preparation of the tetragonal spinels in the series (1) CuxMn3−xO4 (0 ⩽ x ⩽ 0,2), (2) Zn1+xMn2−xO4 (0 ⩽ x ⩽ 0,2), (3) MgxMn3−xO4 (0 ⩽ x ⩽ 1) and (4) ZnxMn3−xO4 (0 ⩽ x ⩽ 1) is described. Studies of the crystal symmetry as a function of composition and of electrical conductivity as a function of composition and temperature, are reported. All the compositions exhibit a thermal hysteresis of the electrical conductivity in the range of the tetragonal to cubic transition. Analysis of crystal-chemical and electrical properties leads to the probable cation distribution formulae for each system. The electrical conductivity involves the hopping mechanism of electrons between octahedral-site Mn3+ and Mn4+ ions. In the systems (1) and (2) the presence of Cu2+ and Zn2+ ions in the octahedral positions together with the equivalent distorting power of the octahedral Cu2+ and Mn3+ ions has been established.

Device quality PbS photoconducting films were obtained by laser plasma vapour deposition on special glass substrates at room temperature and their structure was investigated by X-ray diffraction. High Tc superconducting films of composition Dy 0.2 Y 0.8 Ba 2 Cu 3 O~ 7 and YBa 2 Cu 3 O~ 7 were deposited on sapphire substrate. It was shown for laser deposited YBa 2 Cu 3 O~ 7 that a buffer layer of the same composition predeposited by rf sputtering allows for getting high quality superconducting films.