S.R. Sheen

Synthesis of nanocrystalline AgPd alloys by chemical reduction method

Abstract Chemical reduction was used to produce nanocrystalline Ag-Pd alloy particles for applications in electronic component fabrication. The alloy particles were prepared at room temperature from formaldehyde-sodium hydroxide solutions and silver and palladium nitrates. Characterization of these particles by X-ray powder diffraction (XRD) reveals bi-metal and nanocrystalline silver-palladium alloys. Energy dispersive and XRD analysis of these products shows a complete range of solid solution. Characterization of the morphology, and the oxidation behavior of the Ag-Pd alloy powders are reported.

Preparation of Superconducting Bi-Pb-Sr-Ca-Cu-O Compounds by Oxalate Coprecipitation

Bi1.6Pb0.4Sr2Ca2Cu3Oy has been prepared from an ethylene glycol medium that contains starting metal nitrates via homogeneous coprecipitation using triethylammoniumoxalate and oxalic acid. The theoretical estimate on the conditions of oxalate coprecipitation and preliminary characterization of the properties of the Bi1.6Pb0.4Sr2Ca2Cu3Oy obtained after the high-temperature treatment are reported. The preparation of this five-component system of Bi, Pb, Sr, Ca and Cu using triethylammoniumoxalate and oxalic acid has worked successfully in a large-quantity preparation of up to 200 g per batch.

Superconductivity in Ru-based double pervoskite - the possible existence of a new superconducting pairing state

Abstract We have studied the double perovskite structure A 2 Y(Ru 1−x Cu x )O 6 (A  Ba or Sr) system. The parent compounds are antiferromagnetic insulators. Partial substitution of the Ru-ion by Cu was found to induce superconductivity in these compounds. We also observed in these materials the coexistence of superconductivity and anomalous magnetic orders. Resistive measurements in high magnetic fields show that the field dependence of the transition temperatures is significantly different from those observed in high T c cuprates. Our results suggest that the pairing state of these new materials may be of odd parity such as p-wave.

Synthesis and characterization of high-Tc Y-Ba-Cu-O superconducting oxides by coprecipitation from triethylamine-oxalate media

Abstract A modification of the oxalate coprecipitation process for preparing high-Tc superconducting oxides is reported. The superconducting Y-Ba-Cu-O powders with Tc up to 95.5 K have been prepared from an aqueous solution which contains Y, Ba and Cu salts via homogeneous coprecipitation using triethylamine and oxalic acid solution. The dried precipitate was found to have a specific surface area of 38.4 m2/g. Characterizations of the YBa2Cu3Oy powder by X-ray diffraction (XRD), thermal analysis, scanning electron microscope (SEM), electron microprobe analysis (EMPA), electrical resistivity and magnetic susceptibility measurements are reported. This process might be employed for large quantity preparation of the YBa2Cu3Oy system.

Synthesis of PrBa2Cu4O8 at ambient oxygen pressure

Abstract We report for the first time the successful synthesis of polycrystalline PrBa 2 Cu 4 O 8 (Pr124) at ambient oxygen pressure. Pr124 has been prepared by nitric pyrolysis and oxalate coprecipitation methods, and both methods yield qualitatively the same results. Powder X-ray-diffraction patterns of samples prepared by both methods show a nearly single “124” phase. Thermogravimetric analysis indicates no oxygen loss until the temperature is higher than 800°C, revealing that its thermal stability is distinct from that of PrBa 2 Cu 3 O 7 . Electrical resistivity ϱ ( T ) shows metallic behavior at temperatures. These results show that the quality of the samples prepared at ambient oxygen pressure is comparable to that by hot isostatic pressing technique.

Effect of solution pH on the synthesis of the YBa2Cu4O8 superconductor via an EDTA solution process

High critical temperature, Tc, YBa2Cu4O8 superconducting oxides have been prepared by a solution technique using ethylenediaminetetraacetic acid (EDTA) as complexing agent. The use of an appropriate complexing agent leads to complete mixing of the Y3+, Ba2+ and Cu2+ cations, and thus facilitates formation of YBa2Cu4O8 at a lower temperature. The effect of pH on the homogeneity of samples is discussed. The superconducting YBa2Cu4O8 sample with Tc = 80 K was obtained after heat treatment at 820°C. Characterization of the sample by X-ray diffraction (XRD), thermal analysis, scanning electron microscopy (SEM), electron-probe microanalysis (EPM), electrical resistivity, magnetic susceptibility and other spectroscopic analyses are reported.

Rapid formation of the high-Tc phase in (Bi,Pb)-Sr-Ca-Cu-O superconductor via the sol-gel method

Bi-Pb-Sr-Ca-Cu-O powder was synthesized by the oxalate gel method. A sample with the composition of Bi1.7Pb0.4Sr1.6Ca2.4Cu3.6Oy was used in this study. After pyrolysis of the gel precursor at 500 °C for 5 h, the resulting powder was calcined at 850 °C for another 5 h. The black powder was pressed into pellets and sintered at 852 °C for 5 h. The high-Tc phase was formed more easily in the sample with excess calcium and copper than in the theoretical composition. (Bi,Pb)2Sr2Ca2Cu3Oy (above 90%) was prepared as above within a relatively short time. Characterization of (Bi,Pb)2Sr2Ca2Cu3Oy superconductor by X-ray diffraction, scanning electron microscopy, electron probe microanalysis, resistivity measurement and magnetic measurement, is reported.

A study of Bi-Pb-Sr-Ca-Cu-O/Ag tape prepared by the jelly-roll process

We combined the electrophoretic deposition technique and the jelly-roll process to fabricate Bi-Pb-Sr-Ca-Cu-O (BPSCCO)/Ag tapes. A thick film of BPSCCO on an Ag substrate was prepared by electrophoretic deposition. The BPSCCO/Ag sheet was rolled up to the jelly-roll configuration and then cold rolled to form tape with six superconducting layers. The critical current density of the tape determined by magnetic measurement at 5 K in a 0.2 T field was about 6.5*104 A cm-2 and decreased to 1.3*104 A cm-2 near 5 T.

Synthesis and properties of the Y(Ba1−xSrx)2Cu3−0.15xMo0.15xO7−z superconductors

Abstract We have achieved the preparation of the Y ( Ba 1− x Sr x ) 2 Cu 3 O 7 compounds with x ranging from 0 to 1 by substituting a small amount of Cu with Mo. It was observed that the superconducting transition temperature, T c , first decreases linearly with Sr-content until x 0.4, where a sharp drop in T c occurs. The T c for the sample with x =0.1 is about 40 K. X-ray structure analysis shows that a sharp drop in T c at a critical concentration corresponds to the crystal change from orthorhombic to tetragonal.

Oxygen stabilization in YBa2Cu3O6+δ:Ta2O5 sintered in air

YBa2Cu3O6+δ pellets doped with different amounts of Ta2O5 in the range of 0.1-4 wt% and sintered in air before being cooled to room temperature have been observed to exhibit superconductivity onset at 91 K, even without post sintering oxygen annealing. While the width of the transition varies with the Ta2O5 content, the onset temperature differs marginally. The best results are obtained for a Ta2O5 concentration of 1 wt% for which these parameters are 2.5 and 92 K, respectively. On the other hand, air grown single crystals of YBa2Cu3O6+δ doped with Ta2O5 need a post growth 48 h anneal in air at 500 °C to observe a broad superconducting transition at 80 K (onset temperature). Powder x-ray diffraction analysis shows the stabilization of the orthorhombic 123 phase and the presence of the BaTa2O6 and YBa2TaO6 phases in small amounts. These results are interpreted in terms of the incorporation and stabilization of the stoichiometric excess of oxygen in the presence of pentavalent Ta (Ta5+). This interpretation is supported by oxygen titration analysis and thermogravimetry measurements on these samples.