Effect of processing parameters on the characteristics of high- T c superconductor YBa 2 Cu 3 O y
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I. INTRODUCTION Since the discovery of high-temperature superconductivity in the La-Ba-Cu-O system at ~ 35 K by Bednorz and Muller1 and in the Y-Ba-Cu-O system at ~ 93 K by Wu et al.,2 a large number of research reports on various aspects of these new cuprate-based high-!Tc superconducting materials have appeared. The phase responsible for superconductivity in the Y-Ba-Cu-O system has been identified3 as YBajC^O^, (y> 6.5) having a distorted, oxygen-deficient, perovskitelike orthorhombic structure. Presently most of the research efforts are being aimed at the YBa2Cu3O>, system with or without substitution by other elements. The YBa 2 Cu 3 O y compound undergoes a reversible tetragonal to orthorhombic structural phase transformation on changing the temperature 4 or the oxygen content.5'6 The phase above - 7 0 0 °C is tetragonal(T) whereas below ~-, powders were synthesized by the solid-state reaction method 12 ' 13 from Y 2 O 3 , BaCO3, and CuO powders. Stoichiometric proportions of the oxide powders were slurry mixed in acetone or alcohol and oven dried at ~ 110 °C. The resulting mixture was treated in two different ways. In one process, the mixed powder was slowly heated to ~ 880 "C in air and held at this temperature for ~ 9 0 h in alumina crucibles. It was cooled, pulverized, and recalcined at ~923 °C for another 17 h. It was then cooled, reground, and stored. This material will henceforth be called "powder A." In another process, the slurry mixed and dried powder was baked at —940 °C for - 1 5 h in air. The resulting cake was cooled, ground, and fired again at ~ 940 °C in air. The cycle of heating, cooling, and grinding was repeated two more times. This material will be referred to as "powder B." From powder XRD, both powders A and B were found to be virtually single-phase orthorhombic YBa2Cu3Oj, with only trace amounts of impurities (BaCuO 2 ). Both the powders A and B were dry pressed into rectangular,bars or discs. The bars were isostatically pressed at - 6 0 000 psi. The bars 77-900 to 77-990B (Table I) were made from powder A and were heated at 5 °C/min to the desired sintering temperatures, held for a specific period of time, cooled to 450 °C at 3 °C/min, annealed for 10 h, and cooled slowly to room temperature. The complete sintering and annealing cycle was
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Narottam P. Bansal: Effect of processing parameters on YBa 2 Cu 3 0 r
TABLE I. Properties of YBaj
superconductor processed under different conditions (powder A).
Sintering parameters"
Sample number
Density, g/cm 3 Temperature Time (percent h °C Atmosphere theoretical)
77-900
900
15
Flowing O 2
77-925
925
15
Flowing O 2
77-950
950
15
Flowing O 2
77-970
970
15
Flowing O 2
77-990A
990
4
Flowing O 2
77-990B
990
15
Flowing O 2
Roomtemperature • .•
Tc (K)
-j
AT (K) " • ' c V. » - V Zero 10% to 90% A
Superconducting fraction at 77 K
mfl cm
Onset
Midpoint
9.0
90.6
89.3
87
2.4
14
5.3
90.8
90.0
88.
