The Effects of Seeding on the Formation of High-T c Bi-Pb-Sr-Ca-Cu-O Compounds
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THE EFFECTS OF SEEDING ON THE FORMATION OF HIGH-T. Bi-Pb-Sr-Ca-Cu-O COMPOUNDS Cheol J. Kim', Mark R. DeGuire*, and Narottam P. Bansal'" * Case Western Reserve University, Dept. of Materials Science and Engineering, Cleveland, OH 44106 ** NASA Lewis Research Center, Materials Division, Cleveland, OH 44135 ABSTRACT The effect of adding of Bij.80Pb 0.33Srj.8 7Ca 2Cu 3O, seeds(the 37A phase) to the polycrystalline and glassy Bi-(Pb)-Sr-Ca-Cu-O materials was studied using XRD and SEM. The seeding effect was observed at 835CC, 855°C, and 870oC in several compositions. Adding pre-synthesized 37A phase powder to a mixture of unreacted starting materials accelerates the formation of additional 37A phase. I. Introduction In the Bi-Sr-Ca-Cu-O superconductors, Bi2(Ca,Sr)3 Cu 20 with T,=85K and a caxis spacing-30A is produced in a relatively wide range of compositions and
temperatures while the
Bi 2 (Ca,Sr)4 Cu 3Oy
with T,=l10K and a c-axis spacing-37A is
considerably more difficult to produce. Various methods have been reported to be partially successful at producing large amounts of 37A phase, such as long heat treatment, Pb doping,1' 2 low oxygen pressure annealing, 3 and precise control of the starting composition.3'4 The present study was undertaken to determine whether slow nucleation causes the observed slow rates of formation of the 37A phase. II. Experimental
Samples with nominal compositions of Bi 2Sr 2CaCu 2O,("2212"), Bi2Sr 2Ca 2Cu 30y ("2223"), and Bil.soPbo.33Sr 1.87Ca 2Cu 30O("Pb-2223") were prepared by solid state reaction. The starting materials were high-purity powders of Bi 20 3, CaO, SrO, CuO, and Pb(N0 3)2. These powders were mixed in an agate mortar and pestle and coldpressed. To ensure homogeneity, the powders were ground and pelletized twice during heat-treatment at 800 0C for a total of 24 hours. Glass samples < 1 mm thick were made by melting the pre-reacted 2223 composition at 1100CC in an alumina crucible for 30 minutes, and quenching between copper twin-rollers. Differential thermal analysis (DTA) was performed on the powders of the melt-quenched glass and the solid state reacted crystalline powders using a Perkin-Elmer DTA 1500. Heating rates were 10, 5, and 2CC. Seeds of the 37A phase were synthesized by solid state reaction in air of the Pb-2223 composition. The starting mixture was calcined at 800CC for 1 day and furnace cooled to room temperature. The calcined mixture was ground with agate mortar and pestle and then heat treated at 850CC for 7 days with several intermediate grindings. The XRD pattern in Fig.1 shows this material to be mainly the 37A phase. Seeded polycrystalline specimens were prepared by adding 5 wt% seeds to several matrix compositions.(Table 1) Glassy specimens were seeded by placing a small amount of seeds on the glass surface. These were heat-treated for various times and temperatures. Powder x-ray diffraction (XRD) patterns were recorded at room temperature with Cu Ka radiation in the 2-theta range 3C- 60 o using a Philips ADP 3600 automated diffractometer. The effec
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