Processing Effects on (110K) Phase Development and Properties in Bismuth-Strontium-Calcium-Copper-Oxygen Superconducting System
Antony, Gmon
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https://hdl.handle.net/2142/72169
Description
Title
Processing Effects on (110K) Phase Development and Properties in Bismuth-Strontium-Calcium-Copper-Oxygen Superconducting System
Author(s)
Antony, Gmon
Issue Date
1993
Doctoral Committee Chair(s)
Buchanan, R.C.
Department of Study
Materials Science and Engineering
Discipline
Ceramics Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Abstract
The high T$\sb{\rm c}$ (110K) $\rm Bi\sb2Sr\sb2Ca\sb2Cu\sb3O\sb{x}$ (2223) phase was prepared by distributed liquid phase sintering of combined oxide precursors, $\rm Bi\sb2O\sb3.2SrO$ and (2CaO + 3CuO) from a starting composition of (2.8 224). The samples were characterized using four point probe resistance measurement, SQUID, SEM, TEM, XRD, EDS, EPMA, DTA and TGA. In order to confine and distribute the liquid uniformly, the specimens were rotated during sintering. The effects of starting composition, sintering atmosphere and annealing in producing the high T$\sb{\rm c}$ phase were investigated. The optimum sintering temperature for the formation of the high T$\sb{\rm c}$ phase increased with oxygen partial pressure and with excess amounts of Ca and Sr in the starting composition. In decreased with increasing concentrations of Bi and Cu, which in excess favored the formation of the high T$\sb{\rm c}$ phase. The optimum sintering atmosphere ranged from 80:20 to 70:30 N$\sb2$/O$\sb2$ ratio. Variation of the melting point with sintering atmosphere was attributed to the reduction of Cu$\sp{2+}$ to Cu$\sp+$ on melting. Improvement in resistance behavior after annealing at 400$\sp\circ$C-20h was ascribed to better connectivity between the superconducting (2223) grains by the absorption of oxygen into regions which had become deficient in oxygen during liquid phase sintering. Five different phases were found to coexist in the liquid phase sintered sample after short time (10-36h) sintering; viz. (2223), (2212), two Ca-Sr-Cu-O phases and a Bi-Cu rich liquid phase. It was observed that sintering for long periods of time (100h) resulted in 110K single transition samples with respect to resistance and magnetic susceptibility behavior. Based on the microstructures of samples sintered for different times, it is proposed that the (2223) phase is formed by a reaction between the low T$\sb{\rm c}$ and (Bi,Ca,Sr)$\sb $Cu$\sb{24}$O$\sb{41}$ phases with the aid of the Bi-Cu rich liquid phase. The (Bi,Ca,Sr)$\sb $Cu$\sb{24}$O$\sb{41}$ phase is readily formed from the combined oxide precursors. This explains the relative ease of formation of the 110K phase from these precursors. The role of excess Bi and Cu in the starting composition is also clear as this facilitates the formation of the liquid phase.
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