Processing-property relationships for high critical temperature ceramic superconductors in the bismuth-calcium-strontium-copper-oxygen system
Asthana, Ashish
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https://hdl.handle.net/2142/23475
Description
Title
Processing-property relationships for high critical temperature ceramic superconductors in the bismuth-calcium-strontium-copper-oxygen system
Author(s)
Asthana, Ashish
Issue Date
1992
Doctoral Committee Chair(s)
Payne, David A.
Department of Study
Engineering, Materials Science
Discipline
Engineering, Materials Science
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Language
eng
Abstract
This thesis reports research on high T$\sb{\rm c}$ Bi-Ca-Sr-Cu-O (BCSC) superconductors. The BCSC family of superconductors has three known superconducting phases: Bi$\sb2$Sr$\sb2$Cu$\sb1$O$\sb6$ (abbreviated as 2021, with a reported T$\sb{\rm c}$ of 22 K), Bi$\sb2$Ca$\sb1$Sr$\sb2$Cu$\sb2$O$\sb8$ (abbreviated as 2122, and a T$\sb{\rm c}$ near 80 K), and Bi$\sb2$Ca$\sb2$Sr$\sb2$Cu$\sb3$O$\sb{10}$ (abbreviated as 2223, and a T$\sb{\rm c}$ near 105 K).
Results on the characterization of twinned and untwinned crystals of 2122 are reported. Layer-growth morphology, and anisotropy in properties associated with two-dimensional growth, are discussed. Symmetric cruciform twins, with $\{$110$\}$ twin boundaries, were observed in certain crystals, and determined to be growth twins. Twin morphology and structure were investigated, and related to superconducting properties. Electrical transport measurements within the same twin, and across the twin boundary, determined that the twin boundary per se did not have a significant effect on the resistance-temperature characteristics.
Difficulties encountered in crystal growth of 2223 served to underscore the importance of optimizing processing conditions for phase development in polycrystalline ceramics. Experiments were designed to prepare amorphous materials by rapid solidification, followed by appropriate heat treatment.
A series of experiments on the effect of starting composition on phase composition and superconducting properties demonstrated that: (i) excess Ca and Cu in the starting composition gave higher yields of 2223 and (ii) PbO and Sb$\sb2$O$\sb3$ additives substantially improved 2223 yields. Development of the 2223 phase required extended heat treatment ($\sim$several days) in a narrow temperature window. The yield for 2223 was observed to be sensitive to heat-treatment conditions (time, temperature, and atmosphere). The best results were obtained for heat treatment at 848$\sp\circ$C in 10% O$\sb2$. Sharp resistive transitions with T$\sb{\rm c}$s greater than 100 K were measured.
In an attempt to promote textured growth of 2223 grains and thereby improve critical current densities, heat treatment in small thermal gradients (2-6$\sp\circ$C/cm) were carried out. The resistance-temperature characteristics for the specimens were found to depend on the magnitude of the thermal gradient, as well as the heat-treatment conditions. The best specimens were obtained after heat treatment in a thermal gradient of 2$\sp\circ$C/cm, and exhibited sharp resistance transitions with zero resistance near 110 K.
A Ca- and Cu-rich liquid phase, which was frequently observed in close proximity of superconducting phases, was observed in specimens with high 2223 contents. A liquid-phase-mediated transformation of 2122 to 2223 was proposed based on experimental results.
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