Abstract

This chapter discusses the theory, crystalline structure, properties, and applications of ceramic superconductors. The importance of the work of Bednorz and Müller was that their discovery of superconductivity in ceramics with a perovskite-like structure that led directly to superconductivity above liquid nitrogen temperatures. Although no detailed mechanistic theory yet exists, there is a phenomenological theory because of Ginzburg and Landau (GL theory) that makes some extremely useful predictions for high- T c superconductivity. Superconductors are classified as type I or type II. Most pure elemental superconductors are type I, whereas most alloy conventional superconductors and all high- T c ceramic superconductors are type II. The very short coherence length of the electron pairs in all high- T c superconductors implies that processing has a key role to play. YBa 2 Cu 3 O 7− x was the first ceramic superconductor discovered with T c greater than liquid nitrogen temperature. Texturing is essential to achieve high current densities in polycrystalline wires or the thick films of ceramic superconductors. The high- T c field is continually expanding. The most significant new materials are bismuth-based and thallium-based. These systems exhibit superconducting phases with a variety of values of T c dependent upon their compositions.

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