MELT PROCESSING OF YBaCuO SUPERCONDUCTORS AND CRITICAL CURRENTS

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Abstract
In the Y–Ba–Cu–O system, the superconducting phase is produced by a peritectic reaction: Y2 BaCuO 5(211) + L → 2YBa 2 Cu 3 O x (123). In order to promote the continuous growth of the 123 phase, uniform distribution of 211 in the liquid is required. In this review, I first present the melt process which enables us to achieve fine dispersion of 211 in the liquid. Large 123 crystals can be grown from this precursor structure by slow cooling in temperature gradient. As a result of incomplete peritectic reaction, the crystals contain fine 211 inclusions. The fine dispersion of 211 inclusions in the 123 matrix is considered to have three beneficial effects: 1) to suppress crack formation; 2) to promote oxygen diffusion; and 3) to provide pinning centers. Such crystals exhibit large J c values exceeding 104 A/cm 2 at 77 K and 1 T according to both transport and magnetic measurements. Magnetization measurements also show that the critical state model is established in the sample indicating that no weak-link is present. Flux creep rate is reasonably small compared to that of single crystals. Attainable J c due to the 211 inclusions is estimated based on the conventional flux pinning theory. Finally, I propose the application of such crystals as a superconducting permanent magnet.

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