Nanoscale Pinning in the LRE-123 System - the Way to Applications up to Liquid Oxygen Temperature and High Magnetic Fields

Abstract

The discovery of superconductivity in oxides (Bednorz, et al., 1986), especially in the system of YBa2Cu3Oy “Y-123” (Wu et al., 1987), having a transition temperature well above boiling point of liquid nitrogen and capable of carrying critical current densities at a level necessary for practical use, moreover in rather high magnetic fields, placed cuprate composites into center of the present material physics and technology. Liquid nitrogen cooling has promised construction of cryogenic systems greatly simplified, more realistic and economical in operation. Note that not only the critical temperatures of the new superconductors have been much higher than those of the conventional materials. The upper critical field of the order of 100 T has been estimated and also measured, making from these materials ideal candidates for high field applications (Welp et al., 1989). On the other hand, it has also been found that high-Tc materials in a polycrystalline form carry only low critical current densities, due to grain boundary weak links and crystal anisotropy (Cava et al., 1987). Attempts to improve the critical current density of the Y-123 material by texturing substrates and identifying coupling mechanisms at interface started immediately worldwide (Jin et al., 1988; Babcock et al., 1990). U.S. Pat. No. 5,061,682, issued to Aksay et al., 1991 disclosed a process for preparing conductive and superconductive ceramics composed of Y2BaCuO5, YBa2Cu3O7, and YBa2Cu4O8. The most successful process at present is melttexturing, which controls to a high degree lattice orientation of the crystalline material. In this way the superconducting phase (YBa2Cu3Ox) is formed by a peritectic reaction of Y2BaCuO5 (211) with a liquid phase. The growth process of the superconducting phase is accelerated by means of finely and homogeneously dispersed 211 phase in the liquid phase; at the same time, however, the 211 phase serves as a pinning medium dispersed in the superconducting phase. During the following slow cooling nucleation often occurs. This secondary nucleation forms parasitic grains that consume the material intended for the growth of superconducting grains. In this way high-angle grain boundaries are created that