The formation and role of a phase with glassy appearance in the Tl-Ca-Ba-Cu system

Abstract

A phase with a glassy appearance has been detected in samples of the Tl–Ca–Ba–Cu system. The presence of such a phase is expected to have a negative effect on the critical current density in a bulk superconductor. Samples of 2-2–2-3 starting composition were prepared from Tl₂O₃, BaO₂, and relatively large particles of CaO and CuO. These samples were then heat-treated at 867°C for various periods of time. The use of large particles of CaO and CuO facilitates the examination of the morphological changes associated with the formation of the superconducting phases. Using scanning electron microscopy, a layer of the 2122 phase was found to form on the surface of the large CaO particles whereas near the surface of the large CuO particles, large voids were observed. By varying some of the experimental conditions, a liquid phase with a composition close to 2-0–2-3 was inferred to form. The voids observed in the surroundings of the CuO particles may be due to the formation of such a melt. This liquid phase, after cooling, apparently leads to the “glassy” phase detected in the reacted samples. This glassy phase may play a role as an intermediate in the formation of the 2122 phase, which could result by a direct reaction between the liquid phase and CaO, A part may remain, however, among the superconducting grains, thus having a negative effect on the critical current density.