Unsubstituted Tl-1223: a possible candidate for high current applications of superconductivity

Abstract

The Tl–Ba–Ca–Cu–O system has been investigated. We report on the superconducting properties and the pathway for the formation of the Tl-1223 phase and on equilibrium phase relations. The formation of Tl-1223 implies the double TlO layer phases and occurs at high temperature (890–910°C) by a progressive de-intercalation of TlO layers from Tl-2223. As a consequence of this non-equilibrium process, transient intergrowth phases may be formed and have been unambiguously identified. This path of formation results in a platelet-like morphology for Tl-1223 grains which, in the view of the fabrication of tapes, is much more interesting than the globular shape observed for (Tl,Pb)(Sr,Ba) 2Ca 2Cu 3O z . Moreover, the superconducting properties ( T c and irreversibility line) of the unsubstituted Tl-1223 phase compare well with those of (Tl,Pb)(Sr,Ba) 2Ca 2Cu 3O z . With regard to phase equilibria, we show the existence of a three phase field Tl 2Ca 3O 6–(Tl-2212)–(Ca,Tl) 1− x CuO y which prevents the formation of Tl-2223 for Tl contents higher than two.