Electron microscopy studies of high Tc phase development in melt-quenched Bi-Ca-Sr-Cu oxides

Abstract

A Bi-Ca-Sr-Cu oxide composition (2:4:2:5) was rapidly solidified from the melt, and the crystallization behavior examined on heat-treatment. Annealing conditions were 865°C for up to 11 days. The high Tc 2223 phase (105 K) evolved from the 2122 phase (80 K), which in turn developed from the 2021 phase (12 K). The high Tc phase developed only in the presence of a liquid phase at 865 °C. Lattice imaging was used to follow the conversion of 2122 phase to 2223. Data are reported for syntactic intergrowths, which became less frequent with time at temperature. EDS results are consistent with the conversion of 2122 to 2223. Crystals of 2223 could not be grown from the melt, nor crystallized from the solid at temperatures below 820 °C. The presence of a Cu- and Ca-rich liquid was essential for the development of 2223 at 865 °C. A tentative model for the formation of 2223 via a liquid mediated reaction is proposed. EDS confirmed the liquid was rich in Ca and Cu near the solid-liquid interface, and precipitates of secondary phases were identified by SEM, TEM, and XRD methods. The presence of CuO and (Ca,Sr)2CuO3 verified the enrichment of Cu and Ca at the solid-liquid interface. The results are consistent with the evolution of structure of a 2223 from a 2425 starting composition.