PROCESSING CHARACTERISTICS AND PROPERTIES OF BISRCACUO SUPERCONDUCTING GLASS-CERAMICS PREPARED BY MELT-QUENCHING

In the framework of an extensive research program for the production of textured and ductile high T(c) BiSrCaCuO (BiSCO) wires and tapes, the influence of processing (by melt-quenching) parameters on the crystallization behavior, the quantitative and qualitative evolution of the crystallized phases, the chemical changes in the bulk, and the superconductive properties of various initial compositions of bulk BiSCO glass ceramics, prepared by melt-quenching, have been studied. The elemental composition of the samples changes drastically during heat treatments, affecting mainly Pb, but Sr and Ca also. The identified crystallographic phases, by XRD, were the low T(c) superconducting ''2201'' (Bi2Sr2CuO6) phase, the high T(c) superconducting ''2212'' and ''2223'' phases, and the Ca2PbO4, Ca2CuO3, CuO, CaO, Bi2Sr3-xCaxOy, and (Ca, Sr)3Cu5O8 ''impurities'' compounds. A crystallization sequence from the amorphous state is proposed, involving a reaction at 800-degrees-C between ''2223'', CaO, Ca2CuO3, and Bi2Sr3-xCaxOy to form ''2212'' + Ca2PbO4 + CuO and a 2(''2212'') --> ''2223'' + ''2201'' disproportionation reaction that takes place with the intake of oxygen at a higher temperature. Decomposition of Ca2PbO4, which occurs also at high temperature, causes an increase of ''2212'', which favors the increase of ''2223'' through the disproportionation reaction. The glass transition starts around T(g) = 400-degrees-C, and the crystallization reactions from the amorphous state proceed in two steps, at T(x1) = 465-degrees-C and T(x2) = 504-degrees-C. The Bi2Sr3-xCaxOy ''2212'' and ''2223'' phases are among the first to crystallize as early as after a 1 h treatment (in air) at 488-degrees-C. A gain in weight is observed by thermogravimetry, caused by intake of the oxygen necessary for the formation of the high T(c) superconducting phases. The oxygen intake starts as early as 600-degrees-C. The T(c) onset for the ''2223'' phase is at 122 K, and at 85.5-degrees-C for the ''2212'' phase. Coefficients of thermal expansion have been measured and shown to differ according to crystallographic direction of expansion. The resistivity is increased on cooling, indicating semiconducting behavior of the 2223 BiSCO ceramic (semiconductor-to-metal transition temperature: 210-220 K).