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

In the framework of an extensive research program for the production o f textured and ductile high T(c) BiSrCaCuO (BiSCO) wires and tapes, th e influence of processing (by melt-quenching) parameters on the crysta llization behavior, the quantitative and qualitative evolution of the crystallized phases, the chemical changes in the bulk, and the superco nductive properties of various initial compositions of bulk BiSCO glas s ceramics, prepared by melt-quenching, have been studied. The element al composition of the samples changes drastically during heat treatmen ts, affecting mainly Pb, but Sr and Ca also. The identified crystallog raphic phases, by XRD, were the low T(c) superconducting ''2201'' (Bi2 Sr2CuO6) phase, the high T(c) superconducting ''2212'' and ''2223'' ph ases, and the Ca2PbO4, Ca2CuO3, CuO, CaO, Bi2Sr3-xCaxOy, and (Ca, Sr)3 Cu5O8 ''impurities'' compounds. A crystallization sequence from the am orphous state is proposed, involving a reaction at 800-degrees-C betwe en ''2223'', CaO, Ca2CuO3, and Bi2Sr3-xCaxOy to form ''2212'' + Ca2PbO 4 + CuO and a 2(''2212'') --> ''2223'' + ''2201'' disproportionation r eaction that takes place with the intake of oxygen at a higher tempera ture. 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 th e 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 a re among the first to crystallize as early as after a 1 h treatment (i n air) at 488-degrees-C. A gain in weight is observed by thermogravime try, caused by intake of the oxygen necessary for the formation of the high T(c) superconducting phases. The oxygen intake starts as early a s 600-degrees-C. The T(c) onset for the ''2223'' phase is at 122 K, an d at 85.5-degrees-C for the ''2212'' phase. Coefficients of thermal ex pansion have been measured and shown to differ according to crystallog raphic direction of expansion. The resistivity is increased on cooling , indicating semiconducting behavior of the 2223 BiSCO ceramic (semico nductor-to-metal transition temperature: 210-220 K).