Processing characteristics and properties of BiSrCaCuO superconducting glass ceramics prepared by melt-quenching

Abstract

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, Bi₂Sr_3-xCa_xO_y, and (Ca, Sr)₃Cu₅O₈ “impurities” compounds. A crystallization sequence from the amorphous state is proposed, involving a reaction at 800 °C between “2223”, CaO, Ca₂CuO₃, and Bi₂SrCa_xO_y 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 °C, and the crystallization reactions from the amorphous state proceed in two steps, at T_xl = 465 °C and T_x2 = 504 °C. The Bi₂Sr_3-xCa_xO_y “2212” and “2223” phases are among the first to crystallize as early as after a 1 h treatment (in air) at 488 °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 °C. The T_c onset for the “2223” phase is at 122 K, and at 85.5 °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).