Solidification of Bi2Sr2CaCu2Oy and Bi2Sr1.75Ca0.25CuOy

Abstract

<p>The solidification processes for the compositions Bi₂Sr₂CaCu₂O_<em>y</em> (2212) and Bi₂Sr_1.75Ca_0.25CuO_<em>y</em> (2201) were determined as a function of oxygen partial pressure. During solidification in argon, the superconducting phases were generally not observed to form for either composition. In both cases, the solidus is lowered to approximately 750 °C. Solidification of Bi₂Sr_1.75Ca_0.25CuO_<em>y</em> in Ar resulted in a divorced eutectic structure of Bi₂Sr_{2−<em>x</em>}Ca_<em>x</em>O_<em>y</em> (22<em>x</em>) and Cu₂O while solidification of Bi₂Sr₂CaCu₂O_<em>y</em> in Ar resulted in a divorced eutectic structure of Bi₂Sr_{3−<em>x</em>}Ca_<em>x</em>O_<em>y</em> (23<em>x</em>) and Cu₂O. Solidification of Bi₂Sr_1.75Ca_0.25CuO_<em>y</em> in O₂ resulted in large grains of 2201 interspersed with small regions containing the eutectic structure of 22<em>x</em> and CuO/Cu₂O. Solidification of Bi₂Sr₂CaCu₂O_<em>y</em> in partial pressures of 1%, 20%, and 100% oxygen resulted in multiphase samples consisting of 2212, 2201, some alkaline-earth cuprates, and both divorced eutectic structures found during solidification in Ar. For both compositions, these latter structures can be attributed to oxygen deficiencies present in the melt regardless of the overpressure of oxygen. These eutectic structures are unstable and convert into the superconducting phases during subsequent anneals in oxygen. The formation process of the 2212 phase during solidification from the melt was determined to proceed through an intermediate state involving the 2201 phase.</p>