Formation of the bismuth strontium calcium copper oxide superconductor from metal-bearing precursors

This research investigated the formation of the Bi₂Sr ₂CaCu₂O₈ (Bi-2212) superconductor from a metal-bearing Bi₂O₃, Sr, Ca, and Cu precursor powder prepared via powder metallurgy. The optimal milling schedule was determined to be 1 h high-energy vibratory milling of Bi₂O₃, Sr, and Ca followed by 8 h low-energy rotary-ball milling of Bi₂O₃-Sr-Ca and Cu. The resulting powder consisted of an intimate mixture of particles less than 20 $m$m in size. This powder mixture was packed into a silver tube and then drawn and rolled into tapes. Tapes of $\sim$150 $m$m total thickness could be prepared without appreciable variation in the precursor core thickness (i.e., sausaging).; A series of isothermal annealing treatments at 300–400°C, 750°C, and then 860°C were used to oxidize the metallic phases in the core of the tape and to allow for conversion into Bi-2212. In the first oxidation stage, an insufficient supply of O₂ for Sr and Ca oxidation would result in a displacement reaction between alkaline earth metal and Bi ₂O₃. The resulting liquid Bi metal would migrate outward through the silver sheath, forming a Bi-rich scale on tape surface and leaving a Bi-depleted core. The removal of one side of the precursor tape introduced an easy-access path for O₂ transport; tapes prepared in this manner did not form a Bi-rich scale.; The reaction sequence of metal-bearing precursor to Bi-2212 was investigated. At 350°C the core had reacted with O₂ to form simple oxides. The formation of a multicomponent oxide phase, Bi₉Sr₁₁Ca ₅O_x, was observed to occur at temperatures as low as 350°C. At 650°C, the core consisted of Bi-2201, Bi₉Sr₁₁Ca ₅O_x, CuO, and a Sr-rich (Sr,Ca)O phase. This phase assemblage remained until 840°C, where Bi-2212 began to form. After only 2 h at 860°C, the tape core was comprised primarily of Bi-2212 (by X-ray diffraction). The formation of multi-component oxide phases at relatively low temperatures and modest times illustrates a primary advantage of using metal-bearing precursors. Metal-precursor derived tapes were melt-textured side-by-side with traditional oxide-powder-in-tube Bi-2212 tapes. The measured critical transport current densities were comparable, thus demonstrating the viability of using metal-bearing precursors in Bi-2212 fabrication.