Isothermal melt processing of Bi-2212 tapes

Abstract

Isothermal melt processing (IMP) has been used to produce phase-pure, high critical current density (J,) Bi-2212 tapes at temperatures as low as 780°C. Bi-2212 tapes processed by IMP have yielded J, values up to 250 kA/cm2 (I, = 345 A) at liquid helium temperatures. Small test coils have been produced with 9, values up to 150 kA/cm2. The effects of the oxidation rate, processing temperature, and time spent in the partial melt on the superconducting properties and microstructure were investigated. Optimal superconducting properties result from the interplay of these parameters such that the grain size of the Bi-2212 phase in the polycrytalline core is allowed to coarsen and align itself with the silver sheath to form a well-connected superconductor. Deviations from optimal conditions result in either too little coarsening to form a well-aligned structure or instabilities in the coarsening process that lead to the formation of large secondary phases which disrupt connectivity..

Understanding processing–microstructure–properties relationships in Bi2Sr2CaCu2Ox/Ag round wires and enhanced transport through saw-tooth processing

“…Oxygen partial pressure was also studied as an important variable [17,18]. The effects of hold time and temperature in the melt, t melt and T max , were studied [7,19,20]. High J c was obtained by controlling t melt ; when t melt was longer than 2.5 h, J c decreased linearly [21].…”

Heat treatment control of Ag–Bi2Sr2CaCu2Oxmultifilamentary round wire: investigation of time in the melt

“…Melting 2212 powder has been shown to be critical for developing J c , as evidenced by J c being ∼0 A mm −2 in wires processed with T max below the 2212 melting temperature [17,21]. We wondered what mechanism is responsible for the dramatic decrease in J c when T max is increased by just a few degrees after exceeding the optimum T max .…”