Effect of Resolidification Conditions on ${\rm Bi}_{2}{\rm Sr}_{2}{\rm CaCu}_{2}{\rm O}_{\rm x}/{\rm Ag/AgMg}$ Coil Performance

The performance of Bi₂Sr₂CaCu₂O_x (Bi2212) wires is very sensitive to the heat treatment conditions, and in particular to the conditions immediately after partial-melting. In this paper, the effect of solidification conditions on Bi2212/Ag/AgMg coil performance is investigated using a split melt process. After partial melting, Bi2212 is first cooled with a relatively fast cooling rate, 10degC/hr, for a short time until reaching ldquoT₁rdquo and subsequently by a slower cooling rate, 2.5degC/hr, for a much longer period of time. Here we study the effects of varying T₁. With decreasing T₁, the overall effective cooling rate during resolidification, particularly the initial stage of resolidification, is increased. As a result, the Bi2212 grain size and the bridges between filaments are inhibited. For short witness samples heat treated with the coils, the transport current and the connectivity decrease with decreasing T₁. A similar tendency is observed in coils, however the coils also show inhomogeneous performance within the conductor. In coils the end sections have higher transport critical current and better connectivity than the middle sections. With decreasing T₁ the difference between end sections and middle sections also decreases. Microanalysis shows that with the insulation on the conductor (both witness samples and short samples cut from coil sections) during heat treatment, increased copper-free phases are found in the Bi2212 filaments as compared to the witness samples heat treated without insulation. In short samples cut from coils, microanalysis also shows an increase in the number of outer filaments that are lost. EDS analysis indicates that Ag and Cu react with the insulation fiber. The Cu diffuses through the Ag sheath and reacts with the insulation fiber, leading to Cu deficiency in the filaments.