Quantitative analysis of the effects of strain-state on the microstructure and Jc of BSCCO tapes

After considerable optimization efforts, conventional thermomechanical processing of long, high temperature superconductors has not produced critical current densities (Jc) adequate for most liquid nitrogen temperature applications. New approaches are needed to improve the Jc of superconducting tape produced by co-deforming a ductile silver sheath containing the superconducting oxide using the powder-in-tube process. This study investigates improvements in Jc generated by modifying the strain-state during rolling of silver-sheathed Bi2Sr2Ca2Cu3O10+x (BSCCO-2223) tape using quantitative image analysis of the different phases. Pure compression and combined compression-shear loading was achieved by embedding BSCCO-2223 tapes at different locations within thick steel blocks. High hydrostatic compressive stress was imposed by confining the tape width. Tapes deformed with combined shear-compression exhibited measurably higher Jc values than tapes subjected to pure compression, but their microstructures showed little difference in the amount of nonconducting (including porosity) phase content. However, constraining the tape width resulted in the most significant increase in Jc which corresponded to a much lower porosity and nonconducting phase volume in the oxide near the tape edge