Multiscale studies of processing-microstructure-transport relationships in over- pressure processed Bi₂Sr₂CaCu₂Ox/Ag multifilamentary round wire

Recently, significantly improved transport properties in Bi 2 Sr 2 CaCu 2 O x /Ag (Bi2212/Ag) multifilamentary round wire (RW) has been achieved by applying 100 atm over-pressure (OP) during partial melt processing (PMP). Despite this significant progress, the microstructural details and phase evolution during the high pressure partial melt processing are poorly understood, and previous studies have focused solely on densification. Here we investigate the processing-microstructure-transport relationships of OP-PMP RWs by comparing the filament microstructure of a PMP wire with that of an OP-PMP wire on multiple length scales. OP affects the microstructure differently in each stage of PMP: it causes grain growth during pre-annealing, increases the peritectic melting temperature in the partial melt, improves oxygen uptake during solidification, and enhances grain connectivity during sintering. It is also found that the increase in transport of OP-PMP wire is related to increase in Bi2212 filament density on multiple length scales. Yet OP-PMP wire shows higher area fraction of filaments with large Bi2201 grains. Thus, to further improve transport, optimization of OP-PMP is essential; in particular the maximum heat treatment temperature should be increased, consistent with the increase in peritectic melt temperature, such that less Bi2201 grains form during processing.