Influencing factors on the electrical transport properties of split-melt processed Bi2Sr2CaCu2Ox round wires

Abstract

Here we study the effects of varying split-melt processing (SMP) heat treatment temperatures on multifilamentary Bi2Sr2CaCu2Ox (Bi2212) wires to understand the factors that influence the electrical transport properties. Sharp threshold behaviors result in an order-of-magnitude difference in critical current density and distinct difference in magnetization behavior. Quantitative image analysis of SEM micrographs shows a strong correlation between the content of 2–4 μm interfilamentary bridges and electrical performance. Avrami analysis of re-solidification indicates that Bi2212 grain nucleation is heterogeneous and site-saturation limited, with a transition in the reaction mechanism corresponding to the depletion of liquid. During the first SMP heat treatment, oxygen is fully recovered, accelerating Bi2201 and/or Bi2212 phase nucleation. Large, flat Bi2201 grains form from the Bi2201 liquid at the start of the second heat treatment and subsequently act as Bi2212 grain nucleation sites. During cooling from high temperature, Bi2212 grains form via a solid/liquid reaction that is enhanced by the liquid, that facilitates second phase dissolution and cation diffusion. At lower processing temperatures, and when the liquid is depleted, Bi2212 grain growth is from a slow solid-state reaction.