Influencing factors on the electrical transport properties of split-melt processed Bi₂Sr₂CaCu₂O_xround wires

Abstract

Here we study the effects of varying split-melt processing (SMP) heat treatment temperatures on multifilamentary Bi 2 Sr 2 CaCu 2 O x (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.

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

“…Based on a physically significant special case studied by Cahn [32], where the phase transformation kinetics follows the Avrami equation, nucleation occurs early in the course of the transformation and is site-saturation limited. Also in [12], the values obtained for the Avrami exponents indicate that the formation of Bi2212 is associated with a diffusion mechanism for both the nucleation and the growth of Bi2212 plate-like grains. Rapid cation diffusion at the early stage of solidification causes the rapid formation of Bi2201 and/or Bi2212 grains, resulting in larger Avrami exponents.…”

“…In the subsequent resolidification, the reverse peritectic reaction reforms the Bi2212 phase in a network of grains and grain colonies with end-to-end connectivity. Due to phase separation in the partial-melt and kinetic limitations of resolidification, the reverse reaction is not complete; Bi 2 Sr 2 CuO x (Bi2201) intergrowths and grains form within and beside Bi2212 grains, and AEC and CF phases may remain [11][12][13][14].…”

“…Further study of SMP [12] showed that the enhanced transport is a result of creating more recrystallization sites for the Bi2212 phase. In [12] it was shown that the phase transformation kinetics of the resolidification of Bi2212 wires during non-isothermal cooling, at a constant cooling rate, obeys the Avrami equation.…”

“…Further study of SMP [12] showed that the enhanced transport is a result of creating more recrystallization sites for the Bi2212 phase. In [12] it was shown that the phase transformation kinetics of the resolidification of Bi2212 wires during non-isothermal cooling, at a constant cooling rate, obeys the Avrami equation. Based on a physically significant special case studied by Cahn [32], where the phase transformation kinetics follows the Avrami equation, nucleation occurs early in the course of the transformation and is site-saturation limited.…”

“…The impingement among Bi2201 and/or Bi2212 grains in the latter growth stage results from the retardation of cation diffusion and the decreased growth dimension of the Bi2201 and Bi2212 grains. The slower cation diffusion, after the liquid is consumed, results in the low Avrami exponent for this stage of cooling [12].…”