Abstract

Understanding what makes Bi$_2$Sr$_2$Ca$_1$Cu$_2$O$_x$ (Bi-2212) the only high critical current density ($J_c$), high temperature superconductor (HTS) capable of being made as a round wire (RW) is important intellectually because high $J_c$ RW Bi-2212 breaks the paradigm that forces biaxially textured REBCO and uniaxially textured (Bi,Pb)$_2$Sr$_2$Ca$_2$Cu$_3$O$_x$ (Bi-2223) into tape geometries that reproduce the strong anisotropy of the native crystal structure and force expensive fabrication routes to ensure the best possible texture with minimum density of high angle grain boundaries. The biaxial growth texture of Bi-2212 developed during a partial melt heat treatment should favor high $J_c$, even though its $\sim$15$^{\circ}$ full width at half maximum (FWHM) grain-to-grain misorientation is well beyond the commonly accepted strong-coupling range. Its ability to be strongly overdoped should be valuable too, since underdoped cuprate grain boundaries are widely believed to be weakly linked. Accordingly, we here study property changes after oxygen underdoping the optimized, overdoped wire. While $J_c$ and vortex pinning diminish significantly in underdoped wires, we were not able to develop the prominent weak-link signature (a hysteretic $J_c$(H) characteristic) evident in even the very best Bi-2223 tapes with a $\sim$ 15$^{\circ}$ FWHM uniaxial texture. We attribute the high $J_c$ and lack of weak link signature in our Bi-2212 round wires to the high-aspect ratio, large-grain, basal-plane-faced grain morphology produced by partial-melt processing of Bi-2212 which enables $c$-axis Brick-Wall current flow when $ab$-plane transport is blocked. We conclude that the presently optimized biaxial texture of Bi-2212 intrinsically constitutes a strongly coupled current path, regardless of its oxygen doping state.

Highlights

  • Oriented grain boundaries (GBs) are known to be a major impediment to the realization of high Jc in polycrystalline high-temperature superconductors (HTSs) [1,2]

  • This is a quite unexpected result, incompatible with the standard explanation of current transport in polycrystalline HTS cuprates that strong links and weak links coexist in the active current path and that the path to higher Jc is to ameliorate the doping state of the weak-link grain boundaries to one of higher carrier and superfluid density and, higher Jc

  • We found no evidence of weak-link behavior, even in the most underdoped state when both grains and grain boundaries are strongly underdoped

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Summary

Introduction

Oriented grain boundaries (GBs) are known to be a major impediment to the realization of high Jc in polycrystalline high-temperature superconductors (HTSs) [1,2]. The most widely studied and clearly problematic GBs are planar [001] tilt GBs [1]. Dimos et al [3,4] showed early on that Jc across planar GBs decreases exponentially with an increasing grain-to-grain planar [001] tilt misorientation angle in YBa2Cu3O7−δ (YBCO), with the critical angle for the onset of weak-link behavior later shown to be as low as 2◦–3◦ [5,6]. The [001] tilt GBs in Bi2Sr2Ca1Cu2Ox (Bi2212) [7] and (Bi,Pb)2Sr2Ca2Cu3Ox (Bi-2223) [8,9] display a similar dependence.

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