How resistive must grain boundaries in polycrystalline superconductors be, to limit J_c?

Abstract

Although we can use misorientation angle to distinguish the grain boundaries that can carry high critical current density (J_c) in high temperature superconductors (HTS) from those that cannot, there is no established normal state property equivalent. In this paper, we explore the superconducting and normal state properties of the grains and grain boundaries of polycrystalline YBa2Cu3O7–x (YBCO) using complementary magnetisation and transport measurements, and calculate how resistive grain boundaries must be to limit J_c in polycrystalline superconductors. The average resistivity of the grain boundaries, ρ_GB, in our micro and nanocrystalline YBCO are 0.12 Ωm and 8.2 Ωm, values which are much higher than that of the grains (ρ_G) and leads to huge ρ_GB/ρ_G values of 2 × 103 and 1.6 × 105 respectively. We find that the grain boundaries in our polycrystalline YBCO are sufficiently resistive that we can expect the transport J_c to be several tens of orders of magnitude below the potential current density of the grains in our YBCO samples, as is found experimentally. Calculations presented show that increasing J_c values by ~ 2 orders of magnitude in high fields is still possible in all state-of-the-art technological high-field superconductors. We conclude: grain boundary engineering is unlikely to improve J_c sufficiently in randomly aligned polycrystalline YBCO, to make it technologically useful for high-field applications; in low temperature superconducting intermetallics, such as Nb3Sn, large increases in J_c may be achieved by completely removing the grain boundaries from these materials and, as is the case for thin films of Nb, Ba(FeCo)2As2 and HTS materials, by incorporating additional artificial pinning.