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

Abstract

Although we can use misorientation angle to distinguish the grain boundaries that can carry high critical current density 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 in polycrystalline superconductors. The average resistivity of the grain boundaries, in our micro- and nanocrystalline YBCO are 0.12 Ωm and 8.2 Ωm, values which are much higher than that of the grains and leads to huge 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 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 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 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 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.

The cause of ‘weak-link’ grain boundary behaviour in polycrystalline Bi2Sr2CaCu2O8and Bi2Sr2Ca2Cu3O10superconductors

“…Furthermore, the materials with the highest c / DSc [1], whether it is the cuprate material YBCO [18], Fe-based material Ba(FeCo)2As2 [79] or the elemental material Nb [80], are tapes with no large angle grain boundaries and strong pinning.…”

“…In these cases, c * is predominantly determined by the grains. This interpretation is most often valid for polycrystalline metals and alloys where even high angle grain boundaries can have a similar resistivity to the grains [1]. At the other end of the spectrum, one can consider the grains to have good critical properties and the grain boundaries to be non-superconducting and highly resistive.…”

“…The peaks at 650-680 °C in cycle 2 of the B2 materials may be Bi2Sr2CuO6 + Ca2CuO3 + CuO → Bi2Sr2CaCu2O8 + Ca2CuO5 + CuO [36]. Generally, in nanocrystalline or amorphous materials, small exothermic peaks are observed in the milled materials, which A c c e p t e d M a n u s c r i p t 7 identifies the temperature at which crystallisation occurs [1,[37][38][39]. B1 and B2 are very soft, thus the energy associated with crystallisation is expected to be small.…”

“…In this paper, we apply the quantitative approach used to identify the cause of the 'weak-link' grain boundary behaviour in YBa2Cu3O7 [1], to the Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10 materials that we have fabricated. Magnetic and transport measurements are used to characterise the grain and grain boundary properties of micro-and nanocrystalline material.…”

“…Table 1 shows the DSc for Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10, and the parameters used to calculate them [1]. The angular averaged depairing current densities (⟨ DSc (0, )⟩) were calculated using and 7 [22].…”