A Review of the Influence of Grain Boundary Geometry on the Electromagnetic Properties of Polycrystalline YBa₂Cu₃O_7−x Films

Abstract

Shortly after the discovery of high‐temperature superconducting (HTS) materials in the late 1980s, it was revealed that grain boundaries in these complex oxides are strong barriers to current flow. This fact has remained one of the most significant challenges to a viable HTS conductor, and necessitated the development of technologies capable of producing biaxially textured substrates in long lengths. Multiple studies have reported that the critical current density (Jc) across grain boundaries in the perovskite‐like superconductor YBa2Cu3O7−x (YBCO) falls off exponentially below the intragrain Jc beyond a critical misorientation angle θc of only ≈2°–3°. Here we review our recent work demonstrating that certain grain boundary geometries permit significant enhancements of Jc well beyond the conventional Jc(θ) limit, and also that the grain boundary structure in YBCO films is tied closely to the films' deposition technique. Pulsed laser deposition, a physical vapor deposition technique, results in a columnar grain structure and planar grain boundaries that exhibit the typical Jc(θ) dependence. Ex situ growth processes, where the YBCO film is converted from a previously deposited precursor, can result in laminar grain growth with highly meandered grain boundaries. These latter grain boundary structures are directly correlated to greatly improved Jc values over a wide range of applied magnetic fields. Consequently, very high Jc values are possible in polycrystalline HTS wire even when significant misorientations between grains are present.

Neutron irradiation of coated conductors

“…Numerous experiments on bicrystals 18 showed that, depending on the grain boundary misorientation angle, the current transport is controlled by J c,GB at low fields, whereas at fields in the Tesla range J c,G limits the current transport. Although it is not fully established that the behaviour of a single well-defined grain boundary is representative for the complex grain boundary network of a coated conductor 19,20 , the current limitation by the grains and an increase of J c,G after irradiation are straightforward explanations for the observed J c improvement in high fields. Further evidence for this scenario is provided by neutron irradiation of YBCO thin films grown on single-crystalline substrates, which can be regarded as single macroscopic grains and thus serve as a model system for the individual microscopic grains in a coated conductor.…”

The critical current density of grain boundary channels in polycrystalline HTS and LTS superconductors in magnetic fields

“…Since w is a few microns in Nb 3 Sn, about a factor of 5 -10 larger than the grain size 54 , it suggests that several grains act together as a single channel or equivalently not all triple points separate the channels (probably because they are not aligned sufficiently with the field direction). The HTS YBCO and BiSCCO have much smaller values of w than the grain sizes in these materials [55][56][57] which implies that faceting or grain boundary dislocations divide a grain boundary into many SNS junctions, or to use the language of flux pinning, there are strong pinning sites along the grain boundaries 41,58,59 . In parameterising the free-parameter d, This paper uses a single functional form that naturally leads to power law behaviour at low field and exponential behaviour in high fields without introducing two mechanisms.…”

Strong pinning in thick YBa2Cu3O7 layers mediated by catalysis of a new long-period metastable cuprate phase

“…14 However, this is a minor disadvantage considering that the transport superconducting properties are only weakly sensitive to the grain mis-orientations up to 4°. 16,17 The fluorinated metal-organic deposition ͑MOD͒ precursor layers were deposited by American Superconductor Corporation by spin-coating precursors identical to those used in the production of the second-generation superconducting wire. 18 The precursor film had the cation composition of Y:Dy:Ba:Cu=1:0.5:2:3; Dysprosium metal was added to enhance pinning at high fields, at the same time Dy addition suppressed formation of Y248-type stacking faults ͑SFs͒.…”