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

Abstract

We provide evidence that a single mechanism-flux flow along channels-can explain the functional form of the critical current density (J c) in the low temperature superconductor Nb 3 Sn and in the high temperature superconductors (HTS) YBa 2 Cu 3 O 7-δ (YBCO) and (Bi,Pb) 2 Sr 2 Ca n-1 Cu n O x (BiSCCO) in low and high magnetic fields. In this paper, we show that standard flux pinning theories, used for the last four decades to describe J c in low temperature superconductors (LTS), cannot explain the strain dependence of J c in YBCO because J c is a function of strain but the average superconducting properties are not. We conclude that in the polycrystalline samples presented here, the channels are grain boundaries that are narrow and metallic in Nb 3 Sn and YBCO but wide and semiconducting in BiSCCO. Strain alters J c by changing the superconducting properties of the grains in Nb 3 Sn but by changing the grain boundaries in YBCO.

Mechanism for the uniaxial strain dependence of the critical current in practical REBCO tapes

“…We are putting significant effort into developing the mathematical framework for flux flow along channels. As the community develops better experimental tools to measure the local structural and chemical homogeneity along 29 channels between pinning sites, and completes more comprehensive Ic measurements on many different samples, we expect that it will become increasingly apparent that the local properties of the channels, along which flux flows, will need to be explicitly included in any accurate description of current density [43,44].…”

Review of progress in electromechanical properties of REBCO coated conductors for electric device applications

“…Under θ=0° (B//c-axis), however the I c is significantly affected by the magnetic field. Recently, Sunwong et al reported that the strain effect on J c in YBCO is related to its induced change in the grain boundaries properties [51]. Since the irreversibility in I c is attributed to a presence of permanent deformation or fracture in REBCO coating film such as micro cracks, these will likely affect the correlated pinning under B//c-axis such as grain boundary effect.…”

“…The intensity and frequency of the stress/strain applied to the CC tapes depends on the specific performance condition of the device. In this cyclic loading, the tapes were tested up to failure and in other case up to 10 6 cycles in order to determine the electrical fatigue limit of EDDC-SmBCO CC tapes, i. e., with the 95% I c retention [51]. From this criterion, the electrical fatigue limit was determined to be about 500 MPa (0.85σ y ) in Fig.…”

The critical current density of an SNS Josephson-junction in high magnetic fields

“…We have long known that although characterizing grain boundary pinning using just the size of the grains is useful for describing similar superconducting materials with different grain sizes 35 , it is very simplistic. Visualisation of solutions to the TDGL equations 14 for polycrystalline materials has shown that fluxons cross the superconductor by flowing along the grain boundaries 36 37,38 .…”