The Critical Current Density in Polycrystalline HTS and LTS   Superconductors in High Magnetic Fields
  Flux pinning scaling laws were developed to explain the observed increase in the critical current density Jc caused by increased density of grain boundaries in polycrystalline low temperature superconductors (LTS) such as Nb3Sn. For four decades they have provided the framework for the successful development of the LTS materials that carry high Jc in high magnetic fields. However, the discovery of the weak-link problem suggested that transmission of supercurrent flow through the grain boundaries limited Jc in high temperature superconductors (HTS) such as YBa2Cu3O7-{\delta} (YBCO). Here we provide evidence that a single mechanism - flux flow along grain boundaries - confirmed by time-dependent Ginzburg-Landau (TDGL) computational visualisation in LTS materials, explains the functional form of Jc in polycrystalline LTS Nb3Sn and HTS YBCO and (Bi,Pb)2Sr2Can-1CunOx (BiSCCO) materials in low and high magnetic fields. Data presented show that standard flux pinning theory cannot explain comprehensive Jc data for YBCO because Jc is a function of strain but the superconducting properties are not. We conclude that grain boundaries are narrow and metallic in Nb3Sn and YBCO but wide and semiconducting in BiSCCO. Strain alters Jc by changing the superconducting properties of the grains in Nb3Sn but by changing the grain boundaries in YBCO 
P. Sunwong, J. S. Higgins, Y. Tsui, M. J. Raine and D. P. Hampshire
cond-mat.supr-con