Factors Influencing Critical Current Densities in High T sub c Superconductors

A key requirement for many practical application of the new class of high T sub c superconductors is the ability to sustain high current densities in the presence of magnetic fields. The new materials present two substantially different behaviors. For Ba sub 2 YCu sub 3 O sub 7 critical current behavior of polycrystalline material is dominated by intergranular transport. We will review a model for transport involving the effects of correlation of defects with grain boundary orientation, the presence of impurities at the grain boundaries, measurements of tunneling characteristics through the boundaries, and the effect of grain alignment on critical current. The case is substantially different in the 84K superconducting phase Bi sub (2.2) Sr sub 2 Ca sub (0.8) Cu sub 2 O sub (8+delta) where there is a very strong dependence of critical current densities at 30-84K on magnetic field for single crystals. There is a lack of threshold behavior in the I-V characteristics in finite applied fields larger than H sub (c1) (T) which is attributed to flux creep. This suggests that in the Bi material there is insufficient pinning of vortices in the intrinsic material to prevent them moving in the presence of an applied transport current, causing dissipation. The implications of these various factors on potential applications of these materials will be discussed.