Flux pinning and microstructure in YBa2Cu3O7

Critical current densities J(sub c) of superconductors are one of the most important properties of superconductors in the majority of applications. Unfortunately, in spite of the fact that tremendous efforts have been mounted internationally in studies of the high temperature oxide superconductors, the current values of J(sub c) are disappointingly low except in single crystal bulk specimens and films with the current flowing in the basal planes. When 2 percent of the Cu in YBa2Cu3O7 is replaced by either Al or Fe, it is found that the intragranular critical current densities of these alloyed oxides were reduced. On the other hand, it was also found that substitution for Cu by 2 percent Ni did not show any effect on J(sub c) at 10 K. In order to illustrate what are the primary controlling parameters in reducing the values of J(sub c) when small amounts of Cu were replaced by Fe or Al, some elementary aspects of flux pinning mechanisms are first reviewed. Then it will be shown how those parameters which are pertinent to flux pinning may be estimated for these specimens. Finally, what is probably the most prominent cause for the reduction in J(sub c) is deduced for the specimens discussed.