CRITICAL CURRENT-DENSITY AND MICROSTRUCTURE OF YBA2CU4O8

We investigated the intragranular critical current density J(c) in YBa2Cu4O8 prepared by two routes with different oxygen pressures at various temperatures and applied fields. A relationship between the J(c)'s and the microstructures observed by transmission electron microscopy is discussed. The magnitude of J(c) for the low oxygen processed sample is higher by a factor of 5 than that of the high oxygen pressure sample. Transmission electron microscopy reveals that the sample prepared at high oxygen pressure shows an almost perfect stacking sequence, but the low oxygen pressure prepared one has a severely distorted structure. We conclude that the difference in J(c) (H) for the samples must be related to the different microstructures. J(c) at a fixed magnetic field shows an exponential temperature dependence similar to that seen in other high-T(c) superconductors. The field dependence is J(c) proportional H-n with n between 0.5 and 1.5, and with the n value becoming larger at higher temperatures. This type of the behavior may arise from the two-dimensional character of YBa2Cu4O8.