Influence of microstructure on power dissipation in bulk Y-Ba-Cu-O structures

The relatively low values of critical current density (J/sub C/) in bulk high T/sub C/ materials are a problem for many applications, but these materials are now ending use in developmental fault current limiters of various design. Intended primarily for AC power applications, these devices rely on materials that transition quickly between a state of effectively lossless conduction and a state which dissipates significant power directly or significantly changes the magnetic coupling between other circuit elements. Most bulk material processes have been adjusted to maximize the value of J/sub C/ rather than to provide the well defined, sharp transition just mentioned. This study compares the dissipative properties of sintered YBCO with equiaxed, unoriented grains to that of two different types of melt-processed thick film material with plate-like grains in c-axis orientation. Dissipative properties are measured under current densities of 10 times J/sub C/ or more. Isothermal conditions are maintained through the use of submillisecond feedback-controlled current pulses. Significant differences are noted in the high-current flux flow properties, with the sintered samples developing Ohmic behavior and the samples with oriented microstructures developing voltages proportional to I/sup 2/. Sharp transitions at J/sub C/ and extreme dependence on applied magnetic field were noted in all cases.