Microstructural Aspects of High and Low Tc Superconductors

The U.S. Navy has been developing superconducting homopolar motors for ship propulsion since 1969, initially using conventional NbTi superconducting material for the magnets. With the advent of high critical temperature (high Tc ) superconductors, NbTi has been replaced by bismuth-strontium-calcium-copper-oxide (BSCCO). Performance of these motors depends critically on the properties of the superconducting material specifically of the magnitude of the current density and its stability with time. Flux creep is a major concern in these materials, since it limits high Tc superconductor performance at temperatures above about 30K. As is well known these properties are strongly influenced by the high Tc superconductor microstructure. The level of current transport in a given high Tc superconductor depends upon several intrinsic microstructure-property relationships. In the typical orthorhombic crystal structure, superconducting current flows primarily in the a-b planes. In polycrystalline materials, superconducting current drops off as grain boundary misorientation increases. High current densities in polycrystalline materials needs strong c-axis alignment where current is expected to flow through those grains connected by low angle boundaries. When a magnetic field is applied, the flux vortices may shift due to the force from the current or to thermal activation, resulting in a loss of superconducting properties known as flux creep. Flux vortices may be pinned by microstructural defects such as grain boundaries or dislocations, if present in sufficient quantities, hence the importance of microstructure. The present paper deals with the microstructural aspects of superconductivity, specifically the role played by microstructure in determining superconducting properties. Examples from both the low and high Tc materials will be cited and future trends discussed.