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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been replaced by bismuth-strontium-calcium-copper-oxide (BSCCO).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.
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