Transport properties of low-angle grain boundaries and granular coated conductors

Grain boundaries in oxide superconductors are of central importance both for engineering applications and for issues in fundamental science. Although they have been closely studied for more than a decade, there is still by no means a consensus on the properties of individual boundaries, or their collective behaviour when current percolates in a granular material. A quantitative description of current and flux percolation is essential for the practical specification of conductors and for charting routes to conductor optimization. There are three components to the problem: (i) the properties of individual grains and grain boundaries, (ii) microstructure, texture and grain morphology, and (iii) the macroscopic geometrical constraints such as conductor size and shape. On the basis of new results on the angular variation in an applied magnetic field of the grain boundary critical current, it will be argued that grain morphology as well as texture is important for the critical current of a coated conductor. Progress in characterizing and modelling current and flux percolation will be surveyed and particular gaps in understanding and areas of neglect discussed.

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