Anisotropic grain morphology, crystallographic texture and their implications for flux pinning mechanisms in MgB2pellets, filaments and thin films

Grain morphology and crystallographic texture were investigated by electron microscopy in four different polycrystalline forms of superconducting MgB 2 . The materials included a hot-pressed sintered MgB 2 pellet, a pellet reacted in situ from Mg and B, an in situ reacted MgB 2 filament and a pulsed-laser-deposited thin film grown on a single crystalline [111] oriented SrTiO 3 substrate. Thick plate-shaped grains with an aspect ratio of ∼3 and large faces parallel to (0001) planes dominated the microstructure in all four types of sample. The intermediate-sized plate-shaped grains (0.1 μm × 0.3 μm on average) in the electromagnetically most homogeneous parts of the hot-pressed pellets were strongly facetted, but not textured. Large (3–5 μm) plate-shaped grains were seen in the pellet reacted directly from stoichiometric Mg and B. A tendency for parallel alignment of the [0001] axes of the considerably larger grains (∼0.25 μm × 1 μm) in the filament was observed near its W core, but degradation of this texture away from the core was apparent. The very small grains (∼10 nm) of the thin film possessed a well-defined fibre texture with [0001] parallel to the film normal and no preferred orientation in the plane of the film. Electrical resistivity of the finest grain samples was some 10 3 times higher than the largest grain sample and their critical current density about one order of magnitude higher. We conclude that, in contrast to the cuprate-based high- T c superconductors, grain boundaries do not limit the critical current density of polycrystalline MgB 2 and indeed act as flux-pinning centres, which enhance the critical current density.