Grain morphology and crystallographic texture were investigated by electron microscopy in four different polycrystalline forms of superconducting MgB2. The materials included a hot-pressed sintered MgB2 pellet, a pellet reacted in situ from Mg and B, an in situ reacted MgB2 filament and a pulsed-laser-deposited thin film grown on a single crystalline  oriented SrTiO3 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  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  parallel to the film normal and no preferred orientation in the plane of the film. Electrical resistivity of the finest grain samples was some 103 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-Tc superconductors, grain boundaries do not limit the critical current density of polycrystalline MgB2 and indeed act as flux-pinning centres, which enhance the critical current density.
All Science Journal Classification (ASJC) codes
- Ceramics and Composites
- Condensed Matter Physics
- Metals and Alloys
- Electrical and Electronic Engineering
- Materials Chemistry