The grain morphology and crystallographic texture were investigated by using electron microscopy of 4 different polycrystalline forms of superconducting MgB2. The materials included hot-pressed sintered MgB2 pellets, pellets reacted in situ from Mg and B, in situ reacted MgB2 filaments and pulsed-laser deposited thin films grown onto a monocrystalline [111]-oriented SrTiO3 substrate. Thick plate-shaped grains with an aspect ratio of ~3 and large faces parallel to (00▪1) planes dominated the microstructure in all 4 types of sample. The intermediate-sized plate-shaped grains (0.1μm x 0.3μm) in the electromagnetically most homogeneous parts of the hot-pressed pellets were strongly faceted, but not textured. Large (3–5μm) plate-shaped grains were seen in the pellet reacted directly from stoichiometric Mg and B. A tendency for parallel alignment of the [00▪1] axes of the considerably larger grains (~0.25μm x 1μm) in the filament was observed near its W core, but degradation of this texture away from the core was apparent. The very small grains (~10nm) of the thin film possessed a well-defined fiber texture with [00▪1] 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 an order of magnitude higher. It was concluded that grain boundaries did not limit the critical current density of polycrystalline MgB2 and indeed acted as flux-pinning centers, which enhance the critical current density.

Anisotropic Grain Morphology, Crystallographic Texture and their Implications for Flux Pinning Mechanisms in MgB2 Pellets, Filaments and Thin Films. X.Song, S.E.Babcock, C.B.Eom, D.C.Larbalestier, K.A.Regan, R.J.Cava, S.L.Budko, P.C.Canfield, D.K.Finnemore: Superconductor Science and Technology, 2002, 15, 511-8