High density MgB2 superconductor: structure and morphology through microtomography and SEM investigations

Recently, MgB2 was discovered to be a superconductor with a surprisingly high critical temperature of about 39 K and with significant potential for applications. This material is usually prepared from a mixture of Mg and B in sealed tubes with excess Mg. As-prepared samples usually are very porous and mechanically weak. High density superconducting samples of MgB2 undoped and doped with SiC and B4C were produced by Field Assisted Sintering Technique (FAST) (Tc = 38.5 K). Samples are relatively large of 1.9 cm in diameter, apparently uniform, and can be easily extracted from the die. Processing times are short for easily attainable and relatively low temperatures. The effects of FAST consolidation on crystalline morphology revealed by microtomography and Scanning Electron Microscopy (SEM) and bulk density were investigated. Most samples have bulk density above 90% of the theoretical one. Despite such high bulk density, microtomography in combination with SEM revealed that in the samples there are some regions of low and high density. High density regions are not the consequence of FAST processing and they were observed in the raw material.