Effect of Mg content in precursors on the volume expansion of MgB₂ by two-step reaction method

A series bulk samples of (MgB₂)_x(Mg + 2B)_1-x (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) were prepared by the single step reaction (x = 0) and the two-step reaction (x ≠ 0) in Ar atmosphere. The compositions, porosity and microstructure in MgB₂ samples were investigated by using X-ray diffraction (XRD) and scanning electronic microscope (SEM). XRD analysis indicates that the MgB₂ is easy oxidized during heating. SEM observations show that if comparing with the sample by single step reaction the porosity of sample by two-step reaction decreases clearly due to the effective decrease of Mg content in precursors. The grain connection of sample by two-step reaction is worse because the recrystallization behavior of pre-reacted MgB₂ powders in precursors is not sufficient under our sintering processing. The voluminal expansion rates of bulks decrease when x varies from 0 to 1. We propose that for the sample of x = 0 the increase in volume is resulted from the formation of MgB₂ phase and a possible Kirkendall effect during Mg and B converting into MgB₂, and the decrease in volume of sample of x = 1.0 is attributed to the sintering shrinkage effect of the pre-reacted MgB₂ powders. These evidences indicate that the two-step reaction is a good choice for improving the density and decreasing the voluminal expansion rate of MgB₂ sample.