In situ resistance analysis of MgB2 formation process from Mg(BH4)2

Abstract

Mg(BH₄)₂ was previously studied as a promising hydrogen storage material, because of its high gravimetric storage capacities for hydrogen and suitable thermodynamic properties. Mg(BH₄)₂ began to decompose at about 300 ℃, and formed MgB₂ at the end of hydrogen desorption process with the weight content of 14.9% of hydrogen lost. Aside from the prominent hydrogen storage property, the decomposition process from Mg(BH₄)₂ to MgB₂ can be a potential method for fabricating superconducting MgB₂ at a low sintering temperature. In this paper, MgB₂ bulk was prepared by an <i>in-situ</i> reaction, using the Mg(BH₄)₂ pressed block as a precursor. The resistance change of the sample was monitored during the Mg(BH₄)₂ decomposition process and the resistance-temperature (<i>R</i>-<i>T</i>) curve of this process was recorded. Phase of MgH₂, Mg and B were formed as the block slowly release its hydrogen before MgB₂ occurred. According to the <i>R</i>-<i>T</i> curve, the phase formation of MgB₂ started in a relatively low temperature of 410 ℃. Because MgB₂ was critically formed by Mg and B derived from Mg(BH₄)₂, we can compare our formation temperature with previous study on MgB₂ prepared by Mg and B in different particle size. The fitting result indicated that the particle size of Mg and B harvest from Mg(BH₄)₂ decomposition was only 3.4 nm on average. The nearly atomic level mixture of Mg and B resulted in a high chemical reactivity, which was the main reason for low sintering temperature. X-ray diffraction results showed that the purity of MgB₂ was 95.2%, and the size of MgB₂ grains was 10–18 nm. SEM images showed that the MgB₂ bulk had a porous structure and poor connectivity, which was caused by large amount the hydrogen release during the decomposition. MgB₂ nanofibers can also be observed inside the bulk. In the superconductivity test, the superconducting transition temperature of the bulk was 35 K. After all, such <i>in situ</i> method to fabricate MgB₂ showed a great advantage in some aspects, as its low-cost precursors, low sintering temperature, small grain-size and high superconducting transition temperature in the formed MgB₂, which have the potential in industrial scale fabrication of MgB₂ bulks and wires.