Critical temperature and carbon substitution in MgB2 prepared through the decomposition of Mg(BH4)2

We have prepared Mg(BH₄)₂ and then obtained MgB₂ through the decomposition of the Mg(BH₄)₂. Precursor viscous liquid was first prepared by the metathesis reaction of MgCl₂ and NaBH₄ based on mechanochemical and solution processes. Subsequent drying of the liquid at 180 °C results in the formation of α-Mg(BH₄)₂, whereas the treatment at high drying temperature of 220 °C forms β-Mg(BH₄)₂. Heat treatment in a flow of Ar gas decomposes the Mg(BH₄)₂ into MgH₂ and B at 300 °C, and subsequently the reaction of the MgH₂ and B occurs at 450 °C, followed by the formation of MgB₂ as main phase at 500 °C. Although drying of the liquid at 300 °C forms not Mg(BH₄)₂ but nearly amorphous powder, the heat treatment of the amorphous powder brings about MgB₂ as main phase as well with slightly increased impurity phases. The thus obtained MgB₂ shows a critical temperature (T_c) of 35 K. This low T_c is attributed to the poor crystallinity and the substitution of carbon supplied from the solvent in the solution process.