Mechanosynthesis of Nanocrystalline MgB<sub>2</sub> Ceramic Powders in Hydrogen Alloying Mills via Amorphous Hydride Intermediate

Abstract

In the present work we report on the synthesis of nanocrystalline MgB2 by mechanochemical reaction (mechanosynthesis) conducted in a high-energy mechanical alloying mill filled with hydrogen. The solid-state reaction of mechanochemical alloying between Mg and B with H (hydrogen alloying) leads to formation of an intermediate amorphous (Mg,B)Hx hydride. This amorphous intermediate is subsequently annealed (devitrified) to nucleate and grow nanocrystalline boride. The first stage of synthesis was carried out at room temperature from elemental Mg and B powders in a high-energy ball mill under sequential supply of hydrogen. The subsequent annealing of the amorphous product led to nearly single-phase MgB2, with only small fraction of MgO impurity. The easy room-temperature synthesis renders the method promising for production of MgB2, which recently gained attention as a new 39K ceramic superconductor. The amorphous intermediate itself can be studied further for its capacity to store ca. 2 wt% H in a metastable hydride phase. The effort was undertaken to predict formation of amorphous hydride phase through analysis of atomic volume mismatch between atoms of Mg, B, and H.