MgH2 synthesis during reactive mechanical alloying studied by in-situ pressure monitoring

The synthesis of MgH2 by reactive mechanical milling has been studied by monitoring H2 pressure changes inside a milling chamber. Mg and a Mg-10 wt.% C mixture were used as starting materials and milled under 0.5 MPa of H2. The addition of C doubles the MgH2 synthesis efficiency due to C acting as a process control agent. MgH2 formation has been observed throughout milling and during the rest periods between milling stages. Mg hydriding during the rest periods has been found to be controlled by hydrogen diffusion through MgH2. High-diffusivity paths along grain boundaries seem to be operative during the process. A lower bound for the diffusion coefficient of H in MgH2 at room temperature of 10−25 m2 s−1 has been estimated from the data.