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.