Influence of sintering parameters on formation of Mg–Co hydrides based on their thermodynamic characterization

Ternary hydrides Mg2CoH5 and Mg6Co2H11 were obtained by a combined technique that included ball milling of a 2Mg–Co mixture under Ar followed by sintering in hydrogen atmosphere (5900 kPa) at 425 °C. Formation and decomposition Van’t Hoff diagrams for these hydrides were deduced from pressure–composition isotherms and related changes of enthalpy and entropy were afterwards estimated. While decomposition enthalpy values for both hydrides are lower than previously published ones, formation enthalpy for Mg2CoH5 is closed to the only reported data. Calculated formation enthalpy for Mg6Co2H11 is a novel contribution for the thermodynamic characterization of the system. Based on Van’t Hoff diagrams for hydride formation obtained in this work, several synthesis routes were designed as consecutive sintering stages of constant temperature and hydrogen pressure. They allow the study of the influence of both parameters and treatment duration on reaction kinetics and resulting phases. Synthesis procedures for production of MgH2, Mg6Co2H11 or a mixture of ternary hydrides were identified and involved mechanisms during hydride formation were proposed. These experiments evidenced the key role that the dependence of Mg and Co mobilities with temperature has in the production of these ternary hydrides.