Highlighting of a Single Reaction Path during Reactive Ball Milling of Mg and TM by Quantitative H₂ Gas Sorption Analysis To Form Ternary Complex Hydrides (TM = Fe, Co, Ni)

An experimental procedure has been developed to quantify in situ hydrogen absorption in a reactive ball-milling (RBM) device. It provides an internal calibration of the hydrogen temperature during milling and considers hydrogen as a real gas. Its accuracy has been evaluated to 5% by comparison with crystallographic and thermal decomposition experiments on magnesium hydride and Mg₂TM (TM = Fe, Co, Ni) complex hydrides. Mg and Mg₂TM complex hydrides have been successfully synthesized by RBM of 2Mg + TM powder mixtures under 7.5 MPa of hydrogen. The synthesis is accomplished for reaction times shorter than 240 min, with no apparent incubation stage and with a typical yield of 80%. All Mg-based hydrides are nanocrystalline with crystallite size ranging from 3 to 10 nm. The analysis of in situ hydrogen absorption curves show that the synthesis of the complex hydrides follows the same reaction path and is preceded in all cases by the formation of the MgH₂ phase. Kinetics of MgH₂ formation as an intermediate phase is enhanced by the presence of transition metals.