Mechanochemical study of the hydriding properties of nanostructured Mg₂Ni–Ni composites

A comprehensive approach is presented for defining hydrogen activation and absorbing kinetics in heterogeneous Mg₂Ni/Ni powder composites that were subjected to mechanical refinement. Hydriding tests were performed under conventional hydrogen dissolving and under reactive milling. Irrespective of the absorbing mode, the absorption kinetics is deceleratory throughout. Under conventional thermodynamic conditions, the hydriding rate depends strongly on the microstructural features of both the absorbing Mg₂Ni intermetallic and the Ni phase. The latter plays an important role in the dissociative chemisorption of hydrogen. Under milling the hydrogen uptake and the hydriding kinetics also depend on the intensity of the milling processing, I_M (watt g⁻¹), with the absorption rate increasing exponentially with I_M. The mechanical treatment was found effective even when thermodynamic absorption reached saturation level. Hydriding rates, mechanochemical gains, and instantaneous mechanochemical yields (mol J⁻¹) were used to compare the processes on an absolute scale and to spotlight possible mechanisms controlling kinetics trends and absorbing features under milling.