Structural Stability and Dehydrogenation of (MgH₂+Al, Nb) Powder Mixtures during Mechanical Alloying

Abstract

High-energy ball milling of MgH₂ with appropriate alloying elements is reported to improve the hydrogenation kinetics of MgH₂ but the relevant information on dehydrogenation is scarce. Here, a systematic study was carried out to clarify the effects of two key alloying elements, Nb and Al, on the microstructure and the dehydrogenation behaviour of the MgH₂. Intensive mechanical alloying was carried out to synthesise (MgH₂ + M) powder mixtures (M=Nb, Al). XRD Rietveld analysis revealed the formation of a new bcc phase in the (MgH₂+Nb) mixture and a (Al, Mg) solid solution in the (MgH₂+Al) mixture. The amount of the newly formed phase in each case increased with milling time, while the level of Nb or Al decreases. SEM analysis of the milled powders showed the existence of nano-particles within 20 hours of milling. Thermogravimetry (TG) results showed that the mechanically alloyed (MgH₂+Nb) mixture released about 3.9 mass% H₂ and the milled (MgH₂ + Al) about 5.4 mass% H₂ at 300^°C within 10 minutes, compared with only 1.5 mass% of the milled MgH₂ powder and 1.0 mass% of the as-received MgH₂ under the same conditions.