Intermediate products during the hydriding combustion synthesis of Mg ₂NiH₄

Magnesium-based hydrogen storage alloy is one of the most attractive mediums, as a hydrogen storage tank for a fuel cell. However, the practical application is still limited because of its poor hydriding and dehydriding kinetics and relatively high reaction temperature. To overcome such problems, the hydriding combustion synthesis (HCS) was established in 1997, as one of the most effective production methods. The understanding of these microstructures is strongly related to the improvement of the reaction property. However, up to now, little efforts have been made as to the microstructures of final product and intermediate products. Therefore, the purpose of this paper is to prepare intermediate products at several stages during the HCS of Mg₂NiH ₄, and to study their microstructures by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM) for the first time. Interestingly, the results revealed that: (1) the intermediate product generated at lower temperature than 800 K caused the unique particle morphology of final product of Mg₂NiH₄; (2) severe thermal corrosion, cotton-like material, was also observed on the surface of intermediate products at 705 K, which became enhanced with increasing temperature; (3) after the dehydriding reaction of MgH₂ at 750 K, a large amount of the Mg2Ni phase existed in the intermediate product; (4) hydrogen present during the heating period makes the product to have smaller size. Further investigation on the mechanism of the gas-solid reaction deserves attention, because lowering of the synthesis temperature by as much as 100' is very attractive and significant for industrializing this process.