Intermediate products of hydriding combustion synthesis of Mg ₂NiH₄ studied by optical microscopy and field-emission scanning electron microscopy

In our previous paper, the intermediate products formed during the process of hydriding combustion synthesis of Mg₂NiH₄ were studied by means of X-ray diffraction and scanning electron microscopy. Several interesting results were reported, for example, a large amount of Mg ₂Ni existed in the intermediate product obtained after the dehydriding reaction of MgH₂ at 750 K under 4.0 MPa of hydrogen. This result is very significant because a synthesis temperature lower than the normal synthesis temperature of 873 K by more than 100° is very attractive from the point of view of industrializing this process. Therefore, in this paper, the intermediate products were further studied with a focus on the microstructure by means of optical microscopy and field emission scanning electron microscopy (FE-SEM). The conclusions are as follows: (1) the reaction mechanism of hydriding and dehydriding of magnesium in the process of hydriding combustion synthesis of Mg₂NiH₄ was explicated intuitively and micro-structurally for the first time, (2) It was confirmed that the hydrogen atmosphere used during the heating period resulted in a small particle size of the intermediate products prior to the eutectic temperature. (3) The synthesis mechanism of the gas-solid reaction of intermetallics was elucidated by direct observation of the intermediate products obtained at 755 K and also at 705 K by means of FE-SEM. The Mg₂Ni synthesized surrounded the particles of nickel but was separated from those of magnesium.