Intermediate products of hydriding combustion synthesis of Mg2NiH4 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 Mg2Ni 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.