In situ X-ray diffraction study of the hydriding combustion synthesis of Mg2NiH4

An in situ X-ray diffraction study of the hydriding combustion synthesis of Mg2NiH4 directly from the compact of magnesium and nickel mixture in a pressurized hydrogen atmosphere has been conducted. During two cycles of heating (hydriding) and cooling (dehydriding) in a temperature range from 300 to 823 K, the X-ray diffraction patterns were obtained in a pressure of 1.0 MPa at eleven points of the temperature. The obtained X-ray diffraction patterns reveal nine reactions existing in two cycles of heating and cooling. In the first cycle, (1) Mg+H-2-->MgH2, partially, (2) MgH2-->Mg+H-2, (3) 2Mg+Ni-->Mg2Ni, in the heating period, and (4) Mg2Ni+ 2H(2)-->Mg2NiH4 (HT), partially, (5) Mg2NiH4 (HT)-->Mg2NiH4 (LT), in the cooling period. In the second cycle, (6) Mg2NiH4 (LT)-->Mg2NiH4 (HT), (7) Mg2NiH4 (HT)-->Mg2Ni+2H(2), in the heating period, and (8) Mg2Ni+2H(2)-->Mg2NiH4 (HT), partially, (9) Mg2NiH4 (HT)-->Mg2NiH4 (LT), in the cooling period. The X-ray diffraction intensity of Mg2NiH4 after the second cycle of hydriding decreases to 1/6 times of that after the first cycle of hydriding. In contrast, the X-ray diffraction intensity of Mg2NiH4 after the second cycle of hydriding is six times of that after the first cycle of hydriding, although only one single phase of Mg,Ni exists at 823 K before hydriding either the first cycle or the second cycle. (C) 1998 Elsevier Science S.A. All rights reserved.