Dehydrogenation-hydrogenation characteristics of nanocrystalline Mg2Ni powders compacted by high-pressure torsion

Abstract

High-pressure torsion technique was applied on nanocrystalline Mg2Ni powders to produce bulk disks by simultaneous uniaxial compression and severe shear deformation. The hydrogen absorption and desorption behavior of the disks has been characterized by high-pressure calorimetry. During desorption, the decomposition of the Mg2NiH4 phase takes place, which is followed by the dehydrogenation of Mg2NiH0.3 solid solution. In order to monitor the sorption properties in details, partially dehydrogenated states of the fully absorbed disk have been performed by interrupting the desorption process at 75%, 50% and 25% hydrogen contents in a Sieverts' type apparatus. Microstructural evolution during dehydrogenation has been investigated by X-ray diffraction. The variation of average crystallite size, lattice parameters and unit cell of the competing phases has been determined by the Rietveld refinement method of X-ray diffractograms. The unit cell volume of the Mg2NiH0.3 hydride solid solution decreases with decreasing hydrogen content. Coupled differential scanning calorimetry and thermogravimetry measurements were also taken on the partially desorbed states in order to determine the activation energy of hydrogen release.