Investigations on the synthesis, structural and microstructural characterizations of Mg-based K2PtCl6 type (Mg2FeH6) hydrogen storage material prepared by mechanical alloying
Résumé
This paper deals with the formation of new ternary hydride Mg2FeH6 (K2PtCl6type) in a single-step procedure following the process of mechanicalalloying of initial stoichiometric ingredients Mg and Fe under hydrogen. The optimum yield of formation of single phase Mg2FeH6 was achieved by hydrogen (∼10 atm.) milling of constituent elements at a speed of 400 rpm for various milling durations. The structuralcharacterization of the ball-milled (2 Mg+Fe) powder was carried out using Philips X-ray diffractometer by taking samples from the attritor mill at regular intervals of time. It was found that the Mg2FeH6 phase starts forming at a milling duration of 14 h and the optimum Mg2FeH6 phase formation was obtained at 20 h. The proportion of this phase was estimated by employing Rietveld refinement analysis of the X-ray powder diffraction data and it was found to be 63%. This is the highest phase proportion reported so far for the Mg2FeH6 phase when formed from elemental Mg and Fe following the route of mechanicalalloying. Together with the Mg2FeH6 phase, some quantity of Fe (about 37%) is also present. Fe, being a magnetic impurity, can be removed leaving the Mg2FeH6 content to be nearly 90-100%. However, such purification was not done in the present investigation. We also investigated synthesis of the material obtained by longer milling durations of 25, 28 and 30 h. The XRD patterns for the 25, 28 and 30 h ball-milled materials revealed that the intensity of Mg2FeH6 peaks is reduced in comparison to the Fe peaks. This implies that beyond 20 h, there is no further increase in the phase proportion and the amorphization starts taking place. The post-sintering process of these mechanically alloyed samples did not improve the Mg2FeH6 phase proportion and yield as evidenced by XRD. The X-ray structuralcharacterizations revealed that the as-milled Mg2FeH6material (milling duration of 20 h; under H2 pressure ∼10 atm., speed ∼400 rpm) corresponds to the known face centered cubic structure with lattice parameter a=0.6446(2) nm. The elemental (chemical) compositional analysis was carried out for the mechanically alloyed Mg2FeH6materials using the EDAX technique. The results confirm the correct stoichiometric ratio of the initial mixture (2Mg+Fe). The surface morphologies of the (2Mg+Fe) mixture before and after mechanicalalloying are performed using scanning electron microscopic technique. The SEM explorations reveal the spongy like feature of Mg2FeH6 agglomerates.