Microstructural characterization and hydrogenation study of extruded MgFe alloy

Abstract

Mg-based nanocrystalline alloys or nanocomposites are promising materials for hydrogen storage in the solid state, which is a more effective and safer storage medium than pressurized or liquefied hydrogen. Among the many Mg-based hydrides of interest for hydrogen storage, Mg 2FeH 6 is in a special position due to its relatively high gravimetric capacity of 5.5% and excellent volumetric density of 150 kg H 2/m 3. This work involved a study of the synthesis and processing of Mg-based alloys of this type, produced by high-energy ball milling and hot extrusion. A mixture of 2Mg–Fe was prepared by high-energy ball milling under argon gas. The resulting powder was cold-pressed to produce cylindrical pre-forms, which were then extruded and the sorption properties were analyzed in a microbalance and in a Sieverts apparatus. Phase formation, microstructural evolution, desorption temperatures, and hydrogen storage capacity were analyzed by X-ray diffraction, scanning electron microscopy (SEM), differential scanning calorimetry, and thermogravimetric techniques. The results showed that Mg 2FeH 6 was formed and the hydrogen reaction was reversible. SEM observations indicate a microstructure composed of nanosized grains in the range of 30–80 nm inside particles of about 50 μm, and Mg 2FeH 6 formed mainly at the particle interfaces. The desorption temperature started at about 225 °C, reaching a maximum at 440 °C with low capacity of absorption, indicating low absorption/desorption kinetics, probably due to bulk diffusion limitations.