Hydrogen storage properties of 2 Mg–Fe mixtures processed by hot extrusion: Effect of ram speeds

Abstract

Hot extrusion processing was used for producing bulk samples of 2MgFe mixtures adequate to hydrogen storage. High-energy ball milling was used to prepare 2MgFe powder mixtures, which were cold-pressed into cylindrical pre-forms. Such pre-forms were then processed by hot extrusion (at 300 °C and extrusion ratio of 3/1) to produce bulk samples. In this work, it was analyzed the influence of the ram speed (1, 5, and 10 mm/min) on the microstructure and hydrogen sorption properties of obtained bulks. Nanograins, which resulted from the milling process, remained in the nanosize after hot extrusion conditions. More porous samples were produced at a ram speed of 1 mm/min, which also kept the smaller grain size. These features led the sample processed at 1 mm/min to absorb about 4.8 wt% of H, which was higher than precursor powders that absorbed around 4.3 wt% of H. This behavior was assigned to a redistribution of Fe during extrusion, which was kept agglomerated in the powders. The results also showed that desorption temperatures of bulks were very similar to that of 2MgFe powders. Such behavior is very interesting, considering the lower surface area of bulks. When compared to MgH2 powders, samples processed in this work presented much lower desorption temperatures. Except for the sample processed at 10 mm/min, all other samples fully desorbed in less than 8 min at 350 °C, while commercial MgH2 would take much more than 40 min for complete desorption.