Investigations on the Synthesis, Characterization and Hydrogenation Behaviour of Mg Based Composite Materials Mg-xwt.% CFMmNi₅ Prepared Through R.F. Induction Melting and Mechanical Alloying — A Comparative Study

Abstract

Alloys with general formula Mg-xwt.% CFMmNi₅ (x=30) have been successfully synthesized through R.F. induction melting and mechanical alloying employing high energy attritor ball mill. The hydrogenation and dehydrogenation behaviour of these ew composite materials has been extensively investigated. In the present study, the high magnesium content of the composite alloy typically 70wt.% Mg + 30wt.% CFMmNis synthesized through conventional melting using radio frequency induction furnace and unconventional route of mechanical alloying (ball milling). The various experimental parameters such as time duration, speed of milling and medium of milling have been optimized in order to obtain composite alloys with favourable sorption characteristics. The as-synthesized composite materials have been activated at around 400° C under a hydrogen pressure of ∼40 kg cm^-2 and their hydrogen storage capacities and desorption kinetics have been evaluated. The new composite hydrogen storage materials milled under hexane medium for 5 hrs duration at a speed of 400 rev. min^-1 exhibits higher storage capacity of ∼5.4 wt.% in contrast to their thermally melted counterparts (∼4.9 wt.%) at 350° C. It is also established that the rate of desorption of hydrogen from the mechanically alloyed sample are much faster (about 90 cm³ min¹ gm^-1) than the R.F. induction melted alloys (about 50–60 cm³ min^-1 gm^-1). The hydriding rate and the improved hydrogen storage capacity of these composite materials have found to be strongly correlated with the structural and microstructural characteristics as brought out through XRD and SEM explorations. From the extensive analysis, it was found that the mechanically alloyed samples are superior over the conventional melting samples in regard to the scalability and hydrogenation-dehydrogenation characteristics. This may be due to the availability of large number of grain boundaries, reduction in particle size and presence of fresh surface by which the hydrogen gets adsorbed in addition to bulk absorption.