On the mechanically pulverized MmNi4.6Fe0.4 as a viable hydrogen storage material

The present study deals with investigations on the AB(5)-type MmNi(4.6)Fe(0.4) hydrogen storage alloy with improved storage capacity of similar to 1.7 wt%. The as-synthesized MmNi(4.6)Fe(0.4) intermetallic alloy has a storage capacity of similar to 1.5 wt% with kinetics approximate to 25 cm(3)/min/g. In search of effects, which may lead to improvement of storage capacity and kinetics, we have carried out several possible material modifications including substitution of 3d-transition elements such as Fe, Co, Mn, etc, at Ni sites, surface treatment and ball-milling. It has been found that the last material modification, i.e. ball-milling technique gives optimum results. The ball-milling technique parameters like speed. time duration, ball to powder ratio and medium of milling has been optimized. The maximum storage capacity of similar to 1.7 wt% with kinetics of approximate to 35 cm(3)/min/g is obtained when pulverization of the as-synthesized (RF melted) material employing an attritor mill. By varying the medium, duration and speed of ball milling. it was found that the optimum conditions and estimates correspond to medium: hexane, duration: 10 min, speed: 200 rpm. The PCT evaluation was carried out using Sievert's type apparatus. The structural and microstructural characterizations were explored using XRD and SEM. XRD explorations revealed that full-width at half-maximum (FWHM) before and after pulverizations are 0.20 and 0.25 degrees, i.e. there is a broadening of similar to 25% in the width of the peaks after pulverization. The microstructural investigations revealed that the average particle size of MmNi(4.6)Fe(0.4) after pulverization was similar to 2.5 times less than that of the as-synthesized alloy. Smaller particles together with fresh surfaces are the most likely cause of enhanced hydrogenation (storage capacity and kinetics) behaviour. (C) 2001 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy.