Hydrogenation Properties of TiFe Doped with Zirconium
The goal of this study was to optimize the activation behaviour of hydrogen storage alloy TiFe. We found that the addition of a small amount of Zr in TiFe alloy greatly reduces the hydrogenation activation time. Two different procedural synthesis methods were applied: co-melt, where the TiFe was melted and afterward re-melted with the addition of Zr, and single-melt, where Ti, Fe and Zr were melted together in one single operation. The co-melted sample absorbed hydrogen at its maximum capacity in less than three hours without any pre-treatment. The single-melted alloy absorbed its maximum capacity in less than seven hours, also without pre-treatment. The reason for discrepancies between co-melt and single-melt alloys was found to be the different microstructure. The effect of air exposure was also investigated. We found that the air-exposed samples had the same maximum capacity as the argon protected samples but with a slightly longer incubation time, which is probably due to the presence of a dense surface oxide layer. Scanning electron microscopy revealed the presence of a rich Zr intergranular phase in the TiFe matrix, which is responsible for the enhanced hydrogenation properties of these Zr-doped TiFe alloys.
First hydrogenation enhancement in TiFe alloys for hydrogen storage
In this paper, we report the first hydrogenation kinetics of TiFe  +  X wt.% Zr with X  =  4, 8, 12, and 16. We found that these alloys are multiphase with a TiFe matrix having about 1 at.% of zirconium in it and two secondary phases, which are rich in zirconium but also contain Ti and Fe. When X increases, the chemical composition of one of the secondary phases stays constant but the other secondary phase presents a change of composition. The sample with X  =  8 presented the fastest first hydrogenation kinetics. Hydrogen capacity increases with increasing X but for X greater than 8, there was the appearance of an incubation time. The secondary phases are probably acting as a gateway for hydrogen, thus facilitating the hydrogen absorption in the TiFe phase.
Surface segregation of iron by hydrogenation of FeTi: An 57Fe mössbauer conversion electron study☆
Abstract Mossbauer spectroscopy of 57 Fe conversion electrons was used to study FeTi subjected to 51 hydrogen absorption-desorption cycles. This technique probes both the surface and part of the bulk, and it can also distinguish between magnetic and non-magnetic iron in the lattice. Our measurements suggest the presence of a magnetic overlayer on FeTi made up of iron clusters and the absence of magnetic moments on the iron contained in the bulk. This results shows that the magnetic moment observed in bulk magnetization studies in fact originates from this segregated iron layer on the surface of FeTi.