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.
Mechanical activation of air exposed TiFe + 4 wt% Zr alloy for hydrogenation by cold rolling and ball milling
Abstract In this paper we report the effect of air exposure on TiFe + 4% Zr alloy and how the hydrogen capacity could be recovered by processing the alloy by cold rolling and ball milling. It has been noted that the alloy could not absorb hydrogen after 7 days of air exposure. To restore the hydrogenation capability of the alloy, it was mechanically treated using cold rolling and ball milling. It was found that the air exposed alloy could be hydrogenated after 30 min of ball milling under argon or after cold rolling. Effect of cold rolling and ball milling on first hydrogenation for pure TiFe alloy was also investigated.
Irreversible Change of the Magnetic Properties of TiFe by Hydrogenation
When TiFe is charged with hydrogen, an irreversible change of the magnetic properties of this intermetallic compound is observed, brought about by the formation of iron clusters. A simple method is suggested for deriving the main features of the band structure of TiFe. On this basis it can be understood that TiFe, charged with hydrogen, is not stable in strongly ordered structure, but that Ti- and Fe-clusters are formed. The exchange of positions of metal atoms, necessary for this precipitation, obviously happens at temperatures as low as 100°C under the influence of the distortions of the lattice, brought about by the dissolved hydrogen. When hydrogen is withdrawn from the lattice, the strongly ordered compound would in turn be more stable than the cluster structure, but no rearrangement of metal atoms occurs at 100°C. 
 
 
 
Wenn die intermetallische Verbindung TiFe mit Wasserstoff beladen wird, beobachtet man eine irreversible Anderung ihrer magnetischen Eigenschaften, die durch Bildung von Eisenclustern zustande kommt. Mit einer einfachen Methode konnen die wesentlichen Aspekte der Bandstruktur von TiFe hergeleitet werden. Auf dieser Basis wird verstandlich, das mit Wasserstoff beladenes TiFe in streng geordneter Struktur nicht stabil ist, sondern das sich Ti- und Fe-Cluster bilden. Der Platzwechsel von Metallatomen, der fur diese Ausscheidungsreaktion erforderlich ist, erfolgt schon bei relativ niedrigen Temperaturen (100°C), offensichtlich unter dem Einflus der Gitterverzerrungen, die der geloste Wasserstoff hervorruft. Nach Desorption des Wasserstoffs ist die streng geordnete Struktur wieder stabiler als die Clusterstruktur, doch findet bei 100°C keine entsprechende Neuverteilung der Metallatome statt.