F.D. Manchester

The β → α phase transformation in palladium-hydrogen alloys

Abstract The β → α transformation in thin foils of β palladium hydride (PdH 0.6 ) has been followed at room temperature by electron microscopy and diffraction. In the initial stage of the reaction, coherent precipitation of α close to the unconstrained surface of the foil was observed. The coherency strain fields were consistent with those expected for α precipitation in β. The com-plete transformation of β → α proceeded by a discontinuous reaction that was characterized by the generation of a high dislocation density in the α phase but with no change in orientation. The time interval before this discontinuous reaction started was extremely variable, ranging from less than 5 min to several days. The rate-controlling step in the transformation is believed to be associated with the reaction to form molecular hydrogen at the surface of the foil.

Thermal and motional aspects of the 50 K transition in PdH and PdD

Abstract The “50 K transition” in PdH has been known for some time, but at present considerable doubt remains concerning the structural changes involved. We discuss several recent experimental results dealing with nonstructural aspects of this transition, which are both interesting in themselves and useful in considering the possibilities for structural changes. The measurements involved are: a new look at the specific heat, measurements of the spontaneous heat generation and its relaxation characteristics, the internal friction, and N.M.R., all for β-phase PdH in the 50 K temperature region.

Hydrogen absorption characteristics of an FeTi + misch metal alloy

Abstract Measurements have been made of the hydrogen absorption characteristics of an FeTi + 4.5 wt.% misch metal alloy. This alloy can be activated for hydrogen absorption at room temperature without any thermal cycling. It is suggested that this activation behaviour is due to cracks produced in the FeTi matrix by the differential expansion of misch metal inclusions as a result of preferential hydrogen absorption. These cracks can provide fresh oxide-free surfaces through which the hydrogen can pass readily into the bulk FeTi. Data from scanning electron microscopy, measurements of hydrogen absorption and its response to annealing of the alloy, and the hydrogen absorption characteristics of the majority constituents of misch metal (lanthanum and cerium) are used to support this picture of an inclusion-generated crack mechanism.

Lattice gas aspects of metal-hydrogen system

Abstract Recent work with metal-hydrogen systems (those in which the metal is a transition metal) has given considerable support to the idea, advanced by Hill and by Alefeld, that such metal-hydrogen systems are excellent examples of a lattice gas system—something more usually regarded in physics as a theoretical model, as developed by Yang and Lee. These metalhydrogen systems do not behave as simple lattice gases, because the metal atoms providing the lattice gas cells are influenced by the actions of the interstitial hydrogen so that these cells are not of fixed size. Nevertheless, the lattice gas description is a very useful way of regarding these hydrogen transition metal systems and the points of similarity between lattice gas model and metal-hydrogen system are reviewed. The critical point behaviour of some, at least, of these metal-hydrogen systems, indicates that for the equivalent of the vapour-liquid transition they may usefully be regarded as lattice gases in the mean field approximation, i.e., the attractive forces between the hydrogens which are effective for this transition are of very long range. Recent theoretical work by Hall and Stell on lattice gas models has demonstrated the effect on the model phase-diagram of varying the ratio of the long range attraction to the short range repulsion in the pair potential for the gas particles. This work broadens the scope of comparisons between lattice gas models and metal-hydrogen systems to include the whole phase diagram. Prospects for future mutual interaction between development of models and measurements on metal-hydrogen systems are considered.

Some effects of activation for hydrogen absorption in FeTi powder

Abstract The effect of activation, for hydrogen absorption, on the surface of FeTi has been studied using transmission and reflection electron diffraction to examine structure changes, and magnetization, M , to re-examine the magnetic evidence for surface segregation of elemental iron as reported by others. Following activation, a number of oxides of iron and titanium were found on the FeTi surface and also, between 290 and 1070 K, a number of magnetic species showing pronounced temperature and time dependences and significant hysteresis in the magnetization M ( T ). Part of the observed M ( T ) contribution is compatible with ferrimagnetic behaviour which could come from the presence of magnetic titano-ferrous oxides. The contribution of pure iron was about 0.6% of the room temperature value of M , corresponding to a concentration of Fe below the level which could be detected with TEM. The magnetic behaviour was observed to be a concomitant, and, apparently, independent result of the activation treatment. Our magnetization observations were incompatible with particles of elemental iron behaving superparamagnetically, to give the major contribution to the magnetization, as reported by others, and our overall experimental evidence is that surface segregation giving particles of elemental iron on the FeTi surface does not occur and is not necessary for hydrogen absorption into FeTi; whereas the role of surface oxides in the activation for, and inhibition of, hydrogen absorption in FeTi, appears to be important.

Some observations on activation of FeTi for hydrogen absorption

Abstract We used reflection and transmission electron microscopy and measurements of magnetization to examine structural changes at and near the surface of FeTi which had been activated for hydrogen gas absorption. These measurements were coupled with determinations of the absorption kinetics and absorption isotherms for such activated samples. We present a description of the observed changes resulting from hydrogen activation and, in particular, evidence that formation of metallic iron in the cycling process used to give these samples their ability to absorb hydrogen is, at most, a minor feature of these changes.

The effect of the addition of carbon on the hydrogen-absorbing properties of FeTi

Abstract Examination of the structure and the hydrogen absorption characteristics of FeTi containing 2 and 5 at.% excess carbon and titanium showed that a precipitate of TiC is formed which does not significantly affect the hydrogenabsorbing properties of the FeTi. Oxygen scavenging in the FeTi melt by formation of an oxycarbide, as has been predicted, was not observed.

Conversion electron Mössbauer spectroscopy on surface layers of activated FeTi

Abstract A brief general account is given of conversion electron Mossbauer spectroscopy measurements made on FeTi subjected to activation processes aimed at promoting hydrogen absorption. The principal result of this study was that the absorption of hydrogen through oxide layers on the FeTi surface was not dependent on the presence or absence of the Mossbauer active substances produced, at or near that surface, by the activation processes employed.

The activation of FeTi for hydrogen absorption: progress and possibilities ☆

The intermetallic compound FeTi has attracted attention as a candidate for use as a hydrogen-storage alloy since the early days of hydrogen-storage alloy research, principally because of the abundance and relatively low cost of its constituents and its acceptable hydrogen absorption parameters for many potential applications. However, FeTi is, so far, not the easiest of the hydrogen-storage alloys to activate and thus it has remained as something of a challenge to see if this economically attractive alloy could be made more readily useable. This review deals with results from all the major experimental techniques which have been used on the activation for hydrogen absorption (AFHA) problem in FeTi since it became of topical interest. Various mechanisms which have been suggested as contributing to the activation process are discussed in relation to the experimental evidence available to date. Some treatment is also given of the known structures and properties of oxides and other compounds which have been found, from work in various laboratories, to be present on the FeTi surface, before, during, and after the AFHA treatment. Features of these properties are discussed which are of potential interest in providing hydrogen “passage” mechanisms through layers of surface compounds on FeTi, particularly the oxides. Parallels with the the behaviour of surface layers on other hydrogenabsorbing metals and alloys are also drawn.

Measurements of the Magnetization of Activated FeTi

We have measured the magnetization of activated FeTi samples from room temperature to 800°C. The magnetization measurements, together with structural information from diffraction experiments, indicate that metallic iron is not formed in the activation process and, apparently, is not responsible for observed increases in the hydrogen absorption rate of FeTi after activation treatment.