|M.Sc Student||Ben David Roey|
|Subject||Kinetic Study of Isothermal and Non-Isothermal|
Dehydrogenation Process of Titanium Hydride
|Department||Department of Materials Science and Engineering||Supervisors||Professor Eugen Rabkin|
|Dr. Dror Cohen|
The kinetics of hydrogen desorption (dehydrogenation) from TiH2 powders plays an important role in various applications such as hydrogen storage, synthesis of aluminum metallic foams with the aid of TiH2 as a blowing agent, and production of titanium powders. Better understanding of the kinetics of hydrogen desorption from TiH2 powders and the factors influencing the dehydrogenation may help to optimize the above mentioned technological processes.
TiH2 exhibits a relatively complex dehydrogenation kinetics due to the occurrence of several intermediate hydride phase transformations during hydrogen desorption process. Moreover, hydrogen desorption may also be affected by such powder parameters as particle size, surface oxidation and initial hydrogen content. It was shown recently that some degree of particles sintering occurs during hydrogen desorption process, which proceeds up to relatively high temperatures, exceeding in some cases 0.5 Tm (Tm being the melting temperature of Ti). Nevertheless, the evolution of TiH2 powder morphology as a result of sintering and coarsening processes during dehydrogenation and its effect on hydrogen desorption were not explicitly studied.
In the present work, the effect of sintering/coarsening induced powder morphology evolution on hydrogen desorption was investigated by a comparative study of hydrogen desorption from as-received and pre-oxidized TiH2 powders. In addition, the kinetics of hydrogen desorption from as-received TiH2 was studied by non-isothermal and isothermal temperature programmed desorption - mass spectrometry (TPD-MS) analysis, and the kinetic parameters were evaluated.
The pre-oxidized TiH2 samples were prepared by thermal oxidation of the as-received powder at 400 ˚C for 2-6 h in air, and characterized in terms of hydride phase (XRD) and oxygen content. Hydrogen desorption kinetics was studied by means of TPD-MS under He flow conditions, and powder morphologies at different dehydrogenation stages were examined in SEM.
The thermal analysis results show that above 750 ˚C, hydrogen desorption from the thermally decomposed as-received powder is kinetically delayed compared to that from the pre-oxidized samples, and is accomplished at higher temperature. The scanning electron microscopy observations suggest that the delay stems from simultaneous surface coarsening and particles sintering that occur predominantly in the as-received powder at lower temperatures. It is also suggested that the difference in morphology evolution of the as-received and pre-oxidized powders originates from the dependence of Ti self-diffusion on the concentration of oxygen dissolved in Ti, since the oxide layer itself was found to dissociate at lower temperatures (~500 ˚C). The mechanisms of oxide layer decomposition during TiH2 dehydrogenation were tested with the aid of quantitative analysis of H2O evolution during dehydrogenation. It was found that the process of oxide layer reduction by hydrogen atoms is insignificant compared to oxide layer dissolution.
Besides the thermal analysis and microscopic results, the phase analysis of the as-received and the pre-oxidized TiH2 powders indicates the occurrence of phase transformation of the hydride phase followed by oxidation treatment at 400 ˚C. It was found that the initial δ phase (cubic) of the as-received powder was transformed to ε phase (tetragonal) after oxidation treatment. The possible mechanisms for the observed oxidation treatment induced phase transformation were discussed.