| Description |
Magnesium hydride was proposed to be used for on-board heating and cooling systems due to its high energy density. Low-temperature hydrogenation has been achieved via ball milling and adding appropriate additives. However, further details regarding the mechanism for hydrogenation of magnesium hydride is not available yet. This work first developed a methodically designed technique to test hydrogenation kinetics under isothermal condition across a wide temperature range, from room temperature to 200 ºC using a Sieverts type apparatus, in order to minimize the thermal gradient effect, which is often neglected in the literature. The tested material was ball-milled magnesium mixed with titanium hydride, a combination that had been demonstrated to have excellent hydrogenation and dehydrogenation kinetics in recent studies, and is considered to be a promising material for hydrogen storage and thermal energy storage applications. It was found that the hydrogenation kinetics under isothermal conditions were significantly different from those under nonisothermal conditions. Additionally, it was determined that the hydrogenation kinetics under isothermal conditions were numerically best fit by the Johnson-Mehl-Arrami (JMA) model. The second part of this work further investigated the possible effect of various processing variables, including milling parameters, catalyst, as well as the effect of hydrogenation conditions, such as temperature and pressure, on the hydrogenation kinetics of magnesium hydrides. Moreover, the effect of different processing variables on grain size and specific surface area was investigated as well. Various kinetic models have been employed to examine the hydrogenation kinetics of samples prepared by different parameters and tested under different conditions. The JMA model was found again to be the best numerical model to describe the hydrogenation behavior of the ball-milled magnesium hydride prepared in this work. In addition, the JMD (Jander Diffusion Model) model has high consistency with low-temperature hydrogenation of pure magnesium hydrides. It was found that longer milling time and smaller milling load will lead to smaller grain size of the as-milled powder, thus better hydrogenation rate and smaller activation energy. Adding catalyst could not only assist in reducing grain size of the as-milled MgH2 powder, but also facilitate the hydrogen diffusion in Mg/MgH2, thus reducing the activation energy of hydrogenation in both ways. |
