Hydriding kinetics of Mg[sbnd]TiH2 fine dispersions obtained by mechanosynthesis

Abstract

Mg80[sbnd]Ti20 hydride systems were prepared via reactive ball milling following two different procedures: grinding Mg with Ti or TiH2, in both cases under H2 atmosphere. Such systems were subjected to several cycles of hydrogen uptake and release in volumetric Sieverts apparatus under different pressure and temperature conditions. A structural characterization from SEM images and XRD patterns reveals no essential microstructural differences between the samples obtained by different routes, although a growth in domain size with temperature is observed. Also, the slowing of the kinetics when the temperature goes down is more pronounced in the specimen fabricated by the second route. The influence of the transformed and untransformed phase fractions on the absorption kinetics can be well described using a double Hill function. Such complex function takes into account two distinguishable processes, with and without nucleation, that differentiate themselves as temperature increases. The first one may be related to surface absorption and the second tentatively ascribed to reactions at grain boundaries, dislocations and other extended defects. The fitted values may then be associated to physical constants of the processes occurring during hydrogen absorption in both regions.