The processing of mg-ti powder for hydrogen storage

Çakmak, Gülhan
A study was carried out on the selection of processing condition that would yield Mg-Ti with most favourable hydrogenation properties. Processing routes under consideration were; mechanical milling under inert atmosphere, reactive milling i.e. milling under hydrogen atmosphere, ECAP (equal channel angular pressing) and thermal plasma synthesis. Structure resulting from each of these processing routes was characterized with respect to size reduction, coherently diffracting volume and the distribution of Ti catalyst. Mechanical milling yielded a particulate structure made up of large Mg agglomerates with embedded Ti fragments with a uniform distribution. Mg agglomerates have sizes larger than 100 µm which arises as a result of a balance between cold welding process and ductile fracture. Repeated folding of Mg particles entraps Ti fragments inside the Mg agglomerates resulting in a very uniform distribution. Coherently diffracting volumes measured by X-ray Rietveld analysis have small sizes ca. 26 nm which implies that the agglomerates typically comprise 1011 crystallites. Mechanical milling under hydrogen, i.e. reactive milling, led to drastic reduction in particle size. Mg and Ti convert to MgH2 and TiH2 which are milled efficiently due to their brittleness resulting in particle sizes of sub-micron range. Hydrogenation experiments carried out on Mg-10 vol % Ti milled under argon yields enthalpy and entropy values of -76.74 kJ/mol-H2 and -138.64 J/K.mol-H2 for absorption and 66.54 kJ/mol H2 and 120.12 J/K.mol H2 for desorption, respectively. For 1 bar of hydrogen pressure, this corresponds to a hydrogen release temperature of 280 °C. This value is not far off the lowest desorption temperature reported for powder processed Mg based alloys. ECAP processing is a bulk process where the powders, consolidated in the first pass, have limited contact with atmosphere. This process which can be repeated many times lead to structural evolution similar to that of milling, but for efficient mixing of phases it was necessary to employ multi-pass deformation. An advantage of ECAP deformation is strain hardening of the consolidated powders which has improved milling ability. Based on this, a new route was proposed for the processing of ductile hydrogen storage alloys. This involves several passes of ECAP deformation carried out in open atmosphere and a final milling operation of short duration under inert atmosphere. The plasma processing yields Mg particles of extremely small size. Evaporation of Mg-Ti powder mixture and the subsequent condensation process yield Mg particles which are less than 100 nm. Ti particles, under the current experimental condition used, have irregular size distribution but some could be quite small, i.e. in the order of a few tens of nanometers. Of the four processing routes, it was concluded that both reactive milling and thermal plasma processing are well suited for the production of hydrogen storage alloys. Reactive milling yield particles in submicron range and plasma processing seems to be capable of yielding nanosize Mg particles which, potentially, could be decorated with even smaller Ti particles.