Evolution of the physico-chemical properties of nanostructured MgH2 composites upon hydriding cycle
Résumé
The co-milling process of MgH2 with a transition metal has been developed to produce nanostructured and highly reactive powders. High performances in terms of thermal and mechanical behaviour have been achieved introducing Expanded Graphite and compacting the mixture under the form of composite materials.
However, the hydrogenation reaction of magnesium is accompanied by a significant increase in volume (30%). Moreover, the MgH2 nano-crystallites tend to recrystallize and to induce a progressive swelling of the compacted disks upon cycling. This is a critical phenomenon because it induces mechanical strains on the wall of the MgH2 tank. The purpose of this study is to quantify and to understand this irreversible phenomenon, through correlations with microstructural evolutions. In-situ dilatometry measurements were performed on samples prepared with different additives. The irreversible phenomenon increases progressively up to about 50 cycles where a stabilisation is achieved. Granulometry measurements show a bi-modal distribution of the as-milled powders. Upon cycling, we observe the coalescence of the “small” MgH2 particles, which tends to create large agglomerates and results - again after about 50 cycles - in mono-disperse powders. This behaviour induces an increase in porosity, which explain the progressive swelling of the composites. The maximum of deformation strongly depends of the additive. A relaxation of the maximum strain is observed after 50 cycles for Vanadium whereas it remains constant for Ti-V-Cr. At nanoscale, these different behaviours have been correlated to the ability of the fine additive particles to prevent the motion of the grain boundaries, then to limit the recrystallization of MgH2 particles.