Correlation between microstructural and mechanical behavior of nanostructured MgH2 upon hydrogen cycling
Résumé
Reversible hydrogen storage in magnesium hydride is the most suitable solution for stationary applications, which require large-scale storage systems with an expected lifetime exceeding 5000 hydrogen cycles. Highly reactive nanostructured powders are produced on a large scale by co-milling of MgH2 with transition metals. These powders present a very stable weight capacity upon cycling. However, the MgH2 grains tend to recrystallize and to induce a progressive swelling of the compacted disks. The purpose of this study was to quantify and 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 tend to create large agglomerates and results - again after about 50 cycles - in mono-disperse powders. This evolution 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. This could be correlated to the ability of the fine additive particles to prevent the motion of the grain boundaries, then to limit the agglomeration of MgH2 particles.