Correlation between microstructural and mechanical behavior of nanostructured MgH2 upon hydrogen cycling

Abstract

Highly reactive nanostructured powders are produced on a large scale by co-milling of MgH2 with transition metals. Composite materials with high thermal conductivity are produced by uniaxial compaction of these powders with expanded graphite. However, upon cycling, the MgH2 grains tend to recrystallize and a progressive swelling of the composites is observed. The purpose of this study is to understand this irreversible phenomenon, through correlations with microstructural evolutions.In-situ dilatometry measurements were performed on samples prepared with 2 different additives (pure vanadium or Ti–V–Cr alloy). We systematically observe a stabilization of the irreversible expansion after about 50cycles. A bi-modal distribution of the as-milled powders is observed by granulometry measurements. Upon cycling, the coalescence of the “small” MgH2 particles tends to create large agglomerates and results in mono-disperse powders. This behavior induces an increase in porosity, which explain the progressive swelling of the composites. The maximum of deformation strongly depends on the additive and is lower with vanadium addition than with Ti–V–Cr alloy.