In situ dilatometry measurements of MgH2 compacted disks

•Composites made of ball-milled magnesium hydride powders tend to expanse upon hydrogen cycling.•The irreversible swelling has been quantified by in situ dilatometry measurements.•It increases progressively up to about 30cycles where a stabilisation is achieved.•Natural Expanded Graphite (NEG) appears very efficient to limit the radial stress to a reasonable stress (3.4MPa). In situ dilatometry measurements were performed in order to characterize the progressive swelling which is observed upon cycling on compacted disks made of ball-milled MgH2. Each sample was submitted to 200 hydrogen cycles at 310°C, under 0.1MPa for desorption and 1MPa for absorption. These measurements clearly show a dilatation on absorption and a contraction on desorption especially in the radial direction. However, at the end of the desorption, the initial position is not recovered and a “cumulate displacement” is observed which increases progressively up to about 30cycles where a stabilisation of the phenomenon is achieved. The introduction of Natural Expanded Graphite (NEG) appears very efficient to limit the radial expansion, with a maximal amplitude three times lower than without NEG and a radial stress equivalent to an applied pressure of 3.4MPa.