Magnesium hydride thin film deposition on flexible substrate by reactive plasma sputtering

Magnesium hydride was successfully deposited by reactive plasma sputtering on graphite and polyimide foil flexible substrates. As the same time as this study, deposition was also done on silicon nitride substrate. XRD analysis revealed an as-deposited thin film consisting of tetragonal magnesium hydride P42/mnm (a = b = 4.5170 A; c = 3.0205 A) and hexagonal magnesium P63/mmc (a = b = 3.2094 A; c = 5.2112 A). Thin films hydrogen sorption properties were studied using a HERA volumetric (Sieviert type) apparatus. Sample deposited on graphite was cycled 28 times (70 h) and sample deposited on silicon nitride was cycled 15 times (137 h). Desorption was done under a hydrogen pressure of 4 kPa and absorption was done under 1040 kPa. Temperature was maintained at 350 and 300°C respectively. For practical reasons, absorptions were cycled in automatic mode at approximately 86 % of their final value for sample deposited on graphite. First desorption last respectively 2 and 4 times longer than following absorptions and desorptions. Good cycling stability was observed for sample deposited on graphite. From hydrogen quantity desorbed during first desorption, deduction of respectively 73 and 85 % magnesium hydride as-deposited thin film proportion was made. For sample deposited on graphite, SEM observations revealed a microstructure after cycling made of porous aggregates. Two different grain sizes were revealed: 2 μm-sized grains which could correspond to magnesium hydride, and 300 nm-sized grains which could correspond to magnesium. After cycling, an interface was formed between silicon nitride and magnesium hydride thin film. TEM analysis was done and electronic diffraction pattern analysis revealed that interface was amorphous. Electronic diffraction pattern analysis was also done on the thin film after cycling showing that it was constituted of 30 nm-sized magnesium grains and 8 nm-sized magnesium hydride grains.