The effect of milling energy input and molar ratio on the dehydrogenation and thermal conductivity of the (LiNH2 + nMgH2) (n = 0.5, 0.7, 0.9, 1.0, 1.5 and 2.0) nanocomposites

Hydride nanocomposites in the (LiNH2 + nMgH2) system have been synthesized by ball milling with varying input of milling energy injected into powder particles, QTR (kJ/g). The grain (crystallite) size of LiNH2 and MgH2 decreases rapidly with increasing QTR up to approximately 150–200 kJ/g and subsequently more or less saturates at the value of 10–20 nm. For the injected energy QTR ≈ 250–350 kJ/g the specific surface area (SSA) increases from the initial 2.4 m2/g for powder mixtures before milling to 30–37 m2/g for nanocomposites after milling. After injecting QTR ≈ 550 kJ/g there is a further increase of SSA to 52 m2/g which is over 20-fold increase of SSA from its initial value. That clearly indicates that a profound reduction of particle size has occurred. The hydride phases formed during ball milling with relatively low QTR are identified as a-Mg(NH2)2 (amorphous magnesium imide) and LiH. The ball milled (LiNH2 + nMgH2) nanocomposite system with n = 0.5–0.9 can effectively desorb about 4–5 wt.% H2 with a reasonable rate at the temperature range close to 200 °C. Within a low temperature range up to ∼250 °C, regardless of the molar ratio n and the injected energy QTR the thermal desorption of the (LiNH2 + nMgH2) nanocomposites occurs without any release of ammonia, NH3. For all molar ratios, n, the hydride nanocomposites are fully reversible at 175 °C under a relatively mild pressure of 50 bar H2. The quantity of H2 desorbed decreases with increasing molar ratio n, due to increasing fraction of inactive, retained MgH2. However, at 125 °C the dehydrogenation rate is very sluggish and the quantity of released H2 is minimal. At the temperature range lower than ∼250 °C dehydrogenation of ball milled nanocomposites occurs through formation of the Li2Mg(NH)2 hydride phase. The value of the measured dehydrogenation enthalpy change of 46.7 kJ/molH2 is relatively low and apparently, it is not responsible for sluggish dehydrogenation at 125 °C. The measurements of thermal conductivity for non-milled powders and ball milled nanocomposites show a dramatic reduction of thermal conductivity after ball milling. It seems that this could be a principal factor responsible for such a low dehydrogenation rate at low temperatures.