Hydriding/dehydriding properties of nanocrystalline Mg2Ni alloy

Nanocrystalline Mg2Ni synthesized by mechanical alloying absorbs hydrogen rapidly at room temperature without activation and it shows a marked increase in the initial hydrogen absorption rate and the hydrogen capacity as the temperature increase; The plateau pressure and the hydrogen capacity of nano-structured Mg2Ni increase with the increasing temperature, the value of enthalpy (DeltaH(0)) calculated from the linearity of van't Hoff plot is -30.4kJ/molH(2). The value of enthalpy is in good agreement with the LT-phase Mg2NiH4, which is only half of the value of Mg2Ni alloy and the HT-phase Mg2NiH4, so it's a promising candidate for clear energy materials; The grain size of nanocrystalline Mg2Ni is stable during the long time heating process when the temperature is below 200 and the hydrogen absorption rate and capacity are better than the conventional materials. If the temperature is above 300, the hydrogen absorption rate and capacity are decreased as the time increasing. The remarkable degradation is due to the decreasing of surface and grain boundaries region with the increasing of heating time. So the reaction temperature must be kept blow 200; there are two obvious regions for hydrogen desorption, which corresponds to the two endotherms on the DSC curve. These are due to the decomposition of Mg2NiH4:: Mg2NiH4-->Mg2Nix+H-2-->Mg2Ni+H-2, the hydrogen desorption temperature increases with the growth of grain size, and the energy needed for dehydriding also increases. Results indicated that the condition of hydriding-dehydriding reaction must be blow 200square in order to keep the enhanced properties of nanocystalline Mg2Ni.