Low Temperature Hydrogen Cycling in Mechanically Alloyed Mg2Ni
Abstract
We have observed hydrogen cycling at reduced temperatures (100-175^oC) in nanocrystalline Mg_2Ni using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). When synthesized from the elemental powders by mechanical alloying, Mg_2Ni was found by x-ray diffraction analysis to have a crystallite size of about 7 nm. This nanocrystalline sample was successfully activated for hydrogen absorption in 1 atm of H2 gas at 100^oC, and reached a hydrogen capacity of 2.3 wt%. Subsequently the sample could be dehydrided at 175^oC in flowing inert gas, and rehydrided at 100125^oC in 1 atm of H2 gas. These temperatures are about 100^oC below those conventionally used in hydrogen cycling of Mg_2Ni. Heating the sample to 450^oC mostly destroyed the sample's ability to absorb hydrogen at low temperature, associated with growth in the average crystallite size to about 36 nm. The characteristic dehydriding energy, E = 65 kJ/mole H_2, is about the same in magnitude as the enthalpy of formation of Mg_2NiH_4. We infer that low temperature hydrogen cycling is most likely kinetically enabled by rapid diffusion of hydrogen through an extensive network of grain boundaries and disordered regions in the nanocrystalline microstructure, rather than through any substantial change in the thermodynamics of the Mg_2Ni phase.
- Publication:
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APS March Meeting Abstracts
- Pub Date:
- March 2004
- Bibcode:
- 2004APS..MARS39014P