Particle size, grain size and γ-MgH2 effects on the desorption properties of nanocrystalline commercial magnesium hydride processed by controlled mechanical milling

The hydrogen desorption properties of commercial nanocrystalline magnesium hydride (Tego Magnan (R) from Degussa-Goldschmidt) processed by controlled mechanical milling (CMM) are investigated. A profound effect of the powder particle size on the hydrogen desorption characteristics has been observed. The onset (TON) and peak hydrogen desorption temperatures measured by differential scanning calorimetry (DSC) decrease initially slowly with decreasing mean particle size of hydride, and when the particle size reaches a certain critical threshold value, the desorption temperatures start decreasing more rapidly with further decrease of particle size. The total drop of desorption temperature from its initial value for the as-received MgH2 to the value attained for the milled MgH2 having a particle size of similar to 500-600 nm is within the range 40-60 degrees C. The metastable gamma-MgH2 hydride coexists with the stable nanocrystalline beta-MgH2 in the microstructure of the MgH2 powders ball milled for 10 h and longer. Quantitative evidence shows that two factors, namely the refined powder particle size and the gamma-MgH2 phase residing within the powder particles, acting additively, are responsible for a substantial reduction of the hydrogen desorption temperature of MgH2 hydride.