TEM analysis and sorption properties of high-energy milled MgH2 powders

The main focus of this work was the use of cryo-stage transmission electron microscopy (TEM) to elucidate features of the milled MgH2 powder microstructure not accessible via "conventional" hydride characterization techniques such as X-ray analysis. We chose a classic problem in hydride research: the effect of high energy milling on the microstructure and the desorption properties of magnesium hydride (MgH2) powder. High-energy mechanical milling provided for marked hydrogen desorption enhancement, while subsequent annealing reduced and ultimately eliminated it. Milling created a significant amount of microstrain in the alpha-MgH2. It also reduced the particle size and changed the size distribution to be multimodal. Milling reduced alpha-MgH2 grain size by a factor of two, which can be considered a fairly minor effect. Post-milling anneals completely eliminated the microstrain, increased the grain size to a similar value as the as-received powder, and smoothed the particle size distribution. Using TEM we were able to image the individual hydride grains within the powder particles. TEM analysis of the milled microstructure identified a high density of nano-scale twins within the hydride grains. Annealing reduced the twin density and coarsened their size. Elasticity analysis indicates that the energy stored by the accumulated dislocations as the result of the milling is negligible compared to the heat of the hydride formation. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.