Relation between Melting and Dehydrogenation Temperatures of LiAlH4

The effect of melting of LiAlH4 on its dehydrogenation properties was examined using Differential Scanning Calorimetry (DSC). LiAlH4 samples purified in advance were examined on their hydrogen desorption behaviors in the various heating conditions in which some of them were melted and the others not. Some of the as-received samples were mechanically ground in an Ar atmosphere for a short time of period using a ball milling machine and then supplied to DSC measurements. The color of the sample after milling changes from gray-beige to grey, probably due to contamination of metal elements such as iron and chromium from hardened steel vessel and balls during grinding. The rapid hydrogen desorption was observed for the samples melted by heating, whereas the insignificant hydrogen desorption was observed for the ones which were not melted by heating, in spite that heating and cooling were repeated three times. The time from melting to decomposition of LiAlH4 accompanied by hydrogen desorption was 90(plus-minus) 30 s, independent of hydrogen desorption temperature. The samples milled in hardened steel vessel using a planetary-type ball milling machine exhibited about 2 K lower melting temperature and 15 K lower hydrogen desorption temperature than the as-purified samples. These changes can be considered to happen because contamination metal elements contributed to lowering a melting temperature and worked as a catalyst to decrease the activation energy for decomposition of liquefied LiAlH4. These results indicate that metal elements such as iron and chromium certainly contribute to improve hydrogen desorption temperatures of alanates, as reported in many papers, but it is more important to lower their melting temperatures in order to improve their hydrogen desorption kinetics.