Effect of reaction medium on the mechanochemical decomposition of ZrH2 and on the reactivity of hydride hydrogen

The effect of gas composition on the mechanism of mechanically induced decomposition of ZrH2 was studied under the milling conditions. Also, the reactivity of Zr-hydride hydrogen was studied as a function of milling conditions. Degree of ZrH2 decomposition was estimated through the amount of hydrogen emitted from the sample during the treatment via H2 and CH4, respectively. The changes in the integral intensity of the characteristic peaks of ZrH2 in XRD spectra were also examined. The concentration of mechanically induced defects, as well as the binding energy of hydride hydrogen with different type of defects was estimated from the thermal desorption experiments. The morphology of the samples was studied by high-resolution transmission electron microscopy. It was found that the milling conditions define the kinetics and special features of the decomposition process. No decomposition was observed under the treatment in vacuum at static conditions, due to hydrogen re-adsorption. The steady state regime of the decomposition took place during milling in a He flow because a well developed dislocation network was formed in the bulk of ZrH2. The decomposition process was suppressed after 60-80 min of milling in a 21 Vol.% CO/He mixture because the surface of ZrH2 particles was covered and passivated by carbon. The decomposition in a flow of 77 Vol. %CO/He mixture was accompanied by drastic structural transformation caused by the gas-solid reaction. It was found that the reactivity of Zr-hydride hydrogen strongly depends on the structural distortions.