Hydrogen storage characteristics of mechanically alloyed amorphous metals

The hydrogen storage properties of a series of mechanically alloyed (MA) amorphous Ni_1xZr_x alloys are studied, using both gas phase and electrochemical techniques, and are compared to H storage of rapidly quenched (RQ) amorphous Ni_1−xZr_x. In the MA alloys, hydrogen resides in the Ni_4−nZr_n (n = 4,3,2) tetrahedral interstitial sites, with a maximum hydrogen-to-metal ratio of 1.9(4 n)xⁿ(1 − x)^{4 − n}. These features are identical to those of the RQ alloys and indicate that the topological and chemical order of the MA and RQ materials are essentially the same. However, the typical binding energy of hydrogen in a Ni_4−nZr_n site, E_n, is shifted in the MA alloys relative to the RQ alloys and the distribution of binding energies centered on E_n is significantly broader in the MA samples. Thus, the MA and RQ alloys are not identical and sample annealing does not alter this subtle distinction. The sensitivity of H storage to the presence of chemical order in binary alloys are analyzed theoretically and the data are found to be most consistent with little or no chemical order (random alloys).