Amorphous hydrides

Z.S. Wronski, R.A. Vatin, Ch. Chiu, T. Czujko, Mechanosynthesis of nanocrystaUine MgB ceramic powders in hydrogen alloying mills via amorphous hydride intermediate, Adv. Sci. Technol. 45 (2006) 309-314. 

R.A. Vaiin, S. Li, A. Calka, D. Wexler, Formation and environmental stability of nanocrystalline and amorphous hydrides in the 2Mg-Fe mixture processed by controlled reactive mechanical alloying (CRMA) , J. Alloys Compd. 313 (2004) 270-286. 

Key words hydrogen storage/nanostructured light metal-based hydrides/controlled reactive mechanical alloying/magnesium hydride/amorphous hydrides... 

The intent of the present paper is to provide an overview of the information that can be gleaned from NMR measurements on transition metal hydrides. Since particular emphasis is placed upon comparison between crystalline and amorphous hydrides with similar (nominally identical) compositions, most examples are taken from the work of the author and his colleagues, which are the most extensive results currently available on these properties. It is hoped that other NMR research groups will find the present results and unresolved questions sufficiently challenging to conduct further work on this very interesting class of materials. 

The first proton NMR experiments of diffusion in an amorphous hydride were done > on TiCuH where various relaxation times were measured. Fig. 2 compares the temperature behavior of for c-TiCuHQ g and a-TiCuHj 4 that were obtained by BPP analyses of the proton data. These results are also compared with similar parameters for cubic TiHj gQ. Significant differences in hydrogen diffusion behavior are immediately apparent in Fig. 2. A structural restriction accounts for the smaller values and larger activation energies... 

Perhaps, a more-or-less common diffusion mechanism is operative for these amorphous hydrides that is rather insensitive to the metal constituents or hydrogen stoichiometry. However, a decrease in with increasing hydrogen content is seen for glassy a-Zr2PdH samples which is opposite to the trend observed for the crystalline phases. Changes in the proton... 

This paper has presented a review of the proton NMR results that directly compare the properties of crystalline and amorphous metal hydrides with similar compositions. The types of information on structure, diffusion behavior, and electronic properties that can be extracted from the experimental proton NMR parameters have been described. While these studies have shown some rather remarkable differences between the crystalline and amorphous hydride phases, most of the current evaluations have been of a qualitative nature. Additional, more detailed, experiments and supporting quantitative analyses are needed to explain the specific sources of these differences. The existing NMR data should be extended to other suitable metal-hydrogen systems while as many complimentary techniques as feasible should also be brought to bear on these issues. 

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