Hydrogen absorption amorphous alloys

Said this, we can let the reader to recall Fig. 1.15, where we depicted amorphous-like phase regions at grain boundaries as the pathways open for preferential diffusion of hydrogen atoms. Apparently, an alloy can benefit from some fraction of amorphous phase to improve kinetics of hydrogen absorption, but complete amorphization of crystalline lattice lowers capacity for storing hydrogen [156]. Mechanochemical activation is therefore a complex process where kinetic and thermodynamic effects must be firstly well understood, and then optimized. 

K. Aoki, M. Kamachi, T. Masumoto, Thermodynamics of hydrogen absorption in amorphous Zr-Ni alloys, J. Non-Crystalline Solids 61-62 (1984) 679-684. 

Miscellaneous properties of amorphous alloys are treated in section 9. These comprise properties that have been investigated in only relatively few cases. Examples of such properties are the specific heat, hydrogen absorption, the concentration dependence of the Mbssbauer isomer shift and the few results of ESR on Gd-doped samples. 

Model descriptions for hydrogen absorption in amorphous alloys have been given by Kirchheim et al. (1982), Kirchheim (1982) and Griessen (1983). Griessen notes that the absence of a plateau pressure in these materials may not always be primarily due to disorder. In his model he describes the hydrogen-hydrogen interaction by means of a mean field approximation and shows that the critical temperature (below which the pressure composition isotherms may exhibit a plateau) is positive if a Stoner-like criterion is satisfied ... 

Robbins et al. (1982) investigated the effect of hydrogen absorption on the magnetic properties of amorphous alloys of the type (Ro. o o.2o)iThe quantity of hydrogen absorbed corresponds to hydrogen-to-metal ratios ranging from 0.2 in the alloy with R = Pr and T = Fe up to 1.7 in the alloy with R = Tb and T = Fe. 

Fig. 96. Schematic representation of the distribution of magnetic Fe-Fe interactions in amorphous Y, j,Fe alloys before hydrogen absorption (full line, middle part) after hydrogen absorption (broken line, top part) and after applying external pressure (broken line, bottom part).

K. Aoki, A. Horata and T. Masumoto, Hydrogen Absorption and Desorption Properties of the Amorphous Zr-Ni Alloys, in "Proc. 4th. Int. Conf. on Rapidly Quenched Metals," T. Masumoto and K. Suzuki, eds., Japan Inst. Metals, Sendai, Japan (1Q8P) K. Aoki,... 

After hydrogenation, powder diffraction X-ray scans of all alloys were made. The scans showed no measurable crystallization. In all cases the mean position of the broad amorphous peak moved to lower angles, consistent with the sample dilation caused by hydrogen absorption. 

It has been well known for a couple of decades that noncrystalline metallic alloys can be made by vapour- and melt-quenching. Recent results show that an amorphous phase can also be formed directly when a crystalline metallic alloy is subjected to various types of disordering processes. Solid-state amorphization can be induced through a variety of methods including absorption of atomic hydrogen, thermal interdiffusion reaction along the interface separating... 

The amorphization of a metallic alloy was first observed and investigated systematically by Yeh et al. in 1982 through the absorption of atomic hydrogen into the crystalline Zr3Rh [2.5]. Hints for the possibility of such a transition had been found earlier by Osterreicher et al. on LaNi3 [2.6] and vanDiepgen et al. on CeFe2 [2.31], but were not analyzed in any detail. 

The accuracy of determining the hydrogen content from IR absorption spectra can be undermined in several ways. For example, light-induced degradation of photoconductivity of weakly doped films and alloys of amorphous hydrogenated silicon, known as the Staebler-Wronski effect [135], is clearly observed upon illumination by radiation with energy /iv > 1.17 eV over several hundreds of hours [150]. Usually, the formation of dangling bonds when weak Si—Si bonds are broken as a result of the nomadiative recombination of an electron-hole pair in the tails of the conductivity and valence bands is responsible for the effect [133, 151]. 

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