Hydrogen in disordered alloys

Flanagan T.B. Sakamoto, Y. (1993). Hydrogen in Disordered and Ordered Palladium Alloys, Platinum Metals Rev., Vol.37(l), pp.26-37. 

In the case of the FeNi3-H alloy at temperature T 500°C the atomic order-disorder phase transition takes place. In this alloy in accordance with Eq. (25) for f0 with the value of dissolution heat (see Table 2), the atomic order must increase the hydrogen solubility. This theoretical conclusion is also in accordance with the experimental data. 

From either hydrogenated binary Mg3lr alloy or by sintering powder mixtures of the elements at up to 773 K under a H2 pressure of up to 150 bar. Red colored hydride partially disordered structure (Figure 45) from npd on deuteride, P2 /c, Z = 8 contains four symmetry independent Ir hydride complexes a spy [IrHs]" " and three sad [lrFl4] complexes of which two are disordered, and five hydride anions are exclusively bonded to Mg all complexes are surrounded by Mg in distorted cubic... 

Interstitial impurities (e.g., hydrogen of helium atoms) can easily move over the interstitial cavities in LRC but comparatively large size partial vacancies on the boundaries are traps for small-size atoms. It was established in a series of experiments that amorphous alloys contain traps for hydrogen and helium atoms and that the concentration of these traps is rather high [6.56]. The concentration dependence of interstitial diffusivity in disordered structures was considered phenomenologically [6.56] and by mean-field approximation [6.57],... 

Electrochemical charging with hydrogen increases disordering in Mg-based amorphous alloys. Herein a very disordered amorphous phase forms, which produce no reflection from the coordinate sphere. 

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 ... 

To be specific we shall in the following consider the case of compositionally disordered intermetallic alloys with a fcc-structure (see Fig. lb). We assume that the hydrogen atoms occupy only octahedral interstitial sites. Such sites are coordinated by 6 nearest-neighbours and 8 second nearest-neighbour metal atoms. In an alloy Aj y By the various sites are specified by the number m of B-atoms on nearest neighbour... 

A review is presented of some recent applications of neutron vibrational spectroscopy to the study of disordered metal-hydrogen systems. The examples discussed cover a range of systems from simple dilute solutions in bcc or fee metals to amorphous alloy hydrides. It is shown that neutron inelastic scattering studies of the vibrational density of states provide a powerful and sensitive probe of the local potentials and bonding sites of hydrogen in metals and often reveal critical information on the novel microscopic physical properties and behavior of disordered metals-hydrogen systems, including those influenced by interstitial or substitutional defects. 

In recent years a large number of disordered alloy systems have been studied as hydride-formers and hydrogen host materials. These fall into two general categories (1) substitutional solid solutions, e.g., (Zr,Ti)-Mn2, Zr(Fe,V)2> etc., and (2) non-stoichiometric systems, e.g., ZrMn T where T = Cr,Mn,Fe,Co,Ni or Cu. In the latter materials a more accurate representation is as follows 2-u u quantities... 

A comparison is made of the solubility of hydrogen in ordered and disordered forms of palladium alloys. In PdyCe the solubility of hydrogen is greatest in the disordered form and in Pd Mn and Pd Fe the solubility is greatest in the ordered forms. In both Pd. Mn and PdyCe there are more interstices surrounded by only palladium atom nearest neighbors in the disordered form than in the ordered form. In Pd Fe there are more such interstices in the ordered form. There seems to be no simple pattern of behavior for the explanation of the solubility differences between the ordered and disordered forms of these alloys,... 

In this paper the solubility of hydrogen in ordered and disordered forms of several substitutional palladium alloys will be discussed. Some new data will be presented for the Pdr Ge and Pd Mn alloy-hydrogen systems,... 

Fig. 1 The solubility of hydrogen in Pd Ge at 323 K., ordered alloy O. disordered alloy.

In Fig.6 a comparison of the solubilities of hydrogen in ordered and disordered Pd Mn is shown at 19 K. The disordered sample has been obtained by cold-working as this was found to be an effective way to disorder this alloy,however, it may still not be fully disordered. The solubility is 3.7 times as great at this temperature in the ordered form than in the disordered form. 

In the case of metallic systems, there were early indications that metallic glasses and disordered alloys may be more corrosion resistant, less susceptible to embrittlement by hydrogen and have a higher hydrogen mobility than ordered metals or intermetallics. All of these properties are desirable for hydrogen storage. Subsequent research has shown that thermodynamic instability is a severe problem in many amorphous metal hydrides. The present ASI has provided an appropriate forum to focus on these issues. 

According to earlier investigations (refs.4-5) in the case of a Pd+Cu catalyst prepared by sitajltaneous reduction, four different, Cu-containing phases may form (i) bulk Cu, (ii) adsorbed Cu, (iii) disordered alloy and (iv) ordered alloy (PdCuj) phases. In the absence of Cl"" or Pd ion, no alloy phase formation occurs during consecutive reduction (ref.6). In the latter case bulk metal and adsorbed metal formations occur simultaneously. This phenomenon was observed e.g. in the case of Cu deposition by hydrogen reduction from an aqueous solution of /Cu(NHj) /(0H)2 onto a Pd/C catalyst (Fig. 1). The doublet in the range of 0.5-0.4 V points to bulk Cu deposition on various crystal faces of Pd (ref.7), whereas the peak around 0.55 V is due to ionization of the adsorbed Cu (refs.6-7). 

From the solubility measurements of hydrogen in Al-Fe alloys (28 at.% Al), [1997Xu] ealeulates an enthalpy of dissolution of H atoms varying from -159.7 kJ per mole of H between 125 and 550°C, stability domain of the ordered ai solid solution to -144.0 kJ per mole of H between 550 and 770°C, stability domain of the ordered a.2 solid solution and -132.1 kJ per mole of H between 770 and 1100°C, stability domain of the disordered a solid solution. 

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