Hydrides lattice expansions

It is of interest to note that VH in the hydride phase is significantly less than in AB5 hydrides. Consequently, lattice expansion is also significantly reduced. However, the corrosion rate of the electrodes in Table 9 is still appreciable. Indeed, for the electrode with x - 0.25 the... 

There are few systematic guidelines which can be used to predict the properties of AB2 metal hydride electrodes. Alloy formulation is primarily an empirical process where the composition is designed to provide a bulk hydride-forming phase (or phases) which form, in situ, a corrosion— resistance surface of semipassivating oxide (hydroxide) layers. Lattice expansion is usually reduced relative to the ABS hydrides because of a lower VH. Pressure-composition isotherms of complex AB2 electrode materials indicate nonideal behaviour. 

H-H interaction due to the lattice expansion becomes important and the hydride phase (P phase) nucleates and grows. The hydrogen concentration in the hydride phase is often found to be H M = 1. The volume expansion between the coexisting a- and P-phases corresponds in many cases to 10-20% ofthe metal lattice. Therefore, at the phase boundary high stress is built up and often leads to decrepitation of brittle host metals such as intermetaiiic compounds. The final hydride is a powder with a typical particle size of 10-100 pm (Figure 5.24). 

In a metal, certainly the transition metals, the electrons are more or less free to move in conduction bands. This fact is responsible for the high electrical conductivity of metals. When hydrogen atoms are present in the holes between the atoms, the movement of the electrons is somewhat impaired. As a result, the metal hydrides of this class are poorer conductors than the pure metals. The presence of hydrogen atoms makes the metal atoms less mobile and more restricted to particular lattice sites. Accordingly, the interstitial metal hydrides are more brittle than the parent metal. Also, the inclusion of the hydrogen atoms causes a small degree of lattice expansion so that the interstitial hydrides are less dense than the parent metal alone. 

In addition, a correlation between the unit cell volume and the hydriding properties for LaNi, and Mg Ni is discussed from a view of the nature of the chemical bond between atoms in small polyhedra and also of the possible lattice expansion induced by the hydrogenation. 

In many cases a sloping plateau appears, possibly due to different equilibrium pressure lattice expansions, relaxation of residual forces to relieve the stress in the metal matrix are attributed to this distinctive feature of metal hydride systems as shown in Fig. 12.1(a), the slope of the plateau has been defined as dlnpd/dhi(H/M). 

The introduction of other metals to form palladium based alloys has had promising results. In particular doping of the palladium with silver has been shown to improve the stability of the film and increase the solubility of hydrogen. Further, the temperature above which the a palladium hydride occurred was lowered with increasing silver content (Uemiya et al.,1991 Kikuchi Uemiya, 1991). The hydrogen permeability was optimized when the silver content of the alloy was aroimd 23 wt%. Silver occupies interstitial sites in the palladium lattice and so moderates the lattice expansion and contraction due to hydrogen absorption/desorption. 

In section 3 the phenomenon of hydrogen sorption in intermetallic compounds is described. Topics dealt with in this section are activation treatment, pressure-composition isotherms, miscibility gap, sorption hysteresis, lattice expansion, diffusion, sorption kinetics, surface effects, poisoning, impurity effects and decomposition of ternary hydrides. The sorption characteristics of all ternary rare-earth hydrides investigated are listed in several tables given in the appendix. For comparison the sorption characteristics of some selected ternary hydrides based on non-rare-earth metals are given in a separate table. 

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