Metal-hydride composite materials

HYDROGEN CONCENTRATION DEPENDENCE ON THERMAL AND ELECTRICAL CONDUCTIVITIES OF METAL-HYDRIDE COMPOSITE MATERIALS... 

Keywords Metal-hydride composite materials Thermal diffusivity Thermal conductivity ... 

Develop formulations and proeedures to make siliea eneapsulated metal hydride composite material. 

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. 

The catalysts for oxygen reduction and oxygen oxidation are materials based on substances like Co, Ni, Fe, Mn [16, 19]. One of new applications of oxygen reduction catalysts is air-metal hydride accumulator. Electrodes based on La0.1Cao.4Co03, La0. Cao.,jMn03 [18] are used in this battery. The electrodes of similar composition could be used in SOFC. An insertion of the oxides in their composition (CuO for example) leads to increasing the conductivity of system and efficiency of catalyst [30]. 

As more sophisticated metal hydrides are developed (nanocrystalline, multicomponent systems, composites and nanocomposites, graphite/metals or similar hybrid systems, clusters, etc.), it is important to be a vare that, for practical applications, a large volume of material should be processed in a fast, inexpensive and reliable vay, for example casting. Techniques such as cold vapor deposition may be impossible to scale up but this does not mean they should be discarded as a means of studying new metal hydrides. On the contrary, laboratory techniques allow much better control of the end product and permit the elaboration of new compounds. Once an attractive compound is found then another challenge w ill have to be faced scaling up the synthesis. In this respect, it is important for the community of metal hydrides researchers to also study large-scale production techniques in order to make the transition from laboratory to industrial scale easier. 

As is shown in Section XII, it is possible to produce electrodeposited composite materials, for example, of Ni and Mo (75) or Ni and W, that have high specific real areas and exhibit quite different Tafel slopes (much lower values) from those of corresponding bulk alloys having the same nominal compositions. It is believed that this arises on account of the much lower effective real current densities that then obtain at ordinary practical current densities and possible involvement of micro-metal clusters having intrinsically better catalytic activity as referred to above in the case of Raney materials. Codeposited, sorbed H may also be important for HER catalysis, giving rise to hydridic phases (75,134). 

It must be mentioned that composite materials produced by reduction of oxide mixtures in the Hj at 500°C (161) are not, by any means, true intermetallic phases such as could be prepared by melting and cooling the pure metal components in the same nominal composition ratios. For example, it is found that such composites produced thermally from oxides, reductively at a metal substrate from oxides, or by electrodeposition at a metal substrate from appropriate salts in solution, for example, NiS04 plus (NH4)2Mo04, do not show well-defined X-ray diffraction patterns, and their mode of preparation almost certainly results in important quantities of H remaining in a sorbed hydridic state in such preparations (75). 

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