Electrical insulators, crystalline solids

A variety of solids is used as catalysts metals, alloys, clays, metal oxides, sulphides, nitrides, carbides and so on. Catalysts may be single-phase substances or multiphasic mixtures they may be crystalline, microcrystalline or even amorphous. Catalysts can be electrically insulating, semiconducting or metallic. Some examples of heterogeneously catalysed reactions are given in Table 8.1. Two aspects of catalysts are important activity and selectivity. Activity refers to the ability of catalysts to accelerate chemical reactions so that equilibrium is achieved rapidly. The degree of acceleration... 

In electrically insulating solids, heat is transferred in the form of elastic waves or phonons [1], Anything that affects the propagation of the phonons through the solid affects the thermal conductivity of the solid. In a pure crystalline ceramic, the intrinsic thermal conductivity is limited by the energy dissipated during phonon-phonon collisions or so-called Umklapp processes [15], Commonly, the intrinsic thermal conductivity of solids is described by (5). 

In theory, heat in crystalline solids is transferred by three mechanisms (i) electrons (ii) lattice vibrations and (iii) radiation [44], Since zirconia is an electronic insulator (electrical conductivity occurring at high temperatures by oxygen ion diffusion), electrons play no part in the total thermal conductivity of the system. Hence, thermal conduction in zirconia-based ceramics is mainly by lattice vibrations (phonons) or by radiation (photons). 

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