Diffusion path, networks in oxides

An understanding of diffusion mechanisms in oxides, particularly cation diffusion, must take account of the various possible paths which an atom may follow in going from one normally occupied site to another equivalent site. Such paths may be branched and will likely include stops at metastable points along the way. This chapter describes methods whereby path networks within oxides can be characterized, and discusses diffusion in the structures NaCl, ZnS, FeS, and BeO. 

The oxide ion conductor (Lao.8Sro.2)(Gao.8Mgo.i5Coo.o5)02.8. which has an ideal perovskite-type structure, exhibits diffusion paths along the [110], [110], [011], [Oil], [101] and [101] directions to form a three-dimensional network of equivalent diffusion pathways (Fig. 6.5(b)) [10]. In contrast, in the present double perovskite-type material Lao.64(Tio.92Nbo.o8)02.99, a two-dimensional diffusion pathway, by which 03 atoms migrate along the [110] and [110] directions (Fig. 6.7(b)), is present. This two-dimensional feature is attributable to the layered structure of the material, which consists of La-occupied Lal-Ol,... 

The sulphide usually forms an interconnected network of particles within a matrix of oxide and thus provides paths for rapid diffusion of nickel to the interface with the gas. At high temperatures, when the liquid Ni-S phase is stable, a duplex scale forms with an inner region of sulphide and an outer porous NiO layer. The temperature dependence of the reaction is complex and is a function of gas pressure as indicated in Fig. 7.40 . A strong dependence on gas pressure is observed and, at the higher partial pressures, a maximum in the rate occurs at about 600°C corresponding to the point at which NiS04 becomes unstable. Further increases in temperature lead to the exclusive formation of NiO and a large decrease in the rate of the reaction, due to the fact that NijSj becomes unstable above about 806°C. 

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