Structure-sensitive diffusion

It is usual to classify diffusion processes into volume, grain-boundary, and surface diffusions. While the preceding and following chapters show that the problems of volume and surface diffusion have been attacked in a fairly adequate manner, the data with which the present chapter has to deal are much more fragmentary. Jost, Wagner, Frenkel, and Schottky(i) have developed the theory of disorder in equilibrium with order in crystals. It transpires from theory and experiment that the transitions 

A second kind Of structure-sensitive diffusion is also possible. The crystal, whether metallic or ionic, consists of a mosaic of small blocks, separated by grain boundaries, or submicro-scopic flaws. It is well known that diffusion on an external surface (Chap. VIII) proceeds more easily than volume diffusion. By an analogy, which will later be supported by experimental data, one may suppose that diffusion along 

When the conductivity cannot be attributed to excess of one component, i.e. when the conductivity is not sensitive to changes in partial pressure of a surrounding gas atmosphere of that component, and yet irreversible conductivity or diffusion phenomena are observed, the behaviour may be attributed either to grain-boundary diffusion, or to lattice disorder, frozen by rapid cooling of the high-temperature 

Some properties of crystals depend upon the previous history of the specimen, whil others are insensitive to all treatments. On this basis a classification (3) of crystal properties may be made (Table 77). The structure sensitivity of some of these 

Additive, contribution of anomalous parts of smaller orderofmagnitude than that of normal parts 

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A structure sensitive diffusion down faults, and cracks of molecular dimensions, predominant when argon, nitrogen and air diffuse. 

As a result of a large number of researches it has been established that two types of feult system may exist in crystals which may be called reversible and irreversible fault systems. The former have reproducible properties in many respects, but properties of the latter depend upon the previous history of the specimen. In particular, irreversible fault systems give rise to the structure-sensitive diffusion and conductivity data described in the following chapter, while reversible fault systems give rise to reproducible conductivity and diffusion phenomena. Some of the reproducible and equilibrium types of fault systems may be briefiy discussed before the conductivity and diffusion data are given. 

Structure-sensitive diffusion in metallic systems is of fairly common occurrence. The self-diffusion of bismuth, while strongly anisotropic, is to a certain extent dependent upon the bismuth crystal(43) employed ... 

One may conclude this chapter by giving in Table 83 a list of those systems in which grain-boundary diffusion can play an important part. There are undoubtedly many others which have not been studied, or have been inadequately studied, and a number of properties of structure-sensitive diffusion not yet revealed. 

Very recently, considerable effort has been devoted to the simulation of the oscillatory behavior which has been observed experimentally in various surface reactions. So far, the most studied reaction is the catalytic oxidation of carbon monoxide, where it is well known that oscillations are coupled to reversible reconstructions of the surface via structure-sensitive sticking coefficients of the reactants. A careful evaluation of the simulation results is necessary in order to ensure that oscillations remain in the thermodynamic limit. The roles of surface diffusion of the reactants versus direct adsorption from the gas phase, at the onset of selforganization and synchronized behavior, is a topic which merits further investigation. 

As in the case of the diffusion properties, the viscous properties of the molten salts and slags, which play an important role in the movement of bulk phases, are also very structure-sensitive, and will be referred to in specific examples. For example, the viscosity of liquid silicates are in the range 1-100 poise. The viscosities of molten metals are very similar from one metal to another, but the numerical value is usually in the range 1-10 centipoise. This range should be compared with the familiar case of water at room temperature, which has a viscosity of one centipoise. An empirical relationship which has been proposed for the temperature dependence of the viscosity of liquids as an Arrhenius expression is... 

The continuous decrease in the propane dehydrogenation activity, with constant HD formation rate can have different origins. Calculations indicate that diffusion limitations may play a role in the propane dehydrogenation, but not in the H2-D2 reaction, when the amount of coke in the pores of the catalyst is high. Different structure sensitivity for the two reactions might also contribute to this effect. Somorjai [8] showed that the H2-D2 reaction is structure sensitive. For the propane dehydrogenation, on the other hand, fiiloen et al. [9] found that only one Ft atom is necessary for the reaction to proceed. 

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