Vapour phase transportation

The process of sintering by vapour phase transport is probably important to the growth of single crystals of solids from polycrystalline samples in a closed system, where the vapour species are the transporting gas, which may have the same or different chemical composition from die solid. An example where... 

There is a qualitative distinction between these two types of mass transfer. In the case of vapour phase transport, matter is subtracted from the exposed faces of the particles via dre gas phase at a rate determined by the vapour pressure of the solid, and deposited in the necks. In solid state sintering atoms are removed from the surface and the interior of the particles via the various diffusion vacancy-exchange mechanisms, and the centre-to-cenU e distance of two particles undergoing sintering decreases with time. 

Both these diffusion controlled and the vapour phase transport processes may be described by tire general equation... 

Figure 3.1 (a) (b) The efficiency of 1 1 and 1 4 vapour phase transport reactions, showing the marked dependence of the optimum for the 1 4 reaction on the pressure, (c) The dependence of the 1 2 and 1 4 reaction Gibbs energy on temperature and pressure, showing that the formation of nickel carbonyl is favoured by high pressure, and that of zirconium tetra-iodide, which is much more stable, is favoured by low pressures... 

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