Vapour phase transport

Vapour phase transport consists in the formation of a volatile, instable, intermediate species which contains at least one of the components of the final product. 

Schematically, in its simplest version, the reactant X (to be transported) is placed at the end of a vacuum sealed tube together with a small quantity of the transporting agent Z. Inside the tube a temperature gradient must be maintained. 

The transport depends on the existence of a reversible reaction such as  

The related formation equilibrium constant should have a small value if it is zero no XZ would form if, on the other hand, it is very large the stable XZ would not re-decompose and no X re-deposition could be obtained. Notice that if the synthesis of XZ is endothermic, the equilibrium will be displaced to the right with increasing temperature (the opposite is true if the reaction corresponds to an exothermic formation of XZ). Therefore, in order to have the transport of X, (synthesis of the intermediate at one end of the tube and re-decomposition with deposition of X at the other end) this must be placed at the hot end if the formation of XZ is endothermic (or the cold end, if exothermic). 

A classical example of this process is given by the van Arkel method for the preparation (purification) of several metals. If impure Cr, for instance, is contained together with a small quantity of iodine in a vacuum tube maintained at a temperature at which chromium iodide volatilizes, and a hot zone is created by means of, say, a W filament heated by an electric current, the following reaction will be observed  

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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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