Grain growth by vapour phase transport

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 the solid. An example where 

It is clear that the achievement of equilibrium is assisted by the maximum contact between the reactant and the transporting gas, but the diffusion problem is complex, especially in a temperature gradient, when a process known as thermal diffusion occurs. The ordinary concentration-dependent diffusion process occurs across the direction of gas flow, but the thermal diffusion occurs along the direction of gas flow, and thus along the temperature gradient. 

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