Vapour phase transport processes

In most circumstances, it can be assumed that the gas-solid reaction proceeds more rapidly than the gaseous transport, and therefore that local equilibrium exists between the solid and gaseous components at the source and sink. This implies that the extent and direction of the transport reaction at each end of the temperature gradient may be assessed solely from thermodynamic data, and that the rate of transport across the interface between the gas and the solid phases, at both reactant and product sites, is not rate-determining. Transport of the gaseous species between the source of atoms and the sink where deposition takes place is the rate-determining process. 

The choice of die transporting reagent for a given material is made so diat die reaction is as complete as possible in one direction, in die uptake, and die reverse reaction in die opposite direction at die deposition site. This requires diat not only die choice of die reagent, but also die pressure and temperature ranges under which die reaction is most effectively, or quantitatively, performed, must be calculated (Alcock and Jeffes, 1967 1968). There will always be limitations placed on diis choice by die demands of die chemical ineruiess and temperature stability of die containing materials in which die reaction is canied out. 

These considerations apart, die selection of die optimum conditions for die performance of a transporting reaction requires die choice of die best average value of die equilibrium constant. The effect of die range in die 

The optimal choice depends on the total pressure of tire system, and on tire stoichiometty of tire reaction. As an example, the uansportation of zirconium as the tetra-iodide is made at low pressure, while the purification of nickel by tetracarbonyl formation is made at high pressure. These reactions may be written as 

Botli reactions involve the formation of a vapour-uatisporting species from four gaseous reactant molecules, but whereas the tetra-iodide of zirconium is a stable molecule, the nickel teU acarbonyl has a relatively small stability. The equilibrium constatits for these reactions are derived from the following considerations  

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

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