Thermodynamics and the optimization of vapour phase transport

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

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

The optimal choice depends on the total pressure of the system, and on the stoichiometry of the reaction. As an example, the transportation 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 

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