Predicting the rate of chemical transport using the flux function method

A systematic approach to the selection of independent variables for a theoretical analysis of the rate of chemical transport is discussed. The methodology is based on the use of the transport flux function, which enables optimum conditions to be found on the basis of a minimum number of the variables most likely to have the greatest effect on the transport rate. A more precise quantitative comparison between experimental and theoretical results can be made by taking into account the effects of convection, uncertainties in thermochemical data, and differences in the diffusion properties of vapour species. The latter effect can lead to a redistribution of components in the vapour, and this may have important consequences in certain CVT processes. The theoretical methodology is exemplified by consideration of transport of CoP₃ in the Co-P-Br system, FeP in the Fe-P-I system, and YIG in the Y-Fe-O-Cl and Y-Fe-O-HCl systems. The complex behaviour revealed would probably have been difficult to rationalize on the basis of experimental studies alone.