Diffusion equations expressed in molar fractions: Theory and application to ionic diffusion and demixing.

Abstract

Molar fractions are used in applied diffusion kinetics for incorporating thermodynamic and kinetic databases. Molar quantities (molar concentration and molar flux) and reduced molar quantities (molar fraction and reduced molar flux) usually are regarded to be equivalent; i.e., molar quantities are replaced with their reduced forms. However, as the fluxes are related to material properties, the diffusion equations expressed in molar fractions are not consistent with the normalization condition of molar fractions. We develop diffusion kinetics consistent with this condition. Our method is applicable to diffusion with total reduced flux, such as diffusion in an external field or flow field. As two case studies, the developed method is used to investigate ionic diffusion in an electrolyte solution and ionic demixing in a semiconductor oxide.