The diffusion of moisture in sorbent solids—I. Diffusion cell and pore diffusion

A complete theory of diffusion of absorbing gases in porous solids is developed. Both ordinary and thermal diffusion are considered. The thermodynamics of irreversible processes is used to derive the general flux equations.A new theoretical model of surface diffusion is presented. Moreover, the proper method of combining surface and gas-phase fluxes is established. The total flux in the pores is represented by a 5 parameter model, which includes the effects of (1) free (gaseous) diffusion, (2) Knudsen diffusion, and (3) surface diffusion.The design of a new diffusion cell is presented along with a stagewise method for analyzing results. The cell is unique in that specimens mounted in copper clad circuit board material permits nonisothermal as well as isothermal operation. Earlier designs were confined to isothermal operation due to mounting specimens in metallic materials such as brass and aluminum.