Molecular Diffusion in Solids

We can broadly classify transport in solids into two types of diffusion diffusion that can be considered to follow Pick s law and does not depend primarily on the actual structure of the solid, and diffusion in porous solids where the actual structure and void channels are important. These two broad types of diffusion will be considered. 

Generally, simplified equations are used. Using the general Eq. (6.2-14) for binary 

Integration of Eq. (6.5-1) for a solid slab at steady state gives 

For the case of diffusion radially through a cylinder wall of inner radius r, and outer and length L, 

EXAMPLE 6J-1. Diffusion of Through Neoprene Membrane The gas hydrogen at TC and 0.010 atm partial pressure is diffusing through a membrane of vulcanized neoprene rubber 0.5 mm thick. The pressure of on the other side of the neoprene is zero. Calculate the steady-state flux, assuming that the only resistance to diffusion is in the membrane. The solubility S of gas in neoprene at 17°C is 0.051 m (at STP of 0°C and 1 atm)/m solid atm and the diffusivityD is 1.03 x 10 ° m /s at 17°C. 

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