Elizabeth J. Opila

The Surface Diffusion of High Temperature Vapors in Porous Alumina

In porous alumina barriers ~ 1 mm thick, 45% porosity, and ~1.2 pm average pore diameter, transmission coefficients for NaCl, NaF, and LiF vapors are ~2.6, 6, and 12 times that of He, indicating that surface diffusion contributes to the transmission. For Zn vapor, transmission coefficients are 1.2–3.5 that of He through the ~1.2 µm pores and 18 times that of He through 0.14 p.m pores. The temperature dependences of LiF and NaCl transmission by surface diffusion are close to those for Knudsen effusion, a result that indicates that the activated complex for surface diffusion is very weakly bound to the surface. Above their melting points, the transmissions of LiF and NaCl are irreproducible, but are usually markedly increased, perhaps because a liquid-like film partially penetrates the barrier.