tailieunhanh - Diffusion Solids Fundamentals Diffusion Controlled Solid State Episode 3 Part 2

Tham khảo tài liệu 'diffusion solids fundamentals diffusion controlled solid state episode 3 part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Gas Permeation 529 Permeation of gases through glasses is of technological interest. If the diffusing species is a gas it is possible to expose one face of a glass membrane thickness Ax to a known pressure of the gas whilst the other side of the membrane is connected to a mass spectrometer. In one method the permeating gas is continuously removed from the spectrometer side by pumping and maintaining a pressure difference Ap across the membrane and the steady state diffusion flow of gas J through the membrane is measured. From such experiments the permeability K can be determined via K JAXP- If one assumes that the gas concentration in the glass C is given by Henry s law C Csp where Cs is the ambient pressure solubility and p the gas pressure the permeability can be written as K DCs where D is the diffusivity of the gas. Usually diffusivity and solubility are both Arrhenius activated. Gas Permeation A number of gases permeate through glasses at rates which can have serious consequences for practical applications. Helium can readily permeate through many glasses used for vacuum tubes. Hydrogen permeation can result in coloration of glasses by the reduction of ions to a lower valency or to the metallic state and by the reaction with optically active defects. Oxygen permeating through the wall of an electric lamp can react with filament material causing failure of the bulb. Permeation rates of gases in vitreous silica are shown in Fig. . The data indicate that the permeability decreases as the atomic or molecular diameter of the diffusing species increases. The permeability decreases in the order He H2 Ne N2 O2 Ar Kr. The trend for He Ne and hydrogen isotopes in all glasses is similar as in vitreous silica. Permeation varies linearly with the partial pressure of the gas for pressures up to many atmospheres. The effect of glass composition on helium permeation has been studied for a variety of oxide glasses including silicate borate germanate and