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Transactions of the Japan Institute of Metals, Vol. 14 No. 4 (1973)
pp.303-308

© 1973 The Japan Institute of Metals

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Effective Knudsen Diffusivity through a Porous Solid

Yoichi Maru^{*} and Yoshio Kondo^{*}

In order to measure the effective diffusivity of gas in a porous solid, the permeability was measured by using the pressure gradient technique in the pressure range between 10^{-3} mmHg and atmospheric pressure. The specimen employed was a disc-shaped porous metallic molybdenum pellet prepared by the thermal decomposition of molybdenite pellet at 1290^{°}C in vacuum.

Below the pressure of 1 mmHg, the measured permeability converges into constant values which are 1.98, 2.40 and 6.51 cm^{2}/sec at 15^{°}C for Ar, N_{2} and He gases, respectively. These values correspond to the effective Knudsen diffusivities of each gas and they satisfy the relationship that the effective Knudsen diffusivity is inversely proportional to the square root of molecular weight of diffusing gas. From these results, the mean pore radius is calculated at a = 1.11 µ according to the random pore model of monodisperse pore distribution.

The effective Knudsen diffusivity of diatomic sulfur gas at 1290^{°}C in the same pellet was estimated at 3.60 cm^{2}/sec.
This value is in excellent coincidence with the estimated diffusivity of 3.52 cm^{2}/sec which was calculated from the thermogravimetric data on the assumption that the diffusion of diatomic sulfur gas through the porous metallic molybdenum layer determines the overall rate of thermal decomposition of MoS_{2} pellet.

(Received October 18, 1972)

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