Theory of the time-lag diffusion method for the case of an outgassing solid

Abstract

In the ordinary application of the time-lag method to the measurement of the diffusion coefficient of a gas passing through a plane sheet of an inert solid, the gas is pressurized on one side of the sheet and evacuated on the other. After decay of transients, the cumulative amount,Q(t), of gas diffused through the sheet in time,t, assumes the “time-lag” form,Q(t)=A(t – L). Measurements of the slope,A, and the intercept,L, can be used to determine the diffusion coefficient and the solubility of the gas in the solid. We have rederived this law for the case of a solid that is actively evolving this same gas at an arbitrary rate and have used it to predict the rate of outgassing of the solid upon standing. Practical applications of the theory include radioactive decay of minerals, rejection of plasticizers by plastics, and the decomposition of solid rocket propellants.