Permeation time-lag analysis of “anomalous” diffusion. Part 2.—Helium and nitrogen in graphite powder compacts

Abstract

The results of the first experimental application of the method of time-lag analysis previously developed for the identification and study of different types of anomalous diffusion behaviour are reported. The permeation of He and N₂ in a graphite compact prepared by (uniaxial) compression of successive increments of powder was found to exhibit time-lag anomalies, which were broadly similar to those observed in extensive studies of gaseous permeation through other powder compacts constructed by the same procedure. The analysis covered different temperatures and was extended by varying the method of construction of the plug and by considering transient permeation curves also. The results indicate considerable macroscopic nonhomogeneity of the compact in the axial direction; thus throwing doubt on the view that, in diffusion barriers constructed as above, non-homogeneity on the macroscopic scale cannot be the main cause of the relevant observed time-lag anomalies.