Electrical conductivity of carbonaceous powders

The present paper deals with the electrical conductivity of a number of carbonaceous powders undergoing a low compaction. It is shown that the contributions to the conductivity of such samples are too numerous and too complex to be solved exactly. Hence, several criteria are proposed in order to quantify the behaviour of moderately compressed powders, and thus derive several useful parameters characterizing their grains. Thus, their morphological characterization is shown to be feasible from such electrical measurements. For that purpose, a known weight of material is poured into a thick glass cylinder and is moderately compressed between two stainless steel pistons. A number of different carbonaceous powders having various morphologies, from almost spherical to disk- or needle-shaped, have been investigated. During the compaction, both the electrical conductivity sigma and the volume of the sample are simultaneously measured. The results are found to depend strongly on the initial height h of the powder column. The non-monotonic behaviour of sigma versus h is studied for each available material. Arguments are given for retaining only the sample size which leads to the highest conductivity. The so-called general effective media (GEM) equation is then applied to the corresponding data of sigma versus Phi, where Phi is the volume fraction of grains in the column. In this context, the grains are assumed to behave like either oblate or prolate spheroids, depending on the material. The fitting procedure requires the knowledge of both the apparent density of the powder in the non-compacted state, and that of the grains, and thus uses only two adjustable parameters. For each material, average aspect ratios are derived from the fits of the GEM equation and are shown to be strongly consistent with the expected morphologies. (C) 2002 Elsevier Science Ltd. All rights reserved.