Simple method for characterizing synthetic graphite powders

Abstract

A simple method for characterizing synthetic graphite powders is presented in this paper. A thick glass cylinder contains a known weight of material, which is held between two conducting pistons. These allow the measurement of the electrical conductivity of the sample, and enable its compaction. The graphite powder is gently pressed, and its volume and conductivity variation are simultaneously measured. From these results, the relation between conductivity and the volume fraction of grains is derived: an equation based on the effective medium theory (EMT) is shown to fit the experimental data accurately. The adjustable parameters are directly linked with both the anisotropic conductivity and the morphology of the grains. On the one hand, conductivity measurements achieved on single particles give values of the same order of magnitude as those derived from the fits. On the other hand, the other parameters of the equation perfectly agree with the aspect ratios obtained from apparent density measurements. The percolation thresholds which would be expected from composites made of graphite powders imbedded in an insulating medium are also calculated via the EMT equation. Comparison with other theoretical and experimental values always leads to very good agreement, showing the accuracy of the EMT equation in supplying fair geometrical parameters for the particles.

Microstructure, elastic and electromagnetic properties of epoxy-graphite composites

“…4,5 Here the percolation threshold of composites filled with graphite fillers has been estimated 10,11 according to procedures described in details in Refs. [15][16][17][18][19], based on either percolation or on effective-medium theories. All the size/geometry data of used graphite fillers along with the estimated percolation thresholds are gathered in Table II.…”

Predicting effective permittivity of composites containing conductive inclusions at microwave frequencies

“…Moreover, even assuming that the actual geometry and inclusion size are well characterized, existing percolation theory generally only predicts a range for v c . 13 This range becomes more dubious given the challenge of accurately accounting for certain behavior on a microscopic behavior, such as particle-to-particle interaction or particle alignment, which can alter v c .…”

“…13,15 For our initial fits, we assumed that A should be fixed for a given inclusion aspect ratio, so we set it to different values and determined best-fit values of s and t for each experimental frequency and volume fraction of the SS and iron sphere data. For these calculations, we assumed σ = 1.4 × 10 6 S/m and 1 × 10 7 S/m for SS and iron, respectively.…”

Flocculation of cellulose fibre suspensions: the contribution of percolation and effective-medium theories

“…In a previous work dealing with ellipsoidal particles having an infinite range of sizes and semi-axes a = b = c (Celzard et al 2000), application of EMT to anisotropic particles was shown to lead to the following expression of the percolation threshold:…”

“…In the case of ellipsoids of revolution, called spheroids, i.e., with principal semi-axes such that a = b = c, the depolarization factors obey L a = L b = 1/2 (1 -L c ), and always L a + L b + L c = 1. A figure showing the way L a and L c change with the aspect ratio of the spheroids may be found in Celzard et al (2000). For a prolate ellipsoid, i.e., such that a = b \ c (see Fig.…”