J.F. Marêché

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 σ 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 σ 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 σ versus Φ, where Φ 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.

Composites based on micron-sized exfoliated graphite particles: Electrical conduction, critical exponents and anisotropy

Abstract Electrically conducting composites, made of epoxy resin and micron-sized graphite platelets with diameter-to-thickness ratios of the order of 100 are presented. Samples of various volume percentages of graphite particles are characterized by X-ray diffraction and electron microscopy: orientation of the filler and porosity of the polymer matrix are observed. DC conductivity measurements have been made both within and perpendicular to the plane of the film. Use of percolation theory shows that taking into account the swelling effects of the composite occurring during reticulation yields quasi-universal values for the conductivity critical exponent in both directions and a low critical volume concentration of Φ c = 1.3%.

Electrical conductivity of anisotropic expanded graphite-based monoliths

The electrical conductivity of cubic samples (monoliths) made of moderately compressed expanded graphite has been measured along two orthogonal directions, namely parallel and perpendicular to the pressing force. It is found that these highly porous materials exhibit a percolation behaviour, with a transition from insulator to conductor occurring at a very low critical density. Besides, the monoliths are found to be quite isotropic as their porosity is high; conversely, increasing their apparent density makes them become more and more anisotropic thanks to the orientation of their constitutive graphite particles. Both percolation and effective media theories are shown to accurately fit the conductivity data as far as isotropic materials are concerned. Microstructural parameters derived from the fits are discussed and found to be in fair agreement with the expected description of compressed expanded graphite, i.e. insulating voids surrounded by thin conducting flakes.

Conduction mechanisms in some graphite - polymer composites: the effect of a direct-current electric field

This paper deals with the possible conduction mechanisms in highly anisotropic composites comprising 0 - 10 vol% graphite flakes within a polymer host. Conductivity measurements as a function of DC electric field have been made. In most cases, a non-linear behaviour of the current - voltage relationships is observed. A number of theoretical models are considered and we show that none of them is, by itself, able to explain our results. We further develop several arguments which lead us to consider the existence of a combined tunnelling effect and ionic conduction mechanism.

Porous electrodes-based double-layer supercapacitors: pore structure versus series resistance

Abstract The correlation between the porous structure of electrodes designed for double-layer supercapacitors and the series resistances of the latter is addressed in this paper. It is shown that in such composite films, made of active carbon embedded in a porous polymer, the internal electrolyte resistance within the pores governs the series resistance. In order to evidence this, various compositions of polymer–carbon mixtures are manufactured. Their porosities are determined, both before and after impregnating them with the solvent of the electrolyte. For the compositions under study, the nature of the solvent is found to induce very different effects on the permeability of the films. However, the transport properties within the pore space of the composites are also investigated via ion diffusion experiments. Whatever the solvent, a strong correlation is found between formation factors of the porous films and the series resistances of the final double-layer supercapacitors.

Preparation, electrical and elastic properties of new anisotropic expanded graphite-based composites

Abstract New composite materials with application to catalyst supports or adsorbents are presented. They are made of compressed expanded graphite of various densities first impregnated by polyfurfuryl alcohol and next pyrolyzed and activated. The resultant materials are monoliths comprising a graphite backbone coated by a thin layer of active carbon. The electrical conductivity and the dynamic elastic moduli are measured on each kind of material, namely before and after carbonization, and finally after activation. The results are shown to be consistent with a percolation phenomenon: the conductivity and the rigidity thresholds are derived, and several theoretical considerations and comparisons with pure expanded graphite are made. The discussion leads to a better understanding of the structure of the materials before and after impregnation, namely the graphite backbone and the graphite–polymer or carbon composites. Besides, their conductive and elastic properties are shown to be very good. Hence, the materials are expected to have fair thermal conductivities, to be electrically regenerable (application as adsorbents) and to have an interesting life time (application as catalyst supports).

Surface area of compressed expanded graphite

In a previous work [Carbon (2002) in press], the densification of expanded graphite was modelled. This very light material consists of worm- or accordion-like particles in a loose packing. Worm-like particles are made up of thin disc-shaped graphite sheets, and the average disorientation angle of the thin discs was estimated based on the excluded volume concept. Then, using a simple picture of particle densification, the mosaicity of compressed expanded graphite was calculated and compared with the results of XRD experiments. In the present paper, the same model and assumptions are taken into account for predicting the surface area variations of two kinds of expanded graphite undergoing compaction. The constitutive graphite sheets are assumed to bend and to make flat contacts with their neighbours, thus decreasing the surface of the material. It is worth noting that the proposed modelization is free of any adjustable parameter. The calculations are shown to be in good agreement with experimental results obtained from krypton adsorption at liquid nitrogen temperature.

Densification of expanded graphite

A brief theoretical investigation of the behavior of exfoliated graphite (EG) undergoing compaction is presented. Simple arguments and assumptions allow one to calculate the density of the initial individual worm-like particles of EG for any final porosity of the resultant consolidated blocks. These results as well as excluded volume considerations are used to derive the mean disorientation angle of the graphite sheets within the blocks of compressed EG. X-ray experiments were performed on such autoconsolidated samples. The overall mosaic spreads thus obtained are shown to be in a very good agreement with the model developed in this paper.

De nouveaux isotypes de U2Co3Si5 ou Lu2Co3Si5 dans les systems R-T-Ge (R = Elements Des Terres Rares; T = Ru, Co, Rh, Ir). Supraconductivite de Y2Ir3Ge5

New ternary germanides R2T3Ge5 have been prepared at 1173 K. From their X-ray powder diagrams, some of them are isostructural with U2Co3Si5: R2Ru3Ge5 (R = Y, La-Sm, Gd-Tm), Ce2Co3Ge5, R2Rh3Ge5 (R = La-Pr) and R2Ir3Ge5 (R = Y, La-Sm, Gd-Dy); others crystallize with the Lu2Co3Si5 - type structure, a monoclinic deformation variant of the U2Co3Si5 type: R2Co3Ge5 (R = Y, Pr-Sm, Gd-Er) and R2Rh3Ge5 (R = Y, Nd, Sm, Gd-Tm). The Lu2Co3Si5-type structure of Ho2Rh3Ge5 was refined from single-crystal X-ray data to a conventional R value of 0.034 for 371 independent reflections (Rw = 0.034). Superconductivity above 1.3 K was found only in Y2Ir3Ge5.

Synthesis and electrical properties of some new ternary graphite intercalation compounds

Abstract New ternary graphite intercalation compounds (GICs) of general formula MBi x C 4n (M=K, Rb, Cs; x∼-0.6 ; n=stage) have been synthesized. The electrical resistivity, both parallel (π a ) and perpendicular (π c ) to the basal planes has been studied from 1.4 to 295 K. All in-plane behaviour is metallic ; the c axis conductivity is thermally activated except for the richest compounds. The cesium compounds are superconducting.