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M. S. Dresselhaus Department of Electrical Engineering and Computer Science and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

M. Endo Department of Electrical Engineering, Faculty of Engineering, Shinshu University, Nagano 380, Japan

J-P. Issi, L. Piraux, and V. Bayot Unite de Physico-Chimie et de Physique des Materiaux, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium (Received 18 October 1990; accepted 11 December 1990)

The transport properties of isotropic pitch-based carbon fibers with surface area 1000 m2/g have been investigated. We report preliminary results on the electrical conductivity, the magnetoresistance, the thermal conductivity, and the thermopower of these fibers as a function of temperature. Comparisons are made to transport properties of other disordered carbons.

I. INTRODUCTION

Activated carbons are characterized by their extremely high porosity and their large specific surface area, which can reach several thousands m2/g. As a result, activated carbons are useful in applications where a large surface is required, such as filtration for anti-air pollution systems, the cleaning of drinking water, and blood filtering. Another potentially far-reaching application is to use activated carbons as the electrode material of new supercapacitors.1"3 Many techniques have been used to characterize activated carbons, such as electron microscopy,4 x-ray scattering, gas adsorption,5 and mercury porosimetry.5 From these analyses, the pore size distribution of the material is derived, and the pores are classified into micropores with diameters smaller than 2 nm, mesopores with diameters between 2 nm and 50 nm, and macropores larger than 50 nm. In this paper, we characterize activated carbons using transport properties. We take advantage of the recent fabrication of activated carbon in fibrous form. Previously, activated carbon had been fabricated only in the form of grains or particles, which made quantitative transport measurements of this material impossible. Transport properties can be used as a useful tool to characterize the activated carbon fibers (ACFs). Indeed, transport properties provide a measure of the relaxation time of the carriers (whether electrical charge carriers or heat carriers), yielding information on the density as well as the nature of defects. We expect two kinds of defects in ACFs: boundary scattering and scat778 http://journals.cambridge.org

J. Mater. Res., Vol. 6, No. 4, Apr 1991 Downloaded: 22 Feb 2015

tering caused by the bulk defects related to the nongraphitic nature of the carbon material. Nonactivated carbon fibers have typically a specific surface area of 10 m2/g, so that boundary scattering is negligible in these fibers. Boundary scattering, however, plays a significant role in ACFs, because of their huge specific surface areas. Whereas electron scattering is affected by local defects, phonons are scattered mainly by extended defects, such as dislocations, grain boundaries, and other boundaries, such as pores and