Measurement of electrical properties of a carbon black

Abstract

The direct measurement of electrical properties of a high-surface-area turbostratic carbon black by microwave techniques is reported. This approach obviates many difficulties associated with powder samples and allows absolute values of conductivity and mobility to be obtained, from which charge carrier concentrations can be calculated and compared to those obtained by ESR methods. After a treatment in Ar at 1223 K to remove sulfur, this carbon had conductivity σ = 2.4 (Ω · cm) −1, mobility μ = 5.4 cm 2/V · s, and a carrier concentration of 1.5 × 10 18 g −1 at 300 K. The adsorption of O 2 at 300 K increased σ, decreased μ, and increased carrier concentration to a small extent. After oxidizing this carbon in HNO 3, all three parameters were decreased slightly and adsorbed O 2 had little effect. However, the concentration of charge carriers in both carbon blacks was 10–40 times lower than the conduction carrier concentrations determined from a typical analysis of ESR signal intensity obtained at different temperatures. These preliminary results indicate that holes are the primary charge carriers in these carbon materials, which have small crystallite size and a small extent of graphitization. Oxygen chemisorbed on the pre-cleaned carbon black surface increases the concentration of both localized spin centers and holes and creates a higher electrical conductivity. Intensive oxidation by HNO 3 increases the number of localized spin centers but decreases the concentration of holes, thus also decreasing conductivity. A comparison of these microwave and ESR results implies that the traditional way of identifying and quantifying conduction charge carriers from the temperature-dependent susceptibility of ESR signals may not be satisfactory.