Intrinsic Electrical and Thermal Conductivity of Carbon Black

Carbon black materials are semiconducting with intrinsic electrical resistivity values between 10¹ and 10~² О cm. Figure 5 shows the electrical resistivity of carbon black grades under compression at the corresponding compression density of the sample. For all carbon black types, the intrinsic electrical resistivity is decreasing with increasing applied pressure. A certain electrical resistivity level is usually achieved at lower volume density of the carbon black material the higher its structure is. Elevated resistivity values at high compression density observed for some of the carbon black materials could be explained by the microstructure or the high volatile content.

The influence of the particle size on the intrinsic conductivity of carbon black does not seem to be established, but porosity plays a role once the porosity level is very high: Ultra-conductive carbon black composed of hollow particles shows a higher intrinsic electrical conductivity as it could be expected from its specific volume. On the contrary, the high carbon black structure of conductive carbon black causes high specific volume and consequently low intrinsic conductivity (Probst 1993). Voet et al. showed that for most carbon black grades, the logarithm of the resistivity is a function of the cube root of the specific volume, the limiting element being the interparticle distance (Donnet and Voet 1976). It was concluded

Table 1 Physical properties of commercial conductive carbon black grades (according to supplier information)

that the resistivity of the carbon black is an exponential function of the average particle distance at prevailing pressure and that the predominant conduction mechanism is electron tunneling.

At very high compression, most carbon blacks approach the same limiting level of conductivity. With increasing pressure, since the aggregates becoming more tightly packed and pressed against each other, and with very low contact resistance, the conduction mechanism is considered to be by direct contact and therefore more graphitic in nature (Hess and Herd 1993). In this regime, due to the mostly concentric nature of the superficial carbon layers in the carbon black particles, most of the contacts are between graphitic basal planes. The electronic conductivity along the graphite basal planes in the chemical structure of graphite is four orders of magnitude higher than perpendicular to them. This explains the higher intrinsic electrical resistivities found for carbon black compared to graphite powders usually having a higher probability of contacts between prismatic edges. Surface oxides and impurities absorbed at the carbon black surface significantly increase the electrical resistance. Due to these reasons, heat-treated carbon black materials show a resistivity minimum at about 1200 °C when all surface oxides are completely desorbed from the surface. Above this temperature, the layers show an increased tendency to form

Fig. 5 Electrical volume resistivity obtained from two-point electrical resistivity measurements of pressed carbon black samples at the corresponding compression density. The carbon black samples were compacted by increasing the pressure in equal increments from 1 to 5 kN cm”² with two brass pistons that at the same time acted as sensing electrodes for the electrical resistivity measurement. The numbers given in brackets in the graph legend indicate the OAN of the corresponding carbon black grades in mL (100 g carbon)”¹

around the aggregates a continuous encapsulating shell consequently causing an increase of the electrical contact resistance.

Compared to graphite powders, carbon black materials show significantly lower level of thermal conductivity ranging from 0.1 to 0.5 W (mK)”¹. Graphitic materials therefore would be preferred usually as carbon filler if a high thermal conductivity was required. The electrical and thermal conductivity of graphitic carbons is described in more details elsewhere in this encyclopedia (? Chap. 20, “Graphitic Carbon Powders for Polymer Applications”).