Carbon black is a semiconductor material, and its electrical properties are usually expressed by electrical conductivity or its reciprocal resistivity. The conductivity of carbon black is closely related to its microstructure, particle size, structure, and surface properties.
The microstructure of carbon black imparts electrical conductivity to carbon black. The electrical properties of natural graphite single crystals or highly oriented pyrolytic stones are similar to semi-metallic properties, that is, the energy band between its valence electrons and conductive electron bands is very low, less than 0.04eV. Due to the orientation of the graphite layer, there is also obvious anisotropy in terms of electrical properties. The specific resistance of graphite single crystal in the axis direction is about 5X 10-8 • cm, but the graphite layer is in the Greek diameter. The Forbe resistance in the axial direction is 5 X 10-9 • cm, which is 10 times higher. For carbon black aggregates, due to the concentric nature of the graphite layer orientation, most of the contacts are along the C-axis direction, and its resistivity is higher than that of graphite.
The physical and chemical properties of carbon black are related to its electrical conductivity. Generally speaking, the smaller the particles, the better the conductivity. This is due to increasing the number of carbon black particles per unit volume, thereby increasing the contact point or reducing the particle distance in the dispersion system, reducing the resistance and improving the conductivity. Increase. Fine particle carbon black has high electrical conductivity. The structure of carbon black is also the most important factor affecting the conductivity of carbon black. High-structure carbon black has better conductivity than normal structure or low-structure carbon black. This is obviously due to the existence of the chain branch structure (fibrous structure) of carbon black, and the interweaving connection forms more conductive paths. Volatile matter or residual tar-like substances on the surface of carbon black (that is, solvent extracts) cover the surface of carbon black with a film of oxygen-containing compounds to form an oily molecular film, forming an insulating layer, increasing the resistance of carbon black, and making the conductivity reduce. After the carbon black is heated in a vacuum or in an inert atmosphere to remove oxygen-containing groups and oily substances, the conductivity will increase significantly. Carbon black surface roughness, ie porosity, also affects the conductivity of carbon black. The conductivity of carbon black with rough and porous surface increases, because when the filling amount is constant, the particle distance of porous carbon black particles is smaller than that of solid particles. Therefore, carbon black with good electrical conductivity should have the characteristics of fine particles, high structure, pure surface, rough and porous.
The measurement of carbon black resistance is generally carried out in a compressed state. Because the volume of uncompressed powdered carbon black is variable and contains up to 90% of the void. There are still about 80% voids in the granulated and dense carbon black, and most carbon blacks have a void volume of about 40% based on particle accumulation. There are so many voids in carbon black, and it is variable, and its specific resistance cannot be measured accurately without a certain degree of compression. Compression is mainly to eliminate excessive voids in carbon black, increase the contact between aggregates or reduce the distance between particles, so the conductivity of different carbon blacks can be compared under certain pressure conditions. In general, the resistivity of compressed carbon black decreases with increasing applied pressure (pressure).