Role of grain boundaries in the dielectric behavior of graphite

Electric-polarization-causing carrier-atom interaction occurs in graphite at grain boundaries, as indicated by the electric permittivity (the main dielectric property) being linearly related to the reciprocal of the grain size. A smaller grain size increases the permittivity by increasing the fraction of carriers that participate in the polarization and by facilitating the carrier hopping from one grain boundary to an adjacent one during polarization. The linearity further means that the strength of the carrier-atom interaction is independent of the grain size. Specifically, the relative permittivity (2 kHz) of isotropic graphite increases from 380 to 5100 with decreasing grain size (coke raw material particle size) from 12 to 1 μm. The fraction of carriers that participate in the polarization (≤7.8 × 10⁻¹¹) also decreases with increasing grain size, but is less sensitive to and less well-correlated with the grain size than the permittivity. Extrapolation of the abovementioned linear curve to infinite grain size gives relative permittivity 69 (corresponding to the fraction of carriers that participate 1 × 10⁻¹¹). These two values are much smaller than those for finite grain sizes, indicating that the permittivity and participating carriers are essentially completely due to the carrier-atom interaction at the grain boundaries. The grain size influences the resistivity slightly.