Factors that govern the electric permittivity of carbon materials in the graphite allotrope family

Abstract

Through comparing carbons in the graphite allotrope family, namely isotropic graphite (IG), exfoliated-graphite-based flexible graphite (FG), graphite-flake thick film (not sintered), carbon fibers (PAN, pitch) and highly-oriented pyrolytic graphite (HOPG), all differing in the microstructure, this work addresses for the first time the factors (including the structure-property relationship) that govern the electric permittivity (kHz frequency) of carbons. The permittivity is more sensitive to the microstructural heterogeneities than the resistivity, which only slightly influences the permittivity. Smaller crystallite dimensions promote the permittivity. PAN-based carbon fibers exhibit higher permittivity than pitch-based carbon fibers, IG, FG or HOPG. The crystallographic preferred orientation is not influential to the permittivity, though it influences the resistivity. Interfaces (e.g., the grain boundaries in IG and the interface between the carbon layer ribbons in a fiber) promote the permittivity, being sites for the carrier-atom interaction that enables the polarization. A large number of sites is more important than a large site size for promoting the permittivity. The fiber diameter is not influential to both permittivity and resistivity. The thick film exhibits the highest relative permittivity (5 × 105), due to its low electrical connectivity. IG (grain size 12 μm) gives the lowest relative permittivity (5 × 102), due to its coarse microstructure.