Crystal-grain size, phonon and carrier mean free paths in the basal plane, and carrier density of graphite films prepared from aromatic polyimide films

The electrical and thermal conductivities in the basal plane of graphite films prepared from aromatic polyimide films were evaluated with reference to previously measured electrical and thermal conductivities along their longitudinal directions at room temperature with the aid of magnetoresistance anisotropy measurement at 77 k. The average crystal-grain size of each graphite film was obtained from the relation between thermal conductivity and grain size originally derived by Klemens and Pedraza. It was found that at room temperature the crystal-grain boundary scattering of phonons could dominantly occur for crystal-grains with sizes smaller than ∼520 nm or the average d002 value of grains larger than ∼0.3357 nm. When the grain size exceeds the intrinsic phonon mean free path in a single graphite crystal, i.e. ∼1760 nm, or the average d002 value of the crystal-grains is less than ∼0.3357 nm, the three phonon process in thermal conduction becomes dominant. The mean free paths of carriers were found to be much shorter than those of phonons and it was shown that at 77 k grain sizes smaller than ∼220 nm could cause boundary scattering of carriers. The total carrier density at 77 k evaluated from the magnetoresistance and electrical conductivity data shows a decrease from the value of a single crystal of graphite as the crystallinity of the graphite film is reduced.