Thermal and electrical conductivity and magnetoresistance of graphite films prepared from aromatic polyimide films

Highly crystallized graphite films were prepared from carbonized films derived from commercially available aromatic polyimide films by simple heat treatment at temperatures exceeding 3000 °C. Thermal conductivity κ along the longitudinal direction of each graphite film was measured at room temperature in air by a steady-state method. Measurements of the electrical conductivity σ along the longitudinal direction of each graphite film at room temperature, the ratio of the resistivity ρRT along the longitudinal direction of each graphite film at room temperature to that at 77 K ρ77 K, ρRT/ρ77 K, and the maximum transverse magnetoresistance Δρ/ρ)max at 77 K in a magnetic field of 1 T were also carried out. In these measurements, commercially available graphite sheets, pyrolytic graphite with high thermal conductivity, and pure Cu and Al were included for comparison. Measured values of κ were corrected using the known values of κ for Cu and Al and the catalogue values of the commercially available graphite sheets and the pyrolytic graphite. The corrected values of κ for the present highly crystallized graphite films were comparable to those of the commercially available thermally conductive graphite sheets. Good correlations between κ and σ as well as those between κ and (Δρ/ρ)max and also κ and ρRT/ρ77 K were obtained. In all the curves of κ vs. σ, κ vs. ρRT/ρ77 K and κ vs. (Δ ρ/ρ)max, κ tends to saturate in the region with high κ values. The dominant conduction mechanism of phonons is considered to change from phonon scattering at the boundaries of crystallites in the region with lower κ values to phonon–phonon interaction (three-phonon process) in the region indicating saturation of κ.