Microtexture and Crystallinity of Highly Crystallized Graphite Films Prepared from Aromatic Polymide Films II

Graphite films with very high crystal perfection were prepared from carbonized aromatic polyimide films by successive heat treatments at 3100°C for 40 min and then 3200°C for 23 min without mechanical stress. The starting materials were a commercially available Kapton with 25 μm in thickness and laboratory prepared high-modulus polyimide films via polyamic acid gel with 18 and 45 μm in thickness. Crystallinity and microtexture of the graphite films were examined by the measurements of X-ray diffraction, maximum transverse magnetoresis-tance (Δρ/ρ) max and Hall coefficient RH at 4.2 K, and by the observation of electron channel-ing contrast image (ECI) by scanning electron microscope. The ratios of peak intensities of 101 diffraction peak to 100 diffraction peak, I101/I100, were 0.000-0.037, mean free path of carriers, λ, estimated from (Δρ/ρ) max at 4.2K, 0.1T were 5.7-6.5μm, and mean crystal grain diameters D evaluated from ECI were 11-12 μm for the graphite films. Quite large (Δρ/ρ) max was observed at 4.2 K for the graphite films. The Shubnikov-de Haas oscillations were also observed clearly at 4.2 K in (Δρ/ρ) max and especially in RH, and the Landau levels of n=2-6 were found. The graphite films obtained can be comparable with an HOPG sample heat treated at 3600°C under high mechanical pressure (HOPG3600) in crystallinity and degree of c-axis orientation of graphite layers. These results indicate that the crystallinity and c-axis preferred orientation of graphite films can be improved remarkably by extention of the heat treat-ment time at high temperatures without mechanical stress. There are differences between the electronic properties of HOPG3600 and these of the graphite films because of their different microtextures i.e. different defect distributions.