Microtexture and Crystallinity of Highly Crystalized Graphite Films Prepared from Aromatic Polyimide Films

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Abstract
High quality graphite films have been prepared from carbonized aromatic polyimide films by short time heat treatment at 3100 and 3200°C without any pressurization. The starting materials were Kapton and Novax with 25μm thick and a high-modulus polyimide film (PPT), prepared via polyamic acid gel, with 45μ thick. Microtexture and crystallinity of these graphite films were investigated by the measurements of X-ray diffraction, residual resistivity ratio (RRR), magnetoresistance and Hall coefficient at temperatures of 77 and 4.2 K, and by the observation of electron channeling contrast image in scanning electron microscope. From these experiments, interlayer specing d002, mosaic spread MS, magneto-resistance anisotropy ratio r, mean free path of majority carriersλ at 4.2 K and mean crystal grain size D were evaluated for the graphite films. The graphite films showed high crystallinity (d002=0.3354 nm and RRR=2.67-3.45), and were found to consist of large crystal grains (D=8μm), which were well oriented parallel (MS=5.7-6.9°and r=0.0113-0.0173). Especially, the graphite films prepared from Kapton and PPT showed the highest reported values of maximum transverse magnetoresistance (Δρ/ρ) max at 77 K in the field of 1T (12.54 and 12.06, respectivily), and their λ values were also large (3.5 and 2.6μm, respectively). They exhibited Shubnikov de-Haas oscillation at 4.2 K in (Δρ/ρ) max and very clearly in RH. These results obtained indicate that the graphite films can be comparable with highly oriented pyrolytic graphites or high quality pyrolytic graphites in crystallinity.

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