Structural and textural changes from polyimide Kapton to graphite: Part II. Magnetoresistance and x-ray diffraction
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Yoshihiro Hishiyama Musashi Institute of Technology, Tamazutsumi, Setagaya-ku, Tokyo, 158 Japan (Received 6 March 1991; accepted 19 December 1991)
Structural and textural changes in carbon films prepared from the polyimide Kapton with heat-treatment temperatures up to 3000 °C were followed by measuring the magnetoresistance at 77 K, the ratio of electrical resistivity at room temperature to that at 77 K, and x-ray powder patterns. The maximum magnetoresistance (Ap//9) m a x as a measure of graphitization changed its sign from negative to positive around 2500 °C and then increased its value abruptly. The resistivity ratio PR.T./PWK increased above 2500 °C treatment. The values of ( A p / p ) m a x and PR.T./P77K after 3000 °C treatment reached 275% and 0.93, respectively. The average interlayer spacing J002 after the 3000 °C treatment was 0.3354 nm, the same as natural graphite. The changes in structure and texture in the film were consistent with the model proposed in the previous paper from observation by transmission electron microscopy.
I. INTRODUCTION
HTT (°C)
Highly crystallized graphite films, some of them being comparable with highly oriented pyrolytic graphite (HOPG), were obtained from heat treatment of polyimide films of Kapton. J~3 The carbonization process of the polyimide Kapton, which has been known to have a high degree of orientation of rigid molecules, was studied in detail4 and compared with another polyimide Upilex which has a twisted molecular structure.5 Three fundamental conditions necessary for obtaining a graphite film were pointed out: (1) flatness of the molecule of the precursor polyimide, (2) their high degree of preferred orientation along the film surface, and (3) simple release of non-carbon atoms during the carbonization process. In order to have a high degree of orientation of the polyimide molecules, it is essential to use thin films. A nongraphitizing nature was observed on a bulk carbon specimen prepared from Kapton.1 In the first part of this series of papers, the textural changes from the polyimide Kapton to graphite were studied using high-resolution transmission electron microscopy techniques and polarized-light microscopy.6 The results are schematically summarized in Fig. 1 by showing the change in nanometric texture on the cross section of the film. The films heat-treated from 1000 to 2500 °C showed porous and weak anisotropy in texture; textural change with heat treatment was discussed in comparison with anthracite.6 These textural changes must be consistent with the structural change observed by other techniques, such as x-ray diffraction and magnetoresistance measurements. 1174 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 5, May 1992 Downloaded: 17 Mar 2015
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FIG. 1. Scheme of textural change in polyimide Kapton film with heat tr
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