Structural and textural changes from polyimide Kapton to graphite: Part I. Optical microscopy and transmission electron
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M. Inagaki Department of Applied Chemistry, Faculty of Engineering, Hokkaido University, Sapporo, 060 Japan (Received 6 March 1991; accepted 19 December 1991)
Kapton films, 25 fim thick, were carbonized and then graphitized at various temperatures up to 3000 °C. They were studied by optical microscopy and transmission electron microscopy and were prepared by thin-sectioning for transmission electron microscopy. The residual embedded blocks were used as polished sections for optical microscopy. Kapton thin film heat-treated at 650 °C showed thin areas of preferred orientation at its two opposite faces. This orientation extends all over the film thickness between 800 °C and 1000 °C. It corresponds to a statistical orientation of small aromatic layer stacks, parallel to the film plane. From 1000 °C up to 2475 °C flattened pores develop everywhere. At 2475 °C, they suddenly collapse and partial graphitization occurs. The Kapton film is completely graphitized between 2820 °C and 3000 °C. The mechanism of graphitization is similar to that of anthracites and also to that of nongraphitizing carbons heat-treated under pressure.
I. INTRODUCTION The studies carried out on a large range of various carbons are summarized in Ref. 1. They led to a model suitable for most carbons of relatively high HTT (>1200 °C) and particularly for nongraphitizing ones (Fig. 1). These carbons are porous solids the texture of which is that of crumpled and entangled sheets of papers.2 The more marked the crumpling, the smaller the pores and the lower the final degree of graphitization obtained above 2800 °C. This model was verified by stereographic pictures 34 giving direct evidence. Then it was verified by establishing various projections of the model owing to hkl dark-field images and lattice fringes.5-6 Starting from the fact that a pore wall is the flat part of the crumpled paper, the sketch of Fig. 2(a) explains the images of Figs. 2(b) and 2(c) taken with two orthogonal positions of the objective aperture relative to the 002 ring (002 darkfield images, i.e., 002 DF). These micrographs image, respectively, areas 1 and 2 of the sketch (and those running in parallel). 022 lattice fringe images (LF) give similar results, but they resolve aromatic layers in every direction [Fig. 2(d)]. However, they do not prove the occurrence of pores. To verify the occurrence of pores it was necessary to image also the areas perpendicular or oblique relative to the incident beam, i.e., area 3 in the sketch of Fig. 2(a) [arrows in Fig. 2(e)]. For that, both 10 and 11 DF were performed7-9 [Fig. 2(f)]. The SAD 1158 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 5, May 1992 Downloaded: 14 Mar 2015
FIG. 1. Model of crumpled sheets of paper.
pattern gives a Debye-Scherrer pattern since the pore walls (1,2...) are small. 002 arcs appear only if one of the pore walls is of the same size as the intermediate aperture. The 002 arcing represents the misorientation of BSUs inside the pore wall. For the same carbons but of lower HTT, the pores are not yet for
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