Carbonization and graphitization of polyimide Upilex
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Tempaku-cho,
Yoshihiro Hishiyama Musashi Institute of Technology, 1-28-1, Tamazutsumi, Setagaya-ku, Tokyo, 158 Japan (Received 13 April 1989; accepted 28 January 1991) Two-step carbonization was found on a polyimide film, Upilex, similar to that observed on another polyimide, Kapton: large weight loss up to about 22 wt.% and linear shrinkage along the film surface to 30% in a narrow temperature range from 500 to 650 °C, and an additional small weight loss and shrinkage in a wide range between 700 and 1000 °C. The yield of carbonization agreed roughly with that calculated by assuming the release of non-carbon atoms as simple species of CO, O2, and N2. A 25 fira Upilex film yielded a relatively high degree of graphitization after heat treatment at 3000 °C: a layered structure under SEM, high magnetoresistance, and a sharp 002 x-ray diffraction peak at a J-spacing of 0.336 nm. The degree of graphitization was lower than for a similar Kapton film, which is attributed to the lack of flatness in the Upilex molecule. A very much lower degree of graphitization was observed for 50 /xm Upilex films. This difference is tentatively attributed to constraints that must have influenced the orientation of the Upilex molecules near the surface of the films as they were formed.
I. INTRODUCTION
Highly crystallized and oriented graphite films were prepared from the commercially available polyimide film Kapton by high temperature heat-treatment.1 The crystallinity of the films, which are characterized by galvanomagnetic properties, is comparable with, or even better than, that of a pyrolytic graphite heattreated up to 3200 "C.1"3 The study of Kapton4 shows that its carbonization process consists of two steps; in the first step abrupt weight loss and shrinkage occur at a rather narrow temperature range of 550-650 °C, and in the second step small additional weight loss and shrinkage are found over a wide range of temperatures, 700-1000 °C, associated with growth of carbon layers and consequent rapid increase of electrical conductivity. The high degree of orientation in the original Kapton film, whose molecules are rather flat and rigid, seems to be retained during carbonization even though there is a rather large weight loss (up to 40 wt. %) and also a large shrinkage along the film surface (up to 23%). The yield after carbonization, 60 wt.%, agrees with the calculated one by assuming the release of all non-carbon atoms as simple species of CO, O2, and N2. From these experimental results, three conditions resulting in highly graphitizable carbon films might be expected: (1) flatness of the original organic molecules, (2) high degree of their orientation, and (3) simple rea)
Address correspondence to this author at Faculty of Engineering, Hokkaido University, Sapporo, 060 Japan.
1108
J. Mater. Res., Vol. 6, No. 5, May 1991
http://journals.cambridge.org
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lease of non-carbon atoms during carbonization. Condition (1) is determined definitely by the structure of original organic molecules. The molecules of Kapt
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