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Formation of pores in carbonized polyimide film kapton by high temperature heat treatment
Carh Vol. 30, No. 3, pp. 517-519.1992 Printed in Grelu Britain
~8~2~~2 $5.00 + ‘00 Copyright % 1992 Pergmon Press Ltd
LETTERS TO THE EDITOR
Formation of pores in carbonized polyimide film kapton by high t~rn~ratu~ heat treatment (Received2 January 1992; accepted I6 January 1992) Key Words - Kapton, graphite film, graphitization, exfoliation, magnetoresistance, SEM
Highly crystallized and oriented graphite films were prepared from the commercially available pclyimide tihn Kapton by high temperature heat treatment [l-3]. The crystallinity of the film, characterized by ~aivanomagnetic properties, is comparable with, or even better than, that of a pyrolytic graphite heat-treated up to 32OO*C. Sometimes, however, the graphite titms were found to have rather high values of the anisotropy ratios rT and rrL, which were calculated from the measured values of the magnetoresistance at liquid nitrogen temperature, in other words, a low degree of preferred orientation along the film surface, even though the crystallinity determined from magnetoresistance was very high. In this letter magnetoresistance measurements and scanning electron microscope (SEM) observations on the same specimen show that the low degree of orientation in the film is due to the formation of pores during the graphitization process. A large area Kapton film with thickness of 25 m was cut into ribbons -of about 1 x 30 mmz. These ribbons were carbonized at 800°C for 1 hr with a heating rate of 4O@‘C/br,and then heat-treated at temperatures oyf 2100, 2400, 2700 and 3000°C for 1 hr by the method described in our previous paper [l]. For heat-treated specimens the magnetoresistance was measured at liquid nitrogen temperature, and then the fractured surface of exactly the same specimen was observed by SEM with an acceleration voltage of 2 kV. Parameters of the magnetoresistance (Ap/pjr,,,t,, and (Ap/p)TL,tn [4] were measured at liquid nitrogen temperature and a magnetic field of 1 T. The anisotrouv ratios rr and rrL w&e calculated from the ratios b’f (AP/P)Tmin and (Ap/p)rhin to @p/p),,,, respectively,
which are the measure of orientation of graphite layers parallel to the film surface. For the perfect plane orientation, these ratios rr and rrL must be zero. The value of (Apl )max is known to increase with improvement o P ~stallinity, i.e. it is a measure Of degree of ~aphiti~tion 141. In Table 1, these measured parameters are listed. The present Kapton carbon is found to be graphitized above a temperature between 2100 and 2400°C. In the carbon film heat-treated below 24of)‘C carbon layer planes are highly oriented along the film surface. With the progress of graphitization, however, this orientated texture is disturbed to a large extent, as shown by the large values of r-r and rrL. Figure 1 shows SEM micrographs of the fracture surfaces of each specimen. For the specimen heattreated at 2100°C a dense fracture surface is observed with low magnification (Fig. la). A high magnification micrograph (Fig. lb) indicates that carbon fayers align almost parallel to the film surface, though their extent is limited. This observation is consistent with the IOW values of rr and rrL on this specimen. The specimen heat-treated at 2400°C exhibits the formation of pores in the film (Fig. lc) and can be called exfoliated. This suggests the formation of gas and the softening of the texture around 24OO’C. The main part of this sample consists of large extended layers along the film surface (Fig. Id). The exfoliated regions were probably those poorly graphitized and may have a negative magnetoresistance. The magnetoresistance for the regions of extended graphite layers, however, is positive, and this more than compensates for the negative component. Thus the magnetoresistance measured for
Table 1 Magneto~sis~nce results measured at liquid nitrogen temperature and magnetic field of 1 T for the samples. The values for the specimen heat-treated at 2950°C are those from reference 131. H’IT (“(2 2100 2400 2700 3000 2950
(AP/Phnax(%I
(Ap/Phmin (%I
(AP/P)Tmin (a)
-0.105 2.53 83.34 275.20 606.3
-0.08 1 0.0639 12.17 91.09 19.36
-0.008 1 0.0639 16.75 102.09 14.53
517
i’r 0.077 0.0251 0.146 0.331 0.0319
IR 0.077 0.0253 0.201 0.37 1 0.0240
Fig. 1 SEM micrographs of fracture surfaces of the samples heat-treated at various temperatures (I-ITT’s): (a,b), 21OO’C; (c,d), 24OO’C; (e,f), 27OO’C; (g,h); 3000°C. this specimen is mainly due to the regions of extended layers, and the measured anisotropy ratios rr and r+r~of this specimen are small. Exfoliated regions are extended in the samples heat-treated at 2700 and 3000°C (Figs. le and g), even though graphite layers are developed much more by heat treatment (Fig. If and h). All of the regions were graphitized, and so a large positive magnetoresistance which increased rapidly with increasing heat treatment temperature was measured for these samples. Since the exfoliated texture was almost
built up by high temperature heat-treatment, large values of the anisotropy ratios were obtained for these samples. The exfoliation for the present high temperature-treated specimens was occasional, since in some cases we succeeded in preparing highly oriented graphite films from Kapton of 25 pm in thickness by heat treatment at 2950°C [3]. The magnetoresistance parameters for such specimens are also listed in Table 1 for comparison. The reason for the occasional development of the exfoliated region in the graphite film prepared from
Lettertothe
polyimide Kapton during heat treatment around 24OO’C has to be further examined. The present result shows that the formation of the exfoliated regions in the film may be monitored by the anisotropy ratios of the magnetoresistance measured at liquid nitrogen temperature, and that it is unnecessary to break the film specimens. Acknowledgement - This work was partly supported by
a Grant for International Joint Research Project from the NBDO, Japan Musashi Institute of Technology I-28-1 Tamazutsumi, Setagaya-ku Tokyo 158, JAPAN Faculty of Engineering Hokkaido Vniversiry Kita-ku, Sapporo 060 JAPAN
Y. HISHIYAMA
A. YOSHIDA Y. KABURAGI
M. INAGAKI
519
Editor
REFERENCES 1. 2.
3. 4.
Y. Hishiyama, S. Yasuda, A. Yoshida and M. Inagaki, J. Mater. Sci. 23, 3272 (1988). C. Bouregerette, A. Oberlin and M. Inagaki, J. Mat. Res. (in press). Y. Hishiyama, A. Yoshida, Y. Kaburagi and M. Inagaki, Carbon 30 (1992) in press. Y. Hishiyama, Y. Kaburagi and M. Inagaki, Chem. Phys. of Carbon, Vol. 23, Ed. by P.A. Thrower, Marcel Dekker, New York, 1991, p. 1.
~8~2~~2 $5.00 + ‘00 Copyright % 1992 Pergmon Press Ltd
LETTERS TO THE EDITOR
Formation of pores in carbonized polyimide film kapton by high t~rn~ratu~ heat treatment (Received2 January 1992; accepted I6 January 1992) Key Words - Kapton, graphite film, graphitization, exfoliation, magnetoresistance, SEM
Highly crystallized and oriented graphite films were prepared from the commercially available pclyimide tihn Kapton by high temperature heat treatment [l-3]. The crystallinity of the film, characterized by ~aivanomagnetic properties, is comparable with, or even better than, that of a pyrolytic graphite heat-treated up to 32OO*C. Sometimes, however, the graphite titms were found to have rather high values of the anisotropy ratios rT and rrL, which were calculated from the measured values of the magnetoresistance at liquid nitrogen temperature, in other words, a low degree of preferred orientation along the film surface, even though the crystallinity determined from magnetoresistance was very high. In this letter magnetoresistance measurements and scanning electron microscope (SEM) observations on the same specimen show that the low degree of orientation in the film is due to the formation of pores during the graphitization process. A large area Kapton film with thickness of 25 m was cut into ribbons -of about 1 x 30 mmz. These ribbons were carbonized at 800°C for 1 hr with a heating rate of 4O@‘C/br,and then heat-treated at temperatures oyf 2100, 2400, 2700 and 3000°C for 1 hr by the method described in our previous paper [l]. For heat-treated specimens the magnetoresistance was measured at liquid nitrogen temperature, and then the fractured surface of exactly the same specimen was observed by SEM with an acceleration voltage of 2 kV. Parameters of the magnetoresistance (Ap/pjr,,,t,, and (Ap/p)TL,tn [4] were measured at liquid nitrogen temperature and a magnetic field of 1 T. The anisotrouv ratios rr and rrL w&e calculated from the ratios b’f (AP/P)Tmin and (Ap/p)rhin to @p/p),,,, respectively,
which are the measure of orientation of graphite layers parallel to the film surface. For the perfect plane orientation, these ratios rr and rrL must be zero. The value of (Apl )max is known to increase with improvement o P ~stallinity, i.e. it is a measure Of degree of ~aphiti~tion 141. In Table 1, these measured parameters are listed. The present Kapton carbon is found to be graphitized above a temperature between 2100 and 2400°C. In the carbon film heat-treated below 24of)‘C carbon layer planes are highly oriented along the film surface. With the progress of graphitization, however, this orientated texture is disturbed to a large extent, as shown by the large values of r-r and rrL. Figure 1 shows SEM micrographs of the fracture surfaces of each specimen. For the specimen heattreated at 2100°C a dense fracture surface is observed with low magnification (Fig. la). A high magnification micrograph (Fig. lb) indicates that carbon fayers align almost parallel to the film surface, though their extent is limited. This observation is consistent with the IOW values of rr and rrL on this specimen. The specimen heat-treated at 2400°C exhibits the formation of pores in the film (Fig. lc) and can be called exfoliated. This suggests the formation of gas and the softening of the texture around 24OO’C. The main part of this sample consists of large extended layers along the film surface (Fig. Id). The exfoliated regions were probably those poorly graphitized and may have a negative magnetoresistance. The magnetoresistance for the regions of extended graphite layers, however, is positive, and this more than compensates for the negative component. Thus the magnetoresistance measured for
Table 1 Magneto~sis~nce results measured at liquid nitrogen temperature and magnetic field of 1 T for the samples. The values for the specimen heat-treated at 2950°C are those from reference 131. H’IT (“(2 2100 2400 2700 3000 2950
(AP/Phnax(%I
(Ap/Phmin (%I
(AP/P)Tmin (a)
-0.105 2.53 83.34 275.20 606.3
-0.08 1 0.0639 12.17 91.09 19.36
-0.008 1 0.0639 16.75 102.09 14.53
517
i’r 0.077 0.0251 0.146 0.331 0.0319
IR 0.077 0.0253 0.201 0.37 1 0.0240
Fig. 1 SEM micrographs of fracture surfaces of the samples heat-treated at various temperatures (I-ITT’s): (a,b), 21OO’C; (c,d), 24OO’C; (e,f), 27OO’C; (g,h); 3000°C. this specimen is mainly due to the regions of extended layers, and the measured anisotropy ratios rr and r+r~of this specimen are small. Exfoliated regions are extended in the samples heat-treated at 2700 and 3000°C (Figs. le and g), even though graphite layers are developed much more by heat treatment (Fig. If and h). All of the regions were graphitized, and so a large positive magnetoresistance which increased rapidly with increasing heat treatment temperature was measured for these samples. Since the exfoliated texture was almost
built up by high temperature heat-treatment, large values of the anisotropy ratios were obtained for these samples. The exfoliation for the present high temperature-treated specimens was occasional, since in some cases we succeeded in preparing highly oriented graphite films from Kapton of 25 pm in thickness by heat treatment at 2950°C [3]. The magnetoresistance parameters for such specimens are also listed in Table 1 for comparison. The reason for the occasional development of the exfoliated region in the graphite film prepared from
Lettertothe
polyimide Kapton during heat treatment around 24OO’C has to be further examined. The present result shows that the formation of the exfoliated regions in the film may be monitored by the anisotropy ratios of the magnetoresistance measured at liquid nitrogen temperature, and that it is unnecessary to break the film specimens. Acknowledgement - This work was partly supported by
a Grant for International Joint Research Project from the NBDO, Japan Musashi Institute of Technology I-28-1 Tamazutsumi, Setagaya-ku Tokyo 158, JAPAN Faculty of Engineering Hokkaido Vniversiry Kita-ku, Sapporo 060 JAPAN
Y. HISHIYAMA
A. YOSHIDA Y. KABURAGI
M. INAGAKI
519
Editor
REFERENCES 1. 2.
3. 4.
Y. Hishiyama, S. Yasuda, A. Yoshida and M. Inagaki, J. Mater. Sci. 23, 3272 (1988). C. Bouregerette, A. Oberlin and M. Inagaki, J. Mat. Res. (in press). Y. Hishiyama, A. Yoshida, Y. Kaburagi and M. Inagaki, Carbon 30 (1992) in press. Y. Hishiyama, Y. Kaburagi and M. Inagaki, Chem. Phys. of Carbon, Vol. 23, Ed. by P.A. Thrower, Marcel Dekker, New York, 1991, p. 1.