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Accelerated graphitization of exfoliated carbon fibers
Letters to the editor / Carbon 40 (2002) 617 – 636
628
pressure at increasing total pressure on infiltration rate and degree of pore filling. Carbon 1998;36(2):181–93. ¨ [39] Benzinger W, Huttinger KJ. Chemistry and kinetics of chemical vapor infiltration of pyrocarbon—IV. Investigation of methane / hydrogen mixtures. Carbon 1999;37(6):931–40. ¨ [40] Benzinger W, Huttinger KJ. Chemistry and kinetics of chemical vapor infiltration of pyrocarbon—V. Infiltration of carbon fiber felt. Carbon 1999;37(6):941–6.
¨ [41] Benzinger W, Huttinger KJ. Chemistry and kinetics of chemical vapor infiltration of pyrocarbon—VI. Mechanical properties of infiltrated carbon fiber felt. Carbon 1999;37(8):1311–22. [42] Lavenac J, Langlais F, Feron O, Naslain R. Microstructure of the pyrocarbon matrix in carbon / carbon composites. Compos Sci Technol 2001;61(3):339–45.
Accelerated graphitization of exfoliated carbon fibers Masahiro Toyoda a , Yutaka Kaburagi b , Akira Yoshida b , Hiroyuki Iwata c , Michio Inagaki c , * b
a Fukui National College of Technology, Sabae, Fukui 916 -8507, Japan Musashi Institute of Technology, Tamazutsumi, Setagaya-ku, Tokyo 158 -8557, Japan c Aichi Institute of Technology, Yakusa, Toyota 470 -0392, Japan
Received 3 December 2001; accepted 8 December 2001 Keywords: A. Carbon fibers, Exfoliated graphite; B. Graphitization
Previously, we reported marked exfoliation of carbon fibers by rapid heating of their intercalation compounds up to 1000 8C, which had been obtained through electrolysis in either nitric acid [1,2] or formic acid [3]. After exfoliation, a single fiber of mesophase-pitch-based and PAN-based carbon fibers was found to be converted into a bundle of thin filaments split along the original fiber axis. This morphological change seemed to be consistent with the structure models for these carbon fibers [4], where splitting of a fiber due to exfoliation runs preferentially along the hexagonal carbon layers. As a consequence, each filament formed by exfoliation consists of well-aligned carbon layers along the filament. According to experimental results for aromatic polyimide films [5], the degree of preferred orientation of hexagonal carbon layers, which were formed from imide molecules through thermal decomposition, seemed to be an important factor for high graphitizability. Therefore, a high degree of graphitization was expected for exfoliated carbon fibers after heat treatment at high temperatures. Mesophase-pitch-based carbon fibers heat treated to 3000 8C were selected and exfoliated at 1000 8C for 5 s after being intercalated via electrolysis in nitric acid. Carbon fibers thus exfoliated were heat treated again at 3000 8C for 10 min in a flow of high-purity Ar. Graphitization was explored by Raman spectroscopy on a single
*Corresponding author. Tel.: 181-565-48-8121; fax: 181565-48-0076. E-mail address: [email protected] (M. Inagaki).
filament. The structure and texture of the filament were studied by high-resolution TEM. Morphological changes due to exfoliation were observed by FE-SEM. Fig. 1 shows SEM micrographs of an exfoliated carbon fiber. The original single fiber was separated into a number of thin filaments after exfoliation, but no pronounced change in morphology was detected by reheating at 3000 8C. Fig. 2 shows Raman spectra for the exfoliated fiber and that heated at 3000 8C. The exfoliated fiber shows two peaks, the so-called D-band around 1360 cm 21 and the G-band around 1600 cm 21 , revealing no marked development of graphite structure. This spectrum is the same as that for the original mesophase-pitch-based carbon fibers before exfoliation. After reheating this exfoliated fiber at 3000 8C for only 10 min, however, there was a strong G-band and only a trace of the D-band, that is, marked development of the graphite structure was observed. The difference in the structure observed by Raman spectroscopy between the exfoliated carbon fibers and that heated at 3000 8C is very clear; exfoliation into filaments was effective in accelerating the subsequent graphitization. HRTEM observation also revealed a high degree of graphitization of the reheated exfoliated carbon fibers; the filaments had a well-oriented texture of hexagonal carbon layers along the filament axis, which was shown by small 002 arcs in the electron diffraction pattern and brightening of the whole area of the filament in a 002 dark field image. It also had high crystallinity as shown by thin diffraction rings for all lines in the electron diffraction pattern, and was composed of straight lattice fringes.
0008-6223 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 01 )00315-3
Letters to the editor / Carbon 40 (2002) 617 – 636
629
Fig. 1. SEM micrographs of (a) an exfoliated carbon fiber and (b) after 3000 8C heat treatment.
fibrous morphology, strongly depresses the development of the graphite structure in each fiber crystallite. This is the reason why even carbon fibers with a highly oriented texture cannot achieve a high degree of graphitization.
References
Fig. 2. Raman spectra for a single filament (a) exfoliated and (b) after 3000 8C heat treatment.
The present results show that graphitization is markedly accelerated by exfoliation of carbon fibers into thin filaments. In other words, the constraint to maintain the
[1] Toyoda M, Shimizu A, Iwata H, Inagaki M. Exfoliation of carbon fibers through intercalation compounds synthesized electrochemically. Carbon 2001;39:1697–707. [2] Toyoda M, Kato H, Inagaki M. Intercalation compounds of carbon fibers synthesized electrochemically and its intercalation mechanism. Carbon 2001;39:2231–7. [3] Toyoda M, Sedalacik J, Inagaki M. Intercalation of formic acid into carbon fibers and their exfoliation. Synth Met (submitted for publication). [4] Oberlin A, Bonnamy S, Lafdi K. Structure and texture of carbon fibers. In: Donnet JB et al., editor, Carbon fibers, New York: Dekker, 1998, pp. 85–159. [5] Inagaki M, Hishiyama Y, Takeichi T, Oberlin A. High quality graphite films produced from aromatic polyimides. In: Thrower PA, Radovic L, editors, Chemistry and physics of carbon, vol. 26, New York: Dekker, 1999, pp. 246–333.
628
pressure at increasing total pressure on infiltration rate and degree of pore filling. Carbon 1998;36(2):181–93. ¨ [39] Benzinger W, Huttinger KJ. Chemistry and kinetics of chemical vapor infiltration of pyrocarbon—IV. Investigation of methane / hydrogen mixtures. Carbon 1999;37(6):931–40. ¨ [40] Benzinger W, Huttinger KJ. Chemistry and kinetics of chemical vapor infiltration of pyrocarbon—V. Infiltration of carbon fiber felt. Carbon 1999;37(6):941–6.
¨ [41] Benzinger W, Huttinger KJ. Chemistry and kinetics of chemical vapor infiltration of pyrocarbon—VI. Mechanical properties of infiltrated carbon fiber felt. Carbon 1999;37(8):1311–22. [42] Lavenac J, Langlais F, Feron O, Naslain R. Microstructure of the pyrocarbon matrix in carbon / carbon composites. Compos Sci Technol 2001;61(3):339–45.
Accelerated graphitization of exfoliated carbon fibers Masahiro Toyoda a , Yutaka Kaburagi b , Akira Yoshida b , Hiroyuki Iwata c , Michio Inagaki c , * b
a Fukui National College of Technology, Sabae, Fukui 916 -8507, Japan Musashi Institute of Technology, Tamazutsumi, Setagaya-ku, Tokyo 158 -8557, Japan c Aichi Institute of Technology, Yakusa, Toyota 470 -0392, Japan
Received 3 December 2001; accepted 8 December 2001 Keywords: A. Carbon fibers, Exfoliated graphite; B. Graphitization
Previously, we reported marked exfoliation of carbon fibers by rapid heating of their intercalation compounds up to 1000 8C, which had been obtained through electrolysis in either nitric acid [1,2] or formic acid [3]. After exfoliation, a single fiber of mesophase-pitch-based and PAN-based carbon fibers was found to be converted into a bundle of thin filaments split along the original fiber axis. This morphological change seemed to be consistent with the structure models for these carbon fibers [4], where splitting of a fiber due to exfoliation runs preferentially along the hexagonal carbon layers. As a consequence, each filament formed by exfoliation consists of well-aligned carbon layers along the filament. According to experimental results for aromatic polyimide films [5], the degree of preferred orientation of hexagonal carbon layers, which were formed from imide molecules through thermal decomposition, seemed to be an important factor for high graphitizability. Therefore, a high degree of graphitization was expected for exfoliated carbon fibers after heat treatment at high temperatures. Mesophase-pitch-based carbon fibers heat treated to 3000 8C were selected and exfoliated at 1000 8C for 5 s after being intercalated via electrolysis in nitric acid. Carbon fibers thus exfoliated were heat treated again at 3000 8C for 10 min in a flow of high-purity Ar. Graphitization was explored by Raman spectroscopy on a single
*Corresponding author. Tel.: 181-565-48-8121; fax: 181565-48-0076. E-mail address: [email protected] (M. Inagaki).
filament. The structure and texture of the filament were studied by high-resolution TEM. Morphological changes due to exfoliation were observed by FE-SEM. Fig. 1 shows SEM micrographs of an exfoliated carbon fiber. The original single fiber was separated into a number of thin filaments after exfoliation, but no pronounced change in morphology was detected by reheating at 3000 8C. Fig. 2 shows Raman spectra for the exfoliated fiber and that heated at 3000 8C. The exfoliated fiber shows two peaks, the so-called D-band around 1360 cm 21 and the G-band around 1600 cm 21 , revealing no marked development of graphite structure. This spectrum is the same as that for the original mesophase-pitch-based carbon fibers before exfoliation. After reheating this exfoliated fiber at 3000 8C for only 10 min, however, there was a strong G-band and only a trace of the D-band, that is, marked development of the graphite structure was observed. The difference in the structure observed by Raman spectroscopy between the exfoliated carbon fibers and that heated at 3000 8C is very clear; exfoliation into filaments was effective in accelerating the subsequent graphitization. HRTEM observation also revealed a high degree of graphitization of the reheated exfoliated carbon fibers; the filaments had a well-oriented texture of hexagonal carbon layers along the filament axis, which was shown by small 002 arcs in the electron diffraction pattern and brightening of the whole area of the filament in a 002 dark field image. It also had high crystallinity as shown by thin diffraction rings for all lines in the electron diffraction pattern, and was composed of straight lattice fringes.
0008-6223 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 01 )00315-3
Letters to the editor / Carbon 40 (2002) 617 – 636
629
Fig. 1. SEM micrographs of (a) an exfoliated carbon fiber and (b) after 3000 8C heat treatment.
fibrous morphology, strongly depresses the development of the graphite structure in each fiber crystallite. This is the reason why even carbon fibers with a highly oriented texture cannot achieve a high degree of graphitization.
References
Fig. 2. Raman spectra for a single filament (a) exfoliated and (b) after 3000 8C heat treatment.
The present results show that graphitization is markedly accelerated by exfoliation of carbon fibers into thin filaments. In other words, the constraint to maintain the
[1] Toyoda M, Shimizu A, Iwata H, Inagaki M. Exfoliation of carbon fibers through intercalation compounds synthesized electrochemically. Carbon 2001;39:1697–707. [2] Toyoda M, Kato H, Inagaki M. Intercalation compounds of carbon fibers synthesized electrochemically and its intercalation mechanism. Carbon 2001;39:2231–7. [3] Toyoda M, Sedalacik J, Inagaki M. Intercalation of formic acid into carbon fibers and their exfoliation. Synth Met (submitted for publication). [4] Oberlin A, Bonnamy S, Lafdi K. Structure and texture of carbon fibers. In: Donnet JB et al., editor, Carbon fibers, New York: Dekker, 1998, pp. 85–159. [5] Inagaki M, Hishiyama Y, Takeichi T, Oberlin A. High quality graphite films produced from aromatic polyimides. In: Thrower PA, Radovic L, editors, Chemistry and physics of carbon, vol. 26, New York: Dekker, 1999, pp. 246–333.