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pyrolytic carbon composite
610
Letters to the Editor / Carbon 41 (2003) 579 – 625
Effects of the interface on the graphitization of a carbon fiber / pyrolytic carbon composite Fu-qin Zhang*, Bai-yun Huang, Qi-zhong Huang, Xian Xiong, Teng-fei Cheng State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, China Received 8 November 2002; accepted 10 November 2002 Keywords: A. Carbon / carbon composites; B. Graphitization; C. Raman spectroscopy; D. Interfacial properties
Stress-induced graphitization is an important factor affecting the graphitization of carbon / carbon (C / C) composites. It is well known that stress-induced graphitization occurs in resin-based carbon matrix composites [1,2]. The stress build-up caused by the large volume shrinkage during carbonization results in the graphitization of the resin-based carbon matrix at high temperature. Hishiyama et al. [1] and Zaldivar and Rellick [2] observed this kind of graphitization by polarized light and SEM respectively. However, no such kind of graphitization in chemical vapor deposited pyrolytic carbon matrix composites has been reported. In this work we examined micro-structural changes during graphitization of a chemical vapor infiltrated (CVI) PAN-based carbon fiber (T700 heat treated at 2300 8C) composite, especially at the fiber / matrix interface. Heat treatment of the composite was carried out under flowing argon at temperatures between 2100 and 2800 8C using a graphite resistance furnace and heating rates of about 10 8C / min below 2000 8C and 5 8C / min above 2000 8C. The soaking time was 1 h. Raman spectra of the composite were recorded on a JOBIN YVON-Lab Ram HR800 Raman spectroscope using He–Ne laser (632.8 nm), with the laser beam normal to and focused to about 1 mm in diameter on the polished surface of the composite. The laser power was maintained at about 1 mV at the sample. Details of the matrix microstructure of the composite, under polarized light illumination, are shown in Fig. 1. The matrix has a relatively smooth optical texture. Extinction crosses are seen every 908 in the end-on view. This matrix has a smooth laminar microstructure [3]. No obvious changes in microstructure with heat treatment temperatures were seen. Fibers (and the pyrolytic carbon sheath around them) with their axes normal to the observation surface of the samples were selected for Raman micro-spectroscopy analysis. As indicated in Fig. 1, spectra at CF1 (the fiber core) and CF2 (|1.5 mm from the interface) and PyC1 (|2
mm from the interface) and PyC2 (at the center of the sheath) were recorded. Fig. 2 shows the spectra of pyrolytic carbon in the C / C composite at different HTT. Two bands can be seen. One 21 21 is at about 1335 cm , the other is at about 1584 cm . 21 The intensity of the 1584 cm band has a distinct increase with increasing HTT, which reveals changes in the carbon structure [4–6]. The ratio of the intensities of the 1335 21 21 cm band to the 1584 cm band, R, is inversely proportional to the microcrystalline planar size La [4,5], i.e. R 21 is proportional to La . Fig. 3 shows the changes of Raman parameter R 21 with HTT in the pyrolytic carbon. When HTT increases from 21 2100 8C to 2800 8C, the value of R at PyC1 increases from 0.4 to 0.67, whereas only from 0.44 to 0.56 at PyC2, 21 i.e. PyC1 shows a greater increase in R than does PyC2. This indicates that abnormal graphitization occurs in the pyrolytic carbon sheath near the fiber / matrix interface. 21 Fig. 4 shows the changes of Raman parameter R with HTT in the carbon fiber. A similar tendency was obtained although the extent of the increase is less than for the pyrolytic carbon. This indicates that abnormal graphitiza-
*Corresponding author. Tel.: 186-731-8656-323; fax: 186731-8716-052. E-mail address: [email protected] (F.-q. Zhang).
Fig. 1. Optical photographs (under polarized light illumination) and Raman analysis locations in the C / C composites: CF—carbon fiber, PyC—pyrolytic carbon.
0008-6223 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 02 )00398-6
Letters to the Editor / Carbon 41 (2003) 579 – 625
611
Fig. 3. Changes of Raman parameter R 21 with HTT in pyrolytic carbon.
Graphitization is a process accompanied with volume shrinkage. Expansions of components in a C / C composite during graphitization can be quite different, resulting in thermal stresses. The thermal stress can be estimated by the Whitney and Riley model [7] as shown in Fig. 5. According to this model, the stress reaches a maximum at the interface and decreases with increasing distance from the interface. The stress at the interface, sf and sm , can be calculated by Eq. (1) [7]:
F F
G G
Ef EmVm B sf 5 ]]] 1 2nf ] (am 2 af ) DT EfVf 1 EmVm A Ef EmVf B sm 5 ]]] 1 2nm ] (af 2 am ) DT EfVf 1 EmVm A
(1)
where 1 2 nf (1 1 nf ) 1 (1 2 nf )Vf A 5 ]] 1 ]]]]] Ef EmVm
nfVm Em nfVf Ef B 5 1 1 ]]] 1 ]]] EfVf 1 EmVm EfVf 1 EmVm For the C / C composite shown in Fig. 1, a53.7 mm; b515 mm, i.e., Vf 56%; Vm 594%. Ef 5230 GPa , Em 55 GPa ; nf 50.25; nm 50.25 [8]. Then Eq. (1) transforms to Eq. (2):
sm 5 6(af 2 am ) DT
Fig. 2. Raman spectra of pyrolytic carbon (PyC) in C / C composite at different HTT: (a) PyC1 and (b) PyC2.
tion also occurs in the carbon fiber near the fiber / matrix interface.
(2)
where sm is in GPa . For example, when graphitized at 2800 8C, the change in temperature from chemical vapor deposition (1000 8C) to graphitization is about 1800 8C, i.e. DT51800 8C, suppose af 2 am 51310 26 8C 21 (the expansion of carbon fiber and pyrolytic carbon is difficult to measure at such a high temperature, and no such data have been reported), then sm is about 10 MPa .
612
Letters to the Editor / Carbon 41 (2003) 579 – 625
Fig. 4. Changes of Raman parameter R 21 with HTT in carbon fiber.
ites in which the stresses at the interface are essentially residual stresses which result from the decrease in temperature between the carbonization temperature and room temperature.
Acknowledgements We thank Professor Huiming Cheng and Dr. Feng Li of the Institute of Metal Research, Chinese Academy of Sciences for their help with the Raman spectroscopy analysis.
References
Fig. 5. Whitney and Riley model [7].
Stress in carbon may change the graphitization process. Hishiyama and Inagaki [1] found that a few hundred MPa of stress at the interface in natural graphite / glassy carbon will result in the glassy carbon being graphitized at high temperature. Noda and Kato [9] indicated that glassy carbon heat treated at 2500 8C under 100 MPa pressure is more highly graphitized than at 3000 8C under atmospheric pressure. It therefore appears that it is the stress induced at the interface by the difference of thermal expansion between carbon fiber and pyrolytic carbon during graphitization at high temperature which is responsible for the abnormal graphitization near the interface in this PAN-based carbon fiber / smooth laminar pyrolytic carbon matrix composite. This is different from resin-based carbon matrix compos-
[1] Hishiyama Y, Inagaki M, Kimura S, Yamada S. Graphitization of carbon fiber / glassy carbon composites. Carbon 1974;12:249–58. [2] Zaldivar RJ, Rellick GS. Some observations on stress graphitization in carbon–carbon composites. Carbon 1991;29(8):1155–63. [3] Granoff B. Kinetics of graphitization of carbon-felt / carbonmatrix composites. Carbon 1974;12:405–16. [4] Tuinstra F, Koenig JL. Raman spectrum of graphite. J Chem Phys 1970;53(3):1126–30. [5] Nakamizo M, Kammereck R, Walker PL. Laser Raman studies on carbons. Carbon 1974;12:259–67. [6] Nikiel L, Jagodzinski PW. Raman spectroscopic characterization of graphites: a re-evaluation of spectra / structure correlation. Carbon 1993;31(8):1313–7. [7] Wo DZ, Li SL, Wang XY, Liou XY, Wang ZR, Zhang GD. In: Handbook of composite materials, Beijing: Chemical Industry Press, 2000, pp. 220–1. [8] Li SH. In: Carbon and graphite, Beijing: Metallurgical Industry Press, 1983, pp. 41–5. [9] Noda T, Kato H. Heat treatment of carbon under high pressure. Carbon 1965;3:289–97.
Letters to the Editor / Carbon 41 (2003) 579 – 625
Effects of the interface on the graphitization of a carbon fiber / pyrolytic carbon composite Fu-qin Zhang*, Bai-yun Huang, Qi-zhong Huang, Xian Xiong, Teng-fei Cheng State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, China Received 8 November 2002; accepted 10 November 2002 Keywords: A. Carbon / carbon composites; B. Graphitization; C. Raman spectroscopy; D. Interfacial properties
Stress-induced graphitization is an important factor affecting the graphitization of carbon / carbon (C / C) composites. It is well known that stress-induced graphitization occurs in resin-based carbon matrix composites [1,2]. The stress build-up caused by the large volume shrinkage during carbonization results in the graphitization of the resin-based carbon matrix at high temperature. Hishiyama et al. [1] and Zaldivar and Rellick [2] observed this kind of graphitization by polarized light and SEM respectively. However, no such kind of graphitization in chemical vapor deposited pyrolytic carbon matrix composites has been reported. In this work we examined micro-structural changes during graphitization of a chemical vapor infiltrated (CVI) PAN-based carbon fiber (T700 heat treated at 2300 8C) composite, especially at the fiber / matrix interface. Heat treatment of the composite was carried out under flowing argon at temperatures between 2100 and 2800 8C using a graphite resistance furnace and heating rates of about 10 8C / min below 2000 8C and 5 8C / min above 2000 8C. The soaking time was 1 h. Raman spectra of the composite were recorded on a JOBIN YVON-Lab Ram HR800 Raman spectroscope using He–Ne laser (632.8 nm), with the laser beam normal to and focused to about 1 mm in diameter on the polished surface of the composite. The laser power was maintained at about 1 mV at the sample. Details of the matrix microstructure of the composite, under polarized light illumination, are shown in Fig. 1. The matrix has a relatively smooth optical texture. Extinction crosses are seen every 908 in the end-on view. This matrix has a smooth laminar microstructure [3]. No obvious changes in microstructure with heat treatment temperatures were seen. Fibers (and the pyrolytic carbon sheath around them) with their axes normal to the observation surface of the samples were selected for Raman micro-spectroscopy analysis. As indicated in Fig. 1, spectra at CF1 (the fiber core) and CF2 (|1.5 mm from the interface) and PyC1 (|2
mm from the interface) and PyC2 (at the center of the sheath) were recorded. Fig. 2 shows the spectra of pyrolytic carbon in the C / C composite at different HTT. Two bands can be seen. One 21 21 is at about 1335 cm , the other is at about 1584 cm . 21 The intensity of the 1584 cm band has a distinct increase with increasing HTT, which reveals changes in the carbon structure [4–6]. The ratio of the intensities of the 1335 21 21 cm band to the 1584 cm band, R, is inversely proportional to the microcrystalline planar size La [4,5], i.e. R 21 is proportional to La . Fig. 3 shows the changes of Raman parameter R 21 with HTT in the pyrolytic carbon. When HTT increases from 21 2100 8C to 2800 8C, the value of R at PyC1 increases from 0.4 to 0.67, whereas only from 0.44 to 0.56 at PyC2, 21 i.e. PyC1 shows a greater increase in R than does PyC2. This indicates that abnormal graphitization occurs in the pyrolytic carbon sheath near the fiber / matrix interface. 21 Fig. 4 shows the changes of Raman parameter R with HTT in the carbon fiber. A similar tendency was obtained although the extent of the increase is less than for the pyrolytic carbon. This indicates that abnormal graphitiza-
*Corresponding author. Tel.: 186-731-8656-323; fax: 186731-8716-052. E-mail address: [email protected] (F.-q. Zhang).
Fig. 1. Optical photographs (under polarized light illumination) and Raman analysis locations in the C / C composites: CF—carbon fiber, PyC—pyrolytic carbon.
0008-6223 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 02 )00398-6
Letters to the Editor / Carbon 41 (2003) 579 – 625
611
Fig. 3. Changes of Raman parameter R 21 with HTT in pyrolytic carbon.
Graphitization is a process accompanied with volume shrinkage. Expansions of components in a C / C composite during graphitization can be quite different, resulting in thermal stresses. The thermal stress can be estimated by the Whitney and Riley model [7] as shown in Fig. 5. According to this model, the stress reaches a maximum at the interface and decreases with increasing distance from the interface. The stress at the interface, sf and sm , can be calculated by Eq. (1) [7]:
F F
G G
Ef EmVm B sf 5 ]]] 1 2nf ] (am 2 af ) DT EfVf 1 EmVm A Ef EmVf B sm 5 ]]] 1 2nm ] (af 2 am ) DT EfVf 1 EmVm A
(1)
where 1 2 nf (1 1 nf ) 1 (1 2 nf )Vf A 5 ]] 1 ]]]]] Ef EmVm
nfVm Em nfVf Ef B 5 1 1 ]]] 1 ]]] EfVf 1 EmVm EfVf 1 EmVm For the C / C composite shown in Fig. 1, a53.7 mm; b515 mm, i.e., Vf 56%; Vm 594%. Ef 5230 GPa , Em 55 GPa ; nf 50.25; nm 50.25 [8]. Then Eq. (1) transforms to Eq. (2):
sm 5 6(af 2 am ) DT
Fig. 2. Raman spectra of pyrolytic carbon (PyC) in C / C composite at different HTT: (a) PyC1 and (b) PyC2.
tion also occurs in the carbon fiber near the fiber / matrix interface.
(2)
where sm is in GPa . For example, when graphitized at 2800 8C, the change in temperature from chemical vapor deposition (1000 8C) to graphitization is about 1800 8C, i.e. DT51800 8C, suppose af 2 am 51310 26 8C 21 (the expansion of carbon fiber and pyrolytic carbon is difficult to measure at such a high temperature, and no such data have been reported), then sm is about 10 MPa .
612
Letters to the Editor / Carbon 41 (2003) 579 – 625
Fig. 4. Changes of Raman parameter R 21 with HTT in carbon fiber.
ites in which the stresses at the interface are essentially residual stresses which result from the decrease in temperature between the carbonization temperature and room temperature.
Acknowledgements We thank Professor Huiming Cheng and Dr. Feng Li of the Institute of Metal Research, Chinese Academy of Sciences for their help with the Raman spectroscopy analysis.
References
Fig. 5. Whitney and Riley model [7].
Stress in carbon may change the graphitization process. Hishiyama and Inagaki [1] found that a few hundred MPa of stress at the interface in natural graphite / glassy carbon will result in the glassy carbon being graphitized at high temperature. Noda and Kato [9] indicated that glassy carbon heat treated at 2500 8C under 100 MPa pressure is more highly graphitized than at 3000 8C under atmospheric pressure. It therefore appears that it is the stress induced at the interface by the difference of thermal expansion between carbon fiber and pyrolytic carbon during graphitization at high temperature which is responsible for the abnormal graphitization near the interface in this PAN-based carbon fiber / smooth laminar pyrolytic carbon matrix composite. This is different from resin-based carbon matrix compos-
[1] Hishiyama Y, Inagaki M, Kimura S, Yamada S. Graphitization of carbon fiber / glassy carbon composites. Carbon 1974;12:249–58. [2] Zaldivar RJ, Rellick GS. Some observations on stress graphitization in carbon–carbon composites. Carbon 1991;29(8):1155–63. [3] Granoff B. Kinetics of graphitization of carbon-felt / carbonmatrix composites. Carbon 1974;12:405–16. [4] Tuinstra F, Koenig JL. Raman spectrum of graphite. J Chem Phys 1970;53(3):1126–30. [5] Nakamizo M, Kammereck R, Walker PL. Laser Raman studies on carbons. Carbon 1974;12:259–67. [6] Nikiel L, Jagodzinski PW. Raman spectroscopic characterization of graphites: a re-evaluation of spectra / structure correlation. Carbon 1993;31(8):1313–7. [7] Wo DZ, Li SL, Wang XY, Liou XY, Wang ZR, Zhang GD. In: Handbook of composite materials, Beijing: Chemical Industry Press, 2000, pp. 220–1. [8] Li SH. In: Carbon and graphite, Beijing: Metallurgical Industry Press, 1983, pp. 41–5. [9] Noda T, Kato H. Heat treatment of carbon under high pressure. Carbon 1965;3:289–97.