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Correlation between graphite crystallite distribution morphology and the mechanical properties of carbon fiber during heat treatment
Materials Letters 65 (2011) 3444–3446
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Correlation between graphite crystallite distribution morphology and the mechanical properties of carbon fiber during heat treatment Aijun Gao, Chun Zhao, Sha Luo, Yuanjian Tong, Lianghua Xu ⁎ National Carbon Fiber Engineering Research Center, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 16 June 2011 Accepted 18 July 2011 Available online 22 July 2011 Keywords: Carbon fiber Crystallite Mechanical property Microstructure Grain boundaries
a b s t r a c t The growth mechanism for the graphite crystallites in polyacrylonitrile (PAN)-based carbon fibers, heat-treated at various temperatures, has been proposed. The evolution of distribution morphologies for graphite crystallites is investigated, in relation to variations in the tensile properties of the fiber samples. At high temperatures, dangling bonds are created via the cleavage of weak bonds, such as C\N, C\H and C\C at the edges of the graphite crystallites. The graphite crystallites grow through the bonding reactions between different dangling or dangling bonds and agraphitic carbon atoms. This results in graphite crystallites that increase in size with elevated temperatures that lead to changes in graphite crystallite distribution morphology. Between 1400 and 2400 °C, the distribution morphologies of the graphite crystallites go though three states: (I) a dispersed state, (II) a network state, and (III) a transfixion state. The tensile strength decreased rapidly with increasing heat treatment temperature for fibers with a dispersed state structure, but lowers with fibers bearing a network state structure. The decrease was more rapid for fibers with a transfixion state structure. The tensile modulus increased slowly in states I and II, and rapidly in state III. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiment
Polyacrylonitrile (PAN)-based carbon fibers, which are formed with polycrystalline graphite and amorphous carbon, are most widely used as reinforcement for composites. Using X-ray and transmission electron microscopy, the microstructures of carbon fibers have been studied by several authors, and various models have been proposed. These include a ribbon model [1,2], a circumferential-radial model [1,3], a two-type structure (sheath-core model) [1], and a three-dimensional model [2,4]. Extensive work has been conducted to probe the texture and mechanical properties of carbon fibers [5–10]. The modulus has been successfully modeled [11–13], but strength measurements are more challenging. The aromatic layers in the microstructures of high-modulus and high-strength PAN-based carbon fibers differ mainly in their different curvature radius, which can play a key role in terms of their mechanical properties [5,6]. From the HRTEM images in previous studies [1–8], we observed that the graphite crystallites of different carbon fibers have different morphologies, but the relationship between the different graphite crystallites is not yet understood. In the present study, carbon fibers were heat treated at various temperatures and changes in the tensile properties were measured in relation to the evolution of graphite crystallite distribution morphology.
2.1. Materials Each single tow of fiber used contained 3000 strands of monofilament (JH300, Research Institute of Jilin Petrochemical Company, China). The heat treatments were carried out in a graphite resistance furnace, under a high-purity nitrogen gas atmosphere in a continuous system. The fiber length was kept constant throughout the heat treatments. The heat treatment temperature was set at 1400–2400 °C, and at each temperature the constant temperature heat treatment time was 36 s. 2.2. Apparatus and procedures The lattice fringes of the resulting fibers were observed using high resolution transmission electron microscopy (HRTEM: Jeol ARM-1250) with an acceleration voltage of 300 kV. A tensile test for each fiber multifilament was also carried out using a universal testing machine (AG-1S, Shimadzu, Japan), with a load cell of 1 kN at a crosshead speed of 10 mm/min, according to GB/T 3362-1982. 3. Results and discussion 3.1. Growth mechanism of graphite crystallites
⁎ Corresponding author. Tel./fax: + 86 10 64435913. E-mail address: [email protected] (L. Xu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.07.057
Within carbon fibers, C\C bonds at the edge of the graphite crystallites (bond energyb 400 kJ/mol) are weaker than those inside the hexagonal graphitic network (bond energyN 500 kJ/mol). Chemical
A. Gao et al. / Materials Letters 65 (2011) 3444–3446
3445
Fig. 1. Schematic diagram for the growth of graphite crystals.
bonds involving heteroatoms, such as C\N (bond energy b 350 kJ/mol) or C\H (bond energy b 430 kJ/mol) are also weaker. During heat treatment, these weaker bonds initially break to form highly active dangling bonds (Fig. 1, Structure A to B). Those crystallites with dangling bonds grow by the dangling bonds either reacting to each other or with agraphitic carbon atoms to form more stable structure as soon as they are generated. Different bonding ways (Fig. 1C) lead to crystallites with different profiles (Fig. 1D). 3.2. Evolution of distribution morphologies The distribution morphologies of the graphite crystallites in carbon fibers can be observed intuitively based on HRTEM. Fig. 2 shows the lattice-fringe images (002) for carbon fibers heat treated at various temperatures. In the HRTEM image, a lattice-fringe stands for a single layer of graphite, and a group of lattice-fringes represents a graphite
crystallite. As shown in Fig. 2, from 1400 to 2300 °C the number of lattice-fringes significantly increases. The graphite crystallites in the carbon fibers grew with increasing heat treatment temperature, and their distribution morphologies changed. Between 1400 and 1700 °C, the carbon fibers had small crystallites dispersed within their fibers (analogous to that in Fig. 3A), with amorphous carbon or singe graphite layers in between (double arrows). A few crystallites near one another were bonded together, possibly due to the highly reactive dangling bonds. At elevated temperatures, the crystallite size increased and the probability of the crystallites coming into contact with each other increased. Therefore, cross-linking points (black single arrows) between the crystallites increased. As more and more cross-linking points appeared, a network structure (like that in Fig. 3B) was formed for almost all the graphite crystallites in the carbon fibers (1700 °C). In the crystallite growth process, the rearrangement of carbon atoms induces an internal stress, which can act on the edge of
Fig. 2. Lattice fringe images (002) of carbon fibers heat treated at various temperatures.
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A. Gao et al. / Materials Letters 65 (2011) 3444–3446
Fig. 3. Schematic diagrams for the three distribution morphologies of graphite crystallites.
crystallite and improve the degree of orientation. Under further elevated temperatures, the carbon atom activity increases, and at some stronger cross-linking points, the rearrangement of carbon atoms induces the combination of crystallites to form larger networks. With increasing crystallite size, mobility decreases; it becomes difficult to release internal stress by improving the crystallites degree of orientation. This increased internal stress may make some weaker cross-linking points susceptible to cleavage, and the larger crystallites can become, once again, dispersed in the carbon fibers. The sizes of these dispersed crystallites continue to increase, and the interstices (white single arrows) between them increase because of the internal contractions caused by crystallite growth. The larger crystallites can be considered to transfix the fiber like that shown in Fig. 3C. 3.3. Evolution of mechanical property The mechanical properties of carbon fibers change with increasing heat treatment temperatures, and the variations of mechanical properties can be associated with changes in crystallite distribution morphology. The tensile strength and modulus at different temperatures are shown in Fig. 4. The changes in tensile strength can be divided into three zones (I, II, III). From 1400 to 1600 °C (zone I), the tensile strength decreased rapidly with elevated temperature. In this temperature range, the crystallites were small and dispersed in the carbon fibers. According to the Griffith theory, the strength of carbon fibers is influenced by the size of inherent cracks. The sizes of inherent cracks are proportional to the sizes of the graphite crystallites, and it can be seen that larger sizes lead to a weaker tensile strength. From 1700 to 2000 °C (zone II), the tensile strength decreased slowly due to the cross-linking points between the crystallites consuming energy and preventing crack growth. Above 2000 °C (zone III), the tensile strength decreased rapidly again, because of the destruction of cross-linking points. The crystallites combined with each other and the grain boundaries reduced, the crack easily went through the grain boundaries to destroy the fibers completely. The tensile modulus reflects the contribution of a graphite structure; a network structure does not influence it. The modulus increased slowly below 2000 °C (zone I and II), due to the smaller size and lower orientation degree of the crystallites. In zone III, more and more small crystallites combine into larger crystallites, leading the modulus to increase rapidly. 4. Conclusions Systematic research has been carried out into carbon fibers, heat treatment at various temperatures using HRTEM. At higher tempera-
Fig. 4. Mechanical properties of fibers heat-treated at various temperatures.
tures, dangling bonds play a key role in the growth mechanism of graphite crystallites. Dangling bonds are created via the cleavage of weak bonds, such as C\N, C\H and C\C bonds at the edges of the graphite crystallites. These dangling bonds react with each other or with agraphitic carbon to induce graphite crystallite growth. The graphite crystallite size becomes larger with higher temperature and their distribution morphology changes. Crystallites with a small size are dispersed throughout fibers that are heat treated at lower temperatures, and the crystallites begin to interlink with one another with increasing crystallite size. The crystallites combine and become larger in size above 2100 °C and the internal stress induces the weaker interlinking points to break, re-dispersing the crystallites. The tensile strength decreased rapidly with increasing treatment temperature for fibers with a dispersed crystallite structure. The decrease was slower for fibers with a network structure, because the cross-linking points between the crystallites can effectively prevent crack propagation. The decrease was again more rapid for fibers with a transfixion structure, due to the destruction of cross-linking. The tensile modulus increased slowly for the dispersed and network structures, but rapidly for the transfixion structure. Acknowledgment Financial support from the National Basic Research Program of China (2006CB605302) is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Bennett SC, Johnson DJ. Carbon 1979;17:25–39. Diefendorf RJ, Tokarsky E. Polym Eng Sci 1975;15:150–9. Knibbs RH. J Microsc 1971;94:273–81. Crawford D, Johnson DJ. J Microsc 1971;94:51–62. Guigon M, Oberlin A, Desarmot G. Fibre Sci Technol 1984;20:177–98. Guigon M, Oberlin A, Desarmot G. Fibre Sci Technol 1984;20:55–72. Deurbergue A, Oberlin A. Carbon 1992;30(7):981–7. Guigon M, Oberlin A. Compos Sci Technol 1986;27(1):1–23. Huang Y, Young RJ. Carbon 1995;33(2):97–107. Paris O, Loidl D, Peterlik H. Carbon 2002;40(4):551–5. Johnson W. The structure of PAN based carbon fibres and relationship to physical properties. In: Watt W, Perov BV, editors. Strong fibers, vol. 1. Amsterdam: Elsevier; 1985. p. 389–443. [12] Oberlin A, Guigon M. The structure of carbon fibres. In: Bunsell AR, editor. Fibre reisnforcements for composite materials, vol. 2. Amsterdam: Elsevier; 1988. p. 149–210. [13] Northolt MG, Veldhuizen LH, Jansen H. Carbon 1991;29(8):1267–79.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Correlation between graphite crystallite distribution morphology and the mechanical properties of carbon fiber during heat treatment Aijun Gao, Chun Zhao, Sha Luo, Yuanjian Tong, Lianghua Xu ⁎ National Carbon Fiber Engineering Research Center, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 16 June 2011 Accepted 18 July 2011 Available online 22 July 2011 Keywords: Carbon fiber Crystallite Mechanical property Microstructure Grain boundaries
a b s t r a c t The growth mechanism for the graphite crystallites in polyacrylonitrile (PAN)-based carbon fibers, heat-treated at various temperatures, has been proposed. The evolution of distribution morphologies for graphite crystallites is investigated, in relation to variations in the tensile properties of the fiber samples. At high temperatures, dangling bonds are created via the cleavage of weak bonds, such as C\N, C\H and C\C at the edges of the graphite crystallites. The graphite crystallites grow through the bonding reactions between different dangling or dangling bonds and agraphitic carbon atoms. This results in graphite crystallites that increase in size with elevated temperatures that lead to changes in graphite crystallite distribution morphology. Between 1400 and 2400 °C, the distribution morphologies of the graphite crystallites go though three states: (I) a dispersed state, (II) a network state, and (III) a transfixion state. The tensile strength decreased rapidly with increasing heat treatment temperature for fibers with a dispersed state structure, but lowers with fibers bearing a network state structure. The decrease was more rapid for fibers with a transfixion state structure. The tensile modulus increased slowly in states I and II, and rapidly in state III. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiment
Polyacrylonitrile (PAN)-based carbon fibers, which are formed with polycrystalline graphite and amorphous carbon, are most widely used as reinforcement for composites. Using X-ray and transmission electron microscopy, the microstructures of carbon fibers have been studied by several authors, and various models have been proposed. These include a ribbon model [1,2], a circumferential-radial model [1,3], a two-type structure (sheath-core model) [1], and a three-dimensional model [2,4]. Extensive work has been conducted to probe the texture and mechanical properties of carbon fibers [5–10]. The modulus has been successfully modeled [11–13], but strength measurements are more challenging. The aromatic layers in the microstructures of high-modulus and high-strength PAN-based carbon fibers differ mainly in their different curvature radius, which can play a key role in terms of their mechanical properties [5,6]. From the HRTEM images in previous studies [1–8], we observed that the graphite crystallites of different carbon fibers have different morphologies, but the relationship between the different graphite crystallites is not yet understood. In the present study, carbon fibers were heat treated at various temperatures and changes in the tensile properties were measured in relation to the evolution of graphite crystallite distribution morphology.
2.1. Materials Each single tow of fiber used contained 3000 strands of monofilament (JH300, Research Institute of Jilin Petrochemical Company, China). The heat treatments were carried out in a graphite resistance furnace, under a high-purity nitrogen gas atmosphere in a continuous system. The fiber length was kept constant throughout the heat treatments. The heat treatment temperature was set at 1400–2400 °C, and at each temperature the constant temperature heat treatment time was 36 s. 2.2. Apparatus and procedures The lattice fringes of the resulting fibers were observed using high resolution transmission electron microscopy (HRTEM: Jeol ARM-1250) with an acceleration voltage of 300 kV. A tensile test for each fiber multifilament was also carried out using a universal testing machine (AG-1S, Shimadzu, Japan), with a load cell of 1 kN at a crosshead speed of 10 mm/min, according to GB/T 3362-1982. 3. Results and discussion 3.1. Growth mechanism of graphite crystallites
⁎ Corresponding author. Tel./fax: + 86 10 64435913. E-mail address: [email protected] (L. Xu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.07.057
Within carbon fibers, C\C bonds at the edge of the graphite crystallites (bond energyb 400 kJ/mol) are weaker than those inside the hexagonal graphitic network (bond energyN 500 kJ/mol). Chemical
A. Gao et al. / Materials Letters 65 (2011) 3444–3446
3445
Fig. 1. Schematic diagram for the growth of graphite crystals.
bonds involving heteroatoms, such as C\N (bond energy b 350 kJ/mol) or C\H (bond energy b 430 kJ/mol) are also weaker. During heat treatment, these weaker bonds initially break to form highly active dangling bonds (Fig. 1, Structure A to B). Those crystallites with dangling bonds grow by the dangling bonds either reacting to each other or with agraphitic carbon atoms to form more stable structure as soon as they are generated. Different bonding ways (Fig. 1C) lead to crystallites with different profiles (Fig. 1D). 3.2. Evolution of distribution morphologies The distribution morphologies of the graphite crystallites in carbon fibers can be observed intuitively based on HRTEM. Fig. 2 shows the lattice-fringe images (002) for carbon fibers heat treated at various temperatures. In the HRTEM image, a lattice-fringe stands for a single layer of graphite, and a group of lattice-fringes represents a graphite
crystallite. As shown in Fig. 2, from 1400 to 2300 °C the number of lattice-fringes significantly increases. The graphite crystallites in the carbon fibers grew with increasing heat treatment temperature, and their distribution morphologies changed. Between 1400 and 1700 °C, the carbon fibers had small crystallites dispersed within their fibers (analogous to that in Fig. 3A), with amorphous carbon or singe graphite layers in between (double arrows). A few crystallites near one another were bonded together, possibly due to the highly reactive dangling bonds. At elevated temperatures, the crystallite size increased and the probability of the crystallites coming into contact with each other increased. Therefore, cross-linking points (black single arrows) between the crystallites increased. As more and more cross-linking points appeared, a network structure (like that in Fig. 3B) was formed for almost all the graphite crystallites in the carbon fibers (1700 °C). In the crystallite growth process, the rearrangement of carbon atoms induces an internal stress, which can act on the edge of
Fig. 2. Lattice fringe images (002) of carbon fibers heat treated at various temperatures.
3446
A. Gao et al. / Materials Letters 65 (2011) 3444–3446
Fig. 3. Schematic diagrams for the three distribution morphologies of graphite crystallites.
crystallite and improve the degree of orientation. Under further elevated temperatures, the carbon atom activity increases, and at some stronger cross-linking points, the rearrangement of carbon atoms induces the combination of crystallites to form larger networks. With increasing crystallite size, mobility decreases; it becomes difficult to release internal stress by improving the crystallites degree of orientation. This increased internal stress may make some weaker cross-linking points susceptible to cleavage, and the larger crystallites can become, once again, dispersed in the carbon fibers. The sizes of these dispersed crystallites continue to increase, and the interstices (white single arrows) between them increase because of the internal contractions caused by crystallite growth. The larger crystallites can be considered to transfix the fiber like that shown in Fig. 3C. 3.3. Evolution of mechanical property The mechanical properties of carbon fibers change with increasing heat treatment temperatures, and the variations of mechanical properties can be associated with changes in crystallite distribution morphology. The tensile strength and modulus at different temperatures are shown in Fig. 4. The changes in tensile strength can be divided into three zones (I, II, III). From 1400 to 1600 °C (zone I), the tensile strength decreased rapidly with elevated temperature. In this temperature range, the crystallites were small and dispersed in the carbon fibers. According to the Griffith theory, the strength of carbon fibers is influenced by the size of inherent cracks. The sizes of inherent cracks are proportional to the sizes of the graphite crystallites, and it can be seen that larger sizes lead to a weaker tensile strength. From 1700 to 2000 °C (zone II), the tensile strength decreased slowly due to the cross-linking points between the crystallites consuming energy and preventing crack growth. Above 2000 °C (zone III), the tensile strength decreased rapidly again, because of the destruction of cross-linking points. The crystallites combined with each other and the grain boundaries reduced, the crack easily went through the grain boundaries to destroy the fibers completely. The tensile modulus reflects the contribution of a graphite structure; a network structure does not influence it. The modulus increased slowly below 2000 °C (zone I and II), due to the smaller size and lower orientation degree of the crystallites. In zone III, more and more small crystallites combine into larger crystallites, leading the modulus to increase rapidly. 4. Conclusions Systematic research has been carried out into carbon fibers, heat treatment at various temperatures using HRTEM. At higher tempera-
Fig. 4. Mechanical properties of fibers heat-treated at various temperatures.
tures, dangling bonds play a key role in the growth mechanism of graphite crystallites. Dangling bonds are created via the cleavage of weak bonds, such as C\N, C\H and C\C bonds at the edges of the graphite crystallites. These dangling bonds react with each other or with agraphitic carbon to induce graphite crystallite growth. The graphite crystallite size becomes larger with higher temperature and their distribution morphology changes. Crystallites with a small size are dispersed throughout fibers that are heat treated at lower temperatures, and the crystallites begin to interlink with one another with increasing crystallite size. The crystallites combine and become larger in size above 2100 °C and the internal stress induces the weaker interlinking points to break, re-dispersing the crystallites. The tensile strength decreased rapidly with increasing treatment temperature for fibers with a dispersed crystallite structure. The decrease was slower for fibers with a network structure, because the cross-linking points between the crystallites can effectively prevent crack propagation. The decrease was again more rapid for fibers with a transfixion structure, due to the destruction of cross-linking. The tensile modulus increased slowly for the dispersed and network structures, but rapidly for the transfixion structure. Acknowledgment Financial support from the National Basic Research Program of China (2006CB605302) is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Bennett SC, Johnson DJ. Carbon 1979;17:25–39. Diefendorf RJ, Tokarsky E. Polym Eng Sci 1975;15:150–9. Knibbs RH. J Microsc 1971;94:273–81. Crawford D, Johnson DJ. J Microsc 1971;94:51–62. Guigon M, Oberlin A, Desarmot G. Fibre Sci Technol 1984;20:177–98. Guigon M, Oberlin A, Desarmot G. Fibre Sci Technol 1984;20:55–72. Deurbergue A, Oberlin A. Carbon 1992;30(7):981–7. Guigon M, Oberlin A. Compos Sci Technol 1986;27(1):1–23. Huang Y, Young RJ. Carbon 1995;33(2):97–107. Paris O, Loidl D, Peterlik H. Carbon 2002;40(4):551–5. Johnson W. The structure of PAN based carbon fibres and relationship to physical properties. In: Watt W, Perov BV, editors. Strong fibers, vol. 1. Amsterdam: Elsevier; 1985. p. 389–443. [12] Oberlin A, Guigon M. The structure of carbon fibres. In: Bunsell AR, editor. Fibre reisnforcements for composite materials, vol. 2. Amsterdam: Elsevier; 1988. p. 149–210. [13] Northolt MG, Veldhuizen LH, Jansen H. Carbon 1991;29(8):1267–79.