CARBON PERGAMON Carbon38(2000)805-815 Mesoscopic texture at the skin area of mesophase pitch-based bon fiber Seong-Hwa Hong", Yozo Korai, Isao Mochida Received 16 December 1998; accepted 26 July 1999 Abstract The development of mesoscopic texture, which describes the structural units 10-100 nm in size, at the transverse skin area in mesophase pitch-based carbon fibers as a result of heat-treatment were examined using a high resolution scanning electron microscope(HR-SEM). The graphitized carbon fiber was found to be composed of plate-like mesoscopic structural units defined as rectangular microdomains whose dimensions were 20 nm thick, 30-50 nm wide, and 50-100 nm long along the fiber axis Graphitized fibers spun at 300 and 310.C contained microdomains which were usually arranged with their longer axis perpendicular to the fiber surface in the transverse skin area. Fibers spun at 300 and 310.C exhibited radial and random cross-sectional textures in their major core areas, respectively. The longer edges of the domains and microdomains formed the tops of the fibril and microfibril, respectively, in the longitudinal surface. The graphitized fiber spun at 340C exhibited an onion-like texture in overall area and several layers of zig-zag microdomains formed the concentric surface. The encountering edge of two zig-zag microdomain units forming the top of fibrils exhibited smooth curvature where parallel along the fiber surface. The edge of rectangular microdomain faced directly to the surface in the carbonized fiber after the removal of the soluble component. Spurs in the surface were no longer observed to run parallel to the carbonized surface of the extracted fibers regardless of the transverse textures, suggesting that the basal planes observed in the surface of the unextracted fiber originate from the soluble fraction in the mesophase pitch. 2000 Elsevier Science Ltd. All rights eserved Keywords: A. Carbon fibers, Mesophase; B. Heat treatment; C. Scanning electron microscopy (SEM); D. Textures 1. Introduction pitch and its derived carbon fibers [10]. Such a structural unit maintained its size in the transverse cross-section and Mesophase pitch-based carbon fibers have attracted longitudinal surface of the fiber up to graphitization worldwide attention because of their superior performance temperatures. The microdomains in the mesophase pitch [1-3]. The carbon fibers prepared from mesophase pit are aligned during the spinning step to form the fibers synthesized from aromatic hydrocarbon by aid of HF/B surface texture such as the pleat and fibril, and transverse as a catalyst, have excellent mechanical, thermal, and cross-section showing linear, bent or looped domains electrical properties [4-6], promising broad applications in aligned in radial, random and onion-like textures. The commercial as well as other advanced areas. In spite of pleats and fibrils appear first in the carbonized fiber their anticipated future, much higher performance is still because the carbonization of the soluble fraction follows expected through the control of their mesoscopic textures the morphology of the insoluble microdomain [11. Curved 7-9 domains in the transverse cross-section of the carbonized The present authors have reported microdomains of ca. fiber become straight during graphitization because of the 50 nm as a mesoscopic structural unit in the mesophase growth of graphene layers [12] Corresponding author. Tel. +81-92-583-7279, fax: +81-92- sIze) at the transverse skin area 10 nm deep from the 83-779 surface in the mesophase pitch based carbon fibers through E-mail address: sihong( @endomoribu shinshu-uLac jp(S. H. hea ent up to2400° and sol observed using a high resolution scanning electron micro- 0008-6223/00/S-see front matter 2000 Elsevier Science Ltd. All rights reserved PII:S0008-6223(99)00175-X
PERGAMON Carbon 38 (2000) 805–815 Mesoscopic texture at the skin area of mesophase pitch-based carbon fiber Seong-Hwa Hong , Yozo Korai, Isao Mochida * Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Received 16 December 1998; accepted 26 July 1999 Abstract The development of mesoscopic texture, which describes the structural units 10–100 nm in size, at the transverse skin area in mesophase pitch-based carbon fibers as a result of heat-treatment were examined using a high resolution scanning electron microscope (HR-SEM). The graphitized carbon fiber was found to be composed of plate-like mesoscopic structural units defined as rectangular microdomains whose dimensions were 20 nm thick, 30–50 nm wide, and 50–100 nm long along the fiber axis. Graphitized fibers spun at 300 and 3108C contained microdomains which were usually arranged with their longer axis perpendicular to the fiber surface in the transverse skin area. Fibers spun at 300 and 3108C exhibited radial and random cross-sectional textures in their major core areas, respectively. The longer edges of the domains and microdomains formed the tops of the fibril and microfibril, respectively, in the longitudinal surface. The graphitized fiber spun at 3408C exhibited an onion-like texture in overall area and several layers of zig-zag microdomains formed the concentric surface. The encountering edge of two zig-zag microdomain units forming the top of fibrils exhibited smooth curvature where spurs run parallel along the fiber surface. The edge of rectangular microdomain faced directly to the surface in the carbonized fiber after the removal of the soluble component. Spurs in the surface were no longer observed to run parallel to the carbonized surface of the extracted fibers regardless of the transverse textures, suggesting that the basal planes observed in the surface of the unextracted fiber originate from the soluble fraction in the mesophase pitch. 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibers, Mesophase; B. Heat treatment; C. Scanning electron microscopy (SEM); D. Textures 1. Introduction pitch and its derived carbon fibers [10]. Such a structural unit maintained its size in the transverse cross-section and Mesophase pitch-based carbon fibers have attracted longitudinal surface of the fiber up to graphitization worldwide attention because of their superior performance temperatures. The microdomains in the mesophase pitch [1–3]. The carbon fibers prepared from mesophase pitch are aligned during the spinning step to form the fiber’s synthesized from aromatic hydrocarbon by aid of HF/BF surface texture such as the pleat and fibril, and transverse 3 as a catalyst, have excellent mechanical, thermal, and cross-section showing linear, bent or looped domains electrical properties [4–6], promising broad applications in aligned in radial, random and onion-like textures. The commercial as well as other advanced areas. In spite of pleats and fibrils appear first in the carbonized fiber their anticipated future, much higher performance is still because the carbonization of the soluble fraction follows expected through the control of their mesoscopic textures the morphology of the insoluble microdomain [11]. Curved [7–9]. domains in the transverse cross-section of the carbonized The present authors have reported microdomains of ca. fiber become straight during graphitization because of the 50 nm as a mesoscopic structural unit in the mesophase growth of graphene layers [12]. In the present study, the mesoscopic texture (10–100 nm size) at the transverse skin area 10 nm deep from the *Corresponding author. Tel.: 181-92-583-7279; fax: 181-92- 583-7798. surface in the mesophase pitch based carbon fibers through E-mail address: shhong@endomoribu.shinshu-u.ac.jp (S.-H. heat-treatment up to 24008C and solvent extraction was Hong). observed using a high resolution scanning electron micro- 0008-6223/00/$ – see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223(99)00175-X
S.-H. Hong et al. Carbon 38(2000)805-815 scope (HR-SEM). The at the edge where the a copper grid so that they stood parallel to the electron transverse and longitudina meet tell us the three beam. The transverse cross-sectional edge of the surface dimensional arrangement nt and mo orphology of microdo- was observed by tilting the fiber 10 to the electron mains and domains. Carbon fibers spun at 300, 310, and transverse textures, respectively [12]. The extreme edges of microdomains and domains at the fiber surface may 3. Results uggest either basal planes or prismatic edges of graphene units that may form in the surface of the graphitized fiber. 3.1. HR-SEM textures of graphitized fiber The extracted fiber tells us the origins of mesoscopic texture and the influences of the extracted soluble fraction Fig. I shows HR-SEM photographs of the transverse on the surface texture cross-sectional surface in the graphitized fibers heat-treated at 2400.C, which were spun at 300, 310, and 340.C. The fiber spun at 300C(Fig. la) showed a radial texture in the 2. Experimental verall transverse surface at low magnification. The core area about I um from the fiber center showed a random 2. Material texture. The fiber spun at 310C showed a random transverse texture (Fig. 1b). An onion-like texture was A naphthalene-derived mesophase pitch of 237C sof- found in the graphitized fiber spun at 340C, exhibiting a tening point and 100% anisotropy prepared with HF/BF, hollow of I um diameter in its center(Fig. Ic). A crack as a catalyst was supplied by Mitsubishi Gas Chemical was already visible in the center of a fiber carbonized at Company [13]. The toluene and pyridine insoluble frac- 1000"C as reported in a previous paper [12] tions of tch were 48 and 32 wt% Fig. 2 shows HR-SEM photographs of the skin of the espectively graphitized fiber spun at 310C observed by holding the fiber axis parallel (0%)and tilted at 30 to the electron 2.2. Preparation of fibers beam. The photograph observed along the fiber axis(Fig 2a) shows plate-like microdomains(ca. 20 nm thickness, 340C through a spinneret with a round nozzle 03 mm in 500 nm length). Bright spurs were faun hase pitch was melt-spun at 300, 310 and 50 nm length) and domains anning along the diameter and L/D=3, using a laboratory scale monofila longer axis of the domain. a domain was found to contain ment spinning apparatus [14]. The average diameter of the several microdomains. Both domains and microdomains arbon fiber was controlled to ca. 10 um by spinning and were arranged with their longer axes perpendicular to the extrusion rates of 300 m/min and 50 mg/min, respecti fiber surface ly The photograph observed by tilting 30% to the fiber axis The as-spun fiber was extracted with pyridine in a shows the microfibrils, fibrils and pleats in the longitudinal Soxhlet apparatus at its boiling point. Extraction was surface of the fiber as well as microdomains and domains carried out for I week without agitation. The pyridine in the transverse section(Fig. 2b). The thin plate microdo- insoluble fraction(Pl) of the as-spun fiber was ca. 40 mains and domains meet perpendicular to the fiber surface wt.%. Both the as-spun and the PI fibers extracted were where their edges form the tops of the microfibrils and oxidatively stabilized in air at 270.C for 30 min using a fibrils, respectively. The domains at the skin area along the heating rate of 0,5C/ min. Both stabilized fibers were fiber axis form fibrils ca 50-150 nm thickness A rela carbonized at 300-1500.C in an Ar flow using a heating tively thick domain composed of several microdomains rate of 10C/min. The carbonized fibers were further formed a thick fibril ca. 100-150 nm wide. while a thin graphitized at 2000 or 2400.C for 30 min using a heating fibril of ca 30-50 nm width was formed by the arrange. rate of 6.7C/ min in Ar flow ment of relatively thin domains. The arrangement of microdomains at the skin area formed microfibrils of ca 20 2.3. HR-sEM observation of carbon fiber m within the fibrils. Several microdomains in the skin area formed the domains perpendicular to the surface The texture at the skin area of the mesophase pitch- which merged with the fibrils based carbon fibers was observed by a high resolution In the graphitized carbon fiber, the three-dimensional canning electron microscope (HR-SEM, JEOL JSM olate-like shape of the microdomain was observed to form 6320F)at magnifications of 100,000 and 200,000X. The both longitudinal and transverse cross-sectional surfaces as-spun fibers were observed after coating with about 0.2 The long edge of a plate was oriented along the fiber axis nm of platinum using ion beam sputtering. Fibers heat- and the other edge merged perpendicular to the fiber treated above 700%C were observed without such a coating. surface at the skin area. Although the microdomains were All fibers were cut in liquid nitrogen and were attached to continuously connected along the longitudinal axis of the
806 S.-H. Hong et al. / Carbon 38 (2000) 805 –815 scope (HR-SEM). The textures at the edge where the a copper grid so that they stood parallel to the electron transverse and longitudinal surfaces meet tell us the three- beam. The transverse cross-sectional edge of the surface dimensional arrangement and morphology of microdo- was observed by tilting the fiber 108 to the electron beam. mains and domains. Carbon fibers spun at 300, 310, and 3408C were reported to show radial, random, and onion transverse textures, respectively [12]. The extreme edges of microdomains and domains at the fiber surface may 3. Results suggest either basal planes or prismatic edges of graphene units that may form in the surface of the graphitized fiber. 3.1. HR-SEM textures of graphitized fibers The extracted fiber tells us the origins of mesoscopic texture and the influences of the extracted soluble fraction Fig. 1 shows HR-SEM photographs of the transverse on the surface texture. cross-sectional surface in the graphitized fibers heat-treated at 24008C, which were spun at 300, 310, and 3408C. The fiber spun at 3008C (Fig. 1a) showed a radial texture in the 2. Experimental overall transverse surface at low magnification. The core area about 1 mm from the fiber center showed a random 2.1. Material texture. The fiber spun at 3108C showed a random transverse texture (Fig. 1b). An onion-like texture was A naphthalene-derived mesophase pitch of 2378C sof- found in the graphitized fiber spun at 3408C, exhibiting a tening point and 100% anisotropy prepared with HF/BF hollow of 1 mm diameter in its center (Fig. 1c). A crack 3 as a catalyst was supplied by Mitsubishi Gas Chemical was already visible in the center of a fiber carbonized at Company [13]. The toluene and pyridine insoluble frac- 10008C as reported in a previous paper [12]. tions of the mesophase pitch were 48 and 32 wt.%, Fig. 2 shows HR-SEM photographs of the skin of the respectively. graphitized fiber spun at 3108C observed by holding the fiber axis parallel (08) and tilted at 308 to the electron 2.2. Preparation of fibers beam. The photograph observed along the fiber axis (Fig. 2a) shows plate-like microdomains (ca. 20 nm thickness, The mesophase pitch was melt-spun at 300, 310 and 50 nm length) and domains (ca. 100–150 nm thickness, 3408C through a spinneret with a round nozzle 0.3 mm in 500 nm length). Bright spurs were found running along the diameter and L/D 5 3, using a laboratory scale monofila- longer axis of the domain. A domain was found to contain ment spinning apparatus [14]. The average diameter of the several microdomains. Both domains and microdomains carbon fiber was controlled to ca. 10 mm by spinning and were arranged with their longer axes perpendicular to the extrusion rates of 300 m/min and 50 mg/min, respective- fiber surface. ly. The photograph observed by tilting 308 to the fiber axis The as-spun fiber was extracted with pyridine in a shows the microfibrils, fibrils and pleats in the longitudinal Soxhlet apparatus at its boiling point. Extraction was surface of the fiber as well as microdomains and domains carried out for 1 week without agitation. The pyridine in the transverse section (Fig. 2b). The thin plate microdoinsoluble fraction (PI) of the as-spun fiber was ca. 40 mains and domains meet perpendicular to the fiber surface wt.%. Both the as-spun and the PI fibers extracted were where their edges form the tops of the microfibrils and oxidatively stabilized in air at 2708C for 30 min using a fibrils, respectively. The domains at the skin area along the heating rate of 0.58C/min. Both stabilized fibers were fiber axis form fibrils ca. 50–150 nm thickness. A relacarbonized at 300–15008C in an Ar flow using a heating tively thick domain composed of several microdomains rate of 108C/min. The carbonized fibers were further formed a thick fibril ca. 100–150 nm wide, while a thin graphitized at 2000 or 24008C for 30 min using a heating fibril of ca. 30–50 nm width was formed by the arrangerate of 6.78C/min in Ar flow. ment of relatively thin domains. The arrangement of microdomains at the skin area formed microfibrils of ca. 20 2.3. HR-SEM observation of carbon fibers nm within the fibrils. Several microdomains in the skin area formed the domains perpendicular to the surface The texture at the skin area of the mesophase pitch- which merged with the fibrils. based carbon fibers was observed by a high resolution In the graphitized carbon fiber, the three-dimensional scanning electron microscope (HR-SEM, JEOL JSM plate-like shape of the microdomain was observed to form 6320F) at magnifications of 100,000 and 200,0003. The both longitudinal and transverse cross-sectional surfaces. as-spun fibers were observed after coating with about 0.2 The long edge of a plate was oriented along the fiber axis nm of platinum using ion beam sputtering. Fibers heat- and the other edge merged perpendicular to the fiber treated above 7008C were observed without such a coating. surface at the skin area. Although the microdomains were All fibers were cut in liquid nitrogen and were attached to continuously connected along the longitudinal axis of the
S.-H. Hong et al. Carbon 38(2000)805-815 807 indicating curved basal planes parallel to the surface of the fiber(Fig. 2a-B) Fibrils and pleats were also observed on the surface fractured parallel to the fiber axis as well as on the outer surface (Fig. 2b-A). Cracks developed between the do- mains in the transverse cross-sectional surface. The frac tures induced by these cracks were observed to propagate to gaps between the fibrils parallel to the fiber axis as Fig 3 shows HR-sEM photographs of the skin areas in the three fibers spun at 300, 310, and 340.C and graphit ized at 2400C The fibers spun at 300(Fig 3a)and 310C (Fig. 3b)showed almost the same mesoscopic textures in the skin area in spite of the different overall transverse texture. The size and shape of the fibrils, microfibrils and pleats in the longitudinal surface were almost the same those of the two graphitized fibers. Spherical alignment of purs was commonly observed along the top of the microdomain(points A in Fig. 3a and b) The thickness of the fibrils in the longitudinal surface of the graphitized fiber spun at 340.C (Fig. 3c)was much larger than observed in the graphitized fibers spun at 300 nd 310C. The fiber with the onion-like texture exhibited definite zig-zag layers of microdomains along the fiber surface. The encountering edges of two microdomains connected to each other formed the curved top merging into the fibril in the longitudinal surface as shown at A in 249015KU Fig. 3c. The spurs also run parallel to the surface within the microdomain, exhibiting smooth curvature where they et(point B in Fig 3c) 3. 2. HR-SEM textures of carbonised fibers Fig 4 shows HR-SEM photographs of the skin area in the carbon fibers heat-treated at 1000C. Although the domains exhibited a round shape and vague contour which were more bent at the skin area than in the respective fibers graphitized at 2400"C, the mesoscopic textures in the transverse skin area were almost the same as those of the respective graphitized fibers. The shorter edges of the 34-24 rectangular domains and microdomains also faced the surface, forming fibrils and microfibrils, respective Fig. 1. HR-SEM photographs of transverse cross-section of the shown at points A in Fig 4a and b. The carbonized fiber mesophase pitch-based carbon fibers heat-treated at 2400 In at 340C exhibited zig-zag layers of microdomains spinning temperature(a) 300, (b)310, and(c)340.C along the fiber surface as shown at a in Fig. 4c. The fibrils were almost the same thickness(ca. 50-150 nm) as those of the graphitized fiber. The parallel line up of spurs along fiber, valleys between microdomains were clearly observed the fiber periphery at the surface was also observed, in the surface although the thickness of the surface layer was larger edge of the rectangular microdomain within ca. 10 (around 50 nm) than that of the graphitized fibers nm of the fiber surface showed a round top which alignee parallel to the surface. The high resolution closed up spurs 3.3. HR-SEM textures of as-spun and extracted fibers running within the microdomain. They were basically aligned parallel to the longer axis of the microdomain as Fig 5 shows HR-SEM photographs of the skin area in shown in Fig. 2a-A. However, the spurs were observed the three as-spun fibers. Th scopic textures in the spherically aligned at the very top of the microdomain, transverse skin areas of these three kinds of as-spun fibers
S.-H. Hong et al. / Carbon 38 (2000) 805 –815 807 indicating curved basal planes parallel to the surface of the fiber (Fig. 2a-B). Fibrils and pleats were also observed on the surface fractured parallel to the fiber axis as well as on the outer surface (Fig. 2b-A). Cracks developed between the domains in the transverse cross-sectional surface. The fractures induced by these cracks were observed to propagate to gaps between the fibrils parallel to the fiber axis as shown at B in Fig. 2b. Fig. 3 shows HR-SEM photographs of the skin areas in the three fibers spun at 300, 310, and 3408C and graphitized at 24008C. The fibers spun at 300 (Fig. 3a) and 3108C (Fig. 3b) showed almost the same mesoscopic textures in the skin area in spite of the different overall transverse texture. The size and shape of the fibrils, microfibrils and pleats in the longitudinal surface were almost the same as those of the two graphitized fibers. Spherical alignment of spurs was commonly observed along the top of the microdomain (points A in Fig. 3a and b). The thickness of the fibrils in the longitudinal surface of the graphitized fiber spun at 3408C (Fig. 3c) was much larger than observed in the graphitized fibers spun at 300 and 3108C. The fiber with the onion-like texture exhibited definite zig-zag layers of microdomains along the fiber surface. The encountering edges of two microdomains connected to each other formed the curved top merging into the fibril in the longitudinal surface as shown at A in Fig. 3c. The spurs also run parallel to the surface within the microdomain, exhibiting smooth curvature where they meet (point B in Fig. 3c). 3.2. HR-SEM textures of carbonized fibers Fig. 4 shows HR-SEM photographs of the skin area in the carbon fibers heat-treated at 10008C. Although the domains exhibited a round shape and vague contour which were more bent at the skin area than in the respective fibers graphitized at 24008C, the mesoscopic textures in the transverse skin area were almost the same as those of the respective graphitized fibers. The shorter edges of the rectangular domains and microdomains also faced the surface, forming fibrils and microfibrils, respectively, as shown at points A in Fig. 4a and b. The carbonized fiber Fig. 1. HR-SEM photographs of transverse cross-section of the spun at 3408C exhibited zig-zag layers of microdomains mesophase pitch-based carbon fibers heat-treated at 24008C: along the fiber surface as shown at A in Fig. 4c. The fibrils spinning temperature (a) 300, (b) 310, and (c) 3408C. were almost the same thickness (ca. 50–150 nm) as those of the graphitized fiber. The parallel line up of spurs along fiber, valleys between microdomains were clearly observed the fiber periphery at the surface was also observed, in the surface. although the thickness of the surface layer was larger The edge of the rectangular microdomain within ca. 10 (around 50 nm) than that of the graphitized fibers. nm of the fiber surface showed a round top which aligned parallel to the surface. The high resolution closed up spurs 3.3. HR-SEM textures of as-spun and extracted fibers running within the microdomain. They were basically aligned parallel to the longer axis of the microdomain as Fig. 5 shows HR-SEM photographs of the skin area in shown in Fig. 2a-A. However, the spurs were observed the three as-spun fibers. The mesoscopic textures in the spherically aligned at the very top of the microdomain, transverse skin areas of these three kinds of as-spun fibers
S.-H. Hong et al. Carbon 38(2000)805-815 B ,m;b°8 Fig. 2. HR-SEM photographs of the skin area observed from(a)parallel (0)and(b)30% to the fiber axis in the mesophase pitch-based carbon fiber heat-treated at 2400 C: spinning temperature 310.C. The longer axes of the microdomains are aligned parallel to the fiber surface as shown in A. The spurs are also aligned parallel to the longer axis of the microdomains within them as shown in B did not show any differences. Neither fibrils nor pleats 4. Discussion were observed in the longitudinal surface, although wavy ripples were evident. Microdomains of ca. 50 nm in 4.1. Mesoscopic texture at the skin area of mesophase diameter were observed in the transverse cross-sectional surface, although no particular transverse texture was The present study was aimed at clarifying the mesos- Fig 6 shows HR-SEM photographs of the skin area copic texture of mesophase pitch-based graphitized carbon ne three as-spun fibers after extraction with pyridine and fibers especially at the skin area in its transverse section successively heat-treated at 1000C. These fibers had where the texture exhibits three-dimensional shape and definitely more straight domains than those of the carbon- dimension of the microunits in the carbon fiber. The ized fibers obtained without pyridine extraction regardless graphitized fiber consists of rectangular plate -like micro of the macroscopic transverse textures. The spurs and mains of dimensions 20 nm thick, 30-50 nm wide, and graphene edges along the longer axis of microdomain 100-150 nm long. Several such microdomains form faced directly to the fiber surface after the removal of the domains with linear, bent and loop shapes in the transverse soluble component. The microdomains and spurs run section. The domains are macroscopically arranged typi parallel to the fiber surface disappeared in the carbonized cally in radial, random, and onion-like texture as often fiber after the extraction, although the orientation of reported [15,16] microdomains was basically the same as for carbonized The edges of the microdomains in the transverse section fibers without extraction. A much sharper angle wa are of special interest in the present study. The longer observed at the junction of two zig- zag microdomains at dges of the rectangular plate-like microdomains are the skin area in the extracted as-spun fiber spun at 340C oriented perpendicular to the surface in the radial and an in the corresponding graphitized fiber. Hence, no spur random textures, indicating apparently that the prismatic running around the top or any encounter edge of the edges may meet perpendicular to the fiber surface, al microdomains was observed any longer as shown at A in though domains in the former and latter textures are Fig 6a-c principally radial and very random, as illustrated in Fig. 7
808 S.-H. Hong et al. / Carbon 38 (2000) 805 –815 Fig. 2. HR-SEM photographs of the skin area observed from (a) parallel (08) and (b) 308 to the fiber axis in the mesophase pitch-based carbon fiber heat-treated at 24008C: spinning temperature 3108C. The longer axes of the microdomains are aligned parallel to the fiber surface as shown in A. The spurs are also aligned parallel to the longer axis of the microdomains within them as shown in B. did not show any differences. Neither fibrils nor pleats 4. Discussion were observed in the longitudinal surface, although wavy ripples were evident. Microdomains of ca. 50 nm in 4.1. Mesoscopic texture at the skin area of mesophase diameter were observed in the transverse cross-sectional pitch-based graphitized fiber surface, although no particular transverse texture was recognized. The present study was aimed at clarifying the mesosFig. 6 shows HR-SEM photographs of the skin area in copic texture of mesophase pitch-based graphitized carbon the three as-spun fibers after extraction with pyridine and fibers especially at the skin area in its transverse section successively heat-treated at 10008C. These fibers had where the texture exhibits three-dimensional shape and definitely more straight domains than those of the carbon- dimension of the microunits in the carbon fiber. The ized fibers obtained without pyridine extraction regardless graphitized fiber consists of rectangular plate-like microdoof the macroscopic transverse textures. The spurs and mains of dimensions 20 nm thick, 30–50 nm wide, and graphene edges along the longer axis of microdomain 100–150 nm long. Several such microdomains form faced directly to the fiber surface after the removal of the domains with linear, bent and loop shapes in the transverse soluble component. The microdomains and spurs running section. The domains are macroscopically arranged typiparallel to the fiber surface disappeared in the carbonized cally in radial, random, and onion-like texture as often fiber after the extraction, although the orientation of reported [15,16]. microdomains was basically the same as for carbonized The edges of the microdomains in the transverse section fibers without extraction. A much sharper angle was are of special interest in the present study. The longer observed at the junction of two zig-zag microdomains at edges of the rectangular plate-like microdomains are the skin area in the extracted as-spun fiber spun at 3408C oriented perpendicular to the surface in the radial and than in the corresponding graphitized fiber. Hence, no spur random textures, indicating apparently that the prismatic running around the top or any encounter edge of the edges may meet perpendicular to the fiber surface, almicrodomains was observed any longer as shown at A in though domains in the former and latter textures are Fig. 6a–c. principally radial and very random, as illustrated in Fig. 7
S.-H. Hong et al. Carbon 38(2000)805-815 ⅹ100,000 X200,000 15KU X10 39-2 A B 34-241sK0198 34-24 I photographs of the skin area in the ase pitch-based carbon fibers heat-treated at 2400C: spinning temperature (a) 300,(b)310, and(c)340C. Spherical alignment of spurs was observed along the top of the microdomain as shown at points A in(a)and b). The encountering edges of two microdomains formed the curved top merging into the fibril as shown at A in(c). The spurs also run parallel to the surface within the microdomain as shown at B in(c). In marked contrast, several layers of microdomains in the layer was under 10 nm. Hence, basal planes appear to onion-like texture are arranged in a zig-zag manner with the major surface of the fiber as expected from the their longer edges along the surface, connecting smoothly
S.-H. Hong et al. / Carbon 38 (2000) 805 –815 809 Fig. 3. HR-SEM photographs of the skin area in the mesophase pitch-based carbon fibers heat-treated at 24008C: spinning temperature (a) 300, (b) 310, and (c) 3408C. Spherical alignment of spurs was observed along the top of the microdomain as shown at points A in (a) and (b). The encountering edges of two microdomains formed the curved top merging into the fibril as shown at A in (c). The spurs also run parallel to the surface within the microdomain as shown at B in (c). In marked contrast, several layers of microdomains in the layer was under 10 nm. Hence, basal planes appear to form onion-like texture are arranged in a zig-zag manner with the major surface of the fiber as expected from the their longer edges along the surface, connecting smoothly macroscopic view. two microdomains at their longer edges where the small The longer edge of a microdomain in the transverse basal planes bridge these two edges. The thickness of the section of radial and random textures appears to merge into
10 S.-H. Hong et al. Carbon 38(2000)805-815 A Fig 4. HR-SEM photographs of the skin area in the mesophase pitch-based carbon fibers heat-treated at 1000.C: spinning temperature(a) 300,(b)310, and(c)340C. The shorter edges of the rectangular microdomains and domains faced the surface, forming microfibrils and fibrils, respectively, as shown at points A in(a)and(b). Zig-zag layers of microdomains along the fiber surface were observed as shown at A a microfibril at the surface of the fiber. Hence, the mains corresponds to the hill within a fibril in the fiber of thickness of a microfibril reflects the width of a microdo- onion -like texture main plate along the fiber axis. In contrast, the longer axis Such a carbon fiber mesostructure raises the question of of a plate-like microdomain forms a half of the fibril in the whether graphene prismatic edges or basal planes cover the onion-like texture. Hence, the thickness of a fibril in this surface. The arrangement of the rectangular microdomains texture is much larger than that observed in the radial or eems to suggest that prismatic edges do this for the radial random textures. The connection point of two microdo- and random textures while basal planes do it for the
810 S.-H. Hong et al. / Carbon 38 (2000) 805 –815 Fig. 4. HR-SEM photographs of the skin area in the mesophase pitch-based carbon fibers heat-treated at 10008C: spinning temperature (a) 300, (b) 310, and (c) 3408C. The shorter edges of the rectangular microdomains and domains faced the surface, forming microfibrils and fibrils, respectively, as shown at points A in (a) and (b). Zig-zag layers of microdomains along the fiber surface were observed as shown at A in (c). a microfibril at the surface of the fiber. Hence, the mains corresponds to the hill within a fibril in the fiber of thickness of a microfibril reflects the width of a microdo- onion-like texture. main plate along the fiber axis. In contrast, the longer axis Such a carbon fiber mesostructure raises the question of of a plate-like microdomain forms a half of the fibril in the whether graphene prismatic edges or basal planes cover the onion-like texture. Hence, the thickness of a fibril in this surface. The arrangement of the rectangular microdomains texture is much larger than that observed in the radial or seems to suggest that prismatic edges do this for the radial random textures. The connection point of two microdo- and random textures while basal planes do it for the
S.-H. Hong et al. Carbon 38(2000)805-815 Nds I b Nndst2i Nnldst2 Fig. 5. HR-SEM photographs of the skin area in the mesophase pitch-based as-spun fibers: spinning temperature (a)300,(b)310, and(c) onion-like texture. However, observation under very high magnification exhibits a round surface in the former two 如 rbon surface in the oxi fibers where spurs of graphene layers were arranged to follow the morphology of the top of the microdomain, 4.2. Development of mesoscopic texture suggesting that basal planes also cover the surfaces of both radial and random textures. STM of the graphitized surface Mesoscopic texture has been reported to develop during suggests the dominance of basal planes on the surface of the spinning, carbonization, and graphitization steps the carbon fiber [17]. The first layer of the surface must be [11, 12]. The mesophase pitch carries mesoscopic units of microscopically analyzed, since the surface may govern od-like microdomains which are arranged by the spinning
S.-H. Hong et al. / Carbon 38 (2000) 805 –815 811 Fig. 5. HR-SEM photographs of the skin area in the mesophase pitch-based as-spun fibers: spinning temperature (a) 300, (b) 310, and (c) 3408C. onion-like texture. However, observation under very high the reactivity of carbon surface in the oxidative pretreatmagnification exhibits a round surface in the former two ment of the carbon fiber as a composite filler. fibers where spurs of graphene layers were arranged to follow the morphology of the top of the microdomain, 4.2. Development of mesoscopic texture suggesting that basal planes also cover the surfaces of both radial and random textures. STM of the graphitized surface Mesoscopic texture has been reported to develop during suggests the dominance of basal planes on the surface of the spinning, carbonization, and graphitization steps the carbon fiber [17]. The first layer of the surface must be [11,12]. The mesophase pitch carries mesoscopic units of microscopically analyzed, since the surface may govern rod-like microdomains which are arranged by the spinning
S.-H. Hong et al. Carbon 38(2000)805-815 X50,000 X200,000 6己 A ∩s ulIdIa 品。三 d 8 a tIde A Fig. 6. HR-SEM photographs of the skin area in the mesophase pitch-based as-spun fibers extracted with pyridine and successively heat-treated at 1000'C: spinning temperature(a)300,(b)310, and (c)340C. No spur running around the top or any encounter edge of the microdomains was observed any longer as shown at points A in(a)-(c) process [10]. Any microdomain in the mesophase pitch texture and hence a macroscopic texture, the soluble and as-spun fiber is not distinguishable unless extraction fraction being converted into infusible carbon by maintain- closes up the arrangement of the insoluble fractions. It ing the arrangement of the insoluble fraction by virtue of must be emphasized that the arrangement of the insoluble the stabilization [12]. Such an gin and development microdomains is basically maintained in the graphitized scheme of the mesoscopic textures is illustrated in Fig. 7 iber, acting as the skeleton of the mesoscopic texture he as-spun fibers do not show any particular transvers Carbonization above 700C develops a definite mesoscopic texture because of the soluble fraction. However, solvent
812 S.-H. Hong et al. / Carbon 38 (2000) 805 –815 Fig. 6. HR-SEM photographs of the skin area in the mesophase pitch-based as-spun fibers extracted with pyridine and successively heat-treated at 10008C: spinning temperature (a) 300, (b) 310, and (c) 3408C. No spur running around the top or any encounter edge of the microdomains was observed any longer as shown at points A in (a)–(c). process [10]. Any microdomain in the mesophase pitch texture and hence a macroscopic texture, the soluble and as-spun fiber is not distinguishable unless extraction fraction being converted into infusible carbon by maintaincloses up the arrangement of the insoluble fractions. It ing the arrangement of the insoluble fraction by virtue of must be emphasized that the arrangement of the insoluble the stabilization [12]. Such an origin and development microdomains is basically maintained in the graphitized scheme of the mesoscopic textures is illustrated in Fig. 7. fiber, acting as the skeleton of the mesoscopic texture. The as-spun fibers do not show any particular transverse Carbonization above 7008C develops a definite mesoscopic texture because of the soluble fraction. However, solvent
S.-H. Hong et al. Carbon 38(2000)805-815 Extraction H PI Carbonization M 200 Graphitization 2400°C Radial Extractio 1000°C PI-1000C Graphitization (Random Fig. 7. Origin and development scheme of the transverse skin area in the mesophase pitch-based carbon fibers through the heat-treatment
S.-H. Hong et al. / Carbon 38 (2000) 805 –815 813 Fig. 7. Origin and development scheme of the transverse skin area in the mesophase pitch-based carbon fibers through the heat-treatment
S.-H. Hong et al. Carbon 38(2000)805-815 W/N=0 Carbonization 尖終终 1000°C PI-1000°C 2400°C Onion extraction closes up the transverse textures such as radial, parallel to the surface as observed in the former radial random, and onion shape according to the spinning tem- random, and the latter onion-like textures, respectively peratures, although no line up of spurs in the microdomain is observable in the fibers at this stage. In the radial fiber, 4.3. Spinning linear domains are dominant in the skin area, being placed perpendicular to the fiber surface, and the linearity of The rod-like microdomains in the mes pitch are domains increases during graphitization. The bent domains deformed by spinning into plates with ci in the random fiber are also maintained up to the graphiti- in the as-spun fiber. Such plates are zation temperature of 2400C. The fiber spun at 340C, longitudinal direction to form a microfibril. An importan which is macroscopically of an onion-like texture, exhibits factor in orientation is whether the shorter or longer edges a zig-zag alignment of microdomains in the skin area and of the microdomain plate face the surface. While a lower shows a sharper angle between two microdomains in the viscosity seems to favor shorter edges, a higher viscosity arbonized fiber than for the graphitized fiber. The favors longer edges. The viscosity-orientation correlation graphitization enlarges the graphene sheet to flatten the has been discussed from a macroscopic view to explain the encounter of the graphene units because of the shrinkage radial, random and onion textures of the domains [14] along the radial direction Hence the typical microdomain orientation-viscosity cor The graphite structure is developed by graphitization relation may be restricted to a skin area of a few hundred- above 1500C where the graphene layers grow in two nanometres thickness where strong interaction with the directions of stacking height and area, increasing both L. spinneret wall governs the orientation. and La values, respectively, in the graphitizable carbon Whether basal plane or prismatic edges cover the Such crystal growth changes the rod-like microdomain into surface of the fiber cannot be exclusively concluded with a rectangular shape and gently curved-domains consisting the resolution of HR-SEM of the present study. The spurs of several microdomains are forced to take linear, bent and and graphene edges directly face the fiber surface after the loop shapes with sharper angles at their connections. Such removal of the soluble component and successive carboni- development of mesoscopic texture is also true at the skin zation as described above, suggesting that the basal plane area in the transverse section. The straight rectangular observed in the fiber surface may originate from smaller plates are arranged perpendicularly or in a zig-zag fashion molecules of the soluble fraction which become stacked
814 S.-H. Hong et al. / Carbon 38 (2000) 805 –815 Fig. 7. (continued) extraction closes up the transverse textures such as radial, parallel to the surface as observed in the former radial, random, and onion shape according to the spinning tem- random, and the latter onion-like textures, respectively. peratures, although no line up of spurs in the microdomain is observable in the fibers at this stage. In the radial fiber, 4.3. Spinning linear domains are dominant in the skin area, being placed perpendicular to the fiber surface, and the linearity of The rod-like microdomains in the mesophase pitch are domains increases during graphitization. The bent domains deformed by spinning into plates with curved peripheries in the random fiber are also maintained up to the graphiti- in the as-spun fiber. Such plates are arranged in the zation temperature of 24008C. The fiber spun at 3408C, longitudinal direction to form a microfibril. An important which is macroscopically of an onion-like texture, exhibits factor in orientation is whether the shorter or longer edges a zig-zag alignment of microdomains in the skin area and of the microdomain plate face the surface. While a lower shows a sharper angle between two microdomains in the viscosity seems to favor shorter edges, a higher viscosity carbonized fiber than for the graphitized fiber. The favors longer edges. The viscosity–orientation correlation graphitization enlarges the graphene sheet to flatten the has been discussed from a macroscopic view to explain the encounter of the graphene units because of the shrinkage radial, random and onion textures of the domains [14]. along the radial direction. Hence the typical microdomain orientation–viscosity corThe graphite structure is developed by graphitization relation may be restricted to a skin area of a few hundredabove 15008C where the graphene layers grow in two nanometres thickness where strong interaction with the directions of stacking height and area, increasing both L spinneret wall governs the orientation. c and L values, respectively, in the graphitizable carbon. Whether basal plane or prismatic edges cover the a Such crystal growth changes the rod-like microdomain into surface of the fiber cannot be exclusively concluded with a rectangular shape and gently curved-domains consisting the resolution of HR-SEM of the present study. The spurs of several microdomains are forced to take linear, bent and and graphene edges directly face the fiber surface after the loop shapes with sharper angles at their connections. Such removal of the soluble component and successive carbonidevelopment of mesoscopic texture is also true at the skin zation as described above, suggesting that the basal plane area in the transverse section. The straight rectangular observed in the fiber surface may originate from smaller plates are arranged perpendicularly or in a zig-zag fashion molecules of the soluble fraction which become stacked