CARBON Carbon37(1999)917-930 Development of mesoscopic textures in transverse croSs-section of mesophase pitch-based carbon fibers Seong-Hwa Hong, Yozo Korai, Isao Mochida Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Received 10 June 1998, accepted 29 August 1998 Abstract Development of the transversal texture in the mesophase pitch-based as-spun at 310.C, stabilized, carbonized, calcined and graphitized fibers was examined using a high resolution scanning electron microscope(Hr-SEM). The cross-sectional surface of the as-spun fiber showed randomly oriented microdomains of spherical or ellipsoidal shape, ca. 50 nm long in their longer axis. Such shape and orientation of the microdomain stayed almost the same up to 500C, where the microdomain took a plate shape with some curvatures. The heat treatment up to 700"C induced the connection and orientation of microdomains through the shrinkage due to the evolution of volatile gas, forming the definite domain. Some oids were also observed among the microdomains. The heat treatment at 700C enhanced the growth of carbon planes and their layer stacking in the microdomain. The cross-sectional domain showed gentle curvatures in the carbonized fiber at 700 to 1500.C owing to smaller hexagons. The domain shape was basically not changed up to graphitization. The curved domain became straight during graphitization up to 2000 C, owing to the growth of graphitic planes. The as-spun fiber was extracted with pyridine to observe its microtexture. The extracted as-spun fiber spun at 310C showed macroscopically the same random transversal texture at low magnification to that of the carbonized fiber. The microdomain in the extracted as-spun fiber was already straight and oriented in the same direction as that observed in the graphitized fiber, no curvature being observable. Two other fibers spun at 300 and 340C to show the radial and onion transverse textures, respectively, exhibited the same trend of as-spun and graphitized fibers. Such a series of observation suggests that the insoluble fraction frames the shape, orientation and arrangement of microdomains in the graphitized fiber. The soluble fraction in the as-spun fiber formed the curvature of the mesophase microdomain to be carbonized to follow the shape of the oriented insoluble fraction. The texture in the extracted as-spun fiber was basically maintained after the carbonization, although bright spurs appeared in the insoluble microdomain after the carbonization of extracted as-spun fiber. The carbonization of extracted as-spun fiber induced the very straight periphery of the shorter domain. Less bending was observable in the heat-treated fibers after extraction of as-spun fiber, suggesting the connecting roles of the soluble fraction in the growth of domain. The stacking of carbon layers observed as spurs in the microdomain is basically directed along the longer axis of the shape and alignment of microdomains. o 1999 Elsevier Science Ltd. All rights reserved Keywords: A. Carbon fibers, Mesophase: B. Heat treatment; C Scanning electron py(SEM); D. Textures 1. Introduction control of the meso and micro structures of the mesophase pitch-based carbon fiber. Higher compressive strength and Mesophase pitch-based carbon fiber has been recognized thermal conductivity are targeted to emphasize the advan- as a strategic materials because of its excellent mechanical, tages of the mesophase pitch-based carbon fiber. The electrical, and thermal properties [1-5]. Although the control of its meso and microscopic textures appears to be arbon fiber prepared from the synthetic mesophase pitch the key to obtain such performances of much higher derived from pure aromatic hydrocarbon by the aid of quality [6-10] HF/BF3 has been commercialized, much higher perform- The control of transverse cross-sectional texture during ances and lower cost were still expected through the spinning has been believed to be one of the most relevant problems [11]. The carbon fiber has been reported to Corresponding author exhibit variable transverse textures, which basically reflect 7S-see front matter 1999 Elsevier Science Ltd. All rights reserve S0008-6223(98)00236-X
Carbon 37 (1999) 917–930 Development of mesoscopic textures in transverse cross-section of mesophase pitch-based carbon fibers Seong-Hwa Hong, Yozo Korai, Isao Mochida* Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Received 10 June 1998; accepted 29 August 1998 Abstract Development of the transversal texture in the mesophase pitch-based as-spun at 3108C, stabilized, carbonized, calcined and graphitized fibers was examined using a high resolution scanning electron microscope (HR–SEM). The cross-sectional surface of the as-spun fiber showed randomly oriented microdomains of spherical or ellipsoidal shape, ca. 50 nm long in their longer axis. Such shape and orientation of the microdomain stayed almost the same up to 5008C, where the microdomain took a plate shape with some curvatures. The heat treatment up to 7008C induced the connection and orientation of microdomains through the shrinkage due to the evolution of volatile gas, forming the definite domain. Some voids were also observed among the microdomains. The heat treatment at 7008C enhanced the growth of carbon planes and their layer stacking in the microdomain. The cross-sectional domain showed gentle curvatures in the carbonized fiber at 700 to 15008C owing to smaller hexagons. The domain shape was basically not changed up to graphitization. The curved domain became straight during graphitization up to 20008C, owing to the growth of graphitic planes. The as-spun fiber was extracted with pyridine to observe its microtexture. The extracted as-spun fiber spun at 3108C showed macroscopically the same random transversal texture at low magnification to that of the carbonized fiber. The microdomain in the extracted as-spun fiber was already straight and oriented in the same direction as that observed in the graphitized fiber, no curvature being observable. Two other fibers spun at 300 and 3408C to show the radial and onion transverse textures, respectively, exhibited the same trend of as-spun and graphitized fibers. Such a series of observation suggests that the insoluble fraction frames the shape, orientation and arrangement of microdomains in the graphitized fiber. The soluble fraction in the as-spun fiber formed the curvature of the mesophase microdomain to be carbonized to follow the shape of the oriented insoluble fraction. The texture in the extracted as-spun fiber was basically maintained after the carbonization, although bright spurs appeared in the insoluble microdomain after the carbonization of extracted as-spun fiber. The carbonization of extracted as-spun fiber induced the very straight periphery of the shorter domain. Less bending was observable in the heat-treated fibers after extraction of as-spun fiber, suggesting the connecting roles of the soluble fraction in the growth of domain. The stacking of carbon layers observed as spurs in the microdomain is basically directed along the longer axis of the shape and alignment of microdomains. 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibers; Mesophase; B. Heat treatment; C. Scanning electron microscopy (SEM); D. Textures 1. Introduction control of the meso and micro structures of the mesophase pitch- based carbon fiber. Higher compressive strength and Mesophase pitch-based carbon fiber has been recognized thermal conductivity are targeted to emphasize the advanas a strategic materials because of its excellent mechanical, tages of the mesophase pitch-based carbon fiber. The electrical, and thermal properties [1–5]. Although the control of its meso and microscopic textures appears to be carbon fiber prepared from the synthetic mesophase pitch the key to obtain such performances of much higher derived from pure aromatic hydrocarbon by the aid of quality [6–10]. HF/BF3 has been commercialized, much higher perform- The control of transverse cross-sectional texture during ances and lower cost were still expected through the spinning has been believed to be one of the most relevant problems [11]. The carbon fiber has been reported to *Corresponding author. exhibit variable transverse textures, which basically reflect 0008-6223/99/$ – see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S0008-6223(98)00236-X
918 S.H. Hong et al./ Carbon 37(1999)917-930 the arrangement in the hexagonal carbon planes in the fiber 2. 2. Preparation of fibers cross-section, although their assemblies form the cluster, microdomain. and domain to show the textures. The The mesophase pitch was melt-spun at 300, 310 and transverse texture of the carbon fiber is typically classified 340C, respectively, through a spinneret with a round into thee categories at rather macroscopic level: radial, nozzle of 0.3 mm in diameter and L/D=3 using a random, and onion [12]. Such arrangements are reported to laboratory scale monofilament spinning apparatus [21] e controlled by the spinning conditions, mainly the Before the spinning, the mesophase pitch in the reservoir viscosity, which is governed by the temperature, shear rate was soaked first at 290C and then 330C for I h by a and molecular assemblies of the molten pitch in the heating rate of 3.5"C/min to remove the volatile gas. The pinning nozzle during spinning [13]. Although the nano- mesophase pitch was soaked at the spinning temperature scale texture has been observed in the transversal secti for 2 h before the spinning. The average diameter of by transmission electron microscope (TEM)[14, 15, a carbon fiber was controlled to ca 10 um by a spinning rate high resolution scanning electron microscope(HR-SEM) of 300 m/min and amount of extrudate of 50 mg/min. revealed some mesoscopic structural units in the fiber The as-spun fibers were extracted with pyridine in surface along the fiber axis especially, fibril, pleat units Soxhlet apparatus at its boiling point. Extraction was and their alignment which are believed to be more relevant carried out for I week without agitation. the amount of to the mechanical properties [10, 16]. Its development pyridine-insoluble fraction(PD) of as-spun fiber was ca 40 should be elucidated mesoscopically at key steps of fiber wt% formatio The as-spun and its PI fibers were oxidatively stabilize The present authors reported that the insoluble microdo- at 270.C for 30 min in air at a heating rate of 0.5"C/min mains of the mesophase pitch are inherited in the spun The stabilized fibers were carbonized from 300 to 1500 fiber aligned along the fiber axis [17]. Such aligned at 100C intervals by a heating rate of 10oC/min, the microdomains form the structure of the fiber developing soaking time being I h at respective temperatures in Ar the fibril and pleat units in the carbonized fiber above flow. The PI form of as-spun fiber was also carbonized at 700C as observed by a HR-SEM [18 700 and 1000.C under the same conditions as described In the present study, the development of the transverse above. The carbonized fibers were further graphitized at cross-sectional mesoscopic textures was followed in a 2000 and 2400C for 30 min by a heating rate of 6.7C/ eries of carbon fibers at the key steps of the heat min in ar flow treatment from the spinning to the graphitization. The transverse cross-sectional textures at nano-scale of as-spun, 23. HR -SEM obseration of carbon fibers stabilized, carbonized and graphitized fibers were succes- sively examined by HR-SEM. The as-spun fiber was The transverse cross-sectional surface was observed by extracted with pyridine to observe the microdomain and its high resolution scanning electron microscope(HR-SEM, alignment in the transversal cross-section by Hr-SEM and JEOL JSM 6320F)at a magnification of 200 000. The to identify the origin of transversal texture. As-spun fibers as-spun pyridine-insoluble, stabilized and carbonized fibers spun at 300, 310 and 340.C were reported to show the heat-treated at 300 to 600C were observed after platinum radial, random, and onion textures, respectively [19]. Such coating about 0.2 nm thickness using ion beam sputter fibers of varieties were examined through the solvent (IBS/TM 200S, VCR Group). The heat-treated fibers at extraction and heat treatment at 1000%C 700C or higher were observed without coating. All fibers The formation and change of the transverse cross-sec- were cut in liquid nitrogen to approximately 0.5 cm long, tional mesoscopic texture in the carbon fiber as well as the and they were attached to a copper grid to stand parallel to microdomain and its orientation in mesophase pitch by the electron beam with carbon tape to observe the trans- spinning are discussed based on the above mesoscopic verse cross-sectional texture from the perpendicular view 2. Experimental 3. 1. Development of transverse cross-sectional textures A naphthalene-derived mesophase pitch of 237C sof- Fig. I shows HR-SEM photographs of the transverse tening point and 100% anisotropy prepared with HF/BF3 cross-sectional surface of fiber as-spun at 310.C, as-s as a catalyst [20] was supplied by Mitsubishi Gas Chemi- fiber after extraction with pyridine, and carbon fiber heat- treated at 1000 and 2400C. The cross-sectional surface of insoluble fractions in the mesophase pitch were 48.0 and the as-spun fiber did not show any particular texture at low 31.8 wt%, respectively magnification(Fig. la). The high magnification photograph
918 S.-H. Hong et al. / Carbon 37 (1999) 917 –930 the arrangement in the hexagonal carbon planes in the fiber 2.2. Preparation of fibers cross-section, although their assemblies form the cluster, microdomain, and domain to show the textures. The The mesophase pitch was melt-spun at 300, 310 and transverse texture of the carbon fiber is typically classified 3408C, respectively, through a spinneret with a round into thee categories at rather macroscopic level: radial, nozzle of 0.3 mm in diameter and L/D53 using a random, and onion [12]. Such arrangements are reported to laboratory scale monofilament spinning apparatus [21]. be controlled by the spinning conditions, mainly the Before the spinning, the mesophase pitch in the reservoir viscosity, which is governed by the temperature, shear rate, was soaked first at 2908C and then 3308C for 1 h by a and molecular assemblies of the molten pitch in the heating rate of 3.58C/min to remove the volatile gas. The spinning nozzle during spinning [13]. Although the nano- mesophase pitch was soaked at the spinning temperature scale texture has been observed in the transversal section for 2 h before the spinning. The average diameter of by transmission electron microscope (TEM) [14,15], a carbon fiber was controlled to ca. 10 mm by a spinning rate high resolution scanning electron microscope (HR–SEM) of 300 m/min and amount of extrudate of 50 mg/min. revealed some mesoscopic structural units in the fiber The as-spun fibers were extracted with pyridine in surface along the fiber axis especially, fibril, pleat units Soxhlet apparatus at its boiling point. Extraction was and their alignment which are believed to be more relevant carried out for 1 week without agitation. The amount of to the mechanical properties [10,16]. Its development pyridine-insoluble fraction (PI) of as-spun fiber was ca. 40 should be elucidated mesoscopically at key steps of fiber wt%. formation. The as-spun and its PI fibers were oxidatively stabilized The present authors reported that the insoluble microdo- at 2708C for 30 min in air at a heating rate of 0.58C/min. mains of the mesophase pitch are inherited in the spun The stabilized fibers were carbonized from 300 to 15008C fiber aligned along the fiber axis [17]. Such aligned at 1008C intervals by a heating rate of 108C/min, the microdomains form the structure of the fiber developing soaking time being 1 h at respective temperatures in Ar the fibril and pleat units in the carbonized fiber above flow. The PI form of as-spun fiber was also carbonized at 7008C as observed by a HR–SEM [18]. 700 and 10008C under the same conditions as described In the present study, the development of the transverse above. The carbonized fibers were further graphitized at cross-sectional mesoscopic textures was followed in a 2000 and 24008C for 30 min by a heating rate of 6.78C/ series of carbon fibers at the key steps of the heat- min in Ar flow. treatment from the spinning to the graphitization. The transverse cross-sectional textures at nano-scale of as-spun, 2.3. HR–SEM observation of carbon fibers stabilized, carbonized and graphitized fibers were successively examined by HR–SEM. The as-spun fiber was The transverse cross-sectional surface was observed by a extracted with pyridine to observe the microdomain and its high resolution scanning electron microscope (HR–SEM, alignment in the transversal cross-section by HR–SEM and JEOL JSM 6320F) at a magnification of 200 000. The to identify the origin of transversal texture. As-spun fibers as-spun pyridine-insoluble, stabilized and carbonized fibers spun at 300, 310 and 3408C were reported to show the heat-treated at 300 to 6008C were observed after platinum radial, random, and onion textures, respectively [19]. Such coating about 0.2 nm thickness using ion beam sputter fibers of varieties were examined through the solvent (IBS/TM 200S, VCR Group). The heat-treated fibers at extraction and heat treatment at 10008C. 7008C or higher were observed without coating. All fibers The formation and change of the transverse cross-sec- were cut in liquid nitrogen to approximately 0.5 cm long, tional mesoscopic texture in the carbon fiber as well as the and they were attached to a copper grid to stand parallel to microdomain and its orientation in mesophase pitch by the electron beam with carbon tape to observe the transspinning are discussed based on the above mesoscopic verse cross-sectional texture from the perpendicular view. observation. 3. Results 2. Experimental 3.1. Development of transverse cross-sectional textures 2.1. Material through the heat treatment A naphthalene-derived mesophase pitch of 2378C sof- Fig. 1 shows HR–SEM photographs of the transverse tening point and 100% anisotropy prepared with HF/BF3 cross-sectional surface of fiber as-spun at 3108C, as-spun as a catalyst [20] was supplied by Mitsubishi Gas Chemi- fiber after extraction with pyridine, and carbon fiber heatcal Company. The contents of toluene and pyridine treated at 1000 and 24008C. The cross-sectional surface of insoluble fractions in the mesophase pitch were 48.0 and the as-spun fiber did not show any particular texture at low 31.8 wt%, respectively. magnification (Fig. 1a). The high magnification photograph
S.H. Hong et al. Carbon 37(1999)917-930 L01 SPUF I ISKU loaf sk SPUN I3 b°8a Fig. 1. HR-SEM photographs of transverse cross-section of the mesophase pitch-based(a, b, c)as-spun,(d, e, f) xtraction with pyridine, and carbon fibers heat-treated at(g, h, i)1000C and (, k, I)2400C: spinning temperature 310C of as-spun fiber random orientation of microdo- mains of rectangular shapes with ca. 500-1000 nm size mains of ca. 50 size(Fig. Ic). The whole cross- and pores scattered among the domains(Fig. Id). The high sectional surface fiber appears to consist of such magnification photograph(Fig. If of the extracted as-spu microdomains, showing that the skin and core are homoge- fiber exhibited insoluble microdomains of ca 50-100 nm hous at the magnification of size to show almost the same size compared to that of the The photograph of low magnification in the extracted as-spun fiber without extraction, which formed the do- s-spun fiber showed randomly distributed insoluble do- mains of variable shapes by connection of microdomains
S.-H. Hong et al. / Carbon 37 (1999) 917 –930 919 Fig. 1. HR–SEM photographs of transverse cross-section of the mesophase pitch-based (a, b, c) as-spun, (d, e, f) as-spun fibers after extraction with pyridine, and carbon fibers heat-treated at (g, h, i) 10008C and (j, k, l) 24008C: spinning temperature 3108C. of as-spun fiber exhibited random orientation of microdo- mains of rectangular shapes with ca. 500–1000 nm size mains of ca. 50–100 nm size (Fig. 1c). The whole cross- and pores scattered among the domains (Fig. 1d). The high sectional surface of the fiber appears to consist of such magnification photograph (Fig. 1f) of the extracted as-spun microdomains, showing that the skin and core are homoge- fiber exhibited insoluble microdomains of ca. 50–100 nm neous at the magnification of 50 000 (Fig. 1b). size to show almost the same size compared to that of the The photograph of low magnification in the extracted as-spun fiber without extraction, which formed the doas-spun fiber showed randomly distributed insoluble do- mains of variable shapes by connection of microdomains
S-H. Hong et al. /Carbon 37(1999)917-930 1 5KU 点5K256 1的h ig. I.(continued to each other. No significant orientation was observed in ly a random transversal texture at low magnification the alignment of domains. The periphery of the insoluble 1g). The high magnification photograph of the microdomains in the extracted as-spun fiber appeared showed variable shape of domains of ca. 500-1000 nm much more sharp and twisted, compared with that of the ize(Fig. Ii). The shapes of domains exhibited curvature as-spun fiber, indicating that the removal of the soluble formed through the curved connection of microdomains fraction from the microdomain in the mesophase pitch The cross-sectional surface of fiber heat-treated at 2400oC modifies the shape of domains clearly exhibited the random transversal texture at low The fiber heat-treated at 1000.C showed macroscopical magnification( Fig. 1j). The domain shape of the fiber
920 S.-H. Hong et al. / Carbon 37 (1999) 917 –930 Fig. 1. (continued) to each other. No significant orientation was observed in ly a random transversal texture at low magnification (Fig. the alignment of domains. The periphery of the insoluble 1g). The high magnification photograph of the fiber microdomains in the extracted as-spun fiber appeared showed variable shape of domains of ca. 500–1000 nm much more sharp and twisted, compared with that of the size (Fig. 1i). The shapes of domains exhibited curvature as-spun fiber, indicating that the removal of the soluble formed through the curved connection of microdomains. fraction from the microdomain in the mesophase pitch The cross-sectional surface of fiber heat-treated at 24008C modifies the shape of domains. clearly exhibited the random transversal texture at low The fiber heat-treated at 10008C showed macroscopical- magnification (Fig. 1j). The domain shape of the fiber
S-H. Hong et al. /Carbon 37(1999)917-930 began to show a very periphery. The connection of and random arrangement of such microdomains, showing raight microdomain 60-100 length exhibited very that the skin and core were homogeneous at the magnifica- sharp angles forming tion of 50 000(Fig. lk carbon fiber(Fig. I). The size of the microdomain in the Fig. 2 shows HR-sEM photographs under high magni- as-spun fiber was maintained basically the same after the fication for a series of the transverse cross-sectional heat treatment as well as the extraction. The whole cross- surface of as-spun, stabilized and carbon fibers heat-treated sectional surface exhibited the random transverse texture at 300 to 2400C, which were spun at 310.C, showing 1 danm SPUN 15KU 3200, 00e 400 15kU28 b SKU x2 s15k0×20598 688 15KU x20 Fig. 2. HR-SEM photographs of transverse cross-section of the mesophase pitch-based (a) n,(b) stabilized, and carbon fibers heat-treated at(c)300,d)400,(e)500,()600,(g)700,(h)1000,()1200,0)1500,(k)2000(l)2400: spinning temperature3l0°C
S.-H. Hong et al. / Carbon 37 (1999) 917 –930 921 began to show a very straight periphery. The connection of and random arrangement of such microdomains, showing straight microdomains of ca. 50–100 length exhibited very that the skin and core were homogeneous at the magnificasharp angles forming straight domains in the graphitized tion of 50 000 (Fig. 1k). carbon fiber (Fig. 1l). The size of the microdomain in the Fig. 2 shows HR–SEM photographs under high magnias-spun fiber was maintained basically the same after the fication for a series of the transverse cross-sectional heat treatment as well as the extraction. The whole cross- surface of as-spun, stabilized and carbon fibers heat-treated sectional surface exhibited the random transverse texture at 300 to 24008C, which were spun at 3108C, showing Fig. 2. HR–SEM photographs of transverse cross-section of the mesophase pitch-based (a) as-spun, (b) stabilized, and carbon fibers heat-treated at (c) 300, (d) 400, (e) 500, (f) 600, (g) 700, (h) 1000, (i) 1200, (j) 1500, (k) 2000, (l) 2400: spinning temperature 3108C