0008-6223/9651 0086223(95)00172-7 MICROSTRUCTURE OF MESOPHASE PITCH-BASED CARBON FIBER AND ITS CONTROLN I MOCHIDA, S.H. YooN, N. TAKANo, F FoRtiN, Y KoRAI and K. YoKogawa Institute of Advanced Material Study, Kyushu University 86, Kasuga, Fukuoka 816, Japan uasi-onion. High resolution SEM clarified the domains to have typical shapes of linear, bent, multi-fold ent and loop. The shapes and alignment of the domains depended upon the spinning conditions and which defined the texture in a rather complex manner, varying in th ansverse locations of the fibers of any overall textures. Each domain appeared to possess micro-domain which were arranged to define the size and shape of the domain. the shape of the micro-domains was unit grew during carbonization and graphitization within the micro-domain. Such crystal e g row tr apitis about shrinkage of the micro-domain to change the size of the domains, strongly influencing overall structure and the properties of the carbon and graphitized fibers. The unique pleat shaped structural fibers, were found by high resolution SEM and STM. Such structures were formed in the spinning ster with the fiber of liquid crystal polymers and mesophase pitch-based carbon fiber. Neither PAN, isotropic tch-bascd fiber necdle coke exhibited such a structure, The size and alignment in the pleat like structure were also dependent upon the nature of mesophase pitch and spinning conditions, although the Formation mechanism is not clarified yet. Such overall structure is briefly compared to the properties of he carbon fiber to find guidelines for the preparation of carbon fibers with better pe Key Words--mesophase pitch, pitch-based carbon fiber, micro-structure, domain, micro-domain, pleat In the pre dy, four mesoph Mesophase pitch-based carbon fibers have been rec. carbon fibers with representative transverse align- gnized as one of the key materials for the next century because of their excellent performances in polarized optical microscope (OM), scanning mechanical, electrical and thermal applications [1]. electron microscope EM)of conventional method- The cost and quality are problems which need to be improved [2]. The present authors have introduced scope(HR-SEM), transmission electron microscope novel types of mesophase pitches which are derived (TEM), wide angle X-ray reflection diffractometer from aromatic hydrocarbons by the aid of HF/BF, (XRD)and scanning tunneling microscope(STM) [3-5]. The high yield and structural control of the [ioj, in their transverse and longitudinal sections to mesophase pitch appear to offer a solution to these clarify their microscopic, mesoscopic, and macro- problems. Control of the fiber structure is now a scopic views. Ihe structure of the carbon fibers was major aim in order to improve the performance of compared to the properties of mesophase pitches to the resultant carbon fiber. Unfortunately, the struc- discuss how such overall structure is influenced or ture has not been fully defined from microscopic as controlled well as macroscopic views. The SEM of limited magnification classifies the transverse texture of the fiber as radial, onion, random, and their mixture [6] 2. EXPERIMENTAL TEM has revealed units of graphitic layers and their 2.1 Materials and preparation of mesophase orientation [7-9]. However, it has not yet been pitch-based carbon fibers solved how the units compose the texture. Hence, Table 1 summarizes some properties of mesophase objective fiber structure for the best performance is pitches used in this study. Mesophase pitches were not clarified prepared from aromatic hydrocarbons, such as naph Presented at the 1994 American Carbon s thalene and methylnaphthalene, at Mitsubishi Gas orkshop on Carbon Materials for Advanced Technologies Chemical Co. using HF/ BF3 as a catalyst [11, 12] ak Ridge, Tennessee, May 19 Naphthalene, methylnaphthalene derived meso- chugoku National Industrial Research Institute Kure, phase pitches, and their mixtures were spun under irushima 737-01, Japar nitrogen pressure through a circular shaped spinning
Carbon Vol. 34, No. 8, pp. 941-956,1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0008-6223/96 $15.00 + O._oO 0008-6223(95)00172-7 MICROSTRUCTURE OF MESOPHASE PITCH-BASED CARBON FIBER AND ITS CONTROL* I. MOCHIDA, S.-H. YOON, N. TAKANO, F. FORTIN, Y. KORAI and K. YOK~GAWA~ Institute of Advanced Material Study, Kyushu University 86, Kasuga, Fukuoka 816, Japan (Received 15 May 1994; accepted in revisedform 12 September 1995) Abstract-The macro as well as microscopic structure of four mesophase pitch-based carbon fibers of typical textures is observed using optical, scanning electron, transmission electron, and scanning tunneling microscopes. The four fibers had transverse textures of radial, radial-skin/random-core, random and quasi-onion. High resolution SEM clarified the domains to have typical shapes of linear, bent, multi-fold bent and loop. The shapes and alignment of the domains depended upon the spinning conditions and mesophase pitch properties, which defined the texture in a rather complex manner, varying in the transverse locations of the fibers of any overall textures. Each domain appeared to possess micro-domains which were arranged to define the size and shape of the domain. The shape of the micro-domains was basically unchanged by carbonization and graphitization, while the carbon plane cluster, or graphitic unit grew during carbonization and graphitization within the micro-domain. Such crystal growth brings about shrinkage of the micro-domain to change the size of the domains, strongly influencing overall structure and the properties of the carbon and graphitized fibers. The unique pleat shaped structural units, which are aligned in a zig-zag manner in the longitudinal section of the carbon and graphitized fibers, were found by high resolution SEM and STM. Such structures were formed in the spinning step with the fiber of liquid crystal polymers and mesophase pitch-based carbon fiber. Neither PAN, isotropic pitch-based fibers nor needle coke exhibited such a structure. The size and alignment in the pleat-like structure were also dependent upon the nature of mesophase pitch and spinning conditions, although the formation mechanism is not clarified yet. Such overall structure is briefly compared to the properties of the carbon fiber to find guidelines for the preparation of carbon fibers with better performances. Copyright 0 1996 Elsevier Science Ltd Key Words-mesophase pitch, pitch-based carbon fiber, micro-structure, domain, micro-domain, pleat shaped structure. 1. INTRODUCTION Mesophase pitch-based carbon fibers have been recognized as one of the key materials for the next century because of their excellent performances in mechanical, electrical and thermal applications [ 11. The cost and quality are problems which need to be improved [Z]. The present authors have introduced novel types of mesophase pitches which are derived from aromatic hydrocarbons by the aid of HF/BF, [3-51. The high yield and structural control of the mesophase pitch appear to offer a solution to these problems. Control of the fiber structure is now a major aim in order to improve the performance of the resultant carbon fiber. Unfortunately, the structure has not been fully defined from microscopic as well as macroscopic views. The SEM of limited magnification classifies the transverse texture of the fiber as radial, onion, random, and their mixture [ 61. TEM has revealed units of graphitic layers and their orientation [7-91. However, it has not yet been solved how the units compose the texture. Hence, objective fiber structure for the best performance is not clarified. *Presented at the 1994 American Carbon Society Workshop on Carbon Materials for Advanced Technologies, Oak Ridge, Tennessee, May 1994. +Chugoku National Industrial Research Institute, Kure, Hiroshima 737-01, Japan. In the present study, four mesophase pitch-based carbon fibers with representative transverse alignments prepared in our laboratory were observed by the polarized optical microscope (OM), scanning electron microscope (SEM) of conventional methodology and high resolution scanning electron microscope (HR-SEM), transmission electron microscope (TEM), wide angle X-ray reflection diffractometer (XRD) and scanning tunneling microscope (STM) [lo], in their transverse and longitudinal sections to clarify their microscopic, mesoscopic, and macroscopic views. The structure of the carbon fibers was compared to the properties of mesophase pitches to discuss how such overall structure is influenced or controlled. 2. EXPERIMENTAL 2.1 Materials and preparation of mesophase pitch-based carbonjbers Table 1 summarizes some properties of mesophase pitches used in this study. Mesophase pitches were prepared from aromatic hydrocarbons, such as naphthalene and methylnaphthalene, at Mitsubishi Gas Chemical Co. using HF/BF, as a catalyst [ 11,121. Naphthalene, methylnaphthalene derived mesophase pitches, and their mixtures were spun under nitrogen pressure through a circular shaped spinning 941
MOcHIDA et al Table 1. Some al properties of mesophase and isotropic pitches prepared using H SPa Ac Solubility X-ray properties Code Raw materialC vol% BS Bl-Ps PI(QI) H/C fa d002(A) Lc(002)(nm) mNP Methylnaphthalene20510052193329)0690.863.524 mNP/NPI(7/3)d) mNP( +197100 0690.823.550 NP(30 wt%o NPI 168 0 51 9 40(26) 0.71 0.79 Indistinct Indistinct 237 948(31)0.620.803.574 (a)Softening point. (b) Anisotropic contents. (c) Carbon aromaticity. (d) Pitch thermally blended at 300 C in the nitrogen atmosphere nozzle(Diameter =0.3 mm, Length over Gakushin(JSPS)[15], using an X-ray diffractometer (L/D)=3). Spinning Rigaku Geigerflex: CuKa. 0.15406 nm, 40 k v, 30 Spinning conditions of the present study are A commercial STM Nanoscope II in table 2 cnts Inc )was applied at ambient conditions, The As-spun fibers were oxidatively stabilized in air at scanning areas of 200 x 200 and 2000 x 2000 nm were 270C for 20 minutes at a heating rate of 5C/min used for the observation of the sample surface. The scanning head, tip, and mechanical actuator were Stabilized fibers were carbonized in an argon atmo- mounted on an anti-vibration isolation platform. The sphere at 1000 C for 10 minutes at a heating rate of constant tunnel current mode was employed, and the 20 C/min, and successively graphitized at 2500C for surface relief was expressed by different shades of I minute at a heating rate of 100C/min grey. The parameters applied were current 1.02-1.03 nA; bias voltage 500 m V 2.2 Characterization of graphitized fibers Tensile strength, Youngs modulus The transverse and longitudinal sections of graphi- break were measured at room temperature using ced fibers,(fibers with the typical radial open wedge mono-filaments according to the procedure defined ransverse texture, F-A; fibers with the radial skin in Japanese Industrial Standard(JIS R-7601), using and random core texture, F-B; fibers with the typical an Instron type tensile testing machine (Instron 4200 random texture, F-C; fibers with quasi onion texture, series)at a head speed of 0.5 mm/min a gauge length F-D), were observed by OM, SEM, HR-SEM, and of 25 mm was employed to obtain the tensile proper TEM. STM was also used for observing the longitudi- ties. Tensile strength and Youngs modulus were nal sections of a graphitized fiber the results of 16 tests [161. All fiber samples were observed under a polarized their values distributing within 15%. Compressive light optical microscope(Olympus B061) Fibers were strength was measured according to the composite mounted and polished by conventional procedures. method(Vr(fiber volume)=60%)mono-filament To examine the transverse alignments of graphi- test [17]. The results of 5 tests were averaged tized fibers under sEM (Jeol JSM 5400)and HR- SEM (Jeol JSM 6403F), all fiber samples were cut in liquid nitrogen to approximately 0.5 cm length 3. RESULTS and attached to a copper grid with graphite paste. 3. 1 Transverse alignments of the mesophase The transverse section exposed by the cutting was pitch-based carbon fibe open wedge For the TEM observation, ultra-thin sections, transverse texture Figure 1(a) shows the OM 40-60 nm thickness, perpendicular and parallel to SEM, HR-SEM images of the transverse section thc fibcr axis were prepared by ultramicrotomy (Leica the naphthalene derived mesophase pitch Ultracut S), using a diamond knife, and observed by based graphitized fiber with the typical radial open TEM (Jeol JEM100CX), using bright field(BF), 002 wedge texture(F-A) dark field(DF) and 002 lattice The fibers showed PAC-man cross-sectional shape Lc(002)and La(110)of graphitized fibers were with typical radial open wedge transverse alignment measured according to the method defined by Three isochromatic color regions, blue, red, and Table 2. Conditions of spinning Spinning rate Amounts of extrudate Diameter of pitch Code fibers(m 60⊥10 50+5 mNP/NPI (7/3) 120(±1.0) 123(+0.5
942 I. MOCHIDA etal. Table 1. Some analytical properties of mesophase and isotropic pitches prepared using HF/BF, SPa AC? Solubilities(wt%) X-ray properties Code Raw material “C ~01% BS BI-PS PI(QI) H/C fa” d002(A) Lc(OOZ)(nm) mNP Methylnaphthalene 205 100 52 19 33(29) 0.69 0.86 3.524 7.5 mNP/NPI(7/3)d) mNP(70 wt%) + 197 100 0.69 0.82 3.550 5.0 NPI NPM NP(30 wt%) Naphthalene Naphthalene 168 0 51 9 40(2 6) 0.71 0.79 Indistinct Indistinct 237 100 33 19 48(31) 0.62 0.80 3.574 3.2 (a) Softening point. (b) Anisotropic contents. (c) Carbon aromaticity. (d) Pitch thermally blended at 300°C in the nitrogen atmosphere. nozzle (Diameter = 0.3 mm, Length over diameter (L/D) = 3). Spinning was carried out using a laboratory scale mono-filament spinning apparatus [ 131. Spinning conditions of the present study are detailed in Table 2. As-spun fibers were oxidatively stabilized in air at 270°C for 20 minutes at a heating rate of S”C/min c141. Stabilized fibers were carbonized in an argon atmosphere at 1000°C for 10 minutes at a heating rate of 20”C/min, and successively graphitized at 2500°C for 1 minute at a heating rate of lOO”C/min. 2.2 Characterization of graphitized$bers The transverse and longitudinal sections of graphitized fibers, (fibers with the typical radial open wedge transverse texture, F-A; fibers with the radial skin and random core texture, F-B; fibers with the typical random texture, F-C; fibers with quasi onion texture, F-D), were observed by OM, SEM, HR-SEM, and TEM. STM was also used for observing the longitudinal sections of a graphitized fiber. All fiber samples were observed under a polarized light optical microscope (Olympus B061). Fibers were mounted and polished by conventional procedures. To examine the transverse alignments of graphitized fibers under SEM (Jeol JSM 5400) and HR-SEM (Jeol JSM 6403F), all fiber samples were cut in liquid nitrogen to approximately 0.5 cm length, and attached to a copper grid with graphite paste. The transverse section exposed by the cutting was observed without any coating. For the TEM observation, ultra-thin sections, 40-60 nm thickness, perpendicular and parallel to the fiber axis were prepared by ultramicrotomy (Leica Ultracut S), using a diamond knife, and observed by TEM (Jeol JEMlOOCX), using bright field (BF), 002 dark field (DF) and 002 lattice fringe mode. Lc (002) and La (110) of graphitized fibers were measured according to the method defined by Gakushin (JSPS) [ 151, using an X-ray diffractometer (Rigaku Geigerflex; CuKa, 0.15406 nm, 40 kV, 30 mA). A commercial STM Nanoscope II (Digital instruments Inc.) was applied at ambient conditions. The scanning areas of 200 x 200 and 2000 x 2000 nm were used for the observation of the sample surface. The scanning head, tip, and mechanical actuator were mounted on an anti-vibration isolation platform. The constant tunnel current mode was employed, and the surface relief was expressed by different shades of grey. The parameters applied were current 1.02-1.03 nA; bias voltage 500 mV. Tensile strength, Young’s modulus and strain-tobreak were measured at room temperature using mono-filaments according to the procedure defined in Japanese Industrial Standard (JIS R-7601), using an Instron type tensile testing machine (Instron 4200 series) at a head speed of 0.5 mm/min. A gauge length of 25 mm was employed to obtain the tensile properties. Tensile strength and Young’s modulus were evaluated by averaging the results of 16 tests [ 161, their values distributing within 15%. Compressive strength was measured according to the composite method (V, (fiber volume) = 60%) mono-filament test [ 171. The results of 5 tests were averaged. 3. RESULTS 3.1 Transverse alignments of the mesophase pitch-based carbon fiber 3.1.1 Fiber with typical radial open wedge transverse texture Figure l(a) shows the OM, SEM, HR-SEM images of the transverse section in the methylnaphthalene derived mesophase pitchbased graphitized fiber with the typical radial open wedge texture (F-A). The fibers showed PAC-man cross-sectional shape with typical radial open wedge transverse alignment. Three isochromatic color regions, blue, red, and Table 2. Conditions of spinning Code F-A F-B F-C F-D Material mNP NPM mNP/NPI (7/3) mNP Spinning Spinning rate temperature (“C) (mjmin) 285 400 310 400 280 300 290 400 Amounts of extrudate (mgimin) 60 + 10 50 * 5 55 k 6 60 k 10 Diameter of pitch fibers (mm) 12.5 (& 0.5) 9.0 (k 1.0) 12.0 (+_ 1.0) 12.3 (k 0.5)
Microstructure of mesophase pitch-based carbon fiber and its control OM photograph 10 um SEM photograph Transverse texture 1 um Core section Middle section Outer section 0.1 um HR-SEM photographs Fig. 1.(a)OM, SEM, and HR-SEM photographs of transverse sections in the mesophase carbon fiber with (b)OM, SEM, and HR-SEM photographs of transverse sections in the mesophase sections in the mesophase pi ed carbon fiber with typical random alignment. (d)OM, d HR-SEM photo- graphs of transverse sections in the mesophase pitch-based carbon fiber with quasi-onio ellow, were definitely identified, whereas the sh radial type domain alignment, whereas 1-2 um of the and size of domains were hardly distinguished. A skin section appeared definitely to carry different radial orientation of the constituent domains without domain alignment from that of the core area 2r arrangement was inferred in F-A, suggesting that However, full identification of domain shape, size, each domain in F-a was linearly arranged and devel oped into the center of the fiber. A SEM photograph and conventional SEM photographs. HR-SEM of F-A also showed that the fiber had a well defined photographs of F-a clearly showed that shapes of
Microstructure of mesophase pitch-based carbon fiber and its control 943 OM photograph 10 pm H Transverse texture 1 km SEM photograph 1 pm H Core section Middle section Outer section 0.1 pm H (a) HR-SEM photographs Fig. 1. (a) OM, SEM, and HR-SEM photographs of transverse sections in the mesophase pitch-based carbon fiber with typical radial open wedge alignment. (b) OM, SEM, and HR-SEM photographs of transverse sections in the mesophase pitch-based carbon fiber with radial skin-random core alignment. (c) OM, SEM, and HR-SEM photographs of transverse sections in the mesophase pitch-based carbon fiber with typical random alignment. (d) OM, SEM, and HR-SEM photographs of transverse sections in the mesophase pitch-based carbon fiber with quasi-onion alignment. yellow, were definitely identified, whereas the shape radial type domain alignment, whereas l-2 pm of the and size of domains were hardly distinguished. A skin section appeared definitely to carry different radial orientation of the constituent domains without domain alignment from that of the core area. 27t arrangement was inferred in F-A, suggesting that However, full identification of domain shape, size, each domain in F-A was linearly arranged and devel- and distribution is still unsatisfactory with the OM oped into the center of the fiber. A SEM photograph and conventional SEM photographs. HR-SEM of F-A also showed that the fiber had a well defined photographs of F-A clearly showed that shapes of
I. MOCHIDA et al OM photograph 10m SEM photograph I um Transverse texture I pill Core section Middle section Outer section 0.1 WIn HR-SEM photographs Fig. 1.(cont) the constituent domains were different according to The thickness and length of domains appeared below location, gradually changing from the linear and above 100 nm, respectively. In the middle area, at the outer zone into a circular one in the all domains were of looped and bent shapes. The ore area. Higher magnification definitely identified length assessed above 200 nm and the thickness again the shapes of domains. In the core area, there below 100 nm. The domains are approximately red no definite alignments of domains. Most aligned, focusing to the center. In the outer area, the ains were loop shaped, showing independent constituent domains showed a well developed linear orientation from that of the neighboring domain unit. shape, being aligned in the perfect radial orientation
I. MOCHIDA et al OM photograph 10 km H SEM photograph Transverse texture E Core section Middle section Outer section 0.1 pm t---l (‘4 HR-SEM photographs Fig. 1. (cont.). the constituent domains were different according to their location, gradually changing from the linear shape at the outer zone into a circular one in the core area. Higher magnification definitely identified the shapes of domains. In the core area, there appeared no definite alignments of domains. Most domains were loop shaped, showing independent orientation from that of the neighboring domain unit. The thickness and length of domains appeared below and above 100 nm, respectively. In the middle area, all domains were of looped and bent shapes. The length assessed above 200 nm, and the thickness again below 100 nm. The domains are approximately aligned, focusing to the center. In the outer area, the constituent domains showed a well developed linear shape, being aligned in the perfect radial orientation
Microstructure of pitch-based carbon fiber and its control OM Photograph 10m SEM photograph Transverse texture I um Core section Middle section Outer section 0. 1 um HR- SEM photographs Fig. 1.(cont. No looped or bent shaped domains were found ere identified from closer examination(See smalle the outer area. The length and thickness of domain arrows in Fig. 2). Such a smaller unit, defined as a were above 300 nm and below 100 nm, respectively. micro-domain, had thickness and length around 10 Figure 2 showed further higher magnification and 10-100 nm in the transverse section, respectively HR-SEM photographs of F-A. The loop and lincar Such units were confirmed in TEM bright ficld (BF) aped domains in the core and outer areas of the and inge images(Figs. 3 and 4). Particularly, transverse sections, respectively were well observed. the TEM BF image showed that a domain unit was The smaller units which composed the domain unit composed of several pieces of micro-domain units
Microstructure of mesophase pitch-based carbon fiber and its control 945 OM photograph 10 p,rn H SEM photograph 1 Fm H Transverse texture 1 pm Core section Middle section Outer section 0.1 p,m H (4 HR-SEM photographs Fig. 1. (cont.). No looped or bent shaped domains were found in the outer area. The length and thickness of domain were above 300 nm and below 100 nm, respectively. Figure 2 showed further higher magnification HR-SEM photographs of F-A. The loop and linear shaped domains in the core and outer areas of the transverse sections, respectively were well observed. The smaller units which composed the domain unit were identified from closer examination (See smaller arrows in Fig. 2). Such a smaller unit, defined as a micro-domain, had thickness and length around 10 and lo-100 nm in the transverse section, respectively. Such units were confirmed in TEM bright field (BF) and lattice fringe images (Figs. 3 and 4). Particularly, the TEM BF image showed that a domain unit was composed of several pieces of micro-domain units