CARBON PERGAMON Carbon38(2000)741-747 Structure of melt-blown mesophase pitch-based carbon fiber Fumitaka Watanabe a, Yozo Korai@, Isao Mochida@,*, Yoshiyuki Nishimura Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816-8580, Japan PETOCA LId, Kamisumachi, Ibaraki 314-0102, Japan Received 4 November 1998; accepted 17 July 1999 Abstract Mesophase pitch-based short carbon fibers prepared by the melt blown method exhibited diameters 6 to 16 um distributed along the filament. The transverse surfaces of the thicker and thinner parts of the filament were very different. The thicker art showed a skin/core texture where very thick and long domains run parallel to the core while the thinner one had a PAN-AM texture where sets of straight domains run along the equator. Such unique shapes and textures were prepared through melt blown spinning and different extents of stabilization according to the diameter of the filament. 2000 Elsevier Science Ltd. All rights reserved Keywords: A. Carbon fibers, Mesophase pitch; C. Scanning electron microscopy (SEM): D. Textures 1. Introduction in the transverse surface, and fibril and pleats units he longitudinal surface, as described for a melt spun Since the synthetic mesophase pitch from naphtha in a previous paper [6] was commercialized [1, two kinds of mesophase pitch- based carbon fibers have been prepared by melt spinning and melt blowing methods. The former fiber is applied for 2. Experimental high strength structural material [2], while the latter is now used after graphitization as an anode material for a lithium 2./. Materials ion battery, expanding its production volume very rapidly [3,4] The anode performance characteristics such as capacity Mesophase pitch-based short fibers produced from the naphthalene mesophase pitch through the melt-blowing discharge potential, coulomb efficiency, cycle stability and method were supplied by PEtoCa Co structural factors of the fiber [5] The present paper reports the structure of the melt blown 2.2. Carboni=ation and graphitization fiber in detail, using optical and high resolution scanning electron copes over a wide range of magnifications The as-received fiber (carbonized at 650C) was further to clarify its structural characteristics in comparison wi calcined under an argon flow at 1200"C for 60 min. It graphitized at 2400C at a heating rate of 25.C/min in as well as the axial surfaces of the fiber were observed argon flow for 30 min after the primary carbonization at because the mesophase pitch develops very unique mesos- 700C with no soaking time copic(several hundred nm scale) structure in the fiber which can be described in terms of domains and microdo- 2.3. Characterisation of short fibers *Corresponding author. Tel :+81-92-583-7797; fax: +81-92- The longitudinal section of as-spun fiber was observed 83-779 by a polarized light optical microscope. The cross sectional E-mail address: mochida(@cm. kyushu-uac jp(I Mochida). as well as longitudinal surfaces of the fiber were observed 0008-6223/00/S-see front matter 2000 Elsevier Science Ltd. All rights reserved PII:S0008-6223(99)00148-7
PERGAMON Carbon 38 (2000) 741–747 Structure of melt-blown mesophase pitch-based carbon fiber a a a, b Fumitaka Watanabe , Yozo Korai , Isao Mochida , Yoshiyuki Nishimura * a Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816-8580, Japan b PETOCA Ltd., Kamisumachi, Ibaraki 314-0102, Japan Received 4 November 1998; accepted 17 July 1999 Abstract Mesophase pitch-based short carbon fibers prepared by the melt blown method exhibited diameters 6 to 16 mm distributed along the filament. The transverse surfaces of the thicker and thinner parts of the filament were very different. The thicker part showed a skin/core texture where very thick and long domains run parallel to the core while the thinner one had a PAN-AM texture where sets of straight domains run along the equator. Such unique shapes and textures were prepared through melt blown spinning and different extents of stabilization according to the diameter of the filament. 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibers, Mesophase pitch; C. Scanning electron microscopy (SEM); D. Textures 1. Introduction mains in the transverse surface, and fibril and pleats units on the longitudinal surface, as described for a melt spun Since the synthetic mesophase pitch from naphthalene fiber in a previous paper [6]. was commercialized [1], two kinds of mesophase pitchbased carbon fibers have been prepared by melt spinning and melt blowing methods. The former fiber is applied for 2. Experimental high strength structural material [2], while the latter is now used after graphitization as an anode material for a lithium 2.1. Materials ion battery, expanding its production volume very rapidly [3,4]. Mesophase pitch-based short fibers produced from the The anode performance characteristics such as capacity, naphthalene mesophase pitch through the melt-blowing discharge potential, coulomb efficiency, cycle stability and method were supplied by PETOCA Co. safety are believed to depend strongly on the respective structural factors of the fiber [5]. 2.2. Carbonization and graphitization The present paper reports the structure of the melt blown fiber in detail, using optical and high resolution scanning The as-received fiber (carbonized at 6508C) was further electron microscopes over a wide range of magnifications calcined under an argon flow at 12008C for 60 min. It was to clarify its structural characteristics in comparison with graphitized at 24008C at a heating rate of 258C/min in those by the melt spinning. The transverse cross sectional argon flow for 30 min after the primary carbonization at as well as the axial surfaces of the fiber were observed 7008C with no soaking time. because the mesophase pitch develops very unique mesoscopic (several hundred nm scale) structure in the fiber which can be described in terms of domains and microdo- 2.3. Characterization of short fibers *Corresponding author. Tel.: 181-92-583-7797; fax: 181-92- The longitudinal section of as-spun fiber was observed 583-7798. by a polarized light optical microscope. The cross sectional E-mail address: mochida@cm.kyushu-u.ac.jp (I. Mochida). as well as longitudinal surfaces of the fiber were observed 0008-6223/00/$ – see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223(99)00148-7
F, Watanabe et al./ Carbon 38(2000)741-747 electron microscope(Hr- 3. Rest 3. 1. Shape and size of short carbon fiber Fig. 1 shows optical micrographs of the longitudinal surface of a melt blown carbon fiber filament to examine its diameter. A piece of fiber was sandwiched between two glass slides and was observed by optical microscopy. The diameters found in the photographs were 16, 1l and 7 um. It was found that these different diameters existed in a piece of filament. Fig. 2 illustrates the distribution of the diameters found under the optical microscope. They 810121416 ranged from 6 to 16 um with the majority between 9 and Diameter /um 12 um and a peak between 10 and 1l um. The as-received fibers were 10 cm long. Thus, the average filament had the Fig. 2. Distribution of the diameters of a short fiber largest diameter of 16 um at its center, decreasing to 6 um 3.2. Transverse surface under higher magnification SEM at the ends. The average diameter most frequently ob- served was 10 to 11 um Figs 4 and 5 show SEM photographs of the transverse ig. 3 illustrates transverse cross-sectional surfaces of a surfaces of filaments with different diame filament at its medium (11 um)and smallest(6 pr of the transverse section, especially in the center region diameters. The surface at medium diameter showed a kind differed, depending on the diameter. Fig. 4 shows the of skin/core structure where long and short domains were transverse surface of a filament of medium diameter (11 dominant at the core and skin areas, respectively. The long um)under higher magnification(X40 000). The surface domains in the center were curved gently to form the core. was classified into four areas(a),(b),(c)and(d) according The cross-section of the smaller diameter showed a texture to size and shape of the domain in the areas. Region(a) halogous to the Pan American Air lines Logo(PAN-A consisted of a gently snaking belt of a few long(several texture), or basically similar to the texture found in micrometres) domains, which run at the equator part from Brooks-Taylor type texture, where domains were thinner an edge to another of the cross-sectional circle of filament. and more homogeneous, being arranged in rather radial The domain in this area was basically straight to meet manner, although straight domains formed a definite radially the edge of the cross-section Region(b) consisted quator zone which ran through the center of the cross of long gently snaking domains located next to the(a) sectIo region in the core of the filament. Regions (a)and(b) 16um Ilum Fig. 1. Different diameters of a filament
742 F. Watanabe et al. / Carbon 38 (2000) 741 –747 under a high resolution scanning electron microscope (HRSEM, JEOL 6320F). 3. Results 3.1. Shape and size of short carbon fiber Fig. 1 shows optical micrographs of the longitudinal surface of a melt blown carbon fiber filament to examine its diameter. A piece of fiber was sandwiched between two glass slides and was observed by optical microscopy. The diameters found in the photographs were 16, 11 and 7 mm. It was found that these different diameters existed in a piece of filament. Fig. 2 illustrates the distribution of the diameters found under the optical microscope. They ranged from 6 to 16 mm with the majority between 9 and 12 mm and a peak between 10 and 11 mm. The as-received Fig. 2. Distribution of the diameters of a short fiber. fibers were 10 cm long. Thus, the average filament had the largest diameter of 16 mm at its center, decreasing to 6 mm 3.2. Transverse surface under higher magnification SEM at the ends. The average diameter most frequently observed was 10 to 11 mm. Figs. 4 and 5 show SEM photographs of the transverse Fig. 3 illustrates transverse cross-sectional surfaces of a surfaces of filaments with different diameters. The texture filament at its medium (11 mm) and smallest (6 mm) of the transverse section, especially in the center region, diameters. The surface at medium diameter showed a kind differed, depending on the diameter. Fig. 4 shows the of skin/core structure where long and short domains were transverse surface of a filament of medium diameter (11 dominant at the core and skin areas, respectively. The long mm) under higher magnification (340 000). The surface domains in the center were curved gently to form the core. was classified into four areas (a), (b), (c) and (d) according The cross-section of the smaller diameter showed a texture to size and shape of the domain in the areas. Region (a) analogous to the Pan American Air lines Logo (PAN-AM consisted of a gently snaking belt of a few long (several texture), or basically similar to the texture found in micrometres) domains, which run at the equator part from Brooks-Taylor type texture, where domains were thinner an edge to another of the cross-sectional circle of filament. and more homogeneous, being arranged in rather radial The domain in this area was basically straight to meet manner, although straight domains formed a definite radially the edge of the cross-section. Region (b) consisted equator zone which ran through the center of the cross of long gently snaking domains located next to the (a) section. region in the core of the filament. Regions (a) and (b) Fig. 1. Different diameters of a filament
F, Watanabe et al./ Carbon 38(2000)741-747 43 the equator from an edge to another of the filament circle Region(b) consisted of fairly long and curved semi pherical domains along the domains of region(a) Regions (c)and(d) at the very surface were located at the filament skin and their domains at the fiber surface were com arranged in a radial texture. Region(c), which was located next to region(a), consisted of straight domains of 1000 nm long and 100 nm thick. Region (d), located next to egion (b), consisted of rather short domains. Some mains were randomly arranged especially at the inner area The major difference between the thick(Fig 4)and thin L 0 1 segments(Fig. 5) was observed in the core region(regions SKU (a)and(b) 3.3. Longitudinal surface of the filament Thin part(6um Fig 6 shows the longitudinal surfaces of the filament at representative locations corresponding to areas of (a) and (d)(in Fig 4)in the transverse cross section. Both surfaces consisted of fibrils and pleats, whose dimensions varied according to the location. Region (a) contained thinner fibrils less than 100 um wide, while region(d) contained thicker fibrils around 100 to 150 um thick. The valley between fibrils was very definite for region(a) while broad for region(d) 4. Discussion The present paper describes the unique texture of mesophase pitch-based carbon fiber prepared through melt blown spinning and continuous stabilization. Its structure model and formation mechanism have merits to discuss Thick part(llum) Fig. 7 illustrates a model structure of a filament, average is 10 cm long, consisting of several regions according to 3. SEM photographs of transverse surface for different cross-sectional diameter(Figs. I and 2). The filament is cut at the smallest regions to restrict its length during spinning. The small and large diameter regions exhibit different textures where the domains of variable size and constituted the melt core part. Region (c)which is located shape are arranged differently. The difference of texture next to region (a) near the surfaces exhibited the straight between thin and thick filaments was especially apparent at but short domains with some bends, their edges being the center region. The larger diameter region showed a perpendicular to the surface in a radial alignment. Region SO-called skin/ texture because a thick region of (d)exhibited much shorter and bent or semi-circular filament is difficult to sufficiently stabilize at its center domains of 200-400 nm, of which edges faced the surface, Because the center part of a thick filament is not fully being arranged in a random manner. Such a complex stabilized by oxidation (stabilization), the core region texture under higher magnification may reflect the melt melted during carbonization and hence exhibited long and blown spinning and stabilization heterogeneity, the latter snaking domains. Such a unique texture is characteristic of of which depended on the distance from the fiber surface the skin/core texture. The smaller diameter region showed because of the thick diar a PAN-AM texture in which stabilization reached to the Fig. 5 shows the surface of a filament at a center region. Fig. I shows the diameter distributions in a thinner diameter(6 higher magnification. The piece of fiber. The thick part exhibited a skin/core texture surface was again classified into four areas(a),(b),(c)and while the thin part had a PAN-AM texture. Such different (d)according to the size and shape of domains Region(a) textures are noted to exist in a single filament. When the consisted of slightly curved or straight domains that run at fiber is stabilized under severer conditions(higher tem-
F. Watanabe et al. / Carbon 38 (2000) 741 –747 743 the equator from an edge to another of the filament circle. Region (b) consisted of fairly long and curved semispherical domains along the domains of region (a). Regions (c) and (d) at the very surface were located at the filament skin and their domains at the fiber surface were commonly arranged in a radial texture. Region (c), which was located next to region (a), consisted of straight domains of 1000 nm long and 100 nm thick. Region (d), located next to region (b), consisted of rather short domains. Some domains were randomly arranged especially at the inner area. The major difference between the thick (Fig. 4) and thin segments (Fig. 5) was observed in the core region (regions (a) and (b)). 3.3. Longitudinal surface of the filament Fig. 6 shows the longitudinal surfaces of the filament at representative locations corresponding to areas of (a) and (d) (in Fig. 4) in the transverse cross section. Both surfaces consisted of fibrils and pleats, whose dimensions varied according to the location. Region (a) contained thinner fibrils less than 100 mm wide, while region (d) contained thicker fibrils around 100 to 150 mm thick. The valley between fibrils was very definite for region (a) while broad for region (d). 4. Discussion The present paper describes the unique texture of mesophase pitch-based carbon fiber prepared through melt blown spinning and continuous stabilization. Its structure model and formation mechanism have merits to discuss. Fig. 7 illustrates a model structure of a filament, average is 10 cm long, consisting of several regions according to Fig. 3. SEM photographs of transverse surface for different cross-sectional diameter (Figs. 1 and 2). The filament is diameter. cut at the smallest regions to restrict its length during spinning. The small and large diameter regions exhibit different textures where the domains of variable size and constituted the melt core part. Region (c) which is located shape are arranged differently. The difference of texture next to region (a) near the surfaces exhibited the straight between thin and thick filaments was especially apparent at but short domains with some bends, their edges being the center region. The larger diameter region showed a perpendicular to the surface in a radial alignment. Region so-called skin/core texture because a thick region of (d) exhibited much shorter and bent or semi-circular filament is difficult to sufficiently stabilize at its center. domains of 200–400 nm, of which edges faced the surface, Because the center part of a thick filament is not fully being arranged in a random manner. Such a complex stabilized by oxidation (stabilization), the core region texture under higher magnification may reflect the melt melted during carbonization and hence exhibited long and blown spinning and stabilization heterogeneity, the latter snaking domains. Such a unique texture is characteristic of of which depended on the distance from the fiber surface the skin/core texture. The smaller diameter region showed because of the thick diameter. a PAN-AM texture in which stabilization reached to the Fig. 5 shows the transverse surface of a filament at a center region. Fig. 1 shows the diameter distributions in a thinner diameter (6 mm) under higher magnification. The piece of fiber. The thick part exhibited a skin/core texture surface was again classified into four areas (a), (b), (c) and while the thin part had a PAN-AM texture. Such different (d) according to the size and shape of domains. Region (a) textures are noted to exist in a single filament. When the consisted of slightly curved or straight domains that run at fiber is stabilized under severer conditions (higher tem-
F, Watanabe et al./ Carbon 38(2000)741-747 Edge of circle Pole 5KU×8.8lmb1 1 L01 Fig. 4. SEM photographs of as-received carbonized short fiber(1l um diamete erature(270C), longer time (>60 min) and slower fibril and pleat units which are characteristic of the heating rate(1200C)[7]. Melt blown and pull down The transverse section exhibited the PAN-AM alignment spinning may not produce any differences in the deforma- of domains where a few long domain belts run at the tion of the microdomains during the spinning process. The equator, short domains formed the latitudinal lines, and interesting point is that the dimension of the fibril de- very short or semi-circular domains were aligned with their termines the texture of the transverse section. The straight edges to the surface at the poles. The thick fiber showed domain gives rather thinner pleat at the equator. The pole basically the same alignment as the thin fiber although regions have an approximate radial texture where the short very thick and snaking domains were found at the equator. but rather broad domains are arranged perpendicularly to The domains in the core tended to be larger the surface The longitudinal surface of the present fiber contains Such unique textures of the present fiber are ascribed to
744 F. Watanabe et al. / Carbon 38 (2000) 741 –747 Fig. 4. SEM photographs of as-received carbonized short fiber (11 mm diameter). perature (.2708C), longer time (.60 min) and slower fibril and pleat units which are characteristic of the heating rate (,18C/min)), all parts of the fiber exhibited mesophase pitch-based fiber, reflecting microdomain units exclusively the similar PAN-AM texture regardless of the of several nm size in the mother pitch [7]. All mesophase diameter of filament (this is not shown here). Thus, the pitch-based fibers derived from coal tar, petroleum as well fiber used in this study is estimated to be stabilized under as synthesized pitches contain fibril and pleat units after rather mild conditions. the calcination (.12008C) [7]. Melt blown and pull down The transverse section exhibited the PAN-AM alignment spinning may not produce any differences in the deformaof domains where a few long domain belts run at the tion of the microdomains during the spinning process. The equator, short domains formed the latitudinal lines, and interesting point is that the dimension of the fibril devery short or semi-circular domains were aligned with their termines the texture of the transverse section. The straight edges to the surface at the poles. The thick fiber showed domain gives rather thinner pleat at the equator. The pole basically the same alignment as the thin fiber although regions have an approximate radial texture where the short very thick and snaking domains were found at the equator. but rather broad domains are arranged perpendicularly to The domains in the core tended to be larger. the surface. The longitudinal surface of the present fiber contains Such unique textures of the present fiber are ascribed to
F, Watanabe et al./ Carbon 38(2000)741-747 Equator Edge of circle Fig. 5. SEM photographs of as-received carbonized short fiber(6 um diameter) melt blown spinning and the extent of stabilization and flattens the domains of the radial edges at the pole to which depends strongly on the distance from the fiber align the domains like latitudinal lines, although such surface. The spinning is suspected to have been carried out domains are severely bent or deformed to be semi-circular at a relatively high temperature under a large shear-stress. in a PAN-AM type alignment. The pitch suffers die-swell to form a droplet at the outlet Just after spinning, the transverse section of the larger of the nozzle that is pulled down into a 10-cm long fiber of diameter filament should contain the same texture to that variable diameters. Hence the filament inherits the shape of of the smaller diameter filament. However this filament the droplet to provide the regions of both large and small showed a skin/core texture after carbonization. The do- diameters. The filament is cut occasionally at the smallest mains at the center region of the filament with a skin/core diameter to restrict its length to 10 cm long. texture tends to be continuous and bent. The size and shape Fig. 8 illustrates a schematic mechanism for the intro- of domains depend on the macroscopic structure(diameter duction of PAN-AM texture. The strong shear stress of fiber) and stabilization conditions. The stabilization provides a basically radial alignment of domains in the reaction may suffer some barrier to progress into the center transverse texture. The domains are essentially connected of the thick fiber [9]. when the fiber prepared by the melt in a straight line at the equator. The mesophase pitch blown method is sufficiently stabilized under much severer expands at the outlet of the spinning nozzle to give a larger conditions(longer time and/or slower heating-rate), al diameter due to die-swell [8]. Air blows at a high velocity regions in the filament exhibit the PAN-AM texture along the fiber axis at the two ends of the fiber diameter regardless of filament diameter
F. Watanabe et al. / Carbon 38 (2000) 741 –747 745 Fig. 5. SEM photographs of as-received carbonized short fiber (6 mm diameter). the melt blown spinning and the extent of stabilization and flattens the domains of the radial edges at the pole to which depends strongly on the distance from the fiber align the domains like latitudinal lines, although such surface. The spinning is suspected to have been carried out domains are severely bent or deformed to be semi-circular at a relatively high temperature under a large shear-stress. in a PAN-AM type alignment. The pitch suffers die-swell to form a droplet at the outlet Just after spinning, the transverse section of the larger of the nozzle that is pulled down into a 10-cm long fiber of diameter filament should contain the same texture to that variable diameters. Hence the filament inherits the shape of of the smaller diameter filament. However this filament the droplet to provide the regions of both large and small showed a skin/core texture after carbonization. The dodiameters. The filament is cut occasionally at the smallest mains at the center region of the filament with a skin/core diameter to restrict its length to 10 cm long. texture tends to be continuous and bent. The size and shape Fig. 8 illustrates a schematic mechanism for the intro- of domains depend on the macroscopic structure (diameter duction of PAN-AM texture. The strong shear stress of fiber) and stabilization conditions. The stabilization provides a basically radial alignment of domains in the reaction may suffer some barrier to progress into the center transverse texture. The domains are essentially connected of the thick fiber [9]. When the fiber prepared by the melt in a straight line at the equator. The mesophase pitch blown method is sufficiently stabilized under much severer expands at the outlet of the spinning nozzle to give a larger conditions (longer time and/or slower heating-rate), all diameter due to die-swell [8]. Air blows at a high velocity regions in the filament exhibit the PAN-AM texture along the fiber axis at the two ends of the fiber diameter regardless of filament diameter
F, Watanabe et al./ Carbon 38(2000)741-747 1) 1L1 5KU J x120,800 9 Fig. 6. SEM photographs of the longitudinal surface 10cm Cut Thick part Fig. 7. Relation between the transverse structure and the longitudinal figure
746 F. Watanabe et al. / Carbon 38 (2000) 741 –747 Fig. 6. SEM photographs of the longitudinal surface. Fig. 7. Relation between the transverse structure and the longitudinal figure
F, Watanabe et al./ Carbon 38(2000)741-747 Air nozzle (b) (a) Spinning nozzle (a) (b) ●小 Radial Die-swell PANAM Fig. 8. Production mechanism of PAN-AM texture 5 Inagaki M. J Electrochem Soc 1993: 140: 315. [6] Mochida I, Yoon SH, Takano N, Fortin F, Korai [1] Mochida 1, Shimizu K, Korai Y, Fujiyama S, Toshima H Yokogawa K. Carbon 1996: 34: 941 ono T Carbon 1992. 30- 55 [7 Korai Y, Hong SH, Mochida I Carbon 1998: 36:79-85. 2 Edie DD. Carbon 1998: 36: 345. [8] Elias 3] Takami N, Satoh A, Hara M, Ohsaki T J Electrochem Soc 1977,pp.481-2 995;142:371 [9] Mochida I, Toshima H, Korai Y J Mater Sci 1989, 24: 57-62 4 Takami N, Satoh A, Hara M, Ohsaki T. J Electrochem Soc 1995;142:2564
F. Watanabe et al. / Carbon 38 (2000) 741 –747 747 Fig. 8. Production mechanism of PAN-AM texture. References [5] Inagaki M. J Electrochem Soc 1993;140:315. [6] Mochida I, Yoon SH, Takano N, Fortin F, Korai Y, Yokogawa K. Carbon 1996;34:941. [1] Mochida I, Shimizu K, Korai Y, Fujiyama S, Toshima H, [7] Korai Y, Hong SH, Mochida I. Carbon 1998;36:79–85. Hono T. Carbon 1992;30:55. [8] Elias HG, editor, Macromolecules I, New York: Plenum, [2] Edie DD. Carbon 1998;36:345. 1977, pp. 481–2. [3] Takami N, Satoh A, Hara M, Ohsaki T. J Electrochem Soc [9] Mochida I, Toshima H, Korai Y. J Mater Sci 1989;24:57–62. 1995;142:371. [4] Takami N, Satoh A, Hara M, Ohsaki T. J Electrochem Soc 1995;142:2564