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
Mesophase pitch-based carbon fibers have attracted
worldwide attention because of their superior performance
[1–3]. The carbon fibers prepared from mesophase pitch
synthesized from aromatic hydrocarbon by aid of HF / BF 3
as a catalyst, have excellent mechanical, thermal, and
electrical properties [4–6], promising broad applications in
commercial as well as other advanced areas. In spite of
their anticipated future, much higher performance is still
expected through the control of their mesoscopic textures
[7–9].
The present authors have reported microdomains of ca.
50 nm as a mesoscopic structural unit in the mesophase
*Corresponding author. Tel.: 181-92-583-7279; fax: 181-92583-7798.
E-mail address: shhong@endomoribu.shinshu-u.ac.jp (S.-H.
Hong).
pitch and its derived carbon fibers [10]. Such a structural
unit maintained its size in the transverse cross-section and
longitudinal surface of the fiber up to graphitization
temperatures. The microdomains in the mesophase pitch
are aligned during the spinning step to form the fiber’s
surface texture such as the pleat and fibril, and transverse
cross-section showing linear, bent or looped domains
aligned in radial, random and onion-like textures. The
pleats and fibrils appear first in the carbonized fiber
because the carbonization of the soluble fraction follows
the morphology of the insoluble microdomain [11]. Curved
domains in the transverse cross-section of the carbonized
fiber become straight during graphitization because of the
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
surface in the mesophase pitch based carbon fibers through
heat-treatment up to 24008C and solvent extraction was
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
806
S.-H. Hong et al. / Carbon 38 (2000) 805 – 815
scope (HR-SEM). The textures at the edge where the
transverse and longitudinal surfaces meet tell us the threedimensional arrangement and morphology of microdomains 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
suggest either basal planes or prismatic edges of graphene
units that may form in the surface of the graphitized fiber.
The extracted fiber tells us the origins of mesoscopic
texture and the influences of the extracted soluble fraction
on the surface texture.
2. Experimental
2.1. Material
A naphthalene-derived mesophase pitch of 2378C softening point and 100% anisotropy prepared with HF / BF 3
as a catalyst was supplied by Mitsubishi Gas Chemical
Company [13]. The toluene and pyridine insoluble fractions of the mesophase pitch were 48 and 32 wt.%,
respectively.
2.2. Preparation of fibers
The mesophase pitch was melt-spun at 300, 310 and
3408C through a spinneret with a round nozzle 0.3 mm in
diameter and L /D 5 3, using a laboratory scale monofilament spinning apparatus [14]. The average diameter of the
carbon fiber was controlled to ca. 10 mm by spinning and
extrusion rates of 300 m / min and 50 mg / min, respectively.
The as-spun fiber was extracted with pyridine in a
Soxhlet apparatus at its boiling point. Extraction was
carried out for 1 week without agitation. The pyridine
insoluble fraction (PI) of the as-spun fiber was ca. 40
wt.%. Both the as-spun and the PI fibers extracted were
oxidatively stabilized in air at 2708C for 30 min using a
heating rate of 0.58C / min. Both stabilized fibers were
carbonized at 300–15008C in an Ar flow using a heating
rate of 108C / min. The carbonized fibers were further
graphitized at 2000 or 24008C for 30 min using a heating
rate of 6.78C / min in Ar flow.
2.3. HR-SEM observation of carbon fibers
The texture at the skin area of the mesophase pitchbased carbon fibers was observed by a high resolution
scanning electron microscope (HR-SEM, JEOL JSM
6320F) at magnifications of 100,000 and 200,0003. The
as-spun fibers were observed after coating with about 0.2
nm of platinum using ion beam sputtering. Fibers heattreated above 7008C were observed without such a coating.
All fibers were cut in liquid nitrogen and were attached to
a copper grid so that they stood parallel to the electron
beam. The transverse cross-sectional edge of the surface
was observed by tilting the fiber 108 to the electron beam.
3. Results
3.1. HR-SEM textures of graphitized fibers
Fig. 1 shows HR-SEM photographs of the transverse
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
overall transverse surface at low magnification. The core
area about 1 mm from the fiber center showed a random
texture. The fiber spun at 3108C showed a random
transverse texture (Fig. 1b). An onion-like texture was
found in the graphitized fiber spun at 3408C, exhibiting a
hollow of 1 mm diameter in its center (Fig. 1c). A crack
was already visible in the center of a fiber carbonized at
10008C as reported in a previous paper [12].
Fig. 2 shows HR-SEM photographs of the skin of the
graphitized fiber spun at 3108C observed by holding the
fiber axis parallel (08) and tilted at 308 to the electron
beam. The photograph observed along the fiber axis (Fig.
2a) shows plate-like microdomains (ca. 20 nm thickness,
50 nm length) and domains (ca. 100–150 nm thickness,
500 nm length). Bright spurs were found running along the
longer axis of the domain. A domain was found to contain
several microdomains. Both domains and microdomains
were arranged with their longer axes perpendicular to the
fiber surface.
The photograph observed by tilting 308 to the fiber axis
shows the microfibrils, fibrils and pleats in the longitudinal
surface of the fiber as well as microdomains and domains
in the transverse section (Fig. 2b). The thin plate microdomains and domains meet perpendicular to the fiber surface
where their edges form the tops of the microfibrils and
fibrils, respectively. The domains at the skin area along the
fiber axis form fibrils ca. 50–150 nm thickness. A relatively thick domain composed of several microdomains
formed a thick fibril ca. 100–150 nm wide, while a thin
fibril of ca. 30–50 nm width was formed by the arrangement of relatively thin domains. The arrangement of
microdomains at the skin area formed microfibrils of ca. 20
nm within the fibrils. Several microdomains in the skin
area formed the domains perpendicular to the surface
which merged with the fibrils.
In the graphitized carbon fiber, the three-dimensional
plate-like shape of the microdomain was observed to form
both longitudinal and transverse cross-sectional surfaces.
The long edge of a plate was oriented along the fiber axis
and the other edge merged perpendicular to the fiber
surface at the skin area. Although the microdomains were
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 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. 1. HR-SEM photographs of transverse cross-section of the
mesophase pitch-based carbon fibers heat-treated at 24008C:
spinning temperature (a) 300, (b) 310, and (c) 3408C.
fiber, valleys between microdomains were clearly observed
in the surface.
The edge of the rectangular microdomain within ca. 10
nm of the fiber surface showed a round top which aligned
parallel to the surface. The high resolution closed up spurs
running within the microdomain. They were basically
aligned parallel to the longer axis of the microdomain as
shown in Fig. 2a-A. However, the spurs were observed
spherically aligned at the very top of the microdomain,
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
spun at 3408C exhibited zig-zag layers of microdomains
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
the fiber periphery at the surface was also observed,
although the thickness of the surface layer was larger
(around 50 nm) than that of the graphitized fibers.
3.3. HR-SEM textures of as-spun and extracted fibers
Fig. 5 shows HR-SEM photographs of the skin area in
the three as-spun fibers. The mesoscopic textures in the
transverse skin areas of these three kinds of as-spun fibers
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
were observed in the longitudinal surface, although wavy
ripples were evident. Microdomains of ca. 50 nm in
diameter were observed in the transverse cross-sectional
surface, although no particular transverse texture was
recognized.
Fig. 6 shows HR-SEM photographs of the skin area in
the three as-spun fibers after extraction with pyridine and
successively heat-treated at 10008C. These fibers had
definitely more straight domains than those of the carbonized fibers obtained without pyridine extraction regardless
of the macroscopic transverse textures. The spurs and
graphene edges along the longer axis of microdomain
faced directly to the fiber surface after the removal of the
soluble component. The microdomains and spurs running
parallel to the fiber surface disappeared in the carbonized
fiber after the extraction, although the orientation of
microdomains was basically the same as for carbonized
fibers without extraction. A much sharper angle was
observed at the junction of two zig-zag microdomains at
the skin area in the extracted as-spun fiber spun at 3408C
than in the corresponding graphitized fiber. Hence, no spur
running around the top or any encounter edge of the
microdomains was observed any longer as shown at A in
Fig. 6a–c.
4. Discussion
4.1. Mesoscopic texture at the skin area of mesophase
pitch-based graphitized fiber
The present study was aimed at clarifying the mesoscopic texture of mesophase pitch-based graphitized carbon
fibers especially at the skin area in its transverse section
where the texture exhibits three-dimensional shape and
dimension of the microunits in the carbon fiber. The
graphitized fiber consists of rectangular plate-like microdomains of dimensions 20 nm thick, 30–50 nm wide, and
100–150 nm long. Several such microdomains form
domains with linear, bent and loop shapes in the transverse
section. The domains are macroscopically arranged typically in radial, random, and onion-like texture as often
reported [15,16].
The edges of the microdomains in the transverse section
are of special interest in the present study. The longer
edges of the rectangular plate-like microdomains are
oriented perpendicular to the surface in the radial and
random textures, indicating apparently that the prismatic
edges may meet perpendicular to the fiber surface, although domains in the former and latter textures are
principally radial and very random, as illustrated in Fig. 7.
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
onion-like texture are arranged in a zig-zag manner with
their longer edges along the surface, connecting smoothly
two microdomains at their longer edges where the small
basal planes bridge these two edges. The thickness of the
layer was under 10 nm. Hence, basal planes appear to form
the major surface of the fiber as expected from the
macroscopic view.
The longer edge of a microdomain in the transverse
section of radial and random textures appears to merge into
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
thickness of a microfibril reflects the width of a microdomain plate along the fiber axis. In contrast, the longer axis
of a plate-like microdomain forms a half of the fibril in the
onion-like texture. Hence, the thickness of a fibril in this
texture is much larger than that observed in the radial or
random textures. The connection point of two microdo-
mains corresponds to the hill within a fibril in the fiber of
onion-like texture.
Such a carbon fiber mesostructure raises the question of
whether graphene prismatic edges or basal planes cover the
surface. The arrangement of the rectangular microdomains
seems to suggest that prismatic edges do this for the radial
and random textures while basal planes do it for the
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
magnification exhibits a round surface in the former two
fibers where spurs of graphene layers were arranged to
follow the morphology of the top of the microdomain,
suggesting that basal planes also cover the surfaces of both
radial and random textures. STM of the graphitized surface
suggests the dominance of basal planes on the surface of
the carbon fiber [17]. The first layer of the surface must be
microscopically analyzed, since the surface may govern
the reactivity of carbon surface in the oxidative pretreatment of the carbon fiber as a composite filler.
4.2. Development of mesoscopic texture
Mesoscopic texture has been reported to develop during
the spinning, carbonization, and graphitization steps
[11,12]. The mesophase pitch carries mesoscopic units of
rod-like microdomains which are arranged by the spinning
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
and as-spun fiber is not distinguishable unless extraction
closes up the arrangement of the insoluble fractions. It
must be emphasized that the arrangement of the insoluble
microdomains is basically maintained in the graphitized
fiber, acting as the skeleton of the mesoscopic texture.
Carbonization above 7008C develops a definite mesoscopic
texture and hence a macroscopic texture, the soluble
fraction being converted into infusible carbon by maintaining the arrangement of the insoluble fraction by virtue of
the stabilization [12]. Such an origin and development
scheme of the mesoscopic textures is illustrated in Fig. 7.
The as-spun fibers do not show any particular transverse
texture because of the soluble fraction. However, solvent
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.
814
S.-H. Hong et al. / Carbon 38 (2000) 805 – 815
Fig. 7. (continued)
extraction closes up the transverse textures such as radial,
random, and onion shape according to the spinning temperatures, although no line up of spurs in the microdomain
is observable in the fibers at this stage. In the radial fiber,
linear domains are dominant in the skin area, being placed
perpendicular to the fiber surface, and the linearity of
domains increases during graphitization. The bent domains
in the random fiber are also maintained up to the graphitization temperature of 24008C. The fiber spun at 3408C,
which is macroscopically of an onion-like texture, exhibits
a zig-zag alignment of microdomains in the skin area and
shows a sharper angle between two microdomains in the
carbonized fiber than for the graphitized fiber. The
graphitization enlarges the graphene sheet to flatten the
encounter of the graphene units because of the shrinkage
along the radial direction.
The graphite structure is developed by graphitization
above 15008C where the graphene layers grow in two
directions of stacking height and area, increasing both Lc
and La values, respectively, in the graphitizable carbon.
Such crystal growth changes the rod-like microdomain into
a rectangular shape and gently curved-domains consisting
of several microdomains are forced to take linear, bent and
loop shapes with sharper angles at their connections. Such
development of mesoscopic texture is also true at the skin
area in the transverse section. The straight rectangular
plates are arranged perpendicularly or in a zig-zag fashion
parallel to the surface as observed in the former radial,
random, and the latter onion-like textures, respectively.
4.3. Spinning
The rod-like microdomains in the mesophase pitch are
deformed by spinning into plates with curved peripheries
in the as-spun fiber. Such plates are arranged in the
longitudinal direction to form a microfibril. An important
factor in orientation is whether the shorter or longer edges
of the microdomain plate face the surface. While a lower
viscosity seems to favor shorter edges, a higher viscosity
favors longer edges. The viscosity–orientation correlation
has been discussed from a macroscopic view to explain the
radial, random and onion textures of the domains [14].
Hence the typical microdomain orientation–viscosity correlation may be restricted to a skin area of a few hundrednanometres thickness where strong interaction with the
spinneret wall governs the orientation.
Whether basal plane or prismatic edges cover the
surface of the fiber cannot be exclusively concluded with
the resolution of HR-SEM of the present study. The spurs
and graphene edges directly face the fiber surface after the
removal of the soluble component and successive carbonization as described above, suggesting that the basal plane
observed in the fiber surface may originate from smaller
molecules of the soluble fraction which become stacked
S.-H. Hong et al. / Carbon 38 (2000) 805 – 815
parallel to the surface. The soluble fraction in the mesophase pitch may cover the surface of the fiber during
spinning thus reducing the friction and acting as a lubricant
against the spinneret wall. Thus the soluble fraction
provides the basal plane on the surface as observed by
STM [17], aligning parallel to the fiber surface regardless
of the inner textures of the fiber.
Acknowledgements
This research was partially supported by the Ministry of
Education, Science, Sports and Culture, Grant-in-Aid for
Scientific Research on Priority Areas (Carbon Alloys),
09243101, 1998 and Grant-in-Aid for Scientific Research
(c), 09650941, 1998.
References
[1] Fitzer F. Carbon 1989;27(5):625.
[2] Carbon Fiber, Tokyo: Kindai Henshusha, 1984, p. 231.
815
[3] Matsui J. In: Development and evaluation method of carbon
fiber, Tokyo: Japan Carbon Society, 1988, p. 1.
[4] Edie DD, Fain CC, Robinson KE, Harper AM, Rogers DK.
Carbon 1994;32(6):1045.
[5] Mochida I, Yoon S-H, Takano N, Fortin F, Korai Y,
Yokogawa K. Carbon 1996;34(8):941.
[6] Mochida I, Shimizu K, Korai Y, Fujiyama S, Otsuka H,
Sakai Y. Carbon 1990;28(2):311.
[7] Kumar S, Anderson DP, Crasto AS. J Mater Sci
1993;28(3):423.
[8] Huang Y, Young RJ. Carbon 1995;33(1):97.
[9] FitsGerald JD, Pennock GM, Taylor GH. Carbon
1991;29(1):139.
[10] Korai Y, Hong S-H, Mochida I. Carbon 1998;36(1):79.
[11] Korai Y, Hong S-H, Mochida I. Carbon 1999;37(2):203.
[12] Hong S-H, Korai Y, Mochida I. Carbon 1999;37(6):917.
[13] Mochida I, Shimizu K, Korai Y et al. Carbon 1992;30(1):55.
[14] Yoon S-H, Korai Y, Mochida I, Kato I. Carbon
1994;32(2):273.
[15] Yoon S-H, Korai Y, Mochida I. Carbon 1993;31(6):849.
[16] Mochida I, Toshima H, Korai Y, Matsumoto T. J Mater Sci
1989;24(1):57.
[17] Yoon S-H, Korai Y, Mochida I. Carbon 1996;34(1):83.