Availableonlineatwww.sciencedirect.com DIRECTO CARBON ELSEVIER Carbon42(2004)2567-2572 ww.elsevier. com/locate/carbon Acceleration of graphitization in carbon fibers through exfoliation Masahiro Toyoda a, " Yutaka Kaburagi Akira Yoshida b, Michio Inagaki c Faculty of Engineering, Oita University, 700 Dannoharu, Oita 870-1192, Japan b Musashi Institute of Technology, Tamazutuni, Setagaya, Tokyo 158-8557, Japan chi Institute of Technology, Yakusa, Toyota 470-0392, Japan Received 25 October 2003: accepted 25 May 2004 Available online 10 July 2004 Abstract Exfoliated carbon fibers were found to be graphitized, even those derived from PAN-based carbon fibers. Mesophase-pitch-based d PAN-based carbon fibers heat-treated at a high temperature in advance were exfoliated via their intercalation compounds with nitric acid, single fiber of the former splitting into thin filaments and from the latter small scale- like particles being peeled off. Since exfoliation introduced a large amount of structural defects, 002 X-ray diffraction profile was broadened and intensity of d band i Raman spectrum increased. By re-heat-treatment of these exfoliated carbon fibers at a high temperature as 2800C, however, higher degree of graphitization than the pristine fibers evaluated by the parameters of X-ray diffraction and Raman spectroscopy was obtained, particularly marked acceleration of graphitization being observed in the exfoliated PAN-based carbon fiber c 2004 Elsevier Ltd. All rights reserved Keywords: A. Carbon fibers: Exfoliated graphite: B Graphitization 1. Introduction it had been heated to a temperature very near the Graphitization behavior of carbon materials, includ- carbon materials with plane orientation, such as pyro- ing carbon fibers, has been pointed out by a number of lytic carbons [7] and some of polyimide-derived carbon papers to depend predominantly on their precursors [1 ms [8,9, were graphitized emi 3]. One of the authors(Mi)claimed that the nanotexture treatment under normal pressure. Carbon fibers with carbons, which has been formed through pyrolysis axial orientation were reported to have a wide range of and carbonization of the precursor, is responsible for graphitizability on the basis of the measurement of their graphitization of carbons [3]. Random orientation magnetoresistance on single fibers [10, 11]. Random of crystallites in carbons led to poor graphitization de- orientation in the cross-section of fibers strongly sup- gree even after the heat treatment at a high temperature presses the growth of crystallites and consequently their as 3000 C, which may be demonstrated by poor graphitization, as can be seen in isotropic pitch-based graphitizability of so-called glass-like carbons. In order carbon fibers, but radial texture in the sections to develop the graphitic structure in these carbons, their results in high graphitization degree after the heat nanotexture had to be destroyed, the heat treatmen treatment, as in mesophase-pitch-based carbon fibers under a high pressure as 30 MPa being necessary These experimental facts suggest that constraint in car graphitize [4, 5]. When a rod of glass-like carbon was bon materials to keep their nanotexture and morphol melted by passage of electrical current directly in argon, ogy, such as fibrous one in carbon fibers and spherical a ball with graphite structure was obtained in a crater- one in carbon blacks, is so strong that a simple heat type cavity at the middle of rod, but the wall of crater treatment to high temperatures, even near the melting kept the characteristics of glass-like carbon even though point, is not enough to overcome Previously, we reported marked exfoliation of carbon fibers after rapid heating of their intercalation com- nding author. Fax: +81-97-554-7904 pounds up to 1000C, which had been obtained by the toyoda 22(@cc. oita-u ac JP(M. Toyoda) electrolysis in either nitric acid [12-14] or formic acid 0008-6223/S ont matter 2004 Elsevier Ltd. All rights reserved. doi:10.10l6 carbon2004.05051
Acceleration of graphitization in carbon fibers through exfoliation Masahiro Toyoda a,*, Yutaka Kaburagi b , Akira Yoshida b , Michio Inagaki c a Faculty of Engineering, Oita University, 700 Dannoharu, Oita 870-1192, Japan b Musashi Institute of Technology, Tamazutumi, Setagaya, Tokyo 158-8557, Japan c Aichi Institute of Technology, Yakusa, Toyota 470-0392, Japan Received 25 October 2003; accepted 25 May 2004 Available online 10 July 2004 Abstract Exfoliated carbon fibers were found to be graphitized, even those derived from PAN-based carbon fibers. Mesophase-pitch-based and PAN-based carbon fibers heat-treated at a high temperature in advance were exfoliated via their intercalation compounds with nitric acid, single fiber of the former splitting into thin filaments and from the latter small scale-like particles being peeled off. Since exfoliation introduced a large amount of structural defects, 0 0 2 X-ray diffraction profile was broadened and intensity of D band in Raman spectrum increased. By re-heat-treatment of these exfoliated carbon fibers at a high temperature as 2800 C, however, a higher degree of graphitization than the pristine fibers evaluated by the parameters of X-ray diffraction and Raman spectroscopy was obtained, particularly marked acceleration of graphitization being observed in the exfoliated PAN-based carbon fibers. 2004 Elsevier Ltd. All rights reserved. Keywords: A. Carbon fibers; Exfoliated graphite; B. Graphitization 1. Introduction Graphitization behavior of carbon materials, including carbon fibers, has been pointed out by a number of papers to depend predominantly on their precursors [1– 3]. One of the authors (MI) claimed that the nanotexture of carbons, which has been formed through pyrolysis and carbonization of the precursor, is responsible for their graphitization of carbons [3]. Random orientation of crystallites in carbons led to poor graphitization degree even after the heat treatment at a high temperature as 3000 C, which may be demonstrated by poor graphitizability of so-called glass-like carbons. In order to develop the graphitic structure in these carbons, their nanotexture had to be destroyed, the heat treatment under a high pressure as 30 MPa being necessary to graphitize [4,5]. When a rod of glass-like carbon was melted by passage of electrical current directly in argon, a ball with graphite structure was obtained in a cratertype cavity at the middle of rod, but the wall of crater kept the characteristics of glass-like carbon even though it had been heated to a temperature very near the melting point of carbon [6]. On the other hand, the carbon materials with plane orientation, such as pyrolytic carbons [7] and some of polyimide-derived carbon films [8,9], were graphitized by the high-temperature treatment under normal pressure. Carbon fibers with axial orientation were reported to have a wide range of graphitizability on the basis of the measurement of magnetoresistance on single fibers [10,11]. Random orientation in the cross-section of fibers strongly suppresses the growth of crystallites and consequently their graphitization, as can be seen in isotropic pitch-based carbon fibers, but radial texture in the cross-sections results in high graphitization degree after the heat treatment, as in mesophase-pitch-based carbon fibers. These experimental facts suggest that constraint in carbon materials to keep their nanotexture and morphology, such as fibrous one in carbon fibers and spherical one in carbon blacks, is so strong that a simple heat treatment to high temperatures, even near the melting point, is not enough to overcome. Previously, we reported marked exfoliation of carbon fibers after rapid heating of their intercalation compounds up to 1000 C, which had been obtained by the electrolysis in either nitric acid [12–14] or formic acid * Corresponding author. Fax: +81-97-554-7904. E-mail address: toyoda22@cc.oita-u.ac.jp (M. Toyoda). 0008-6223/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.05.051 Carbon 42 (2004) 2567–2572 www.elsevier.com/locate/carbon
M. Toyoda et al/ Carbon 42(2004)2567-2572 [15]. After exfoliation, a single fiber of mesophase-pitch- ExCFs using a 514.5 nm line of an argon ion laser Jobin based carbon fibers was found to convert to a bundle of Yon Ramanor, T-64000) thin filaments split along the original fiber axis. In our previous paper [16], graphitization of these exfoliated filaments prep ared from the mesophase-pitch-based 3 Results and discussion carbon fibers was accelerated by the high-temperature heat treatment 3. 1. Surfa In the present work, PAN-based carbon fibers, which were difficult to be graphitized under usual heat treat In Fig. 1(a) and(b), a change in surface morphology ment, were exfoliated and then heat-treated at a high of pristine and exfoliated mesophase-pitch-based carbon temperature, in order to study the acceleration fibers, respectively, is shown. A single fiber was split inte raphitization after exfoliation. For comparison, exactly a number of thin filaments after exfoliation, which was the same processes were applied on mesophase-pitch- explained to be due to a sudden decomposition of based carbon fibers. Degree of graphitization was eval- intercalates (derivatives from nitric acid) to gaseous uated from X-ray diffraction and Raman spectroscopy. species [12]. In Fig. I(c)and(d), a change in surface morphology with high-temperature heat treatment is shown. By the high-temperature treatment, splitting into thin filaments looks to become clearer just after the exfoliation, a memory of the original single fiber could be recognized(Fig. I(c)), but it seems to be less obvious Mesophase-pitch-based carbon fibers, which were after the high-temperature treatment(Fig. I(d) heat-treated to 3000 oC and had so-called corrugate In the case of pan-based carbon fibers howey radial texture in their cross-sections, and PAN-based surface morphology change due to exfoliation is quite arbon fibers, which were heat-treated up to 2800C different from that of mesophase-pitch-based carbon fi and had random texture in their cross-sections, were bers; scale- like small fragments are peeled off from the selected, mainly because the former had relatively high original fibers and a number of cracks and fissures are graphitization degree but the latter had almost no formed preferentially along with the original fiber axis, developments of graphitic structure. The intercalation as shown in Fig. 2(b)and (d). No change in surface compounds of these fibers were prepared by the elec- morphology in the exfoliated PAN-based carbon fibers trolysis to the total electric charge of 2400C using a is observed after the high-temperature treatment constant current of o5 a in a 13 mol/dm nitric acid and The cracks and fissures with exfoliation were rea- then changed to the residue compounds by washing sonably supposed to be formed by cleaving between water and drying in air. The residue compounds pre- hexagonal carbon layers, just like in the exfoliation of red thus were quickly inserted into the furnace at 1000 natural graphite flakes [12]. This morphological chang oC and kept for 5 s in order to make them exfoliate. with the exfoliation of carbon fibers seemed to be con- Carbon fibers thus exfoliated(exfoliated carbon fibers, sistent with the structure models for these carbon fibers ExCFs) were heat-treated again at 900C and then ei- [20]; in the pristine PAN-based carbon fibers, growth of ther 2800 or 3000C for 10 min in a flow of high-purity crystallites are not marked even after the high-temper ature treatment, but the crystallites grow in the case of Surface morphology of ExCFs before and after the the mesophase-pitch-based carbon fibers, as reported on high-temperature treatment was observed with an FE- carbon fibers from different precursors. This is the rea- SEM(Hitachi, S-4100). The degree of graphitization of son why peering-off of scale-like small fragments was these fibers was evaluated by X-ray diffraction and apparently observed on the present PAN-based carbon Raman spectroscopy. X-ray diffraction measurement fibers. As a consequence, each of filaments and scales with Cu Ko radiation was performed for a bundle of formed by exfoliation consisted of well-aligned carbon ExCFs mounted on a specially designed sample holder layers, as proved in our previous paper [12]. Therefore, using a wide angle diffractometer(Rigaku Rint 2100). either filaments or scales consisting of well-aligned car The average values of the interlayer spacing doo] and of bon layers were reasonably expected to graphitize crystallite size along c-axis Lc(002) were determined after correcting the diffraction profile of 002 line for 3. 2. X-ray diffraction Lorentz-polarization and atomic scattering factors, referring to the outer standard of an HOPG specimen In Fig. 3, 002 diffraction profiles are compared on with a flat cleaved surface, of which door was calibrated the pristine, exfoliated and high-temperature-treated fi- as 0.3354 nm by a powder method with the inner stan- bers. The structural parameters determined from 002 dard of high-purity silicon powder [17]. First order diffraction line, interlayer spacing door and crystallite Raman spectrum was measured on a single filament of size along c-axis Lc(002), are listed in Table l(Fig 4)
[15]. After exfoliation, a single fiber of mesophase-pitchbased carbon fibers was found to convert to a bundle of thin filaments split along the original fiber axis. In our previous paper [16], graphitization of these exfoliated filaments prepared from the mesophase-pitch-based carbon fibers was accelerated by the high-temperature heat treatment. In the present work, PAN-based carbon fibers, which were difficult to be graphitized under usual heat treatment, were exfoliated and then heat-treated at a high temperature, in order to study the acceleration in graphitization after exfoliation. For comparison, exactly the same processes were applied on mesophase-pitchbased carbon fibers. Degree of graphitization was evaluated from X-ray diffraction and Raman spectroscopy. 2. Experimental Mesophase-pitch-based carbon fibers, which were heat-treated to 3000 C and had so-called corrugate radial texture in their cross-sections, and PAN-based carbon fibers, which were heat-treated up to 2800 C and had random texture in their cross-sections, were selected, mainly because the former had relatively high graphitization degree but the latter had almost no developments of graphitic structure. The intercalation compounds of these fibers were prepared by the electrolysis to the total electric charge of 2400 C using a constant current of 0.5 A in a 13 mol/dm3 nitric acid and then changed to the residue compounds by washing water and drying in air. The residue compounds prepared thus were quickly inserted into the furnace at 1000 C and kept for 5 s in order to make them exfoliate. Carbon fibers thus exfoliated (exfoliated carbon fibers, ExCFs) were heat-treated again at 900 C and then either 2800 or 3000 C for 10 min in a flow of high-purity Ar. Surface morphology of ExCFs before and after the high-temperature treatment was observed with an FESEM (Hitachi, S-4100). The degree of graphitization of these fibers was evaluated by X-ray diffraction and Raman spectroscopy. X-ray diffraction measurement with Cu Ka radiation was performed for a bundle of ExCFs mounted on a specially designed sample holder using a wide angle diffractometer (Rigaku Rint 2100). The average values of the interlayer spacing d002 and of crystallite size along c-axis Lc(0 0 2) were determined after correcting the diffraction profile of 0 0 2 line for Lorentz-polarization and atomic scattering factors, referring to the outer standard of an HOPG specimen with a flat cleaved surface, of which d002 was calibrated as 0.3354 nm by a powder method with the inner standard of high-purity silicon powder [17]. First order Raman spectrum was measured on a single filament of ExCFs using a 514.5 nm line of an argon ion laser (Jobin Ybon Ramanor, T-64000). 3. Results and discussion 3.1. Surface morphology In Fig. 1(a) and (b), a change in surface morphology of pristine and exfoliated mesophase-pitch-based carbon fibers, respectively, is shown. A single fiber was split into a number of thin filaments after exfoliation, which was explained to be due to a sudden decomposition of intercalates (derivatives from nitric acid) to gaseous species [12]. In Fig. 1(c) and (d), a change in surface morphology with high-temperature heat treatment is shown. By the high-temperature treatment, splitting into thin filaments looks to become clearer. Just after the exfoliation, a memory of the original single fiber could be recognized (Fig. 1(c)), but it seems to be less obvious after the high-temperature treatment (Fig. 1(d)). In the case of PAN-based carbon fibers, however, surface morphology change due to exfoliation is quite different from that of mesophase-pitch-based carbon fi- bers; scale-like small fragments are peeled off from the original fibers and a number of cracks and fissures are formed preferentially along with the original fiber axis, as shown in Fig. 2(b) and (d). No change in surface morphology in the exfoliated PAN-based carbon fibers is observed after the high-temperature treatment. The cracks and fissures with exfoliation were reasonably supposed to be formed by cleaving between hexagonal carbon layers, just like in the exfoliation of natural graphite flakes [12]. This morphological change with the exfoliation of carbon fibers seemed to be consistent with the structure models for these carbon fibers [20]; in the pristine PAN-based carbon fibers, growth of crystallites are not marked even after the high-temperature treatment, but the crystallites grow in the case of the mesophase-pitch-based carbon fibers, as reported on carbon fibers from different precursors. This is the reason why peering-off of scale-like small fragments was apparently observed on the present PAN-based carbon fibers. As a consequence, each of filaments and scales formed by exfoliation consisted of well-aligned carbon layers, as proved in our previous paper [12]. Therefore, either filaments or scales consisting of well-aligned carbon layers were reasonably expected to graphitize. 3.2. X-ray diffraction In Fig. 3, 0 0 2 diffraction profiles are compared on the pristine, exfoliated and high-temperature-treated fi- bers. The structural parameters determined from 0 0 2 diffraction line, interlayer spacing d002 and crystallite size along c-axis Lc(0 0 2), are listed in Table 1. (Fig. 4) 2568 M. Toyoda et al. / Carbon 42 (2004) 2567–2572
M. Toyoda et al /Carbon 42(2004)2567-2571 (b) 60 (d) Fig. 1. SEM images of exfoliated mesophase-pitch-based carbon fibers: (a) pristine single fiber and(b)after exfoliation, (c)after 2800C treatment (b) 10.0pm 10.0pm c) 60.0m 3.0μm Fig. 2. SEM images of PAN-based carbon fibers: (a) pristine single fiber and (b)after exfoliation, (c)after 2800C treatment with low magnification and(d) with high magnification. The pristine mesophase-pitch-based carbon fibers, profile is markedly broadened and the graphitization which have been heat-treated at 3000C in advance, degree evaluated from dooz and lco 02) is lowered. have a sharp 002 diffraction profile with door of 0. 3376 After the heat treatment at 3000C, acceleration of nm and Lc(002)of 28 nm. By the exfoliation, the 002 graphitization in this ExCFs is observed clearly; 002
The pristine mesophase-pitch-based carbon fibers, which have been heat-treated at 3000 C in advance, have a sharp 0 0 2 diffraction profile with d002 of 0.3376 nm and Lc(0 0 2) of 28 nm. By the exfoliation, the 0 0 2 profile is markedly broadened and the graphitization degree evaluated from d002 and Lc(0 0 2) is lowered. After the heat treatment at 3000 C, acceleration of graphitization in this ExCFs is observed clearly; 0 0 2 Fig. 2. SEM images of PAN-based carbon fibers: (a) pristine single fiber and (b) after exfoliation, (c) after 2800 C treatment with low magnification and (d) with high magnification. Fig. 1. SEM images of exfoliated mesophase-pitch-based carbon fibers: (a) pristine single fiber and (b) after exfoliation, (c) after 2800 C treatment with low magnification and (d) with high magnification. M. Toyoda et al. / Carbon 42 (2004) 2567–2572 2569
2570 M. Toyoda et al/ Carbon 42(2004)2567-2572 a)Mesophase-pitch-based carbon fibers b)PAN-based carbon fibers 3000C-treated 900°- treated 3000°C- treat Exfoliated 2223242526272829303122232425262728293031 20/degree Fig. 3. Changes in 002 diffraction profile with exfoliation and following high-temperature treatment. Structural parameters determined by X-ray diffraction and Raman spectroscopy Carbon fibers Pristine Exfoliated Heat-treated at 900C Re-heat-treated at 3000oC mesophase- pitch-based 0.3376 0.341 0.3380 0.3375 Lc(002)(nm) 0.3±0.1 1.2±0.1 0.9±0.1 0.05±0.02 0.03±0.01 0.3±0 .02±0.01 FWHM of D band (in) 32±2 85±4 l10±6 FWHM of G band(Win) 65±3 70±3 PAN-based doo?(nm) 0.343 0.343 0.343 0.3370 10±0.1 17±0.2 19±0.2 09±0.03 0.2±0.1 0.4±0 0.08±0.0 FWHM of D band (Win)p 35±2 ±3 9±2 FWHM of G band (Wi)G 34±1 56±2 20±1 profile becomes sharper than the pristine, and Lc(002) subsequent re-heat-treatment at a high temperature as value becomes much higher, 42 nm, although dooz value 2800C, the former has much larger Lc value than the does not change much latter. This difference is reasonably supposed to be due The pristine PAN-based carbon fibers have low de- to the morphology after exfoliation; the mesophase- gree of graphitization, a broad 002 profile, a high door pitch-based fibers split into thin but long filaments but value of 0. 343 nm and relatively low Lc(002)value of 6 small sized scales are peeled off and many cracks and nm, even though they have been heat-treated to 2800c fissures are introduced into the PAN-based carbon fi- in advance, being consistent with previously reported bers. data on PAN-based carbon fibers [10]. Pronounced happening of 002 profile, decrease in dooz from 0. 343 3.3. Raman spectroscopy to 0. 337 nm and growth of crystallite size lc(00 2)from 6 to 16 nm are observed after the re-heat -treatment to mesophase-pitch-based and PAN-based car 2800 oC after exfoliation Raman spectrum is shown on pristine, Comparing between mesophase-pitch-based and exfo re-heat-treated at 900oC and at a high tem- PAN-based carbon fibers experienced exfoliation and perature as 3000 and 2800oC, respectively. In each
profile becomes sharper than the pristine, and Lc(0 0 2) value becomes much higher, 42 nm, although d002 value does not change much. The pristine PAN-based carbon fibers have low degree of graphitization, a broad 0 0 2 profile, a high d002 value of 0.343 nm and relatively low Lc(0 0 2) value of 6 nm, even though they have been heat-treated to 2800 C in advance, being consistent with previously reported data on PAN-based carbon fibers [10]. Pronounced sharpening of 0 0 2 profile, decrease in d002 from 0.343 to 0.337 nm and growth of crystallite size Lc(0 0 2) from 6 to 16 nm are observed after the re-heat-treatment to 2800 C after exfoliation. Comparing between mesophase-pitch-based and PAN-based carbon fibers experienced exfoliation and subsequent re-heat-treatment at a high temperature as 2800 C, the former has much larger Lc value than the latter. This difference is reasonably supposed to be due to the morphology after exfoliation; the mesophasepitch-based fibers split into thin but long filaments but small sized scales are peeled off and many cracks and fissures are introduced into the PAN-based carbon fi- bers. 3.3. Raman spectroscopy On the mesophase-pitch-based and PAN-based carbon fibers, Raman spectrum is shown on pristine, exfoliated, re-heat-treated at 900 C and at a high temperature as 3000 and 2800 C, respectively. In each Fig. 3. Changes in 0 0 2 diffraction profile with exfoliation and following high-temperature treatment. Table 1 Structural parameters determined by X-ray diffraction and Raman spectroscopy Carbon fibers Pristine Exfoliated Heat-treated at 900 C Re-heat-treated at 3000 C Mesophase-pitch-based d002 (nm) 0.3376 0.341 0.3380 0.3375 Lc(0 0 2) (nm) 28 9 14 42 ID=IG 0.3 ± 0.1 1.2 ± 0.1 0.9 ± 0.1 0.05 ± 0.02 ID0=IG 0.03 ± 0.01 0.3 ± 0.1 0.2 ± 0.1 0.02 ± 0.01 FWHM of D band ðW1=2ÞD 32 ± 2 85 ± 4 110 ± 6 40 ± 2 FWHM of G band ðW1=2ÞG 20 ± 1 65 ± 3 70 ± 3 20 ± 1 PAN-based d002 (nm) 0.343 0.343 0.343 0.3370 Lc(0 0 2) (nm) 6 5 6 16 ID=IG 1.0 ± 0.1 1.7 ± 0.2 1.9 ± 0.2 0.09 ± 0.03 ID0=IG 0.2 ± 0.1 0.4 ± 0.1 0.3 ± 0.1 0.08 ± 0.03 FWHM of D band ðW1=2ÞD 35 ± 2 63 ± 3 49 ± 3 39 ± 2 FWHM of G band ðW1=2ÞG 34 ± 1 56 ± 2 48 ± 2 20 ± 1 2570 M. Toyoda et al. / Carbon 42 (2004) 2567–2572
M. Toyoda et al. Carbon 42(2004)2567-2572 a) Mesophase-pitch-based carbon fibers b)PAN-based carbon fibers 3000° c-treated D band D band 900.C-treated 900°- treated Exfoliated Exfoliated Raman shift/ cm Raman shift cn Fig. 4. Changes in Raman spectrum with exfoliation and following high-temperature treatment of carbon fibers. pectrum,three bands are observed, g band with the phase-pitch-based carbon fibers. The pristine carbon fi Raman shift of about 1590 cm, D band around 1360 bers are not graphitized, showing a strong d band wi cm- and D band around 1620 cm-. G band is due to almost the same intensity as g band and a clear d' E2g vibration mode on ordered graphitic structure [18]. band. The exfoliation introduced large amount of D and D bands have been interpreted to be due to structural disorder, as being revealed by growth and discontinuity of hexagonal carbon layer planes such as broadening of D band. The re-heat-treatment at 900C finite crystallite size and also to edge planes of crystal- could not anneal these structural disorders. The re-heat lites [18, 19 the latter appearing in the high frequency treatment at 2800C, however, results in marked de- tail ofg band. The relative intensities of d band tog crease in intensities of d and d bands and g band band, Ip/IG, and of D band to G band, Ip/IG, and also becomes strong and sharp, as shown numerically by the all width at half maximum intensity(FWHM) forG intensity ratios and FWHMs in Table l, which reveals band,(Wip)G, and that of D band,(Wi/)p, therefore, high degree of graphitization. The graphitization degree materials measured For eao, graphitization of carbon of ExCFs is clearly seen to be much higher than that of depend on the degree spectrum, the integrated the pristine fibers in the case of PAN-based ones. These intensities of the G, D and d bands, and FWHMs ofg results are consistent with those of X-ray diffraction and d bands were determined after separation of these analysis. Accelerated graphitization observed on PAN- bands using a fitting program. In Table l, the values of based carbon fibers was remarkable, though graphiti Ip/IG, Ip/IG,(Wip)G and(Win)p thus obtained are zation degree was a little lower than mesophase-pitch- listed with scattering in measured values based carbon fibers The pristine mesophase-pitch-based carbon fibers These results revealed that the exfoliation through heat-treated at 3000C show very weak D and D bands the decomposition of intercalation compounds of car- and relatively sharp G band, which is consistent with X- bon fibers released the constraint of the fibrous mor- ray parameters observed on the same fibers. By the ex- phology, associated with the introduction of various foliation, a large amount of structural disorders was structural defects. In order to keep fibrous morphology, supposed to be introduced, because of marked broad- certain constraint is reasonably supposed to play ening of 002 X-ray diffraction line, which resulted in important role. One of the evidences for the presence of marked increases in the intensities of d and d bands. this constraint in carbon fibers is the fact that inter- and also in broadening of G and d bands. The re-heat- calation of nitric acid is not possible through chemical treatment to 3000C, however, makes the intensities of process and is necessary to be forced by electrochemical D and d bands markedly weaker and the g band process [12-15]. Formation of cracks and fissures along stronger and sharper, as numerically shown by the fiber axis is reasonably understood to reveal that intensity ratios, Ip/IG and /p/IG and FWHMs, (Win) he constraint in the pristine fibers was released, al and (Win)p, in Table 1. The results by Raman spec- though quantitative discussion on the constraint and its troscopy reveal high degree of graphitization of ExCFs, release is not possible. This release of constraints to higher than the pristine, more clearly than those by X- keep pristine fibrous morphology by exfoliation and ray diffraction. he following annealing of structural defects by the In the case of PAN-based carbon fibers, the structural re-heat-treatment at a high temperature as 2800C changes with exfoliation and re-heat-treatment were caused the improvement in the graphitization of carbon observed markedly, much more than the case of meso- fibers
spectrum, three bands are observed, G band with the Raman shift of about 1590 cm1, D band around 1360 cm1 and D0 band around 1620 cm1. G band is due to E2g vibration mode on ordered graphitic structure [18]. D and D0 bands have been interpreted to be due to discontinuity of hexagonal carbon layer planes such as finite crystallite size and also to edge planes of crystallites [18,19], the latter appearing in the high frequency tail of G band. The relative intensities of D band to G band, ID=IG, and of D0 band to G band, ID0=IG, and also full width at half maximum intensity (FWHM) for G band, ðW1=2ÞG, and that of D band, ðW1=2ÞD, therefore, depend on the degree of graphitization of carbon materials measured. For each spectrum, the integrated intensities of the G, D and D0 bands, and FWHMs of G and D bands were determined after separation of these bands using a fitting program. In Table 1, the values of ID=IG, ID0=IG, ðW1=2ÞG and ðW1=2ÞD thus obtained are listed with scattering in measured values. The pristine mesophase-pitch-based carbon fibers heat-treated at 3000 C show very weak D and D0 bands and relatively sharp G band, which is consistent with Xray parameters observed on the same fibers. By the exfoliation, a large amount of structural disorders was supposed to be introduced, because of marked broadening of 0 0 2 X-ray diffraction line, which resulted in marked increases in the intensities of D and D0 bands, and also in broadening of G and D bands. The re-heattreatment to 3000 C, however, makes the intensities of D and D0 bands markedly weaker and the G band stronger and sharper, as numerically shown by the intensity ratios, ID=IG and ID0=IG and FWHMs, ðW1=2ÞG and ðW1=2ÞD, in Table 1. The results by Raman spectroscopy reveal high degree of graphitization of ExCFs, higher than the pristine, more clearly than those by Xray diffraction. In the case of PAN-based carbon fibers, the structural changes with exfoliation and re-heat-treatment were observed markedly, much more than the case of mesophase-pitch-based carbon fibers. The pristine carbon fi- bers are not graphitized, showing a strong D band with almost the same intensity as G band and a clear D0 band. The exfoliation introduced large amount of structural disorder, as being revealed by growth and broadening of D band. The re-heat-treatment at 900 C could not anneal these structural disorders. The re-heattreatment at 2800 C, however, results in marked decrease in intensities of D and D0 bands, and G band becomes strong and sharp, as shown numerically by the intensity ratios and FWHMs in Table 1, which reveals high degree of graphitization. The graphitization degree of ExCFs is clearly seen to be much higher than that of the pristine fibers in the case of PAN-based ones. These results are consistent with those of X-ray diffraction analysis. Accelerated graphitization observed on PANbased carbon fibers was remarkable, though graphitization degree was a little lower than mesophase-pitchbased carbon fibers. These results revealed that the exfoliation through the decomposition of intercalation compounds of carbon fibers released the constraint of the fibrous morphology, associated with the introduction of various structural defects. In order to keep fibrous morphology, certain constraint is reasonably supposed to play an important role. One of the evidences for the presence of this constraint in carbon fibers is the fact that intercalation of nitric acid is not possible through chemical process and is necessary to be forced by electrochemical process [12–15]. Formation of cracks and fissures along the fiber axis is reasonably understood to reveal that the constraint in the pristine fibers was released, although quantitative discussion on the constraint and its release is not possible. This release of constraints to keep pristine fibrous morphology by exfoliation and the following annealing of structural defects by the re-heat-treatment at a high temperature as 2800 C caused the improvement in the graphitization of carbon fibers. Fig. 4. Changes in Raman spectrum with exfoliation and following high-temperature treatment of carbon fibers. M. Toyoda et al. / Carbon 42 (2004) 2567–2572 2571
M. Toyoda et al/ Carbon 42(2004)2567-2572 4. Conclusions [6 Noda T, Inagaki M. The melting of glassy carbon. Bull Chem Soc pn1964:37:170910. Two carbon fibers used in the present work, a wide [7 Bokros JC. Deposition, structure, and properties of pyrolytic raphitizability of carbon fibers was tried to arbon. In: Walker PL, editor. Chemistry of physics of carbon, vol 5. New York: Marcel Dekker; 1969. p 8- graphitization degree in mesophase-pitch [8 Hishiyama Y, Yasuda S, Yoshida A, Inagaki M. Structure and and low degree in PAN-based carbon fibers. The properties of highly crystallized graphite films based on polyimide exfoliation of those carbon fibers was observed to occur Kapton. J Mater Sci 1988: 23: 3277-3277. preferentially to split the original single fiber into thin 9 Inagaki M, Hishiyama Y, Takeichi T, Oberlin A. High quality filaments by the formation of large cracks and small graphite films produced from aromatic polyimides. In: Thrower fissures preferentially along with the fiber axis, which PA, Radovic L, editors. Chemistry and physics of carbon, vol. 26 New York: Marcel Dekker; 1999. p. 246-333 was clearly observed on mesophase-pitch-based carbon [10] Hishiyama Y, Kaburagi Y, Yoshida A Magnetoresistance and fibers(Fig. 1). On PAN-based carbon fibers, peering-off preferred on in carbon fibers. In: Inagaki M. editor. of scale -like fragments from the fiber appeared, but a Science and new applications of carbon fibers. Toyohashi: Toyohashi University: 1984. p. 21-51 number of cracks or fissures along with the fibers axis [l] Hishiyama Y, Kaburagi Y, Inagaki M. Characterization of were also observed after exfoliation The acceleration of ructure and microtexture of carbon materials by magnetoresis. graphitization was observed on exfoliated carbon fibers tance technique. In: Thrower PA, editor. Chemistry and physics of after the re-heat-treatment at high temperatures as 2800 carbon, vol. 23. New York: Marcel Dekker; 1991. p. 1-68. C, particularly on those derived from PAN-based car [12] Toyoda M, Shimizu A, Iwata H, Inagaki M. Exfoliation of bon fibers carbon fibers through intercalation compounds synthesized elec- On the electrochemical capacitor where the exfoliated chemically. Carbon 2001: 39: 1697-707 [13] Toyoda M, Kato H, Inagaki M. Intercalation compounds of carbon fibers were used as the electrode materials. a carbon fibers synthesized electrochemically and its intercalation huge capacitance was obtained in concentrated sulfuric mechanism. Carbon 2001: 39: 2231-7. acid, which was supposed to be due to intercalation of [14] Toyoda M, Katoh H, Shimizu A, Inagaki M Exfoliation of nitric sulfuric acid [21]. If this mechanism supposed is the case, acid intercalated carbon fibers: effects of heat-treatment temper. high degree of graphitization of exfoliated carbon fibers ature of pristine carbon fibers and electrolyte concentration on the foliation behavior. Carbon 2003: 41: 731-8. might be an advantage to get much higher capacitance. [15] Toyoda M, Sedalacik J, Inagaki M. Intercalation of formic acid Electron emitting performance from these exfoliated bon fibers and their exfoliation. Synth Metals carbon fibers showed even better than that from a 2002;130:39-43 bundle of carbon nanotubes [22]. High graphitization [16] Toyoda M, Kaburagi Y, Yoshida A, Iwata H, Inagaki M. degree of these exfoliated carbon fibers after high-tem Accelerated graphitization of exfoliated carbon fibers. Carbon 2002:40628-9 perature treatment might be preferable for this appli- [17] Hishiyama Y, Igarashi K, Kanaoka I, Fujii H, Kaneda T, cation in the terms of high electrical conductivity, high Koideasawa T, et al. Graphitization behavior of Kapton-derived temperature stability. etc. arbon film related to structure, microtexture and transport roperties. Carbon 1997; 35(5): 657-68 [18 Nemanich RJ, Solin SA. First- and second-order Raman References catering from finite-size crystals of graphite. Phys Rev B 1979; 20:392-401 [9 Katagiri G, Ishida H, Ishitani A Raman spectra of graphite edge [] Pacault A, editor. Les carbons. Masson et C, 1965 ane. Carbon1988:26:565-71 2 Marsh H. Rodriguez-Reinoso, editors. Sciences of carbon mate- [20] Oberlin A, Bonnamy S, Lafdi K. Structure and texture of carbon rials. Univ. Alicante. 2000 fibers. In: Donnet JB et al.. editors. Carbon fibers. New York: 3 Inagaki M. New carbons--control of structure and functions. Marcel Dekker; 1998. p. 85-159 Oxford: Elsevier: 2000 [21] Soneda Y, Toyoda M, Hayashi K, Yamashita J, Kodama M, 4 Inagaki M. Graphitization under high pressure. TANSO Hatori H, et al. Huge electrochemical capacitance of exfoliated 1987: (No. 129): 68-80 [ in Japanese] carbon fibers. Carbon 2003: 41- 2680-2 5 Inagaki M, Meyer AP. Stress graphitization. In: Thower P, [22 Toyoda M, Sugimoto N, Inagaki M. Study of characterization of Radovic L, editors. Chemistry and physics of carbon. New Yor emitter using exfoliated carbon fibers. Annual Meeting of Carbon Marcel Dekker: 1998. p. 149-244 Society of Japan, 4-6 December 2003, Chiba, Japan. p. 8-9
4. Conclusions Two carbon fibers used in the present work, a wide range of graphitizability of carbon fibers was tried to cover, high graphitization degree in mesophase-pitchbased and low degree in PAN-based carbon fibers. The exfoliation of those carbon fibers was observed to occur preferentially to split the original single fiber into thin filaments by the formation of large cracks and small fissures preferentially along with the fiber axis, which was clearly observed on mesophase-pitch-based carbon fibers (Fig. 1). On PAN-based carbon fibers, peering-off of scale-like fragments from the fiber appeared, but a number of cracks or fissures along with the fibers axis were also observed after exfoliation. The acceleration of graphitization was observed on exfoliated carbon fibers after the re-heat-treatment at high temperatures as 2800 C, particularly on those derived from PAN-based carbon fibers. On the electrochemical capacitor where the exfoliated carbon fibers were used as the electrode materials, a huge capacitance was obtained in concentrated sulfuric acid, which was supposed to be due to intercalation of sulfuric acid [21]. If this mechanism supposed is the case, high degree of graphitization of exfoliated carbon fibers might be an advantage to get much higher capacitance. Electron emitting performance from these exfoliated carbon fibers showed even better than that from a bundle of carbon nanotubes [22]. High graphitization degree of these exfoliated carbon fibers after high-temperature treatment might be preferable for this application in the terms of high electrical conductivity, hightemperature stability. etc. References [1] Pacault A, editor. Les carbones. Masson et Cie, 1965. [2] Marsh H. Rodriguez-Reinoso, editors. Sciences of carbon materials. Univ. Alicante, 2000. [3] Inagaki M. New carbons––control of structure and functions. Oxford: Elsevier; 2000. [4] Inagaki M. Graphitization under high pressure. TANSO 1987;(No. 129):68–80 [in Japanese]. [5] Inagaki M, Meyer AP. Stress graphitization. In: Thower P, Radovic L, editors. Chemistry and physics of carbon. New York: Marcel Dekker; 1998. p. 149–244. [6] Noda T, Inagaki M. The melting of glassy carbon. Bull Chem Soc Jpn 1964;37:1709–10. [7] Bokros JC. Deposition, structure, and properties of pyrolytic carbon. In: Walker PL, editor. Chemistry of physics of carbon, vol. 5. New York: Marcel Dekker; 1969. p. 8–23. [8] Hishiyama Y, Yasuda S, Yoshida A, Inagaki M. Structure and properties of highly crystallized graphite films based on polyimide Kapton. J Mater Sci 1988;23:3277–3277. [9] Inagaki M, Hishiyama Y, Takeichi T, Oberlin A. High quality graphite films produced from aromatic polyimides. In: Thrower PA, Radovic L, editors. Chemistry and physics of carbon, vol. 26. New York: Marcel Dekker; 1999. p. 246–333. [10] Hishiyama Y, Kaburagi Y, Yoshida A. Magnetoresistance and preferred orientation in carbon fibers. In: Inagaki M, editor. Science and new applications of carbon fibers. Toyohashi: Toyohashi University; 1984. p. 21–51. [11] Hishiyama Y, Kaburagi Y, Inagaki M. Characterization of structure and microtexture of carbon materials by magnetoresistance technique. In: Thrower PA, editor. Chemistry and physics of carbon, vol. 23. New York: Marcel Dekker; 1991. p. 1–68. [12] Toyoda M, Shimizu A, Iwata H, Inagaki M. Exfoliation of carbon fibers through intercalation compounds synthesized electrochemically. Carbon 2001;39:1697–707. [13] Toyoda M, Kato H, Inagaki M. Intercalation compounds of carbon fibers synthesized electrochemically and its intercalation mechanism. Carbon 2001;39:2231–7. [14] Toyoda M, Katoh H, Shimizu A, Inagaki M. Exfoliation of nitric acid intercalated carbon fibers: effects of heat-treatment temperature of pristine carbon fibers and electrolyte concentration on the exfoliation behavior. Carbon 2003;41:731–8. [15] Toyoda M, Sedalacik J, Inagaki M. Intercalation of formic acid into carbon fibers and their exfoliation. Synth Metals 2002;130:39–43. [16] Toyoda M, Kaburagi Y, Yoshida A, Iwata H, Inagaki M. Accelerated graphitization of exfoliated carbon fibers. Carbon 2002;40:628–9. [17] Hishiyama Y, Igarashi K, Kanaoka I, Fujii H, Kaneda T, Koideasawa T, et al. Graphitization behavior of Kapton-derived carbon film related to structure, microtexture and transport properties. Carbon 1997;35(5):657–68. [18] Nemanich RJ, Solin SA. First- and second-order Raman scattering from finite-size crystals of graphite. Phys RevB 1979; 20:392–401. [19] Katagiri G, Ishida H, Ishitani A. Raman spectra of graphite edge plane. Carbon 1988;26:565–71. [20] Oberlin A, Bonnamy S, Lafdi K. Structure and texture of carbon fibers. In: Donnet JB et al., editors. Carbon fibers. New York: Marcel Dekker; 1998. p. 85–159. [21] Soneda Y, Toyoda M, Hayashi K, Yamashita J, Kodama M, Hatori H, et al. 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