- Email: [email protected]
polymer blend
Carbon 44 (2006) 682–686 www.elsevier.com/locate/carbon
Preparation of highly crystalline carbon nanofibers from pitch/polymer blend Hisayoshi Ono, Asao Oya
*
Graduate School of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan Received 1 June 2004; accepted 22 September 2005 Available online 7 November 2005
Abstract High crystalline carbon nanofibers were prepared by using polymer blend technique. Naphthalene-based mesophase pitch (AR pitch) was dispersed finely in polymethylpentene matrix, spun by using a melt-blown spinning machine, stabilized at 160 °C in an oxygen atmosphere and carbonized at 900 °C in a nitrogen atmosphere. Bundles of the carbon nanofibers with ca. 100 nm in diameter were obtained after removal of polymethylpentene at the carbonization process. No impurity carbon was observed. The carbon nanofibers consisted of fine carbon crystallites with preferred orientation along the fiber axis. After heating to 3000 °C, the carbon crystallites grew drastically to have an interlayer spacing of 0.3367 nm and a crystallite thickness of 56.9 nm, respectively, with remarkable improvement of the preferred orientation of the crystallites. Advantages and disadvantages of the present method were discussed briefly. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon nanofiber; Graphitization; Stabilization; Electron microscopy; X-ray diffraction; Crystal structure
1. Introduction Previously the present authors reported on carbon nanofibers prepared from novolac-type phenol-formaldehyde resin by using polymer blend technique [1], of which the basic idea was described elsewhere in detail [2]. The nanofibers had 200–300 nm in diameter and were referred to thin carbon fibers in the paper [1]. They were consisting in a typical non-graphitizing carbon as expected from the polymeric nature of the precursor. So the resulting carbon nanofibers are predicted to have poor mechanical properties as well as the conventional non-graphitizing carbon fibers [3]. At present, high crystalline carbon nanofibers are prepared by a catalytic chemical vapor deposition (CCVD) method [4], and they are commercially available by Showa Denko Co. Ltd. in Japan. However, we believe that the CCVD method is not necessarily suitable for mass-production because of a low concentration of raw *
Corresponding author. Tel.: +81 277 30 1350; fax: +81 277 30 1353. E-mail address: [email protected] (A. Oya).
0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.09.021
hydrocarbon gas at the reaction process. The disadvantage is compensated with a large reaction rate, but the large reaction rate makes it difficult to control the structure of carbon nanofibers and to suppress the formation of carbon impurities. This is why we tried to develop a novel preparation method of highly crystalline carbon nanofibers instead of the CCVD method. We thought that the purpose will be accomplished successfully by applying the polymer blend technique to a graphitizable carbon precursor. After many trials, we choose a naphthalene-based mesophase pitch precursor [5,6]. The present paper reports on preparation process and structure of the carbon nanofibers derived from the mesophase pitch using the polymer blend technique. 2. Experimental 2.1. Materials Naphthalene-based mesophase pitch referred to AR pitch was prepared by Mitsubishi Gas Chemical Co. Ltd. [5,6], of which some properties are shown in Table 1. Polymethylpentene (Mitsui-Sumitomo Chemical
H. Ono, A. Oya / Carbon 44 (2006) 682–686 Co. Ltd.: TPX RT-18, abbreviation: PMP) with a softening point of 176 °C was used as a thermally decomposable matrix polymer without carbon residue.
683
microscope (200 kV). X-ray diffraction patterns were taken by a RIGAKU RINT2000/PC diffractometer with monochromatized Cu Ka radiation, operated at 40 kV and 40 mA. Raman spectra were taken by use of a NIPPON BUNKO NSR-1000 F spectrometer.
2.2. Preparation procedure
3. Results and discussion Both polymers were blended by weight to a ratio of AR pitch:PMP = 3:7 and then mixed by a planetary ball mill (Fritsch P-7). The mixture was subsequently blended by a kneader (Atlas Electric Devices Co., Laboratory Mixing Extruder Model CS-194AV) at 240 °C to homogenize and to remove air included in the mixture to improve the spinnability. The resulting polymer blend was spun by use of a meltblown spinning machine with monohole of 0.5 mm in diameter. The machine was specially made by Nippon Nozzle Co. Ltd. according to our order. The spinning conditions are as follows; pot pressure: 0.4 MPa, blow pressure: 3.5 MPa, spinning temperature: ca. 380 °C. The fundamental mechanism of melt-blown spinning is explained in detail elsewhere [7]. The polymer blend fiber was put in a furnace held at 160 °C and kept for 24 h under an oxygen atmosphere for stabilization. The stabilized fiber was carbonized at 900 °C for 1 h in a nitrogen atmosphere, and a part of the carbonized fiber was further graphitized at 3000 °C for 30 min in an argon atmosphere.
2.3. Characterization Both the polymers and the polymer blend were analyzed thermogravimetrically by use of a Rigaku Thermo plus TG8120 apparatus with a heating rate of 10 °C/min in a nitrogen atmosphere. Polarized light microphotographs were kindly taken by Mitsubishi Gas Chemical Co. Ltd. Scanning electron microscopic (SEM) and field-emission SEM (FESEM) observations were made by using a JEOL JFC-1500 and a JEOL JSM-6700FS microscopes, respectively. Transmission electron microscopic (TEM) observation was carried out by use of a JEOL JEM-2010
Fig. 1 shows TG curves of AR pitch, PMP and the polymer blend. All specimens were heated in advance at 160 °C for 24 h under an oxygen atmosphere before the measurement, which are the same conditions as used for the stabilization. PMP disappeared without any residue below 450 °C, and AR pitch resulted in about 70 wt% of carbon yield after heating to 1000 °C. Thermal behavior of the polymer blend coincided with that calculated from the behavior of each component and the mixing ratio, which suggests no occurrence of reaction between both the polymers to change their thermal behaviors. So, the combination of AR pitch and PMP can be reasonably applied to the polymer blend method [2]. Fig. 2 shows SEM photographs of the polymer blend fibers before and after the stabilization. There was no clear difference between them. The fiber diameters were not uniform and ranged from 300 nm to 10 lm (the average is several lm), because it was not easy to operate the spinning machine skillfully. Nevertheless, the diameters were far
Table 1 Properties of AR pitch Specific gravity Softening point (°C) Mesophase content (%) H/C (atom ratio) Aromaticity (d3030/d2920)
1.23 275 100 0.58–0.64 0.5/0.6
Solubility (%) Hexane insoluble Toluene insoluble Pyridine insoluble
98–99 60–65 40–50
Fig. 1. TG curves of AR pitch, PMP and the polymer blend in a nitrogen atmosphere, after heating at 160 °C for 24 h in an oxygen atmosphere.
Fig. 2. SEM photographs of the polymer blend fibers before (left) and after (right) stabilization.
684
H. Ono, A. Oya / Carbon 44 (2006) 682–686
small compared with ca. 10 lm of diameter for pitch-based carbon fibers spun by using a conventional melt-spinning apparatus [8]. The result also shows a possibility to spin the homogeneous fibers with several hundreds nm in diameter through improvement of the spinning technique. The thinner polymer blend fiber seems to result in thinner carbon nanofibers finally. The melt-blown spinning machine cannot spin continuous fibers and, therefore, has been used conventionally for mass production of non-woven fabrics [7]. Fortunately continuous fiber is not required for the preparation of carbon nanofibers by the present method, because the AR pitch nanofibers elongated in PMP matrix are far short compared with the polymer blend fibers as seen in Fig. 2. The melt-blown spinning machine must be the most promising in mass-production of carbon nanofibers by using the polymer blend technique. The stabilization mechanism of the AR pitch fiber was previously reported by Mochida et al. [6]. After the carbonization of the stabilized blend fiber, carbon nanofibers as seen in Fig. 3 were left after the removal of PMP. These fibers formed bundles as noted later. Diameter of the nanofibers was around 100 nm and was relatively uniform. We predicted initially a large scatter of the diameter from following two points. (1) There was a large dispersion in the diameter of polymer blend fibers as seen in Fig. 2. (2) The dispersed AR pitch in the PMP
Fig. 3. A FE-SEM photograph of carbon nanofibers heated at 900 °C.
matrix coagulated to form particles with various sizes at the spinning process. Fig. 4(left) shows a polarized light micrograph of the polymer blend just after the mechanical kneading, on which irregular black lines represent AR pitch. This polymer blend was molten in the spinneret to 380 °C of the spinning temperature. Various sizes of AR pitch particles formed (Fig. 4(right)), possibly leading to the dispersion in the diameter of the resulting carbon nanofibers, although the result was opposed as stated above. We suppose that a larger AR pitch particle in the PMP matrix is favorably elongated because of a larger tensile shear applied at spinning process. In addition, the authors would like to emphasize no formation of any
Fig. 4. Polarized light micrographs of the polymer blends before and after melting at 380 °C.
Fig. 5. TEM photographs of a carbon nanofiber heated at 900 °C. Inset is the SAED pattern. A white arrow shows the direction of preferred orientation of carbon crystallites.
H. Ono, A. Oya / Carbon 44 (2006) 682–686
carbon impurity such as carbon particles and flakes. The present method therefore makes possible the preparation of high purity carbon nanofibers with no any carbon impurity. The carbonized nanofibers were served for TEM observation. As seen in Fig. 5(left), the fiber surface was smooth and no defect was observed. The carbon nanofibers consisted in fine carbon crystallites with a preferred orientation along the fiber axis (Fig. 5(right)). The preferred orientation was more clearly shown by the selected area electron diffraction (SAED) pattern. The structure was changed drastically after heating to 3000 °C. Fig. 6 shows SEM photographs of the nanofibers after heating to 3000 °C. The fibers seen in the photographs are bundles of carbon nanofibers, as suggested by the splitting of the fibers as indicated by arrow 2 (Fig. 6(left)). Very thin bundles were sometimes observed, see arrow 1. The bundles were ground to serve TEM observation. Fig. 7 shows a TEM photograph of the nanofibers. The diameters were about 100 nm. Fig. 7(right) shows an irregular broken edge suggesting a highly crystalline carbon structure [9]. This prediction was certified clearly by the TEM image and the SAED pattern shown in Fig. 8. The carbon crystals were well-aligned along the fiber axis. The degree of graphitization of the carbon nanofibers were examined by use of X-ray diffraction analysis. Fig. 9 shows the X-ray diffraction profile of the 002 diffraction peak of polyaromatic carbon together with the 111 diffraction peak of silicon used as an internal standard. A broad
685
Fig. 8. An enlarged TEM photograph of a carbon nanofiber heated at 3000 °C (left) and its SAED pattern (right).
diffraction profile of the 900 °C nanofibers sharpened drastically after heating to 3000 °C. The interlayer spacing (d002) and the crystallite thickness (Lc(002)) were 0.3379 nm and 1.6 nm for the 900 °C fiber, and 0.3367 nm and 56.9 nm for the 3000 °C fiber, respectively. Such a low d002 spacing suggested a high graphitization state (i.e., with a 3D crystal structure similar or close to that of graphite, as opposed to the 2D structure of turbostratic carbons). This was confirmed by the evidence for specific reflections such as 112 found in the SAED pattern in Fig. 8(right) as arrowed. Though the accurate display of the graphenes within the fibers is still questionable (specifically in fiber crosssection), it was stated above that highly crystalline carbon nanofibers with a high preferred orientation of crystallites along the fiber axis were prepared successfully by using a
Fig. 6. SEM photographs of bundles of carbon nanofibers heated at 3000 °C.
Fig. 7. TEM photographs of carbon nanofibers separated from the bundles, after heating at 3000 °C.
686
H. Ono, A. Oya / Carbon 44 (2006) 682–686
spinning machine, resulting in the fibers with an average diameter of several lm. The fibers were stabilized at 160 °C in an oxygen atmosphere and carbonized at 900 °C in a nitrogen atmosphere. After removal of the matrix at the carbonization process, the bundles of the carbon nanofibers were left. The nanofiber diameter was ca. 100 nm. The carbon nanofibers consisted of fine carbon crystallites with preferred orientation along the fiber axis. The carbon crystallites grew drastically after heating to 3000 °C. X-ray parameters of the graphitized nanofibers were 0.3367 nm as interlayer spacing and 57 nm as crystallite thickness. Acknowledgements
Fig. 9. 002 X-ray diffraction profiles of carbon nanofibers heated at 900 °C and 3000 °C.
polymer blend technique. Main reason for the success is that AR mesophase pitch was spun by the melt-blown spinning machine. No impurity carbon was included in the resulting carbon nanofibers. The present method has a high potentiality for mass-production of high purity carbon nanofibers. However, some problems were remained unsolved. The most serious one is that the AR pitch nanofibers were stabilized under a severe condition, because the diffusion of oxygen is suppressed by the PMP matrix. So the thinner polymer blend fiber is more favorable to the stabilization in view of a short diffusion distance for oxygen. The melt-blown spinning machine is more suitable for this purpose than the conventional melt spinning apparatus [3]. Nevertheless 24 h at 160 °C was required. How to shorten the stabilization time during the process seems a key point for the viable commercialization of the present preparation method of carbon nanofiber. 4. Conclusion Naphthalene-based mesophase pitch (AR pitch) particles were dispersed in polymethylpentene matrix. The resulting polymer blend was spun by using a melt-blown
The authors wish to thank Prof. Y. Korai for supplying AR pitch, and Dr. F. Watanabe for taking polarized optical microscopic pictures. References [1] Oya A, Kasahara N. Preparation of thin carbon fiber from phenolformaldehyde polymer micro-beads dispersed in polyethylene matrix. Carbon 2000;38:1141–4. [2] Hulicova D, Oya A. Polymer blend technique as a method to design fine carbon materials. Carbon 2003;41:1443–50. [3] Otani S. Carbonaceous mesophase and carbon fibers. Mol Cryst Liq Cryst 1981;63:249–64. [4] Endo M, Kim YA, Takeda T, Hong SH, Matsusita T, Hayashi T, Dresselhaus MS. Structural characterization of carbon nanofibers obtained by hydrocarbon pyrolysis. Carbon 2001;39:2003–10. [5] Mochida I, Shimizu K, Korai Y, Otsuka H, Fujiyama S. Structure and carbonization properties of pitches produced catalytically from aromatic hydrocarbons with HF/BF3. Carbon 1988;26:843–52. [6] Mochida I, Shimizu K, Korai Y, Otsuka H, Sakai Y, Fujiyama S. Preparation of mesophase pitch from aromatic hydrocarbons by the acid of HF/BF3. Carbon 1990;28:311–9. [7] Balk H. Method of producing plastic fibers or filaments, preferably in conjunction with the formation of non-woven fabric. United State Patent No. 5,098,636, 1992. [8] Bahl OP, Shen Z, Lavin JG, Ross RA. Manufacture of carbon fibers. In: Donnet JB, Wang TK, Peng JM, Rebouillat S, editors. Carbon Fibers. 3rd ed. New York: Marcel Dekker; 1998. p. 1–83. [9] Oberlin A, Bonnamy S, Lafdi K. Structure and texture of carbon fibers. In: Donnet JB, Wang TK, Peng JM, Rebouillat S, editors. Carbon Fibers. 3rd ed. New York: Marcel Dekker; 1998. p. 85–160.
Preparation of highly crystalline carbon nanofibers from pitch/polymer blend Hisayoshi Ono, Asao Oya
*
Graduate School of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan Received 1 June 2004; accepted 22 September 2005 Available online 7 November 2005
Abstract High crystalline carbon nanofibers were prepared by using polymer blend technique. Naphthalene-based mesophase pitch (AR pitch) was dispersed finely in polymethylpentene matrix, spun by using a melt-blown spinning machine, stabilized at 160 °C in an oxygen atmosphere and carbonized at 900 °C in a nitrogen atmosphere. Bundles of the carbon nanofibers with ca. 100 nm in diameter were obtained after removal of polymethylpentene at the carbonization process. No impurity carbon was observed. The carbon nanofibers consisted of fine carbon crystallites with preferred orientation along the fiber axis. After heating to 3000 °C, the carbon crystallites grew drastically to have an interlayer spacing of 0.3367 nm and a crystallite thickness of 56.9 nm, respectively, with remarkable improvement of the preferred orientation of the crystallites. Advantages and disadvantages of the present method were discussed briefly. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon nanofiber; Graphitization; Stabilization; Electron microscopy; X-ray diffraction; Crystal structure
1. Introduction Previously the present authors reported on carbon nanofibers prepared from novolac-type phenol-formaldehyde resin by using polymer blend technique [1], of which the basic idea was described elsewhere in detail [2]. The nanofibers had 200–300 nm in diameter and were referred to thin carbon fibers in the paper [1]. They were consisting in a typical non-graphitizing carbon as expected from the polymeric nature of the precursor. So the resulting carbon nanofibers are predicted to have poor mechanical properties as well as the conventional non-graphitizing carbon fibers [3]. At present, high crystalline carbon nanofibers are prepared by a catalytic chemical vapor deposition (CCVD) method [4], and they are commercially available by Showa Denko Co. Ltd. in Japan. However, we believe that the CCVD method is not necessarily suitable for mass-production because of a low concentration of raw *
Corresponding author. Tel.: +81 277 30 1350; fax: +81 277 30 1353. E-mail address: [email protected] (A. Oya).
0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.09.021
hydrocarbon gas at the reaction process. The disadvantage is compensated with a large reaction rate, but the large reaction rate makes it difficult to control the structure of carbon nanofibers and to suppress the formation of carbon impurities. This is why we tried to develop a novel preparation method of highly crystalline carbon nanofibers instead of the CCVD method. We thought that the purpose will be accomplished successfully by applying the polymer blend technique to a graphitizable carbon precursor. After many trials, we choose a naphthalene-based mesophase pitch precursor [5,6]. The present paper reports on preparation process and structure of the carbon nanofibers derived from the mesophase pitch using the polymer blend technique. 2. Experimental 2.1. Materials Naphthalene-based mesophase pitch referred to AR pitch was prepared by Mitsubishi Gas Chemical Co. Ltd. [5,6], of which some properties are shown in Table 1. Polymethylpentene (Mitsui-Sumitomo Chemical
H. Ono, A. Oya / Carbon 44 (2006) 682–686 Co. Ltd.: TPX RT-18, abbreviation: PMP) with a softening point of 176 °C was used as a thermally decomposable matrix polymer without carbon residue.
683
microscope (200 kV). X-ray diffraction patterns were taken by a RIGAKU RINT2000/PC diffractometer with monochromatized Cu Ka radiation, operated at 40 kV and 40 mA. Raman spectra were taken by use of a NIPPON BUNKO NSR-1000 F spectrometer.
2.2. Preparation procedure
3. Results and discussion Both polymers were blended by weight to a ratio of AR pitch:PMP = 3:7 and then mixed by a planetary ball mill (Fritsch P-7). The mixture was subsequently blended by a kneader (Atlas Electric Devices Co., Laboratory Mixing Extruder Model CS-194AV) at 240 °C to homogenize and to remove air included in the mixture to improve the spinnability. The resulting polymer blend was spun by use of a meltblown spinning machine with monohole of 0.5 mm in diameter. The machine was specially made by Nippon Nozzle Co. Ltd. according to our order. The spinning conditions are as follows; pot pressure: 0.4 MPa, blow pressure: 3.5 MPa, spinning temperature: ca. 380 °C. The fundamental mechanism of melt-blown spinning is explained in detail elsewhere [7]. The polymer blend fiber was put in a furnace held at 160 °C and kept for 24 h under an oxygen atmosphere for stabilization. The stabilized fiber was carbonized at 900 °C for 1 h in a nitrogen atmosphere, and a part of the carbonized fiber was further graphitized at 3000 °C for 30 min in an argon atmosphere.
2.3. Characterization Both the polymers and the polymer blend were analyzed thermogravimetrically by use of a Rigaku Thermo plus TG8120 apparatus with a heating rate of 10 °C/min in a nitrogen atmosphere. Polarized light microphotographs were kindly taken by Mitsubishi Gas Chemical Co. Ltd. Scanning electron microscopic (SEM) and field-emission SEM (FESEM) observations were made by using a JEOL JFC-1500 and a JEOL JSM-6700FS microscopes, respectively. Transmission electron microscopic (TEM) observation was carried out by use of a JEOL JEM-2010
Fig. 1 shows TG curves of AR pitch, PMP and the polymer blend. All specimens were heated in advance at 160 °C for 24 h under an oxygen atmosphere before the measurement, which are the same conditions as used for the stabilization. PMP disappeared without any residue below 450 °C, and AR pitch resulted in about 70 wt% of carbon yield after heating to 1000 °C. Thermal behavior of the polymer blend coincided with that calculated from the behavior of each component and the mixing ratio, which suggests no occurrence of reaction between both the polymers to change their thermal behaviors. So, the combination of AR pitch and PMP can be reasonably applied to the polymer blend method [2]. Fig. 2 shows SEM photographs of the polymer blend fibers before and after the stabilization. There was no clear difference between them. The fiber diameters were not uniform and ranged from 300 nm to 10 lm (the average is several lm), because it was not easy to operate the spinning machine skillfully. Nevertheless, the diameters were far
Table 1 Properties of AR pitch Specific gravity Softening point (°C) Mesophase content (%) H/C (atom ratio) Aromaticity (d3030/d2920)
1.23 275 100 0.58–0.64 0.5/0.6
Solubility (%) Hexane insoluble Toluene insoluble Pyridine insoluble
98–99 60–65 40–50
Fig. 1. TG curves of AR pitch, PMP and the polymer blend in a nitrogen atmosphere, after heating at 160 °C for 24 h in an oxygen atmosphere.
Fig. 2. SEM photographs of the polymer blend fibers before (left) and after (right) stabilization.
684
H. Ono, A. Oya / Carbon 44 (2006) 682–686
small compared with ca. 10 lm of diameter for pitch-based carbon fibers spun by using a conventional melt-spinning apparatus [8]. The result also shows a possibility to spin the homogeneous fibers with several hundreds nm in diameter through improvement of the spinning technique. The thinner polymer blend fiber seems to result in thinner carbon nanofibers finally. The melt-blown spinning machine cannot spin continuous fibers and, therefore, has been used conventionally for mass production of non-woven fabrics [7]. Fortunately continuous fiber is not required for the preparation of carbon nanofibers by the present method, because the AR pitch nanofibers elongated in PMP matrix are far short compared with the polymer blend fibers as seen in Fig. 2. The melt-blown spinning machine must be the most promising in mass-production of carbon nanofibers by using the polymer blend technique. The stabilization mechanism of the AR pitch fiber was previously reported by Mochida et al. [6]. After the carbonization of the stabilized blend fiber, carbon nanofibers as seen in Fig. 3 were left after the removal of PMP. These fibers formed bundles as noted later. Diameter of the nanofibers was around 100 nm and was relatively uniform. We predicted initially a large scatter of the diameter from following two points. (1) There was a large dispersion in the diameter of polymer blend fibers as seen in Fig. 2. (2) The dispersed AR pitch in the PMP
Fig. 3. A FE-SEM photograph of carbon nanofibers heated at 900 °C.
matrix coagulated to form particles with various sizes at the spinning process. Fig. 4(left) shows a polarized light micrograph of the polymer blend just after the mechanical kneading, on which irregular black lines represent AR pitch. This polymer blend was molten in the spinneret to 380 °C of the spinning temperature. Various sizes of AR pitch particles formed (Fig. 4(right)), possibly leading to the dispersion in the diameter of the resulting carbon nanofibers, although the result was opposed as stated above. We suppose that a larger AR pitch particle in the PMP matrix is favorably elongated because of a larger tensile shear applied at spinning process. In addition, the authors would like to emphasize no formation of any
Fig. 4. Polarized light micrographs of the polymer blends before and after melting at 380 °C.
Fig. 5. TEM photographs of a carbon nanofiber heated at 900 °C. Inset is the SAED pattern. A white arrow shows the direction of preferred orientation of carbon crystallites.
H. Ono, A. Oya / Carbon 44 (2006) 682–686
carbon impurity such as carbon particles and flakes. The present method therefore makes possible the preparation of high purity carbon nanofibers with no any carbon impurity. The carbonized nanofibers were served for TEM observation. As seen in Fig. 5(left), the fiber surface was smooth and no defect was observed. The carbon nanofibers consisted in fine carbon crystallites with a preferred orientation along the fiber axis (Fig. 5(right)). The preferred orientation was more clearly shown by the selected area electron diffraction (SAED) pattern. The structure was changed drastically after heating to 3000 °C. Fig. 6 shows SEM photographs of the nanofibers after heating to 3000 °C. The fibers seen in the photographs are bundles of carbon nanofibers, as suggested by the splitting of the fibers as indicated by arrow 2 (Fig. 6(left)). Very thin bundles were sometimes observed, see arrow 1. The bundles were ground to serve TEM observation. Fig. 7 shows a TEM photograph of the nanofibers. The diameters were about 100 nm. Fig. 7(right) shows an irregular broken edge suggesting a highly crystalline carbon structure [9]. This prediction was certified clearly by the TEM image and the SAED pattern shown in Fig. 8. The carbon crystals were well-aligned along the fiber axis. The degree of graphitization of the carbon nanofibers were examined by use of X-ray diffraction analysis. Fig. 9 shows the X-ray diffraction profile of the 002 diffraction peak of polyaromatic carbon together with the 111 diffraction peak of silicon used as an internal standard. A broad
685
Fig. 8. An enlarged TEM photograph of a carbon nanofiber heated at 3000 °C (left) and its SAED pattern (right).
diffraction profile of the 900 °C nanofibers sharpened drastically after heating to 3000 °C. The interlayer spacing (d002) and the crystallite thickness (Lc(002)) were 0.3379 nm and 1.6 nm for the 900 °C fiber, and 0.3367 nm and 56.9 nm for the 3000 °C fiber, respectively. Such a low d002 spacing suggested a high graphitization state (i.e., with a 3D crystal structure similar or close to that of graphite, as opposed to the 2D structure of turbostratic carbons). This was confirmed by the evidence for specific reflections such as 112 found in the SAED pattern in Fig. 8(right) as arrowed. Though the accurate display of the graphenes within the fibers is still questionable (specifically in fiber crosssection), it was stated above that highly crystalline carbon nanofibers with a high preferred orientation of crystallites along the fiber axis were prepared successfully by using a
Fig. 6. SEM photographs of bundles of carbon nanofibers heated at 3000 °C.
Fig. 7. TEM photographs of carbon nanofibers separated from the bundles, after heating at 3000 °C.
686
H. Ono, A. Oya / Carbon 44 (2006) 682–686
spinning machine, resulting in the fibers with an average diameter of several lm. The fibers were stabilized at 160 °C in an oxygen atmosphere and carbonized at 900 °C in a nitrogen atmosphere. After removal of the matrix at the carbonization process, the bundles of the carbon nanofibers were left. The nanofiber diameter was ca. 100 nm. The carbon nanofibers consisted of fine carbon crystallites with preferred orientation along the fiber axis. The carbon crystallites grew drastically after heating to 3000 °C. X-ray parameters of the graphitized nanofibers were 0.3367 nm as interlayer spacing and 57 nm as crystallite thickness. Acknowledgements
Fig. 9. 002 X-ray diffraction profiles of carbon nanofibers heated at 900 °C and 3000 °C.
polymer blend technique. Main reason for the success is that AR mesophase pitch was spun by the melt-blown spinning machine. No impurity carbon was included in the resulting carbon nanofibers. The present method has a high potentiality for mass-production of high purity carbon nanofibers. However, some problems were remained unsolved. The most serious one is that the AR pitch nanofibers were stabilized under a severe condition, because the diffusion of oxygen is suppressed by the PMP matrix. So the thinner polymer blend fiber is more favorable to the stabilization in view of a short diffusion distance for oxygen. The melt-blown spinning machine is more suitable for this purpose than the conventional melt spinning apparatus [3]. Nevertheless 24 h at 160 °C was required. How to shorten the stabilization time during the process seems a key point for the viable commercialization of the present preparation method of carbon nanofiber. 4. Conclusion Naphthalene-based mesophase pitch (AR pitch) particles were dispersed in polymethylpentene matrix. The resulting polymer blend was spun by using a melt-blown
The authors wish to thank Prof. Y. Korai for supplying AR pitch, and Dr. F. Watanabe for taking polarized optical microscopic pictures. References [1] Oya A, Kasahara N. Preparation of thin carbon fiber from phenolformaldehyde polymer micro-beads dispersed in polyethylene matrix. Carbon 2000;38:1141–4. [2] Hulicova D, Oya A. Polymer blend technique as a method to design fine carbon materials. Carbon 2003;41:1443–50. [3] Otani S. Carbonaceous mesophase and carbon fibers. Mol Cryst Liq Cryst 1981;63:249–64. [4] Endo M, Kim YA, Takeda T, Hong SH, Matsusita T, Hayashi T, Dresselhaus MS. Structural characterization of carbon nanofibers obtained by hydrocarbon pyrolysis. Carbon 2001;39:2003–10. [5] Mochida I, Shimizu K, Korai Y, Otsuka H, Fujiyama S. Structure and carbonization properties of pitches produced catalytically from aromatic hydrocarbons with HF/BF3. Carbon 1988;26:843–52. [6] Mochida I, Shimizu K, Korai Y, Otsuka H, Sakai Y, Fujiyama S. Preparation of mesophase pitch from aromatic hydrocarbons by the acid of HF/BF3. Carbon 1990;28:311–9. [7] Balk H. Method of producing plastic fibers or filaments, preferably in conjunction with the formation of non-woven fabric. United State Patent No. 5,098,636, 1992. [8] Bahl OP, Shen Z, Lavin JG, Ross RA. Manufacture of carbon fibers. In: Donnet JB, Wang TK, Peng JM, Rebouillat S, editors. Carbon Fibers. 3rd ed. New York: Marcel Dekker; 1998. p. 1–83. [9] Oberlin A, Bonnamy S, Lafdi K. Structure and texture of carbon fibers. In: Donnet JB, Wang TK, Peng JM, Rebouillat S, editors. Carbon Fibers. 3rd ed. New York: Marcel Dekker; 1998. p. 85–160.