- Email: [email protected]
Diamond synthesis by CO2 laser irradiation
190
Diamond and Related Materials, 2 (1993) 190-196
Diamond synthesis by C O
2
laser irradiation
Noribumi Kikuchi, Yuzo Ohsawa and Ikuro Suzuki Central Research Institute of Mitsubishi Materials Corporation, 1-297 Kitabukuro, Omiya, Saitama 330 (Japan)
Abstract The laser-beam-induced phase transformation of graphite, amorphous carbon and glassy carbon to diamond was investigated. A selection of these carbon sources was irradiated and cut by a continuous wave CO2 laser in a vacuum chamber which was filled with helium at a pressure of 500 Torr. In vaporized fine powder from these plates, diamond, graphite, chaoite and amorphous carbon were detected. Some craters with average diameter 0.05 mm were observed on the surface of the glassy carbon plate near the cutting edges and their number increased with holding time at room temperature. At the bottom of these craters, fine powder with average size 1 ~tm was seen and identified as crystalline diamonds by electron beam diffraction and laser Raman spectroscopy. The number of craters on the glassy carbon plate increased progressivelyfor 3 years after irradiation and the amount of synthesized diamond powder increased. This was not observed for other carbon sources such as graphite and amorphous carbon. From these results, we can conclude that glassy carbon is the preferred starting material for diamond synthesis by laser irradiation.
1. Introduction D i a m o n d is the hardest material known and has been widely used in m a n y industrial applications such as grinding tools and polishing powders. D i a m o n d has been obtained by mining for a long time but was synthesized by the ultrahigh pressure method in 1955 by Bundy et al. in G E [1]. The ultrahigh pressure method has become the general method of producing diamond powder on an industrial scale. In 1981, a new method of diamond synthesis from gas phases was developed by Derjaguin and Spitsyn in the former USSR [2]. This method has been widely studied by m a n y researchers in the world and is now established as another method of diamond synthesis [3-5]. However, Derjaguin and coworkers announced another interesting possibility of diamond synthesis in 1983, i.e., laser-beam-induced phase transformation from graphite to diamond [6, 7]. They irradiated ultrafine graphite powder with a CO2 laser beam and confirmed that the phase transformation from graphite to diamond occurred. Roy and coworkers also carried out laser beam studies and reported that high-pressure phases such as diamond, cubic-BN and stitiovite (SIO2) could be synthesized by the same process [8-10]. Therefore, this process is expected to become a promising new technique for the synthesis of high-pressure phases. However, the exact process conditions for the synthesis are not yet known. In this work, we carried out similar experiments to investigate the process conditions, especially the effect of carbon source, in order to determine which is the best
0925-9635/93/$6.00
source material for the phase transformation from graphite to diamond.
2. Experimental details Three carbon sources were chosen. Cold isostatic pressed (CIP) pieces of graphite powder, amorphous carbon plates (Toyo Carbon Corp., sintered amorphous carbon, 50 m m diameter and 3 m m height) and glassy carbon plates (Tokai Materials Corp., GC-30, 50 m m diameter and 3 m m height) were used as targets. The experiments were performed in a vacuum chamber with a diameter of 700 m m and a height of 250 mm. A schematic drawing of the experimental apparatus is shown in Fig. 1. The target plates were placed at the center of the vacuum chamber on the target holder which could be moved in the plane and the axial directions. The chamber was filled with helium gas at a pressure of 500 or 100 Torr. A continuous wave CO2 laser beam generated by a Simada-Rika SH-102 laser was introduced through a ZnSe window and focused onto the target with a ZnSe lens. The gas pressure, laser beam power density and irradiation times are given in Table 1. The target was cut in two pieces by laser irradiation by moving the holder. Vaporized phases formed fine powder and condensed on the chamber wall. These fine powders and the cut plates were examined by X-ray diffraction (XRD), laser Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and transmission electron diffraction (TED).
© 1993 - - Elsevier Sequoia. All rights reserved
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N. Kikuchi et al. / Synthesis by CO 2 laser irradiation
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Fig. 2. XRD of target materials: (a) CIP graphite powder, (b) amorphous carbon, and (c) glassy carbon. I
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TABLE 1. Experimental conditions Target material
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3. Results (b) amorphouscarbon
3.1. CIP graphite powder targets The results of XRD of the three target materials, (a) a CIP graphite powder target, (b) an amorphous carbon target and (c) a glassy carbon target, are shown in Fig. 2. The results of XRD of vaporized fine powder from these targets are shown in Fig. 3. Figure 2(a) reveals peaks of crystalline graphite; no other peaks are observed for the graphite target. However, broad peaks of diamond are revealed for the vaporized fine powder and graphite peaks are not observed in Fig.3(a). These results are in good agreement with those of Derjaurgin and Roy [6, 7, 9, 10]. Figure 4 shows the results of laser Raman spectroscopy measurements of the three target materials, (a) a CIP graphite powder target, (b) an amorphous carbon target and (c) a glassy carbon target. Figure 5 shows the results of laser Raman spectra of vaporized fine powder from these targets. In Fig. 4(a), sharp peaks of graphite at 1585 cm -1 and 1375cm -1 are seen from the CIP graphite powder target, but these graphite peaks become broad and the broad peak of amorphous carbon at around 1440 cm- 1 appears for the vaporized fine powder in Fig. 5(a). The sharp peak of crystalline diamond at 1332 cm-1 is not observed but a small peak is observed at 1330 cm 1. This indicates that the vaporized fine
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powder from the CIP graphite powder target consists of crystalline diamond and graphite with a defective crystal structure. Figure 6 shows the results of TEM and TED of vaporized fine powder from (a) a CIP graphite powders target, (b) an amorphous carbon target and (c) a glassy carbon target. In Fig. 6(a), the TEM picture shows that the powders consist of two phases, a dark core and a fine gray surrounding phase. In the TED figure, diamond
192
N. Kikuchi et al. / Synthesis by CO 2 laser irradiation
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Fig. 4. Laser Raman spectra of target materials: (a) CIP graphite powder, (b) amorphous carbon, and (c) glassy carbon.
and graphite ring patterns are detected with a strong halo pattern which is due to amorphous carbon. From the TED pattern, we can infer that the dark core is crystalline diamond or graphite and the surrounding phase is amorphous carbon.
3.2. Amorphous carbon target The results of XRD of an amorphous carbon target and the fine powder from it are shown in Fig. 2(b) and Fig. 3(b) respectively. Broad peaks of graphite are observed from the amorphous carbon target and no peaks appear for the vaporized fine powder except for weak broad peaks of graphite. The results of laser Raman spectroscopy measurements of an amorphous carbon target and the fine powder from it are shown in Fig. 4(b) and Fig. 5(b) respectively. Sharp peaks of graphite at 1580 cm- 1 and 1360 cm -1 are observed from the amorphous carbon target but broad peaks around 1600 cm -~ and 1360 cm- I appear for the fine powder. The diamond
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Fig. 5. Laser Raman spectra of vaporized fine powder from {a) a CIP graphite powder target, (b) an amorphous carbon target, and (c) a glassy carbon target.
peak at 1332 cm-1 is detected faintly on the broad peak at around 1360 cm -1. These results indicate that essentially the same reaction occurs with a CIP graphite target and with an amorphous carbon target, but the phase transformation from graphite to diamond occurs more easily in graphite powder than in an amorphous carbon plate. The results of TEM and TED of the fine powders from an amorphous carbon target are shown in Fig. 6(b). Very fine particles with a uniform size of about 20 nm are observed using TEM, TED reveals a strong halo pattern. These results indicate that the powder consists mostly of amorphous carbon.
3.3. Glassy carbon target The results of XRD of a glassy carbon target and the fine powder from it are shown in Fig. 2(c) and Fig.3(c) respectively. Broad graphite peaks, as for the amorphous carbon target, are observed for the glassy carbon target and no peaks appear in the patterns for fine powder.
N. Kikuchi et al. / Synthesis by CO 2 laser irradiation
193
shown in Fig. 4(c) and Fig. 5(c) respectively. Sharp peaks of graphite appear for the target but these peaks are broad, more like those for the amorphous carbon target and powder. The results of TEM and TED of fine powder from a glassy carbon target are shown in Fig. 6(c). Fine powder with average size 50 nm and coarse powder with average size 0.6 rtm were observed using TEM. From the TED patterns, a single crystal of chaoite is detected with a. strong halo pattern. These results indicate that most of the fine powder is amorphous and some chaoite powder is also produced. These data are similar to those reported by Fedoseev et al. [6]. However, a very peculiar phenomenon occurred on the glassy carbon target after laser irradiation. About 1 year after the laser irradiation, many craters with an average diameter of 50 ~tm appeared on the surface of the irradiated target. SEM images are shown in Fig. 7(a). These craters are homogeneously distributed on the surface of the target in Fig. 7(a). Coarse particles with average size 30 lam (Fig. 7(b)) and fine particles with average size 1 lain (Fig. 7(c)) were found on the bottoms of these craters. The number of craters increased progressively for 3 years after the irradiation. Figure 8 shows fine particles which appeared again at the same place. In other words, some fine particles appeared which were then removed. After I year, new fine particles appeared in the same place. Figure 9 shows the results of laser Raman spectroscopy measurements of these targets and particles. For the particles, microscopic laser Raman measurements with an average focus area 1 ~tm in diameter were used. The result for the irradiated target is shown in Fig. 9(a). Two broad peaks around 1600 cm -1 and 1350 cm -1 are detected but, in addition, a sharp peak of diamond at 1332 cm -1 is seen on the broad peak around 1350 cm- a. This indicates that crystalline diamond exists on the target surface. In Fig. 9(b) and (c), the results for coarse particles and fine particles are shown respectively. A very sharp peak of crystalline diamond at 1332 c m is seen for both particles, indicating that they are crystalline diamond. Figure 10 shows the results of TEM and TED of particles in the craters. A ring pattern of diamond is observed in the TED pattern showing that crystalline diamond can be synthesized on the surface of glassy carbon. Fig. 6. TEM and TED images of vaporized fine powder from (a) a CIP graphite powder target, (b) an amorphous carbon target, and (c) a glassy carbon target.
These data are very similar to those for the amorphous carbon target. The results of laser Raman spectroscopy measurements of a glassy carbon target and the fine powder are
3.4. Reproducibility In these experiments, reproducibility is one of the most important problems. Unfortunately, reproducibility of our experiments is not good and we could not reproduce the crystalline diamond on glassy carbon targets under the same experimental conditions. However, a very similar phenomenon was observed, i.e.,
194
N. Kikuchi et al. / Synthesis by CO 2 laser irradiation
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Fig. 8. Fine particles appearing at the same place.
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Fig. 7. SEM images of (a) craters appearing on the surface of the irradiated glassy carbon target, (b) a coarse particle on the bottom of the crater, and (c) fine particles on the bottom of the crater.
m a n y craters were f o r m e d o n the surface of the glassy c a r b o n target. I n Fig: 11 t h e s e craters a r e s h o w n , b u t c r y s t a l l i n e d i a m o n d c o u l d n o t be d e t e c t e d in these craters by laser Raman spectroscopy measurements.
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Fig. 9. Laser Raman spectra of (a) the irradiated glassy carbon target, (b) a coarse particle on the bottom of the crater, and (c) a fine particle on the bottom of the crater.
4. Discussion I n the e x p e r i m e n t s of F e d 0 s e e v a n d D e r j a g u i n et al. [6, 7], p o l y c r y s t a l l i n e g r a p h i t e p o w d e r a n d fine c a r b o n
N. Kikuchi et al. / Synthesis by CO 2 laser irradiation
0.2gm
Fig. 10. TEM and TED images of fine particles in the crater.
Fig. 11. Craters on a glassy carbon target after a reproduced experiment. black were used as carbon sources. These powders were irradiated by a continuous wave CO2 laser. They expected that the rapid cooling of materials heated to elevated temperatures would contribute to the formation of metastable structures. Diamond, lonsdalite and carbines were detected by electron beam diffraction in the products which were almost entirely amorphous carbon. They concluded that this transformation from graphite to diamond was induced by compressive stress due to the difference in thermal expansion coefficients between the basal plane and transferal direction of the graphite structure. The calculated value of induced pressure is 1500 kbar at 3000°C and this is enough to transfer graphite to diamond. Roy and coworkers did the same experiment and obtained similar results [8 10]. They explained that the phase transformation was induced by the very large stress at high temperature. When graphite particles are exposed to a laser beam, their temperatures rise so
195
rapidly that equilibrium thermal expansion cannot occur. As a result, the particles experience very large stress at high temperature. The high cooling rate is also important to ensure preservation of the transformed metastable phases. In our experiments, almost the same reaction occurred in a CIP graphite target, an amorphous carbon target and a glassy carbon target. From Figs. 2 6, it is seen that diamond, graphite, chaoite and amorphous carbon coexist in vaporized fine powder. However, the reaction on glassy carbon targets seems to be a little different from the reaction on CIP graphite and amorphous carbon targets. In this case, graphite transformed to diamond not only in the vaporized fine powder but also on the surface of the target. This is a useful process for obtaining diamond since there is no need to select the diamond from graphite and other material powders. This indicates that glassy carbon is a preferred material for obtaining diamond. Glassy carbon is made from furfuryl alcohol heat treated at high temperature. The structure of glassy carbon is known as a ribbon-like structure of the basal plane of graphite [11]. We cannot clearly propose the reason why glassy carbon is a preferred material for transformation to diamond. However, one reason seems to be the low porosity of glassy carbon. The porosity of glassy carbon is a few per cent and this value is much lower than that of graphite and amorphous carbon. It can therefore store the high stress induced by rapid heating and cooling. The stored stress seems to promote the phase transformation progressively and a volume change occurs which creates craters. Other data of Miyamoto et al. indicated that the glassy carbon is easily transformed to diamond [12]. They reported that the addition of glassy carbon is effective for sintering diamond powder by an ultrahigh pressure process. In this case, they supposed that most diamond newly formed from glassy carbon worked to promote diamonddiamond direct bonding. However, the reproducibility of this experiment is poor and this indicates that the optimum condition for glassy carbon to transform to diamond is not yet established. It seems that many parameters such as the structure of the glassy carbon, the heat capacity of the target, heat transfer in the target, heating rate, cooling rate, laser power etc., are interrelated with each other. In this experiment, we present the possibility of a new method of diamond synthesis but there are many problems to be solved. 5. Conclusions
Laser beam irradiation on several carbon targets, i.e., cold isostatic pressed (CIP) graphite powder, amorphous carbon and glassy carbon, was carried out and the degree of phase transformation was investigated by
196
N. Kikuchi et al. / Synthesis by CO 2 laser irradiation
XRD, laser Raman spectroscopy, SEM, TEM and TED. The results are as follows. (1) Diamond, graphite and amorphous carbon were detected in the vaporized powder from CIP graphite targets and glassy carbon targets but only amorphous carbon was detected in the vaporized powder from amorphous carbon targets. (2) In addition, chaoite was detected in the vaporized powder from glassy carbon targets. (3) After the laser beam irradiation, many craters were generated on the surface of the glassy carbon target and diamond powder was synthesized on the bottoms of these craters. (4) The number of craters increased progressively for 3 years after irradiation and the amount of synthesized diamond powder increased. This was not observed for other targets. (5) From these results, we conclude that glassy carbon is a preferred starting material for diamond synthesis by laser irradiation.
References 1 F. P. Bundy, H. T. Hall, H. M. Strong, and R. H. Wentorf, Jr., Nature, 176 (1955) 51. 2 B. V. Spitsyn, L. L. Bouilov and B. V. Derjaguin, J. Cryst. Growth, 52 (1981) 219. 3 S. Matsumoto, Y. Sato, M. Kamo and N. Setaka, Jpn. J. Appl. Phys., 21 (1982) L183. 4 M. Kamo, Y. Sato, S. Matsumoto and N. Setaka, J. Cryst. Growth, 62 (1985) 341. 5 S. Matsumoto, J. Mater. Sci. Lett., 4 (1985) 600. 6 D. V. Fedoseev, V. L. Bukhovest, I. G. Varshavskaya, A. V. Lavrent'ev and B. V. Derjaguin, Carbon, 21 (1985) 125. 7 D. V. Fedoseev, I. G. Varshavskaya, A. V. Lavrent'ev and B. V. Derjaguin, Powder Technol., 44 (1985) 125. 8 M. Alam, T. DebRoy, R. Roy and E. Breval, Appl. Phys. Lett., 53 (1988) 1687. 9 M. Alam, T. DebRoy, R. Roy and E. Breval, Carbon, 27 (1989) 289. 10 E. Breval, M. Alam, T. DebRoy and R. Roy, J. Mater. Sci. Lett., 9 (1990) 1071. 11 E. Fitzer and W. Schafter, Carbon, 8 (1970) 353. 12 M. Miyamoto, K. Kitahata, M. Akaishi and O. Fukunaga, Refract. Met. Hard Mater., 10 (1991) 27.
Diamond and Related Materials, 2 (1993) 190-196
Diamond synthesis by C O
2
laser irradiation
Noribumi Kikuchi, Yuzo Ohsawa and Ikuro Suzuki Central Research Institute of Mitsubishi Materials Corporation, 1-297 Kitabukuro, Omiya, Saitama 330 (Japan)
Abstract The laser-beam-induced phase transformation of graphite, amorphous carbon and glassy carbon to diamond was investigated. A selection of these carbon sources was irradiated and cut by a continuous wave CO2 laser in a vacuum chamber which was filled with helium at a pressure of 500 Torr. In vaporized fine powder from these plates, diamond, graphite, chaoite and amorphous carbon were detected. Some craters with average diameter 0.05 mm were observed on the surface of the glassy carbon plate near the cutting edges and their number increased with holding time at room temperature. At the bottom of these craters, fine powder with average size 1 ~tm was seen and identified as crystalline diamonds by electron beam diffraction and laser Raman spectroscopy. The number of craters on the glassy carbon plate increased progressivelyfor 3 years after irradiation and the amount of synthesized diamond powder increased. This was not observed for other carbon sources such as graphite and amorphous carbon. From these results, we can conclude that glassy carbon is the preferred starting material for diamond synthesis by laser irradiation.
1. Introduction D i a m o n d is the hardest material known and has been widely used in m a n y industrial applications such as grinding tools and polishing powders. D i a m o n d has been obtained by mining for a long time but was synthesized by the ultrahigh pressure method in 1955 by Bundy et al. in G E [1]. The ultrahigh pressure method has become the general method of producing diamond powder on an industrial scale. In 1981, a new method of diamond synthesis from gas phases was developed by Derjaguin and Spitsyn in the former USSR [2]. This method has been widely studied by m a n y researchers in the world and is now established as another method of diamond synthesis [3-5]. However, Derjaguin and coworkers announced another interesting possibility of diamond synthesis in 1983, i.e., laser-beam-induced phase transformation from graphite to diamond [6, 7]. They irradiated ultrafine graphite powder with a CO2 laser beam and confirmed that the phase transformation from graphite to diamond occurred. Roy and coworkers also carried out laser beam studies and reported that high-pressure phases such as diamond, cubic-BN and stitiovite (SIO2) could be synthesized by the same process [8-10]. Therefore, this process is expected to become a promising new technique for the synthesis of high-pressure phases. However, the exact process conditions for the synthesis are not yet known. In this work, we carried out similar experiments to investigate the process conditions, especially the effect of carbon source, in order to determine which is the best
0925-9635/93/$6.00
source material for the phase transformation from graphite to diamond.
2. Experimental details Three carbon sources were chosen. Cold isostatic pressed (CIP) pieces of graphite powder, amorphous carbon plates (Toyo Carbon Corp., sintered amorphous carbon, 50 m m diameter and 3 m m height) and glassy carbon plates (Tokai Materials Corp., GC-30, 50 m m diameter and 3 m m height) were used as targets. The experiments were performed in a vacuum chamber with a diameter of 700 m m and a height of 250 mm. A schematic drawing of the experimental apparatus is shown in Fig. 1. The target plates were placed at the center of the vacuum chamber on the target holder which could be moved in the plane and the axial directions. The chamber was filled with helium gas at a pressure of 500 or 100 Torr. A continuous wave CO2 laser beam generated by a Simada-Rika SH-102 laser was introduced through a ZnSe window and focused onto the target with a ZnSe lens. The gas pressure, laser beam power density and irradiation times are given in Table 1. The target was cut in two pieces by laser irradiation by moving the holder. Vaporized phases formed fine powder and condensed on the chamber wall. These fine powders and the cut plates were examined by X-ray diffraction (XRD), laser Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and transmission electron diffraction (TED).
© 1993 - - Elsevier Sequoia. All rights reserved
19l
N. Kikuchi et al. / Synthesis by CO 2 laser irradiation
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Gas pressure (Torr)
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Irradiation time (min)
500 He 100 He 500 He
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Fig. 2. XRD of target materials: (a) CIP graphite powder, (b) amorphous carbon, and (c) glassy carbon. I
CIP graphite Amorphous carbon Glassy carbon
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TABLE 1. Experimental conditions Target material
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3. Results (b) amorphouscarbon
3.1. CIP graphite powder targets The results of XRD of the three target materials, (a) a CIP graphite powder target, (b) an amorphous carbon target and (c) a glassy carbon target, are shown in Fig. 2. The results of XRD of vaporized fine powder from these targets are shown in Fig. 3. Figure 2(a) reveals peaks of crystalline graphite; no other peaks are observed for the graphite target. However, broad peaks of diamond are revealed for the vaporized fine powder and graphite peaks are not observed in Fig.3(a). These results are in good agreement with those of Derjaurgin and Roy [6, 7, 9, 10]. Figure 4 shows the results of laser Raman spectroscopy measurements of the three target materials, (a) a CIP graphite powder target, (b) an amorphous carbon target and (c) a glassy carbon target. Figure 5 shows the results of laser Raman spectra of vaporized fine powder from these targets. In Fig. 4(a), sharp peaks of graphite at 1585 cm -1 and 1375cm -1 are seen from the CIP graphite powder target, but these graphite peaks become broad and the broad peak of amorphous carbon at around 1440 cm- 1 appears for the vaporized fine powder in Fig. 5(a). The sharp peak of crystalline diamond at 1332 cm-1 is not observed but a small peak is observed at 1330 cm 1. This indicates that the vaporized fine
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powder from the CIP graphite powder target consists of crystalline diamond and graphite with a defective crystal structure. Figure 6 shows the results of TEM and TED of vaporized fine powder from (a) a CIP graphite powders target, (b) an amorphous carbon target and (c) a glassy carbon target. In Fig. 6(a), the TEM picture shows that the powders consist of two phases, a dark core and a fine gray surrounding phase. In the TED figure, diamond
192
N. Kikuchi et al. / Synthesis by CO 2 laser irradiation
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and graphite ring patterns are detected with a strong halo pattern which is due to amorphous carbon. From the TED pattern, we can infer that the dark core is crystalline diamond or graphite and the surrounding phase is amorphous carbon.
3.2. Amorphous carbon target The results of XRD of an amorphous carbon target and the fine powder from it are shown in Fig. 2(b) and Fig. 3(b) respectively. Broad peaks of graphite are observed from the amorphous carbon target and no peaks appear for the vaporized fine powder except for weak broad peaks of graphite. The results of laser Raman spectroscopy measurements of an amorphous carbon target and the fine powder from it are shown in Fig. 4(b) and Fig. 5(b) respectively. Sharp peaks of graphite at 1580 cm- 1 and 1360 cm -1 are observed from the amorphous carbon target but broad peaks around 1600 cm -~ and 1360 cm- I appear for the fine powder. The diamond
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Fig. 5. Laser Raman spectra of vaporized fine powder from {a) a CIP graphite powder target, (b) an amorphous carbon target, and (c) a glassy carbon target.
peak at 1332 cm-1 is detected faintly on the broad peak at around 1360 cm -1. These results indicate that essentially the same reaction occurs with a CIP graphite target and with an amorphous carbon target, but the phase transformation from graphite to diamond occurs more easily in graphite powder than in an amorphous carbon plate. The results of TEM and TED of the fine powders from an amorphous carbon target are shown in Fig. 6(b). Very fine particles with a uniform size of about 20 nm are observed using TEM, TED reveals a strong halo pattern. These results indicate that the powder consists mostly of amorphous carbon.
3.3. Glassy carbon target The results of XRD of a glassy carbon target and the fine powder from it are shown in Fig. 2(c) and Fig.3(c) respectively. Broad graphite peaks, as for the amorphous carbon target, are observed for the glassy carbon target and no peaks appear in the patterns for fine powder.
N. Kikuchi et al. / Synthesis by CO 2 laser irradiation
193
shown in Fig. 4(c) and Fig. 5(c) respectively. Sharp peaks of graphite appear for the target but these peaks are broad, more like those for the amorphous carbon target and powder. The results of TEM and TED of fine powder from a glassy carbon target are shown in Fig. 6(c). Fine powder with average size 50 nm and coarse powder with average size 0.6 rtm were observed using TEM. From the TED patterns, a single crystal of chaoite is detected with a. strong halo pattern. These results indicate that most of the fine powder is amorphous and some chaoite powder is also produced. These data are similar to those reported by Fedoseev et al. [6]. However, a very peculiar phenomenon occurred on the glassy carbon target after laser irradiation. About 1 year after the laser irradiation, many craters with an average diameter of 50 ~tm appeared on the surface of the irradiated target. SEM images are shown in Fig. 7(a). These craters are homogeneously distributed on the surface of the target in Fig. 7(a). Coarse particles with average size 30 lam (Fig. 7(b)) and fine particles with average size 1 lain (Fig. 7(c)) were found on the bottoms of these craters. The number of craters increased progressively for 3 years after the irradiation. Figure 8 shows fine particles which appeared again at the same place. In other words, some fine particles appeared which were then removed. After I year, new fine particles appeared in the same place. Figure 9 shows the results of laser Raman spectroscopy measurements of these targets and particles. For the particles, microscopic laser Raman measurements with an average focus area 1 ~tm in diameter were used. The result for the irradiated target is shown in Fig. 9(a). Two broad peaks around 1600 cm -1 and 1350 cm -1 are detected but, in addition, a sharp peak of diamond at 1332 cm -1 is seen on the broad peak around 1350 cm- a. This indicates that crystalline diamond exists on the target surface. In Fig. 9(b) and (c), the results for coarse particles and fine particles are shown respectively. A very sharp peak of crystalline diamond at 1332 c m is seen for both particles, indicating that they are crystalline diamond. Figure 10 shows the results of TEM and TED of particles in the craters. A ring pattern of diamond is observed in the TED pattern showing that crystalline diamond can be synthesized on the surface of glassy carbon. Fig. 6. TEM and TED images of vaporized fine powder from (a) a CIP graphite powder target, (b) an amorphous carbon target, and (c) a glassy carbon target.
These data are very similar to those for the amorphous carbon target. The results of laser Raman spectroscopy measurements of a glassy carbon target and the fine powder are
3.4. Reproducibility In these experiments, reproducibility is one of the most important problems. Unfortunately, reproducibility of our experiments is not good and we could not reproduce the crystalline diamond on glassy carbon targets under the same experimental conditions. However, a very similar phenomenon was observed, i.e.,
194
N. Kikuchi et al. / Synthesis by CO 2 laser irradiation
1 gm
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Fig. 7. SEM images of (a) craters appearing on the surface of the irradiated glassy carbon target, (b) a coarse particle on the bottom of the crater, and (c) fine particles on the bottom of the crater.
m a n y craters were f o r m e d o n the surface of the glassy c a r b o n target. I n Fig: 11 t h e s e craters a r e s h o w n , b u t c r y s t a l l i n e d i a m o n d c o u l d n o t be d e t e c t e d in these craters by laser Raman spectroscopy measurements.
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Fig. 9. Laser Raman spectra of (a) the irradiated glassy carbon target, (b) a coarse particle on the bottom of the crater, and (c) a fine particle on the bottom of the crater.
4. Discussion I n the e x p e r i m e n t s of F e d 0 s e e v a n d D e r j a g u i n et al. [6, 7], p o l y c r y s t a l l i n e g r a p h i t e p o w d e r a n d fine c a r b o n
N. Kikuchi et al. / Synthesis by CO 2 laser irradiation
0.2gm
Fig. 10. TEM and TED images of fine particles in the crater.
Fig. 11. Craters on a glassy carbon target after a reproduced experiment. black were used as carbon sources. These powders were irradiated by a continuous wave CO2 laser. They expected that the rapid cooling of materials heated to elevated temperatures would contribute to the formation of metastable structures. Diamond, lonsdalite and carbines were detected by electron beam diffraction in the products which were almost entirely amorphous carbon. They concluded that this transformation from graphite to diamond was induced by compressive stress due to the difference in thermal expansion coefficients between the basal plane and transferal direction of the graphite structure. The calculated value of induced pressure is 1500 kbar at 3000°C and this is enough to transfer graphite to diamond. Roy and coworkers did the same experiment and obtained similar results [8 10]. They explained that the phase transformation was induced by the very large stress at high temperature. When graphite particles are exposed to a laser beam, their temperatures rise so
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rapidly that equilibrium thermal expansion cannot occur. As a result, the particles experience very large stress at high temperature. The high cooling rate is also important to ensure preservation of the transformed metastable phases. In our experiments, almost the same reaction occurred in a CIP graphite target, an amorphous carbon target and a glassy carbon target. From Figs. 2 6, it is seen that diamond, graphite, chaoite and amorphous carbon coexist in vaporized fine powder. However, the reaction on glassy carbon targets seems to be a little different from the reaction on CIP graphite and amorphous carbon targets. In this case, graphite transformed to diamond not only in the vaporized fine powder but also on the surface of the target. This is a useful process for obtaining diamond since there is no need to select the diamond from graphite and other material powders. This indicates that glassy carbon is a preferred material for obtaining diamond. Glassy carbon is made from furfuryl alcohol heat treated at high temperature. The structure of glassy carbon is known as a ribbon-like structure of the basal plane of graphite [11]. We cannot clearly propose the reason why glassy carbon is a preferred material for transformation to diamond. However, one reason seems to be the low porosity of glassy carbon. The porosity of glassy carbon is a few per cent and this value is much lower than that of graphite and amorphous carbon. It can therefore store the high stress induced by rapid heating and cooling. The stored stress seems to promote the phase transformation progressively and a volume change occurs which creates craters. Other data of Miyamoto et al. indicated that the glassy carbon is easily transformed to diamond [12]. They reported that the addition of glassy carbon is effective for sintering diamond powder by an ultrahigh pressure process. In this case, they supposed that most diamond newly formed from glassy carbon worked to promote diamonddiamond direct bonding. However, the reproducibility of this experiment is poor and this indicates that the optimum condition for glassy carbon to transform to diamond is not yet established. It seems that many parameters such as the structure of the glassy carbon, the heat capacity of the target, heat transfer in the target, heating rate, cooling rate, laser power etc., are interrelated with each other. In this experiment, we present the possibility of a new method of diamond synthesis but there are many problems to be solved. 5. Conclusions
Laser beam irradiation on several carbon targets, i.e., cold isostatic pressed (CIP) graphite powder, amorphous carbon and glassy carbon, was carried out and the degree of phase transformation was investigated by
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N. Kikuchi et al. / Synthesis by CO 2 laser irradiation
XRD, laser Raman spectroscopy, SEM, TEM and TED. The results are as follows. (1) Diamond, graphite and amorphous carbon were detected in the vaporized powder from CIP graphite targets and glassy carbon targets but only amorphous carbon was detected in the vaporized powder from amorphous carbon targets. (2) In addition, chaoite was detected in the vaporized powder from glassy carbon targets. (3) After the laser beam irradiation, many craters were generated on the surface of the glassy carbon target and diamond powder was synthesized on the bottoms of these craters. (4) The number of craters increased progressively for 3 years after irradiation and the amount of synthesized diamond powder increased. This was not observed for other targets. (5) From these results, we conclude that glassy carbon is a preferred starting material for diamond synthesis by laser irradiation.
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