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Efficient synthesis of fullerenes in RF thermal plasma reactor
Chemical Physics Letters 378 (2003) 434–439 www.elsevier.com/locate/cplett
Efficient synthesis of fullerenes in RF thermal plasma reactor B. Todorovic-Markovic a, Z. Markovic a,*, I. Mohai b, Z. K aroly b, L. G al b, K. F€ oglein b, P.T. Szab o c, J. Szepv€ olgyi b a Vin ca Institute of Nuclear Sciences P.O. Box 522, Belgrade, Serbia and Montenegro Research Laboratory of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary b
c
Received 29 March 2003; in final form 31 July 2003 Published online: 26 August 2003
Abstract Formation of fullerene soot was studied in an inductively coupled, radiofrequency (RF) thermal plasma reactor. A previously developed kinetic model of fullerene formation was applied to determine synthesis conditions leading to high fullerene yield. The experimental results verified the kinetic model. Maximum yield of 4.1% was obtained in particular conditions. It corresponded to a fullerene production rate of 6.4 g h1 . Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Many papers were published on the potential applications of fullerene based materials more recently [1,2]. However, the high price of purified fullerenes inhibited their widespread use up to now. For practical applications, fullerenes are mainly produced by evaporating graphite electrodes in thermal arc plasma reactors. The main disadvantage of particular method is the sharp decrease of fullerene yields and hence, its production rate at discharge currents above 100 A. It is due to the high velocity of the plasma gases and hence, to the very short residence time of carbon
*
Corresponding author. Fax. +381-11-344-0100. E-mail address: [email protected] (Z. Markovic).
containing species in the hot plasma region, in particular conditions. Inductively coupled, RF thermal plasmas offer several advantages as compared to arc plasmas. In RF thermal plasma reactors, more voluminous plasma flames are formed, and the gas velocities are much lower than in arc plasmas. Thus, the mean residence time of reactive species in the hot plasma region is longer in RF thermal plasmas as compared to arc ones. Powder-like starting material can be introduced into RF plasmas with ease, contrary to arc plasmas. In addition, feed rate of powders can be independently changed of RF plasma parameters. In spite of the apparent advantages, there have been only a few attempts on fullerene synthesis from carbon powder in inductively coupled RF or hybrid plasmas [3,4]. A production rate of
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0009-2614(03)01320-4
B. Todorovic-Markovic et al. / Chemical Physics Letters 378 (2003) 434–439
2.1 g h1 was achieved by Yoshie et al. [3] by processing high purity carbon black in a hybrid plasma reactor at a power level of 25 kW. Fulcheri et al. [5] described a new synthesis method for production of fullerenes from carbon powders in a three-phase arc plasma. Using acetylene black feedstock, they produced extractable fullerenes with a yield of 3.5%. In this work, we demonstrate that fullerenes can be effectively produced from graphite powder in inductively coupled, RF thermal plasma reactor. The synthesis conditions were estimated by a kinetic model of fullerene formation constructed previously [6]. This model assumes that fullerenes are forming in inert atmosphere at temperatures above 2000 K only [7]. Recent studies [8] revealed that the initial temperature of carbon vapour should be as high as 10 000 K for an efficient fullerene synthesis. In the above model [6], fullerene yield ðY Þ was described by the following equation: Y ¼ f ðX Þ;
ð1Þ
where X ¼K
pffiffiffiffiffi N c T0 r: V
ð2Þ
In Eq. (2), Nc denotes carbon concentration in the plasma flame, V is the mean velocity of plasma gases in the flame region, T0 is the maximum plasma temperature, and K is a constant determined by the mass and radius of carbon atom. Values of r vary in the range of ½0; R where R is the distance between the spot of maximum temperature (T0 ) and the reactor wall of Tw temperature. Calculations by particular model [6] showed that the fullerene yield increased with X up to certain values. However, at higher X values, fullerene yield became of saturation type with a plateau of about 20%. Consequently, the plasma parameters, such as gas velocity, maximum temperature and volume of plasma flame affect fullerene yield, along with the carbon concentration in the hot plasma region. Reaction conditions in RF thermal plasma reactors operating at atmospheric pressure mainly depend on the plate power and the composition of hot gases. The latter one determines the ionization
435
potential, the enthalpy and the heat conductivity of the plasma flame. From inert gases worth of considering as plasma gas, helium has the greatest heat conductivity. Another important process parameter is the concentration of carbon atoms in the plasma flame: it should be as high as possible [6]. In addition, the mean residence time of carbon vapour in the hot plasma should be long enough. Thus, the velocity of gases (V ) should be low in the plasma reactor.
2. Experimental Synthetic graphite powder (Aldrich) having a mean particle size of 1–2 lm and a purity of 99.7% was treated in a thermal plasma reactor, at atmospheric pressure. The RF power was produced by a generator operating at 3–5 MHz. Plate power of 27 kW was inductively coupled to a TEKNA PL-35 torch connected to the water cooled plasma reactor, cyclone and dust filter (Fig. 1). Helium was used as plasma gas (15 slpm) and carrier gas (2–10 slpm), respectively. The sheath gas was argon (40 slpm). The graphite powder was injected axially to the top of the plasma flame with feed rates of 90–468 g h1 . A TRIAX 550 spectrometer (Jobin–Yvon) connected to CCD 3000 detector monitored the optical emission of the plasma flame through a quartz glass window at a height of 10 cm below the tip of feeding nozzle. In our experiments, the main product was solid soot collected mainly from the reactor wall. To separate fullerenes, the soot was extracted in toluene with a 100 mg/10 ml solid/liquid ratio for 1 min at 20 °C. Actually all fullerenes were separated from the soot in particular conditions. Amount of fullerenes in the filtered extracts was measured by UV–Vis spectrophotometry against reference solutions. The extracts were also subjected to MS analysis by nanospray ionization [9].
3. Results and discussion The experimental conditions and results are presented in Table 1. The mean gas velocity ðV Þ was calculated at the viewing height of 10 cm
436
B. Todorovic-Markovic et al. / Chemical Physics Letters 378 (2003) 434–439
Fig. 1. The experimental set-up.
Table 1 Experimental conditions and results
Graphite feed rate (g h1 ) V (m s1 ) Tv (K) Ncp (m3 ) I(C2 )/I(CN) Nc (m3 ) X 0 ¼ Nc Tv0:5 V 1 Y (%)
Run 1
Run 2
Run 3
Run 4
Run 5
90 12.1 4800 0.825 1023 0.4 0.825 1023 4.731023 1.3
108 12.74 4800 0.94 1023 0.93 0.94 1023 5.11 1023 2.9
156 12.736 5000 1.348 1023 1.98 1.348 1023 7.48 1023 4.1
192 13.48 4900 1.57 1023 0.73 0.93 1023 4.82 1023 2.4
468
below the tip of feeding probe. The experimental set-up did not make possible to measure the maximum plasma temperature (T0 ). Instead of T0 , the vibration temperature of C2 radicals at the viewing height (Tv ) was calculated from the emission spectra, using the Boltzmann plot method [10]. In experimental conditions of this work, Tv values were changing in a relatively narrow temperature range (Table 1). The ÔpseudoÕ concentration of evaporated carbon in the plasma flame (Ncp ) was calculated from the feed rate of graphite and the volumetric velocity of gases (Table 1). For a complete evaporation of graphite, Ncp is equal to the actual concentration of carbon atoms (Nc ) in the gas phase.
0 0 0 0.05
The C2 radicals being important in terms of fullerene formation can be characterized by the Swan band of C2 (d3 Pg ) ! C2 (a3 Pu ) located in the visible region of the spectrum (Fig. 2). The vibration excitation of d3 Pg state was estimated from the intensities of different vibration bands in sequence Dv ¼ þ1. In the experimental spectra we observed bands of CN(B2 Rþ ) ! CN(X2 Rþ ) radical, as well (Fig. 2). Nitrogen required for CN formation most probably originated from the impurities of argon sheath gas of 99.995% purity. Intensities of CN bands were nearly constant in each run. We performed thermodynamic calculations by computer code FACTSAGE to compare formation of C2 and CN in equilibrium conditions, in
B. Todorovic-Markovic et al. / Chemical Physics Letters 378 (2003) 434–439
437
Fig. 2. A typical emission spectrum of carbon plasma for experimental conditions of this work. Fig. 3. Concentration of evaporated carbon as plotted against relative emission intensities.
the temperature range of 3000–6000 K. It became clear that in equilibrium conditions the molar fraction of CN was determined by the nitrogen feed rate only, i.e., the purity of argon sheath gas. When nitrogen is present in the system, CN (and C2 N) always form in the temperature range of 4500–5000 K. However, the molar fraction of CN is smaller of two orders of magnitude than that of C2 in given conditions. Above 5000 K, atomic N is the most stable nitrogen-containing species. In our experiments, the feed rate of argon sheath gas was 40 slpm being high enough for CN formation. Thus, the emission intensity ratio of IðC2 Þ=IðCNÞ seems to be a proper indicator of the carbon evaporation in particular conditions. In Table 1 IðC2 Þ represents the mean value of C2 band-heads of Swan band positioned at 473.7 nm, while IðCNÞ represents that of CN radical at 388.3 nm in the experimental spectra. The IðC2 Þ=IðCNÞ ratio increased with the feed rate of graphite up to feed rate of 156 g h1 . It refers to complete evaporation of graphite in these conditions. Further, the IðC2 Þ=IðCNÞ emission intensity ratio decreased with feed rate. There was no C2 signal at feed rate of 468 g h1 (Table 1) detecting incomplete evaporation of carbon at higher loads. In order to calculate the actual concentration of carbon in the plasma flame even in these conditions, the Nc values for Runs 1–3 (i.e., complete evaporation of graphite) were plotted against the
IðC2 Þ=IðCNÞ ratio (Fig. 3). Concentration of evaporated carbon in Run 4 was estimated from Fig. 3 to be of 0.93 1023 m3 . It means that 59% of the fed graphite powder evaporated in that case. No C2 emission was detected in Run 5, in spite of some fullerene formation, as it was determined from the analysis of toluene extract. To estimate the effects of plasma parameters on fullerene yield, values of X 0 were calculated as follows: pffiffiffiffiffi Nc Tv 0 : ð3Þ X ¼ V Thus, X 0 can be regarded as a collective measure of plasma parameters in particular conditions. It is analogous with X defined by the above-referred model of fullerene formation [6]. Fullerene yield increased with X 0 as it can be seen in Fig. 4. In this work the highest fullerene yield of 4.1% and a production rate of 6.4 g h1 was obtained when X 0 had a maximum of 7.48 1023 m4 K0:5 s. Thus, Fig. 4 refers to the validity of kinetic model [6] used for calculating X 0 . The specific energy input which was defined as plasma power related to feed rate of graphite was 0.173 kWh g1 in the optimum case as above. It is a bit higher than the corresponding 0.1 kWh g1 value of the arc plasma method [11]. However, the RF plasma processing of carbon powder to
438
B. Todorovic-Markovic et al. / Chemical Physics Letters 378 (2003) 434–439
unique tools for producing fullerenes of higher molecular weight in a considerable amount. Some C60 -oxide was detected, as well, mainly due to the traces of oxygen in the sheath gas.
4. Summary
Fig. 4. Fullerene yield as plotted against X 0 .
fullerenes can more easily extend to pilot and industrial scale, respectively. A typical MS spectrum of the toluene extract from Run 3 is shown in Fig. 5. Fullerene ions such as C 60 , C70 and C84 can be clearly observed. It is worth mentioning that higher fullerenes such as C84 and C90 are more abundant in the mass spectrum than C70 . RF thermal plasmas seems to be
Processing of graphite powder in an RF plasma reactor at atmospheric pressure resulted in the formation of fullerene soot of remarkable amount. The fullerene production rate has a maximum against feed rate of graphite. A maximum production rate of 6.4 g h1 was achieved in particular experimental conditions. The experimental results confirmed the validity of the kinetic model of fullerene formation constructed by Markovic et al. [6]. The toluene extract of fullerene soot produced in this work contained different fullerenes including C60 , C70 , C78 , C82 , C84 , C86 , C90 , C96 and so on. Individual components of this mixture can be separated by vacuum sublimation [12]. Findings of this work furnish a good basis for optimising synthesis of fullerenes in RF thermal plasma conditions. There is a good chance that the
Fig. 5. Mass spectrum of the toluene extract from Run 3.
B. Todorovic-Markovic et al. / Chemical Physics Letters 378 (2003) 434–439
joint optimization of fullerene synthesis and purification of reaction products may lead to a costeffective production of fullerenes from graphite powders. References [1] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, San Diego, 1995, p. 870. [2] J.C. Withers, R.O. Loutfy, T.P. Lowe, Full. Sci. Tech. 5 (1997) 1. [3] K. Yoshie, S. Kasuya, K. Eguchi, T. Yoshida, Appl. Phys. Lett. 61 (1992) 2782. [4] G. Cota-Sanchez, L. Merlo-Sosa, A. Huczko, G. Soucy, Proc. ISPC, Orleans, France 15, 2001, pp. 515–520.
439
[5] L. Fulcheri, Y. Schwob, F. Fabry, G. Flamant, L.F.P. Chibante, D. Laplaze, Carbon 38 (2000) 797. [6] Z. Markovic, B. Todorovic-Markovic, T. Jokic, P. Pavlovic, P. Stefanovic, J. Blanusa, T. Nenadovic, Full. Sci. Tech. 6 (1998) 1057. [7] S.G. Kim, D. Tomanek, Phys. Rev. Lett. 72 (1994) 2418. [8] Z. Markovic, B. Todorovic-Markovic, M. Marinkovic, T. Nenadovic, Carbon 41 (2003) 369. [9] M.P. Barrow, X. Feng, J.I. Wallace, O.V. Boltalina, R. Taylor, P.J. Derrick, T. Drewello, Chem. Phys. Lett. 330 (2000) 267. [10] R. Bleekrode, Phil. Res. Rep. Suppl. (1967) 1. [11] H. Lange, A. Huczko, H. Shinohara, A. Koshio, P. Byszewski, Proc ISPC, Prague, Check Republic 14, 1999, pp. 2241–2246. [12] R.D. Averitt, J.M. Alford, N.J. Halas, Appl. Phys. Lett. 65 (1994) 374.
Efficient synthesis of fullerenes in RF thermal plasma reactor B. Todorovic-Markovic a, Z. Markovic a,*, I. Mohai b, Z. K aroly b, L. G al b, K. F€ oglein b, P.T. Szab o c, J. Szepv€ olgyi b a Vin ca Institute of Nuclear Sciences P.O. Box 522, Belgrade, Serbia and Montenegro Research Laboratory of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary b
c
Received 29 March 2003; in final form 31 July 2003 Published online: 26 August 2003
Abstract Formation of fullerene soot was studied in an inductively coupled, radiofrequency (RF) thermal plasma reactor. A previously developed kinetic model of fullerene formation was applied to determine synthesis conditions leading to high fullerene yield. The experimental results verified the kinetic model. Maximum yield of 4.1% was obtained in particular conditions. It corresponded to a fullerene production rate of 6.4 g h1 . Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Many papers were published on the potential applications of fullerene based materials more recently [1,2]. However, the high price of purified fullerenes inhibited their widespread use up to now. For practical applications, fullerenes are mainly produced by evaporating graphite electrodes in thermal arc plasma reactors. The main disadvantage of particular method is the sharp decrease of fullerene yields and hence, its production rate at discharge currents above 100 A. It is due to the high velocity of the plasma gases and hence, to the very short residence time of carbon
*
Corresponding author. Fax. +381-11-344-0100. E-mail address: [email protected] (Z. Markovic).
containing species in the hot plasma region, in particular conditions. Inductively coupled, RF thermal plasmas offer several advantages as compared to arc plasmas. In RF thermal plasma reactors, more voluminous plasma flames are formed, and the gas velocities are much lower than in arc plasmas. Thus, the mean residence time of reactive species in the hot plasma region is longer in RF thermal plasmas as compared to arc ones. Powder-like starting material can be introduced into RF plasmas with ease, contrary to arc plasmas. In addition, feed rate of powders can be independently changed of RF plasma parameters. In spite of the apparent advantages, there have been only a few attempts on fullerene synthesis from carbon powder in inductively coupled RF or hybrid plasmas [3,4]. A production rate of
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0009-2614(03)01320-4
B. Todorovic-Markovic et al. / Chemical Physics Letters 378 (2003) 434–439
2.1 g h1 was achieved by Yoshie et al. [3] by processing high purity carbon black in a hybrid plasma reactor at a power level of 25 kW. Fulcheri et al. [5] described a new synthesis method for production of fullerenes from carbon powders in a three-phase arc plasma. Using acetylene black feedstock, they produced extractable fullerenes with a yield of 3.5%. In this work, we demonstrate that fullerenes can be effectively produced from graphite powder in inductively coupled, RF thermal plasma reactor. The synthesis conditions were estimated by a kinetic model of fullerene formation constructed previously [6]. This model assumes that fullerenes are forming in inert atmosphere at temperatures above 2000 K only [7]. Recent studies [8] revealed that the initial temperature of carbon vapour should be as high as 10 000 K for an efficient fullerene synthesis. In the above model [6], fullerene yield ðY Þ was described by the following equation: Y ¼ f ðX Þ;
ð1Þ
where X ¼K
pffiffiffiffiffi N c T0 r: V
ð2Þ
In Eq. (2), Nc denotes carbon concentration in the plasma flame, V is the mean velocity of plasma gases in the flame region, T0 is the maximum plasma temperature, and K is a constant determined by the mass and radius of carbon atom. Values of r vary in the range of ½0; R where R is the distance between the spot of maximum temperature (T0 ) and the reactor wall of Tw temperature. Calculations by particular model [6] showed that the fullerene yield increased with X up to certain values. However, at higher X values, fullerene yield became of saturation type with a plateau of about 20%. Consequently, the plasma parameters, such as gas velocity, maximum temperature and volume of plasma flame affect fullerene yield, along with the carbon concentration in the hot plasma region. Reaction conditions in RF thermal plasma reactors operating at atmospheric pressure mainly depend on the plate power and the composition of hot gases. The latter one determines the ionization
435
potential, the enthalpy and the heat conductivity of the plasma flame. From inert gases worth of considering as plasma gas, helium has the greatest heat conductivity. Another important process parameter is the concentration of carbon atoms in the plasma flame: it should be as high as possible [6]. In addition, the mean residence time of carbon vapour in the hot plasma should be long enough. Thus, the velocity of gases (V ) should be low in the plasma reactor.
2. Experimental Synthetic graphite powder (Aldrich) having a mean particle size of 1–2 lm and a purity of 99.7% was treated in a thermal plasma reactor, at atmospheric pressure. The RF power was produced by a generator operating at 3–5 MHz. Plate power of 27 kW was inductively coupled to a TEKNA PL-35 torch connected to the water cooled plasma reactor, cyclone and dust filter (Fig. 1). Helium was used as plasma gas (15 slpm) and carrier gas (2–10 slpm), respectively. The sheath gas was argon (40 slpm). The graphite powder was injected axially to the top of the plasma flame with feed rates of 90–468 g h1 . A TRIAX 550 spectrometer (Jobin–Yvon) connected to CCD 3000 detector monitored the optical emission of the plasma flame through a quartz glass window at a height of 10 cm below the tip of feeding nozzle. In our experiments, the main product was solid soot collected mainly from the reactor wall. To separate fullerenes, the soot was extracted in toluene with a 100 mg/10 ml solid/liquid ratio for 1 min at 20 °C. Actually all fullerenes were separated from the soot in particular conditions. Amount of fullerenes in the filtered extracts was measured by UV–Vis spectrophotometry against reference solutions. The extracts were also subjected to MS analysis by nanospray ionization [9].
3. Results and discussion The experimental conditions and results are presented in Table 1. The mean gas velocity ðV Þ was calculated at the viewing height of 10 cm
436
B. Todorovic-Markovic et al. / Chemical Physics Letters 378 (2003) 434–439
Fig. 1. The experimental set-up.
Table 1 Experimental conditions and results
Graphite feed rate (g h1 ) V (m s1 ) Tv (K) Ncp (m3 ) I(C2 )/I(CN) Nc (m3 ) X 0 ¼ Nc Tv0:5 V 1 Y (%)
Run 1
Run 2
Run 3
Run 4
Run 5
90 12.1 4800 0.825 1023 0.4 0.825 1023 4.731023 1.3
108 12.74 4800 0.94 1023 0.93 0.94 1023 5.11 1023 2.9
156 12.736 5000 1.348 1023 1.98 1.348 1023 7.48 1023 4.1
192 13.48 4900 1.57 1023 0.73 0.93 1023 4.82 1023 2.4
468
below the tip of feeding probe. The experimental set-up did not make possible to measure the maximum plasma temperature (T0 ). Instead of T0 , the vibration temperature of C2 radicals at the viewing height (Tv ) was calculated from the emission spectra, using the Boltzmann plot method [10]. In experimental conditions of this work, Tv values were changing in a relatively narrow temperature range (Table 1). The ÔpseudoÕ concentration of evaporated carbon in the plasma flame (Ncp ) was calculated from the feed rate of graphite and the volumetric velocity of gases (Table 1). For a complete evaporation of graphite, Ncp is equal to the actual concentration of carbon atoms (Nc ) in the gas phase.
0 0 0 0.05
The C2 radicals being important in terms of fullerene formation can be characterized by the Swan band of C2 (d3 Pg ) ! C2 (a3 Pu ) located in the visible region of the spectrum (Fig. 2). The vibration excitation of d3 Pg state was estimated from the intensities of different vibration bands in sequence Dv ¼ þ1. In the experimental spectra we observed bands of CN(B2 Rþ ) ! CN(X2 Rþ ) radical, as well (Fig. 2). Nitrogen required for CN formation most probably originated from the impurities of argon sheath gas of 99.995% purity. Intensities of CN bands were nearly constant in each run. We performed thermodynamic calculations by computer code FACTSAGE to compare formation of C2 and CN in equilibrium conditions, in
B. Todorovic-Markovic et al. / Chemical Physics Letters 378 (2003) 434–439
437
Fig. 2. A typical emission spectrum of carbon plasma for experimental conditions of this work. Fig. 3. Concentration of evaporated carbon as plotted against relative emission intensities.
the temperature range of 3000–6000 K. It became clear that in equilibrium conditions the molar fraction of CN was determined by the nitrogen feed rate only, i.e., the purity of argon sheath gas. When nitrogen is present in the system, CN (and C2 N) always form in the temperature range of 4500–5000 K. However, the molar fraction of CN is smaller of two orders of magnitude than that of C2 in given conditions. Above 5000 K, atomic N is the most stable nitrogen-containing species. In our experiments, the feed rate of argon sheath gas was 40 slpm being high enough for CN formation. Thus, the emission intensity ratio of IðC2 Þ=IðCNÞ seems to be a proper indicator of the carbon evaporation in particular conditions. In Table 1 IðC2 Þ represents the mean value of C2 band-heads of Swan band positioned at 473.7 nm, while IðCNÞ represents that of CN radical at 388.3 nm in the experimental spectra. The IðC2 Þ=IðCNÞ ratio increased with the feed rate of graphite up to feed rate of 156 g h1 . It refers to complete evaporation of graphite in these conditions. Further, the IðC2 Þ=IðCNÞ emission intensity ratio decreased with feed rate. There was no C2 signal at feed rate of 468 g h1 (Table 1) detecting incomplete evaporation of carbon at higher loads. In order to calculate the actual concentration of carbon in the plasma flame even in these conditions, the Nc values for Runs 1–3 (i.e., complete evaporation of graphite) were plotted against the
IðC2 Þ=IðCNÞ ratio (Fig. 3). Concentration of evaporated carbon in Run 4 was estimated from Fig. 3 to be of 0.93 1023 m3 . It means that 59% of the fed graphite powder evaporated in that case. No C2 emission was detected in Run 5, in spite of some fullerene formation, as it was determined from the analysis of toluene extract. To estimate the effects of plasma parameters on fullerene yield, values of X 0 were calculated as follows: pffiffiffiffiffi Nc Tv 0 : ð3Þ X ¼ V Thus, X 0 can be regarded as a collective measure of plasma parameters in particular conditions. It is analogous with X defined by the above-referred model of fullerene formation [6]. Fullerene yield increased with X 0 as it can be seen in Fig. 4. In this work the highest fullerene yield of 4.1% and a production rate of 6.4 g h1 was obtained when X 0 had a maximum of 7.48 1023 m4 K0:5 s. Thus, Fig. 4 refers to the validity of kinetic model [6] used for calculating X 0 . The specific energy input which was defined as plasma power related to feed rate of graphite was 0.173 kWh g1 in the optimum case as above. It is a bit higher than the corresponding 0.1 kWh g1 value of the arc plasma method [11]. However, the RF plasma processing of carbon powder to
438
B. Todorovic-Markovic et al. / Chemical Physics Letters 378 (2003) 434–439
unique tools for producing fullerenes of higher molecular weight in a considerable amount. Some C60 -oxide was detected, as well, mainly due to the traces of oxygen in the sheath gas.
4. Summary
Fig. 4. Fullerene yield as plotted against X 0 .
fullerenes can more easily extend to pilot and industrial scale, respectively. A typical MS spectrum of the toluene extract from Run 3 is shown in Fig. 5. Fullerene ions such as C 60 , C70 and C84 can be clearly observed. It is worth mentioning that higher fullerenes such as C84 and C90 are more abundant in the mass spectrum than C70 . RF thermal plasmas seems to be
Processing of graphite powder in an RF plasma reactor at atmospheric pressure resulted in the formation of fullerene soot of remarkable amount. The fullerene production rate has a maximum against feed rate of graphite. A maximum production rate of 6.4 g h1 was achieved in particular experimental conditions. The experimental results confirmed the validity of the kinetic model of fullerene formation constructed by Markovic et al. [6]. The toluene extract of fullerene soot produced in this work contained different fullerenes including C60 , C70 , C78 , C82 , C84 , C86 , C90 , C96 and so on. Individual components of this mixture can be separated by vacuum sublimation [12]. Findings of this work furnish a good basis for optimising synthesis of fullerenes in RF thermal plasma conditions. There is a good chance that the
Fig. 5. Mass spectrum of the toluene extract from Run 3.
B. Todorovic-Markovic et al. / Chemical Physics Letters 378 (2003) 434–439
joint optimization of fullerene synthesis and purification of reaction products may lead to a costeffective production of fullerenes from graphite powders. References [1] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, San Diego, 1995, p. 870. [2] J.C. Withers, R.O. Loutfy, T.P. Lowe, Full. Sci. Tech. 5 (1997) 1. [3] K. Yoshie, S. Kasuya, K. Eguchi, T. Yoshida, Appl. Phys. Lett. 61 (1992) 2782. [4] G. Cota-Sanchez, L. Merlo-Sosa, A. Huczko, G. Soucy, Proc. ISPC, Orleans, France 15, 2001, pp. 515–520.
439
[5] L. Fulcheri, Y. Schwob, F. Fabry, G. Flamant, L.F.P. Chibante, D. Laplaze, Carbon 38 (2000) 797. [6] Z. Markovic, B. Todorovic-Markovic, T. Jokic, P. Pavlovic, P. Stefanovic, J. Blanusa, T. Nenadovic, Full. Sci. Tech. 6 (1998) 1057. [7] S.G. Kim, D. Tomanek, Phys. Rev. Lett. 72 (1994) 2418. [8] Z. Markovic, B. Todorovic-Markovic, M. Marinkovic, T. Nenadovic, Carbon 41 (2003) 369. [9] M.P. Barrow, X. Feng, J.I. Wallace, O.V. Boltalina, R. Taylor, P.J. Derrick, T. Drewello, Chem. Phys. Lett. 330 (2000) 267. [10] R. Bleekrode, Phil. Res. Rep. Suppl. (1967) 1. [11] H. Lange, A. Huczko, H. Shinohara, A. Koshio, P. Byszewski, Proc ISPC, Prague, Check Republic 14, 1999, pp. 2241–2246. [12] R.D. Averitt, J.M. Alford, N.J. Halas, Appl. Phys. Lett. 65 (1994) 374.