Analysis of the Growth Behavior of Carbon Nanofibers Synthesized

Apr 3, 2013 - In this work, the growth behavior of carbon nanofibers synthesized using the liquid pulse injection technique was clarified by analyzing...
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Analysis of the Growth Behavior of Carbon Nanofibers Synthesized Using the Liquid Pulse Injection Technique Shin R. Mukai,* Yuusuke Rikima, Riku Furukawa, and Isao Ogino Division of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University, N13W8 Kita-ku, Sapporo, Japan ABSTRACT: In this work, the growth behavior of carbon nanofibers synthesized using the liquid pulse injection technique was clarified by analyzing samples collected at different growth stages. It was confirmed that, due to the high catalytic activity of the ultrafine metal catalyst particles generated through this technique, the fibers elongate at an extremely high rate during the initial stage of growth, which leads to the high efficiency of carbon nanofiber production. Owing to this high elongation rate, the elongation process and thickening process can be practically considered as two consecutive processes. This allows the modification of the production sequence, resulting in a significant enhancement in production efficiency.

1. INTRODUCTION Filamentous carbons have attracted much attention for several decades due to their unique features such as high strength to weight ratio, high chemical resistance, and moderate electrical conductivity. Although the pioneering reports of filamentous carbons were of discontinuous filaments,1,2 the carbons that reached the market first were continuous carbon fibers. Rayon fibers were first used as the precursor, but it was soon discovered that carbon fibers obtained from polyacrylonitrile (PAN) fibers generally possess better properties.3−5 Therefore, PAN based carbon fibers soon dominated the market. Pitch is also commercially used as a precursor as it is inexpensive and as fibers with a fairly high elastic modulus can be easily obtained from it.6,7 Discontinuous filaments gained the spotlight again in the 1970s.8 First, vapor grown carbon fibers (VGCFs), carbon filaments directly grown from hydrocarbon vapors having diameters in the μm range, were the target of interest.9−11 The growth mechanism of such fibers was clarified in the early 1970s,12−16 and this led to the development of methods for mass production.17,18 Among them, the method originally proposed by Endo et al. became the standard of commercial production, as it allows the continuous production of VGCFs.18 Interest then shifted to filaments with smaller diameters, carbon nanotubes (CNTs), and carbon nanofibers (CNFs), mostly owing to the reports about single wall CNTs.19,20 It is well-known that all types of discontinuous filaments, VGCFs, CNFs and CNTs, can be synthesized from hydrocarbon vapors using an integrated process of the catalysis of ultrafine metal particles and thermal chemical vapor deposition (CVD). The catalysis of ultrafine metal particles is used to elongate the filaments, and thermal CVD is used to thicken them.12−16 The type of filament can be changed by controlling the speed and duration of the thermal CVD process. Generally, filaments with smaller diameters possess better properties; however, as only a single filament can be obtained from a single catalyst particle and as the diameter of the initial filament obtained through the catalysis of the particle is extremely small, the productivity of CNTs and CNFs tends to be low when compared with other carbon materials. However, when compared with CNTs, there is still a higher possibility to © 2013 American Chemical Society

significantly improve the productivity of CNFs, as most of the carbon from which it comes from is through the thermal CVD process. Therefore, we have been focusing on the development of an efficient process to produce CNFs. CVD can proceed only where a substrate for deposition exists. Therefore, the key to enhance the productivity of CNFs is to accelerate the elongation process, even though most of its carbon is formed through thermal CVD. The elongation rate of a CNF depends on the catalytic activity of the metal particle used to grow it. This activity is governed by the size and temperature of the particle.8−10,21−23 Although CNFs can be produced in a fairly wide temperature range, the size of the catalyst particle is very crucial, as only particles with a proper size, a few to about 30 nm, can contribute to elongation.21−23 Such particles must also be generated in a highly dense state to efficiently grow CNFs, as the space in a reactor is quite limited. Previously, we developed a method which allows the generation of ultrafine metal particles in a highly dense state, the liquid pulse injection (LPI) technique.23−26 Through attempts to produce VGCFs, it was found that extremely long fibers can be obtained in an extremely short time by the use of this method.24,25 The analysis of the growth behavior of such VGCFs showed that growth rates higher than 3000 μm s−1 can be easily achieved.23−25 We also attempted the production of CNFs using the LPI technique.27−29 Through optimization of process parameters, it was found that CNFs can be produced at carbon yields over 80% by the use of this method. However, the size uniformity of CNFs tends to be low when CNFs are produced at such high carbon yields, as the yield is increased by simply increasing the time allowed for thickening. It is assumed that the analysis of the growth behavior of CNFs produced using the LPI technique will give us clues to how to further improve productivity and product quality. When CNFs are produced using conventional methods, it is generally Special Issue: NASCRE 3 Received: Revised: Accepted: Published: 15281

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injected into the reactor at an interval of 90 s. When 90 s passed after the final liquid pulse, power supply to the electric furnace was cut, and the reactor was quickly purged with nitrogen to terminate reactions. Then, the reactor was cooled down to room temperature, and the produced CNFs located at the bottom of the reactor were collected. As the main purpose of this work is to study the growth behavior of CNFs generated through the LPI technique, experiments using modified procedures were also conducted so that CNF samples at different growth stages could be collected. As mentioned above, in usual production experiments, the fibers increase their length and diameter as they move downward from the top part of the reactor, and they finally accumulate at the bottom of the reactor. Therefore, if the CNFs flowing downward could be directly sampled at different positions within the reactor, it is thought that a series of samples which clearly shows the growth behavior of CNFs can be obtained. As it is difficult to directly collect CNFs at predetermined positions within the reactor, in this study, the flow rate of the carrier gas was rapidly increased to the maximum flow rate of the flow controller (2000 cm3-STP min−1) when CNFs reached the sampling positions, to blow them out of the high temperature zone and quickly terminate their growth. It was confirmed that this procedure hardly affects the temperature distribution within the reactor. Through this procedure, it is thought that fibers representing the CNFs at the designated sampling position can be collected. In this work, we sampled CNFs at positions where their temperature first reaches 1000, 1050, 1100, 1150, and 1200 °C. Finally, CNFs were produced using a modified production sequence. First, the carrier gas was supplied to the reactor at a constant flow rate. After the center position of the reactor reached 1200 °C, a liquid pulse of the initial source was introduced into the reactor. When the generated fibers reached a designated position within the high temperature reaction zone, the supply of the carrier gas was terminated, and after 70 s elapsed from pulse introduction, the carrier gas was supplied again to the reactor at the original constant flow rate. The designated positions were where the temperature of the generated fibers first reached 1100 and 1200 °C. After 20 s from the resupplying of the carrier gas, the next liquid pulse of the initial source was introduced into the reactor. This cycle was repeated 20 times, and the produced CNFs located at the bottom of the reactor were collected. All of the collected CNFs were observed using a scanning electron microscope (JEOL, JSM-6600F), and their dimensions were checked. The carbon yield at which the CNFs were obtained was calculated from the weight of the collected CNFs, and the total amount of carbon in the initial source was introduced into the reactor. The total length of the obtained CNFs was estimated from the total weight of the fibers and their average diameter.

extremely difficult to analyze their growth behavior, especially under conditions adopted in actual production processes, as the elongation process initiates at various positions within the reactor and terminates quite quickly. The short length of the attainable CNFs also makes it difficult to clarify its growth behavior. However, in the LPI technique, fiber growth initiates quite simultaneously and extremely long CNFs can be easily obtained, both factors making it easier to study the growth behavior of CNFs. Therefore, in this work, we challenged the analysis of the growth behavior of CNFs produced using the LPI technique.

2. EXPERIMENTAL SECTION Figure 1 shows a schematic of the experimental apparatus used in this work. It consists of a ceramic tube reactor (i.d.: 42 mm,

Figure 1. Schematic of experimental aparatus.

L: 1000 mm), an electric furnace, and mass flow controllers. The initial source used for CNF production was a mixture of benzene (carbon source), ferrocene (catalyst source), and thiophene (catalyst promoter). The weight ratios among them were set to 94:5:1. Hydrogen was used as the carrier gas, and the flow rate of it was set in the range of 100 to 400 cm3-STP min−1. The growth behavior of CNFs is highly affected by the temperature of the atmosphere they are grown in. In a conventional tubular reactor like the one used in this work, the temperature profile within the reactor is usually parabolic. When CNFs are produced in such reactors using the LPI technique, first ultrafine metal catalyst particles are instantaneously formed at the upper part of the reactor as each liquid pulse of the initial source is introduced. These particles flow downward with the carrier gas and the temperature of them increases according to the temperature profile, leading to the generation of primary fibers using the coexisting hydrocarbons as the carbon source. Concurrently, carbon layers are formed on the primary fibers through thermal CVD. Finally, the generated CNFs reach the bottom of the reactor. This means that the temperature the generated CNFs experience is governed by the temperature profile within the reactor. Therefore, before conducting production experiments, the temperature profile within the reactor was measured. In the production of CNFs, first, the reactor was thoroughly purged with nitrogen and then with hydrogen. Then, the reactor was heated so as the temperature of the center position of the reactor would reach 1200 °C, the maximum temperature. After the temperature and the carrier gas flow within the reactor reached a steady state, 20 liquid pulses of the initial source were

3. RESULTS AND DISCUSSION 3.1. Temperature Profile. Figure 2 shows a typical temperature profile within the reactor used in this work. As was expected, the temperature profile was parabolic, and the temperature reached a maximum at the center of the reactor. This profile was measured at a carrier gas flow rate of 100 cm3STP min−1, but it was confirmed that this profile hardly changes when the carrier gas flow rate was varied, at least when it was set within the range used in this study. The distances 15282

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time allowed for the fibers to grow, and the temperatures the fibers were collected at are also shown in each micrograph. In the LPI technique, CNFs usually initiate growth at temperatures around 1000 °C.26 It can be noticed that, due to the high catalytic activity of the catalyst particles obtained through the LPI technique, long fibers are already formed when the fibers reach the position where the temperature is 1050 °C. Figure 4

Figure 2. Temperature distribution within the reactor used in this work.

from the top of the reactor to the sampling points used in this work are also indicated in Figure 2. 3.2. Time Required to Reach Sampling Positions. Along with the temperature distribution, the growth behavior of CNFs is also affected by how long they reside within the high temperature zone of the reactor. This length is governed by the carrier gas flow rate. The time required for the generated CNFs to reach the positions of sampling after pulse introduction also depends on the carrier gas flow rate. The flow rates adopted in this work and how long it took for the generated CNFs to reach the designated sampling points after pulse introduction at each flow rate are summarized in Table 1.

Figure 4. Growth curve of CNF diameter (carrier gas flow rate: 100 cm3-STP min−1).

shows the diameters of the CNFs as a function of the time elapsed after pulse introduction. The diameter of the fibers hardly increase in the initial stage of reaction, as the thermal CVD of the hydrocarbons existing in the vapor phase is suppressed by the hydrogen carrier,26 but as the temperature increases, the thermal CVD process is accelerated and the diameter of the fibers significantly increase. Figure 5 shows the carbon yields the CNFs of the same series were obtained at, as a function of the time elapsed after liquid pulse introduction. The temperatures at which the CNFs were collected are also shown in the figure. It can be noticed that, after fiber growth initiates, the carbon yield first rapidly increases, and then the rate that the yield increases at becomes gradual and then slightly increases again. As the diameter of a primary fiber is extremely small, the contribution of the catalytic elongation process of CNFs to the increase in carbon yield is low when compared with the thickening process. This fact implies that the rapid increase in carbon yield during CNF production is mainly due to the thickening process. The thickening process is a thermal CVD process, so it can proceed only when a substrate exists. In CNF production, the primary fibers provide the substrates for thermal CVD; therefore, a significant amount of primary fibers must exist for a significant increase in carbon yield to occur. Therefore, from the results shown in Figure 5, it is assumed

Table 1. Time Required to Reach Sampling Positions after Pulse Introduction carrier gas flow rate [cm3-STP min−1] temperature at sampling position [°C]

100

1000 1050 1100 1150 1200

20 24 28 40 46

150

300

time required, s 13 7 16 8 18 9 27 13 31 15

400 5 6 7 10 12

3.3. Growth Behavior of CNFs. Using the sampling sequence explained above, 4 series of CNF samples, each series obtained at a different carrier gas flow rate, were collected. Figure 3 shows SEM micrographs of the series of CNFs obtained at a carrier gas flow rate of 100 cm3-STP min−1. The

Figure 3. SEM images of a series of CNFs produced at a carrier gas flow rate of 100 cm3-STP min−1 (t: time elapsed after pulse introduction; T: temperature at sampling position). 15283

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temperature is thought to be governed by the temperature of where it is located within the reactor, the increase in the carrier gas flow rate leads to the rapid increase in the catalyst temperature, as it reaches the high temperature reaction zone much faster. This is favorable for the elongation of the fiber generated from it, especially when the fiber is in its early stages of growth. However, the increase in the flow rate also leads to the decrease in the time allowed for the generated CNFs to reside in the high temperature reaction zone. Therefore, the achievable carbon yield decreases as the time allowed for thickening is limited. Figure 7 compares SEM images of typical fibers produced at a high and a low carrier gas flow rate. As expected, the fibers obtained at higher carrier gas flow rates are thinner, indicating that there is a high possibility that the carbon yield can be further increased even at high carrier gas flow rates by increasing the time they can reside in the high temperature reaction zone. Interestingly, the CNFs synthesized at higher carrier gas flow rates tended to have uniform diameters. For example, the standard deviation of the diameter of fibers obtained at a carrier gas flow rate of 300 cm3-STP min−1(average diameter, 66 nm) is 15 nm and is much smaller than that of fibers obtained at 100 cm3-STP min−1 (average diameter, 120 nm; standard deviation, 26 nm). This can be explained as follows. In the LPI technique, the generated catalyst particles have to reach the high temperature zone in the reactor in order to get activated, but when the carrier gas flow rate is low, the catalyst particles generated from the same pulse tend to reach this zone at different timings. Therefore, the initial fibers also initiate growth at different timings. This means that the time allowed for thermal CVD varies among fibers, so even though a high carbon yield can be achieved, it will become difficult to obtain products with uniform dimensions. On the other hand, when the carrier gas flow rate is high, the generated catalyst particles will reach the high temperature zone at similar timings, so initial fiber growth initiates fairly simultaneously. This means that the difference in the time allowed for thermal CVD to proceed will be small, so CNFs with uniform dimensions can be easily obtained. 3.4. Modification of Production Sequence. Considering the growth behavior of CNFs which was clarified in this work, it was assumed that the productivity and uniformity of the CNFs could be improved by modifying the production sequence used in the LPI technique. The key points are to first quickly increase the temperature of the catalyst particles and then to maintain a fairly long residence time. Indeed, these points can be fulfilled by simply increasing the length of the reactor and using a high carrier gas flow rate, but this leads to a significant increase in the cost of the production system. Moreover, using a longer reactor means that the amount of heat loss will increase, which is disadvantageous especially in processes which require high temperatures. The key points can be fulfilled even using the same production system by modifying the adopted sequence, i.e., by first supplying the carrier gas at a high flow rate when the liquid pulse is introduced and then terminating the supply when the generated fibers reach the high temperature zone within the reactor. We verified if this strategy actually works by producing CNFs using the modified production sequence explained above. The base carrier gas flow rate was set to 300 cm3-STP min−1. Figure 8 compares the carbon yield and amount of carrier consumption of the normal sequence and modified sequences. In the normal sequence, the total carbon

Figure 5. Relationship between carbon yield, total length, and the time elapsed after pulse introduction of CNFs produced at a carrier gas flow rate of 100 cm3-STP min−1.

that numerous initial fibers were generated in the initial stage of growth and that they elongated quite quickly. These are reasonable assumptions considering the high elongation rates achieved when the LPI technique was used to produce VGCFs.23−26 The following decrease in the rate the carbon yield increases is thought to be due to the decrease in the amount of hydrocarbon species which can contribute to CVD, caused by consumption of the species or transformation of the species to a fairly inactive hydrocarbon, methane. The following increase can be explained by the activation of methane through temperature increase. In order to analyze their elongation behavior, the total lengths of CNFs collected at different positions were calculated. The results are also summarized in Figure 5. As this is an extremely rough method for estimation, the results were rather scattered, but it can be noticed that the values corresponding to the 5 sampling positions are in the same order, and most of the values are quite identical. This result suggests that the fibers elongated at extremely high rates and that elongation terminated at the early stage of growth, maybe even before their temperature reached 1050 °C. Therefore, it seems that it is much more reasonable to regard the 2 processes of elongation and thickening as 2 consecutive processes, rather than 2 processes that proceed concurrently. Next, we checked how the carrier gas flow rate affects the growth behavior of the fibers. The results are summarized in Figure 6. We found that, when the carrier gas flow rate is increased, the increase in the yield during the initial stage of growth becomes much more significant. As the catalyst

Figure 6. Effect of carrier gas flow rate on the carbon yield of CNFs. 15284

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Figure 7. SEM image of CNFs obtained at different carrier gas flow rates.

Figure 8. Results of production sequence modification (base carrier gas flow rate: 300 cm3-STP min−1).

carrier supply at a timing slightly earlier led to an additional 10% increase. The amount of carrier gas consumption can also be significantly decreased by adopting this new production sequence. Figure 9 compares SEM images of CNFs obtained using different production sequences. It can be noticed that the CNFs obtained through the modified sequence are much thicker than those obtained through the normal sequence. Moreover, it can be noticed that the diameters of the thick CNFs are also quite uniform. For example, the standard deviation of the diameter of CNFs obtained using the modified sequence and terminating carrier gas supply when the fibers reached 1100 °C is about 5.4 nm and is much smaller than that of CNFs obtained using the normal sequence at a carrier gas flow rate of 100 cm3-STP min−1 (26 nm), even though their average diameters are quite similar (normal sequence: 118 nm; modified sequence: 97 nm).

yield is about 43%, but by cutting the supply of the carrier gas when the generated fibers reach the temperature of about 1200 °C, the maximum temperature within the reactor, a 10% increase in the carbon yield, could be achieved. Terminating

4. CONCLUSIONS In order to find clues to further enhance productivity, the growth behavior of CNFs synthesized using the LPI technique was clarified by analyzing samples collected at different growth

Figure 9. SEM images of CNFs obtained using different production sequences (base carrier gas flow rate: 300 cm3-STP min−1). 15285

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(18) Endo, M.; Shikata, M. Growth of vapor-grown carbon fibers using fluid ultra-tine particles of metals. Ohyo Butsuri 1985, 54, 507− 510. (19) Iijima, S.; Ichihashi, T. Single-shell carbon nanotubes of 1-nm diameter. Nature 1993, 363, 603−605. (20) Bethune, D. S.; Kiang, C. H.; De Vries, M. S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers, R. Cobalt catalysed growth of carbon nanotubes with single atomic-layer walls. Nature 1993, 363, 605−7. (21) Benissad, F.; Gadelle, P.; Coulon, M.; Bonnetain, L. Formation de fibres de carbone a partir du methane: I Croissance catalytique et epaississement pyrolytique. Carbon 1988, 26, 61−69. (22) Ishioka, M.; Okada, T.; Matsubara, K. Preparation of vaporgrown carbon fibers by floating catalyst method in Linz-Donawitz converter gas: Influence of catalyst size. Carbon 1993, 31, 699−703. (23) Mukai, S. R.; Masuda, T.; Matsuzawa, Y.; Hashimoto, K. The influence of catalysts particle size distribution on the yield of vaporgrown carbon fibers produced using the liquid pulse injection technique. Chem. Eng. Sci. 1998, 53, 439−448. (24) Masuda, T.; Mukai, S. R.; Hashimoto, K. The production of long vapor grown carbon fibers at high growth rates. Carbon 1992, 30, 124−126. (25) Masuda, T.; Mukai, S. R.; Hashimoto, K. The liquid pulse injection technique, a new method to obtain long vapor grown carbon fibers at high growth rates. Carbon 1993, 31, 783−787. (26) Mukai, S. R.; Masuda, T.; Fujikata, Y.; Hashimoto, K. The production of vapor grown carbon fibers from a mixture of benzene, toluene and xylene using the liquid pulse injection technique. Chem. Eng. Sci. 1994, 49, 1909−4916. (27) Mukai, S. R.; Ohtaka, T.; Tamon, H. Production of carbon nanofibers using the liquid pulse injection technique. Proceedings of Carbon 2003, Oviedo, Spain, July 6−10, 2003. (28) Mukai, S. R.; Ohtaka, T.; Tamon, H. High efficiency carbon nanofiber production using the liquid pulse injection technique. Proceedings of Carbon 2004, Providence, RI, July 11−16, 2004. (29) Mukai, S. R.; Ikeshita, A.; Hayashida, Y.; Yamada, I. Scaling-up of the carbon nanofiber production system based on the liquid pulse injection technique. Proceedings of Carbon 2009, Biarittz, France, June 14−19, 2009.

stages. It was confirmed that, due to the high catalytic activity of the ultrafine metal catalyst particles generated through this technique, the fibers elongate at an extremely high rate during the initial stage of growth, which leads to a high efficiency of CNF production. Owing to this high elongation rate, the elongation process and thickening process can be considered as 2 consecutive processes. On the basis of these findings, the production sequence of the LPI technique was modified, resulting in a significant enhancement in production efficiency.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81-11-706-6590. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ABBREVIATIONS PAN = polyacrylonitrile VGCF = vapor grown carbon fiber CNT = carbon nanotubes CNF = carbon nanofibers CVD = chemical vapor deposition LPI = liquid pulse injection

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REFERENCES

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