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Thermal Behavior of Transparent Film Heaters Made of Single-Walled Carbon Nanotubes Duckjong Kim, Hyun-Chang Lee, Ju Yeon Woo, and Chang-Soo Han* Nano Mechanical Systems Research Center, Korea Institute of Machinery and Materials, 171 Jang-dong, Yuseong-gu, Daejeon 305-343, Korea ReceiVed: NoVember 13, 2009; ReVised Manuscript ReceiVed: February 5, 2010
We investigate thermal behavior of transparent film heaters (TFH) made of single-walled carbon nanotubes. We fabricate the TFH by using the spray coating method. We studied the temperature dependence of the electrical resistance of the TFH in terms of Joule and external heating in various gas environments. Test results show that the effect of the electrical current through the TFH on the temperature dependence of the electrical resistance is not important and that the humidity and the degree of vacuum significantly affect the shape of the resistance-temperature curve. We discuss the physical meanings underlying the experimental results and how to make use of these findings. This study improves the understanding of the heating effect on electrical conductance of the TFH made of single-walled carbon nanotubes which could be a good candidate for the heater in many applications requiring both transparency and heating function. Introduction Since the discovery of carbon nanotubes (CNTs), their superior physical properties have led to many practical applications.1,2 In particular, films made of CNTs have the potential of next generation transparent conducting film (TCF) due to high conductivity, mechanical flexibility, simple fabrication, and abundance of raw carbon materials.3-5 CNT-TCFs have been studied for use as transistors, electrodes in photovoltaics and organic light-emitting diodes, sensors, and actuators.6-10 These applications generally require high transparency and low electrical resistance. Recently, we reported the possibility of creating a transparent film heater (TFH) using CNT-TCF.11 We made the TCF by using the vacuum filtration method and demonstrated its potential as a vehicle defroster. Although small test samples were successfully fabricated by using the vacuum filtration method, the TCF size is limited by the filter dimensions for the filtration method and this is a drawback for applications requiring TCF with large area. In this context, the spray coating method, which is a robust approach for TCFs with large area, could be a breakthrough in the commercialization of the TFH. Understanding the thermal behavior of CNT-TCFs is also crucial in a commercially valuable achievement. Several researchers have reported the effect of temperature on the electrical transport properties of the TCFs. The resistivity of TCFs made of single-walled carbon nanotubes (SWCNTs) decreased as the temperature increased at low temperature. This temperature dependence of electrical resistance is called negative temperature dependence. Above a certain transition temperature, the resistivity showed opposite temperature dependence, positive temperature dependence.12-16 Kaiser et al. explained that this so-called U-shaped temperature dependence was related to the transition from semiconducting to metallic behavior.12 Barnes et al. attributed the positive temperature dependence of the resistance to the dopant desorption from their experimental results on the temperature dependence of resistivity for metallic and semiconducting SWCNTs-enriched transparent networks.13 * To whom correspondence should be addressed. E-mail: cshan@ kimm.re.kr. Phone: +82-42-868-7126. Fax: +82-42-868-7884.
However, the previous studies mostly focused on the thermal behavior of the TCFs below room temperature. They did not use Joule heating of the TCFs and just relied on external temperature controller to regulate the temperature of the TCFs. To the best knowledge of the authors, there has been no report on the thermal behavior of the heater using the TCF. In this research, we fabricated TFHs by using the spray coating method and investigated the relationship between temperature and the electrical resistance of the TFH in terms of Joule and external heating in various gas environments. We also discussed physical meanings underlying the experimental results and how to use them. Experimental Details We fabricated TCFs made of SWCNTs by using the spray coating method. We used thermal annealing and acid treatment to purify SWCNTs prepared by the arc-discharge method. Using field emission scanning electron microscopy (FESEM, Model: FEI NOVA 200) and field emission transmission electron microscopy (FETEM, Model: JEOL JEM-2100F), we verified that the purified SWCNTs had almost no large impurities as shown in the inset of Figure 1a. We dispersed SWCNTs in deionized water with 1 wt % sodium dodecyl sulfate (SDS) and sonicated for several hours. The concentration of the SWCNT solution was 1 µg/mL. We sprayed the SWCNT solution on a glass substrate to form the TCF. The size of the glass substrate is 50 mm × 50 mm × 0.5 mm. As a result, we made TCFs with transparency of about 70% at 550 nm. The sheet resistance of the TCFs was in the range of 130-190 Ω/sq. We measured the optical transmittance and the sheet resistance by using the absorption spectroscopy (Optizen 2120UV Plus) and the fourpoint probe method (Jandel CMT 100), respectively. Finally, on the TCF, we formed electrodes by using silver paste to make a low-resistance electrical contact. Figure 1a shows the prepared sample. The Raman spectrum shown in the inset of Figure 1a clearly demonstrates the most characteristic features of SWCNTs in terms of the radial breathing mode (RBM), the D-band, and the G-band. We heated the prepared samples either by Joule heating or external heating. Figure 1b shows the setup for the Joule heating
10.1021/jp910799a 2010 American Chemical Society Published on Web 03/12/2010
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Figure 2. Resistance-temperature curves for (a) Joule heating and (b) external heating in the air.
measurement, we used T-type thermocouples. We calculated the electrical resistance by dividing the applied voltage difference by the measured current. For external heating, we heated the samples on a hot plate. For this case, we measured the electrical resistance directly by using the data acquisition unit (Agilent 34970A). In the present work, we collected measured data once every second. For experiments under vacuum or a specific gas environment, we tested TFHs in a vacuum chamber. For vacuum environment, we decreased the chamber pressure to 3 × 10-6 Torr. For a specific gas environment, we infused the specific gas into the chamber keeping the maximum chamber pressure to the atmospheric pressure as soon as the chamber reached the desired vacuum level. When we filled the chamber with dry air, we dehumidified the air using an air dryer system (Keumsung NRD-12). Results and Discussion
Figure 1. (a) Prepared TFH made of SWCNTs. The inset includes FESEM, FETEM images, and the Raman spectrum of the SWCNTs film. (b) Setup for the Joule heating experiment.
experiment. For Joule heating, we applied the voltage difference of 60 V across the electrodes by using a DC power supply (Agilent E3649A). We measured the voltage difference and the surface temperature of the TFH by using a data acquisition unit (Agilent 34970A) and monitored the electrical current through the TFH by using a current meter (Fluke 189). For temperature
To obtain the resistance-temperature (R-T) curve, we increased the film temperature from room temperature to 160 °C, and then immediately cooled the film to the initial temperature by natural convection. Once the film temperature returned to the initial value, we reapplied heat to the film immediately. We repeated this heating and cooling cycle three times. Parts a and b of Figure 2 show the results for Joule and external heating in the air, respectively. We normalized the TCF resistance by the initial TCF resistance, R0. The results show a mixed behavior with a transition from the negative temperature dependence to the positive temperature dependence, the socalled U-shaped curve for both cases. Although the U-shape is more apparent for the Joule heating case, the effect of the heating method on the transition temperature is not important. Above the transition temperature, the resistance of the TCFs increases as the temperature increases, i.e., positive temperature depen-
TFH Made of Single-Walled Carbon Nanotubes
Figure 3. Resistance changes of the heater for the thermal cyclic experiment in vacuum.
dence. These results show that the temperature dependence of the resistance is not significantly affected by the electrical current through the TFH. Throughout the thermal cycles, the temperature dependence of the resistance showed a repeated pattern except for the first heating cycle. After this cyclic experiment, we left the TFH at room temperature in the air for about one week, after which the electrical conductance of the heater recovered. To clarify the effect of the air on the electrical resistance of the TCF, we performed the Joule heating experiment under vacuum. We heated the TFH to 160 °C. Figure 3 clearly shows that there is no U-shaped temperature dependence of the resistance. Only positive temperature dependence appears in the graph. Initially, the resistance increases rapidly with the temperature rise. The slope of resistance change decreases as the temperature increases. At around 60 °C, the slope of the resistance increase reaches a steady value. In the vacuum environment, the heater resistance increase during the first heating cycle is much larger than that in the air. The experimental results clearly show that some components in the air influence the electrical resistance of the TCF seriously. To investigate the effect of each component in the air, we tested the TFH in various gas environments. Dry air contains 78.1% nitrogen, 21.0% oxygen, 0.9% argon, and so on. Air also contains a variable amount of water vapor. Figure 4 shows the results from the experiments in dry air, nitrogen, oxygen, and argon. Figure 4a shows that the TFH tested in the dry air does not show the U-shaped temperature dependence of the resistance. This indicates that water molecules in the air cause the U-shaped temperature dependence. Some researchers have reported that water molecules act as a donor doping the SWCNTs and that sufficient exposure to humidity decreases the electrical resistance of SWCNTs.17,18 We developed an explanation on the U-shaped temperature dependence of the resistance assuming two conflicting effects. As the temperature increases, the kinetic energy of water molecules in the vicinity of the TFH increases and the adsorption of water molecules to SWCNTs caused by collision between water molecules and SWCNTs increases. On the other hand, as the temperature increases, desorption of water molecules from SWCNTs becomes more active. For temperatures below the transition temperature, the former effect is dominant and the resistance decreases as the temperature increases. For temperatures above the transition temperature, the latter effect is dominant and the positive temperature dependence of the resistance appears. Panels b-d of Figure 4 show that the R-T curve does not seriously depend on the gas filling the test chamber. However, there is a trivial difference among them. For nitrogen and oxygen environments, the temperature dependence of the resistance
J. Phys. Chem. C, Vol. 114, No. 13, 2010 5819 shows a repeated pattern except for the first heating cycle as shown in parts b and c of Figure 4. On the other hand, in the argon gas, the resistance consistently increases even after the first heating cycle as shown in Figure 4d. The resistance increase is more apparent in the R-T curve for vacuum as shown in Figure 3. Therefore we could infer that the nitrogen and oxygen molecules in the air reduce the electrical resistance of the CNTTCFs as reported by several researchers.19-21 To clearly show the effect of each component in the air on the R-T curve of the TFH, we summarized the experimental results as shown in Figure 5. Figure 5a shows the resistance increase during the first heating cycle. The R-T curves in Figure 5a show that the chamber pressure is a predominant factor determining the resistance increase during the initial heating and that the humidity is another one. Except for the case of vacuum environment, we maintained the chamber pressure to the atmospheric pressure. Figure 5a shows that the resistance increase due to the initial heating is less than 20% for the atmospheric pressure. Only for the vacuum environment does the resistance increase by about 150% during the first heating cycle. It is well-known by the Langmuir isotherm that adsorption of molecules on a solid surface decreases as the gas pressure decreases.22 Hence, our results indicate that the resistance increase seriously depends on desorption of some dopants as pointed out by Barnes et al.13 In addition, the R-T curves for air with water vapor and dry air show that the humidity in the air suppresses the resistance increase during the initial heating. The initial resistance increase appears just once during the thermal cyclic test. Therefore, to guarantee the repeatability of the TFH, warming of the TFH would be necessary. Figure 5b shows the R-T curves during the third heating cycle and this temperature dependence of the resistance is repeatable as shown in Figures 2-4. The R-T curves repeated during thermal cyclic condition show that the humidity is the most important factor and that the chamber pressure is another one. The humidity reduces the resistance up to 5%. In the vacuum environment, the resistance increase due to the temperature rise becomes more apparent as the temperature increases. Because the effect of gas filling the test chamber on the temperature dependence of the resistance is negligible as shown in Figure 5, sensing the humidity or the degree of vacuum from the R-T curve would be much easier than gas sensing. For temperatures less than about 100 °C, the humidity effect is predominant and the R-T curve could be used for humidity sensing. For higher temperatures, the effect of the chamber pressure becomes more important and the R-T curve could be used for measurement of the degree of vacuum. Since the humidity in the air seriously affects the R-T curve of the TFH, humidity control or waterproofing the CNT-TCFs would be essential for repeatability and reliability of the TFH. Figure 5b also shows that another effect could become apparent when all dopants are removed from SWCNTs. The positive temperature dependence of the electrical resistance maintains in vacuum environment even after warm-up. This strongly indicates that the positive temperature dependence is also caused by the intrinsic property of the SWCNTs, the so-called metallic behavior, for temperatures above room temperature, as pointed out by Kaiser et al.12 Therefore, the temperature dependence of the electrical resistance of the TFH depends on both the dopant adsorption and intrinsic electrical behavior of the SWCNTs. After the thermal test, we measured the resistance of the TFH for about 20 h in vacuum without heating to determine whether the TFH would recover their initial conductance. As shown in Figure 6, there was no significant recovery. However, when we
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Figure 4. Resistance-temperature curves from thermal cyclic experiments in (a) dry air, (b) nitrogen, (c) oxygen, and (d) argon.
Figure 5. Resistance-temperature curves for (a) the first heating cycle and (b) the third heating cycle.
dopants would be nitrogen, oxygen, and water molecules. The reduction of the film resistance gradually proceeded over a few days. Conclusion
Figure 6. Resistance changes of the heater for long-term cooling.
placed the TCF in the air, the electrical conductance recovered appreciably after a few hours. This conductance recovery indicates that the dopants seriously reducing the resistance of the TFH would be some components of the air. From the experimental results discussed above, the candidates for the
We experimentally studied the thermal behavior of TFH using SWCNTs. We used the spray coating method in heater fabrication. We investigated the temperature dependence of the electrical resistance of the TFH in terms of Joule and external heating in various gas environments. There was no significant difference in the temperature dependence of the resistance between Joule and external heating in the air; we observed similar temperature dependence of the resistance including the U-shaped curve for both heating conditions. However, for the experiments conducted in environments without water vapor, there was only positive temperature dependence in the R-T curve. We could come to the conclusion that the water molecules cause the U-shaped temperature dependence of the resistance. The experimental results show that the humidity and the degree of vacuum are the main parameters affecting the R-T curve. Hence, humidity control or waterproofing the CNT-TCFs would
TFH Made of Single-Walled Carbon Nanotubes be necessary to guarantee the repeatability in the temperature dependence of the electrical resistance of the CNT-TCFs. This study provides useful information on the thermal behavior of the TFH made of SWCNTs and could be a cornerstone for achievements of commercially valuable TCF applications. Acknowledgment. This research was supported by the Center for Nanoscale Mechatronics and Manufacturing, one of the 21st Century Frontier Programs. References and Notes (1) Baughman, R. H.; Zakhidov, A. A.; Heer, W. D. Science 2002, 297, 787. (2) Collins, P. G.; Avouris, P. Sci. Am. 2000, 62. (3) Hu, L.; Hecht, D. S.; Gruner, G. Nano Lett. 2004, 4, 2513. (4) Saran, N.; Parikh, K.; Suh, D.-S.; Munoz, E.; Kolla, H.; Manohar, S. K. J. Am. Chem. Soc. 2004, 126, 4462. (5) Wu, Z. C.; Chen, Z. H.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Herbard, A. F.; Rinzler, A. G. Science 2004, 305, 1273. (6) Artukovic, E.; Kaempgen, M.; Hecht, D. S.; Roth, S.; Gruner, G. Nano Lett. 2005, 5, 757. (7) Pasquier, A. D.; Unalan, H. E.; Kanwal, A.; Miller, S.; Chhowalla, M. Appl. Phys. Lett. 2005, 87, 203511. (8) Kaempgen, M.; Roth, S. J. Electroanal. Chem. 2006, 586, 72.
J. Phys. Chem. C, Vol. 114, No. 13, 2010 5821 (9) Li, J.; Hu, L.; Wang, L.; Zhou, Y.; Gruner, G.; Marks, T. J. Nano Lett. 2006, 6, 2472. (10) Yu, X.; Rajamani, R.; Stelson, K. A.; Cui, T. Sens. Actuators, A 2006, 132, 626. (11) Yoon, Y. H.; Song, J. W.; Kim, D.; Kim, J.; Park, J.; Oh, S.; Han, C. S. AdV. Mater. 2007, 19, 4284. (12) Kaiser, A. B.; Du¨sberg, G.; Roth, S. Phys. ReV. B 1998, 57, 1418. (13) Barnes, T. M.; Blackburn, J. L.; Lagemaat, J.; Coutts, T. J.; Heben, M. J. ACS Nano 2008, 2, 1968. (14) Fischer, J. E.; Dai, H.; Thess, A.; Lee, R.; Hanjani, N. M.; Dehaas, D. L.; Smalley, R. E. Phys. ReV. B 1997, 55, 4921. (15) Itkis, M. E.; Borondics, F.; Yu, A.; Haddon, R. C. Science 2006, 312, 413. (16) Ska´kalova´, V.; Kaiser, A. B.; Woo, Y. S.; Roth, S. Phys. ReV. B 2006, 74, 085403. (17) Zahab, A.; Spina, L.; Poncharal, P. Phys. ReV. B 2000, 62, 10000. (18) Na, P. S.; Kim, H.; So, H.-M.; Kong, K.-J.; Chang, H.; Ryu, B. H.; Choi, Y.; Lee, J.-O.; Kim, B.-K.; Kim, J.-J.; Kim, J. Appl. Phys. Lett. 2005, 87, 093101. (19) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 1801. (20) Jhi, S. H.; Louie, S. G.; Cohen, M. L. Phys. ReV. Lett. 2000, 85, 1710. (21) Mowbray, D. J.; Morgan, C.; Thygesen, K. S. Phys. ReV. B 2009, 79, 195431. (22) Langmuir, I. J. Am. Chem. Soc. 1916, 38, 2221.
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