Letter pubs.acs.org/NanoLett
High-Density Carbon Nanotube Buckypapers with Superior Transport and Mechanical Properties Ling Zhang, Guang Zhang, Changhong Liu,* and Shoushan Fan Tsinghua-Foxconn Nanotechnology Research Center and Department of Physics, Tsinghua University, Beijing 100084, People’s Republic of China ABSTRACT: High-density buckypapers were obtained by using well-aligned carbon nanotube arrays. The density of the buckypapers was as high as 1.39 g cm−3, which is close to the ultimate density of ideal buckypapers. Then we measured the transport and mechanical properties of the buckypapers. Our results demonstrated that its electrical and thermal conductivities could be almost linearly improved by increasing its density. In particular, its superior thermal conductivity is nearly twice that of common metals, which enables it a lightweight and more efficient heat-transfer materials. The Young’s modulus of the buckypapers could reach a magnitude over 2 GPa, which is greatly improved compared with previous reported results. In view of this, our work provided a simple and convenient method to prepare high-density buckypapers with excellent transport and mechanical properties. KEYWORDS: Carbon nanotubes, buckypapers, CNT-based macroscopic materials, thermal conductivity
A
on the properties of the aligned buckypaper. Several effective methods, such as CNT suspension in strong magnetic fields, pushing superaligned CNT arrays down, have been developed to prepare the aligned CNT films.20,23,24 But limitations during these preparation processes, such as CNT dispersion in solution,20 rigorous experimental conditions and complex operations, raise the difficulties in increasing the density and getting good CNT alignment so as to limit the properties of the buckypapers. In this work, we fabricated a kind of aligned buckypapers with very high density, which showed very high thermal conductivity and other superior properties. By increasing the density of samples, the thermal conductivity as high as 766 Wm−1 K−1 was reached. We also indicated a strong positive relationship between the density increasing and the physical properties enhancement. That is, by increasing the density of the aligned buckypaper, the mechanical strength and conducting abilities were obviously enhanced. Accordingly, our work reported here will provide a simple and convenient method to prepare high-quality buckypapers with excellent transport and mechanical properties. The aligned buckypaper in this experiment was prepared through two main steps, that is, the preparation of aligned CNT films and the pressing process to increase the density.20,25 The CNT used in this experiment was superaligned multiwall CNTs (MWNTs) arrays fabricated by chmical vapor deposition (CVD) method. First, by spinning the MWNTs out of the superaligned CNT arrays, layers of CNT film were
ligned carbon nanotube paper, also called aligned buckypaper, made from carbon nanotubes (CNTs), because of its potential in mechanical, thermal, and electrical properties, is one of the most promising CNT-based macroscopic materials.1,2 It has been extensively investigated for attractive candidates such as actuators, electrode, transistors, supercapacitor, and composites on which novel physical and chemical properties were applied.3−7 The outstanding performance of the buckypapers is mainly attributed to several excellent properties of individual CNTs, such as the very high Young’s modulus, and the excellent thermal and electrical properties.8−11 In addition, the paperlike appearance of the buckypapers is very suitable for the flexible and conductive applications such as field-emission device, membrane, solar cell, and thermal composites.12−18 The aligned CNT buckypaper has shown high performance in heat transfers,5,19,20 which will be a kind of ideal heat-transfer material because of its lightweight and corrosion resistance. Recently, represented by the popularity of smart phone and tablet PCs, portable device hardware have a rapid development. The heat-transfer efficiency and overweighted heat-transfer materials have greatly delayed the development of portable device hardware. Nowadays, the rapid development of portable device hardware has aroused huge demand for lighter and more efficient heat-transfer materials, in which aligned buckypaper may be a kind of ideal materials for this application. However, up to now the measured properties of aligned buckypaper are significantly restricted by several reasons. As we know, the mechanical strength and thermal conductivity of buckypapers are remarkably reduced by the crookedness and agglomeration of CNTs, which also occupied the orientation space inefficiently and limit the density of buckypaper.21,22 In addition, the preparation methods also have an obvious impact © 2012 American Chemical Society
Received: June 22, 2012 Revised: August 16, 2012 Published: August 27, 2012 4848
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Figure 1. (A) Photograph and (B) SEM images of the aligned buckypaper samples with different magnifications. The arrows in the images illustrate the CNTs alignment in the buckypaper. (C,D) SEM images of the aligned buckypaper surfaces with different densities, 0.44 and 1.09 g cm−3, respectively. (E,F) SEM images of the aligned buckypaper with different densities (side view), 0.44 and 1.09 g cm−3, respectively.
obtained.25,26 The spinning direction was held in one-way, so as to ensure the well alignment of CNTs (for more spinning processes and setup details, readers can refer to ref 25 and ref 26). Second, the CNT films were divided in squares of 8 mm wide, 20 mm long, by laser cut. Third, the CNT films were drenched into alcohol to shrink the CNTs of different layers. Then the films were pressed with different pressures in the range of 20 to 30 MPa. The density of CNT paper would be modulated by the different pressures applied. Then we acquired a series of aligned CNT papers with different densities. The highest density of our samples could be as high as 1.39 g cm−3, which is close to the ultimate density 1.58 g cm−3 of ideal buckypapers. The ultimate density of ideal buckypapers is calculated by a simple hexagonal close packing model. We have to mention here that the outer and inner diameters of the typical individual MWNTs involved was 12 and 6 nm, respectively, which vary the calculated ultimate density of the buckypapers.
The microstructures of the samples were probed by scanning electron microscope (SEM; Sirion 200, resolution ∼1.0 nm). The density was computed by the mass and volume of samples, and the thickness of CNT paper was measured by a micrometer. Samples were cut into different parts and repeated measurements were carried out to obtain precise density results. The mechanical properties of samples were measured by mechanical test machine (Instron 5848 Microtester). We fixed the aligned buckypaper pieces along the length direction and stretched them with an increasing force. During the stretching, deformation of sample was recorded by the test machine and the Young’s modulus was computed out. Measurements of thermal conductivity at room temperature were carried out by a self-heating method.27 The specific testing device is shown in Figure 2A. The aligned CNT paper was cut into slices with width of 2 mm and a length of 20 mm. Then these slices were hanged over two aluminum substrates. The aluminum substrates were placed on an insulated wood 4849
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substrate. The aligned CNT paper slices were adhered to the aluminum substrates by high-purity silver paste in order to get a perfect contact condition. We added a DC voltage on the substrate and the aligned CNT paper samples will be selfheated. When we changed the DC voltages in the range of 0.05−0.5 V, the heating temperature of aligned CNT paper samples would correspondingly change. The highest heating temperature was about 340 K and the heat loss to the surrounding air was tiny; the error caused by the heat loss is about 1.9−3.4% and is in the range of our experimental error. By measuring the temperatures of middle point and contacted point on substrate with an Optris LS infrared thermometer, we could figure out the thermal conductivity of samples.20 The whole testing device was put in an airtight testing environment in order to prevent the effect of air flow. What’s more, by measuring the current and voltage of samples, electrical conductivities could be computed out. On macroscopic, the surface of aligned buckypapers looks very smooth and glossy (Figure 1A). This appearance indicates the high-density arrangement of CNTs in the buckypapers. Huge bunches of CNTs formed the clear texture on the surface of the sample. Scanning electron microscopy (SEM) images show the specific CNTs arrangement on the surface of samples. The length of these individual CNTs is about several hundred micrometers, which is much larger than the scale of figures. This is why connected parts of individual CNTs were barely observed in our figures. In the microstructure, almost no agglomeration and crookedness is shown, and the space among the CNTs is at a relatively low level (Figure 1B). A fine alignment of CNT bundles is shown clearly in a larger scale (inset figure of Figure 1B). The microstructure of the buckypapers with different densities 0.44 and 1.09 g cm−3 was illustrated in Figure 1C,D (top view), Figure 1E,F (side view), respectively. But the appearances show little difference in the top view (Figure 1C,D). Although agglomeration and crookedness could be observed due to the alcohol dropping process during the preparation, the overall CNTs alignment of top view is apparently evident. While seen from the side view, it is obvious that the sample with larger density (Figure 1F) has a more closely CNTs arrangement. The density difference was mainly determined by the distance between neighboring CNT layers. Higher density samples share space more efficiently between layers than smaller ones. The comparison of thermal conductivities of aligned buckypapers with different densities at the room temperature is shown in Figure 2B. The error bars denoted the data uncertainty for the measurements. The density range of these samples was from 0.81 to 1.39 g cm−3. Accordingly, the value of the thermal conductivity of the sample with density 0.81 g cm−3 is 472 ± 68 W m−1 K−1, and the value increased to 766 ± 77 W m−1 K−1 when the density increased to 1.39 g cm−3. Additionally, measurement uncertainty is increasing as the measured values growing, but the largest relative uncertainty was below 20%. The heat-transfer properties of CNT-based materials have always been a hot research topic. Individual CNT has very high thermal conductivities both in theoretical and experimental conditions.28,29 In this work, we get the result that the aligned buckypaper inherits the high thermal performance of individual CNTs. Moreover, compared with previous reports on aligned or random buckypaper, this study suggested that the high performance of aligned buckypaper can be realized by increasing the density of it.5,14,20
Figure 2. (A) Schematic illustration of the self-heating and testing measurements of the thermal conductivity of the aligned buckypapers in this study. (B) Thermal conductivities of the buckypapers as a function of sample density. (C) Thermal conductivities of individual CNT in the buckypapers as a function of density.
In order to identify the relation between the specific thermal conductivity and the density of aligned buckypaper, we roughly computed the thermal conductivity of individual CNTs in the aligned buckypapers (Figure 2C). It related to the specific quantity of heat conducted by the individual CNT. The computed value fluctuated in the range of 960−1140 W m−1 K−1, which is close but slightly lower than our previously reported value.30 In the Figure 2C, the thermal conductivities of individual CNTs keep a relative stable value at different densities, and this demonstrated that the thermal conductivity of individual CNTs in the buckypapers did not varied apparently with the density enhancement. Here we’d like to give a short discussion on these results. On one hand, in unit cross-section area more CNTs will decrease the gap between contiguous individual CNTs and could 4850
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Figure 3. (A) Young’s modulus of the aligned buckypaper as a function of sample density. These data include results of our work and several reference works. We have to mention here that we could not get densities of buckypapers of refs 21, 32, and 33, which are marked by different horizontal lines. (B) Electrical conductivity of the buckypapers as a function of sample density.
from 31 900−64 000 S m−1. The obvious trend reveals similar increase in electrical conductivity with density. From the measured value 31 900 ± 5500 S m−1 at the density of 0.77 g cm−3 up to a higher value of 63 800 ± 6300 S m−1 at the density of 1.39 g cm−3, the rising density promoted the electrical conductivity with more than 100% increment. Measurement errors were also illustrated in the figure. According to the experimental data, significant positive correlation of electrical conductivity could be determined even when the measurement errors are taken into account. Compared with the previous work, the aligned buckypaper in this paper showed a considerable improvement on the electrical conductivity.19,20 However, considering the value of individual MWNT, there still exists a difference of magnitude.38,39 This disparity could be attributed to the discontinuity between the individual CNTs in the buckypapers. In conclusion, we reported an approach to fabricate the aligned buckypapers with ultrahigh density. The density of these buckypapers could be as high as 1.39 g cm−3, which is close to the ultimate density of ideal buckypapers. Further, these buckypapers have shown superior properties including the excellent thermal conductivity, electrical conductivity and Young’s modulus. By increasing the density to as large as 1.39 g cm−3, we got the thermal conductivity 766 ± 77 W m−1 K−1 along the axis direction, which is remarkably larger than any conventional hear-transfer metals. The Young’s modulus could reach a magnitude over 2 GPa, which is greatly improved compared with previous reported results. Besides, the Young’s modulus and thermal and electrical conductivity of the buckypaper showed an obvious positive correlation with the density of the sample. We attribute these behaviors to tight alignment and orderly orientation of CNTs. By increasing the amount of CNTs in unit space, the mechanic strength and conducted ability of aligned buckypaper are enhanced. Owing to their superior properties, these high-quality buckypapers may have broad application potential as energy conservation materials.
transfer heat more efficiently. This is why increasing density could improve the thermal conductivity performance. On the other hand, the heat conduction also relies on the good orientation of CNTs in the aligned buckypaper. The measured values of thermal conductivities of the aligned buckypapers are superior to that of aluminum or copper metal, which is 217 W and 397 W m−1 K−1, respectively. Aluminum and copper are conventionally used as heat-transfer materials. The buckypapers obtained here possess the superior thermal conducting property. With its much lightweight and relatively convenient preparation process and applicable paper-shape, these buckypapers have an application perspective as heat-transfer materials. The correlation between Young’s modulus and density of the aligned buckypaper is illustrated in Figure 3A. Within the density range of 0.43−1.29 g cm−3, the measured value of Young’s modulus was in the range of 280−2200 MPa. The sample with density 0.43 g cm−3 has a Young’s modulus 283 MPa. When the density increased to 1.29 g cm−3, the measured result is 2190 MPa correspondingly. Several remarkable results of the Young’s modulus of buckypaper have been reported in previous works3,21,31−34 and we listed these results in our Figure 3A. These results were achieved with different buckypapers that is prepared by different methods and different carbon nanotubes. For example, by using the CNT sheet and reinforcement of polymer, Coleman et al. reported CNT sheets sharing modulus around 2 GPa.31 In our experiment, the Young’s modulus could reach a magnitude over 2 GPa, which is greatly improved compared with previous results.3,35 This is also in agreement with previous theoretical researches.36,37 Besides, we have to mention here the Young’s modulus of buckypaper showing an anisotropy characteristic, which is due to the CNTs having a stronger mechanical property along the axis than radial direction. In the radial direction, different individual CNTs are connected by van der Waals force, which is relatively weaker than the chemical bond force in CNT. On the other hand, the experimental value of Young’s modulus was reported as high as 1.8 TPa for the individual MWNTs, which is 1000 times larger than our results. This can be attributed to the weak contact force between individual CNTs along the axis direction. We further studied the electrical conductivity of the aligned buckypaper with different density at the temperature 300 K (Figure 3B). The sample density differs in the range of 0.77− 1.39 g cm−3 and the electrical conductivity changes in the range
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Telephone: 86 10 62796011. Notes
The authors declare no competing financial interest. 4851
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(29) Kim, P.; Shi, L.; Majumdar, A.; McEuen, P. L. Phys. Rev. Lett. 2001, 87, 215502. (30) Li, Q. W.; Liu, C. H.; Wang, X. S.; Fan, S. S. Nanotechnology 2009, 20, 14570214. (31) Coleman, J. N.; Blau, W. J.; Dalton, A. B.; Munoz, E.; Collins, S.; Kim, B. G.; Razal, J.; Selvidge, M.; Vieiro, G.; Baughman, R. H. Appl. Phys. Lett. 2003, 82, 1682−1684. (32) Zhang, X.; Sreekumar, T. V.; Liu, T.; Kumar, S. J. Phys. Chem. B 2004, 108, 16435−16440. (33) Dettlaff-Weglikowska, U.; Skákalová, V.; Graupner, R.; Jhang, S. H.; Kim, B. H.; Lee, H. J.; Ley, L.; Park, Y. W.; Berber, S.; Tománek, D.; Roth, S. J. Am. Chem. Soc. 2005, 127, 5125−5131. (34) Hall, L. J.; Coluci, V. R.; Galvão, D. S.; Kozlov, M. E.; Zhang, M.; Dantas, S. O.; Baughman, R. H. Science 2008, 320, 504−507. (35) Xu, G.; Zhang, Q.; Zhou, W.; Huang, J.; Wei, F. Appl. Phys. A 2008, 92, 531−539. (36) Zaeri, M. M.; Ziaei-Rad, S.; Vahedi, A.; Karimzadeh, F. Carbon 2010, 48, 3916−3930. (37) Cranford, S. W.; Buehler, M. J. Nanotechnology 2010, 21, 265706. (38) Ebbesen, T. W.; Lezec, H. J.; Hiura, H.; Bennett, J. W.; Ghaemi, H. F.; Thio, T. Nature 1996, 382, 54−56. (39) Ando, Y.; Zhao, X.; Shimoyama, H.; Sakai, G.; Kaneto, K. Int. J. Inorg. Mater. 1999, 1, 77−82.
ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (2012CB932301) and the Natural Science Foundation of China (51173098, 10721404).
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