Letter pubs.acs.org/NanoLett
Multilayered Carbon Nanotube/Polymer Composite Based Thermoelectric Fabrics Corey A. Hewitt,† Alan B. Kaiser,‡ Siegmar Roth,§ Matt Craps,∥ Richard Czerw,∥ and David L. Carroll*,† †
Center for Nanotechnology, Wake Forest University, Winston Salem, North Carolina 27105, United States MacDiarmid Institute for Advanced Materials and Nanotechnology, SCPS, Victoria University of Wellington, Wellington 6140, New Zealand § School of Electrical Engineering, WCU Flexible Nanosystems, Korea University, Seoul, Korea ∥ NanoTechLabs, Yadkinville, North Carolina 27055, United States ‡
S Supporting Information *
ABSTRACT: Thermoelectrics are materials capable of the solid-state conversion between thermal and electrical energy. Carbon nanotube/polymer composite thin films are known to exhibit thermoelectric effects, however, have a low figure of merit (ZT) of 0.02. In this work, we demonstrate individual composite films of multiwalled carbon nanotubes (MWNT)/ polyvinylidene fluoride (PVDF) that are layered into multiple element modules that resemble a felt fabric. The thermoelectric voltage generated by these fabrics is the sum of contributions from each layer, resulting in increased power output. Since these fabrics have the potential to be cheaper, lighter, and more easily processed than the commonly used thermoelectric bismuth telluride, the overall performance of the fabric shows promise as a realistic alternative in a number of applications such as portable lightweight electronics. KEYWORDS: Carbon nanotubes, polymer, thermoelectric power, device fabrication, electrical conductivity has a ZT ≈ 1 and a power factor of 7 800 μW m−1 K−2.6 At this performance level, it would require a CNT/polymer thermoelectric module of about 500 cm2 to generate enough power to run a standard wrist watch from a ΔT ≈ 10 K generated by body heat. This is about fifty times the area of a typical wrist watch. There are, however, several potential benefits to CNT/ polymer thermoelectrics. Carbon nanotube/polymer composites and Bi2Te3 have similar low thermal conductivities of about ∼3 W m−1 K−1.4,8 This allows for a sustained temperature difference across the film. The power per unit mass for Bi2Te3 is about 232 mWg−1, while current CNT thermoelectrics have a power per unit mass of 60 mW g−1,9 but have the potential to reach as high as 1300 mW g−1 if a ZT ≈ 0.2 is reached. If CNT/polymer thermoelectrics are produced on a large scale, the cost could be as low as $1/watt due to ease of production and low cost for materials, while currently produced Bi2Te3 thermoelectrics are ∼$7/watt.10 Additionally, CNT/polymer composites are flexible and durable, unlike crystalline thermoelectrics. It is when these benefits are considered, along with ZT and the fact that it can be improved upon, that the use of organics as thermoelectrics may be practical in applications unsuited to Bi2Te3.
T
raditional inorganic crystalline thermoelectrics such as bismuth telluride (Bi2Te3) have been studied and utilized commercially for the last half century, but recent advancements in organic thermoelectrics show promise for their use as alternatives to these materials.1 Organics typically have low electrical conductivities but they have the potential to be used as thermoelectrics as a result of the inverse relationship between the Seebeck coefficient α and electrical conductivity σ due to charge carrier concentration and mobility.2 These competing factors comprise the power factor α2σ in the dimensionless figure of merit (ZT = (α2σ/κ)T where κ is the thermal conductivity and T is temperature) and is important because it is directly related to the usable power attainable from the thermoelectric. A high ZT is achieved by creating a material with a high power factor and low thermal conductivity. This task is further complicated by the direct relationship between the electrical conductivity and the charge carrier contribution to the thermal conductivity.2 The figure of merit does not, however, include several other important considerations such as cost, weight, and processability. These additional factors allow for the consideration of organics as thermoelectric materials. Of particular interest as an organic thermoelectric are carbon nanotube (CNT)/polymer thin films due to their heterogeneous structure that allows for the slight decoupling of these thermoelectric parameters leading to an increased ZT.3−7 Currently the best nanotube/polymer thermoelectrics have a ZT ≈ 0.02 and a power factor of 25 μW m−1 K−2, while Bi2Te3 © 2012 American Chemical Society
Received: October 28, 2011 Revised: January 24, 2012 Published: February 8, 2012 1307
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Figure 1. (a) Layer arrangement for the multilayered fabric. CNT/PVDF conduction layers (B,D) are alternated between PVDF insulation layers (A,C,E). Every other conduction layer contains p-type CNTs (B), while the others contain n-type CNTs (D). The shorter insulating layers allow for alternating p/n junctions when the stack is pressed and heated to the polymer melting point of 450 K to bond the layers. Layers A−D can be repeated to reach the desired number of conduction layers N. When the film is exposed to a temperature gradient ΔT, charge carriers (holes h, or electrons e) migrate from Th to Tc resulting in a thermoelectric current I. (b) The resulting thermoelectric voltage VTEP can be read across the ends of the first and last conduction layers. (c) The thermoelectric fabric remains flexible and lightweight.
VTEP (see Supporting Information Figure S1 for a description of how VTEP is affected by different temperature gradients). Since the voltage contribution of each conducting layer is determined by its Seebeck coefficient and can be added in series due to the alternating p/n junctions, the resulting VTEP magnitude is given by
To produce sufficient power, any thermoelectric material needs to be combined into a module containing many alternating p-type and n-type legs that are connected electrically in series, and thermally in parallel. This arrangement of elements is utilized because it allows for the direct addition of the thermoelectric voltage contribution of each leg while it is subject to the same maximum available ΔT. This module architecture results in the highest attainable thermoelectric voltage for a given number of legs (N) subject to the available temperature difference ΔT. Typically, thermoelectric modules composed of bulk materials such as Bi2Te3 are arranged in a way such that the temperature difference between the two ends of the thermoelectric legs is perpendicular to the surface of the module.11 Since the CNT/polymer thermoelectric materials are thin films, however, this limits the maximum temperature difference attainable perpendicular to the surface of the film; therefore, a different geometry for connecting the legs must be adopted. In this Letter, we report on a method for constructing a thermoelectric module consisting of multiple layers of CNT/ polymer films that allows for the arrangement of the temperature gradient parallel to the surface of the module; this module arrangement results in a feltlike thermoelectric fabric. The fabrication and characterization of the single films that comprise the multilayered fabric have been reported previously.3 To form the multilayered film, individually prepared conducting and insulating layers are arranged as in Figure 1a and then bonded together by pressing the stack at the melting point of the polymer in use (about 450 K). The films used in this study were polyvinylidene fluoride (PVDF) with 95 or 20% CNTs by weight (wt %) for the conducting layers, and pure PVDF for the insulating layers. The resulting single film thicknesses were 25−40 μm, while the multilayer film thickness depends on the total number of layers. The number of conduction layers is given by N = nn + np where nn and np are the number of n-type and p-type layers, respectively. When the fabric is subject to a temperature difference ΔT = Th − Tc parallel to the surface as shown in Figure 1b, the charge carriers (holes h, or electrons e) travel from the Th side to the Tc side due to the Seebeck effect and generate a thermoelectric voltage
VTEP = [nn|a n| + n p|a p|]ΔT
(1)
where αn and αp are the Seebeck coefficients of the n-type and p-type films, respectively. A room temperature measurement of VTEP/ΔT versus the number of conduction layers N for the 95 wt % fabrics was performed, with the results shown in Figure 2a. The theoretical
Figure 2. (a) Thermoelectric voltage generated per 1 K ΔT versus the number of conduction layers N in a multilayered film composed of 95 wt % CNT/PVDF single films. The fit is calculated using eq 1 and the room temperature Seebeck coefficients of αn = −5.04 μV K−1 and αp = 10.05 μV K−1. (b) VTEP versus ΔT for a 72 layer fabric. Tc was held at room temperature while Th was increased to 390 K, at which point the fabric short circuited due to melting of the PVDF. The solid line shows that VTEP increases linearly with a VTEP/ΔT value of 550 μV K−1.
value is calculated from eq 1 using the room temperature Seebeck coefficients of 10.05 μV K−1 for αp and −5.04 μV K−1 for αn. The experimental VTEP/ΔT values follow closely to those calculated with no measurable drop off in VTEP as N is increased. Adding layers to the fabric is equivalent to adding voltage sources in series, so the limiting factor of N in practice 1308
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is the heat source’s ability to produce a sufficient ΔT throughout all N layers. Since the VTEP is proportional to ΔT because of the Seebeck effect, increasing ΔT will also result in an increased VTEP output, which is shown in Figure 2b. A 72 layer fabric was used with a VTEP/ΔT = 550 μV K−1 leading to a 51 mV output at a ΔT = 95 K. The limiting factor for ΔT is when Th = 390 K because at this point the polymer begins deforming, compromising the multilayered structure of the fabric. The temperature dependent behavior of VTEP/ΔT is also important because the fabrics will be subject to varying temperatures during use. The temperature dependence of VTEP/ΔT for the 20 wt % single films and several different multilayered fabrics was measured with the results shown in Figure 3a. The observed temperature dependent behavior of α
1. The results show that the multilayered films retain the characteristic T dependence of the single films, while still producing the expected VTEP. Electrical conductivity measurements were also performed on several multilayered fabrics to determine if layering the films in the module introduces any internal resistance to the overall film due to the p/n junctions. Figure 3b shows the results of σ versus absolute T for the N = 3, 7, 11 layer films, as well as the single film electrical conductivities. These results are typical of CNTs and CNT/polymer composites, and a previously reported thermal fluctuation induced tunneling model has been used to describe the temperature dependent behavior.15,17,18 If no additional resistance is introduced due to layering, the multilayered σ should be between the two single film conductivities. From Figure 3b, the electrical conductivity for N = 3 does fall between the single film values, but there is about a 15% drop off in σ for the N = 7, 11 modules. For the N = 11 module, this resulted in an average decrease in σ of about 1% per p/n junction. The decrease in σ is most likely due to the decreased CNT concentration in the p/n junction region. Figure 4a shows the typical CNT arrangement and concentration for a 20 wt % film, while Figure 4b shows the composition of the junction. Since the films are bonded together by pressing the junction at the melting point of the polymer, the main constituent of the junction is polymer. The decrease in σ could potentially be eliminated by forming the single films in one continuous strip with alternating p-type and n-type segments and then folding the alternating layers over, or by evaporating a high conductivity material such as indium oxide onto the segment of the film that will form the junction.19 Power measurements on the 72 layer fabric were performed for several different load resistances with the results shown in Figure 5. The ΔT was kept at a safe operating temperature of 50 K to avoid deformation of the structure at high temperatures. The maximum power generation of 137 nW occurred when the load resistance matched the internal fabric resistance of 1270 Ω. At this load resistance the VTEP was 13 mV compared to an open circuit VTEP of 26 mV at the same ΔT. Above 1270 Ω, VTEP continues to increase as it approaches the open circuit voltage, but P decreases as the load resistance becomes exponentially larger. The power output as a function of ΔT at a load resistance of 1270 Ω is shown in the inset of Figure 5 and exhibits a squared behavior due to the linear relationship between VTEP and ΔT as seen in Figure 2b. If higher power levels are required, ΔT could be increased as shown in the Figure 5 inset, and the number of conduction layers can be increased provided the heat source can supply a sufficient ΔT. For a fabric composed of 300 layers and exposed to a ΔT = 100 K, the theoretical power output could be as high as 5 μW. Further improvements to power output could potentially be made through optimization of the single film ZT. This could be done by improving the Seebeck coefficient through chemical treatment of the nanotubes,20 increasing electrical conductivity by using conducting polymers,1 or decreasing thermal conductivity by introducing phonon scattering defects along the nanotubes.21 With optimization of the single film ZT, film dimensions, and multilayer interfabric contacts and layer count for a specific application, these fabric-modules could offer a realistic alternative to current thermoelectrics for use in lightweight, flexible, and portable electronics.
Figure 3. (a) Temperature dependence of VTEP for the N = 3, 7, 11 layer fabrics, along with α for the single 20 wt % CNT/PVDF p-type and n-type films. Fits to single film data are calculated using eq 2, while multilayer fits are calculated using the single film results coupled with eq 1. (b) Temperature dependence of electrical conductivity for the N = 3, 7, 11 layer fabrics and the single 20 wt % CNT/PVDF p-type and n-type films.
for the single films is typical of CNTs and CNT composite films.12−16 This trend has been described previously using a heterogeneous model given by ⎡ ⎛ ⎞1/1 + d ⎤ T ⎥ α(T ) = bT + cT1/2 exp⎢ −⎜ 0 ⎟ ⎢⎣ ⎝ T ⎠ ⎥⎦
(2)
where b and c are constants for the metallic and semiconducting contributions, T0 is a constant related to the energy differences for hopping between nanotubes, and d is the dimensionality of hopping that depends on the morphology of inter tube contacts.13−15 The T1/2 term is exponentially weighted to represent the suppression of the semiconducting contribution to α at low T.16 The multilayered fits were calculated using the single n-type layer and p-type layer fits from eq 2 (to determine αn(T) and αp(T)) along with the corresponding nn and np values to calculate VTEP/ΔT using eq 1309
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Figure 4. (a) SEM image of the surface of one of the 20 wt % CNT p-type legs. The CNT/polymer matrix is visible with the ∼30 nm diameter tubes comprising most of the surface while the polymer is coating the CNTs and binding them together. (b) Image of the p/n junction (between green dashed lines), which was formed by heating the junction region to slightly above the polymer melting point (450 K). Nanotubes are visible in the junction, but the main constituent is polymer (brighter regions), as opposed to the surface composition in (a). (4) Yao, Q.; Chen, L.; Zhang, W.; Liufu, S.; Chen, X. ACS Nano 2010, 4 (4), 2445−2451. (5) Yu, C.; Kim, Y. S.; Kim, D.; Grunlan, J. C. Nano Lett. 2008, 8 (12), 4428−4432. (6) Kim, D.; Kim, Y.; Choi, K.; Grunlan, J. C.; Yu, C. ACS Nano 2009, 4 (1), 513−523. (7) Haggenmueller, R.; Guthy, C.; Lukes, J. R.; Fischer, J. E.; Winey, K. I. Macromolecules 2007, 40, 2417−2421. (8) Goldsmid, H. J. Proc. Phys. Soc. London, Sect. B 1956, 69, 203− 209. (9) Bux, S. K.; Blair, R. G.; Gogna, P. K.; Lee, H.; Chen, G.; Dresselhaus, M. S.; Kaner, R. B.; Fleurial, J.-P. Adv. Funct. Mater. 2009, 19, 2445−2452. (10) Rowe, D. M.; Min, G. J. Power Sources 1998, 73, 193−198. (11) Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.; Vashaee, D.; Chen, X.; Liu, J.; Dresselhaus, M. S.; Chen, G.; Ren, Z. Science 2008, 320, 634−638. (12) Baxendale, M.; Lim, K. G.; Amaratunga, G. L. Phys. Rev. B 2000, 61 (19), 12705−12708. (13) Choi, Y.-M.; Lee, D.-S.; Czerw, R.; Chiu, P.-W.; Grobert, N.; Terrones, M.; Reyes-Reyes, M.; Terrones, H.; Charlier, J.-C.; Ajayan, P. M.; Roth, S.; Carroll, D. L.; Park, Y.-W. Nano Lett. 2003, 3 (6), 839−842. (14) Carroll, D. L.; Czerw, R.; Webster, S. Synth. Met. 2005, 155, 694−697. (15) Kaiser, A. B.; Düsberg, G.; Roth, S. Phys. Rev. B 1998, 57 (3), 1418−1421. (16) Kaiser, A. B.; Park, Y. W.; Kim, G. T.; Choi, E. S.; Düsberg, G.; Roth, S. Synth. Met. 1999, 103, 2547−2550. (17) Kymakis, E.; Amaratunga, G. A. J. Appl. Phys. 2006, 99, 084302. (18) Sheng, P. Phys. Rev. B 1980, 21 (6), 2180−2195. (19) Tahar, R. B. H.; Ban, T.; Ohya, Y.; Takahashi, Y. J. Appl. Phys. 1998, 83 (5), 2631−2645. (20) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 1801−1804. (21) Che, J.; Ç ağın, T.; W., A. G. III Nanotechnology 2000, 11, 65− 69.
Figure 5. Thermoelectric power and voltage generated by a 72 layer film at a ΔT = 50 K for varying load resistances. The peak power of 137 nW occurs at a load resistance of 1270 Ω. The inset shows power versus ΔT for a load resistance of 1270 Ω. This squared behavior was expected due to the linear trend of VTEP versus ΔT in Figure 2b and the relation P = V2/R.
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ASSOCIATED CONTENT
S Supporting Information *
Effect of temperature gradient profile on VTEP and power generated. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS S. Roth acknowledges World Class University Project “Flexible Nanosystems” (WCU, R32-2008-000-10082-0) of the Korean Ministry of Education, Science, and Technology.
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REFERENCES
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