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Keywords:

  • chemical doping;
  • expanded graphite;
  • Seebeck coefficient;
  • thermoelectric modules

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

This report demonstrates application of expanded graphite (ExG) for thermoelectric energy conversion, where it serves as a filler for both p- and n-type organic materials. Thin ExG composite films showing improved thermoelectric properties were prepared. In particular, composites with intrinsically conducting polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) yielding high electrical conductivity (up to 104 S m−1) and enhanced thermopower (Seebeck coefficient) provided promising p-type material. Chemical doping experiments performed on ExG dispersed in polyvinyl alcohol (PVA) revealed that the exfoliated graphitic sheets can be efficiently n-doped with polyethyleneimine (PEI). As a result, n-type ExG/PVA/PEI composite thin films showing improved n-type characteristics with thermopower values as high as −25.3 µV K−1 were prepared. With a 25 wt% ratio of PEI to ExG, the electrical conductivity was measured to be ∼103 S m−1, which is remarkably high for n-type polymer composites. Strips of composite films containing 50 wt% of ExG in PEDOT:PSS were used as p-type components, and composite films containing 20 wt% of ExG in PVA doped with PEI were used as n-type components in thermoelectric modules to demonstrate thermoelectric voltage with one, two, and three p-n couples connected in series. The testing modules produced an output voltage of ∼4 mV at a temperature gradient of 50 K. The module generated 1.7 nW power, when a load resistance matched the internal module resistance of 1 kΩ. Our results show that chemical functionalization of ExG in thin composite films resulted in more effective thermoelectric properties.

1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

The crystal structure of graphite is a layered network of covalent bonded carbon atoms. Within the same plane, three of the four valence electrons of each C atom are localized in sp2 hybrid orbitals, but the fourth is in a 2p orbital perpendicular to the plane. These 2p electrons are delocalized in each plane of C atoms. Overall, the bonds within the planes are strong covalent bonds, while the forces operating between adjacent planes are only weak van der Waals interactions. The large interlayer distances and weak forces allow the layers to glide over one another relative easily. These features enable graphite to be used as a solid lubricant and as a writing utensil. The electrical properties of graphite follow directly from its crystal structure. The electrical conductivity is direction-dependent; in a direction parallel to the layers, the electrical conductivity is ∼106 S m−1 but ∼1 S m−1 in a direction perpendicular to the layers.

Graphite is a versatile material widely used in industrial scale engineering due to its extraordinary physical and chemical properties. Its electrical conductivity, which approaches that of a metal, is extensively used in electrodes for batteries and electrolysis reactions. The extremely high thermal stability of graphite (700 K in air and more than 3000 K in inert atmosphere) has an application in the fabrication of crucibles to hold molten metal. High thermal conductivity is exploited in graphite heat sinks for laptop computers, which keep them cool while saving weight. Graphite is also used as electrically conductive filler for manufacturing conductive polymer composites [1, 2]. Although forces of attraction between individual layers in graphite are only weak van der Waals forces, the exfoliation of graphite into randomly dispersed mono- or few-layers is a challenging task. Thus, several chemical intercalation procedures have been developed to support the separation of graphitic layers [3]. The following rapid heating of the graphite intercalation compounds causes individual carbon layers to move apart and expansion of graphite crystals by up to hundreds of times in the direction perpendicular to the layers [4, 5]. The resulting expanded graphite (ExG) with low density and high temperature resistance is an important industrial raw material. Easily processed by lamination or compression, it is used for fabrication of large sheets and shaped parts, such as gaskets, and seals for high-temperature applications [6].

Thermoelectric materials, which directly convert thermal energy into electrical energy, currently attract much attention due to their applications in solid state cooling and power generation from waste heat [7-9]. The efficiency of the thermoelectric devices is determined by the thermoelectric figure of merit, ZT, which is defined as ZT = S2σT/κ, where S, σ, T, and κ represent the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively [10]. Crystalline graphite showing high electrical conductivity, low Seebeck coefficient, and high thermal conductivity has a low ZT value at room temperature, and thus is not suitable for thermoelectric applications. However, it is expected that its thermoelectric properties can be significantly modified by exfoliation. The ExG/polymer thin films, which maintain the high electrical conductivity of exfoliated carbon layers and low thermal conductivity of polymers could be advantageous for thermoelectric applications. Moreover, ExG sheets can be chemically doped to change the thermoelectric characteristics of ExG from p- to n-type, as has been recently reported [11]. Currently, research on ExG/polymer composites is focused on improving the mechanical and electrical properties of the polymer, however, the thermoelectric properties of ExG/polymer composites are completely ignored.

In this paper, we explored using ExG as a filler for thin composite films, and assessed its potential to be used for thermoelectric applications. The following sections focus on the preparation of the composite films, determination of their electrical conductivity and the Seebeck coefficient, and a discussion about achieved results considering corresponding data reported for the multi-walled carbon nanotube (MWCNT) composite films [12].

2 Materials and methods

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

ExG used in this study was fabricated by exfoliation of natural graphite crystals via intercalation of sulfuric and nitric acids and thermal decomposition of intercalation compounds at SGL Carbon, Meitingen, Germany. The microstructure of the provided material was investigated using an optical microscope (Fig. 1a) and a scanning electron microscope (SEM, Hitachi S-4800, Fig. 1b). The composite films were prepared by casting a dispersion of ExG in polymer (Fig. 1c). Bisphenol A polycarbonate (PC) in the form of pellets and polyvinyl alcohol (PVA) as a powder were purchased from Aldrich. Poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) was purchased from H. C. Starck (Clevios) as a suspension containing 1.3 weight % (wt%) of solids (0.5 wt% PEDOT and 0.8 wt% PSS) in water.

image

Figure 1. (a) Optical micrograph of a part of an expanded graphite particle, and (b) its SEM image; (c) dispersion of 10 wt% of ExG in PVA/PEI solution; (d) photograph of an ExG/PEI composite thin film; (e) photograph of a graphite foil obtained by lamination of ExG.

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To prepare the ExG dispersions, PEDOT:PSS was used as purchased, PC was dissolved in chloroform, and PVA was dissolved in water. A corresponding quantity of ExG was dispersed in the polymer suspensions using an ultrasonic probe homogenizer working in a pulsed mode (1/s) within 45 min. The resulting homogeneous viscous slurry was poured onto a pre-cleaned glass plate (ExG/PC, ExG/PVA composites) or onto a poly(ethylene terephthalate) (PET) foil (ExG/PEDOT/PSS composites). After drying for more than 24 h at ambient conditions, composite films were peeled off from the glass surface and used as free standing films (Fig. 1d). Composites with PEDOT:PSS were characterized as a PET foil coatings (coating thickness ∼5 µm). In order to prepare the ExG/PVA/polyethyleneimine (PEI) composites, ExG was dispersed in PVA. Afterwards, a corresponding amount of PEI was added and the slurry was dried under ambient conditions. The resulting thin films (∼30 µm) were washed with water to remove the excess of PVA coated on the surface and subsequently dried before carrying out electrical measurements. Furthermore, ExG flakes were compressed into flexible graphite foils, which were used as reference samples for the composite films. The foils available under the commercial name Sigraflex® (0.18 mm thickness and 1.35 g cm−3 density) were used as received (Fig. 1e).

Electrical conductivity measurements were carried out by cutting the composite films into strips, typically 10 mm × 30 mm. Each film sample was pressure-contacted with four parallel metallic wires to perform a standard four probe measurement at room temperature. A Keithley 238 current source and a 34401A multimeter form Hewlett-Packard (HP) were used to perform current–voltage (IV) sweeps. Electrical conductance was then obtained by taking the slope of the IV curve and converting it into electrical conductivity by multiplying it by a geometric factor. To determine the Seebeck coefficient, a temperature gradient (ΔT) was generated along a film strip by heating one end of the strip and leaving the other end exposed to air. Two platinum (Pt 100 Ω) resistors were clamped with electrical contacts at the ends of the sample strips to measure the sample temperature. The Seebeck coefficient was calculated from the voltage ΔV generated by ΔT using S = −ΔVT.

Thermoelectric p–n junction modules were fabricated by mechanically compressing the ends of the p- and n-type ExG composite strips. The composite films containing 50 wt% of ExG in PEDOT:PSS were used as p-type components, and the composite films containing 20 wt% of ExG in PVA doped with 25 wt% of PEI were used as n-type components. Modules composed of one, two, or three p–n junctions were arranged to produce an alternating assembly of p-type ExG/PEDOT:PSS composite films and PEI-doped n-type composite films with a PET insulating film in between them. In order to test the module composed of three p–n junctions, ΔTs were applied across the module, while the total output voltage (VTEP), current and power generated for several load resistances were determined.

3 Results and discussion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

As presented in the optical image in Fig. 1a, loosely bonded, porous, and worm-like rods were formed after expansion of graphite crystals in a direction perpendicular to the layers as a result of intercalation followed by rapid heating. Exfoliation after expansion created a kind of a honeycomb-like microstructure with inter-lamellar bubbles as revealed by the observation of the SEM images. Figure 1b shows wrinkled graphitic sheets consisting mostly of few-layer graphene, which has different electrical characteristics from that of bulk graphite [13]. The fluffy lamellar structure promotes dispersion of graphitic layers in solvents, in particular, if polymers are present as suspension stabilizers. An example of a homogeneous suspension prepared by ultrasonic treatment of ExG in an aqueous solution of PVA is shown in Fig. 1c. Figure 1d represents thin free standing composite films which were prepared by suspension casting on a smooth glass surface. A strip cut from the film (10 mm × 30 mm), was used for thermoelectric characterization. Flat, smooth, and flexible graphite foils were also formed by rolling and compressing of ExG flakes without a binder due to its ability to be mechanically interlocked. Figure 1e presents a piece of a graphite foil cut from a 295 mm × 210 mm × 0.18 mm large graphite sheet.

Figure 2a and b shows the electrical conductivity and the Seebeck coefficient of the selected ExG composite films measured at room temperature in the film plane together with corresponding data from three metallic foils consisting of copper, lead, constantan, and an ExG foil as a reference sample. The remarkably high electrical conductivity (∼105 S m−1) of the ExG foil is almost the same as that of metals. ExG foils, also, exhibit thermoelectric effects, but the thermoelectric power expressed by the Seebeck coefficient is relative low (10.4 µV K−1), similar to that of lead. Due to the direct relationship between the electrical conductivity and the charge carrier contribution to the thermal conductivity, a high in-plane thermal conductivity (1.8 × 102 W mK−1) was determined at room temperature for the ExG foil. This resulted in a low figure of merit ZT ∼2 × 10−5. By comparison, the most commonly used thermoelectric material, bismuth telluride, has a ZT ∼1. However, it is well known that the thermal conductivity of composites can be reduced by more than two orders of magnitude, if the conducting filler is dispersed in an insulating matrix. Therefore, the ExG/polymer composites were prepared and investigated. Figure 2a reveals that the high electrical conductivity of ExG is preserved in the thin composite films with typical insulating polymers, like PC and PVA, suggesting formation of an electrically interconnected network of graphitic layers within the composite films. High electrical conductivity is an important property required for harvesting current from thermoelectric devices. Thin ExG composite films with intrinsically conductive polymer PEDOT:PSS have the best transport properties with the highest electrical conductivity (∼104 S m−1) and an improved Seebeck coefficient (12.2 µV K−1). An improvement in the thermoelectric behavior of polymer composites with PEDOT:PSS was already reported for carbon nanotubes (CNT) filled polymer composites [14, 15]. Also here, the particles of conducting polymers filling the layer-to-layer spacings enhance the conductivity of the ExG composite films. On the other hand, thermal transport through the PEDOT:PSS junctions remains comparable with typical polymeric materials due to the dissimilar bondings and vibrational features between graphite and PEDOT:PSS. The diagrams shown in Fig. 2 also demonstrate that ExG sheets embedded in polymer can be efficiently doped with PEI providing a promising thermoelectric material exhibiting improved n-type characteristics (−21.5 µV K−1 for 10 wt% ExG in PVA doped with 50 wt% of PEI).

image

Figure 2. (a) In-plane electrical conductivity of metals, ExG foil, ExG thin composite film with a 1:1 weight ratio between ExG and PEDOT: PSS, composite films with PC, PVA containing 10 wt% of ExG, and also doped with PEI with a ExG:PEI weight ratio 1:1, (b) Seebeck coefficients of the corresponding samples.

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The influence of the PEI concentration on the thermoelectric properties of the thin ExG composite films has been systematically studied for composites with a fixed concentration of ExG (20 wt%). Figure 3 shows the effect of increasing the doping level by increasing the weight ratio of PEI with respect to ExG dispersed in PVA. The observed changes are thought to be due to the n-type doping of PEI molecules adsorbed on the surface of ExG sheets, which provide electrons through the lone-pairs of nitrogen atoms of imine functional groups. The remarkable high electrical conductivity of the ExG/PVA film doped with 25 wt% of PEI reached value of 1000 S m−1, which is an increase by factor of 6 by comparison with that of undoped ExG/PVA. The fluctuations in electrical conductivity of doped samples are obviously caused by formation of the surface charge transfer complexes on the ExG sheets with different stoichiometry, which determine the charge redistribution and affect electrical conductivity. However, further investigations are required to fully clarify and understand the observed effect. The interactions of graphitic layers with PEI convert thermopower from p- to n-type but maintain high electrical conductivity within the range 102–103 S m−1. Improved n-type characteristics with thermopower values as high as −25.3 µV K−1 and a high electrical conductivity of ∼103 S m−1 was achieved for doping level of 25 wt%. The effect of n-type doping in ExG yielding high electrical conductivities is very different from that observed for CNT, where doping with PEI caused strong reductions in electrical conductivities [16].

image

Figure 3. Electrical conductivities and Seebeck coefficients of 20 wt% ExG/PVA composite films doped with increasing ExG/PEI weight ratio.

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Thus, the electrical conductivity measured for the n-type ExG composite films appear to be more than 40 times higher than those of typical polymer composites filled with n-doped carbon materials [17]. This could be explained by the two-dimensional transport properties in few-layer graphene sheets embedded in polymer. The electrons provided by the functional imine groups of PEI to the sheets are not localized like in confined one-dimensional carbon nanomaterials but rather delocalized over the whole layers.

With an estimated thermal conductivity κ of ∼0.3 W mK−1, which is typical for polymer composites [14, 18], ZT is calculated to be ∼6 × 10−4 at 300 K for 20 wt% ExG/PVA/PEI composite film, and ∼1.4 × 10−3 for ExG/PEDOT:PSS (1:1) composite films. These values of ZT are significantly enhanced compared to that of a pure ExG foil (∼2 × 10−5).

The combination of high electrical conductivity and high negative thermopower values are desired for an n-type thermoelectric material needed in p–n module devices to produce sufficient power. Thus, the ExG composite films doped with 25 wt% of PEI served as n-type components, while composite films containing 50 wt% of ExG in PEDOT:PSS were used as p-type components in the fabricated thermoelectric modules. The open circuit thermoelectric voltages VTEP produced by modules with one, two, and three p–n couples as a function of temperature gradients applied across the modules are illustrated in Fig. 4. All three devices demonstrate the linear dependence of VTEP according to the Seebeck effect and confirmed that the electrical contacts between the individual strips remained intact. For example, a temperature gradient of 70 K provided an output voltage of ∼2.5 mV. When more composite films were added in series, the generated voltage increased proportionally as the sum of the voltage contributions from each film, up to ∼6.5 mV for three couples.

image

Figure 4. Thermoelectric voltage generated from p–n junctions as a function of temperature gradient. One, two, and three p–n junctions were connected in series for testing.

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It is interesting to note that the single p–n junction composed of two electrically contacted composite films containing 50 wt% of ExG as the p-type component, and 20 wt% of ExG in PVA n-doped with PEI as the n-type component generated a VTEP as large as 35 µV per 1 K ΔT. By comparison, a recently reported [12] single p–n junction fabricated from two polyvinylidene fluoride films with 95 or 20 wt% of MWCNTs produced a VTEP of ∼15 µV per 1 K ΔT. This improved voltage, by more than a factor of 2, is attributed to a larger Seebeck coefficient (−25.3 µV K−1) produced by the chemical functionalization of ExG with PEI. A key additional consideration for using ExG instead of CNTs in composite thermoelectric materials would be the low manufacturing cost of ExG.

In order to determine the power produced by the three p–n junction device, voltages and currents from the device were measured as a function of load resistance when a temperature gradient of ∼50 K was applied (Fig. 5). Maximum power generation (1.7 nW) occurred when the load resistance (1 kΩ) matched the internal module resistance. At this load resistance, the VTEP was ∼4.2 mV compared to an open circuit VTEP of 5 mV. When the load resistance ≥1 kΩ, VTEP increased continuously, approaching open circuit voltage, but the power decreased with an exponentially increasing load resistance. The usable power attainable from this thermoelectric device is still low. However, considering that commercial modules consist of hundreds of p–n junctions with a correspondingly greater power, and the higher power output per unit mass in comparison to the traditional inorganic thermoelectric materials, the modules based on ExG composite films could be considered for thermoelectric applications. This simple and low cost fabrication process producing thin ExG composite films in which the thermoelectric properties can be easily modified by chemical functionalization could play an important role in developing polymer based thermoelectric materials.

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Figure 5. A plot of the power and voltage produced by a module with three p–n junctions as a function of loaded resistance with an applied ΔT = 50 K.

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4 Conclusions

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

In this paper, we explore a more economical source of thermopower by investigating ExG composite thin films. We demonstrated that ExG can be sufficiently dispersed in polymer solution to form stable suspensions. Thin composite films were prepared by casting and drying the suspension. Electrically and thermally insulating polymers, PC and PVA, and an intrinsically conductive polymer PEDOT:PSS served as a matrix material in this study. We also demonstrated that the thermoelectric properties of ExG in polymers can be easily modified by chemical functionalization. In particular, the ExG composite films in PVA showing n-type characteristics were prepared using PEI as the n-type dopant. High electrical conductivity was measured for the n-type composite films, >40 times than those of typical polymer composites with n-doped carbon nanomaterials. High electrical conductivity of ExG sheets was also reflected in the electrical conductivity of the composite films with PEDOT:PSS. Based on our results achieved on the individual ExG composites films, thermoelectric modules composed of p- and n-type films were fabricated and tested. A single p–n junction generated a voltage of ∼35 µV per 1 K. The voltage output increased linearly with additional p–n junctions and with increasing temperature gradients.

Acknowledgements

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

This work was supported by World Class University Project (WCU, R32-2008-000-10082-0) through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology.

References

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References