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Materials Science for Energy Technologies 2 (2019) 551–555
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Materials Science for Energy Technologies
CHINESE ROOTS GLOBAL IMPACT
journal homepage: www.keaipublishing.com/en/journals/materials-science-for-energy-technologies
Flexible thermoelectric cells fabricated by rubbing-in technology with rubber-carbon nanotubes/graphene composites Khasan S. Karimov a,b, Noshin Fatima a,⇑, Khalid J. Siddiqui a, Muhammad I. Khan a a b
GIK Institute of Engineering Sciences and Technology, Topi, District Swabi, KPK 23640, Pakistan Center for Innovative Development of Science and New Technologies of Academy of Sciences, Aini 299/2, Dushanbe 734063, Tajikistan
a r t i c l e
i n f o
Article history: Received 18 March 2019 Revised 3 June 2019 Accepted 4 June 2019 Available online 5 June 2019 Keywords: Flexible thermoelectric cell Carbon nanotubes Graphene Energy conversion Rubbing-in technology
a b s t r a c t In this research, CNT and graphene-rubber composites based thermoelectric cells were fabricated by using the rubbing-in technology. However, the comparative analysis of both elastic thermoelectric cells was also carried out. The carbon nanotubes and graphene composites with rubber were used as an active layer of thermoelectric cells. The films were deposited over rubber substrate (length 7 mm and width 5 mm) and pressure of (7–12) g/cm2. The electric dependence of the cells i.e., voltage (open-circuited), current (short-circuited), resistance and Seebeck coefficient with respect to averaged temperature (with gradient = 10 °C) was measured in the temperature interval of 30 °C–55 °C. The CNT/Graphene-rubber composite based active layer properties were investigated and results showed that with an increment in average temperature, there is an increase in current and voltage as well. In case of short circuit current, the increase in CNT-rubber and graphene-rubber composites was 1.82 and 4.5 times while an increase in open circuit voltage was 1.75 and 6.5 times, respectively. It was also found, that the graphene-rubber composites cells had a higher thermoelectric coefficient as compared to CNT-rubber thermoelectric cells. These cells can be potentially utilized as flexible and vibrations free thermoelectric sensors for measurement of the gradient of temperature and elastic thermoelectric modules for low power applications. Ó 2019 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-ncnd/4.0/).
1. Introduction Nowadays researchers are focusing on flexible devices, which produce electricity and can be bent or stretched during their normal use [1,2]. Usually, electronic devices are fabricated on rigid and flat substrates like the circuits that run smartphones computers, high-temperature power electronics, and solar panels [3,4]. However, they are expensive, limited in shapes and it is difficult to implement them in contoured forms and into soft materials, such as rubber, paper or cloth [5]. Circuits which are mounted over flexible substrate like PEEK, conductive film over polyester, even the human body are known as flexible electronics [6]. Due to the flexible nature, the size and weight of the devices are reduced.
⇑ Corresponding author. E-mail address: [email protected] (N. Fatima). Peer review under responsibility of KeAi Communications Co., Ltd.
Production and hosting by Elsevier
The flex circuits are foldable, hence along with small sizes the packaging have less propensity to break. However, some of the disadvantages of flex devices are that they are difficult to assemble and their repair and rework are usually challenging or unmanageable [7]. Gille and Hansen discovered and patented one of the earliest flex circuits in 1903, they coated a conductive metal layer over paraffin paper [8]. Flex circuits from the same era were specified in books of Thomas Edison, in which patterns on linen paper were coated with the powder of graphite and gum of cellulose [9], although confirmation of its implementation is not found. However, to cover day to day increase in power consumption the researchers are highly interested to achieve such goals through which they can use natural sources for power production such as utilizing the thermal energy and its conversion to electrical energy [10]. A smarter way to achieve such generators that convert heat energy to electrical energy is thermo-electric cells or generators. Exploring different composites by using unique techniques and their comparisons can help in the selection of good thermoelectric materials with high efficiency. It could be used to provide electricity to overcome power deficiency.
https://doi.org/10.1016/j.mset.2019.06.001 2589-2991/Ó 2019 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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K.S. Karimov et al. / Materials Science for Energy Technologies 2 (2019) 551–555
Recently, organic and inorganic semiconductors thermoelectric devices i.e., generators and cells are fabricated and characterized for their Seebeck effect intensively due to the increasing demand of power consumption [11–13]. In the case of semiconductors, the Seebeck effect of thermoelectric cells which converts heat energy to electric energy plays an important role in modern power technology [14,15]. Thermoelectric efficiency (Z) can be obtained from Eq. (1) as follows [16]:
Z¼
a2 r
ð1Þ
K tot
where, a, r represents the Seebeck coefficient and the electrical conductivity, while K tot ¼ K el þ K ph is the sum of thermal conduction of electron ðK el Þ and phonon ðK ph Þ known as total thermal conductivity. The increment in the efficiency of thermoelectric cells is dependent on the decrease of phonon thermal conductivityðK ph Þ. This way, a number of new material including CNT and graphenerubber composites were investigated intensively [17]. Hu et al. determined a CNT based thermo-electrochemical cell using thermal energy from harvested wastage [18]. Sahu et. al., designed a thermoelectric voltage booster generator and achieved up to 50% efficiency [19]. A CNT/graphene-based thermoelectric polymer nanocomposite for low-temperature thermoelectric applications was investigated in Ref. [20]. Another CNT composite based thermoelectric cell was fabricated and characterized, results showed that pure CNT, PANI and CNT/PANI nanocomposite have Seebeck coefficients of 12.2 mV/K, 2.74 mV/K and 22.4 mV/K at 300 K [21]. One more CNT composite based thin film temperature gradient sensor was investigated in the range from 5 to 38 °C where change observed in voltage, current and Seebeck coefficient by a factor of 5, 5.3 and 1.3 times, where the negative sign is indicating decrease [22]. Concerning the fabrication of flexible thermoelectric cells and generators the problem is in an initial state, however, its future seems to be promising due to the facts that they have relatively simple inherent technology and elastic in nature. At the same time, the fabrication of the flexible thermoelectric cells would be entirely different with respect to rigid thermoelectric cells due to a different principle of the devices, their structure and fabrication technology. In continuation of our earlier efforts where thermoelectric cells were fabricated and characterized based on CNT [22] and bismuth telluride composites with CNT and graphene [23], the present work also fabricates thermoelectric cells based on CNT and graphene but with the flexible substrate and rubbing-in technology. 2. Experimental 2.1. Fabrication of thermoelectric cell Initially the commercially produced (Sun Nanotech Co ltd., China) CNT and graphene were used without further purification.
Fig. 1. Schematic diagram of the rubbing-in technology for deposition of rubbergraphene/CNT film.
The thermoelectric cells were fabricated by depositing CNT and graphene thin layers on the rubber substrate with a pressure of (7–12) g/cm2 by using rubbing-in technology as shown in Fig. 1. The films deposition time was 2 min. The material disperse was controlled by spreading 30–40 mg of material over the rubber substrate and the load movement in two perpendicular directions results in deposition of the thin film. Under the condition of strain (produced by load) the size of rubber pores expands making the deposition process easy. Then when the load is removed the pores resize themselves, squeezing the material particles within the substrate resulting in material and rubber composite. The material ratio with rubber was 80 wt% and 20 wt%, respectively. Two groups of thermoelectric cells were prepared: first group was based on the composite of CNT and rubber active layer fabricated by rubbing pure CNT over rubber substrate by rubbing-in technology, while in the second group, graphene and rubber composite based cells were achieved by similar method as mentioned earlier only by using graphene instead of CNT. Later both types of samples were tested as thermoelectric cells and both physical morphology and electronic results were obtained. For microscopy, the AFM was used. Fig. 2 (a) and (b) show 3D images of CNT-rubber and graphene-rubber based samples. The samples were characterized using the atomic force microscope with the model FLEX-AFM, manufactured by NANOSURF. The AFM of CNT-rubber sample shows a relatively smooth surface profile as compared to that of graphene-rubber based sample. The CNT-rubber based sample shows a roughness of 0.267 mm and graphene-rubber surface roughness is 0.295 mm. This difference of surface morphology may be attributed to the initial surface roughness of the rubber substrate. It is expected that the roughness of the initial rubber substrate is slightly higher in the case of graphene-rubber sample as compared to that of CNT-rubber based sample.
Fig. 2. The 3D images of flexible thermoelectric cells based on (a) CNT-rubber and (b) graphene-rubber composites.
K.S. Karimov et al. / Materials Science for Energy Technologies 2 (2019) 551–555
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Fig. 3. Schematic for measurement of thermoelectric effect: H1 and H2 – Miniature electric heaters 1 and 2, TC1 and TC2 – Thermocouple 1 and 2, respectively.
Fig. 5. Open circuit voltage- and short-circuit current–average temperature relationships for graphene-rubber composite.
Fig. 4. Open circuit voltage- and short-circuit current–average temperature relationships for CNT-rubber composite.
The sizes of the rubber substrate were: length – 16 mm, width – 5 mm, height – 5 mm. The rubber-CNT/graphene composite layer thickness, length, and width were equal to 30–40 mm, 7 mm and 5 mm. Fig. 3. showed the schematic diagrams of the side view of the thermoelectric cells. 2.2. Characterization of fabricated thermo-electric cells For the electronic characterizations of the samples, they were kept in a horizontal position, where miniature heaters H1 and H2 were attached at both ends of the cell to create the gradient of temperature between the thermocouples TC1 and TC,2 respectively. The gradient of temperature was 10 °C. The temperatures were measured by two Fluke 87 multi-meters: the thermocouples were connected to the opposite ends of the flexible thermoelectric cell and were also acted as electrodes to measure the generated thermoelectric properties. Electric parameters i.e., VOC and ISC were investigated by the HIOKI-DT4253 digital multi-meter. 3. Results and discussion 3.1. VOC and ISC characteristics of thermo-electric cells Figs. 4 and 5 show relationships of the VOC- and ISC-average temperature (with a constant gradient of 10 °C) for CNT-rubber and graphene-rubber composites. Results showed that VOC and ISC increased with an increase in average temperature by a factor of 1.75 and 4.5 times (CNT-rubber composites), where increment in graphene-rubber composites is 6.5 and 1.82 times, respectively. These results show that the highest increases in open-circuit voltages and short-circuit current are observed in case of graphene-rubber composites. Confirming the observation, Fig. 6 also demonstrates the comparison of the dependence of Seebeck
Fig. 6. Dependence of Seebeck coefficients on temperature for the CNT-rubber and graphene-rubber composites.
coefficients on averaged temperature for the CNT-rubber and graphene-rubber composites. The Seebeck coefficient of CNTrubber and graphene-rubber composites were equal to 5 mV/°C and 20 mV/°C at 30 °C, whereas at 55 °C it was increased to 32.5 mV/°C and 35 mV/°C with a total increase of 1.75 and 6.5 times, respectively. Fig. 7. demonstrates the dependences of the thermoelectric cell’s internal resistance of CNT- and graphene-rubber composites on averaged temperature. It is observed that as temperature increased the resistances decreased. The reason behind is that due to the presence of CNT or graphene, the conductivity of the composite film increased, resulting in a decrease of the internal resistance. As the internal resistance decreases path to flow of charges will be provided which in return increases short-circuit current that was observed experimentally too (Figs. 4 and 5). However, an increase of the thermoelectric voltage of the active film with an increment in the mobility of carriers is due to increment in the energy of charges, electrons, and probably it decreasing of potential barriers. The decrease in resistance with an increase in temperature is 1.17 and 1.31 times in case of CNT-rubber and graphene-rubber composites, respectively.
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Fig. 7. Dependence of the internal resistance of the thermoelectric cells based on CNT-rubber and graphene-rubber composites on averaged temperature.
Table 1 Summary of CNT/Graphene-rubber composites thermoelectric properties. Thin Film
DISC w.r.t T
DVOC w.r.t T
CNT-Rubber composite Graphene-Rubber composite
1.82 4.50
1.75 6.5
The summary of our work is shown in Table 1, which shows that the increase in ISC and VOC of graphene-rubber composite is more than that of CNT-rubber composite. However, the decrease in the internal resistance of the samples determined as a factor of the VOC and ISC is 0.05 and 2.19 times in case of CNT-rubber and graphene-rubber composites, respectively. After introducing the thermoelectric efficiency (Z) (Eq. (1)) the dimensionless figure of merit ZT was introduced [24] by multiplying Z with the average temperature:
T¼
ðT 2 þ T 1 Þ 2
ð2Þ
where T 1 and T 2 are temperatures of two contacts. A greater ZT indicates better thermodynamic efficiency. ZT is a parameter to compare the efficiencies of the devices by utilizing a variety of materials. When ZT ¼ 1 then the value is considered good; when its value in the range between 3 and 4, it considered useful for thermoelectric applications so they can compete in efficiency with mechanical devices. However, the finest value of CNT and graphene ZT are 2 and 4, respectively [25–28]. Mostly researchers are focusing on thermoelectric materials to increase a and reduce ktot by influencing the material structure. Maximum energy efficiency is determined by [29]:
gmax
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi TH TC 1þZT 1 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ TH 1 þ Z T þ TTHC
ð3Þ
where T H is the hot junction temperature and T C is the cooled sur
face temperature. Z T is the modified dimensionless figure of merit, defining the capacity of the thermoelectric material utilized within the device as an active layer. The efficiency of thermoelectric energy conversion is determined by the ZT:Fine value of a means that by suppressing the total thermal conductivity will result in making the Z basically relevant [30]. Even though graphene have exceptionally a highly
intrinsic K tot and by introducing disorder or by using graphene with rough edges can suppress its dominating component K ph [31–33]. Graphene is one of the best choices for TE energy conversion, the reason is that its lattice can be by design disordered, like from either irradiation of electron beam [34], or by varying the dopant concentration [35]. Though, possibilities for improvement of graphene TE properties are still under debates [36]. Graphene Seebeck coefficient and electrical conductivity are relatively high than CNT [37,38]. Conversely, it has high thermal conductivity K tot resulting in limitation of ZT which is not good from the perspective of thermoelectric properties. Hence to decrease the K tot many methods are discussed without affecting the electrical conductivity of the material like reducing the particle size and increase the dopant [39]. However, the CNT and graphene have electrical conductivity properties resembling both to semiconductors and metals i.e., small band gap and depends on the lattice orientation of graphite w.r.t. their axis [40]. The primary role of the rubber in the composite was to make the CNT/graphene layer firm by rubbing-in technology that could provide the stable properties which were observed practically. Secondly, the rubber could partly influence the thermoelectric properties of the composite [41]. The comparison of the Seebeck coefficients of the CNT based thermoelectric cell examined in [22] shows that the effect of the rubber to the thermoelectric properties was negligible as the value of the main parameter, Seebeck coefficient, that we measured was lower than pure CNT as presented in [22]. This might be lower due to the structure of rubber substrate which probably has changed the structural configuration of CNT or graphene strips from the strait. It implies that the contribution of the ‘‘bottom” and ‘‘walls” of the ‘‘well” to the thermoelectric voltage would be different. On the other hand, further investigation is needed to clarify this concept. Generally, the Seebeck coefficient is determined through the type of material [42,43]. Semiconductor materials doped with either p-type or n-type impurity, a is >200–1000 lV°C1 and a decrement is observed with rising in temperature, while a is very low in case of undoped semiconductors. Although, in metals, a is small 0–60 lV°C1 and increase gradually with temperature increase. The literature showed that quasi-one-dimensional crystals of TCNQ complexes have a 1000 lV °C1 [43]. Though, its a not easy to grow sufficiently large size crystals for practical uses. Thus, Bi2Te3 based thermoelectric cells seem reasonable since their properties have improved recently [44]. Not only CNT, but many researchers have also worked on thermoelectric properties of graphene [45] and obtained enhanced values for thermoelectric ZT from disordered armchair [46] and oxygen plasma treatments [47] of graphene nanoribbons. In the present work, the results showed that graphene-rubber composite was more sensitive compared to CNT-rubber composite when deposited by rubbing-in technology which was also observed in the literature [31,37]. For energy conversion using thermoelectric cells, pure graphene has two disadvantages i.e., it has no band gap (leading to a small value of a) and high thermal conduction resulting in small ZT [48]. At the same time, the thermal conductivity of graphene lattice can be reduced by nanostructuring and bandgap engineering for enhancement of a. Therefore, graphene may potentially be used for energy conversion [48]. Analysis of research shows that high efficiency of thermoelectric devices was realized due to selective surfaces and highperformance materials. In case of investigation results presented here, it is difficult to differentiate the role of every component of the composite, especially that of the rubber. Detailed investigations are required to clarify the reasons for the observations in this research which showed the increasing thermoelectric current and voltage.
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4. Conclusion The work presents the successful fabrication of the thermoelectric cells based on CNT and graphene with rubber, composite by rubbing-in technology. Experimental data showed that with an increment in the average temperature range from 30 to 55 °C, the current and voltage increased, respectively. The output of the thermoelectric cell mostly depend on the properties of the CNT and graphene, however, rubber makes the film firm, stable and flexible. Additionally, the ‘‘wells’’ structure of the rubber substrate probably affected the thermoelectric voltage and current as well. The results obtained can be used, first, for further development of the flexible thermoelectric cells technology, second, as a teaching tool to enhance learning and for low power applications. Considering the potential advantages of the flexible power generating devices, in this paper, investigation, and properties of the thermoelectric cells based on CNT-rubber and graphene-rubber composite films which are resistive and stable in vibrating conditions. Declaration of Competing Interest The authors declare that they have no conflict of interest. Acknowledgment The authors are indebted to GIK Institute of Engineering Sciences and Technology of Pakistan providing exclusive support during the work. References [1] C.S. Kim, G.S. Lee, H. Choi, Y.J. Kim, H.M. Yang, S.H. Lim, S.G. Lee, B.J. Cho, Structural design of a flexible thermoelectric power generator for wearable applications, Appl. Energy 214 (2018) 131–138. [2] P. Li, Y. Li, Z. Zhang, J. Chen, Y. Li, Y. Ma, Capillarity-driven assembly of singlewalled carbon nanotubes onto nickel wires for flexible wire-shaped supercapacitors, Mater. Sci. Energy Technol. 1 (2) (2018) 91–96. [3] B. Ma, T.H. Lee, S.E. Dorris, R.E. Koritala, U. Balachandran, Flexible ceramic film capacitors for high-temperature power electronics, Mater. Sci. Energy Technol. 2 (1) (2019) 96–103. [4] S. Prasad, D. Devaraj, R. Boddula, S. Salla, M.S. AlSalhi, Fabrication, device performance, and MPPT for flexible dye-sensitized solar panel based on gelpolymer phthaloylchitosan based electrolyte and nanocluster CoS2 counter electrode, Mater. Sci. Energy Technol. 2 (1) (2019) 319–328. [5] S. Qing, A. Rezania, L.A. Rosendahl, X. Gou, Design of flexible thermoelectric generator as human body sensor, Mater. Today: Proc. 5 (4) (2018) 10338– 10346. [6] S. Qing, A. Rezania, L.A. Rosendahl, A.A. Enkeshafi, X. Gou, Characteristics and parametric analysis of a novel flexible ink-based thermoelectric generator for human body sensor, Energy Convers. Manage. 156 (2018) 655–665. [7] J. Jur, M. Losego, P.E. Hopkins, Flexible thermoelectric devices, methods of preparation thereof, and methods of recovering waste heat therewith. U.S. Patent 9,929,332, 2018. [8] K. Gilleo, J. Murray, The Definitive History of the Printed Circuit, 1999. [9] W. Pennington, History of colour printing in the United Kingdom, J. Soc. Dyers Colour. 19 (2) (1903) 36–40. [10] R.N. Radkar, B.A. Bhanvase, D.P. Barai, S.H. Sonawane, Intensified convective heat transfer using ZnO nanofluids in heat exchanger with helical coiled geometry at constant wall temperature, Mater. Sci. Energy Technol. 2 (2) (2019) 161–170. [11] M. Sumino, K. Harada, M. Ikeda, S. Tanaka, K. Miyazaki, Thermoelectric properties of n-type C60 thin films and their application in organic thermovoltaic devices, Appl. Phys. Lett. 99 (2011) 093308. [12] S.B. Riffat, X. Ma, Thermoelectrics: a review of present and potential applications, Appl. Therm. Eng. 23 (2003) 913. [13] M. Sindhuja, V. Sudha, S. Harinipriya, R. Venugopal, B. Usmani, Electrodeposited Ni/SiC composite coating on graphite for high temperature solar thermal applications, Mater. Sci. Energy Technol. 1 (1) (2018) 3–10. [14] L.E. Bell, Cooling, heating, generating power, and recovering waste heat with thermoelectric systems, Science 321 (2008) 1457. [15] P.H. Maheshwari, Developing the processing stages of carbon fiber composite paper as efficient materials for energy conversion, storage, and conservation, Mater. Sci. Energy Technol. 2 (2019) 490–502. [16] G.C. Christakudis, S.K. Plachkova, L.E. Shelimova, Influence of defects on the (GeTe)1–x [(Ag2Te)1 y (Sb2Te3)y]x phonon thermal resistivity, Phys. Status Sol. (A) 111 (2) (1989) 469–475.
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Materials Science for Energy Technologies
CHINESE ROOTS GLOBAL IMPACT
journal homepage: www.keaipublishing.com/en/journals/materials-science-for-energy-technologies
Flexible thermoelectric cells fabricated by rubbing-in technology with rubber-carbon nanotubes/graphene composites Khasan S. Karimov a,b, Noshin Fatima a,⇑, Khalid J. Siddiqui a, Muhammad I. Khan a a b
GIK Institute of Engineering Sciences and Technology, Topi, District Swabi, KPK 23640, Pakistan Center for Innovative Development of Science and New Technologies of Academy of Sciences, Aini 299/2, Dushanbe 734063, Tajikistan
a r t i c l e
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Article history: Received 18 March 2019 Revised 3 June 2019 Accepted 4 June 2019 Available online 5 June 2019 Keywords: Flexible thermoelectric cell Carbon nanotubes Graphene Energy conversion Rubbing-in technology
a b s t r a c t In this research, CNT and graphene-rubber composites based thermoelectric cells were fabricated by using the rubbing-in technology. However, the comparative analysis of both elastic thermoelectric cells was also carried out. The carbon nanotubes and graphene composites with rubber were used as an active layer of thermoelectric cells. The films were deposited over rubber substrate (length 7 mm and width 5 mm) and pressure of (7–12) g/cm2. The electric dependence of the cells i.e., voltage (open-circuited), current (short-circuited), resistance and Seebeck coefficient with respect to averaged temperature (with gradient = 10 °C) was measured in the temperature interval of 30 °C–55 °C. The CNT/Graphene-rubber composite based active layer properties were investigated and results showed that with an increment in average temperature, there is an increase in current and voltage as well. In case of short circuit current, the increase in CNT-rubber and graphene-rubber composites was 1.82 and 4.5 times while an increase in open circuit voltage was 1.75 and 6.5 times, respectively. It was also found, that the graphene-rubber composites cells had a higher thermoelectric coefficient as compared to CNT-rubber thermoelectric cells. These cells can be potentially utilized as flexible and vibrations free thermoelectric sensors for measurement of the gradient of temperature and elastic thermoelectric modules for low power applications. Ó 2019 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-ncnd/4.0/).
1. Introduction Nowadays researchers are focusing on flexible devices, which produce electricity and can be bent or stretched during their normal use [1,2]. Usually, electronic devices are fabricated on rigid and flat substrates like the circuits that run smartphones computers, high-temperature power electronics, and solar panels [3,4]. However, they are expensive, limited in shapes and it is difficult to implement them in contoured forms and into soft materials, such as rubber, paper or cloth [5]. Circuits which are mounted over flexible substrate like PEEK, conductive film over polyester, even the human body are known as flexible electronics [6]. Due to the flexible nature, the size and weight of the devices are reduced.
⇑ Corresponding author. E-mail address: [email protected] (N. Fatima). Peer review under responsibility of KeAi Communications Co., Ltd.
Production and hosting by Elsevier
The flex circuits are foldable, hence along with small sizes the packaging have less propensity to break. However, some of the disadvantages of flex devices are that they are difficult to assemble and their repair and rework are usually challenging or unmanageable [7]. Gille and Hansen discovered and patented one of the earliest flex circuits in 1903, they coated a conductive metal layer over paraffin paper [8]. Flex circuits from the same era were specified in books of Thomas Edison, in which patterns on linen paper were coated with the powder of graphite and gum of cellulose [9], although confirmation of its implementation is not found. However, to cover day to day increase in power consumption the researchers are highly interested to achieve such goals through which they can use natural sources for power production such as utilizing the thermal energy and its conversion to electrical energy [10]. A smarter way to achieve such generators that convert heat energy to electrical energy is thermo-electric cells or generators. Exploring different composites by using unique techniques and their comparisons can help in the selection of good thermoelectric materials with high efficiency. It could be used to provide electricity to overcome power deficiency.
https://doi.org/10.1016/j.mset.2019.06.001 2589-2991/Ó 2019 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Recently, organic and inorganic semiconductors thermoelectric devices i.e., generators and cells are fabricated and characterized for their Seebeck effect intensively due to the increasing demand of power consumption [11–13]. In the case of semiconductors, the Seebeck effect of thermoelectric cells which converts heat energy to electric energy plays an important role in modern power technology [14,15]. Thermoelectric efficiency (Z) can be obtained from Eq. (1) as follows [16]:
Z¼
a2 r
ð1Þ
K tot
where, a, r represents the Seebeck coefficient and the electrical conductivity, while K tot ¼ K el þ K ph is the sum of thermal conduction of electron ðK el Þ and phonon ðK ph Þ known as total thermal conductivity. The increment in the efficiency of thermoelectric cells is dependent on the decrease of phonon thermal conductivityðK ph Þ. This way, a number of new material including CNT and graphenerubber composites were investigated intensively [17]. Hu et al. determined a CNT based thermo-electrochemical cell using thermal energy from harvested wastage [18]. Sahu et. al., designed a thermoelectric voltage booster generator and achieved up to 50% efficiency [19]. A CNT/graphene-based thermoelectric polymer nanocomposite for low-temperature thermoelectric applications was investigated in Ref. [20]. Another CNT composite based thermoelectric cell was fabricated and characterized, results showed that pure CNT, PANI and CNT/PANI nanocomposite have Seebeck coefficients of 12.2 mV/K, 2.74 mV/K and 22.4 mV/K at 300 K [21]. One more CNT composite based thin film temperature gradient sensor was investigated in the range from 5 to 38 °C where change observed in voltage, current and Seebeck coefficient by a factor of 5, 5.3 and 1.3 times, where the negative sign is indicating decrease [22]. Concerning the fabrication of flexible thermoelectric cells and generators the problem is in an initial state, however, its future seems to be promising due to the facts that they have relatively simple inherent technology and elastic in nature. At the same time, the fabrication of the flexible thermoelectric cells would be entirely different with respect to rigid thermoelectric cells due to a different principle of the devices, their structure and fabrication technology. In continuation of our earlier efforts where thermoelectric cells were fabricated and characterized based on CNT [22] and bismuth telluride composites with CNT and graphene [23], the present work also fabricates thermoelectric cells based on CNT and graphene but with the flexible substrate and rubbing-in technology. 2. Experimental 2.1. Fabrication of thermoelectric cell Initially the commercially produced (Sun Nanotech Co ltd., China) CNT and graphene were used without further purification.
Fig. 1. Schematic diagram of the rubbing-in technology for deposition of rubbergraphene/CNT film.
The thermoelectric cells were fabricated by depositing CNT and graphene thin layers on the rubber substrate with a pressure of (7–12) g/cm2 by using rubbing-in technology as shown in Fig. 1. The films deposition time was 2 min. The material disperse was controlled by spreading 30–40 mg of material over the rubber substrate and the load movement in two perpendicular directions results in deposition of the thin film. Under the condition of strain (produced by load) the size of rubber pores expands making the deposition process easy. Then when the load is removed the pores resize themselves, squeezing the material particles within the substrate resulting in material and rubber composite. The material ratio with rubber was 80 wt% and 20 wt%, respectively. Two groups of thermoelectric cells were prepared: first group was based on the composite of CNT and rubber active layer fabricated by rubbing pure CNT over rubber substrate by rubbing-in technology, while in the second group, graphene and rubber composite based cells were achieved by similar method as mentioned earlier only by using graphene instead of CNT. Later both types of samples were tested as thermoelectric cells and both physical morphology and electronic results were obtained. For microscopy, the AFM was used. Fig. 2 (a) and (b) show 3D images of CNT-rubber and graphene-rubber based samples. The samples were characterized using the atomic force microscope with the model FLEX-AFM, manufactured by NANOSURF. The AFM of CNT-rubber sample shows a relatively smooth surface profile as compared to that of graphene-rubber based sample. The CNT-rubber based sample shows a roughness of 0.267 mm and graphene-rubber surface roughness is 0.295 mm. This difference of surface morphology may be attributed to the initial surface roughness of the rubber substrate. It is expected that the roughness of the initial rubber substrate is slightly higher in the case of graphene-rubber sample as compared to that of CNT-rubber based sample.
Fig. 2. The 3D images of flexible thermoelectric cells based on (a) CNT-rubber and (b) graphene-rubber composites.
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Fig. 3. Schematic for measurement of thermoelectric effect: H1 and H2 – Miniature electric heaters 1 and 2, TC1 and TC2 – Thermocouple 1 and 2, respectively.
Fig. 5. Open circuit voltage- and short-circuit current–average temperature relationships for graphene-rubber composite.
Fig. 4. Open circuit voltage- and short-circuit current–average temperature relationships for CNT-rubber composite.
The sizes of the rubber substrate were: length – 16 mm, width – 5 mm, height – 5 mm. The rubber-CNT/graphene composite layer thickness, length, and width were equal to 30–40 mm, 7 mm and 5 mm. Fig. 3. showed the schematic diagrams of the side view of the thermoelectric cells. 2.2. Characterization of fabricated thermo-electric cells For the electronic characterizations of the samples, they were kept in a horizontal position, where miniature heaters H1 and H2 were attached at both ends of the cell to create the gradient of temperature between the thermocouples TC1 and TC,2 respectively. The gradient of temperature was 10 °C. The temperatures were measured by two Fluke 87 multi-meters: the thermocouples were connected to the opposite ends of the flexible thermoelectric cell and were also acted as electrodes to measure the generated thermoelectric properties. Electric parameters i.e., VOC and ISC were investigated by the HIOKI-DT4253 digital multi-meter. 3. Results and discussion 3.1. VOC and ISC characteristics of thermo-electric cells Figs. 4 and 5 show relationships of the VOC- and ISC-average temperature (with a constant gradient of 10 °C) for CNT-rubber and graphene-rubber composites. Results showed that VOC and ISC increased with an increase in average temperature by a factor of 1.75 and 4.5 times (CNT-rubber composites), where increment in graphene-rubber composites is 6.5 and 1.82 times, respectively. These results show that the highest increases in open-circuit voltages and short-circuit current are observed in case of graphene-rubber composites. Confirming the observation, Fig. 6 also demonstrates the comparison of the dependence of Seebeck
Fig. 6. Dependence of Seebeck coefficients on temperature for the CNT-rubber and graphene-rubber composites.
coefficients on averaged temperature for the CNT-rubber and graphene-rubber composites. The Seebeck coefficient of CNTrubber and graphene-rubber composites were equal to 5 mV/°C and 20 mV/°C at 30 °C, whereas at 55 °C it was increased to 32.5 mV/°C and 35 mV/°C with a total increase of 1.75 and 6.5 times, respectively. Fig. 7. demonstrates the dependences of the thermoelectric cell’s internal resistance of CNT- and graphene-rubber composites on averaged temperature. It is observed that as temperature increased the resistances decreased. The reason behind is that due to the presence of CNT or graphene, the conductivity of the composite film increased, resulting in a decrease of the internal resistance. As the internal resistance decreases path to flow of charges will be provided which in return increases short-circuit current that was observed experimentally too (Figs. 4 and 5). However, an increase of the thermoelectric voltage of the active film with an increment in the mobility of carriers is due to increment in the energy of charges, electrons, and probably it decreasing of potential barriers. The decrease in resistance with an increase in temperature is 1.17 and 1.31 times in case of CNT-rubber and graphene-rubber composites, respectively.
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Fig. 7. Dependence of the internal resistance of the thermoelectric cells based on CNT-rubber and graphene-rubber composites on averaged temperature.
Table 1 Summary of CNT/Graphene-rubber composites thermoelectric properties. Thin Film
DISC w.r.t T
DVOC w.r.t T
CNT-Rubber composite Graphene-Rubber composite
1.82 4.50
1.75 6.5
The summary of our work is shown in Table 1, which shows that the increase in ISC and VOC of graphene-rubber composite is more than that of CNT-rubber composite. However, the decrease in the internal resistance of the samples determined as a factor of the VOC and ISC is 0.05 and 2.19 times in case of CNT-rubber and graphene-rubber composites, respectively. After introducing the thermoelectric efficiency (Z) (Eq. (1)) the dimensionless figure of merit ZT was introduced [24] by multiplying Z with the average temperature:
T¼
ðT 2 þ T 1 Þ 2
ð2Þ
where T 1 and T 2 are temperatures of two contacts. A greater ZT indicates better thermodynamic efficiency. ZT is a parameter to compare the efficiencies of the devices by utilizing a variety of materials. When ZT ¼ 1 then the value is considered good; when its value in the range between 3 and 4, it considered useful for thermoelectric applications so they can compete in efficiency with mechanical devices. However, the finest value of CNT and graphene ZT are 2 and 4, respectively [25–28]. Mostly researchers are focusing on thermoelectric materials to increase a and reduce ktot by influencing the material structure. Maximum energy efficiency is determined by [29]:
gmax
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi TH TC 1þZT 1 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ TH 1 þ Z T þ TTHC
ð3Þ
where T H is the hot junction temperature and T C is the cooled sur
face temperature. Z T is the modified dimensionless figure of merit, defining the capacity of the thermoelectric material utilized within the device as an active layer. The efficiency of thermoelectric energy conversion is determined by the ZT:Fine value of a means that by suppressing the total thermal conductivity will result in making the Z basically relevant [30]. Even though graphene have exceptionally a highly
intrinsic K tot and by introducing disorder or by using graphene with rough edges can suppress its dominating component K ph [31–33]. Graphene is one of the best choices for TE energy conversion, the reason is that its lattice can be by design disordered, like from either irradiation of electron beam [34], or by varying the dopant concentration [35]. Though, possibilities for improvement of graphene TE properties are still under debates [36]. Graphene Seebeck coefficient and electrical conductivity are relatively high than CNT [37,38]. Conversely, it has high thermal conductivity K tot resulting in limitation of ZT which is not good from the perspective of thermoelectric properties. Hence to decrease the K tot many methods are discussed without affecting the electrical conductivity of the material like reducing the particle size and increase the dopant [39]. However, the CNT and graphene have electrical conductivity properties resembling both to semiconductors and metals i.e., small band gap and depends on the lattice orientation of graphite w.r.t. their axis [40]. The primary role of the rubber in the composite was to make the CNT/graphene layer firm by rubbing-in technology that could provide the stable properties which were observed practically. Secondly, the rubber could partly influence the thermoelectric properties of the composite [41]. The comparison of the Seebeck coefficients of the CNT based thermoelectric cell examined in [22] shows that the effect of the rubber to the thermoelectric properties was negligible as the value of the main parameter, Seebeck coefficient, that we measured was lower than pure CNT as presented in [22]. This might be lower due to the structure of rubber substrate which probably has changed the structural configuration of CNT or graphene strips from the strait. It implies that the contribution of the ‘‘bottom” and ‘‘walls” of the ‘‘well” to the thermoelectric voltage would be different. On the other hand, further investigation is needed to clarify this concept. Generally, the Seebeck coefficient is determined through the type of material [42,43]. Semiconductor materials doped with either p-type or n-type impurity, a is >200–1000 lV°C1 and a decrement is observed with rising in temperature, while a is very low in case of undoped semiconductors. Although, in metals, a is small 0–60 lV°C1 and increase gradually with temperature increase. The literature showed that quasi-one-dimensional crystals of TCNQ complexes have a 1000 lV °C1 [43]. Though, its a not easy to grow sufficiently large size crystals for practical uses. Thus, Bi2Te3 based thermoelectric cells seem reasonable since their properties have improved recently [44]. Not only CNT, but many researchers have also worked on thermoelectric properties of graphene [45] and obtained enhanced values for thermoelectric ZT from disordered armchair [46] and oxygen plasma treatments [47] of graphene nanoribbons. In the present work, the results showed that graphene-rubber composite was more sensitive compared to CNT-rubber composite when deposited by rubbing-in technology which was also observed in the literature [31,37]. For energy conversion using thermoelectric cells, pure graphene has two disadvantages i.e., it has no band gap (leading to a small value of a) and high thermal conduction resulting in small ZT [48]. At the same time, the thermal conductivity of graphene lattice can be reduced by nanostructuring and bandgap engineering for enhancement of a. Therefore, graphene may potentially be used for energy conversion [48]. Analysis of research shows that high efficiency of thermoelectric devices was realized due to selective surfaces and highperformance materials. In case of investigation results presented here, it is difficult to differentiate the role of every component of the composite, especially that of the rubber. Detailed investigations are required to clarify the reasons for the observations in this research which showed the increasing thermoelectric current and voltage.
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4. Conclusion The work presents the successful fabrication of the thermoelectric cells based on CNT and graphene with rubber, composite by rubbing-in technology. Experimental data showed that with an increment in the average temperature range from 30 to 55 °C, the current and voltage increased, respectively. The output of the thermoelectric cell mostly depend on the properties of the CNT and graphene, however, rubber makes the film firm, stable and flexible. Additionally, the ‘‘wells’’ structure of the rubber substrate probably affected the thermoelectric voltage and current as well. The results obtained can be used, first, for further development of the flexible thermoelectric cells technology, second, as a teaching tool to enhance learning and for low power applications. Considering the potential advantages of the flexible power generating devices, in this paper, investigation, and properties of the thermoelectric cells based on CNT-rubber and graphene-rubber composite films which are resistive and stable in vibrating conditions. Declaration of Competing Interest The authors declare that they have no conflict of interest. Acknowledgment The authors are indebted to GIK Institute of Engineering Sciences and Technology of Pakistan providing exclusive support during the work. References [1] C.S. Kim, G.S. Lee, H. Choi, Y.J. Kim, H.M. Yang, S.H. Lim, S.G. Lee, B.J. Cho, Structural design of a flexible thermoelectric power generator for wearable applications, Appl. Energy 214 (2018) 131–138. [2] P. Li, Y. Li, Z. Zhang, J. Chen, Y. Li, Y. Ma, Capillarity-driven assembly of singlewalled carbon nanotubes onto nickel wires for flexible wire-shaped supercapacitors, Mater. Sci. Energy Technol. 1 (2) (2018) 91–96. [3] B. Ma, T.H. Lee, S.E. Dorris, R.E. Koritala, U. Balachandran, Flexible ceramic film capacitors for high-temperature power electronics, Mater. Sci. Energy Technol. 2 (1) (2019) 96–103. [4] S. Prasad, D. Devaraj, R. Boddula, S. Salla, M.S. AlSalhi, Fabrication, device performance, and MPPT for flexible dye-sensitized solar panel based on gelpolymer phthaloylchitosan based electrolyte and nanocluster CoS2 counter electrode, Mater. Sci. Energy Technol. 2 (1) (2019) 319–328. [5] S. Qing, A. Rezania, L.A. Rosendahl, X. Gou, Design of flexible thermoelectric generator as human body sensor, Mater. Today: Proc. 5 (4) (2018) 10338– 10346. [6] S. Qing, A. Rezania, L.A. Rosendahl, A.A. Enkeshafi, X. Gou, Characteristics and parametric analysis of a novel flexible ink-based thermoelectric generator for human body sensor, Energy Convers. Manage. 156 (2018) 655–665. [7] J. Jur, M. Losego, P.E. Hopkins, Flexible thermoelectric devices, methods of preparation thereof, and methods of recovering waste heat therewith. U.S. Patent 9,929,332, 2018. [8] K. Gilleo, J. Murray, The Definitive History of the Printed Circuit, 1999. [9] W. Pennington, History of colour printing in the United Kingdom, J. Soc. Dyers Colour. 19 (2) (1903) 36–40. [10] R.N. Radkar, B.A. Bhanvase, D.P. Barai, S.H. Sonawane, Intensified convective heat transfer using ZnO nanofluids in heat exchanger with helical coiled geometry at constant wall temperature, Mater. Sci. Energy Technol. 2 (2) (2019) 161–170. [11] M. Sumino, K. Harada, M. Ikeda, S. Tanaka, K. Miyazaki, Thermoelectric properties of n-type C60 thin films and their application in organic thermovoltaic devices, Appl. Phys. Lett. 99 (2011) 093308. [12] S.B. Riffat, X. Ma, Thermoelectrics: a review of present and potential applications, Appl. Therm. Eng. 23 (2003) 913. [13] M. Sindhuja, V. Sudha, S. Harinipriya, R. Venugopal, B. Usmani, Electrodeposited Ni/SiC composite coating on graphite for high temperature solar thermal applications, Mater. Sci. Energy Technol. 1 (1) (2018) 3–10. [14] L.E. Bell, Cooling, heating, generating power, and recovering waste heat with thermoelectric systems, Science 321 (2008) 1457. [15] P.H. Maheshwari, Developing the processing stages of carbon fiber composite paper as efficient materials for energy conversion, storage, and conservation, Mater. Sci. Energy Technol. 2 (2019) 490–502. [16] G.C. Christakudis, S.K. Plachkova, L.E. Shelimova, Influence of defects on the (GeTe)1–x [(Ag2Te)1 y (Sb2Te3)y]x phonon thermal resistivity, Phys. Status Sol. (A) 111 (2) (1989) 469–475.
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