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Enhanced thermoelectric performance of Bi2Te3 based graphene nanocomposites
Accepted Manuscript Full Length Article Enhanced thermoelectric performance of Bi2Te3 based graphene nanocomposites Kaleem Ahmad, C. Wan, M.A. Al-Eshaikh, A.N. Kadachi PII: DOI: Reference:
S0169-4332(18)32932-5 https://doi.org/10.1016/j.apsusc.2018.10.163 APSUSC 40731
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
26 November 2017 2 October 2018 19 October 2018
Please cite this article as: K. Ahmad, C. Wan, M.A. Al-Eshaikh, A.N. Kadachi, Enhanced thermoelectric performance of Bi2Te3 based graphene nanocomposites, Applied Surface Science (2018), doi: https://doi.org/ 10.1016/j.apsusc.2018.10.163
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Enhanced thermoelectric performance of Bi2Te3 based graphene nanocomposites Kaleem Ahmad1*, C. Wan2, M.A. Al-Eshaikh3, A. N. Kadachi3 1*
Sustainable Energy Technologies Center, College of Engineering, King Saud University, PO Box 800, Riyadh 11421, Saudi Arabia, Email : [email protected] 2 State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China 3 Research Center, College of Engineering, King Saud University, PO Box 800, Riyadh 11421, Saudi Arabia
Abstract: Graphene in different vol% ranging from 0.5, 0.75 and 1.5 were uniformly incorporated in the nanostructured Bi2Te3 that was obtained from ball milling the coarse powder. The composite and the pristine Bi2Te3 powders were consolidated by the high frequency induction heated sintering. The thermoelectric properties of the bulks were investigated in the temperature range from 300-525K. The effective thermal conductivities of the composites decrease with the reduction in the particle size of the pristine Bi2Te3 as well as with the addition of graphene attributed to enhanced phonon scattering from the phase boundaries. The significant increase in electrical conductivity accompanied by less decrease in the Seebeck coefficient at 1.5 vol% of graphene culminates in a high power factor. The results suggest that enhancement in power factor is attributed to quantum confinement effect through introduction of 2D graphene in Bi2Te3. Thus, reduction in thermal conductivity in conjunction with substantial improvement in power factor at 1.5 vol% of graphene leads to considerable enhancement in the thermoelectric figure of merit at ~500 K from pristine bulk. This work presents a novel strategy for development of high performance cost effective and scalable nanostructured thermoelectric bulk materials similar to Bi2Te3 based superlattices. Keywords: Bi2Te3, graphene, thermoelectric, composites 1
1.
Introduction Clean energy is one of the biggest challenges in the 21st century. There are continued
efforts to find solutions for the production of sustainable energy through different emerging technologies [1]. In particular, thermoelectric energy harvesting provides a simple and green solution to sustainable energy challenges by converting some part of low-grade heat which is ubiquitous in the environment and freely available from plenty of sources such as sun light, different energy intensive industrial processes, automobile industries etc. into electricity [2, 3]. Thermoelectric are solid-state devices that provide environmentally friendly source of electrical power, run quietly and are free of any moving parts [4]. Bi2Te3 and its alloys are state of the art thermoelectric materials, which are being used commercially for near room temperature applications. During the last several years, extensive research work has been conducted to improve energy conversion efficiency of these devices to enhance their spectrum of utilization from niche to broader applications. In general, device efficiency depends on, performance of materials, which is closely related to figure of merit. The figure of merit is quantitatively described as where S, σ, k and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature respectively. The thermoelectric parameters are strongly interdependent on each other; therefore, it is a big challenge to manipulate them freely. Hence, marginal increment in ZT is generally observed, since improvement in any parameter leads to detriment in others and vice versa [5]. The groundbreaking work of Dresselhaus and coworkers [6] proposed a solution to enhance power factor by reducing the dimensionality. The authors reported that quantum confinement effect through introduction of low dimensional constituents to the bulk 2
matrix could grant some liberty to manipulate ‘S’ and ‘σ’ quasi independently in concomitant with the reduction in thermal conductivity [6]. Therefore, substantial breakthrough in energy conversion efficiency could be achieved by designing the thermoelectric materials through quantum effects via reducing the dimensionality of nanostructured constituents [5, 6]. Graphene is one of the most promising low dimensional carbon allotropes that has high specific surface area and tremendous electrical, mechanical and thermal properties [7]. The two dimensional (2D) graphene could enhance electrical conductivity of bulk materials at very low vol% due to high aspect ratio and its high intrinsic electrical conductivity [8]. Furthermore, it has been found beneficial to reduce the effective thermal conductivity of the composites due to enhanced phonon scattering at the nanostructured phase boundaries [9-11]. Graphene also improves mechanical stability of the bulk materials that is essential for device fabrication [12]. Several studies were performed by incorporation of graphene in bismuth telluride and its solid solutions [13-16]. For instance, Shin et al. [17] incorporated 0.4 vol% of graphene in bismuth antimony telluride and reported 15% enhancement in thermoelectric figure of merit from the pristine material due to enhanced power factor and reduced thermal conductivity. Kim and Ju [18] fabricated Bi2Te3 nanowire based 1.0 wt% graphene composites and reported improved figure of merit at 300K with respect to the pristine bulk. Similarly, Zong and coworkers [19] showed that by introducing multilayer graphene at the intergranular positions of skutterudite dramatically reduces the thermal conductivity without affecting the electrical conductivity thus significantly improving thermoelectric figure of merit. Recently, Li’s group [20] synthesized the graphene quantum dots (GQDs) hybrid 3
nanosheets via solution based strategy and embedded it in bismuth telluride nanosheet matrix. The authors’ reported improved thermoelectric figure of merit for Bi2Te3/GQDs20 nm at 425 K primarily due to reduction in thermal conductivity. Most recently Kumar et al. [10] reported the enhanced thermoelectric properties of bismuth telluride nanosheets with graphene nanofiller. The enhancement was attributed to substantial decrease in lattice thermal conductivity with improved electron mobility. The uniform distribution of graphene in the bulk material by ball milling and other chemical methods is a big challenge due to its tendency for agglomeration [19]. Whereas, the unique functionalities of graphene could only be translated to the matrix if it has been evenly distributed in it that eventually leads to substantial enhancement in ZT. Based on our experience of uniform dispersion of carbon allotropes in the bulk matrix [8, 21, 22], therefore, in this work using a simple and effective strategy, different 0.5, 0.75 and 1.5 vol% of quantum confined structures of two dimensional graphene were uniformly dispersed in nanostructured Bi2Te3 powder and its effect on the thermoelectric properties of bulk nanocomposites was evaluated. The composites were fabricated through high frequency induction heated sintering and their thermoelectric properties were estimated in the temperature range between 300 and 525K. 2.
Experimental procedure Coarse powder of Bi2Te3 (99.98%) and graphene were purchased from Alfa Aesar,
USA and Emfutur, Spain respectively. The as received Bi2Te3 was ball milled in an inert environment by a planetary micormill P-7 (Fritsch) for 24 hours. The graphene nanoplates were 1-4 nm in thickness. In order to uniformly disperse graphene into ball
4
milled Bi2Te3 powder, a well-tested strategy based on a combination of ultra-sonication and ball milling was employed to process the composite powders [8, 21, 22]. In brief, different 0.5, 0.75 and 1.5 vol% of graphene was dispersed in ethanol for half an hour by ultrasonically. While in parallel the ball milled Bi2Te3 powder was mixed separately in ethanol through ultra-sonication. After half an hour, both the slurries were combined and further mixing was performed through a customized procedure by simultaneous application of mechanical agitation and sonication. Finally, the slurry for additional mixing was ball milled. After drying, the processed composite powders and coarse Bi2Te3 were consolidated in a high frequency, induction heated sintering furnace at 450°C under uniaxial pressure of 35 MPa. The bulk densities of consolidated samples were measured by the Archimedes’s method. Phase identification was performed by the X-ray diffraction (Bruker D-8 Discover, Germany). The scanning electron microscopy and transmission electron microscopy were performed by JEOL JSM-7600F and JEM2100F respectively on bulk samples and powders. The thermal diffusivity of the disk-shaped bulk samples ≈12.5 mm in diameter and ≈1.5 mm in thickness was measured by the LFA-457 (NETZSCH, Germany). The electrical conductivity and Seebeck coefficient were estimated by the ZEM-3 (ULVAC, RIKO, Japan). 3. 3.1
Results and discussion Powder processing, morphology and microstructure The coarse Bi2Te3 as shown in Fig. 1(a) was ball milled for 24 h in an inert
environment. The ball milling transformed the coarse powder into nanostructured Bi2Te3. The graphene (as received) were characterized by the SEM and TEM as shown in Figs. 1
5
(b) and (c) respectively. The low magnification TEM image shows typical morphology of graphene in agreement with the previous report [19]. The main challenge in powder processing is the exfoliation of graphene soft agglomerates and its uniform dispersion in the nanostructured Bi2Te3, so that functionality of 2D quantum confinement effect from graphene could be transferred to the bulk matrix. Based on our experience, a similar methodology reported earlier has been adopted to exfoliate and disperse uniformly the nanosheets of graphene in the Bi2Te3 fine powder [8, 21]. Fig. 2(a) and (b) show typical micrographs of processed 0.75 and 1.5 vol% nanostructured Bi2Te3/graphene composite powders. The Bi2Te3 particles seem to be wrapped by the graphene nanosheets. The XRD analysis of composite and Bi2Te3 powders was performed as shown in Fig. 2 (c). All the peaks correspond well with the standard PDF card 15-0863. The powders were consolidated by the high frequency induction heated sintering at 450°C. The relative densities of pristine Bi2Te3 and its composites with 0.5, 0.75, 1.5 vol% of graphene were found ~ 99 and ~91, ~92, ~91 % of theoretical densities respectively. The fractured surfaces of pristine Bi2Te3 and its composites were characterized by the SEM as a shown in Fig. 3. The microstructure of pristine Bi2Te3 bulk sample in Fig. 3(a) exhibits typically layered structure and large grains due to coarse nature of Bi2Te3 powder. The nanostructured bulk composites with 0.5, 0.75 and 1.5 vol% graphene are shown in Figs. 3(b), (c) and (d) respectively. The average grain size of the composites somewhat decrease with the addition of graphene in agreement with the earlier work [11]. The morphology of fractured surface shows very high degree of randomness of nanostructured grains in the composites, whereas the pristine bulk shows large grains with obvious layered structure. The selected areas in Figs. 3(c) and (d) are further 6
magnified and shown in Figs. 3(e) and (f) respectively. The careful observations of the zoomed figures suggest the presence of graphene nanosheets uniformly distributed at the intergranular positions of the composites. The XRD patterns of pristine Bi2Te3 and its bulk composites are shown in Fig. 4. All the major peaks are matched well with the standard PDF card 15-0863. The nanostructured composite powders were consolidated in the pressure assisted high frequency induction heated furnace as described in Ref. [23]. The consolidation of the fine composite powders at lower sintering temperature and in a shorter timescale severely suppresses the preferential gain growth in the bulk composites. The XRD patterns of composite powders as well as their bulk samples demonstrate isotropic behavior that is corroborated by the highly random morphology of the fractured surfaces of composites. However, the pristine bulk shows somewhat anisotropic behavior due to coarse particle size of Bi2Te3 powder. Therefore, thermoelectric transport properties of the pristine bulk are estimated in the same direction i.e. parallel to the pressing direction, while for the composites the electrical and thermal properties were measured in the direction perpendicular and parallel to the pressing of the composites respectively [24, 25]. It is likely that minimal anisotropy could be present in the composites. 3.2
Thermoelectric properties The electrical conductivity, Seebeck coefficient and power factor of pristine
Bi2Te3 and its bulk composites with graphene are shown in Fig. 5. The electrical conductivity of all the bulk samples decreases with temperature, showing semi-metallic transport behavior typically observed in thermoelectric semiconductors [16, 26]. The temperature dependent electrical conductivity increases with the addition of graphene for 7
all the bulk samples. The largest increase in the electrical conductivity is observed at 1.5 vol% of graphene in Bi2Te3. The substantial increase in electrical conductivity of the composites from pristine bulk is predominantly attributed to the addition of graphene and partly to the ball milling. The mechanical deformation of the coarse Bi2Te3 by ball milling induced donor-like effect that increased the electronic conductivity [24, 27]. Furthermore, the addition of highly conducting graphene increases the effective electrical conductivity of the composites substantially. The highest values of electrical conductivity are achieved at 1.5 vol% due to largest volume fraction of graphene. The increase in electrical conductivity with the addition of graphene indicates that it has been dispersed uniformly in the matrix. For the graphene infused composites, the percolation starts generally at ~0.1 vol% for transport of electrons through the interconnected network of nanosheets spanning across the bulk samples [28, 29]. Therefore, it is believed that a percolation phenomenon exists for transport of electrons through graphene nanosheets in the composites that substantially enhance the electrical conductivity, in particular at 1.5 vol% of graphene. The electrical conductivity of 0.75 vol% graphene is close to 0.5 vol% composite, apparently it should be higher as the vol% increases to 0.75%. This behavior could be attributed to slightly inhomogeneous dispersion of 0.75 vol% graphene as it gives lower conductivity values of the composite in agreement with the literature [17]. The slope (-dσ/dT) slightly increases with the addition of graphene in the composites suggesting transformation of electronic transport towards more metallic behavior [21]. The temperature dependent Seebeck coefficient of pristine Bi2Te3 and composites are shown in Fig. 5(b). The incorporation of graphene has significantly improved the Seebeck coefficient of the composites when compared to pristine bulk. The absolute 8
Seebeck coefficient values of the composites increases with the increase of temperature thus demonstrating a typical behavior observed for Bi2Te3 based materials [14]. The highest value of Seebeck coefficient shifts towards higher temperature due to thermal excitation of minority carriers [30, 31]. The negative values of Seebeck coefficients are an indication that electronic transport predominantly consisting of electrons. The 0.75 vol% composite has the maximum absolute values of Seebeck coefficient and the 1.5 vol% has the second highest values of Seebeck coefficient. In general, an inverse relationship exists between electrical conductivity and Seebeck coefficient due to a tradeoff between these two parameters. However, in the present study, in particular, for 1.5 vol% graphene composite, the Seebeck coefficient showed a lower decrease in apropos to the electrical conductivity but generally both parameters increase simultaneously. The results suggest that the increase in Seebcek coefficient is partly attributed to the potential barrier scattering in combination with the percolation effect as reported by the Zhao et al. [32]. Furthermore, this behavior could be described by considering the quantum mechanical effects of low dimension inclusions in the composites. It was proposed by Hicks and Dresselhaus [33] that as the system size decreases to nanometer scale there is a localized split in the density of states that leads to its increase over a narrow energy range [6]. Since we know that graphene is a 2D quantum sheet [34, 35], therefore, its uniform dispersion in the nanostructured Bi2Te3 is believed to be responsible for generation of sharp features in the density of states of the composites [6]. This phenomenon affects the Seebeck coefficient as its values somehow depends on local increase in the density of states through Mott relation [36, 37]. While density of states in turn has proportional relationship with the effective mass of charge
9
carriers in the composites [36], therefore, it is anticipated that there is an increase in effective mass of charge carriers in the composites that consequently enhanced the Seebeck coefficient. Similar phenomenon was reported by several researchers in bismuth antimony telluride through incorporation of expanded graphene and SiC in the form of composites [25, 38]. In our recent study the increase in effective mass has been observed through introduction of single wall carbon nanotubes in Bi2Te3 [21]. It is worth mentioning that the increase in effective mass somewhat degrade the mobility but the gain in Seebeck is substantial as compared to decreases in mobility [39]. The temperature dependent power factor is shown in Fig. 5(c). The addition of graphene has increased the power factor of the composites in the studied temperature range (300K to 525 K). The highest value of power factor is achieved for 1.5 vol% of graphene/Bi2Te3 composite which is attributed to significant increase in conductivity and Seebeck coefficient. The 0.5 and 0.75 vol% have almost similar values of power factor due to a lower difference of graphene vol% between the two composites. Figs. 6(a) and (b) show thermal properties of the composites and pristine bulk sample. The thermal conductivity of the bulks was evaluated through the equation k = α ρ Cp. The thermal diffusivity ‘α’ was measured between the temperature range of 300 K and 525 K. The specific heat ‘Cp’ of pure bismuth telluride and graphene was evaluated from the thermochemical database [40] in accordance with the literature [41, 42]. For the bulk materials in the form of composite, the heat capacity was deduced from the rule of mixture using pure substance values as described in several reports [21, 43, 44]. Finally, the total thermal conductivity ‘k’ was estimated as shown in Fig. 6(a). The thermal conductivity of the composites decreases significantly with the addition of graphene due 10
to enhanced scattering from the nanostructured Bi2Te3 as well as from graphene. A two pronged strategy has been found effective in thermal conductivity reduction of the composites [23]. The ball milling significantly reduced the particle size of Bi2Te3, furthermore the addition of graphene suppresses the grain growth, thereby enhancing the interface scattering and reducing the thermal conductivity of the composites. Thermal conductivity is generally composed of predominantly charge carrier thermal conductivity ‘ke’, lattice thermal conductivity ‘kl’ as well as negligible contribution from bipolar thermal conductivity near room temperature [26]. The carrier thermal conductivity was estimated through equation LₒσT by applying the Wiedemann–Franz law using the Lorenz number 1.5 ×10-08 V2K-2 in accordance with the literature [30, 45]. The lattice/phonon thermal conductivity ‘kl’ was calculated by subtracting the carrier thermal conductivity from the total thermal conductivity ‘k’ and results are shown in Fig. 6(b). The lattice thermal conductivity substantially decreases with the addition of 1.5 vol% graphene in Bi2Te3 [17]. The evaluated thermoelectric figure of merit of pristine bulk and it composites with graphene are shown in Fig. 6(c). The thermoelectric figures of merit significantly increased with the addition of graphene in nanostructured Bi2Te3. The 0.5 and 0.75 vol% have comparable values of ZT due to less difference in graphene fraction. However, at 1.5 vol% substantial increase in ZT has been observed. This is attributed to the remarkable increase in the power factor and considerable decrease in the thermal conductivity of the composite. The larger ZT achieved in this study for 1.5 vol% of graphene is lower than the typically observed value for Bi2Te3 bulk material but it is acceptable as similar or even smaller values are reported in the literature for the Bi2Te3
11
based composites [46, 47]. Moreover, Agrawal et al. [14] have shown much higher ZT value as compared to present study but their range of improvement is not as substantial as observed in this work i.e. several fold enhancement from the pristine Bi2Te3 bulk. 4.
Conclusions
In summary, graphene in various (0.5, 0.75 and 1.5) vol% were homogeneously dispersed in nanostructured Bi2Te3 through a combination of ultra-sonication, magnetic stirring and ball milling. The bulk samples were fabricated by the high frequency induction heated sintering and their temperature (300-525K) dependent thermoelectric properties were investigated. The electrical conductivity of composites increases with the addition of graphene and substantial improvement has been observed at 1.5 vol%. The Seebeck coefficient improves and highest values are obtained for 0.75 vol% of graphene. Therefore, a considerable improvement in power factor is achieved at 1.5 vol% attributed to the quantum confinement effect of 2D graphene in the bulk composites. The thermal conductivity of the composites decreases with the addition of graphene due to enhanced phonon scattering from the nanostructured interfaces and high surface area of graphene. Therefore, an excellent improvement in power factor with significantly reduced thermal conductivity leads to remarkable increases in the thermoelectric figure of merit at 1.5 vol% of graphene at ~500 K. Acknowledgements This Project was funded by the National Plan for Science, Technology and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award Number (11-NAN1913-02). We appreciate help for sintering the samples 12
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Figure 1
Fig. 1 SEM image of (a) as received coarse Bi2Te3 powder (b) graphene and (c) Low magnification TEM image of graphene 19
Figure 2
Fig. 2 SEM image of (a) 0.75 (b) 1.5, vol% graphene/Bi2Te3 composite powders (c) XRD patterns of pristine and graphene/Bi2Te3 composite powders 20
Figure 3
Fig. 3 Fractured surfaces of (a) pristine Bi2Te3 bulk (b) 0.5 vol% graphene/Bi2Te3 (c) 0.75 vol% graphene/Bi2Te3 and (d) 1.5 vol% graphene/Bi2Te3 bulk composites
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Figure 4
Fig. 4 XRD patterns of pristine Bi2Te3 and graphene/Bi2Te3 bulk composites
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Figure 5
Fig. 5 Temperature dependent (a) Electrical conductivity (b) Seebeck coefficient (c) Power factor of Bi2Te3 based graphene composites 23
Figure 6
Fig. 6 (a) Thermal conductivity, k (b) Lattice thermal conductivity, kl (c) Figure of merit of Bi2Te3 based graphene composites. 24
Highlights
Cost effective and scalable strategy for the development of high performance nanostructured thermoelectric bulk materials Substantial enhancement is observed in electrical conductivity and Seebeck coefficient Significantly improved thermoelectric figure of merit of the composite from the pristine bulk sample
25
S0169-4332(18)32932-5 https://doi.org/10.1016/j.apsusc.2018.10.163 APSUSC 40731
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
26 November 2017 2 October 2018 19 October 2018
Please cite this article as: K. Ahmad, C. Wan, M.A. Al-Eshaikh, A.N. Kadachi, Enhanced thermoelectric performance of Bi2Te3 based graphene nanocomposites, Applied Surface Science (2018), doi: https://doi.org/ 10.1016/j.apsusc.2018.10.163
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Enhanced thermoelectric performance of Bi2Te3 based graphene nanocomposites Kaleem Ahmad1*, C. Wan2, M.A. Al-Eshaikh3, A. N. Kadachi3 1*
Sustainable Energy Technologies Center, College of Engineering, King Saud University, PO Box 800, Riyadh 11421, Saudi Arabia, Email : [email protected] 2 State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China 3 Research Center, College of Engineering, King Saud University, PO Box 800, Riyadh 11421, Saudi Arabia
Abstract: Graphene in different vol% ranging from 0.5, 0.75 and 1.5 were uniformly incorporated in the nanostructured Bi2Te3 that was obtained from ball milling the coarse powder. The composite and the pristine Bi2Te3 powders were consolidated by the high frequency induction heated sintering. The thermoelectric properties of the bulks were investigated in the temperature range from 300-525K. The effective thermal conductivities of the composites decrease with the reduction in the particle size of the pristine Bi2Te3 as well as with the addition of graphene attributed to enhanced phonon scattering from the phase boundaries. The significant increase in electrical conductivity accompanied by less decrease in the Seebeck coefficient at 1.5 vol% of graphene culminates in a high power factor. The results suggest that enhancement in power factor is attributed to quantum confinement effect through introduction of 2D graphene in Bi2Te3. Thus, reduction in thermal conductivity in conjunction with substantial improvement in power factor at 1.5 vol% of graphene leads to considerable enhancement in the thermoelectric figure of merit at ~500 K from pristine bulk. This work presents a novel strategy for development of high performance cost effective and scalable nanostructured thermoelectric bulk materials similar to Bi2Te3 based superlattices. Keywords: Bi2Te3, graphene, thermoelectric, composites 1
1.
Introduction Clean energy is one of the biggest challenges in the 21st century. There are continued
efforts to find solutions for the production of sustainable energy through different emerging technologies [1]. In particular, thermoelectric energy harvesting provides a simple and green solution to sustainable energy challenges by converting some part of low-grade heat which is ubiquitous in the environment and freely available from plenty of sources such as sun light, different energy intensive industrial processes, automobile industries etc. into electricity [2, 3]. Thermoelectric are solid-state devices that provide environmentally friendly source of electrical power, run quietly and are free of any moving parts [4]. Bi2Te3 and its alloys are state of the art thermoelectric materials, which are being used commercially for near room temperature applications. During the last several years, extensive research work has been conducted to improve energy conversion efficiency of these devices to enhance their spectrum of utilization from niche to broader applications. In general, device efficiency depends on, performance of materials, which is closely related to figure of merit. The figure of merit is quantitatively described as where S, σ, k and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature respectively. The thermoelectric parameters are strongly interdependent on each other; therefore, it is a big challenge to manipulate them freely. Hence, marginal increment in ZT is generally observed, since improvement in any parameter leads to detriment in others and vice versa [5]. The groundbreaking work of Dresselhaus and coworkers [6] proposed a solution to enhance power factor by reducing the dimensionality. The authors reported that quantum confinement effect through introduction of low dimensional constituents to the bulk 2
matrix could grant some liberty to manipulate ‘S’ and ‘σ’ quasi independently in concomitant with the reduction in thermal conductivity [6]. Therefore, substantial breakthrough in energy conversion efficiency could be achieved by designing the thermoelectric materials through quantum effects via reducing the dimensionality of nanostructured constituents [5, 6]. Graphene is one of the most promising low dimensional carbon allotropes that has high specific surface area and tremendous electrical, mechanical and thermal properties [7]. The two dimensional (2D) graphene could enhance electrical conductivity of bulk materials at very low vol% due to high aspect ratio and its high intrinsic electrical conductivity [8]. Furthermore, it has been found beneficial to reduce the effective thermal conductivity of the composites due to enhanced phonon scattering at the nanostructured phase boundaries [9-11]. Graphene also improves mechanical stability of the bulk materials that is essential for device fabrication [12]. Several studies were performed by incorporation of graphene in bismuth telluride and its solid solutions [13-16]. For instance, Shin et al. [17] incorporated 0.4 vol% of graphene in bismuth antimony telluride and reported 15% enhancement in thermoelectric figure of merit from the pristine material due to enhanced power factor and reduced thermal conductivity. Kim and Ju [18] fabricated Bi2Te3 nanowire based 1.0 wt% graphene composites and reported improved figure of merit at 300K with respect to the pristine bulk. Similarly, Zong and coworkers [19] showed that by introducing multilayer graphene at the intergranular positions of skutterudite dramatically reduces the thermal conductivity without affecting the electrical conductivity thus significantly improving thermoelectric figure of merit. Recently, Li’s group [20] synthesized the graphene quantum dots (GQDs) hybrid 3
nanosheets via solution based strategy and embedded it in bismuth telluride nanosheet matrix. The authors’ reported improved thermoelectric figure of merit for Bi2Te3/GQDs20 nm at 425 K primarily due to reduction in thermal conductivity. Most recently Kumar et al. [10] reported the enhanced thermoelectric properties of bismuth telluride nanosheets with graphene nanofiller. The enhancement was attributed to substantial decrease in lattice thermal conductivity with improved electron mobility. The uniform distribution of graphene in the bulk material by ball milling and other chemical methods is a big challenge due to its tendency for agglomeration [19]. Whereas, the unique functionalities of graphene could only be translated to the matrix if it has been evenly distributed in it that eventually leads to substantial enhancement in ZT. Based on our experience of uniform dispersion of carbon allotropes in the bulk matrix [8, 21, 22], therefore, in this work using a simple and effective strategy, different 0.5, 0.75 and 1.5 vol% of quantum confined structures of two dimensional graphene were uniformly dispersed in nanostructured Bi2Te3 powder and its effect on the thermoelectric properties of bulk nanocomposites was evaluated. The composites were fabricated through high frequency induction heated sintering and their thermoelectric properties were estimated in the temperature range between 300 and 525K. 2.
Experimental procedure Coarse powder of Bi2Te3 (99.98%) and graphene were purchased from Alfa Aesar,
USA and Emfutur, Spain respectively. The as received Bi2Te3 was ball milled in an inert environment by a planetary micormill P-7 (Fritsch) for 24 hours. The graphene nanoplates were 1-4 nm in thickness. In order to uniformly disperse graphene into ball
4
milled Bi2Te3 powder, a well-tested strategy based on a combination of ultra-sonication and ball milling was employed to process the composite powders [8, 21, 22]. In brief, different 0.5, 0.75 and 1.5 vol% of graphene was dispersed in ethanol for half an hour by ultrasonically. While in parallel the ball milled Bi2Te3 powder was mixed separately in ethanol through ultra-sonication. After half an hour, both the slurries were combined and further mixing was performed through a customized procedure by simultaneous application of mechanical agitation and sonication. Finally, the slurry for additional mixing was ball milled. After drying, the processed composite powders and coarse Bi2Te3 were consolidated in a high frequency, induction heated sintering furnace at 450°C under uniaxial pressure of 35 MPa. The bulk densities of consolidated samples were measured by the Archimedes’s method. Phase identification was performed by the X-ray diffraction (Bruker D-8 Discover, Germany). The scanning electron microscopy and transmission electron microscopy were performed by JEOL JSM-7600F and JEM2100F respectively on bulk samples and powders. The thermal diffusivity of the disk-shaped bulk samples ≈12.5 mm in diameter and ≈1.5 mm in thickness was measured by the LFA-457 (NETZSCH, Germany). The electrical conductivity and Seebeck coefficient were estimated by the ZEM-3 (ULVAC, RIKO, Japan). 3. 3.1
Results and discussion Powder processing, morphology and microstructure The coarse Bi2Te3 as shown in Fig. 1(a) was ball milled for 24 h in an inert
environment. The ball milling transformed the coarse powder into nanostructured Bi2Te3. The graphene (as received) were characterized by the SEM and TEM as shown in Figs. 1
5
(b) and (c) respectively. The low magnification TEM image shows typical morphology of graphene in agreement with the previous report [19]. The main challenge in powder processing is the exfoliation of graphene soft agglomerates and its uniform dispersion in the nanostructured Bi2Te3, so that functionality of 2D quantum confinement effect from graphene could be transferred to the bulk matrix. Based on our experience, a similar methodology reported earlier has been adopted to exfoliate and disperse uniformly the nanosheets of graphene in the Bi2Te3 fine powder [8, 21]. Fig. 2(a) and (b) show typical micrographs of processed 0.75 and 1.5 vol% nanostructured Bi2Te3/graphene composite powders. The Bi2Te3 particles seem to be wrapped by the graphene nanosheets. The XRD analysis of composite and Bi2Te3 powders was performed as shown in Fig. 2 (c). All the peaks correspond well with the standard PDF card 15-0863. The powders were consolidated by the high frequency induction heated sintering at 450°C. The relative densities of pristine Bi2Te3 and its composites with 0.5, 0.75, 1.5 vol% of graphene were found ~ 99 and ~91, ~92, ~91 % of theoretical densities respectively. The fractured surfaces of pristine Bi2Te3 and its composites were characterized by the SEM as a shown in Fig. 3. The microstructure of pristine Bi2Te3 bulk sample in Fig. 3(a) exhibits typically layered structure and large grains due to coarse nature of Bi2Te3 powder. The nanostructured bulk composites with 0.5, 0.75 and 1.5 vol% graphene are shown in Figs. 3(b), (c) and (d) respectively. The average grain size of the composites somewhat decrease with the addition of graphene in agreement with the earlier work [11]. The morphology of fractured surface shows very high degree of randomness of nanostructured grains in the composites, whereas the pristine bulk shows large grains with obvious layered structure. The selected areas in Figs. 3(c) and (d) are further 6
magnified and shown in Figs. 3(e) and (f) respectively. The careful observations of the zoomed figures suggest the presence of graphene nanosheets uniformly distributed at the intergranular positions of the composites. The XRD patterns of pristine Bi2Te3 and its bulk composites are shown in Fig. 4. All the major peaks are matched well with the standard PDF card 15-0863. The nanostructured composite powders were consolidated in the pressure assisted high frequency induction heated furnace as described in Ref. [23]. The consolidation of the fine composite powders at lower sintering temperature and in a shorter timescale severely suppresses the preferential gain growth in the bulk composites. The XRD patterns of composite powders as well as their bulk samples demonstrate isotropic behavior that is corroborated by the highly random morphology of the fractured surfaces of composites. However, the pristine bulk shows somewhat anisotropic behavior due to coarse particle size of Bi2Te3 powder. Therefore, thermoelectric transport properties of the pristine bulk are estimated in the same direction i.e. parallel to the pressing direction, while for the composites the electrical and thermal properties were measured in the direction perpendicular and parallel to the pressing of the composites respectively [24, 25]. It is likely that minimal anisotropy could be present in the composites. 3.2
Thermoelectric properties The electrical conductivity, Seebeck coefficient and power factor of pristine
Bi2Te3 and its bulk composites with graphene are shown in Fig. 5. The electrical conductivity of all the bulk samples decreases with temperature, showing semi-metallic transport behavior typically observed in thermoelectric semiconductors [16, 26]. The temperature dependent electrical conductivity increases with the addition of graphene for 7
all the bulk samples. The largest increase in the electrical conductivity is observed at 1.5 vol% of graphene in Bi2Te3. The substantial increase in electrical conductivity of the composites from pristine bulk is predominantly attributed to the addition of graphene and partly to the ball milling. The mechanical deformation of the coarse Bi2Te3 by ball milling induced donor-like effect that increased the electronic conductivity [24, 27]. Furthermore, the addition of highly conducting graphene increases the effective electrical conductivity of the composites substantially. The highest values of electrical conductivity are achieved at 1.5 vol% due to largest volume fraction of graphene. The increase in electrical conductivity with the addition of graphene indicates that it has been dispersed uniformly in the matrix. For the graphene infused composites, the percolation starts generally at ~0.1 vol% for transport of electrons through the interconnected network of nanosheets spanning across the bulk samples [28, 29]. Therefore, it is believed that a percolation phenomenon exists for transport of electrons through graphene nanosheets in the composites that substantially enhance the electrical conductivity, in particular at 1.5 vol% of graphene. The electrical conductivity of 0.75 vol% graphene is close to 0.5 vol% composite, apparently it should be higher as the vol% increases to 0.75%. This behavior could be attributed to slightly inhomogeneous dispersion of 0.75 vol% graphene as it gives lower conductivity values of the composite in agreement with the literature [17]. The slope (-dσ/dT) slightly increases with the addition of graphene in the composites suggesting transformation of electronic transport towards more metallic behavior [21]. The temperature dependent Seebeck coefficient of pristine Bi2Te3 and composites are shown in Fig. 5(b). The incorporation of graphene has significantly improved the Seebeck coefficient of the composites when compared to pristine bulk. The absolute 8
Seebeck coefficient values of the composites increases with the increase of temperature thus demonstrating a typical behavior observed for Bi2Te3 based materials [14]. The highest value of Seebeck coefficient shifts towards higher temperature due to thermal excitation of minority carriers [30, 31]. The negative values of Seebeck coefficients are an indication that electronic transport predominantly consisting of electrons. The 0.75 vol% composite has the maximum absolute values of Seebeck coefficient and the 1.5 vol% has the second highest values of Seebeck coefficient. In general, an inverse relationship exists between electrical conductivity and Seebeck coefficient due to a tradeoff between these two parameters. However, in the present study, in particular, for 1.5 vol% graphene composite, the Seebeck coefficient showed a lower decrease in apropos to the electrical conductivity but generally both parameters increase simultaneously. The results suggest that the increase in Seebcek coefficient is partly attributed to the potential barrier scattering in combination with the percolation effect as reported by the Zhao et al. [32]. Furthermore, this behavior could be described by considering the quantum mechanical effects of low dimension inclusions in the composites. It was proposed by Hicks and Dresselhaus [33] that as the system size decreases to nanometer scale there is a localized split in the density of states that leads to its increase over a narrow energy range [6]. Since we know that graphene is a 2D quantum sheet [34, 35], therefore, its uniform dispersion in the nanostructured Bi2Te3 is believed to be responsible for generation of sharp features in the density of states of the composites [6]. This phenomenon affects the Seebeck coefficient as its values somehow depends on local increase in the density of states through Mott relation [36, 37]. While density of states in turn has proportional relationship with the effective mass of charge
9
carriers in the composites [36], therefore, it is anticipated that there is an increase in effective mass of charge carriers in the composites that consequently enhanced the Seebeck coefficient. Similar phenomenon was reported by several researchers in bismuth antimony telluride through incorporation of expanded graphene and SiC in the form of composites [25, 38]. In our recent study the increase in effective mass has been observed through introduction of single wall carbon nanotubes in Bi2Te3 [21]. It is worth mentioning that the increase in effective mass somewhat degrade the mobility but the gain in Seebeck is substantial as compared to decreases in mobility [39]. The temperature dependent power factor is shown in Fig. 5(c). The addition of graphene has increased the power factor of the composites in the studied temperature range (300K to 525 K). The highest value of power factor is achieved for 1.5 vol% of graphene/Bi2Te3 composite which is attributed to significant increase in conductivity and Seebeck coefficient. The 0.5 and 0.75 vol% have almost similar values of power factor due to a lower difference of graphene vol% between the two composites. Figs. 6(a) and (b) show thermal properties of the composites and pristine bulk sample. The thermal conductivity of the bulks was evaluated through the equation k = α ρ Cp. The thermal diffusivity ‘α’ was measured between the temperature range of 300 K and 525 K. The specific heat ‘Cp’ of pure bismuth telluride and graphene was evaluated from the thermochemical database [40] in accordance with the literature [41, 42]. For the bulk materials in the form of composite, the heat capacity was deduced from the rule of mixture using pure substance values as described in several reports [21, 43, 44]. Finally, the total thermal conductivity ‘k’ was estimated as shown in Fig. 6(a). The thermal conductivity of the composites decreases significantly with the addition of graphene due 10
to enhanced scattering from the nanostructured Bi2Te3 as well as from graphene. A two pronged strategy has been found effective in thermal conductivity reduction of the composites [23]. The ball milling significantly reduced the particle size of Bi2Te3, furthermore the addition of graphene suppresses the grain growth, thereby enhancing the interface scattering and reducing the thermal conductivity of the composites. Thermal conductivity is generally composed of predominantly charge carrier thermal conductivity ‘ke’, lattice thermal conductivity ‘kl’ as well as negligible contribution from bipolar thermal conductivity near room temperature [26]. The carrier thermal conductivity was estimated through equation LₒσT by applying the Wiedemann–Franz law using the Lorenz number 1.5 ×10-08 V2K-2 in accordance with the literature [30, 45]. The lattice/phonon thermal conductivity ‘kl’ was calculated by subtracting the carrier thermal conductivity from the total thermal conductivity ‘k’ and results are shown in Fig. 6(b). The lattice thermal conductivity substantially decreases with the addition of 1.5 vol% graphene in Bi2Te3 [17]. The evaluated thermoelectric figure of merit of pristine bulk and it composites with graphene are shown in Fig. 6(c). The thermoelectric figures of merit significantly increased with the addition of graphene in nanostructured Bi2Te3. The 0.5 and 0.75 vol% have comparable values of ZT due to less difference in graphene fraction. However, at 1.5 vol% substantial increase in ZT has been observed. This is attributed to the remarkable increase in the power factor and considerable decrease in the thermal conductivity of the composite. The larger ZT achieved in this study for 1.5 vol% of graphene is lower than the typically observed value for Bi2Te3 bulk material but it is acceptable as similar or even smaller values are reported in the literature for the Bi2Te3
11
based composites [46, 47]. Moreover, Agrawal et al. [14] have shown much higher ZT value as compared to present study but their range of improvement is not as substantial as observed in this work i.e. several fold enhancement from the pristine Bi2Te3 bulk. 4.
Conclusions
In summary, graphene in various (0.5, 0.75 and 1.5) vol% were homogeneously dispersed in nanostructured Bi2Te3 through a combination of ultra-sonication, magnetic stirring and ball milling. The bulk samples were fabricated by the high frequency induction heated sintering and their temperature (300-525K) dependent thermoelectric properties were investigated. The electrical conductivity of composites increases with the addition of graphene and substantial improvement has been observed at 1.5 vol%. The Seebeck coefficient improves and highest values are obtained for 0.75 vol% of graphene. Therefore, a considerable improvement in power factor is achieved at 1.5 vol% attributed to the quantum confinement effect of 2D graphene in the bulk composites. The thermal conductivity of the composites decreases with the addition of graphene due to enhanced phonon scattering from the nanostructured interfaces and high surface area of graphene. Therefore, an excellent improvement in power factor with significantly reduced thermal conductivity leads to remarkable increases in the thermoelectric figure of merit at 1.5 vol% of graphene at ~500 K. Acknowledgements This Project was funded by the National Plan for Science, Technology and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award Number (11-NAN1913-02). We appreciate help for sintering the samples 12
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Figure 1
Fig. 1 SEM image of (a) as received coarse Bi2Te3 powder (b) graphene and (c) Low magnification TEM image of graphene 19
Figure 2
Fig. 2 SEM image of (a) 0.75 (b) 1.5, vol% graphene/Bi2Te3 composite powders (c) XRD patterns of pristine and graphene/Bi2Te3 composite powders 20
Figure 3
Fig. 3 Fractured surfaces of (a) pristine Bi2Te3 bulk (b) 0.5 vol% graphene/Bi2Te3 (c) 0.75 vol% graphene/Bi2Te3 and (d) 1.5 vol% graphene/Bi2Te3 bulk composites
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Figure 4
Fig. 4 XRD patterns of pristine Bi2Te3 and graphene/Bi2Te3 bulk composites
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Figure 5
Fig. 5 Temperature dependent (a) Electrical conductivity (b) Seebeck coefficient (c) Power factor of Bi2Te3 based graphene composites 23
Figure 6
Fig. 6 (a) Thermal conductivity, k (b) Lattice thermal conductivity, kl (c) Figure of merit of Bi2Te3 based graphene composites. 24
Highlights
Cost effective and scalable strategy for the development of high performance nanostructured thermoelectric bulk materials Substantial enhancement is observed in electrical conductivity and Seebeck coefficient Significantly improved thermoelectric figure of merit of the composite from the pristine bulk sample
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