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Synthesis of alumina nanoparticle-embedded-bismuth telluride matrix thermoelectric composite powders
Materials Letters 82 (2012) 141–144
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of alumina nanoparticle-embedded-bismuth telluride matrix thermoelectric composite powders Kyung Tae Kim a,⁎, Hye Young Koo a, Gil-Geun Lee b, Gook Hyun Ha a a b
Powder Technology Department, Korea Institute of Materials Science, 797 Changwon-daero, Seongsan-gu, Changwon-si, Gyeongnam 641-831, Republic of Korea Division of Materials Science and Engineering, Pukyong National University, San100, Yongdang-Dong, Nam-Gu, Busan 608-739, Republic of Korea
a r t i c l e
i n f o
Article history: Received 12 March 2012 Accepted 7 May 2012 Available online 23 May 2012 Keywords: Bismuth telluride Thermoelectric Composite Powders Nanoparticles
a b s t r a c t Alumina nanoparticle-embedded-bismuth telluride matrix (Al2O3/Bi2Te3) composite powders were synthesized by chemical routes that employ a polyol reduction process to Bi acetates and Te chlorides in the solvent. With application of the polyol process, the synthesized composite powders show a disc-shaped morphology with 50 nm thickness and 1 μm diameter. High-resolution TEM images reveal that alumina nanoparticles are homogeneously embedded in the Bi2Te3 matrix powders. The Al2O3/Bi2Te3 nanocomposites sintered from the composite powders exhibit improved ZT values at room temperature compared to that of pure Bi2Te3 due to reduced thermal conductivity caused by active phonon and carrier scattering at the newly formed Al2O3/ Bi2Te3 interface. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Energy conversion by using a bismuth telluride (Bi2Te3) as a thermoelectric (TE) material is very attractive for achieving power generation without requiring any driving parts or cooling systems for electronic devices due to the high TE properties of Bi2Te3 at ambient temperature [1]. The energy conversion efficiency of TE devices depends on the dimensionless figure of merit, ZT which is defined as α2σTκ− 1, where α is the Seebeck coefficient, σ is the electrical conductivity, к is the thermal conductivity and T is the absolute temperature. A number of approaches have been explored in recent decades in efforts to improve the ZT by realizing a nanometer-scale microstructure; examples include grain refinement and the incorporation of nanoparticles in TE alloys [2]. It has been reported that thermal conductivity can be effectively reduced by increased phonon-scattering and by controlling electrical conductivity and/or the Seebeck coefficient via electron filtering through the use of a nanocomposite structure consisting of TE materials and nanoinclusions [3–6]. In particular, decreasing the lattice thermal conductivity by using a nano-sized second phase dispersed in the matrix has recently been recognized as an effective means of improving the ZT value. Specifically, effective phonon-scattering can occur while carriers pass through the dispersed nano-materials which are the size below 10 nm due to the difference in the wave-lengths of the phonons and carriers. Thus far, however, few studies on Bi2Te3 matrix nanocomposites with separately added nanomaterials have been reported [6–8], in contrast
⁎ Corresponding author. Tel.: + 82 55 280 3506; fax: + 82 55 280 3392. E-mail addresses: [email protected], [email protected] (K.T. Kim). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.05.053
with work on in-situ formed Te precipitates [2] or addition of amorphous nanocrystals [9]. In this study, we report on the production of alumina nanoparticledispersed bismuth telluride matrix (Al2O3/Bi2Te3) composite powders through a chemical route. The nanocomposites sintered from Al2O3/ Bi2Te3 composite powders show 15% lower thermal conductivity relative to pure Bi2Te3 bulk fabricated by the same process without Al2O3 nanoparticles. 2. Experimental procedure Al2O3 nanoparticles with size of about 10 nm were utilized as nanodispersoid materials. They were chemically treated and rendered into a stable colloidal solution in a solvent by adding a surfactant. The Al2O3/ Bi2Te3 and Bi2Te3 nanopowders was synthesized by using a polyol reduction process of Bismuth (III) acetates and Tellurium (IV) chlorides as precursors, respectively [10]. Dioctylether and 1,2-hexadecanediol were used as a solvent and reducing agent, respectively. Oleylamine, and trioctylphosphine (TOP) were added into the mixed solution as a surfactant and stabilizer, respectively. The mixed solution was heated to a temperature of 250 °C under an Ar gas atmosphere. The reactant after the polyol reaction was air-cooled on a mantle to room temperature. The synthesized Bi2Te3 powders and Al2O3/Bi2Te3 composite powders were consolidated by a spark plasma sintering process (Welltech Co., WL-15-400, Korea) at 350 °C for 10 min. The final size of spark plasma sintered Bi2Te3 and Al2O3/Bi2Te3 nanocomposite was 12.5 mm in diameter and 2.0 mm in thickness. The phase of the powders was characterized by the X-ray diffraction method (model no. X'pert MPD 3040 with Cu Kα radiation). The microstructures of the powders were analyzed by field emission scanning electron microscopy (FESEM, Hitachi
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4700S). Embedding of Al2O3 nanoparticles in Bi2Te3 matrix powders was characterized by a high resolution transmission electron microscopy (HRTEM) (JEM3010, 300 kV). While the electric properties of both sintered bodies were measured in the in-plane direction, the thermal diffusivity was characterized in the perpendicular direction to the buttontype sample. The room-temperature Seebeck coefficient and electrical resistivity of the samples were characterized using a Hall-effect measurement. Thermal conductivity was calculated from the thermal diffusivity measured by the Laser Flash method (LFA 457, Netzsch), and the specific heat and the density of the sintered body.
3. Results and discussion Fig. 1(a) schematically illustrates the key process for making alumina-embedded-bismuth telluride matrix composite powders. The alumina nanoparticles are homogeneously dispersed in the surfactantassisted solvent. The Bi and Te atoms, reduced by polyol agents, can be formed nearby the nanoparticles and a Bi2Te3 phase heterogeneously can be nucleated at the surface of surfactant-capped alumina rather than at the wall of the flask in the solution system. This process provides Al2O3/Bi2Te3 composite powders in which Al2O3 nanoparticles are
Fig. 1. Schematic illustration of (a) heterogeneous nucleation of Bi2Te3 materials onto alumina nanoparticles and Al2O3 nanoparticle-embedded Bi2Te3 matrix composite powders, (b) FEM-SEM image of the composite powders synthesized by chemical route (c) XRD patterns. (d) bright-field TEM image, (e) dark-field TEM image of Al2O3/Bi2Te3 composite powders, (f) EDS results displaying Al2O3 presenting at the region pointed out in (d).
K.T. Kim et al. / Materials Letters 82 (2012) 141–144
143
Fig. 2. (a) Photograph of sintered samples of Bi2Te3 and Al2O3/Bi2Te3 nanocomposites, (b) fracture-surface SEM image of the Al2O3/Bi2Te3 nanocomposite, (c) TEM image showing Al2O3 nanoparticles dispersed in bismuth telluride matrix and enlarged TEM image of (c).
homogeneously dispersed, as shown in Fig. 1(a). Once alumina nanoparticles are embedded in the Bi2Te3 matrix powders, it is expected that the dispersion status of the nanoparticles will remain even after sintering at the elevated sintering temperature. Fig. 1(b) shows FE-SEM surface morphologies of Al2O3/Bi2Te3 composite powders synthesized by the polyol reduction process. The powders have a regular disc-shape with a size of about 500 nm in plane-direction and a few tens of nm thickness. Several powders have transparent characteristics due to very thin thickness. The XRD patterns shown in Fig. 1(c) clearly exhibit a Bi2Te3 phase corresponding to JCPDS card no. 15-0863 while Al2O3 is not observed in the peaks due to its small volume fraction of less than 3%. The various indexed crystal planes including (015) as the main peak indicate that the composite powders consist of polycrystalline Bi2Te3 grains. The Bi2Te3 powders without Al2O3 nanoparticles also display similar XRD patterns. This indicates that the Bi2Te3 phase is well formed by the polyol process regardless of addition of Al2O3. Fig. 1(d) shows a bright-field TEM image of the composite powders. Many bright spots and gray-colored spots in the bismuth telluride matrix powders can be observed, while there are no spots in the Bi2Te3 powders. Fig. 1(e) exhibits a dark-field TEM image that clearly shows a homogeneous dispersion of alumina nanoparticles in the matrix powders. Fig. 1(f) shows the point-Energy Dispersive Spectrometer (EDS) results on the spot marked as “+” in Fig. 1(d). The result displays a clear Al (Kα) peak indicating the presence of Al2O3, since the only Al-related materials are the Al2O3 nanoparticles in the synthesized powders. These results demonstrate that the developed syntheticprocess is very suitable for obtaining homogeneously embedding Al2O3 nanoparticles in thermoelectric Bi2Te3 matrix powders. Fig. 2(a) shows a photograph of two samples sintered by a spark plasma sintering process. The left and right samples are machined-
Bi2Te3 bulk and Al2O3/Bi2Te3 nanocomposite, respectively, with size of 8 mm× 8 mm. Fig. 2(b) shows the FE-SEM morphology of the fracture surface of the nanocomposite revealing grains aligned in the characteristic direction. It appears that the directionality of the Bi2Te3 grains is determined by the disc shape of the initial powders, as shown in Fig. 1(b). Fig. 2(c) shows a bright-field TEM image of Al2O3 nanoparticle-embedded-bismuth telluride nanocomposites consolidated by spark plasma sintering process. The enlarged TEM image of Fig. 2(c) clearly indicates that Al2O3 nanoparticles are dispersed in the Bi2Te3 matrix grain rather than located between the grain boundaries after a sintering process at the elevated temperature. As provided in Table 1, the relative densities of the consolidated bulks are similar at roughly 98%, regardless of the addition of alumina nanoparticles. The electrical resistivity of the nanocomposites is slightly increased from 2.0 × 10 − 5 Ω m to 2.5 × 10 − 5 Ω m with the addition of Al2O3 nanoparticles at room temperature, because the numerous interfaces produced by the alumina nanoparticles in the Bi2Te3 matrix impede carrier movement. Meanwhile, the Seebeck coefficient of the Bi2Te3 materials at room temperature was enhanced 1.3-fold to − 128 μV/K by Al2O3 addition. The enhancement of the Seebeck coefficient is ascribed to the potential formed by the Al2O3/Bi2Te3 interface in the nanocomposite, which provides energy filtering behavior [5]. The measured thermal conductivity of the Al2O3/Bi2Te3 nanocomposite was reduced by 15% compared to that of pure Bi2Te3 at 25 °C. The thermal conductivity (к) is the sum of the electrical (кel), lattice (кph) and ambipolar (кam) conductivities. кel is directly proportional to the Lorentz constant (2.45 × 10 − 8 V 2 K − 2), electrical conductivity (σ), and temperature (T), the value, 0.29 W/mK of the nanocomposite calculated from кel = LσT at 25 °C is 20% lower than the value, 0.37 W/mK of pure Bi2Te3 fabricated by the same process without Al2O3. Hence, the reduction of
Table 1 Comparison of relative densities and thermoelectric properties measured at 300 K for Bi2Te3 and Al2O3/Bi2Te3 nanocomposites.
Bi2Te3 Al2O3/Bi2Te3
Relative density (%)
Electrical resistivity (10− 5 Ω m)
Seebeck coefficient (μV/K)
Total thermal conductivity, кtotal (W/mK)
Electrical–thermal conductivity, кel, (W/mK)
кtotal–кel (W/mK)
Figure-of-merit (ZT) at 300 K
98.5 98.0
2.0 2.5
− 100 − 128
1.05 0.85
0.37 0.29
0.68 0.56
0.16 0.20
144
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4. Conclusions Al2O3/Bi2Te3 composite powders characterized by alumina nanoparticles that are homogeneously embedded in bismuth telluride matrix powders are fabricated by using a polyol process. The thermal conductivity of the nanocomposite prepared from the powders is 20% lower than that of Bi2Te3 materials. The dimensionless ZT value of the nanocomposite is thus increased by alumina addition due to the desirable thermal conductivity and Seebeck coefficient despite degraded electrical conductivity. These findings clarify that incorporating nanoparticles in thermoelectric Bi2Te3 materials via a chemical route is a promising method to obtain reduced total thermal conductivity, which can result in an enhanced ZT value. Fig. 3. Measured thermal conductivities of Bi2Te3 and Al2O3/Bi2Te3 nanocomposites with increasing temperature ranging from 25 °C to 225 °C.
total thermal conductivity in the nanocomposites might originate from newly formed Al2O3/Bi2Te3 interfaces, which cause carrier and phonon dissipation. Fig. 3 shows that the reduced thermal conductivity of the nanocomposite is retained under temperature ranging from 25 °C to 225 °C. Regarding the thermoelectric properties of the samples, the dimensionless figure-of-merit (ZT) of the Al2O3/Bi2Te3 nanocomposites was calculated and compared with that of pure Bi2Te3 materials and the results are shown in Table 1. Although the ZT value of the nanocomposite is slightly increased by the addition of alumina nanoparticles, the absolute value is not as high as expected. To date, n-type Bi2Te3 synthesized by wet-chemical processes [11,12] has shown relatively lower ZT values than that of Bi2Te3 obtained via the traditional powder metallurgy (PM) method [9]. We attribute this to increased electrical resistivity caused by alumina nanoparticles that are partly agglomerated in the bismuth telluride matrix materials.
Acknowledgments This work was supported by the principal R&D Program of Korea Institute of Materials Science and one of the authors (G.H.H.) thanks the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy (MKE), Republic of Korea. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Disalro FJ. Science 2000;287:1024–7. Poudel B, Hao Q, Ma Y, Lan Y, Minnich A, Yu B, et al. Science 2008;320:634–8. Toprak M, Zhang Y, Muhammed M. Mater Lett 2003;57:3976–82. Kim W, Zide J, Gossard A, Klenov D, Stemmer S. Phys Rev Lett 2006;96:045901. Faleev SV, Leonard F. Phys Rev B 2008;77:214304. Zhan G, Kuntz JD, Murkherjee AK, Zhu P, Koumoto K. Scr Mater 2006;54:77–82. Zhao L, Zhang B, Li J, Zhou M, Liu W, Liu J. J Alloys Compd 2008;455:259–64. Fan S, Zhao J, Yan Q, Ma J, Hong HH. J Electron Mater 2011;40:1018–23. Xie W, Tang X, Yan Y, Zhang Q, Tritt TM. Appl Phys Lett 2009;94:102111. Kim KT, Lee HM, Kim DW, Kim KJ, Ha GH. J Korean Phys Soc 2010;57:1037–40. Scheele M, Oeschler N, Meier K, Kornowski A, Klinke C, Weller H. Adv Funct Mater 2009;19:3476–83. [12] Dirmyer MR, Martin J, Nolas GS, Sen A, Badding JV. Small 2009;5:933–7.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of alumina nanoparticle-embedded-bismuth telluride matrix thermoelectric composite powders Kyung Tae Kim a,⁎, Hye Young Koo a, Gil-Geun Lee b, Gook Hyun Ha a a b
Powder Technology Department, Korea Institute of Materials Science, 797 Changwon-daero, Seongsan-gu, Changwon-si, Gyeongnam 641-831, Republic of Korea Division of Materials Science and Engineering, Pukyong National University, San100, Yongdang-Dong, Nam-Gu, Busan 608-739, Republic of Korea
a r t i c l e
i n f o
Article history: Received 12 March 2012 Accepted 7 May 2012 Available online 23 May 2012 Keywords: Bismuth telluride Thermoelectric Composite Powders Nanoparticles
a b s t r a c t Alumina nanoparticle-embedded-bismuth telluride matrix (Al2O3/Bi2Te3) composite powders were synthesized by chemical routes that employ a polyol reduction process to Bi acetates and Te chlorides in the solvent. With application of the polyol process, the synthesized composite powders show a disc-shaped morphology with 50 nm thickness and 1 μm diameter. High-resolution TEM images reveal that alumina nanoparticles are homogeneously embedded in the Bi2Te3 matrix powders. The Al2O3/Bi2Te3 nanocomposites sintered from the composite powders exhibit improved ZT values at room temperature compared to that of pure Bi2Te3 due to reduced thermal conductivity caused by active phonon and carrier scattering at the newly formed Al2O3/ Bi2Te3 interface. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Energy conversion by using a bismuth telluride (Bi2Te3) as a thermoelectric (TE) material is very attractive for achieving power generation without requiring any driving parts or cooling systems for electronic devices due to the high TE properties of Bi2Te3 at ambient temperature [1]. The energy conversion efficiency of TE devices depends on the dimensionless figure of merit, ZT which is defined as α2σTκ− 1, where α is the Seebeck coefficient, σ is the electrical conductivity, к is the thermal conductivity and T is the absolute temperature. A number of approaches have been explored in recent decades in efforts to improve the ZT by realizing a nanometer-scale microstructure; examples include grain refinement and the incorporation of nanoparticles in TE alloys [2]. It has been reported that thermal conductivity can be effectively reduced by increased phonon-scattering and by controlling electrical conductivity and/or the Seebeck coefficient via electron filtering through the use of a nanocomposite structure consisting of TE materials and nanoinclusions [3–6]. In particular, decreasing the lattice thermal conductivity by using a nano-sized second phase dispersed in the matrix has recently been recognized as an effective means of improving the ZT value. Specifically, effective phonon-scattering can occur while carriers pass through the dispersed nano-materials which are the size below 10 nm due to the difference in the wave-lengths of the phonons and carriers. Thus far, however, few studies on Bi2Te3 matrix nanocomposites with separately added nanomaterials have been reported [6–8], in contrast
⁎ Corresponding author. Tel.: + 82 55 280 3506; fax: + 82 55 280 3392. E-mail addresses: [email protected], [email protected] (K.T. Kim). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.05.053
with work on in-situ formed Te precipitates [2] or addition of amorphous nanocrystals [9]. In this study, we report on the production of alumina nanoparticledispersed bismuth telluride matrix (Al2O3/Bi2Te3) composite powders through a chemical route. The nanocomposites sintered from Al2O3/ Bi2Te3 composite powders show 15% lower thermal conductivity relative to pure Bi2Te3 bulk fabricated by the same process without Al2O3 nanoparticles. 2. Experimental procedure Al2O3 nanoparticles with size of about 10 nm were utilized as nanodispersoid materials. They were chemically treated and rendered into a stable colloidal solution in a solvent by adding a surfactant. The Al2O3/ Bi2Te3 and Bi2Te3 nanopowders was synthesized by using a polyol reduction process of Bismuth (III) acetates and Tellurium (IV) chlorides as precursors, respectively [10]. Dioctylether and 1,2-hexadecanediol were used as a solvent and reducing agent, respectively. Oleylamine, and trioctylphosphine (TOP) were added into the mixed solution as a surfactant and stabilizer, respectively. The mixed solution was heated to a temperature of 250 °C under an Ar gas atmosphere. The reactant after the polyol reaction was air-cooled on a mantle to room temperature. The synthesized Bi2Te3 powders and Al2O3/Bi2Te3 composite powders were consolidated by a spark plasma sintering process (Welltech Co., WL-15-400, Korea) at 350 °C for 10 min. The final size of spark plasma sintered Bi2Te3 and Al2O3/Bi2Te3 nanocomposite was 12.5 mm in diameter and 2.0 mm in thickness. The phase of the powders was characterized by the X-ray diffraction method (model no. X'pert MPD 3040 with Cu Kα radiation). The microstructures of the powders were analyzed by field emission scanning electron microscopy (FESEM, Hitachi
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4700S). Embedding of Al2O3 nanoparticles in Bi2Te3 matrix powders was characterized by a high resolution transmission electron microscopy (HRTEM) (JEM3010, 300 kV). While the electric properties of both sintered bodies were measured in the in-plane direction, the thermal diffusivity was characterized in the perpendicular direction to the buttontype sample. The room-temperature Seebeck coefficient and electrical resistivity of the samples were characterized using a Hall-effect measurement. Thermal conductivity was calculated from the thermal diffusivity measured by the Laser Flash method (LFA 457, Netzsch), and the specific heat and the density of the sintered body.
3. Results and discussion Fig. 1(a) schematically illustrates the key process for making alumina-embedded-bismuth telluride matrix composite powders. The alumina nanoparticles are homogeneously dispersed in the surfactantassisted solvent. The Bi and Te atoms, reduced by polyol agents, can be formed nearby the nanoparticles and a Bi2Te3 phase heterogeneously can be nucleated at the surface of surfactant-capped alumina rather than at the wall of the flask in the solution system. This process provides Al2O3/Bi2Te3 composite powders in which Al2O3 nanoparticles are
Fig. 1. Schematic illustration of (a) heterogeneous nucleation of Bi2Te3 materials onto alumina nanoparticles and Al2O3 nanoparticle-embedded Bi2Te3 matrix composite powders, (b) FEM-SEM image of the composite powders synthesized by chemical route (c) XRD patterns. (d) bright-field TEM image, (e) dark-field TEM image of Al2O3/Bi2Te3 composite powders, (f) EDS results displaying Al2O3 presenting at the region pointed out in (d).
K.T. Kim et al. / Materials Letters 82 (2012) 141–144
143
Fig. 2. (a) Photograph of sintered samples of Bi2Te3 and Al2O3/Bi2Te3 nanocomposites, (b) fracture-surface SEM image of the Al2O3/Bi2Te3 nanocomposite, (c) TEM image showing Al2O3 nanoparticles dispersed in bismuth telluride matrix and enlarged TEM image of (c).
homogeneously dispersed, as shown in Fig. 1(a). Once alumina nanoparticles are embedded in the Bi2Te3 matrix powders, it is expected that the dispersion status of the nanoparticles will remain even after sintering at the elevated sintering temperature. Fig. 1(b) shows FE-SEM surface morphologies of Al2O3/Bi2Te3 composite powders synthesized by the polyol reduction process. The powders have a regular disc-shape with a size of about 500 nm in plane-direction and a few tens of nm thickness. Several powders have transparent characteristics due to very thin thickness. The XRD patterns shown in Fig. 1(c) clearly exhibit a Bi2Te3 phase corresponding to JCPDS card no. 15-0863 while Al2O3 is not observed in the peaks due to its small volume fraction of less than 3%. The various indexed crystal planes including (015) as the main peak indicate that the composite powders consist of polycrystalline Bi2Te3 grains. The Bi2Te3 powders without Al2O3 nanoparticles also display similar XRD patterns. This indicates that the Bi2Te3 phase is well formed by the polyol process regardless of addition of Al2O3. Fig. 1(d) shows a bright-field TEM image of the composite powders. Many bright spots and gray-colored spots in the bismuth telluride matrix powders can be observed, while there are no spots in the Bi2Te3 powders. Fig. 1(e) exhibits a dark-field TEM image that clearly shows a homogeneous dispersion of alumina nanoparticles in the matrix powders. Fig. 1(f) shows the point-Energy Dispersive Spectrometer (EDS) results on the spot marked as “+” in Fig. 1(d). The result displays a clear Al (Kα) peak indicating the presence of Al2O3, since the only Al-related materials are the Al2O3 nanoparticles in the synthesized powders. These results demonstrate that the developed syntheticprocess is very suitable for obtaining homogeneously embedding Al2O3 nanoparticles in thermoelectric Bi2Te3 matrix powders. Fig. 2(a) shows a photograph of two samples sintered by a spark plasma sintering process. The left and right samples are machined-
Bi2Te3 bulk and Al2O3/Bi2Te3 nanocomposite, respectively, with size of 8 mm× 8 mm. Fig. 2(b) shows the FE-SEM morphology of the fracture surface of the nanocomposite revealing grains aligned in the characteristic direction. It appears that the directionality of the Bi2Te3 grains is determined by the disc shape of the initial powders, as shown in Fig. 1(b). Fig. 2(c) shows a bright-field TEM image of Al2O3 nanoparticle-embedded-bismuth telluride nanocomposites consolidated by spark plasma sintering process. The enlarged TEM image of Fig. 2(c) clearly indicates that Al2O3 nanoparticles are dispersed in the Bi2Te3 matrix grain rather than located between the grain boundaries after a sintering process at the elevated temperature. As provided in Table 1, the relative densities of the consolidated bulks are similar at roughly 98%, regardless of the addition of alumina nanoparticles. The electrical resistivity of the nanocomposites is slightly increased from 2.0 × 10 − 5 Ω m to 2.5 × 10 − 5 Ω m with the addition of Al2O3 nanoparticles at room temperature, because the numerous interfaces produced by the alumina nanoparticles in the Bi2Te3 matrix impede carrier movement. Meanwhile, the Seebeck coefficient of the Bi2Te3 materials at room temperature was enhanced 1.3-fold to − 128 μV/K by Al2O3 addition. The enhancement of the Seebeck coefficient is ascribed to the potential formed by the Al2O3/Bi2Te3 interface in the nanocomposite, which provides energy filtering behavior [5]. The measured thermal conductivity of the Al2O3/Bi2Te3 nanocomposite was reduced by 15% compared to that of pure Bi2Te3 at 25 °C. The thermal conductivity (к) is the sum of the electrical (кel), lattice (кph) and ambipolar (кam) conductivities. кel is directly proportional to the Lorentz constant (2.45 × 10 − 8 V 2 K − 2), electrical conductivity (σ), and temperature (T), the value, 0.29 W/mK of the nanocomposite calculated from кel = LσT at 25 °C is 20% lower than the value, 0.37 W/mK of pure Bi2Te3 fabricated by the same process without Al2O3. Hence, the reduction of
Table 1 Comparison of relative densities and thermoelectric properties measured at 300 K for Bi2Te3 and Al2O3/Bi2Te3 nanocomposites.
Bi2Te3 Al2O3/Bi2Te3
Relative density (%)
Electrical resistivity (10− 5 Ω m)
Seebeck coefficient (μV/K)
Total thermal conductivity, кtotal (W/mK)
Electrical–thermal conductivity, кel, (W/mK)
кtotal–кel (W/mK)
Figure-of-merit (ZT) at 300 K
98.5 98.0
2.0 2.5
− 100 − 128
1.05 0.85
0.37 0.29
0.68 0.56
0.16 0.20
144
K.T. Kim et al. / Materials Letters 82 (2012) 141–144
4. Conclusions Al2O3/Bi2Te3 composite powders characterized by alumina nanoparticles that are homogeneously embedded in bismuth telluride matrix powders are fabricated by using a polyol process. The thermal conductivity of the nanocomposite prepared from the powders is 20% lower than that of Bi2Te3 materials. The dimensionless ZT value of the nanocomposite is thus increased by alumina addition due to the desirable thermal conductivity and Seebeck coefficient despite degraded electrical conductivity. These findings clarify that incorporating nanoparticles in thermoelectric Bi2Te3 materials via a chemical route is a promising method to obtain reduced total thermal conductivity, which can result in an enhanced ZT value. Fig. 3. Measured thermal conductivities of Bi2Te3 and Al2O3/Bi2Te3 nanocomposites with increasing temperature ranging from 25 °C to 225 °C.
total thermal conductivity in the nanocomposites might originate from newly formed Al2O3/Bi2Te3 interfaces, which cause carrier and phonon dissipation. Fig. 3 shows that the reduced thermal conductivity of the nanocomposite is retained under temperature ranging from 25 °C to 225 °C. Regarding the thermoelectric properties of the samples, the dimensionless figure-of-merit (ZT) of the Al2O3/Bi2Te3 nanocomposites was calculated and compared with that of pure Bi2Te3 materials and the results are shown in Table 1. Although the ZT value of the nanocomposite is slightly increased by the addition of alumina nanoparticles, the absolute value is not as high as expected. To date, n-type Bi2Te3 synthesized by wet-chemical processes [11,12] has shown relatively lower ZT values than that of Bi2Te3 obtained via the traditional powder metallurgy (PM) method [9]. We attribute this to increased electrical resistivity caused by alumina nanoparticles that are partly agglomerated in the bismuth telluride matrix materials.
Acknowledgments This work was supported by the principal R&D Program of Korea Institute of Materials Science and one of the authors (G.H.H.) thanks the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy (MKE), Republic of Korea. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Disalro FJ. Science 2000;287:1024–7. Poudel B, Hao Q, Ma Y, Lan Y, Minnich A, Yu B, et al. Science 2008;320:634–8. Toprak M, Zhang Y, Muhammed M. Mater Lett 2003;57:3976–82. Kim W, Zide J, Gossard A, Klenov D, Stemmer S. Phys Rev Lett 2006;96:045901. Faleev SV, Leonard F. Phys Rev B 2008;77:214304. Zhan G, Kuntz JD, Murkherjee AK, Zhu P, Koumoto K. Scr Mater 2006;54:77–82. Zhao L, Zhang B, Li J, Zhou M, Liu W, Liu J. J Alloys Compd 2008;455:259–64. Fan S, Zhao J, Yan Q, Ma J, Hong HH. J Electron Mater 2011;40:1018–23. Xie W, Tang X, Yan Y, Zhang Q, Tritt TM. Appl Phys Lett 2009;94:102111. Kim KT, Lee HM, Kim DW, Kim KJ, Ha GH. J Korean Phys Soc 2010;57:1037–40. Scheele M, Oeschler N, Meier K, Kornowski A, Klinke C, Weller H. Adv Funct Mater 2009;19:3476–83. [12] Dirmyer MR, Martin J, Nolas GS, Sen A, Badding JV. Small 2009;5:933–7.