Abstract
Alumina nanoparticle-dispersed bismuth-antimony-tellurium matrix (Al2O3/BST) composite powders were fabricated by using ball milling process of alumina nanoparticle about 10 nm and p-type bismuth telluride nanopowders prepared from the mechanochemical process (MCP). The fabricated Al2O3/BST composite powders were a few hundreds of nanometer in size, with a clear Bi0.5Sb1.5Te3 phase. The composite powders were consolidated into p-type bulk composite by spark plasma sintering process. High-resolution TEM images reveal that alumina nanoparticles were dispersed among the grain boundary or in the matrix grain. The sintered 0.3 vol.% Al2O3/BST composite exhibited significantly improved power factor and reduced thermal conductivity in the temperature ranging from 293 to 473 K compared to those of pure BST. From these results, the highly increased ZT value of 1.5 was obtained from 0.3 vol.% Al2O3/BST composite at 323 K.
1. Introduction
Bismuth telluride (Bi2Te3)-based alloys are very attractive thermoelectric (TE) materials due to their high energy conversion efficiency at ambient temperature for achieving power generation without requiring any driving parts or cooling systems in electronic devices [1]. It is well known that the TE conversion efficiency of thermoelectric devices is expressed as the dimensionless figure-of-merit, ZT, which is calculated from , where is the absolute temperature, α is the Seebeck coefficient, and ρ and κ are respectively the electrical resistivity and thermal conductivity. Thus, current researches have concentrated on obtaining a reduced thermal conductivity and increased electrical properties, respectively in order to enhance ZT values. Nanostructuring of the microstructure and the incorporation of nanoparticles in bismuth telluride TE alloys have been reported as efficient approaches to achieve high ZT value by controlling carrier and phonon transporting behaviors [2–5].
Recently, many researchers report that controlling electrical conductivity and/or the Seebeck coefficient can be simultaneously improved by an electron filtering through the use of a composite structure consisting of TE materials and nanoinclusions such as SiC nanoparticles, and carbon nanotubes [6–10]. Furthermore, the concept of the reduction in the lattice thermal conductivity more than increasing electrical resistivity has been widely accepted to improve the ZT value, because phonons are effectively blocked while carriers pass through the dispersed nanoparticles below 10 nm in size due to the difference in the wavelengths between the phonons ad carriers. However, most of the bismuth telluride matrix composite is based on binary Bi2Te3-based materials [6, 8, 9]. There have been a few studies on achieving high ZT values of ternary Bi2Te3 matrix composite including the separately added nanoparticles.
In this study, we report thermoelectric properties of alumina nanoparticle-dispersed p-type Bi0.5Sb1.5Te3 matrix composite fabricated by high-energy ball milling process and followed by spark plasma sintering (SPS) process. The composites sintered from 0.3 vol.% Al2O3/BST composite powders show significantly increased ZT value, 1.5 compared to 1.1 of pure BST materials.
2. Experimental Procedure
Al2O3 nanoparticles used in this study have size of about 10 nm and α-phase. They were chemically treated into a stable colloidal solution in a solvent by adding a surfactant. We used mechanochemical process to synthesize p-type Bi-Sb-Te (BST) powders with Bi0.5Sb1.5Te3 phase from a solution of Bi2(SO4)3, SbCl3, and TeO2 powders that are supplied from byproducts of Cu refining process. During the MCP, Bi sulfate, Sb chloride, and Te oxide were homogeneously mixed by ball milling process, and the milled powders were subsequently dried and calcinated into complex oxide materials. Finally, the calcinated powders were reduced to Bi-Sb-Te-based TE powders (for more detailed experimental procedures, refer to [11]). The synthesized Bi0.5Sb1.5Te3 powders were mixed with Al2O3 nanoparticles by a high-energy ball milling process (Planetary Milling Machine, Fritsch) in a hexane solvent. The Al2O3/BST composite powders were consolidated by a spark plasma sintering process (SPS, Weltech Co., WL-15-400, Republic of Korea) at 350°C for 10 min [12]. The phase of the powders and consolidated bulk was characterized by the X-ray diffraction method (model no. X’pert MPD 3040 with CuKα radiation). The microstructures of the powders and bulk composites were analyzed by field emission scanning electron microscopy (FESEM). Embedding of Al2O3 nanoparticles in BST matrix powders was characterized by a high-resolution transmission electron microscopy (HR-TEM, FE-TEM, 200 kV JEM2100F, JEOL). The weigh percent of the Al2O3 was found to be 0.014 wt%, corresponding to a 0.3 vol.% Al2O3/Bi0.5Sb1.5Te3 composite when a density of 3.96 g/cc of the Al2O3 and a density of 6.88 g/cc of Bi0.5Sb1.5Te3 were applied. The thermoelectric properties of the sintered body were characterized using ZEM-3 (Ulvac-Rico), and the relative density was evaluated by Archimedes method. Thermal conductivity of the BST and Al2O3/BST composites was calculated by the thermal diffusivity measured by the laser flash method (LFA457, Netzsch) and the specific heat measured by a differential scanning calorimeter (DSC) and the density of the sintered body.
3. Results and Discussion
Figure 1(a) shows the surface morphology of dried alumina nanoparticles used in this study. The size of primary nanoparticles is ranging from 10 to 20 nm and the primary nanoparticles are agglomerated into a few hundreds nm in diameters. The shape of the agglomerated nanoparticles is spherical due to reducing the surface energy. Figure 1(b) shows the size distribution of the nanoparticles solved in water solution. When the concentration of Al2O3 nanoparticles is 50 wt%, the size of nanoparticles exhibits a few micrometers in average size because of the agglomeration among them. As the concentration is reduced into 3 wt%, the size of about 1020 nm is revealed in the solvent. Thus, the alumina nanoparticles 3 wt% solution were used for homogeneous mixing with p-type powders. Figure 1(c) shows the morphology of Bi-Sb-Te-based powders prepared from the mechanochemical process (MCP) of the byproducts obtained from the Cu refining process. The BST powders show 100 nm of averaged size and it was found that XRD patterns of the fabricated powders are corresponded to Bi0.5Sb1.5Te3 phase of JCPDS card no. 49-1713. The FE-SEM image shown in Figure 1(e) exhibits the morphology of Al2O3/BST composite powders prepared after ball milling process. The high-resolution TEM image of the composite powders reveals that alumina nanoparticles are embedded or dispersed in the Bi0.5Sb1.5Te3 matrix powders.
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Figure 2(a) shows the fracture surface of the Al2O3/BST composite consolidated by SPS process. The SPS provides fast densification time not to agglomerate Al2O3/nanoparticles during the consolidation, even though hot pressing method also gives high density of p-type bismuth telluride powders. Typical brittle fracture of bismuth telluride-based materials is observed and some closed pores are partly revealed on the surface. A TEM image of Figure 2(b) exhibits grain boundaries of BST matrix and dispersed Al2O3 nanoparticles which shows bright region as indicated by the arrows. From the microstructure, it was analyzed that the distribution of Al2O3 nanoparticles are divided as intragrain type and intergrain type, respectively, as schematically illustrated in Figure 2(c). We expect that this microstructure provides a big benefit reducing total thermal conductivity because of the newly formed extrinsic Al2O3/BST interfaces besides the BST grain boundaries. Figure 2(d) shows the variation of XRD patterns of 0.3, 0.5, and 1.0 vol.% Al2O3/BST composites compared with pure BST materials. All XRD patterns have no big difference nonetheless contents of Al2O3 nanoparticles increase. This implies that the addition of alumina nanoparticles do not affect the phase of BST matrix.
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Figure 3 shows thermoelectric properties of pure BST and Al2O3/BST composites characterized at the temperature ranging from 293 to 473 K. Figure 3(a) shows the electrical resistivities as a function of temperature. The measured electrical resistivity shows an increasing behavior by increasing temperature. The electrical resistivity of 1.0 vol.% Al2O3/BST composite increases from 1.5 to Ωm when compared at 293 K and this tendency is still continuously remained by 473 K. Electrical resistivity of Al2O3/BST composite is increased as volume fraction of nonconductive Al2O3 materials increases. However, 0.3 vol.% Al2O3/BST composite shows similar electrical resistivity values with pure BST materials. This behavior means that addition of 0.3 vol.% Al2O3 nanoparticles does not affect the carrier transporting behaviors of pure BST. As shown in Table 1, carrier mobility of 0.3 vol.% Al2O3/BST composite does not change compared to pure BST.
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The measured Seebeck coefficients of all samples show clearly positive values. These mean that the fabricated samples have p-type semiconducting behaviors. Seebeck coefficients of pure BST vary about 205210 μV/K as a function of temperature. The maximum Seebeck coefficient, 235 μV/K, is obtained from the 0.3 vol.% Al2O3/BST composite at 373 K. As shown in Table 1, carrier density of 0.3 vol.% Al2O3/BST composite shows the lowest value compared to other samples. It can be considered that the lower Fermi energy level caused by decreasing carrier density results in an increased Seebeck coefficient [13]. Furthermore, the results of Hall effect measurement in Table 1 also show that once Al2O3 nanoparticles below 10 nm in diameter are well dispersed under critical volume fraction in the matrix, they affect to carrier mobility and Seebeck coefficient.
Figure 3(c) shows the power factors calculated from the electrical properties. The power factor values of all samples are decreasing with increasing the temperature. However, all power factors of the composites are higher than that of pure BST due to the increased Seebeck coefficients. The peak power factor, 33 μW/K2 cm, was also obtained from the 0.3 vol.% Al2O3/BST composite at 293 K. This value is 1.7 times higher than 22 μW/K2 cm of pure BST materials. Although the difference between power factors is reduced as temperature increases, the tendency is continuously remained by 473 K. Figure 3(d) displays variation of thermal conductivity of the composites compared with pure BST materials as a function of temperature. The measured thermal conductivity of BST is about 0.8 W/mK and the values are decreased by the addition of alumina nanoparticles. As the volume fraction of alumina nanoparticles increases from 0.3 to 1.0 vol.%, total thermal conductivity is decreased down to 0.7 W/mK. It is considered that many interfaces generated from Al2O3 nanoparticles dispersed in the BST matrix result in the reduction of total thermal conductivity. The result implies that a reduction of the total thermal conductivity in the composites may originate from the newly formed 0.3 vol.% Al2O3/BST interfaces, which cause carrier and lattice phonon dissipation, as reported previously by the authors in an alumina nanoparticles/Bi2Te3 system [14].
Figure 4 shows the dimensionless figures-of-merit (ZT) of pure BST and the Al2O3/BST composites calculated from the thermoelectric properties displayed in Figure 3. The ZT of the composites shows higher values than those of pure BST as a function of temperature ranging from room temperature to 474 K. Furthermore, the maximum ZT value, 1.5, has been obtained from the 0.3 vol.% Al2O3/BST composite at 373 K due to significantly increased powder factors.
4. Summary
In summary, p-type BST nanopowders prepared from MCP were mixed with 0.3, 0.5, and 1.0 vol.% Al2O3 nanoparticles by a high-energy ball milling process. The fabricated Al2O3/BST composite powders show clear Bi0.5Sb1.5Te3 phase as designed. It was found that the thermoelectric properties of the sintered Al2O3/BST composite were significantly changed by the contents of alumina nanoparticles. The maximum ZT, 1.5, was obtained from the 0.3 vol.% Al2O3/BST composite that exhibited high power factor and reduced thermal conductivity. These results clarify that the volume fraction of dispersion agents should be considered for the optimization of electrical and thermal transport behaviors in thermoelectric materials.
Acknowledgments
This work was supported by the principal R&D Program of the Korea Institute of Materials Science, Republic of Korea and Kyung Tae Kim appreciates the support by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grant no. 2013R1A1A2010377).