Abstract
We herein report the electronic and thermal transport properties of p-type Bi0.5Sb1.5Te3 polycrystalline bulks with dense pore structure. Dense pore structure was fabricated by vaporization of residual Te during the pressureless annealing of spark plasma sintered bulks of Te coated Bi0.5Sb1.5Te3 powders. The lattice thermal conductivity was effectively reduced to the value of 0.35 W m−1 K−1 at 300 K mainly due to the phonon scattering by pores, while the power factor was not significantly affected. An enhanced of 1.24 at 300 K was obtained in spark plasma sintered and annealed bulks of 3 wt.% Te coated Bi0.5Sb1.5Te3 by these synergetic effects.
1. Introduction
Since 1950s, Bi-Te-based thermoelectric (TE) materials have been intensively investigated in order to realize highly efficient power generation from low-grade heat (<250°C) or solid-state cooling and heating system. However, current applications are restricted to small scale systems such as climate control seat mainly due to their low TE performance. TE conversion efficiency is defined in terms of a dimensionless figure of merit, , where is the electrical conductivity, is the Seebeck coefficient, and is the total thermal conductivity at a given absolute temperature . Because value is controlled by electronic ( and ) and thermal transport properties , we can obtain the enhanced by following two approaches. The first approach is to reduce the lattice contribution () of by promoting phonon scattering while maintaining the electronic transport properties. The other approach is breaking the tradeoff between and through the density of states (DOS) engineering near at Fermi level.
Recently, nanostructuring has shown to be one of the most effective ways to reduce the of Bi2Te3-based TE materials. Many works of the literature have confirmed that enhanced phonon scattering at the interfaces can effectively reduce the without a significant reduction of [1–6]. There are two main approaches to prepare Bi2Te3-based nanostructured bulks. The first is the fabrication of nanograined structure to obtain the high density of the grain boundaries to scatter phonons. By formation of nanograined structure, a further reduction of becomes possible by intensified mid- and long-wavelength phonon scattering. There have been many experimental reports on nanograined structure Bi2Te3-based TE materials with enhanced values [1, 2]. For example, nanograined structure fabricated by high-energy ball milling combined with hot pressing raises the maximum value of p-type Bi0.5Sb1.5Te3 polycrystalline bulk up to 1.4 at 373 K from 1.0 at 300 K for its crystal ingot [1]. The second is the formation of nanoinclusion composite. Phase boundaries between TE material matrix and introduced nanoinclusions can act as effective phonon scattering centers; however, nanoinclusions through this approach limit enhancement up to ~20% so far mainly due to the difficulty of controlling the size and distribution of nanoinclusions [7–9].
Here we report on the newly developed nanostructuring approach for formation of pore structure in Bi-Te-based TE materials. We investigated the relation between the pore structure and TE transport properties. The origin of the enhanced could be elucidated from the viewpoint of the carrier filtering effect in addition to the phonon scattering in the presence of densely formed pore structure.
2. Experimental
Bi0.5Sb1.5Te3 ingots were prepared by conventional solid-state reaction of high-purity elemental Bi, Sb, and Te (>99.99%, 5N Plus). Bi, Sb, and Te granules were weighed with a stoichiometric ratio of the elements and sealed in an evacuated fused silica tube 12 mm in diameter. The tube was heated to 1073 K in a box furnace for 10 h and then quenched in water. The sample was ball-milled in N2-filled stainless steel vessel for 10 min using a high-energy mill (8000D, SPEX, USA). Ground Bi0.5Sb1.5Te3 powders show a wide grain size distribution ranging from a few nanometers to tens of microns as shown in Figure 1(a). Then, Te coated Bi0.5Sb1.5Te3 powders were fabricated by evaporation method. Figure 1(b) illustrates a schematic diagram of evaporation process. Firstly, Te powders (2, 6, 10, 20, 40, 60, and 80 wt.% of Bi0.5Sb1.5Te3 powders) were placed at the end zone of quartz tube and Bi0.5Sb1.5Te3 powders were loaded into the center part of quartz tube. The dead end part of quartz tube (Te powders) was placed in the muffle furnace with the open end of quartz tube connected with the vacuum pump, and then the furnace was heated to 370°C for 1–6 h, while the zone for Bi0.5Sb1.5Te3 powders keeps maintaining at room temperature.
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The Te coated Bi0.5Sb1.5Te3 powders were consolidated in a spark plasma sinter (SPS) using graphite die (diameter = 10.5 mm) in dynamic vacuum under an applied uniaxial pressure of 50 MPa at 753 K for 3 min. Then, the consolidated samples were annealed at 640 K for 20 h by the same evaporation method in order to remove the residual Te. The phases of Te coated Bi0.5Sb1.5Te3 powders were analyzed by powder X-ray diffraction experiments using an X-ray diffractometer equipped with Cu K radiation (). Seebeck coefficient and electrical conductivity measurements from 300 K to 450 K were performed using an ULVAC ZEM-3 system. The values were calculated from measurements taken separately: sample density (), heat capacity (), and thermal diffusivity () measured under vacuum by laser-flash method (TC-9000, ULVAC, Japan), in which was used as a constant value of 0.186 J g−1 K−1.
3. Results and Discussion
In order to fabricate the pore structure in Bi2Te3-based TE materials, we introduced the Te metal element with high vapor pressure (723 K) onto the surface of Bi0.5Sb1.5Te3 powders using evaporation method. Te layer with 8–11 μm thickness was coated onto the surface of Bi0.5Sb1.5Te3 powders (Figure 1(c)). The Te contents of final Te coated Bi0.5Sb1.5Te3 powders did not show significant change with the weight of Te loading (2–80 wt.%) for evaporation and were ranging from 1 to 3 wt.% of Bi0.5Sb1.5Te3 powders. This might be related to the nanowire-like growth of Te with restricted length and diameter as shown in Figure 1(d). The growth mechanism of Te nanowires will be published elsewhere. The powder X-ray diffraction (XRD) patterns of Bi0.5Sb1.5Te3 and 3 wt.% Te coated Bi0.5Sb1.5Te3 powders are shown in Figure 2. All patterns are indexable to the Bi2Te3 as a major phase and small amount of Te was also noticed. Peaks for other phases were not detected in all compositions. From the microstructure and XRD pattern, we concluded that Te coated Bi0.5Sb1.5Te3 powders could be successfully prepared by the present evaporation route.
The relative densities of SPS compacted bulks of wt.% Te coated Bi0.5Sb1.5Te3 powders (TeBST, ~95%) did not show significant change compared with SPS compacted bulks of Bi0.5Sb1.5Te3 powders (BST, ~96%). However, both of and values of 2TeBST and 3TeBST were lower than those of BST as shown in Figures 3(a) and 3(b). This result suggests that coated Te remained within the bulks and deteriorated the electronic transport properties due to its metallic characteristics. To remove the residual Te, TeBST were annealed at 640 K for 20 h under vacuum. Figures 4(a) and 4(b) show the SEM images of fractured surface for SPS compacted and vacuum-annealed 2 (2TeBSTa) and 3 wt.% Te coated Bi0.5Sb1.5Te3 (3TeBSTa). These figures clearly show the formation of dense pore structure with a wide size distribution ranging from a few to hundreds of nanometers. The relative densities of TeBSTa bulks were reduced to the value of 88%, indicating that vaporization of Te during the annealing process generated the pore structure. The values of 2TeBSTa and 3TeBSTa were lower than those of BST (Figure 3(a)) because of their low densities, while the values showed relatively small decrease around room temperature (Figure 3(b)). Thus, the power factors are not much degraded (inset of Figure 3(b)). This might be considered to be related to the carrier filtering effect at the surface of pores. is related to the energy derivative of the electronic density of state and the relaxation time through the Mott relation [10]. Thus enhancement can be attained by a carrier filtering effect, a strong energy dependence of caused by band bending at the surface of pores [11]. Experimental evidence of an enhancement of has also been reported in Bi2Te3-based materials [12].
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We evaluated the temperature dependence of the of 2TeBSTa and 3TeBSTa. The results are shown in Figure 5(a). Those of the BST are shown for comparison. Over the whole measured temperature range the value of shows significant reduction compared to those of BST. In order to clarify the phonon scattering effect by pores we calculated the using the following equation (inset of Figure 5): , where the electronic contribution () is estimated from the Wiedemann-Franz law, , with the Lorentz number W Ω K−2. Extremely low values of ranging from 0.35 to 0.39 W m−1 K−1 at 300 K were obtained in both the 2TeBSTa and 3TeBSTa. In comparison with the value of reference BST ( W m−1 K−1 at 300 K), decreased by about 50%, indicating stronger phonon scattering in the presence of densely formed pore structure.
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Figure 5(b) presents the s for 2TeBSTa and 3TeBSTa as a function of temperature. A high above 1.24 at 300 K was realized in 3TeBSTa, and this high value should be originated from its low value. By formation of pore structure without significantly affecting the charge carriers, this concept is an effective way to improve the TE performance by scattering phonons in addition to enhancement in by carrier filtering effect. Nevertheless, a novel processing technique for the generation of nanosize pores with <50 nm diameter is highly needed to realize the maximum by nanostructuring approach-based nanosized pore. Combined technique of nanosized powders fabrication for increasing the Te coating surface and controlled evaporation for forming the nanoscaled Te layer will give the possibility of further improvement of TE performance.
4. Conclusions
We successfully fabricated the dense pore structure in Bi0.5Sb1.5Te3-based TE materials by evaporation method of Te metal elements. The TE transport properties were investigated in the viewpoint of the pore structure. The characteristic TE properties are summarized as follows.(1)The thermal conductivity was largely reduced due to the significant phonon scattering by nanosized pores, leading to the extremely low lattice thermal conductivity of 0.35 W m−1 K−1 at 300 K.(2)The electronic transport properties were not much affected due to the enhancement of Seebeck coefficient, which is presumably attributed to the carrier filtering effect originated from nanosized pores. (3)The dense pore structured Bi0.5Sb1.5Te3 TE materials show the enhanced dimensionless figure of merit, of 1.24 at 300 K.
Acknowledgments
This research was supported by the Institute for Basic Science (IBS) in Korea and by the Human Resources Development Program (no. 20124010203270) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean Government’s Ministry of Trade, Industry and Energy.