Journal of the Korean Physical Society, Vol. 67, No. 10, November 2015, pp. 1809⇠1813
Thermoelectric and Transport Properties of Mechanically-alloyed Bi2 Te3 y Sey Solid Solutions A-Young Eum and Il-Ho Kim⇤ Department of Materials Science and Engineering, Korea National University of Transportation, Chungju 27469, Korea
Soon-Mok Choi School of Energy, Materials and Chemical Engineering, Korea University of Technology and Education, Cheonan 31253, Korea
Soonil Lee and Won-Seon Seo Energy and Environmental Materials Division, Korea Institute of Ceramic Engineering and Technology, Jinju 52851, Korea
Jae-Soung Park and Seung-Ho Yang Functional Material R&D Team, Heesung Metal, Ltd., Incheon 21697, Korea (Received 8 September 2015) Bi2 Te3 y Sey (y = 0 0.6) solid solutions were prepared by using mechanical alloying and hot pressing. The lattice constants decreased with increasing Se content, which revealed the successful formation of solid solutions by using a planetary mill. The relative densities of hot-pressed specimens were higher than 96%. All specimens indicated n-type conductions in the measuring temperature range from 323 K to 523 K, and the electrical conductivity slightly decreased with increasing temperature. The Seebeck coefficient increased with increasing Se content, and the electrical and the thermal conductivities decreased; thus, the dimensionless figure of merit was improved. A maximum dimensionless figure of merit of 0.76 was obtained at 473 K for Bi2 Te2.55 Se0.45 . PACS numbers: 72.15.Jf, 72.20.Pa Keywords: Thermoelectric, Bismuth telluride, Mechanical alloying, Hot pressing, Charge transport DOI: 10.3938/jkps.67.1809
I. INTRODUCTION Recently, thermoelectric materials have been receiving increasing attention because of energy exhaustion and environmental problems, and their applications to power generation and electronic cooling have been studied for the past decades [1, 2]. The efficiency of thermoelectric materials is evaluated as the dimensionless figure of merit, ZT = ↵2 1 T, where ↵ is the Seebeck coefficient, is the electrical conductivity, is the thermal conductivity, and T is the absolute temperature [3]. Therefore, a large Seebeck coefficient, a high electrical conductivity, and a low thermal conductivity are required to improve the thermoelectric performance of a material [4]. Bi2 Te3 -based materials have good thermoelectric properties at room temperature. Bi2 Te3 has a rhombohedral structure with -Te1 -Bi-Te2 -Bi-Te1 - quintuple layers stacked along the c-axis direction [5, 6]. Bi2 Se3 has ⇤ E-mail:
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the same crystal structure and similar electronic structure as Bi2 Te3 ; thus, homogeneous solid solutions of Bi2 Te3 y Sey can be formed The Se atom occupies the Te sites, thereby decreases the carrier mobility and the lattice thermal conductivity due to the intensified alloy scattering for electrons and phonons [7,8]. In addition, the lattice constants decrease with increasing Se content due to the atomic radius of Se being smaller than that of Te [9]. Therefore, changes in the thermoelectric properties are expected for di↵erent Se contents. The unidirectional crystal-growth method exhibits excellent thermoelectric performance, but poor mechanical performance along the cleavage plane due to the weak van der Waals bonding between Te1 -Te1 [10,11]. Therefore, studies to improve the mechanical properties without the decrease in the thermoelectric properties have been carried out [12,13]. Poudel et al. [14] reported ZT = 1.4 at 373 K for p-type Bi0.5 Sb1.5 Te3 prepared by using mechanical alloying and hot pressing, and Lee et al. [15] obtained ZT = 0.86 at 323 K for p-type Bi0.5 Sb1.5 Te3
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Journal of the Korean Physical Society, Vol. 67, No. 10, November 2015
Table 1. Chemical composition, lattice constant, relative density and transport properties of Bi2 Te3 Composition Nominal Bi2 Te3 Bi2 Te2.85 Se0.15 Bi2 Te2.7 Se0.3 Bi2 Te2.55 Se0.45 Bi2 Te2.4 Se0.6
Actual Bi2.02 Te2.92 Bi1.96 Te2.89 Se0.15 Bi1.98 Te2.73 Se0.29 Bi1.99 Te2.58 Se0.43 Bi1.94 Te2.35 Se0.61
Lattice constant [nm] a c 0.4383 3.0414 0.4368 3.0362 0.4345 3.0344 0.4336 3.0266 0.4323 3.0111
Relative Density [%] 94.8 95.3 96.5 95.9 96.0
Hall Coefficient [cm3 C 1 ] 8.18 ⇥ 10 2 9.09 ⇥ 10 2 1.04 ⇥ 10 1 1.21 ⇥ 10 1 1.45 ⇥ 10 1
y Sey .
Mobility [cm2 V 1 s 1 ] 96.92 84.65 75.74 75.30 73.44
Carrier Concentration [cm 3 ] 7.63 ⇥ 1019 6.87 ⇥ 1019 5.98 ⇥ 1019 5.17 ⇥ 1019 4.29 ⇥ 1019
prepared by using mechanical alloying and hot pressing. Yan et al. [16] obtained ZT = 1.04 at 398 K for n-type Bi2 Te2.7 Se0.3 prepared by using mechanical alloying and hot pressing, and Lee et al. [17] reported ZT = 0.63 at 440 K for n-type Bi2 Te2.4 Se0.6 prepared by using mechanical alloying and hot pressing. In the present study, n-type Bi2 Te3 y Sey solid solutions were synthesized by using mechanical alloying with a planetary mill and were consolidated by using hot pressing. The e↵ects of Se on the crystal lattice, the microstructure, and the transport and the thermoelectric properties were examined.
II. EXPERIMENTS AND DISCUSSION Bi (< 180 µm, purity 99.999%), Te (< 150 µm, purity 99.999%), and Se (< 75 µm, purity 99.999%) were weighed according to the stoichiometric composition. Bi2 Te3 y Sey solid solutions (y = 0 0.6) were prepared by using mechanical alloying (MA) and hot pressing (HP). The mixed powders were then placed with steel balls with a diameter of 5 mm into a hardened steel jar at a ball-to-powder weight ratio of 20 (BPR = 20). The MA was performed at 300 rpm for 8 h in a planetary mill (Fritsch Pulverisette5). The synthesized powders were then hot-pressed in a cylindrical graphite die with internal diameter of 10 mm at 623 K under a pressure of 70 MPa for 1 h in a vacuum. The phases and the lattice constants of the solid solutions were analyzed by using an X-ray di↵ractometer (XRD; Bruker D8-Advance) with CuK radiation in the ✓-2✓ mode (10 90 ) at a step size of 0.02 and a scan speed of 0.4 sec/step. Microstructure distributions were observed via a scanning electron microscope (SEM; FEI Quanta400) with an energy dispersive spectroscope (EDS; Oxford JSM-5800). The Hall coefficient, carrier concentration and mobility were measured by using the van der Pauw method (Keithley 7065) with a magnetic field (1 T) and an electric current (50 mA). The Seebeck coefficient and the electrical conductivity were measured by using the ZEM3 (Ulvac-Riko) system in a helium atmosphere. The thermal conductivity was calculated from the specific heat, density and thermal di↵usivity measured by using the
Fig. 1. (Color online) (a) X-ray di↵raction patterns of Bi2 Te3 y Sey solid solutions and (b) di↵raction peak shifts due to Se substitution.
TC-9000H (Ulvac-Riko) system. The dimensionless figure of merit was evaluated at temperatures from 323 K to 523 K. Figure 1 shows the XRD patterns of the Bi2 Te3 y Sey solid solutions. As presented in Fig. 1(a), all di↵raction peaks corresponded to the rhombohedral crystal structure of Bi2 Te3 (ICDD PDF# 15-0863); thus, Bi2 Te3 y Sey solid solutions were successfully synthe-
Thermoelectric and Transport Properties of Mechanically-alloyed · · · – A-Young Eum et al.
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Fig. 2. (Color online) Microstructures and elemental distributions of Bi2 Te3 y Sey solid solutions.
sized. The enlarged (015) and (10·10) peaks in Fig. 1(b) obviously confirmed the shift to higher angles with increasing Se content, which resulted from a decrease in the lattice constants due to substitution of Se for Te. Table 1 presents the lattice constants for the Bi2 Te3 y Sey solid solutions. The lattice constants decreased with increasing Se content because the atomic radius of Se (0.115 nm) was smaller than that of Te (0.14 nm) [9]. The decrease in the c-axis was larger than that in the a-axis: the a-axis decreased from 0.4383 nm to 0.4323 nm, and the c-axis decreased from 3.0414 nm to 3.0111 nm. Figure 2 shows the microstructures and the elemental distributions of Bi2 Te3 y Sey . The relative densities of all hot-pressed specimens were higher than 96%, and the component elements were distributed homogeneously. The microstructure rarely changed regardless of Se content. The actual compositions were similar to the nominal compositions; thus, precise control of the composition was possible during the synthesis and the sintering processes. The electronic transport properties of Bi2 Te3 y Sey at room temperature are presented in Table 1. The Hall coefficient of all specimens had a negative sign, indicating n-type conduction. The carrier concentration and the mobility decreased with increasing Se content. The electronic transport properties in the Bi2 Te3 -based alloys strongly depended on the antisite defects and anion vacancies [18, 19]. When < 3/2 ( : the ratio of anionic to cationic components), excess Bi atoms will occupy the Te sites to generate BiTe antisite defects acting as electron acceptors. However, when > 3/2, excess Te atoms will occupy the Bi sites to generate TeBi antisite defects acting as electron donors [20]. When y 1.2 in Bi2 Te3 y Sey , the carrier concentration decreases with increasing Se content due to a decrease in the TeBi defects [21]. Figure 3 presents the electrical conductivity of Bi2 Te3 y Sey . The electrical conductivity decreased with increasing Se content due to a decrease in the carrier concentration. Therefore, the electrical conductivity was controllable by the formation of solid solutions between Bi2 Te3 and Bi2 Se3 . The electrical conductivity decreased with increasing temperature, indicating a degenerate semiconductor behavior. This resulted from the high
Fig. 3. Temperature dependence of the electrical conductivity of Bi2 Te3 y Sey .
Fig. 4. Temperature dependence of the Seebeck coefficient of Bi2 Te3 y Sey .
carrier concentration, which was larger than 1019 cm 3 . Figure 4 illustrates the Seebeck coefficient of Bi2 Te3 y Sey . All specimens had negative Seebeck coefficients, indicating n-type semiconductors with majority carriers of electrons. The Seebeck coefficient changed from 131 µVK 1 to 173.6 µVK 1 at 323 K with increasing Se content. The absolute value of the Seebeck coefficient, |↵|, increased with increasing temperature, reaching a maximum value at 473 K. The maximum value of |↵| was | 188| µVK 1 at 473 K for Bi2 Te2.4 Se0.6 . The Seebeck coefficient of an n-type material can be expressed as 2 8⇡ 2 kB m⇤ T ⇣ ⇡ ⌘2/3 ↵= (1) 3eh2 3n where kB is the Boltzmann constant, e is the electronic charge, h is the Plank constant, m⇤ is the e↵ective carrier mass, and n is the carrier concentration [22]. Therefore, the |↵| increased with increasing Se constant due
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Journal of the Korean Physical Society, Vol. 67, No. 10, November 2015
Fig. 5. Temperature dependence of the power factor of Bi2 Te3 y Sey .
to a decrease in the carrier concentration. However, as the temperature increased, the value of |↵| reached its peak value due to an increase in the carrier concentration caused by intrinsic transitions. Figure 5 shows the power factor of Bi2 Te3 y Sey . According to the power factor relation PF = ↵2 [23], the PF can be enhanced by increasing the Seebeck coefficient and/or the electrical conductivity. However, the PF has a trade-o↵ relation with the Seebeck coefficient and the electrical conductivity because there two components depend on the carrier concentration. Despite its low Seebeck coefficient, Bi2 Te3 has the highest PF value because of its high electrical conductivity. The PF values decreased with increasing Se content. Figure 6 presents the thermal conductivity of Bi2 Te3 y Sey . The thermal conductivity () is the sum of the lattice thermal conductivity (L ) and the electronic thermal conductivity (E ), which can be calculated using the Wiedemann-Franz law (E = L T) [24], where the Lorenz number L = 2.0 ⇥ 10 8 V2 K 2 was taken in this study. The thermal conductivity decreased with increasing Se content, and this resulted from the decreased electronic thermal conductivity due to the decreased carrier concentration, as shown in Fig. 6(b) and Table 1. Generally, the Se substitution for Te reduces the lattice thermal conductivity due to alloy scattering for electrons and phonons [7,8], while in this study, the contribution of Se in reducing the lattice thermal conductivity was not significant, as shown in Fig. 6(c). Figure 7 shows the dimensionless figure of merit (ZT) of Bi2 Te3 y Sey . Bi2 Te3 exhibited the lowest ZT value due to the high thermal conductivity despite the high PF. The Seebeck coefficient increased with increasing Se content while the thermal conductivity decreased. Consequently, the ZT value was enhanced at all temperatures measured. A maximum ZTmax = 0.76 was achieved at 473 K for Bi2 Te2.55 Se0.45 , which was lower than ZTmax
Fig. 6. Temperature dependence of the thermal conductivity of Bi2 Te3 y Sey : (a) total thermal conductivity, (b) electronic thermal conductivity and (c) lattice thermal conductivity.
= 1.08 at 303 K obtained by Bi2 Te2.7 Se0.3 prepared by using zone melting [25], but higher than ZTmax = 0.63 at 440 K for Bi2 Te2.4 Se0.6 prepared by using attrition milling and hot pressing [26].
Thermoelectric and Transport Properties of Mechanically-alloyed · · · – A-Young Eum et al.
Fig. 7. Temperature dependence of the dimensionless figure of merit of Bi2 Te3 y Sey .
III. CONCLUSION Bi2 Te3 y Sey solid solutions were prepared by using the MA-HP process. The lattice constants decreased with increasing Se content, which confirmed the successful formation of Bi2 Te3 y Sey solid solutions by using planetary milling. The carrier concentration and the electrical conductivity decreased with increasing Se content. All specimens indicated n-type conductions in the measured temperature range. The decreased carrier concentration caused by Se substitution reduced the electronic thermal conductivity while the phonon scattering had little e↵ect on the lattice thermal conductivity. As a result, the thermoelectric properties were improved by Se substitution, and a maximum ZT value of 0.76 was obtained at 473 K for Bi2 Te2.55 Se0.45 .
ACKNOWLEDGMENTS This study was supported by the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Trade, Industry and Energy, Republic of Korea.
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