Uniformity of Thermoelectric Properties of N-type Bi₂Te_3-ySe_y Bulky Compacts

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Abstract

1. INTRODUCTION

Because thermoelectric parameters (electrical conductivity, Seebeck coefficient, and thermal conductivity) are temperature-dependent, the temperature that exhibits the best thermoelectric performance is different for each material. Skutterudites, clathrates, and silicides are good thermoelectric materials in the mid/high-temperature range, and -based materials exhibit excellent performance in the low-temperature range [1]. In general, is used as p-type (Bi,Sb)₂Te₃ or n-type Bi₂(Te,Se)₃ by forming a solid solution with Sb₂Te₃ or Bi₂Se₃ with the same crystal structure (space group R 3¯ m). However, n-type materials exhibit lower ZT values than p-type materials [2-4], and their thermoelectric property changes are highly sensitive to the composition and carrier concentration [5]. Therefore, because both p-type and n-type materials are used to manufacture thermoelectric devices, it is necessary to improve the thermoelectric performance by optimizing the composition and carrier concentration of the n-type Bi₂Te₃-based materials.

Fig 1 shows the equilibrium phase diagram of a Bi-Te binary system [6]. In addition to Bi₂Te₃, various intermediate phases exist such as Bi₇Te₃, Bi₂Te, Bi4Te₃, etc. The crystal systems of Bi₂Te₃ and Bi₂Se₃ are rhombohedral (rhombohedric hexagonal), and as shown in Fig 2, they exhibit a stacked structure perpendicular to the c-axis in the following order [7,8]:

Fig. 1.

Equilibrium phase diagram of Bi-Te binary system [6].

Fig. 2.

Crystal structure of Bi₂Te3 showing van der Waals gaps between quintuple layers [7,8].

-Te¹(Se¹)-Bi-Te²(Se²)-Bi-Te¹(Se¹)-

where the superscripts indicate crystallographically different positions. While other bonding surfaces have ionic or covalent bonds, the bonding surfaces of Te¹(Se¹)-Te¹(Se¹) have weak van der Waals bonds; thus, cleavage planes exist along the basal plane perpendicular to the c-axis [9]. Therefore, the mechanical properties along the cleavage plane are brittle [10], and in the case of single crystals, strong anisotropy appears in the thermal/electrical properties [11,12].

Because Bi₂Te₃ and Bi₂Se₃ form a solid solution of Bi₂Te_3-ySe_y over the entire composition range, the periodicity of the crystal lattice is maintained; however, the lattice thermal conductivity may be reduced by phonon scattering at short wavelengths due to deformation of the lattice through alloying. In addition, when Bi₂Te₃ and Bi₂Se₃ form a solid solution, the highly electronegative Se occupies the position of Te². Because the atomic radius of Se is smaller than that of Te, the lattice constant decreases as the substitution amount of Se increases, and the bandgap energy increases because of the formation of Bi-Se² bonds, which have stronger binding forces than Bi-Te² bonds [13]. Therefore, changes in the thermal/electrical and lattice properties are expected when Se is substituted for Te.

The substitution of Bi or Te in Bi₂Te₃ by a dopant changes the electronic structure and transport characteristics and thus affects the thermoelectric properties. In addition, doping can decrease the lattice thermal conductivity by increasing phonon scattering due to lattice distortion caused by impurities [14,15]. Halogen elements such as I and Br are known to act as donors, and halides such as SbI₃, AgI, TeI₄, CdBr₂, and AgBr are used because of the high reactivity of these halogen elements [16,17]. The halogen replaces the Te site, donating electrons [18], and is almost completely ionized in Bi₂Te₃ owing to its low ionization energy [19]. Various elements are known to be acceptors for Bi₂Te₃; transition atoms (Cu, Ag, and Zn), group-IV elements (Ge, Sn, and Pb), and group-V elements (As, Sb, and Bi) increase the hole concentration by substituting the Bi sites [17,20]. Therefore, in n-type Bi₂(Te,Se)₃, adopting acceptors can improve the power factor by increasing the Seebeck coefficient, which results from a decrease in carrier concentration.

In this study, Bi₂Te₃-Bi₂Se₃ solid solutions (Bi₂Te_3-ySe_y) were formed to reduce thermal conductivity due to increased phonon scattering by substitution, and various dopants (D) were incorporated to improve the electrical properties. In addition, bulky compacts of n-type Bi₂Te_3-ySe_y:D_m were prepared for application in thermoelectric modules, and the uniformity of their thermoelectric properties were examined.

2. SYNTHESIS AND ANALYSIS

Bi₂Te₃-Bi₂Se₃ solid solutions were synthesized using the encapsulated melting (EM) method. In this study, three types of specimens were prepared; their detailed compositions are as follows [21-26]:

① Bi₂Te_3-ySe_y (y = 0, 0.15, 0.3, 0.45, and 0.6)

② Bi₂Te_2.85Se_0.15:D_m (D = I, Cu, Ag, In, Al, Ni, Zn, Ge, and Sn; m = 0-0.03)

③ Bi₂Te_2.7Se_0.3:D_m (D = I, Cu, and Ag; m = 00.02)

Elemental Bi (purity 99.999%), Te (purity 99.999%), Se (purity 99.999%), and dopants (purity 99.9%-99.95%) were weighed according to their compositions and loaded into a quartz tube sealed in a vacuum. The sealed raw materials were melted at 1073 K for 4 h and subsequently cooled in a furnace. After crushing the obtained ingot to less than 75 μm, the powder was placed into a graphite mold with an inner diameter of 10ϕ (specimens for measuring thermoelectric properties) and 25ϕ (specimens for evaluating uniformity of thermoelectric performance), and sintered by hot pressing (HP) for 1 h at 648-698 K under a pressure of 70 MPa. Fig 3 shows the 10ϕ- and 25ϕ-diameter Bi₂Te_3-ySe_y:D_m sintered bodies prepared using the EM-HP process.

Fig. 3.

Bi₂Te_3-ySe_y:D_m chalcogenide compacts with diameters of 10ϕ and 25ϕ prepared by the EM-HP process (unit: mm).

Solid-solution formation and phase change were investigated by X-ray diffraction (XRD: Bruker, D8-Advance) analysis using Cu K_α radiation. To examine the thermoelectric properties, Seebeck coefficient (α), electrical conductivity (σ), and thermal conductivity (κ) were measured in the temperature (T) range of 323-523 K. To measure the Seebeck coefficient and electrical conductivity, the HP compact was cut into dimension of 3 mm × 3 mm × 9 mm, and the specimen for thermal conductivity measurement was processed into a disk shape with a diameter of 10 mm × thickness 1 mm. The power factor (PF = α²× σ) was calculated from the Seebeck coefficient and electrical conductivity measured using Advance Riko ZEM3 equipment, and the thermal conductivity (κ = d · c_p · r) was obtained from the thermal diffusivity (d), specific heat (cp), and density (r) measured using Advance Riko TC-9000H laser flash system. Finally, the thermoelectric performance was evaluated by calculating the dimensionless figure of merit (ZT = PF · T · κ^-1)

3. THERMOELECTRIC PERFORMANCE

Miller and Li [27] found that antisite (antistructure) defects in Bi₂Te₃-based materials predominantly affect the carrier concentration, while Horak et al. [28] and Cho and Kim [29,30] suggested the following equilibrium defect equations:

Stoichiometric Bi₂Te₃ exhibits n-type properties [31]. However, because of the evaporation of Te at a high vapor pressure, Bi exists in excess, and thus Bi atoms occupy the Te sites, producing Bi_Te^- antisite defects, which donate holes to exhibit p-type characteristics (Equation 1). When Bi₂Te₃ and Bi₂Se₃ form a solid solution, Se atoms are substituted at the Te sites, and Bi_Te^- antisite defects are reduced; thus, the hole concentration decreases, the excess Te atoms form Te_Bi⁺ and V_Te, and electrons are generated, resulting in n-type conduction (Equations 2 and 3). Therefore, according to the manufacturing conditions and dopant addition, the concentration of antisite defects changes and thus greatly affects the Seebeck coefficient and electrical conductivity, which are closely related to the carrier concentration.

In this study, the thermoelectric properties are summarized for n-type Bi₂Te_3-ySe_y:D_m chalcogenides with a diameter of 10ϕ prepared by the EM-HP process. The comparison focused solely on the highest ZT values (as shown in Fig 4) of specimens with optimum compositions [21-26].

Fig. 4.

Summary of thermoelectric performance of Bi₂Te_3-ySe_y:D_m chalcogenide compacts (10ϕ × 10t) prepared by the EM-HP process.

4. UNIFORMITY

To evaluate the two-dimensional uniformity of the thermoelectric properties according to the bulk (diameter increased from 10ϕ to 25ϕ) n-type Bi₂Te_3-ySe_y:D_m thermoelectric materials, sintered bodies with dimensions of 25ϕ × 3t were produced using the method mentioned in Section 2, where ϕ is the diameter in mm and t is the thickness in mm. In this study, for the two types of Bi₂Te_2.85Se_0.15:I_0.005 and Bi₂Te_2.7Se_0.3:I_0.0075, which exhibited the best thermoelectric performances among the 10ϕ-Bi₂Te_3-ySe_y:D_m samples, the power factors were measured after processing the sintered body (25ϕ × 3t) into four (S1-S4) rectangular parallelepiped (3 × 3 × 9 mm³) specimens via electrical discharge machining (Fig 5).

Fig. 5.

Sample preparation for uniformity of Bi₂Te_3-ySe_y:I_m chalcogenide compacts (25ϕ × 3t) prepared by the EM-HP process.

Fig 6(a) shows the power factor of the thermoelectric materials manufactured at an HP temperature of 673 K. Compared with the maximum power factor of the 10ϕ sintered body (PF_max = 2.75 mW/mK² at 323 K for Bi₂Te_2.85Se_0.15:I_0.005 and PF_max = 3.19 mW/mK² at 323 K for Bi₂Te_2.7Se_0.3:I_0.0075), when the diameter of the sintered body increased, the average value of the maximum power factor decreased to PF_ave = 2.09 ± 0.01 mW/mK² at 323 K for Bi₂Te_2.85Se_0.15:I_0.005 and PF_ave = 2.43 ± 0.17 mW/mK² at 323 K for Bi₂Te_2.7Se_0.3:I_0.0075.

Fig. 6.

Thermoelectric power factors of Bi₂Te_3-ySe_y:I_m chalcogenide (a) compacts with diameters of 10ϕ (HP673K) and 25ϕ (HP673K), (b) compacts (25ϕ × 3t) according to HP temperature (BTS_0.15: Bi₂Te_2.85Se_0.15 and BTS_0.3: Bi₂Te_2.7Se_0.3), and (c) compacts with diameters of 10ϕ (HP673K) and 25ϕ (HP648K) prepared by the optimized EM-HP process.

However, in the case of the 25ϕ × 3t sintered body, the distribution ranges (deviations) in the power factor values were not large; thus, the uniformity was acceptable. Optimal HP conditions for thermoelectric materials should be derived while taking into account the differences in size (area and volume) of the sintered body. Fig 6(b) presents the power factor of the bulky (25ϕ × 3t) thermoelectric materials prepared by varying the HP temperature. In the case of the 10ϕ sintered body, the optimum HP temperature was 673 K, but in the case of the 25ϕ sintered body (both Bi₂Te_2.85Se_0.15:I_0.005 and Bi₂Te_2.7Se_0.3:I_0.0075), the optimum HP temperature was 648 K.

In addition, it is necessary to maintain uniform temperature within the material when manufacturing a large-area sintered body. Fig 6(c) shows the power factors of the 25ϕ sintered bodies fabricated again at the HP temperature of 648 K. Power factor values were similar to those of the 10ϕ specimens sintered at the HP temperature of 678 K. For the 25ϕ sintered bodies, the average values of the maximum power factors were PF_ave = 3.05 ± 0.16 mW/mK² at 323 K for Bi₂Te_2.85Se_0.15:I_0.005 and PF_ave = 2.70 ± 0.12 mW/mK² at 323 K for Bi₂Te_2.7Se_0.3:I_0.0075. Additionally, the uniformity of the power factor was satisfactory.

The three-dimensional uniformity of the thermoelectric performance was examined for bulky n-type Bi₂Te_3-ySe_y:D_m. As shown in Fig 7, after processing, the Bi₂Te_2.7Se_0.3:I_0.0075 sintered body with dimensions of 25ϕ × 25t was cut into four (U1, U2, D1, and D2) cylinders (10ϕ 12t) by electrical discharge machining, and the specimens were prepared for XRD phase analysis and thermoelectric property measurements.

Fig. 7.

Sample preparation for uniformity of thermoelectric performance of Bi₂Te_2.7Se_0.3:I_0.0075 chalcogenide compacts (25ϕ × 25t) prepared by the EM-HP process (unit: mm).

Fig 8 shows the XRD phase analysis results for the Bi₂Te_2.7Se_0.3:I_0.0075 sintered body (25ϕ × 25t). The diffraction patterns of all the specimens matched those of the ICDD standard diffraction data (PDF# 15-0863), indicating that the desired phase was successfully synthesized, and no residual elements or secondary phases were found.

Fig. 8.

X-ray diffraction patterns exhibiting a homogeneous single chalcogenide phase of Bi₂Te_2.7Se_0.3:I_0.0075 bulky compacts (25ϕ × 25t) prepared by the EMHP process.

Fig 9 shows the ZT of the bulk Bi₂Te_2.7Se_0.3:I_0.0075 (25ϕ × 25t). The ZT values of the four specimens were evaluated. The maximum thermoelectric performance of ZT = 1.01-1.06 (average ZT_ave = 1.03) was obtained at 423 K. This is slightly lower than the highest thermoelectric performance (ZT = 1.13 at 423 K) of the Bi₂Te_2.7Se_0.3:I_0.0075 sintered body (10ϕ × 10t) in Section 3-3. However, these uniform experimental results are encouraging for the mass/bulky production of thermoelectric materials for fabricating thermoelectric devices.

Fig. 9.

Dimensionless Fig.s of merit of Bi₂Te_2.7Se_0.3:I_0.0075 bulky compacts (25ϕ × 25t) prepared by the EM-HP process.

5. SUMMARY

N-type Bi₂Te₃-Bi₂Se₃ solid solutions were synthesized using the EM?HP process, and the uniformity of their thermoelectric properties were evaluated. Solid solutions of Bi₂Te₃-Bi₂Se₃, doped with various elements including I, Cu, Ag, In, Al, Ni, Zn, Ge, and Sn, were synthesized, and their thermoelectric properties were compared. In the case of undoped Bi₂Te_3-ySe_y, all the samples exhibited n-type conductivity, and as the Se content increased, the electrical conductivity decreased, possibly because of the decreased concentration of Te_Bi⁺ antisite defects. When Se was substituted for Te, the Seebeck coefficient increased and the thermal conductivity decreased, resulting in an improved ZT at all measured temperatures compared with that of Bi₂Te₃; the maximum values were ZT = 0.56 at 323 K when y = 0.15 and ZT = 0.60 at 423 K when y = 0.6. In the case of doping in Bi₂Te_2.85Se_0.15 solid solutions (Bi₂Te_2.85Se_0.15:D_m), while I replaced Te and generated electrons, increasing the carrier concentration, the other dopants occupied the Bi sites and decreased the carrier concentration. Bi₂Te_2.85Se_0.15:I_0.005 exhibited the highest ZT = 0.90 at 423K. For Bi₂Te_2.85Se_0.15:Cu_0.01, Bi₂Te_2.85Se_0.15:Ag_0.01 and Bi₂Te_2.85Se_0.15:In0.005, the ZT values also improved to ZT = 0.76, ZT = 0.75, and ZT = 0.66 at 323 K, mainly because of the increase in the power factor. In the doping cases to Bi₂Te_2.7Se_0.3 solid solution (Bi₂Te_2.7Se_0.3:D_m), I acted as a donor, but Cu and Ag acted as acceptors, similar to Bi₂Te_2.85Se_0.15:D_m. ZT was enhanced through doping with I and Cu, resulting in ZT values of 1.13 for Bi₂Te_2.7Se_0.3:I_0.0075 and 0.73 for Bi₂Te_2.7Se_0.3:Cu_0.01 at 423 K. However, when Ag was doped into Bi₂Te_2.7Se_0.3, there was little improvement in ZT. Bi₂Te_2.7Se_0.3:I_0.0075 with the best thermoelectric performance among n-type Bi₂Te_3-ySe_y:D_m was produced in bulky compacts with dimensions of 25ϕ × 25t, and the threedimensional uniformity of their thermoelectric performance was verified.

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