Electronic Transport and Thermoelectric Properties of Te-Doped Tetrahedrites Cu₁₂Sb_4-yTe_yS₁₃

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Abstract

1. INTRODUCTION

Tetrahedrite (Cu₁₂Sb₄S₁₃), a mineral composed of earth-abundant and eco-friendly elements, has been studied as a potential thermoelectric material since it exhibits excellent performance with high electrical conductivity and low thermal conductivity [1,2]. PbTe-based materials are known to be excellent thermoelectric materials in the temperature range of 500–800 K. However, they consist of rare elements and toxic heavy metals. Cu₁₂Sb₄S₁₃ has a complex and highly symmetric cubic structure (space group I4¯3m) composed of Cu^IS₄ tetrahedra, Cu^IIS₃ triangles, and SbS₃ trigonal pyramids [3]. The low lattice thermal conductivity of tetrahedrite is due to the lone-pair electrons of the Sb atoms, which cause the Cu^II atoms to oscillate anharmonically with low frequency and high amplitude in the S triangle plane [4-6].

Thermoelectric performance is evaluated using a dimensionless figure of merit (ZT = α²σκ^-1T). Good thermoelectric materials should have a power factor (PF = α²σ) and low thermal conductivity (κ) at application temperature (T in Kelvin), where α is the Seebeck coefficient and σ is the electrical conductivity. Carrier concentration affects both the Seebeck coefficient and electrical conductivity, and these two parameters are trade-offs for the PF. In general, PF can be improved by optimizing the carrier concentration. To maximize PF and reduce the thermal conductivity of tetrahedrite simultaneously, studies have been conducted to reduce the hole concentration by substituting various elements for the Cu, Sb, and S sites [7-10]. Most studies on tetrahedrite have focused on the partial substitution of transition elements (e.g., Fe, Co, Ni, Zn) for the Cu sites; however, few studies have been reported on the doping of Sb or S sites.

The Sb sites of natural mineral tetrahedrite usually contain As, Bi, and Te atoms as impurities [11]; this has suggested studies on the substitution of Sb sites in synthetic tetrahedrite. Kwak et al. [12] reported Bi-doped Cu₁₂Sb_4-yBi_yS₁₃ (y = 0.1– 0.4) prepared by mechanical alloying (MA) and hot pressing (HP); ZT = 0.88 at 723 K for Cu₁₂Sb_3.9Bi_0.1S₁₃. Kumar et al. [11] studied Bi-doped Cu₁₂Sb_4-yBi_yS₁₃ (y = 0.2–0.8) synthesized by encapsulated melting (EM) and HP; ZT = 0.84 at 673 K for Cu₁₂Sb_3.9Bi_0.1S₁₃. Bouyrie et al. [13] examined Te-doped Cu₁₂Sb_4-yTe_yS₁₃ (y = 0.5–1.75) prepared by EM and spark plasma sintering (SPS); ZT = 0.65 at 623 K for Cu₁₂Sb₃.39 Te0.61S₁₃. Lu et al. [14] reported Te-doped Cu₁₂Sb_4-yTe_yS₁₃ (y = 0–1) synthesized by EM and HP; ZT = 0.92 at 723 K for Cu₁₂Sb₃TeS₁₃.

In this study, Te-doped tetrahedrites Cu₁₂Sb_4-yTe_yS₁₃ (y = 0.1–0.4) were prepared by MA and HP using a solid-state synthesis method, and charge transport and thermoelectric properties were examined.

2. EXPERIMENTAL PROCEDURE

Te-doped Cu₁₂Sb_4-yTe_yS₁₃ (y = 0.1-0.4) was synthesized by MA. All chemicals were purchased from Kojundo Chemical Laboratory Co., Saitama, Japan. Cu (purity 99.9%, <45 µm), Sb (purity 99.999%, <150 µm), Te (purity 99.999%, <150 µm), and S (purity 99.99%, <75 µm) powders were weighed to obtain a stoichiometric composition. Mixed elemental powders and stainless-steel balls (diameter 5 mm) were loaded in a hardened steel jar at a weight ratio (ball-to-powder ratio) of 20. MA was performed at 350 rpm for 24 h in an Ar atmosphere using a planetary ball mill (Pulverisette5, Fritsch, Germany). To consolidate the synthesized powder, HP was conducted at 723 K for 2 h under a pressure of 70 using a uniaxial vacuum hot pressing system (VHP, Jeongmin VSL, Korea).

X-ray diffraction (XRD; D8-Advance, Bruker, Germany) was performed with Cu-Kα radiation to analyze the phases of the synthetic powders and sintered specimens. Scanning electron microscopy (SEM; Quanta400, FEI, USA) was used to observe the fractured surfaces of the HP specimens, and energy-dispersive X-ray spectroscopy (EDS; Quantax200, Bruker, Germany) was employed to conduct the elemental analysis. The energy levels of the elements were Cu-Lα (0.928 eV), Sb-Lα (3.604 eV), Te-Lα (3.768 eV), and S-Kα (2.309 eV). The electronic transport parameters (Hall coefficient, carrier concentration, and mobility) were examined under a direct current (DC) of 100 mA and a magnetic field of 1 Tesla using the van der Pauw method (7065, Keithley, USA). The thermoelectric properties were measured at temperatures from 323 to 723 K. The DC four-probe method (ZEM-3, Advance Riko, Japan) was used to measure the electrical conductivity and Seebeck coefficient. The laser flash method (TC-9000H, Advance Riko, Japan) was employed to determine the thermal conductivity after measuring the thermal diffusivity, specific heat, and density. Finally, the PF and ZT were evaluated.

3. RESULTS AND DISCUSSION

XRD patterns of the Cu₁₂Sb_4-yTe_yS₁₃ prepared by the MA-HP process are shown in Figure 1. The diffraction peaks of all the specimens matched the ICDD standard diffraction data (PDF#024-1318, space group I4¯3m). The diffraction angles shifted to a lower value with increased Te doping (y). The lattice constant increased from 1.0334 to 1.0346 nm with Te doping, as shown in Table 1. Lu et al. [14] prepared Te-doped tetrahedrites Cu₁₂Sb_4-yTe_yS₁₃ (y = 0.1–1.0) using the EM-HP method and reported slight changes in the lattice constant because the difference between the covalent radii of Sb³⁺ (0.139 nm) and Te⁴⁺ (0.138 nm) was small.

Fig. 1.

XRD patterns of Cu₁₂Sb_4-yTe_yS₁₃ tetrahedrites prepared by the MA-HP process.

Table 1.

Chemical, physical, and electronic transport properties of Cu₁₂Sb_4-yTe_yS₁₃.

Figure 2 displays SEM images of the Cu₁₂Sb_4-yTe_yS₁₃ prepared by the MA-HP process. The samples sintered by HP exhibited high relative densities of 99.7%–100.0%. No significant changes in the microstructure were observed when Te content was varied. Unreacted residual elements and secondary phases were not observed. The EDS elemental maps of Cu₁₂Sb_3.6Te_0.4S₁₃ showed that the constituent elements were uniformly distributed. As shown in Table 1, the experimental composition agreed well with the nominal composition; however, the Cu content was relatively high, and the S content was relatively low. This was interpreted as errors in the compositional analysis and volatilization of S during HP.

Fig. 2.

SEM images of fractured surfaces for Cu₁₂Sb_4-yTe_yS₁₃ and EDS elemental maps of Cu, Sb, S, and Te for Cu₁₂Sb_3.6Te_0.4S₁₃.

Figure 3 presents the carrier concentration and mobility of Cu₁₂Sb_4-yTe_yS₁₃, and Table 1 summarizes their average values from measuring the Hall coefficient 20 times. Te-doped tetrahedrites exhibited positive Hall coefficients, confirming they are p-type semiconductors. The carrier (hole) concentration decreased from 3.56 × 10¹⁸ to 1.74 × 10¹⁸ cm^-3 as Te content increased. Mobility is generally inversely related to carrier concentration in a nondegenerate semiconductor, and its values ranged from 361 to 497 cm² Vs^-1 in this study. Bouyrie et al. [13] conducted a Korringa–Kohn–Rostoker calculation for Cu₁₂Sb₂Te₂S₁₃, which yielded a band gap of 0.9 eV lower than the 1.3 eV of Cu₁₂Sb₄S₁₃. However, its electrical resistivity was higher than that of undoped Cu₁₂Sb₄S₁₃; therefore, they suggested that measurement of the Hall coefficient would be necessary to confirm this accurately. Cu₁₂Sb₄S₁₃ prepared using the same method as in our previous study [15] exhibited a high carrier concentration of 3.12 × 10²⁰ cm^-3 and low mobility of 4.35 cm² V^-1s^-1; however, in the present study, Te doping reduced the carrier concentration to 1.74 × 10¹⁸ cm^-3 and increased the mobility to 497 cm² V^-1s^-1. Measurement of the Hall coefficient of the tetrahedrite was problematic because of its very low value (almost zero) with large errors [16]. This made it difficult to analyze the experimental data on the charge transport properties of tetrahedrite.

Fig. 3.

Carrier concentration and mobility for Cu₁₂Sb_4-yTe_yS₁₃.

Figure 4 shows the electrical conductivity of the Cu₁₂Sb_4-yTe_yS₁₃. The electrical conductivity was relatively constant at temperatures from 323 to 723 K, similar to nondegenerate semiconductor behavior. However, the conductivity decreased as the Te content increased (at a constant temperature), which was consistent with the change in carrier concentration according to the Te concentration (Figure 3). Bouyrie et al. [13] reported that the electrical resistivity of Cu₁₂Sb_4-yTe_yS₁₃ (y = 0.5–2.0) decreased as the temperature increased, while it increased with Te content. Lu et al. [14] studied the electrical resistivity (conductivity) of Cu₁₂Sb_4-yTe_yS₁₃ (y = 0–1), which increased from 1.5 × 10^-5 to 3.7 × 10^-5 Ωm (6.7 × 10⁴ and 2.7 × 10⁴ Sm^-1, respectively) as the Te content increased at 723 K. This is because Te doping at the Sb sites caused electrons to fill the valence band and reduced the number of available holes, that is, charge carriers. The electrical conductivity is affected by the carrier concentration (n) and mobility (μ), as shown by the following relation:

Fig. 4.

Temperature dependence of the electrical conductivity for Cu₁₂Sb_4-yTe_yS₁₃.

where e is the electron charge. In this study, the electrical conductivity decreased from 2.6 × 10⁴ Sm^-1 for Cu₁₂Sb_3.9 Te_0.1S₁₃ to 1.6 × 10⁴ Sm^-1 for Cu₁₂Sb_3.6Te_0.4S₁₃ at 723 K, owing to the decreased carrier concentration caused by Te doping.

Figure 5 shows the Seebeck coefficients of the Cu₁₂Sb_4-yTe_yS₁₃ compounds. All of the specimens exhibited positive values for the p-type semiconductor and agreed with the sign of the Hall coefficients. The Seebeck coefficient is expressed as:

Fig. 5.

Temperature dependence of the Seebeck coefficient for Cu₁₂Sb_4-yTe_yS₁₃.

where k_B is the Boltzmann constant, m* is the effective carrier mass, and h is the Planck constant [17]. In general, the absolute value of Seebeck coefficient reaches a maximum at a certain temperature and then decreases with further increases in temperature because the carrier concentration increases rapidly owing to the intrinsic transition. The carrier concentration was decreased by Te doping; therefore, the intrinsic transition temperature should have been reduced. However, the intrinsic transition did not occur until 723 K in this study, possibly because of the low Te-doping levels. As a result, the intrinsic transition temperature of Cu₁₂Sb_4-yTe_yS₁₃ (y = 0.1-0.4) is expected to be >723 K. The Seebeck coefficient increased as the Te concentration increased at a constant temperature, because it is inversely related to carrier concentration. Therefore, the Seebeck coefficient increased as the Te content increased (the carrier concentration decreased), and a maximum Seebeck coefficient of 204 μVK^-1 was obtained for Cu₁₂Sb_3.6Te_0.4S₁₃ at 723 K. This value was higher than the maximum Seebeck coefficient of 183 μVK^-1 at 723 K for the undoped Cu₁₂Sb₄S₁₃ reported in our previous study [15]. Bouyrie et al. [13] reported that the Seebeck coefficient of Cu₁₂Sb_4-yTe_yS₁₃ (y = 0.5–2.0) increased with increasing temperature and Te content. Lu et al. [14] achieved a Seebeck coefficient of 193 μVK^-1 at 723 K for Cu₁₂Sb₃TeS₁₃, which was a 50% increase over that of undoped Cu₁₂Sb₄S₁₃.

Figure 6 presents the PF of Cu₁₂Sb_4-yTe_yS₁₃. As the temperature increased, the PF increased. PF has a trade-off relationship with the carrier concentration. In this study, the decrease in electrical conductivity had a greater effect than the increase in the Seebeck coefficient; thus, PF decreased as the Te content increased. The highest PF (0.91 mWm^-1K^-2) was achieved at 723 K for Cu₁₂Sb_3.9Te_0.1S₁₃. However, this value was slightly lower than the value of 0.95 mWm^-1K^-2 at 723 K for the undoped Cu₁₂Sb₄S₁₃ in our previous study [15], due to the decrease in electrical conductivity from Te doping.

Fig. 6.

Temperature dependence of the power factor for Cu₁₂Sb_4-yTe_yS₁₃.

Figure 7 shows the thermal conductivity of the Cu₁₂Sb_4-yTe_yS₁₃ compounds. The thermal conductivity exhibited values below 1.0 Wm^-1K^-1 at 323−723 K for all samples, as shown in Fig 7(a). As the Te content increased, the value decreased from 0.69 to 0.61 Wm^-1K^-1 at 323 K and 0.80 to 0.75 Wm^-1K^-1 at 723 K. The thermal conductivity of the undoped Cu₁₂Sb₄S₁₃ ranged from 0.72 to 0.77 Wm^-1K^-1 at 323-723 K [15], and the Te-doped specimens also maintained low thermal conductivity values in this study. Lu et al. [14] reported that as the Te content increased, the thermal conductivity of Cu₁₂Sb_4-yTe_yS₁₃ (y = 0–1) decreased; 0.64–0.77 Wm^-1K^-1 at 300-700 K for Cu₁₂Sb₃TeS₁₃, which was approximately half that of undoped tetrahedrite. The thermal conductivity of a semiconductor is composed of two components: lattice thermal conductivity (κ_L: phonon contribution) and electronic thermal conductivity (κ_E: carrier contribution), and κ_E can be isolated by the Wiedemann-Franz law (κ_E = LσT) [18]. The Lorenz number (L) can be estimated from the relation [19]:

Fig. 7.

Temperature dependence of the thermal conductivities for Cu₁₂Sb_4-yTe_yS₁₃: (a) total thermal conductivity and (b) electronic and lattice thermal conductivities.

and these values can be found in Table 1. As the Te content increased, the Lorenz number decreased from 1.80 × 10^-8 to 1.74 × 10^-8 V2 K^-2 at 323 K because of the increase in the Seebeck coefficient caused by the decreased carrier concentration.

Figure 7(b) presents the separated values of the lattice and electronic thermal conductivity. The electronic thermal conductivity increased with increasing temperature, ranging from 0.07 to 0.32 Wm^-1K^-1 at 323-723 K. The electronic thermal conductivity decreased as the Te doping level increased because the carrier concentration decreased. At temperatures of 323–723 K, the lattice thermal conductivity was < 0.6 Wm^-1K^-1 regardless of the Te concentration. Bouyrie et al. [13] reported a similar lattice thermal conductivity (< 0.5 Wm^-1K^-1) for Cu₁₂Sb_4-yTe_yS₁₃ (y = 0.5–2.0), irrespective of Te content. Tetrahedrite has been reported to have inherently low thermal conductivity due to phonon scattering of the Cu atoms, bringing the lattice thermal conductivity closer to the theoretical minimum value [20].

Figure 8 shows the ZT for Cu₁₂Sb_4-yTe_yS₁₃. ZT increased with temperature because of the temperature dependence of PF and thermal conductivity. The highest value (0.80) was obtained at 723 K for Cu₁₂Sb_3.9Te_0.1S₁₃, which was higher than the ZT of 0.66, at 723 K for undoped Cu₁₂Sb₄S₁₃ prepared under the same conditions by Pi et al. [21]. Barbier et al. [22] achieved a ZT of 0.60, at 723 K for Cu₁₂Sb₄S₁₃ produced by EM and SPS. Therefore, Te doping is considered to be effective at enhancing the thermoelectric performance of tetrahedrite. Bouyrie et al. [13,23] reported ZT values of 0.65 and 0.70, at 623 K for Cu₁₂Sb_2.59Te_1.41S₁₃ and Cu_11.5Ni_0.5Sb_3.25Te_0.75S₁₃ synthesized by EM and SPS, respectively. Lu et al. [14] reported a ZT value of 0.92 at 723 K for Cu₁₂Sb₃TeS₁₃ synthesized by EM and HP. As a result, in this study, Te-doped tetrahedrites with high thermoelectric performance could be synthesized by MA and HP without post-annealing as a solid-state method.

Fig. 8.

Dimensionless figure of merit for Cu₁₂Sb_4-yTe_yS₁₃.

4. CONCLUSIONS

Cu₁₂Sb_4-yTe_yS₁₃ (y = 0.1–0.4) tetrahedrites doped with Te at the Sb sites were successfully synthesized without secondary phases using a MA-HP process. The HP produced densely sintered bodies with a relative density > 99.7%. As the Te content increased, the lattice constant increased from 1.0334 to 1.0346 nm, confirming the successful substitution of Te at the Sb sites. Additionally, as Te content increased, the electrical conductivity decreased, while the Seebeck coefficient increased due to reduced carrier concentration. Cu₁₂Sb_3.9Te_0.1S₁₃ exhibited the highest PF of 0.91 mWm^-1K^-2 at 723 K because the decrease in electrical conductivity dominated the increase in Seebeck coefficient by Te doping. Further, as the Te content increased, the contribution of the decrease in electronic thermal conductivity was greater than that of the decrease in lattice thermal conductivity; thus, the total thermal conductivity decreased. As a result, the highest ZT (0.80) was obtained at 723 K for Cu₁₂Sb_3.9Te_0.1S₁₃, which had high PF and low thermal conductivity.

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