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*Crystals*
**2017**,
*7*(9),
257;
doi:10.3390/cryst7090257

Article

Enhanced Thermoelectric Performance of Te-Doped Bi

_{2}Se_{3−x}Te_{x}Bulks by Self-Propagating High-Temperature SynthesisState Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China

^{*}

Author to whom correspondence should be addressed.

Academic Editor:
George S. Nolas

Received: 12 June 2017 / Accepted: 21 August 2017 / Published: 28 August 2017

## Abstract

**:**

Polycrystalline Bi

_{2}Se_{3−x}Te_{x}(x = 0~1.5) samples were prepared by self-propagating high-temperature synthesis (SHS) combined with spark plasma sintering (SPS) and their thermoelectric properties were investigated. The SHS-SPS process can shorten the time with few energy consumptions, and obtain almost pure Bi_{2}Se_{3}-based phases. Consequently, the Se vacancies and anti-site defects contribute to the converged carrier concentration of ~2 × 10^{19}cm^{−3}while the increased carrier effective mass enhances the Seebeck coefficient to more than −158 μV K^{−1}over the entire temperature range. The lattice thermal conductivity is suppressed from 1.07 Wm^{−1}K^{−1}for the pristine specimen to ~0.6 Wm^{−1}K^{−1}for Te-substitution samples at 300 K because of point defects caused by the difference of mass and size between Te and Se atoms. Coupled with the enhanced power factor and reduced lattice thermal conductivity, a high ZT of 0.67 can be obtained at 473 K for the Bi_{2}Se_{1.5}Te_{1.5}sample. Our results reveal that Te-substitution based on the SHS-SPS method is highly-efficient and can improve the thermoelectric properties of Bi_{2}Se_{3}-based materials largely.Keywords:

Bi_{2}Se

_{3−x}Te

_{x}; thermoelectric; SHS; solid solution

## 1. Introduction

With increasing attention on the environmental protection and renewable resources, thermoelectric (TE) instruments, which can directly convert heat into electricity, are considered as a potential solution for harness waste heat [1,2,3]. Considerable numbers of efforts have been devoted to improving the energy conversion efficiency and the stability of the TE materials [4]. The conversion efficiency depends positively on the dimensionless figure of merit, ZT = S

^{2}σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity and T is the absolute temperature, respectively [5]. To maximize the ZT value of a kind of material, a large Seebeck coefficient, electrical conductivity and low thermal conductivity are needed. However, these parameters have a strong coupling with each other, which makes it a challenging task to enhance ZT significantly. Motivated by achieving high thermoelectric performance, multiple methods have been adopted [6]. Many studies so far have focused on the atomic or molecular scales such as doping or alloying to enhance carrier concentration or carrier mobility and thus electrical conductivity to optimize TE performance [7,8,9,10]. To enhance Seebeck coefficient while maintaining high electrical conductivity, manipulating the band structure offers a new guideline [11]. Meanwhile, effective alteration at nanometer or mesoscopic scales including the quantum confinement [12] and energy filtering effect [13] can drastically elevate electrical properties. And developing multi-scale microstructures can obtain a lower thermal conductivity, which is caused by the phonons scattering from high to low frequencies [14]. Besides, further efforts have been made to explore new TE materials and new synthesis methods [15,16,17].Bismuth selenide (Bi

_{2}Se_{3}) is a V-VI semiconductor with a narrow band gap of ~0.3 eV. Several excellent work describing its application of optical recording system [18] and photoelectrochemical devices [19] can be found elsewhere. Because of good TE properties in the mid-temperature, bismuth chalcogenides gained more attention [20,21,22,23] in thermoelectrics. Bi_{2}Se_{3}has a rhombohedral layered structure, where Se-Se layers are bonded by van der Waals [24]. On the basis of the weak inter-layer bonding, Sun et al. have reported on the enhancement of the thermoelectric properties of Bi_{2}Se_{3}by the interlayer Cu doping [22]. So far, Bi-Te-Se crystals could be fabricated through the zone melting method [25]. Bi_{2}Se_{3}nanostructures have been synthesized by solvothermal method and ZT of 0.096 was obtained at 523 K [26]. Similarly, for Bi_{2}Se_{3−x}Te_{x}(x ≤ 1.5), Liu prepared by ball milling, but only achieved ZT of ~0.3 [9]. Nonetheless, the mentioned methods of zone melting, solvothermal method and ball milling et al. are time and energy consuming. Meanwhile, the ZT of Bi_{2}Se_{3}-based materials is not large enough to meet the requirements of application in mass production. In contrast, self-propagating high-temperature synthesis (SHS) has been proved to be an efficient method to prepare the TE materials alternatively. When the heating rate and temperature are high enough, the reaction wave appears. The heat generated by the exothermic reaction can maintain the whole combustion process, which is exceptionally fast. It shortens the time with few consumptions and can be easily adopted in the commercial application [27]. A wide range of TE materials have been synthesized successfully by this method, including Cu_{2}Se, BiCuSeO, Cu_{3}SbSe_{3}and so on [27,28,29,30].Previous work showed that Bi

_{2}Se_{3}and Bi_{2}Te_{3}can be prepared by SHS method [31]. However, there are few studies focusing on the thermoelectric properties of Bi_{2}Se_{3−x}Te_{x}(x ≤ 1.5) prepared by combustion method. In this work, we successfully synthesized Bi_{2}Se_{3−x}Te_{x}(x = 0, 0.3, 0.6, 0.9, 1.2, 1.5) via the SHS method followed by spark plasma sintering (SPS) and studied the thermoelectric properties from 300 K to 593 K. Our results show that the highest power factor (PF = S^{2}σ) can achieve 11.2 μWcm^{−1}K^{−2}for Bi_{2}Se_{1.5}Te_{1.5}at 300 K and the lattice thermal conductivity (κ_{L}) could be reduced to the lowest value of 0.35 Wm^{−1}K^{−1}at 593 K via Te alloying for Bi_{2}Se_{2.1}Te_{0.9}. The ZT of ~0.67 is finally achieved at 473 K for Bi_{2}Se_{1.5}Te_{1.5}, demonstrating the potential application for energy conversion in the mid-temperature. And SHS process will have more hopeful prospects in commercial applications.## 2. Experimental Procedures

In the initial stage, Bi (99.99%, Aladdin), Se (99.99%, Aladdin), and Te (99.99%, Aladdin) powders were mixed meticulously in stoichiometric amounts. Then the mixture was cold-pressed into pellets with the diameter of 20 mm. The SHS process was started by heating the bottom of the pellets with a hand torch in the air. Once ignited, the hand torch is removed immediately. The heat generated by the combination reaction kept the combustion process propagating until it was finished in several seconds. Then the pellets were grounded into fine powders carefully by hand. The powders were then sintered into pellets of φ 12.7 mm by SPS (Sumitomo Coal Mining Co., Ltd., Tokyo, Japan) at the temperature of 593 K for 5 min under a uniaxial pressure of 40 MPa.

The phase structures were investigated by X-ray diffraction (XRD, RINT2000, Rigaku, Tokyo, Japan) analysis. The morphology and composition of cross-sectional bulks were checked by field-emission scanning electron microscopy (FESEM) (LEO1530, Oxford Instruments, Oxford, UK). The electrical properties including electrical conductivity and Seebeck coefficient were measured from room temperature to 593 K by ZEM-3 (ULVAC, Kanagawa, Japan). The van der Pauw method was used in an Eastchanging Hall measurement station to measure Hall coefficient (R

_{H}). The carrier concentration (n) and mobility (μ) were estimated by the equation n = 1/eR_{H}and μ = σR_{H}. To ensure the accuracy, the samples were polished to be thinner than 0.5 mm for the measurements. The total thermal conductivity is determined by the equation κ = DC_{p}ρ, where D is thermal diffusivity, C_{p}is specific heat and ρ is the density of the bulks. The thermal diffusivity was obtained by the laser flash method and the specific heat was calculated by the Dulong-Petit relation. The density of the bulks was derived with Archimedes method.## 3. Results and Discussion

Figure 1a is the XRD result of all the Bi

_{2}Se_{3−x}Te_{x}samples with x = 0~1.5. All the major peaks in the XRD patterns correspond to a standard card, Bi_{2}Se_{3}, PDF #33-0214. The additional small peaks can be identified as Bi_{2}O_{2}Se (PDF #29-0237), which is possibly generated by oxidation during the ultra-fast combustion process in the air. In this work, we assume that all the samples contain the same amount of Bi_{2}O_{2}Se, and we neglect the effect of existence of Bi_{2}O_{2}Se due to its small amount (small peaks in the XRD result). In Figure 1b, the lattice parameters were calculated according to the position of XRD peaks. With increasing Te content, the lattice constants a and c increase linearly, which indicates Te can successfully substitute for Se atoms to form solid solution by SHS process in a short time. Figure 1c–e show the morphology of cross-sectional bulks (x = 0, 0.3, 1.5). All the samples were sintered well with high density (94% or above). The layer structure can be seen clearly in the Bi_{2}Se_{3−x}Te_{x}bulks.Figure 2 shows the temperature dependence of electrical conductivity and Seebeck coefficient. The electrical conductivity σ (in Figure 2a) of pristine Bi

_{2}Se_{3}maintains at about 400 Scm^{−1}from 300 K to 593 K, which is much higher than Bi_{2}Se_{3}prepared by other method [22]. Se is much easier to evaporate during the combustion process because of low energy of evaporation and thus it leaves Se vacancies and free electrons, which may contribute to higher electrical conductivity. This can be indicated in the following equation:
$$B{i}_{2}S{e}_{3}=2B{i}_{Bi}^{\times}+\left(3-\mathrm{y}\right)S{e}_{Se}^{\times}+\mathrm{y}Se(g)\uparrow +\mathrm{y}{V}_{Se}^{2+}+2\mathrm{y}{e}^{-}$$

As the Te content increases (x > 0), the electrical conductivity at 300 K initially increases to ~870 Scm

^{−1}because of increased carrier concentration (Table 1), then decreases to ~400 Scm^{−1}owing to the change of carrier mobility, which is much lower than the pristine Bi_{2}Se_{3}(Table 1). The carrier concentration increases may be a result of increasing anti-site defects ($B{i}_{Te}^{-}$) [31], which is caused by the fact that Bi can jump from Bi-site to Te-site easily because of small difference in electronegativity [10], as indicated in Equation (2).
$$B{i}_{2}T{e}_{3}=(2-\frac{2}{5}z)B{i}_{Bi}^{\times}+(3-z)T{e}_{Te}^{\times}+zTe(g)\uparrow +(\frac{2}{5}z{V}_{Bi}^{3-}+\frac{3}{5}z{V}_{Te}^{2+})+\frac{2}{5}zB{i}_{Te}^{-}+\frac{2}{5}z{h}^{+}$$

On the contrary, the number of Se vacancies ${V}_{Se}^{2+}$ will be fewer due to the increasing Te content. The decreased Se vacancies and increased anti-site defects make the carrier concentration converges to about 2 × 10

^{19}cm^{−3}. The carrier mobility decreases with higher Te content at 300 K in general due to the enhanced alloy scattering. Interestingly, we found μ of Bi_{2}Se_{1.5}Te_{1.5}(x = 1.5) is slightly larger than Bi_{2}Se_{1.8}Te_{1.2}(x = 1.2) at the room temperature, which is possibly caused by the intrinsic high mobility of Bi_{2}Te_{3}[31].As shown in Figure 2b, the negative Seebeck coefficient of all the samples indicates the dominance of electrons in the transport process. Generally, the value of Seebeck coefficient can be estimated by the equation [32]:
where $e$, ${k}_{B}$, $T$, $h$, ${m}_{d}^{\ast}$, and $n$ are the carrier charge, Boltzmann constant, absolute temperature, Planck constant, the effective mass of the carrier, and carrier concentration. As shown in the formula, because of largely enhanced carrier concentration with increasing Te content (x < 0.9) at 300 K, the Seebeck coefficient decreases. Then the Seebeck coefficient was improved due to the larger carrier effective mass at 300 K (Table 1). It should be noticed that each sample with x ≥ 0.9, as the temperature increases, the value of Seebeck coefficient first increases then decreases, which is caused by the intrinsic excitations. The highest Seebeck coefficient of −180 μV K

$$\left|S\right|=\frac{8{\pi}^{2}{k}_{B}^{2}T}{3e{h}^{2}}{m}_{d}^{\ast}{(\frac{\pi}{3n})}^{2/3}$$

^{−1}is achieved at 473 K for the Bi_{2}Se_{1.5}Te_{1.5}sample. The Bi_{2}Se_{1.5}Te_{1.5}sample attains the largest effective mass of ~0.60 m_{0}, which is in accord with the difference of Seebeck coefficient with different Te contents at room temperature.The variation of power factor (PF = S

^{2}σ) with increasing temperature of all the samples is shown in Figure 3. The Bi_{2}Se_{1.5}Te_{1.5}sample reaches the highest PF of 11.2 μWcm^{−1}K^{−2}at room temperature, which is almost twice higher than that of pristine Bi_{2}Se_{3}. But it drops to about 8 μWcm^{−1}K^{−2}at 593 K owing to the decreased electrical conductivity and Seebeck coefficient.Figure 4a illustrates the total thermal conductivity (κ) as a function of temperature from room temperature to 593 K. The κ of pristine Bi
where κ
where L is the Lorenz constant and σ is electrical conductivity. In the single parabolic band model, L depends on the reduced chemical potential and scattering parameter. It can be estimated by fitting the values of the Seebeck coefficient and the room temperature data has been listed in Table 1. The details can be seen elsewhere [33,34]. As mentioned above (Figure 2b), intrinsic excitaions don’t occur until 423 K or above. Therefore, the κ

_{2}Se_{3}is in the range of 1.04–1.31 Wm^{−1}K^{−1}. As x increases to 1.2, the κ drops into the range of 0.76–0.82 Wm^{−1}K^{−1}from 300 K to 593 K. With further increasing Te content, κ is much larger than the sample of x = 1.2. To have a better understanding of the thermal transport properties, κ is subsequently divided into three parts:
κ = κ

_{e}+ κ_{L}+ κ_{B}_{e}is electron thermal conductivity, κ_{L}is lattice thermal conductivity and κ_{B}is the bipolar thermal conductivity induced by intrinsic excitaions. κ_{e}can be estimated by Wiedemann-Franz relation:
κ

_{e}= LσT_{B}can be ignored at low temperature in Figure 4b. Consequently, κ_{L}and the reciprocal temperature, T^{−1}, follow a linear relationship. As shown in Figure 4c, the lattice thermal conductivity drops substantially after alloying. Note that the κ_{L}of Bi_{2}Se_{2.1}Te_{0.9}achieves the lowest value of 0.35 Wm^{−1}K^{−1}at 593 K. The effective suppression of the κ_{L}of could be attributed to point defects caused by the different mass and size between Te and Se atoms. Similar to the previous literature [9], the κ_{L}rises slightly when x is above 0.9. This may be ascribed to the relatively high κ_{L}of Bi_{2}Te_{3}[31], whose effect is larger than the point defects. Figure 4d shows the temperature dependence of κ_{B}. With low Te content (x ≤ 0.6), κ_{B}is almost zero from 300 K to 593 K, because there are no intrinsic excitaions. Intrinsic excitaions occur and κ_{B}’s contribution to κ becomes larger when x ≥ 0.9, which is owing to narrower band gap with increasing Te content [35].The ZT values for all the Bi

_{2}Se_{3−x}Te_{x}samples (x = 0, 0.3, 0.6, 0.9, 1.2, 1.5) are presented in Figure 5. The enhanced power factor and the effective suppression of lattice thermal conductivity synergistically contribute to the highest ZT value of 0.67 at 473 K for the sample of Bi_{2}Se_{1.5}Te_{1.5}, which is almost twice higher than the pristine Bi_{2}Se_{3}. ZT values of Bi_{2}Se_{3}(ball milling) [22], Bi_{2}Se_{2}Te (ball milling) [9], Bi_{2}Te_{3}(SHS) [31] from the literature are included for comparison. Our results show that the ZT value of Te-substituted Bi_{2}Se_{3}-based materials prepared by SHS is much larger than that by other methods.## 4. Conclusions

In summary, we have investigated the thermoelectric properties (300–593 K) of Bi

_{2}Se_{3−x}Te_{x}samples (x = 0, 0.3, 0.6, 0.9, 1.2, 1.5), which are prepared by SHS-SPS process successfully. Compared with other methods, the SHS-SPS process is much faster and requires less energy, which is desirable in commercial application even though with small amount of second phase Bi_{2}O_{2}Se. Our results show that the power factor of Bi_{2}Se_{1.5}Te_{1.5}achieves 11.2 μWcm^{−1}K^{−2}at 300 K by the increased carrier concentration and the enhancement of Seebeck coefficient. The point defects originate from the difference of mass and size between Te and Se atoms significantly suppresses the lattice thermal conductivity. Benefiting from the improved power factor and the decreased lattice thermal conductivity, a high ZT of 0.67 can be obtained at 473 K for the sample of Bi_{2}Se_{1.5}Te_{1.5}, which demonstrates that the Te-substitution via SHS-SPS method is highly-efficient and can enhance the thermoelectric properties of Bi_{2}Se_{3}-based materials.## Acknowledgments

This work was supported by the National Key Research Programme of China, under grant No. 2016YFA0201003, Ministry of Sci & Tech of China through a 973-Project under grant No. 2013CB632506, and National Science Foundation of China under grand No. 51672155 and 51532003.

## Author Contributions

Rui Liu performed the experiments and wrote the paper. Guangkun Ren, Yaochu Liu, Cewen Nan and Yuanhua Lin revised the manuscript. Chan Liu, Xing Tan and Zhifang Zhou assisted in experiments.

## Conflicts of Interest

The authors declare no conflict of interest.

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**Figure 1.**(

**a**) XRD patterns and (

**b**) lattice parameters of sintered Bi

_{2}Se

_{3−x}Te

_{x}bulk samples; field-emission scanning electron micrographs of Bi

_{2}Se

_{3−x}Te

_{x}, for which, (

**c**) x = 0; (

**d**) x = 0.3; and (

**e**) x = 1.5.

**Figure 2.**Temperature dependence of (

**a**) electrical conductivity and (

**b**) Seebeck coefficient for Bi

_{2}Se

_{3−x}Te

_{x}samples.

**Figure 4.**The temperature dependence of the (

**a**) total thermal conductivity; (

**b**) κ − κ

_{B}; (

**c**) lattice thermal conductivity; and (

**d**) the bipolar thermal conductivity for Bi

_{2}Se

_{3−x}Te

_{x}samples.

**Table 1.**Actual composition, carrier concentration (n), carrier mobility (μ), carrier effective mass (m*), Seebeck coefficient (S), Lorenz constant (L), lattice thermal conductivity (κ

_{L}), κ

_{L}/κ, and density of Bi

_{2}Se

_{3−x}Te

_{x}samples at 300 K.

x | n (10^{18} cm^{−3}) | μ (cm^{2} V^{−1} s^{−1}) | m*/m_{0} | S (μV K^{−1}) | L (10^{−8} V^{2} K^{−2}) | κ_{L} (Wm^{−}^{1} K^{−}^{1}) | κ_{L}/κ | Density (g cm^{−3}) |
---|---|---|---|---|---|---|---|---|

0.0 | 5.94 | 444.48 | 0.19 | −118.23 | 1.83 | 1.07 | 81.7% | 7.01 |

0.3 | 17.31 | 309.19 | 0.25 | −73.65 | 2.06 | 0.80 | 59.4% | 6.92 |

0.6 | 21.94 | 209.36 | 0.31 | −79.95 | 2.03 | 0.62 | 57.4% | 6.77 |

0.9 | 24.42 | 149.33 | 0.45 | −107.68 | 1.88 | 0.52 | 60.4% | 6.65 |

1.2 | 24.37 | 97.45 | 0.53 | −126.49 | 1.81 | 0.60 | 74.0% | 6.83 |

1.5 | 20.73 | 134.96 | 0.60 | −158.72 | 1.71 | 0.59 | 71.6% | 6.96 |

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