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Influence of Ga-doping on the thermoelectric properties of Bi(2−x)GaxTe2.7Se0.3 alloy
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Influence of Ga-doping on the thermoelectric properties of Bi(2 x)GaxTe2.7Se0.3 alloy Xingkai Duann, Konggang Hu, Shifeng Ding, Dahu Man, Haixia Jin Institute of New Energy Materials, School of Mechanical and Materials Engineering, Jiujiang University, Jiujiang 332005, China Received 19 July 2014; accepted 1 December 2014
Abstract Bi(2 x)GaxTe2.7Se0.3 (x ¼0, 0.04, 0.08, 0.12) alloys were fabricated by vacuum melting and hot pressing technique. The structure of the samples was evaluated by means of X-ray diffraction. The peak shift toward higher angle can be observed by Ga-doping. The effects of Ga substitution for Bi on the electrical and thermal transport properties were investigated in the temperature range of 300–500 K. The power factor values of the Ga-doped samples are obviously improved in the temperature range of 300–440 K. Among all the samples, the Bi(2 x)GaxTe2.7Se0.3 (x ¼0.04) sample showed the lowest thermal conductivity near room temperature and the maximum ZT value reached 0.82 at 400 K. & 2015 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved.
Keywords: Bi2Te2.7Se0.3; Electrical transport properties; Thermal transport properties; Carrier concentration
1. Introduction Thermoelectric materials have attracted a considerable amount of attention due to their ability of quietly converting waste heat from different sources into electricity or electrical power directly into cooling [1]. The efficiency of heat–electricity conversion can be defined by the dimensionless figure of merit ZT ¼ (S2σ/κ)T, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity and T is the absolute temperature. Accordingly, a good thermoelectric material has to combine large Seebeck coefficient, high electrical conductivity and low thermal conductivity [2]. Bi2Te3based compounds are the most important commercial thermoelectric materials near room temperature. The relatively high thermoelectric properties of the bulk Bi2Te3-based compounds have been achieved by utilizing hot pressing [3,4], spark plasma sintering [5,6]. Although thermoelectric properties of n Corresponding author. Present address: Qianjin Donglu 551, Lushan District, Jiujiang City 332005, China. Tel.: þ 86 792 8763469; fax: þ86 792 8312891. E-mail address: [email protected] (X. Duan). Peer review under responsibility of Chinese Materials Research Society.
the Bi2Te3-based compounds have been improved by doping and adjusting composition [7–9], the above-mentioned studies mainly focus on improving thermoelectric properties of p-type (Bi,Sb)2Te3. N-type Bi2Te2.7Se0.3 is the state-of-the-art thermoelectric material used in cooling device. However, the ZT values of n-type Bi2(Te,Se)3 are usually lower than those of p-type (Bi,Sb)2Te3. Therefore, it is very necessary to optimize the electrical and thermal transport properties of n-type Bi2Te2.7Se0.3 in order to develop p–n joint cooling device near room temperature. In recent years, many studies have been carried out to improve the thermoelectric properties of n-type Bi2Te3-based materials by doping [10–12] and tuning the carrier concentration. Lee et al. [13] reported that the increase in the carrier concentration induced by I doping led to an increase in the electrical conductivity. The ZT value was improved by I doping due to the increased PF, demonstrating a maximum of ZT ¼ 1.13 at 423 K for Bi2Te2.7Se0.3:I0.0075. Electrical transport properties of Bi2Te3-based materials are closely related to the concentration of atomic vacancies and antisite defects [14]. In the Bi2Te2.7Se0.3 structure, Bi atoms can occupy the sites of Se or Te, and the antisite defects (BiTe or BiSe) can form. Since the differences in the electronegativity between Ga (1.81) and
http://dx.doi.org/10.1016/j.pnsc.2015.01.003 1002-0071/& 2015 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved.
Please cite this article as: X. Duan, et al., Influence of Ga-doping on the thermoelectric properties of Bi(2 x)GaxTe2.7Se0.3 alloy, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.01.003
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Te (2.10) or Se (2.55) are bigger than those between Bi (2.02) and Te (2.10) or Se (2.55) [15], the following trend can be applied to the ability of the antisite defects formation: BiTe 4 BiSe 4GaTe 4 GaSe. In this way, the carrier concentration will be modified via Ga substitution for Bi. On the other hand, alloying with atoms of similar electric potentials but different masses can scatter phonons effectively, where scattering is due to difference in mass and/or bond stiffness [16]. The big difference of the mass exists between Ga and Bi, and the lattice thermal conductivity can be decreased by Ga-doping. So the electrical and thermal transport properties of the Bi(2 x)GaxTe2.7Se0.3 compounds can be tuned by Ga-doping. In this paper, the influences of Ga-doping on the thermoelectric properties of the Bi(2 x)GaxTe2.7Se0.3 compounds were investigated. The goal of this work was to optimize the electrical and thermal transport properties of the Bi2Te2.7Se0.3 by Ga-doping.
2. Materials and methods Elemental lumps of Bi (99.99%), Te (99.99%), Se (99.99%) and Ga (99.99%) were weighed according to the stoichiometry of Bi(2 x)GaxTe2.7Se0.3 (x ¼ 0, 0.04, 0.08, and 0.12). The elemental mixtures were loaded into the quartz tubes. The quartz tubes were sealed under vacuum and placed in a furnace and heated to 1073 K for 6 h, and then they were cooled to room temperature. The obtained ingots were pulverized using the agate mortar under vacuum. The powders were shaped by vacuum hot pressing in the graphite dies under 60 MPa pressure at 673 K for 1 h. The bulk disk-shaped samples of ∅12 mm 20 mm were obtained. X-ray diffraction measurements were performed on an X-ray diffractometer (Bruker, D8Advance with Cu Kα radiation, λ ¼ 1.5406 Å). The Seebeck coefficients were obtained by measuring thermal power under a temperature gradient of 10 K (the dimension of samples: ∅12 mm 4 mm). The electrical resistivity and Hall effects were measured by the Van der Pauw technique. A reversible magnetic field of 2 T is used for the Hall effects measurement in the temperature range of 300–500 K (the dimension of samples: ∅12 mm 1.2–1.5 mm). The thermal diffusivity (λ) was tested by the laser flash diffusivity method using a laser flash method (LFA 457) (the dimension of samples: ∅12 mm 1.2–1.5 mm). The specific heat (Cp) was obtained from TA: DSCQ20. The thermal conductivity was calculated according to the equation: λ¼ κ/(DCp), where D is the density. All of the measurements were carried out at the direction perpendicular to the pressing direction in the temperature range of 300–500 K.
3. Results and discussion The XRD patterns of the Bi(2 x)GaxTe2.7Se0.3 (x¼ 0, 0.04, 0.08, 0.12) are shown in Fig. 1(a). The result in Fig. 1(a) shows that the characteristic diffraction peaks are consistent with the peaks of Bi2Te2.7Se0.3. All the samples have the same rhombohedral lattice structures. The results of XRD patterns
Fig. 1. (a) XRD patterns of the Bi(2 x)GaxTe2.7Se0.3 samples and (b) XRD patterns of the corresponding enlarged for (006) crystal plane.
testify that the obtained materials are the single phase. Besides, Fig. 1(a) also shows that the preferentially oriented c-axis samples can be fabricated by the vacuum melting and hot pressing method. Fig. 1(b) shows the corresponding enlarged (006) peak of the samples. The peak shift toward the higher angle can be observed with the increase of Ga content. It is attributed to the difference of atomic radius between Ga (1.30 Å) and Bi (1.60 Å). It can be concluded that Ga atoms enter into the lattices sites. Fig. 2(a) shows the temperature dependence of the electrical conductivity (σ) for Bi(2 x)GaxTe2.7Se0.3. All samples show a metal conductive behavior in the temperature range of 300– 500 K. The electrical conductivities of the doped samples increase with increasing the content of Ga. It shows the content of Ga-doping has a significant influence on the electrical transport properties. The carrier concentration (nH) as a function of temperature was investigated via Hall measurements in Fig. 2(b). As the content of Ga-doping increases, the carrier concentration increases. Electrical conductivity can be expressed as σ¼ nHμHe, where μH is the carrier mobility and e is the electronic charge. The variation of electrical conductivity
Please cite this article as: X. Duan, et al., Influence of Ga-doping on the thermoelectric properties of Bi(2 x)GaxTe2.7Se0.3 alloy, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.01.003
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fabricated by the vacuum melting and hot pressing method in this work. The electrical conductivity of n-type Bi2Te3based alloys can be also improved by texture enhancement [17,18]. In a pulverized and sintered samples, n-type Bi2Te3based alloys can generate excess negative carriers in the lattice (called donor-like effect) [19], which could vastly improve the electrical conductivity. Fig. 3 depicts the carrier mobility as a function of temperature for Bi(2 x)GaxTe2.7Se0.3. The changes of carrier mobility impose few effects on the electrical conductivity with increasing the content of Ga-doping. So the variation in electrical conductivity may be mainly attributed to the increase of the electron concentration. Fig. 4(a) shows the absolute values of Seebeck coefficient as a function of temperature for the Bi(2 x)GaxTe2.7Se0.3. The Seebeck coefficients of the Ga-doped samples decreased gradually with increasing the content of Ga-doping. The interrelationship between Seebeck coefficient and carrier concentration can be confirmed from relatively simple models of electron transport. For a metal or degenerate semiconductor, the Seebeck coefficient is given by [20] α¼
Fig. 2. (a) Temperature dependence of the electrical conductivity for Bi(2 x)GaxTe2.7Se0.3 and (b) temperature dependence of the carrier concentration for Bi(2 x)GaxTe2.7Se0.3 (measured at perpendicular to the pressing direction).
can be attributed to the alteration of nH and μH. The electrical transport properties of Bi2Te3-based material are closely related to the numbers of atomic vacancies and antisite defects [14]. The volatilization of Se or Te atoms can produce Se or Te vacancies, VSe or VTe. Furthermore, Bi atoms can occupy the sites of Se or Te, and the antisite defects (BiTe or BiSe) can form. Each vacancy can contribute two electrons, and each antisite defect only offers one hole. In this way, the dominating carrier of the Bi2Te3-based material is determined by the numbers of the two kinds of defects. The less physical and chemical difference the two atoms are, the easier the antisite defects form [15]. Due to the differences in the electronegativity of Ga (1.81), Bi (2.02), Te (2.10) and Se (2.55), the following trend can be applied to the ability of the antisite defects formation: BiTe 4 BiSe 4 GaTe 4 GaSe. Ga substitution for Bi will lead to a decrease in the concentration of the antisite defects (BiTe or BiSe) with increasing the content of Ga, and this will result in the increase in the numbers of electron. So the carrier concentration increases and the electrical conductivity goes up with increasing the content of Ga-doping. Besides, the preferentially oriented c-axis samples can be
2 2 8π k B n π 2=3 m T 3n 3eh2
ð1Þ
where α is the Seebeck coefficient and n is the carrier concentration. The carrier concentration dependence of the Seebeck coefficient is shown in Fig. 4(b). Hence the Seebeck coefficient decreases with the increase of the carrier concentration. Fig. 5 exhibits the temperature dependence of the power factor for Bi(2 x)GaxTe2.7Se0.3. The power factors of the Gadoped samples are obviously higher than those of the Bi2Te2.7Se0.3 near room temperature. The reason may be that the electrical conductivity is improved by Ga-doping and the Seebeck coefficients of the Ga-doped samples have not been derogated strongly. As expected, Ga substitution for Bi acts as the important effects and can improve the electrical properties of n-type Bi2Te2.7Se0.3 thermoelectric materials
Fig. 3. Temperature dependence of the carrier mobility for Bi(2 x)GaxTe2.7Se0.3 (measured at perpendicular to the pressing direction).
Please cite this article as: X. Duan, et al., Influence of Ga-doping on the thermoelectric properties of Bi(2 x)GaxTe2.7Se0.3 alloy, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.01.003
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Fig. 4. (a) Temperature dependence of the absolute values of Seebeck coefficient and (b) carrier concentration dependence of the absolute values of Seebeck coefficient for Bi(2 x)GaxTe2.7Se0.3.
Fig. 5. Temperature dependence of the power factor for Bi(2 x)GaxTe2.7Se0.3.
Fig. 6(a) shows the thermal conductivity (κ) as a function of the temperature for the Bi(2 x)GaxTe2.7Se0.3 samples. The thermal conductivity of the samples gradually increases with increasing the content of Ga-doping. But the thermal conductivity
Fig. 6. Temperature dependence of the thermal conductivity (a) and the lattice thermal conductivity (b) for Bi(2 x)GaxTe2.7Se0.3 (measured at perpendicular to the pressing direction).
of the Bi(2 x)GaxTe2.7Se0.3 (x ¼ 0.04) is the lowest near room temperature. The thermal conductivity of a degenerate semiconductor can be described by the following equation: κ¼ κe þ κL, where κe is the electronic thermal conductivity and κL is the lattice thermal conductivity. According to the Wiedemann–Franz relation, κe can be calculated by κe ¼ L0σT, where L0 is Lorentz constant (L0 ¼ 2.45 10 8 V2 K 2), σ is the electrical conductivity and T is the absolute temperature. Because the electrical conductivity increases with increasing the content of Ga-doping, the electronic thermal conductivity of the Bi(2 x)GaxTe2.7Se0.3 certainly goes up. The lattice thermal conductivity of the samples is shown in Fig. 6(b). It can be found that Ga substitution for Bi result in the lower lattice thermal conductivity of the Bi(2 x)GaxTe2.7Se0.3 (x ¼ 0.04, 0.08, 0.12) samples in comparison with that of the Bi2Te2.7Se0.3 samples in the temperature range of 300–500 K, specially at the temperatures near room temperature. The big difference of the mass exists between Ga (69.723) and Bi (208.9804), so the scattering of phonons can be intensified by conjunct effects of alloying and the defects. Because the lattice thermal conductivity can be decreased by Ga-doping, the
Please cite this article as: X. Duan, et al., Influence of Ga-doping on the thermoelectric properties of Bi(2 x)GaxTe2.7Se0.3 alloy, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.01.003
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alloys are significantly improved by Ga substitution for Bi. The highest ZT of the Bi(2 x)GaxTe2.7Se0.3 (x¼ 0.04) sample can reach 0.82 at 400 K. It can be concluded that the appropriate content of Ga-doping are very effective in improving thermoelectric properties of n-type Bi2Te2.7Se0.3 compounds. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (51161009), the China Scholarship Council (201208360039) and the Science Foundation of the Education Department of Jiangxi Province (GJJ13722). The authors also thank Dr. G. Jeff Snyder group at Caltech for kind permission to use TE testing apparatus. Fig. 7. Temperature dependence of the dimensionless thermoelectric figure of merit ZT for Bi(2 x)GaxTe2.7Se0.3.
appropriate Ga-doping can effectively decrease the thermal conductivity of Bi2Te2.7Se0.3 near room temperature. The dimensionless thermoelectric figure of merit ZT values are shown for Bi(2 x)GaxTe2.7Se0.3 in Fig. 7. Compared with the Bi2Te2.7Se0.3 sample, the ZT values of Ga-doped Bi2Te2.7Se0.3 have an obvious enhancement in the temperature range of 300–473 K. Because Ga-doping can improve the power factor and decrease the lattice thermal conductivity of the Bi2Te2.7Se0.3 samples, the Bi(2 x)GaxTe2.7Se0.3 (x¼ 0.04) samples have the maximum ZT values. The highest ZT of the sample can reach 0.82 at 400 K. From the above results it can be concluded that the appropriate content of Ga-doping are very effective in enhancing thermoelectric figure of merit of n-type Bi2Te2.7Se0.3. 4. Conclusion
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
The Bi(2 x)GaxTe2.7Se0.3 (x ¼ 0.04, 0.08, 0.12) alloys can result in substantial increases of the electrical conductivity from 300 to 500 K by Ga-doping. The Seebeck coefficients of the Ga-doped samples have not been derogated strongly within whole testing temperature range. The lattice thermal conductivities of the Bi(2 x)GaxTe2.7Se0.3 (x ¼ 0.04, 0.08, 0.12) are lower than that of Bi2Te2.7Se0.3 in the temperature range of 300–460 K. The Bi(2 x)GaxTe2.7Se0.3 (x¼ 0.04) samples have the lowest thermal conductivity near room temperature. It can be found that the thermoelectric properties of the Bi2Te2.7Se0.3
[14] [15] [16] [17] [18] [19] [20]
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Please cite this article as: X. Duan, et al., Influence of Ga-doping on the thermoelectric properties of Bi(2 x)GaxTe2.7Se0.3 alloy, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.01.003
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Original Research
Influence of Ga-doping on the thermoelectric properties of Bi(2 x)GaxTe2.7Se0.3 alloy Xingkai Duann, Konggang Hu, Shifeng Ding, Dahu Man, Haixia Jin Institute of New Energy Materials, School of Mechanical and Materials Engineering, Jiujiang University, Jiujiang 332005, China Received 19 July 2014; accepted 1 December 2014
Abstract Bi(2 x)GaxTe2.7Se0.3 (x ¼0, 0.04, 0.08, 0.12) alloys were fabricated by vacuum melting and hot pressing technique. The structure of the samples was evaluated by means of X-ray diffraction. The peak shift toward higher angle can be observed by Ga-doping. The effects of Ga substitution for Bi on the electrical and thermal transport properties were investigated in the temperature range of 300–500 K. The power factor values of the Ga-doped samples are obviously improved in the temperature range of 300–440 K. Among all the samples, the Bi(2 x)GaxTe2.7Se0.3 (x ¼0.04) sample showed the lowest thermal conductivity near room temperature and the maximum ZT value reached 0.82 at 400 K. & 2015 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved.
Keywords: Bi2Te2.7Se0.3; Electrical transport properties; Thermal transport properties; Carrier concentration
1. Introduction Thermoelectric materials have attracted a considerable amount of attention due to their ability of quietly converting waste heat from different sources into electricity or electrical power directly into cooling [1]. The efficiency of heat–electricity conversion can be defined by the dimensionless figure of merit ZT ¼ (S2σ/κ)T, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity and T is the absolute temperature. Accordingly, a good thermoelectric material has to combine large Seebeck coefficient, high electrical conductivity and low thermal conductivity [2]. Bi2Te3based compounds are the most important commercial thermoelectric materials near room temperature. The relatively high thermoelectric properties of the bulk Bi2Te3-based compounds have been achieved by utilizing hot pressing [3,4], spark plasma sintering [5,6]. Although thermoelectric properties of n Corresponding author. Present address: Qianjin Donglu 551, Lushan District, Jiujiang City 332005, China. Tel.: þ 86 792 8763469; fax: þ86 792 8312891. E-mail address: [email protected] (X. Duan). Peer review under responsibility of Chinese Materials Research Society.
the Bi2Te3-based compounds have been improved by doping and adjusting composition [7–9], the above-mentioned studies mainly focus on improving thermoelectric properties of p-type (Bi,Sb)2Te3. N-type Bi2Te2.7Se0.3 is the state-of-the-art thermoelectric material used in cooling device. However, the ZT values of n-type Bi2(Te,Se)3 are usually lower than those of p-type (Bi,Sb)2Te3. Therefore, it is very necessary to optimize the electrical and thermal transport properties of n-type Bi2Te2.7Se0.3 in order to develop p–n joint cooling device near room temperature. In recent years, many studies have been carried out to improve the thermoelectric properties of n-type Bi2Te3-based materials by doping [10–12] and tuning the carrier concentration. Lee et al. [13] reported that the increase in the carrier concentration induced by I doping led to an increase in the electrical conductivity. The ZT value was improved by I doping due to the increased PF, demonstrating a maximum of ZT ¼ 1.13 at 423 K for Bi2Te2.7Se0.3:I0.0075. Electrical transport properties of Bi2Te3-based materials are closely related to the concentration of atomic vacancies and antisite defects [14]. In the Bi2Te2.7Se0.3 structure, Bi atoms can occupy the sites of Se or Te, and the antisite defects (BiTe or BiSe) can form. Since the differences in the electronegativity between Ga (1.81) and
http://dx.doi.org/10.1016/j.pnsc.2015.01.003 1002-0071/& 2015 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved.
Please cite this article as: X. Duan, et al., Influence of Ga-doping on the thermoelectric properties of Bi(2 x)GaxTe2.7Se0.3 alloy, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.01.003
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Te (2.10) or Se (2.55) are bigger than those between Bi (2.02) and Te (2.10) or Se (2.55) [15], the following trend can be applied to the ability of the antisite defects formation: BiTe 4 BiSe 4GaTe 4 GaSe. In this way, the carrier concentration will be modified via Ga substitution for Bi. On the other hand, alloying with atoms of similar electric potentials but different masses can scatter phonons effectively, where scattering is due to difference in mass and/or bond stiffness [16]. The big difference of the mass exists between Ga and Bi, and the lattice thermal conductivity can be decreased by Ga-doping. So the electrical and thermal transport properties of the Bi(2 x)GaxTe2.7Se0.3 compounds can be tuned by Ga-doping. In this paper, the influences of Ga-doping on the thermoelectric properties of the Bi(2 x)GaxTe2.7Se0.3 compounds were investigated. The goal of this work was to optimize the electrical and thermal transport properties of the Bi2Te2.7Se0.3 by Ga-doping.
2. Materials and methods Elemental lumps of Bi (99.99%), Te (99.99%), Se (99.99%) and Ga (99.99%) were weighed according to the stoichiometry of Bi(2 x)GaxTe2.7Se0.3 (x ¼ 0, 0.04, 0.08, and 0.12). The elemental mixtures were loaded into the quartz tubes. The quartz tubes were sealed under vacuum and placed in a furnace and heated to 1073 K for 6 h, and then they were cooled to room temperature. The obtained ingots were pulverized using the agate mortar under vacuum. The powders were shaped by vacuum hot pressing in the graphite dies under 60 MPa pressure at 673 K for 1 h. The bulk disk-shaped samples of ∅12 mm 20 mm were obtained. X-ray diffraction measurements were performed on an X-ray diffractometer (Bruker, D8Advance with Cu Kα radiation, λ ¼ 1.5406 Å). The Seebeck coefficients were obtained by measuring thermal power under a temperature gradient of 10 K (the dimension of samples: ∅12 mm 4 mm). The electrical resistivity and Hall effects were measured by the Van der Pauw technique. A reversible magnetic field of 2 T is used for the Hall effects measurement in the temperature range of 300–500 K (the dimension of samples: ∅12 mm 1.2–1.5 mm). The thermal diffusivity (λ) was tested by the laser flash diffusivity method using a laser flash method (LFA 457) (the dimension of samples: ∅12 mm 1.2–1.5 mm). The specific heat (Cp) was obtained from TA: DSCQ20. The thermal conductivity was calculated according to the equation: λ¼ κ/(DCp), where D is the density. All of the measurements were carried out at the direction perpendicular to the pressing direction in the temperature range of 300–500 K.
3. Results and discussion The XRD patterns of the Bi(2 x)GaxTe2.7Se0.3 (x¼ 0, 0.04, 0.08, 0.12) are shown in Fig. 1(a). The result in Fig. 1(a) shows that the characteristic diffraction peaks are consistent with the peaks of Bi2Te2.7Se0.3. All the samples have the same rhombohedral lattice structures. The results of XRD patterns
Fig. 1. (a) XRD patterns of the Bi(2 x)GaxTe2.7Se0.3 samples and (b) XRD patterns of the corresponding enlarged for (006) crystal plane.
testify that the obtained materials are the single phase. Besides, Fig. 1(a) also shows that the preferentially oriented c-axis samples can be fabricated by the vacuum melting and hot pressing method. Fig. 1(b) shows the corresponding enlarged (006) peak of the samples. The peak shift toward the higher angle can be observed with the increase of Ga content. It is attributed to the difference of atomic radius between Ga (1.30 Å) and Bi (1.60 Å). It can be concluded that Ga atoms enter into the lattices sites. Fig. 2(a) shows the temperature dependence of the electrical conductivity (σ) for Bi(2 x)GaxTe2.7Se0.3. All samples show a metal conductive behavior in the temperature range of 300– 500 K. The electrical conductivities of the doped samples increase with increasing the content of Ga. It shows the content of Ga-doping has a significant influence on the electrical transport properties. The carrier concentration (nH) as a function of temperature was investigated via Hall measurements in Fig. 2(b). As the content of Ga-doping increases, the carrier concentration increases. Electrical conductivity can be expressed as σ¼ nHμHe, where μH is the carrier mobility and e is the electronic charge. The variation of electrical conductivity
Please cite this article as: X. Duan, et al., Influence of Ga-doping on the thermoelectric properties of Bi(2 x)GaxTe2.7Se0.3 alloy, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.01.003
X. Duan et al. / Progress in Natural Science: Materials International ] (]]]]) ]]]–]]]
3
fabricated by the vacuum melting and hot pressing method in this work. The electrical conductivity of n-type Bi2Te3based alloys can be also improved by texture enhancement [17,18]. In a pulverized and sintered samples, n-type Bi2Te3based alloys can generate excess negative carriers in the lattice (called donor-like effect) [19], which could vastly improve the electrical conductivity. Fig. 3 depicts the carrier mobility as a function of temperature for Bi(2 x)GaxTe2.7Se0.3. The changes of carrier mobility impose few effects on the electrical conductivity with increasing the content of Ga-doping. So the variation in electrical conductivity may be mainly attributed to the increase of the electron concentration. Fig. 4(a) shows the absolute values of Seebeck coefficient as a function of temperature for the Bi(2 x)GaxTe2.7Se0.3. The Seebeck coefficients of the Ga-doped samples decreased gradually with increasing the content of Ga-doping. The interrelationship between Seebeck coefficient and carrier concentration can be confirmed from relatively simple models of electron transport. For a metal or degenerate semiconductor, the Seebeck coefficient is given by [20] α¼
Fig. 2. (a) Temperature dependence of the electrical conductivity for Bi(2 x)GaxTe2.7Se0.3 and (b) temperature dependence of the carrier concentration for Bi(2 x)GaxTe2.7Se0.3 (measured at perpendicular to the pressing direction).
can be attributed to the alteration of nH and μH. The electrical transport properties of Bi2Te3-based material are closely related to the numbers of atomic vacancies and antisite defects [14]. The volatilization of Se or Te atoms can produce Se or Te vacancies, VSe or VTe. Furthermore, Bi atoms can occupy the sites of Se or Te, and the antisite defects (BiTe or BiSe) can form. Each vacancy can contribute two electrons, and each antisite defect only offers one hole. In this way, the dominating carrier of the Bi2Te3-based material is determined by the numbers of the two kinds of defects. The less physical and chemical difference the two atoms are, the easier the antisite defects form [15]. Due to the differences in the electronegativity of Ga (1.81), Bi (2.02), Te (2.10) and Se (2.55), the following trend can be applied to the ability of the antisite defects formation: BiTe 4 BiSe 4 GaTe 4 GaSe. Ga substitution for Bi will lead to a decrease in the concentration of the antisite defects (BiTe or BiSe) with increasing the content of Ga, and this will result in the increase in the numbers of electron. So the carrier concentration increases and the electrical conductivity goes up with increasing the content of Ga-doping. Besides, the preferentially oriented c-axis samples can be
2 2 8π k B n π 2=3 m T 3n 3eh2
ð1Þ
where α is the Seebeck coefficient and n is the carrier concentration. The carrier concentration dependence of the Seebeck coefficient is shown in Fig. 4(b). Hence the Seebeck coefficient decreases with the increase of the carrier concentration. Fig. 5 exhibits the temperature dependence of the power factor for Bi(2 x)GaxTe2.7Se0.3. The power factors of the Gadoped samples are obviously higher than those of the Bi2Te2.7Se0.3 near room temperature. The reason may be that the electrical conductivity is improved by Ga-doping and the Seebeck coefficients of the Ga-doped samples have not been derogated strongly. As expected, Ga substitution for Bi acts as the important effects and can improve the electrical properties of n-type Bi2Te2.7Se0.3 thermoelectric materials
Fig. 3. Temperature dependence of the carrier mobility for Bi(2 x)GaxTe2.7Se0.3 (measured at perpendicular to the pressing direction).
Please cite this article as: X. Duan, et al., Influence of Ga-doping on the thermoelectric properties of Bi(2 x)GaxTe2.7Se0.3 alloy, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.01.003
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Fig. 4. (a) Temperature dependence of the absolute values of Seebeck coefficient and (b) carrier concentration dependence of the absolute values of Seebeck coefficient for Bi(2 x)GaxTe2.7Se0.3.
Fig. 5. Temperature dependence of the power factor for Bi(2 x)GaxTe2.7Se0.3.
Fig. 6(a) shows the thermal conductivity (κ) as a function of the temperature for the Bi(2 x)GaxTe2.7Se0.3 samples. The thermal conductivity of the samples gradually increases with increasing the content of Ga-doping. But the thermal conductivity
Fig. 6. Temperature dependence of the thermal conductivity (a) and the lattice thermal conductivity (b) for Bi(2 x)GaxTe2.7Se0.3 (measured at perpendicular to the pressing direction).
of the Bi(2 x)GaxTe2.7Se0.3 (x ¼ 0.04) is the lowest near room temperature. The thermal conductivity of a degenerate semiconductor can be described by the following equation: κ¼ κe þ κL, where κe is the electronic thermal conductivity and κL is the lattice thermal conductivity. According to the Wiedemann–Franz relation, κe can be calculated by κe ¼ L0σT, where L0 is Lorentz constant (L0 ¼ 2.45 10 8 V2 K 2), σ is the electrical conductivity and T is the absolute temperature. Because the electrical conductivity increases with increasing the content of Ga-doping, the electronic thermal conductivity of the Bi(2 x)GaxTe2.7Se0.3 certainly goes up. The lattice thermal conductivity of the samples is shown in Fig. 6(b). It can be found that Ga substitution for Bi result in the lower lattice thermal conductivity of the Bi(2 x)GaxTe2.7Se0.3 (x ¼ 0.04, 0.08, 0.12) samples in comparison with that of the Bi2Te2.7Se0.3 samples in the temperature range of 300–500 K, specially at the temperatures near room temperature. The big difference of the mass exists between Ga (69.723) and Bi (208.9804), so the scattering of phonons can be intensified by conjunct effects of alloying and the defects. Because the lattice thermal conductivity can be decreased by Ga-doping, the
Please cite this article as: X. Duan, et al., Influence of Ga-doping on the thermoelectric properties of Bi(2 x)GaxTe2.7Se0.3 alloy, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.01.003
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alloys are significantly improved by Ga substitution for Bi. The highest ZT of the Bi(2 x)GaxTe2.7Se0.3 (x¼ 0.04) sample can reach 0.82 at 400 K. It can be concluded that the appropriate content of Ga-doping are very effective in improving thermoelectric properties of n-type Bi2Te2.7Se0.3 compounds. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (51161009), the China Scholarship Council (201208360039) and the Science Foundation of the Education Department of Jiangxi Province (GJJ13722). The authors also thank Dr. G. Jeff Snyder group at Caltech for kind permission to use TE testing apparatus. Fig. 7. Temperature dependence of the dimensionless thermoelectric figure of merit ZT for Bi(2 x)GaxTe2.7Se0.3.
appropriate Ga-doping can effectively decrease the thermal conductivity of Bi2Te2.7Se0.3 near room temperature. The dimensionless thermoelectric figure of merit ZT values are shown for Bi(2 x)GaxTe2.7Se0.3 in Fig. 7. Compared with the Bi2Te2.7Se0.3 sample, the ZT values of Ga-doped Bi2Te2.7Se0.3 have an obvious enhancement in the temperature range of 300–473 K. Because Ga-doping can improve the power factor and decrease the lattice thermal conductivity of the Bi2Te2.7Se0.3 samples, the Bi(2 x)GaxTe2.7Se0.3 (x¼ 0.04) samples have the maximum ZT values. The highest ZT of the sample can reach 0.82 at 400 K. From the above results it can be concluded that the appropriate content of Ga-doping are very effective in enhancing thermoelectric figure of merit of n-type Bi2Te2.7Se0.3. 4. Conclusion
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The Bi(2 x)GaxTe2.7Se0.3 (x ¼ 0.04, 0.08, 0.12) alloys can result in substantial increases of the electrical conductivity from 300 to 500 K by Ga-doping. The Seebeck coefficients of the Ga-doped samples have not been derogated strongly within whole testing temperature range. The lattice thermal conductivities of the Bi(2 x)GaxTe2.7Se0.3 (x ¼ 0.04, 0.08, 0.12) are lower than that of Bi2Te2.7Se0.3 in the temperature range of 300–460 K. The Bi(2 x)GaxTe2.7Se0.3 (x¼ 0.04) samples have the lowest thermal conductivity near room temperature. It can be found that the thermoelectric properties of the Bi2Te2.7Se0.3
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Please cite this article as: X. Duan, et al., Influence of Ga-doping on the thermoelectric properties of Bi(2 x)GaxTe2.7Se0.3 alloy, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.01.003