- Email: [email protected]
Cation-anion codoping to enhance thermoelectric performance of BiSbSe3
Materials Science in Semiconductor Processing 93 (2019) 299–303
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Cation-anion codoping to enhance thermoelectric performance of BiSbSe3 a
a
a
a
De Zhang , Jingdan Lei , Weibao Guan , Zheng Ma , Zhenxiang Cheng ⁎ ⁎ Chao Wanga, , Yuanxu Wanga, a b
a,b
T
b
, Lijuan Zhang ,
Institute for Computational Materials Science, School of Physics and Electronics, Henan University, Kaifeng 475004, China Institute for Superconducting and Electronic Materials, University of Wollongong, Squires Way, North Wollongong, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermoelectric materials Cation-anion codoping BiSbSe3
To improve the thermoelectric performance of n-type BiSbSe3, the cation and anion codoping was implemented by doping SnCl4. SnCl4 doping mainly regulate the relationship between electrical conductivity and Seebeck coefficient, which is an effective way to enhance the figure of merit ZT . SnCl4 doping can decompose into cation and anion which provide electrons and thus increase carrier concentration. The optimum carrier concentration was obviously increased to 1.17 × 10 20 cm−3 . The electrical conductivity boosts from 0.05 to 5087 Sm−1 at 300 K. Coupled with the intrinsically suppressed thermal conductivity originating from weak chemical band and grain refinement, a higher ZT of 0.61 is obtained at 773 K for (Bi,Sb)1.95Sn0.05Se2.8Cl0.2, which is ten times higher than that of the un-doped BiSbSe3.
1. Introduction Due to the consumption of finite sources and the phenomenon of population explosion, the world is facing an alarming and urgent problem of energy scarcity [1,2]. With most of industrial heat energy wasting, there is a strong desire for the development of thermoelectric (TE) materials, which can directly convert heat into electrical energy by the Seebeck effect and realize electric refrigeration by the Peltier effect. The performance of TE materials is characterized as the dimensionless figure of merit ZT , ZT = S 2σT /(κ e + κL) , where S, σ , T, κ e and κL stand for the Seebeck coefficient, electrical conductivity, absolute temperature, electronic thermal conductivity, and lattice thermal conductivity, respectively [3,4]. An ideal TE materials must have large S, large σ , and a low κ simultaneously [5]. However, as we know, it is not easy to improve all parameters at the same time because TE parameters are interrelated [6]. In the last decades, researchers have found some ways to successfully “tune or design” these properties of TE materials slight independently, such as phonon glass electron crystal (PGEC), [7–9] nanostructuring, [10,11] large lattice anharmonicity, [12,13] band engineering, [14–17], and optimizing carrier concentration, [18,19] etc . Topological insulators (TLs) are fascinating and challenging materials, and some of them have superior TE performance, like Bi2Te3based materials. Bi2Te3-based materials are one of the most important topological insulators, which possess high ZT s of 1.2 ∼ 1.4 near the room temperature [1,6,20]. The rarity of telluride (Te) element,
⁎
however, in the earth's crust impedes their large-scale application [21]. Te-free Bi2Se3, which belongs to the layered narrow-band group AV2 BVI 3 (A=Bi,Sb and B˭Se,Te), has the same tetradymite structure with Bi2Te3, suggesting that it is probably a promising good thermoelectric material. It is worth mentioning that pristine Bi2Se3 has the single valley conduction band minimum, which results in a low power factor. In addition, it has high κL , and thus a low ZT (∼ 0.3) [22,23]. The TE performance of Bi2Se3 can be improved by optimizing the carrier concentration and band engineering etc . Recently, Wang et al. [24] reported that band convergence can be realized by a composition-induced crystal structural transition through Sb alloying, which doubles the band degeneracy Nv from 1 to 2, leading to a large density of state (DOS) effective mass and, thus, the enhancement of power factor. In conjunction with the transition-induced phonon softening and substantial lattice anharmonicity, a significant reduction of lattice thermal conductivity, from ∼ 1.3 (Bi2Se3) to ∼ 0.64 Wm−1K−1 (BiSbSe3) was also received. Although BiSbSe3 presents a low κL and large S, making it a promising candidate for mid-temperature TE material, its low σ limits the TE figure of merit. Unlike Bi2Te3-based materials, which can be confirmed as n- or p-type semiconductors by the adjustment of stoichiometric ratio of Bi and Te, the type of transport carrier for BiSbSe3 is dominated by charged selenium vacancies which were regarded as electron donors, directly resulting in n-type behavior [25]. An optimized electron carrier concentration of BiSbSe3 was realized by means of iodine substitution at anion sites, from 0.17 to 9.1 cm−3, leading to an exciting ZT of ∼ 1.0 at 800 K [24]. Recently, Zhao et al. utilized the
Corresponding author. E-mail addresses: [email protected] (C. Wang), [email protected] (Y. Wang).
https://doi.org/10.1016/j.mssp.2019.01.021 Received 8 September 2018; Received in revised form 17 December 2018; Accepted 15 January 2019 1369-8001/ © 2019 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 93 (2019) 299–303
D. Zhang et al.
graphite dies with an inner diameter of 12.7 mm, and densified by SPS at 748 K for 7 min under a uniaxial pressure of 50 MPa under vacuum. The phase compositions of the samples were characterized by X-ray diffraction (XRD, DX-2700) with Cu K a radiation accelerated in a 2 θ range of 10–80°. The microstructure of freshly broken surface of the bulk samples were observed by a field emission scanning electron microscope (SEM, JSM-7001F). The densified samples were used to simultaneously measure σ and S using a commercial equipment (ZEM-3, ULVAC-RIKO, InC.) under a low-pressure He atmosphere. κ was examined in the same direction as the test direction of electrical conductivity using a DLF-1 (Waters) laser flash apparatus under Ar atmosphere. Carrier concentration n and Hall mobility μ were measured at room temperature using a DC Hall measurement system (ET9005, East Changing Technologies, Inc.). 3. Results and discussion Fig. 1. XRD patterns of (Bi,Sb)2-xSnxSe3–4xCl4x.
Fig. 1 shows the XRD patterns of all samples. The main Bragg reflections of all samples correspond well to the pure orthorhombic phase besides an additional weak peak when x is 0.05 and 0.06, indicating BiSbSe3 as the major phase and suggesting SnCl4 enter the BiSbSe3 matrix. In order to observe the microstructure of cation and anion codoping in BiSbSe3, we performed the FESEM analysis of all samples, as shown in Fig. 2(a-e). It can be noticed that samples contain crystalline grains with different orientation and show layered structure. We observed SEM images in the different directions of BiSbSe3 in order to confirm whether BiSbSe3 contains anisotropy, as shown in Fig. S1. The result shows that, compared to Bi2Se3, the anisotropy of BiSbSe3 is not obvious [27]. The distribution of SnCl4 and Bi2Se3 elements in the sample was determined via elemental mapping analysis, which may illustrates that Sn atoms replace Bi/Sb, as shown in Fig. S2. Additionally, the size of crystalline grains become smaller when x is 0.05 for (Bi,Sb)2-xSnxSe3–4xCl4x. In this work, the cation-anion co-doping gives rise to the grain refinement, which indicates the increase of grain boundaries and thus, the enhancement of phonon scattering [28]. Fig. 3 shows the temperature dependent TE performances of (Bi,Sb)2xSnxSe3–4xCl4x (x = 0–0.06). It is clearly seen that the σ of all the cation-anion co-doped samples are greatly improved, as shown in Fig. 3(a). The σ of pristine BiSbSe3 is around 0.05 Sm−1 at room temperature and it increase to 5087 Sm−1 when x is 0.05. This significant increase of electrical conductivity is mainly ascribed to the contribution of increased carrier concentration, as shown in Fig. 3(c). The carrier
same principle to lift the ZT to 1.4 at 800 K by doping BiBr3 [26]. With the emergence of more ways to improve thermoelectric performance, it is a currently major task that finding more convenient and easy-doping methods to improve the TE performance of BiSbSe3. In this work, we found the TE performance of BiSbSe3 is dramatically improved by doping SnCl4. When SnCl4 is dissolves into BiSbSe3, Sn and Cl atoms replace the Bi/Sb and Se sites, respectively, as electron donor, thus increase the carrier concentration and enhance the σ . The room-temperature σ increases from 0.05 to 5087 Sm−1 when x is 0.05. With the benefits of low thermal conductivities, ranging from 0.44 to 0.51 Wm−1K−1, a high ZT of 0.61 at 773 K is achieved when x is 0.05 for (Bi,Sb)2-xSnxSe3–4xCl4x.
2. Experimental procedure SnCl4 doped BiSbSe3 samples, namely (Bi,Sb)2-xSnxSe3–4xCl4x, were prepared by combining melting-annealing with spark plasma sintering (SPS) technique (SPS-211LX, Fuji Electronic Industrial Co. Ltd.). Bi, Sb, Se, anhydrous SnCl4 were weighed in the glove box, and (Bi,Sb)2xSnxSe3–4xCl4x (x = 0–0.06) samples were prepared [24]. After heat preservation, the quartz tubes were quenched in the mixture of liquid nitrogen and water, and subsequently annealed at 673 K for 48 h. The annealed ingots were ground into fine powders and loaded into
Fig. 2. (a) SEM images of sintered (Bi,Sb)2-xSnxSe3–4xCl4x. (a)x = 0; (b)x = 0.03; (c)x = 0.04; (d)x = 0.05; (e)x = 0.06. 300
Materials Science in Semiconductor Processing 93 (2019) 299–303
D. Zhang et al.
Fig. 3. Temperature dependence of thermoelectric properties for (Bi,Sb)2-xSnxSe3–4xCl4x, (a) electrical conductivity, (b) Seebeck coefficient, (e) total thermal conductivity, (f) lattice thermal conductivity. (c) Carrier concentration and carrier mobility as a function of SnCl4 content. (d) Room-temperature Pisarenko plot for (Bi,Sb)2-xSnxSe3–4xCl4x.
mobility shows almost no change with increasing doping concentration ranging from 2.08 to 3.28 cm2 v−1 s−1. Typically, the pristine BiSbSe3 has a very low carrier concentration of 3.6 × 1015 at room temperature. It presents five orders of magnitude increase, around 11.77 × 1019 cm−3 when x is 0.05. As we know, the 4 + of Sn is stable [29]. When SnCl4 doped into the BiSbSe3, Sn atoms substitute for Bi or Sb sites, while Cl atoms substitute for Se sites, as the electron donors, significantly increase the carrier concentration of BiSbSe3 by cation and anion codoping, [30] as presented by the following defect chemistry reaction: Cl(BiSbSe3) ⟶Cl·Se + e′ Sn(BiSbSe3) ⟶Sn·Bi /Sn·Sb + e′ Fig. 3(b) shows the temperature dependence of S for (Bi,Sb)2xSnxSe3–4xCl4x (x = 0–0.06). The negative S implies n-type carrier
Table 1 Carrier concentration, carriers mobility and effective mass of all the samples at room temperature. Samples
nH (1019cm−3 )
μH (cm2v−1s−1)
m*
x = 0.00
3.6 × 10−4 4.64 7.54 11.77 12.9
26.81
0.043
2.08 2.25 2.71 3.28
2.38 1.90 1.88 1.88
x = 0.03 x = 0.04 x = 0.05 x = 0.06
301
Materials Science in Semiconductor Processing 93 (2019) 299–303
D. Zhang et al.
on the measured S, and the acoustic phonon scattering r is assumed to −1/2. It is obvious that the change in m* from x = 0.03–0.06 shows almost no change in Table 1. According to SPB assumption, carrier mobility μ can be expressed: μ = eτ / m*, thus the unchanged effective mass is corresponded with the unchanged mobility. Additionally, although the impurity phase could be observed in XRD pattern when x is 0.05 and 0.06, the electrical transport properties presented in Fig. 3(ac) showed no obvious degeneration. Fig. 3(e-f) displays the temperature dependence of the total κ and κL for the pristine and co-doped samples. κL was obtained by subtracting the electron thermal conduction κ ele from κ . κ ele can be calculated by the Wiedemann-Franz law (κ ele =L σ T), where L is the Lorenz constant. Generally, L 0 ≈ 2.45 × 10−8 WΩK−2 , but actually, L is less than L 0 for most TE materials, especially for high temperature TE materials. L can be described by Eq. (5) [4,35]. 2
(r + 5/2) Fr + 3/2 (η) ⎞ ⎤ k 2 ⎡ (r + 7/2) Fr + 5/2 (η) L = ⎛ B⎞ ⎢ − ⎜⎛ ⎟ ⎥. ⎝ e ⎠ ⎣ (r + 3/2) Fr + 1/2 (η) ⎝ (r + 3/2) Fr + 1/2 (η) ⎠ ⎦
The finally κL was plotted in Fig. 3(f). Compared with other TE materials with high κL , BiSbSe3 possesses the intrinsically low κL (0.38 Wm−1K−1 at 300 K). which is mainly attributed to the low sound velocity due to its weak chemical bonds. For example, the average sound velocity of BiSbSe3 and α -MgAgSb are 1688 and 1921 ms−1, respectively [24,36]. However, the average sound velocity of half-Heusler FeNbSb reaches 3473 ms−1, and hence higher κL (18 Wm−1 K−1 at 300 K) [37]. Although the κ of the doped sample slightly increased, as shown in Fig. 3(e), it keeps a lower value in the entire test temperature range. The temperature dependent power factors of (Bi,Sb)2-xSnxSe3–4xCl4x are plotted in Fig. 4(a). According to the result of σ and S, the maximum power factor reaches 4 μW cm−1 K−2 when x is 0.05 for (Bi,Sb)2xSnxSe3–4xCl4x at 773 K. The ZT values as a function of temperature for (Bi,Sb)2-xSnxSe3–4xCl4x are shown in Fig. 4(b). With the benefits of the increased σ and the low κ , a high ZT value of 0.61 at 773 K is achieved for (Bi,Sb)1.95Sn0.05Se2.8Cl0.2, being ten times higher than that (0.06) of pristine BiSbSe3.
Fig. 4. (a) Temperature dependence of (a) power factor, (b) ZT for (Bi,Sb)2xSnxSe3–4xCl4x.
conduction for the BiSbSe3 system. Contrary to the enhancement of σ , the S presents obvious reduction with increasing SnCl4 fraction due to the significant increase of carrier concentration (Fig. 3c). Typically, the room-temperature Seebeck coefficient of pristine BiSbSe3 is 530.21 μVK−1, and decreases to 140.77 μVK−1 when x = 0.05. Additionally, the relation between S and n can be explicated according to the Boltzmann transport equation of electrons in a single parabolic band (SPB) approximate. Although BiSbSe3 is a multiband material, most researchers in similar research directions use the SPB model [31–34]. The SPB approximation can be expressed:
S=±
kB ⎡ (r + 5/2) Fr + 3/2 (η) − η ⎤, ⎥ e ⎢ ⎦ ⎣ (r + 3/2) Fr + 1/2 (η)
n = 4π ⎛ ⎝ Fn (η) =
2 m*kB T ⎞3/2 F1/2 (η) , h2 rH ⎠
∫0
∞
χn 1 + e χ −η
dχ ,
(5)
4. Conclusions In summary, we prepared (Bi,Sb)2-xSnxSe3–4xCl4x by melting and spark plasma sintering process. It was found that the cation-anion codoping can effectively improve the electrical transport properties and enhance TE performance. Furthermore, it demonstrates that Sn and Cl atoms act as electron donors at Bi/Sb and Se sites improved the σ and suppressed the grain growth. The σ of (Bi,Sb)1.95Sn0.05Se2.8Cl0.2 achieves a considerable enhancement from 0.05 to 5087 Sm−1 at room temperature by virtue of the significantly increased n, so ZT is dramatically lifted from 0.06 to 0.61. The improved TE performance makes the cation-anion co-doped BiSbSe3 an excellent candidate for mediumtemperature thermoelectric power generation when large-scale applications are desired.
(1)
(2)
(3)
2/3
m* =
h2 ⎡ n ⎤ ⎢ 4πF1/2 (η) ⎦ ⎥ 2kB T ⎣
,
Acknowledgments (4) This research was sponsored by the National Natural Science Foundation of China (No. U1504511, 51571083, 51371076, 11674083), the Science and Technology fund of Henan Province (No. 182102210227, 162102210169, 162300410224), and Foundation of Henan Educational Committee (14A430029).
where kB is the Boltzmann constant, Fn (η) is the nth order Fermi integral, η is the reduced Fermi energy, e is the electron charge, and h is the Planck constant. The rH of −1/2 is considered in SPB model. According to Eq. (1)–(3), pisarenko curve was plotted in Fig. 3(d). Clearly, the augment of carrier concentration reduces the S, and the values of S deviates from theoretical pisarenko curve. From a theoretical perspective, this phenomenon may be due to the sharpening of the bottom of conduction band with the SnCl4 content increasing, which leads to S less than theoretical value. For the carrier effective mass (m*), it can be converted by Eq. (2). The reduced Fermi energy can be calculated based
Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at doi:10.1016/j.mssp.2019.01.021. 302
Materials Science in Semiconductor Processing 93 (2019) 299–303
D. Zhang et al.
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303
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Cation-anion codoping to enhance thermoelectric performance of BiSbSe3 a
a
a
a
De Zhang , Jingdan Lei , Weibao Guan , Zheng Ma , Zhenxiang Cheng ⁎ ⁎ Chao Wanga, , Yuanxu Wanga, a b
a,b
T
b
, Lijuan Zhang ,
Institute for Computational Materials Science, School of Physics and Electronics, Henan University, Kaifeng 475004, China Institute for Superconducting and Electronic Materials, University of Wollongong, Squires Way, North Wollongong, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermoelectric materials Cation-anion codoping BiSbSe3
To improve the thermoelectric performance of n-type BiSbSe3, the cation and anion codoping was implemented by doping SnCl4. SnCl4 doping mainly regulate the relationship between electrical conductivity and Seebeck coefficient, which is an effective way to enhance the figure of merit ZT . SnCl4 doping can decompose into cation and anion which provide electrons and thus increase carrier concentration. The optimum carrier concentration was obviously increased to 1.17 × 10 20 cm−3 . The electrical conductivity boosts from 0.05 to 5087 Sm−1 at 300 K. Coupled with the intrinsically suppressed thermal conductivity originating from weak chemical band and grain refinement, a higher ZT of 0.61 is obtained at 773 K for (Bi,Sb)1.95Sn0.05Se2.8Cl0.2, which is ten times higher than that of the un-doped BiSbSe3.
1. Introduction Due to the consumption of finite sources and the phenomenon of population explosion, the world is facing an alarming and urgent problem of energy scarcity [1,2]. With most of industrial heat energy wasting, there is a strong desire for the development of thermoelectric (TE) materials, which can directly convert heat into electrical energy by the Seebeck effect and realize electric refrigeration by the Peltier effect. The performance of TE materials is characterized as the dimensionless figure of merit ZT , ZT = S 2σT /(κ e + κL) , where S, σ , T, κ e and κL stand for the Seebeck coefficient, electrical conductivity, absolute temperature, electronic thermal conductivity, and lattice thermal conductivity, respectively [3,4]. An ideal TE materials must have large S, large σ , and a low κ simultaneously [5]. However, as we know, it is not easy to improve all parameters at the same time because TE parameters are interrelated [6]. In the last decades, researchers have found some ways to successfully “tune or design” these properties of TE materials slight independently, such as phonon glass electron crystal (PGEC), [7–9] nanostructuring, [10,11] large lattice anharmonicity, [12,13] band engineering, [14–17], and optimizing carrier concentration, [18,19] etc . Topological insulators (TLs) are fascinating and challenging materials, and some of them have superior TE performance, like Bi2Te3based materials. Bi2Te3-based materials are one of the most important topological insulators, which possess high ZT s of 1.2 ∼ 1.4 near the room temperature [1,6,20]. The rarity of telluride (Te) element,
⁎
however, in the earth's crust impedes their large-scale application [21]. Te-free Bi2Se3, which belongs to the layered narrow-band group AV2 BVI 3 (A=Bi,Sb and B˭Se,Te), has the same tetradymite structure with Bi2Te3, suggesting that it is probably a promising good thermoelectric material. It is worth mentioning that pristine Bi2Se3 has the single valley conduction band minimum, which results in a low power factor. In addition, it has high κL , and thus a low ZT (∼ 0.3) [22,23]. The TE performance of Bi2Se3 can be improved by optimizing the carrier concentration and band engineering etc . Recently, Wang et al. [24] reported that band convergence can be realized by a composition-induced crystal structural transition through Sb alloying, which doubles the band degeneracy Nv from 1 to 2, leading to a large density of state (DOS) effective mass and, thus, the enhancement of power factor. In conjunction with the transition-induced phonon softening and substantial lattice anharmonicity, a significant reduction of lattice thermal conductivity, from ∼ 1.3 (Bi2Se3) to ∼ 0.64 Wm−1K−1 (BiSbSe3) was also received. Although BiSbSe3 presents a low κL and large S, making it a promising candidate for mid-temperature TE material, its low σ limits the TE figure of merit. Unlike Bi2Te3-based materials, which can be confirmed as n- or p-type semiconductors by the adjustment of stoichiometric ratio of Bi and Te, the type of transport carrier for BiSbSe3 is dominated by charged selenium vacancies which were regarded as electron donors, directly resulting in n-type behavior [25]. An optimized electron carrier concentration of BiSbSe3 was realized by means of iodine substitution at anion sites, from 0.17 to 9.1 cm−3, leading to an exciting ZT of ∼ 1.0 at 800 K [24]. Recently, Zhao et al. utilized the
Corresponding author. E-mail addresses: [email protected] (C. Wang), [email protected] (Y. Wang).
https://doi.org/10.1016/j.mssp.2019.01.021 Received 8 September 2018; Received in revised form 17 December 2018; Accepted 15 January 2019 1369-8001/ © 2019 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 93 (2019) 299–303
D. Zhang et al.
graphite dies with an inner diameter of 12.7 mm, and densified by SPS at 748 K for 7 min under a uniaxial pressure of 50 MPa under vacuum. The phase compositions of the samples were characterized by X-ray diffraction (XRD, DX-2700) with Cu K a radiation accelerated in a 2 θ range of 10–80°. The microstructure of freshly broken surface of the bulk samples were observed by a field emission scanning electron microscope (SEM, JSM-7001F). The densified samples were used to simultaneously measure σ and S using a commercial equipment (ZEM-3, ULVAC-RIKO, InC.) under a low-pressure He atmosphere. κ was examined in the same direction as the test direction of electrical conductivity using a DLF-1 (Waters) laser flash apparatus under Ar atmosphere. Carrier concentration n and Hall mobility μ were measured at room temperature using a DC Hall measurement system (ET9005, East Changing Technologies, Inc.). 3. Results and discussion Fig. 1. XRD patterns of (Bi,Sb)2-xSnxSe3–4xCl4x.
Fig. 1 shows the XRD patterns of all samples. The main Bragg reflections of all samples correspond well to the pure orthorhombic phase besides an additional weak peak when x is 0.05 and 0.06, indicating BiSbSe3 as the major phase and suggesting SnCl4 enter the BiSbSe3 matrix. In order to observe the microstructure of cation and anion codoping in BiSbSe3, we performed the FESEM analysis of all samples, as shown in Fig. 2(a-e). It can be noticed that samples contain crystalline grains with different orientation and show layered structure. We observed SEM images in the different directions of BiSbSe3 in order to confirm whether BiSbSe3 contains anisotropy, as shown in Fig. S1. The result shows that, compared to Bi2Se3, the anisotropy of BiSbSe3 is not obvious [27]. The distribution of SnCl4 and Bi2Se3 elements in the sample was determined via elemental mapping analysis, which may illustrates that Sn atoms replace Bi/Sb, as shown in Fig. S2. Additionally, the size of crystalline grains become smaller when x is 0.05 for (Bi,Sb)2-xSnxSe3–4xCl4x. In this work, the cation-anion co-doping gives rise to the grain refinement, which indicates the increase of grain boundaries and thus, the enhancement of phonon scattering [28]. Fig. 3 shows the temperature dependent TE performances of (Bi,Sb)2xSnxSe3–4xCl4x (x = 0–0.06). It is clearly seen that the σ of all the cation-anion co-doped samples are greatly improved, as shown in Fig. 3(a). The σ of pristine BiSbSe3 is around 0.05 Sm−1 at room temperature and it increase to 5087 Sm−1 when x is 0.05. This significant increase of electrical conductivity is mainly ascribed to the contribution of increased carrier concentration, as shown in Fig. 3(c). The carrier
same principle to lift the ZT to 1.4 at 800 K by doping BiBr3 [26]. With the emergence of more ways to improve thermoelectric performance, it is a currently major task that finding more convenient and easy-doping methods to improve the TE performance of BiSbSe3. In this work, we found the TE performance of BiSbSe3 is dramatically improved by doping SnCl4. When SnCl4 is dissolves into BiSbSe3, Sn and Cl atoms replace the Bi/Sb and Se sites, respectively, as electron donor, thus increase the carrier concentration and enhance the σ . The room-temperature σ increases from 0.05 to 5087 Sm−1 when x is 0.05. With the benefits of low thermal conductivities, ranging from 0.44 to 0.51 Wm−1K−1, a high ZT of 0.61 at 773 K is achieved when x is 0.05 for (Bi,Sb)2-xSnxSe3–4xCl4x.
2. Experimental procedure SnCl4 doped BiSbSe3 samples, namely (Bi,Sb)2-xSnxSe3–4xCl4x, were prepared by combining melting-annealing with spark plasma sintering (SPS) technique (SPS-211LX, Fuji Electronic Industrial Co. Ltd.). Bi, Sb, Se, anhydrous SnCl4 were weighed in the glove box, and (Bi,Sb)2xSnxSe3–4xCl4x (x = 0–0.06) samples were prepared [24]. After heat preservation, the quartz tubes were quenched in the mixture of liquid nitrogen and water, and subsequently annealed at 673 K for 48 h. The annealed ingots were ground into fine powders and loaded into
Fig. 2. (a) SEM images of sintered (Bi,Sb)2-xSnxSe3–4xCl4x. (a)x = 0; (b)x = 0.03; (c)x = 0.04; (d)x = 0.05; (e)x = 0.06. 300
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Fig. 3. Temperature dependence of thermoelectric properties for (Bi,Sb)2-xSnxSe3–4xCl4x, (a) electrical conductivity, (b) Seebeck coefficient, (e) total thermal conductivity, (f) lattice thermal conductivity. (c) Carrier concentration and carrier mobility as a function of SnCl4 content. (d) Room-temperature Pisarenko plot for (Bi,Sb)2-xSnxSe3–4xCl4x.
mobility shows almost no change with increasing doping concentration ranging from 2.08 to 3.28 cm2 v−1 s−1. Typically, the pristine BiSbSe3 has a very low carrier concentration of 3.6 × 1015 at room temperature. It presents five orders of magnitude increase, around 11.77 × 1019 cm−3 when x is 0.05. As we know, the 4 + of Sn is stable [29]. When SnCl4 doped into the BiSbSe3, Sn atoms substitute for Bi or Sb sites, while Cl atoms substitute for Se sites, as the electron donors, significantly increase the carrier concentration of BiSbSe3 by cation and anion codoping, [30] as presented by the following defect chemistry reaction: Cl(BiSbSe3) ⟶Cl·Se + e′ Sn(BiSbSe3) ⟶Sn·Bi /Sn·Sb + e′ Fig. 3(b) shows the temperature dependence of S for (Bi,Sb)2xSnxSe3–4xCl4x (x = 0–0.06). The negative S implies n-type carrier
Table 1 Carrier concentration, carriers mobility and effective mass of all the samples at room temperature. Samples
nH (1019cm−3 )
μH (cm2v−1s−1)
m*
x = 0.00
3.6 × 10−4 4.64 7.54 11.77 12.9
26.81
0.043
2.08 2.25 2.71 3.28
2.38 1.90 1.88 1.88
x = 0.03 x = 0.04 x = 0.05 x = 0.06
301
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on the measured S, and the acoustic phonon scattering r is assumed to −1/2. It is obvious that the change in m* from x = 0.03–0.06 shows almost no change in Table 1. According to SPB assumption, carrier mobility μ can be expressed: μ = eτ / m*, thus the unchanged effective mass is corresponded with the unchanged mobility. Additionally, although the impurity phase could be observed in XRD pattern when x is 0.05 and 0.06, the electrical transport properties presented in Fig. 3(ac) showed no obvious degeneration. Fig. 3(e-f) displays the temperature dependence of the total κ and κL for the pristine and co-doped samples. κL was obtained by subtracting the electron thermal conduction κ ele from κ . κ ele can be calculated by the Wiedemann-Franz law (κ ele =L σ T), where L is the Lorenz constant. Generally, L 0 ≈ 2.45 × 10−8 WΩK−2 , but actually, L is less than L 0 for most TE materials, especially for high temperature TE materials. L can be described by Eq. (5) [4,35]. 2
(r + 5/2) Fr + 3/2 (η) ⎞ ⎤ k 2 ⎡ (r + 7/2) Fr + 5/2 (η) L = ⎛ B⎞ ⎢ − ⎜⎛ ⎟ ⎥. ⎝ e ⎠ ⎣ (r + 3/2) Fr + 1/2 (η) ⎝ (r + 3/2) Fr + 1/2 (η) ⎠ ⎦
The finally κL was plotted in Fig. 3(f). Compared with other TE materials with high κL , BiSbSe3 possesses the intrinsically low κL (0.38 Wm−1K−1 at 300 K). which is mainly attributed to the low sound velocity due to its weak chemical bonds. For example, the average sound velocity of BiSbSe3 and α -MgAgSb are 1688 and 1921 ms−1, respectively [24,36]. However, the average sound velocity of half-Heusler FeNbSb reaches 3473 ms−1, and hence higher κL (18 Wm−1 K−1 at 300 K) [37]. Although the κ of the doped sample slightly increased, as shown in Fig. 3(e), it keeps a lower value in the entire test temperature range. The temperature dependent power factors of (Bi,Sb)2-xSnxSe3–4xCl4x are plotted in Fig. 4(a). According to the result of σ and S, the maximum power factor reaches 4 μW cm−1 K−2 when x is 0.05 for (Bi,Sb)2xSnxSe3–4xCl4x at 773 K. The ZT values as a function of temperature for (Bi,Sb)2-xSnxSe3–4xCl4x are shown in Fig. 4(b). With the benefits of the increased σ and the low κ , a high ZT value of 0.61 at 773 K is achieved for (Bi,Sb)1.95Sn0.05Se2.8Cl0.2, being ten times higher than that (0.06) of pristine BiSbSe3.
Fig. 4. (a) Temperature dependence of (a) power factor, (b) ZT for (Bi,Sb)2xSnxSe3–4xCl4x.
conduction for the BiSbSe3 system. Contrary to the enhancement of σ , the S presents obvious reduction with increasing SnCl4 fraction due to the significant increase of carrier concentration (Fig. 3c). Typically, the room-temperature Seebeck coefficient of pristine BiSbSe3 is 530.21 μVK−1, and decreases to 140.77 μVK−1 when x = 0.05. Additionally, the relation between S and n can be explicated according to the Boltzmann transport equation of electrons in a single parabolic band (SPB) approximate. Although BiSbSe3 is a multiband material, most researchers in similar research directions use the SPB model [31–34]. The SPB approximation can be expressed:
S=±
kB ⎡ (r + 5/2) Fr + 3/2 (η) − η ⎤, ⎥ e ⎢ ⎦ ⎣ (r + 3/2) Fr + 1/2 (η)
n = 4π ⎛ ⎝ Fn (η) =
2 m*kB T ⎞3/2 F1/2 (η) , h2 rH ⎠
∫0
∞
χn 1 + e χ −η
dχ ,
(5)
4. Conclusions In summary, we prepared (Bi,Sb)2-xSnxSe3–4xCl4x by melting and spark plasma sintering process. It was found that the cation-anion codoping can effectively improve the electrical transport properties and enhance TE performance. Furthermore, it demonstrates that Sn and Cl atoms act as electron donors at Bi/Sb and Se sites improved the σ and suppressed the grain growth. The σ of (Bi,Sb)1.95Sn0.05Se2.8Cl0.2 achieves a considerable enhancement from 0.05 to 5087 Sm−1 at room temperature by virtue of the significantly increased n, so ZT is dramatically lifted from 0.06 to 0.61. The improved TE performance makes the cation-anion co-doped BiSbSe3 an excellent candidate for mediumtemperature thermoelectric power generation when large-scale applications are desired.
(1)
(2)
(3)
2/3
m* =
h2 ⎡ n ⎤ ⎢ 4πF1/2 (η) ⎦ ⎥ 2kB T ⎣
,
Acknowledgments (4) This research was sponsored by the National Natural Science Foundation of China (No. U1504511, 51571083, 51371076, 11674083), the Science and Technology fund of Henan Province (No. 182102210227, 162102210169, 162300410224), and Foundation of Henan Educational Committee (14A430029).
where kB is the Boltzmann constant, Fn (η) is the nth order Fermi integral, η is the reduced Fermi energy, e is the electron charge, and h is the Planck constant. The rH of −1/2 is considered in SPB model. According to Eq. (1)–(3), pisarenko curve was plotted in Fig. 3(d). Clearly, the augment of carrier concentration reduces the S, and the values of S deviates from theoretical pisarenko curve. From a theoretical perspective, this phenomenon may be due to the sharpening of the bottom of conduction band with the SnCl4 content increasing, which leads to S less than theoretical value. For the carrier effective mass (m*), it can be converted by Eq. (2). The reduced Fermi energy can be calculated based
Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at doi:10.1016/j.mssp.2019.01.021. 302
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