Thermoelectric transport properties of Pb–Sn–Te–Se system
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Abstract
IV–VI compounds are considered as promising thermoelectric materials, and high thermoelectric performance was achieved in IV–VI solid solutions. In this work, the thermoelectric properties of Pb–Sn–Te–Se-based solid solutions were systematically investigated. Among these solid solutions, it is found that a figure of merit (ZT) peak value of 1.0 at 873 K can be obtained in (PbTe)0.5(SnTe)0.5, on account of the combination of superior electrical properties in SnTe and low thermal conductivity in PbTe. Furthermore, we investigated and summarized the thermoelectric transport properties and proposed the thermoelectric performance maps for the IV–VI solid solutions in Pb–Sn–Te–Se system. This comprehensive investigation on Pb–Sn–Te–Se-based solid solutions can effectively guide and scan thermoelectric performance for a given unknown composition and enhance the thermoelectric properties in IV–VI compounds.
Graphical Abstract
Keywords
Thermoelectric materials Electrical conductivity Thermal conductivity Pb–Sn–Te–Se system Solid solutions1 Introduction
Thermoelectric materials provide an alternative way to directly and reversibly convert between heat and electricity, making them particularly appealing in waste heat recovery and fuel efficiency improvement [1, 2, 3]. Researchers are making great efforts to explore and design high-performance thermoelectric materials for practical applications [4, 5, 6, 7, 8, 9, 10, 11, 12, 13]. The efficiency of a thermoelectric material is governed by the dimensionless figure of merit, ZT = (S 2 σT/κ), where S, σ, T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity, respectively [2, 14]. It is supposed to exhibit high S and σ, and low κ simultaneously in a superior thermoelectric material. However, the interdependence of these parameters complicates the efforts in thermoelectric designing [2].
Noticeably, some state-of-the-art thermoelectric materials are from the IV–VI systems, including lead chalcogenides (PbTe [15, 16, 17], PbSe [18, 19, 20] and PbS [21, 22]) and tin chalcogenides (SnTe [23, 24, 25, 26], SnSe [7, 8, 27, 28] and SnS [29]). Interestingly, solid solutions between these compounds are confirmed to exhibit superior performance than end-members through successfully manipulating nanostructures and electronic band structures, such as PbTe–PbSe [13, 30, 31, 32], PbTe–PbS [15, 33, 34, 35], PbSe–PbS [36, 37, 38], SnSe–SnS [39], PbSe–SnSe [40], Pb1−x Sn x Te–PbS [41] and PbTe–PbSe–PbS [16, 42]. These outstanding thermoelectric materials motivate us to re-investigate IV–VI systems, and the composition is expected to be optimized through scanning the thermoelectric properties for the solid solutions between every two end-member.
In this paper, solid solutions with different ingredients in Pb–Sn–Te–Se system were synthesized by melting method and pulverizing followed by spark plasma sintering (SPS). It is found that PbSnTeSe possesses strong phonon scattering in solid solutions. Namely, a low lattice thermal conductivity of ~ 0.8 W·m−1·K−1 at 873 K can be achieved when PbTe is alloyed with SnSe. In addition, PbTe–SnTe solid solutions simultaneously possess promising thermal and electrical transport properties, and thus, the ZT value exceeds 1.0 at 873 K in (PbTe)0.5(SnTe)0.5. The present results show that (PbTe)0.5(SnTe)0.5 possesses the best thermoelectric performance among Pb–Sn–Te–Se-based solid solutions. Based on the investigated thermoelectric transport properties of IV–VI solid solutions, we proposed one performance map for Pb–Sn–Te–Se system. The comprehensive investigations on Pb–Sn–Te–Se-based solid solutions can effectively guide and scan thermoelectric performance for a given unknown composition and enhance the thermoelectric properties in IV–VI compounds.
2 Experimental
2.1 Raw materials
The raw materials include Pb granules (99.99%, Aladdin element, China), Sn pieces (99.99%, Aladdin element, China), Te pieces (99.999%, Aladdin element, China), Se pieces (99.999%, Aladdin element, China).
2.2 Synthesis
The ingots of the Pb–Sn–Te–Se systems were prepared by putting the high-purity materials (Pb, Sn, Te and Se) into quartz tubes; then, the tubes were evacuated (< 1.3 × 10−2 Pa) and sealed. The sealed tubes were slowly heated to 823 K over 12 h, then quickly heated to 1423 K over 6 h, kept at this temperature for 6 h and furnace-cooled to room temperature. The obtained ingots were crushed into fine powders and densified using spark plasma sintering (SPS) in a 15-mm-diameter graphite dies under 50 MPa at 873 K for 6 min; disk-shaped samples with dimension of Φ 15 mm × 8 mm were obtained.
2.3 Phase and microstructure
Samples pulverized with an agate mortar were used for X-ray powder diffraction. The diffraction patterns were recorded with Cu Kα (λ = 0.15418 nm) radiation in a reflection geometry on an INEL diffractometer operating at 40 kV and 20 mA using a position-sensitive detector. Scanning electron microscope (SEM, JSM7500, JEOL, Tokyo, Japan) and energy-dispersive spectroscopy (EDS) experiments were carried out. SEM specimens were prepared by conventional methods, including cutting, grinding, dimpling and polishing, etc.
2.4 Electrical transport properties
The obtained SPS-processed pellets were cut into bars with 3 mm × 3 mm × 10 mm that were used for simultaneous measurement of the electrical conductivity and Seebeck coefficient using CTA and Ulvac-Riko ZEM-3 instrument under a helium atmosphere from room temperature to 873 K. The samples were coated with a thin layer of boron nitride to protect the instrument from the influence of evaporation.
2.5 Thermal transport properties
The obtained SPS-processed pellets were cut and polished into a Φ 6-mm disk shape with a 1–2 mm thickness for thermal diffusivity measurements. The disks were coated with a thin layer of graphite to minimize errors from the emissivity of the materials. The thermal conductivity was calculated according to the formula κ = D × ρ×C p , where the thermal diffusivity (D) was measured using laser-flash diffusivity method with a Netzsch LFA457 instrument, ρ is the sample density determined using the dimensions and mass of the sample and C p is the specific heat capacity estimated with Dulong–Petit law. The thermal diffusivity data were analyzed using a Cowan model with pulse correction. Noticeably, all the properties described in this study were measured perpendicular to the sintering pressure direction, which is the same to the direction for electrical transport properties measurement.
3 Results and discussion
Investigated solid solutions in Pb–Sn–Te–Se system
a Powder XRD patterns and b lattice parameters for (PbSe)1−x (SnTe) x
Thermoelectric transport properties as a function of temperature for (PbSe)1−x (SnTe) x : a electrical conductivity, b Seebeck coefficient, c power factor, d total thermal conductivity, e lattice thermal conductivity, f ZT values
As shown in Fig. 3d, the variation tendencies of thermal conductivities for all solid solutions are consistent with those of their electrical conductivities. κ tot is a sum of the electronic (κ ele) and lattice thermal conductivity (κ lat). κ ele is directly proportional to the electrical conductivity (σ) through the Wiedemann–Franz relation, κ ele = LσT, where L is the Lorenz number [50]. The typical method of Lorenz number calculation is adopted [21]. When the SnTe component gradually rises in the solid solution, the Lorenz number of ~ 2.45 × 10−8 W·Ω·K−2 shows a property of degenerate semiconductor [50]. As shown in Fig. 3d, the overall trend of the total thermal conductivity of each sample decreases first and then rises at high temperature, indicating the existence of bipolar effect. The minimum thermal conductivity of solid solutions reaches ~ 1.0 W·m−1·K−1 at 473 K in (PbSe)0.8(SnTe)0.2. At high temperature, the thermal conductivity of each sample is superior to those of PbSe and SnTe; a lattice thermal conductivity of ~ 0.8 W·m−1·K−1 is obtained in (PbSe)0.5(SnTe)0.5 at 873 K (Fig. 3e). Owing to the special valence band characteristic of PbSe and high carrier concentration brought by SnTe, the ZT values of ~ 0.6, ~ 0.5 and ~ 0.6 are obtained in samples with x = 0.5, 0.6 and 0.8 at 773 K. In (PbSe)0.5(SnTe)0.5, the ZT value of ~ 0.72 is achieved at 873 K.
a SEM image and EDS elemental maps (b Pb, c Te, d Sn, e Se) of (PbSe)0.5(SnTe)0.5
Thermoelectric properties of solid solutions in Pb–Sn–Te–Se system
Samples | σ(300 K)/(S·cm−1) | S(max)/(μV·K−1) | PF(max)/(μW·cm−1·K−2) | κ tot (min)/(W·m−1·K−1) | κ lat (min)/(W·m−1·K−1) | ZT(max) |
---|---|---|---|---|---|---|
PbTe | 282.02 | 400.46 | 21.59 | 1.44 | 1.41 | 0.33 |
(PbTe)0.75(SnTe)0.25 | 1326.79 | 238.02 | 10.95 | 1.19 | 0.96 | 0.46 |
(PbTe)0. 5(SnTe)0. 5 | 2554.02 | 215.45 | 20.08 | 1.56 | 0.90 | 1.03 |
(PbTe)0.25(SnTe)0.75 | 4382.89 | 182.06 | 21.87 | 2.08 | 1.03 | 0.92 |
SnTe | 7718.24 | 146.15 | 26.10 | 3.05 | 1.19 | 0.75 |
PbSe | 464.69 | 338.49 | 28.70 | 1.16 | 1.11 | 0.52 |
(PbSe)0.75(SnSe)0.25 | 1121.24 | 150.12 | 4.97 | 1.25 | 0.89 | 0.23 |
(PbSe)0.5(SnSe)0.5 | 1564.86 | 124.53 | 6.62 | 0.91 | 1.02 | 0.36 |
(PbSe)0.25(SnSe)0.75 | 1.05 | 304.86 | 3.70 | 0.36 | 0.35 | 0.67 |
SnSe | 0.02 | 530.45 | 6.17 | 0.60 | 0.58 | 0.75 |
(PbTe)0.75(PbSe)0.25 | 79.01 | 365.91 | 10.97 | 0.99 | 0.97 | 0.30 |
(PbTe)0.5(PbSe)0.5 | 52.70 | 447.48 | 9.02 | 0.91 | 0.89 | 0.27 |
(PbTe)0.25(PbSe)0.75 | 63.42 | 430.03 | 9.88 | 1.00 | 0.98 | 0.27 |
(SnTe)0.75(SnSe)0.25 | 4565.65 | 137.96 | 19.40 | 2.58 | 0.98 | 0.66 |
(SnTe)0.5(SnSe)0.5 | 153.68 | 127.49 | 4.99 | 0.99 | 0.84 | 0.30 |
(SnTe)0.25(SnSe)0.75 | 1.79 | 399.06 | 5.14 | 0.42 | 0.42 | 0.72 |
(PbTe)0.8(SnSe)0.2 | 382.91 | 230.97 | 7.57 | 0.97 | 0.84 | 0.37 |
(PbTe)0.6(SnSe)0.4 | 1029.48 | 177.59 | 8.03 | 1.19 | 0.72 | 0.44 |
(PbTe)0.5(SnSe)0.5 | 1269.91 | 167.49 | 11.22 | 1.23 | 0.64 | 0.72 |
(PbTe)0.4(SnSe)0.6 | 1603.95 | 127.42 | 6.88 | 1.37 | 0.32 | 0.32 |
(PbTe)0.2(SnSe)0.8 | 63.41 | 162.07 | 2.09 | 0.56 | 0.53 | 0.21 |
a Electrical conductivity, b Seebeck coefficient, c power factor of Pb–Sn–Te–Se system
Minimum a total thermal conductivity and b lattice thermal conductivity of Pb–Sn–Te–Se system
Maximum ZT values of Pb–Sn–Te–Se system
4 Conclusion
In this work, a systematic study on solid solutions in Pb–Sn–Te–Se system was presented. In the PbSe–SnTe system, the ZT value of ~ 0.72 is achieved at 873 K in PbSnTeSe. In the SnTe–PbTe system, due to the effective combination of SnTe and PbTe, the complementary performance of two compounds is achieved and both electrical and thermal transport properties are significantly enhanced. In (SnTe)0.5(PbTe)0.5, the ZT value exceeds 1.0 at 773–873 K. Present results indicate that (1) SnTe can effectively enhance the electrical conductivity in PbTe, PbSe and SnSe because of its high carrier concentration; (2) SnSe can largely suppress the lattice thermal conductivity in PbTe, PbSe and SnTe due to its low thermal conductivity; (3) the relatively high ZT values appear in PbSe–SnTe and SnTe–PbTe solid solutions. Noticeably, these results in this work are acquired only using alloying method; therefore, higher performance is worthy to be expected after optimizing the carrier concentration.
Notes
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Nos. 51671015, 51571007 and 51772012), the 111 project (No. B17002), the Beijing Municipal Science and Technology Commission (No. Z171100002017002) and the Shenzhen Peacock Plan Team (No. KQTD2016022619565991).
Compliance with ethical standards
Ethical standards
On behalf of all the authors, we declare that all the experiments comply with the current laws of the country in which they were performed.
Supplementary material
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