Article pubs.acs.org/cm
Extremely Low Lattice Thermal Conductivity and Point Defect Scattering of Phonons in Ag-doped (SnSe)1−x(SnS)x Compounds Chan-Chieh Lin,†,§ R. Lydia,†,§ Jae Hyun Yun,† Ho Seong Lee,‡ and Jong Soo Rhyee*,† †
Department of Applied Physics and KHU-KIST Department of Converging Science and Technology, Kyung Hee University, Yong-in 17104, Korea ‡ School of Materials Science & Metallurgical Engineering, Kyungpook National University, Daegu 41566, Korea S Supporting Information *
ABSTRACT: Single crystalline SnSe has been reported to exhibit the high thermoelectric zT value of 2.6 at 923 K along the b-axis direction, due to its low thermal conductivity [Zhao, L. D.; et al. Nature 2014, 508, 373]. However, the strongly anisotropic properties of the orthorhombic structure degrade the thermoelectric performance of polycrystalline SnSe, resulting in a low zT of 0.6 and 0.8 for Ag- and Na-doped SnSe, respectively. Here, we prepared Ag0.01Sn0.99Se1−xSx (x = 0, 0.10, 0.15, 0.20, and 0.35) compounds by melting and hot press sintering. The compounds showed extremely low thermal conductivity (0.11 W m−1 K−1 at 825 K for x = 0.15). Using transmission electron microscopy images, we found that SnS alloying induced numerous nanoscale point defects. A Debye−Callaway model analysis supported the conclusion that the extremely low lattice thermal conductivity could be attributed to the point defect scattering of phonons. This resulted in a high zT of 1.67 at 823 K for the x = 0.15 sample, which is the state-of-the-art zT value for polycrystalline SnSe. Because the compounds are based on the environmentally friendly and cheap materials Sn, Se, and S, they make promising candidates for thermoelectric applications.
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try.27−29 Although SnSe is a highly stable, nontoxic, earth abundant, and inexpensive compound, it has previously been ignored for thermoelectric research because of its large band gap of about 0.9 eV, which results in a very low zT value.30,31 Recently, an extremely high zT value of 2.6 at 923 K was observed for an SnSe single crystal due to its intrinsic ultralow thermal conductivity along the b-axis.32 The extremely low thermal conductivity comes from the layered structure of SnSe and an anomalously high Grü neisen parameter, which represents the anharmonicity of the lattice vibration.32−34 However, in polycrystalline SnSe, the strongly anisotropic properties of SnSe degrades the thermoelectric performance because its high electrical resistivity, and in that case the zT values have only been reported to be about 0.6 and 0.8 for Agand Na-doped SnSe, respectively.35,36 In this work, Ag0.01Sn0.99Se1−xSx compounds (x = 0, 0.10, 0.15, 0.20, and 0.35) were synthesized by melting and hot press sintering. Using high-resolution transmission microscopy images, we found numerous nanoscale point defects after SnS alloying. An extremely low thermal conductivity of 0.11 W m−1 K−1 was obtained for the x = 0.15 sample, resulting in a high zT value of 1.67 at 823 K. A Debye−Callaway model analysis
INTRODUCTION Thermoelectric (TE) materials can directly convert heat into electricity and vice versa when a thermal gradient and electric bias are applied, respectively. They are commercially used today in electrical power generation and solid state cooling technologies.1−4 The ideal materials for thermoelectric generators should produce a high dimensionless thermoelectric figure-of-merit (zT), which is defined by zT = S2T/ρκ, where S, T, ρ, and κ are the Seebeck coefficient, absolute temperature, electrical resistivity, and thermal conductivity, respectively.5,6 Extensive efforts have been expended to obtain high zT values from metals and alloys using single crystalline and/or nanostructured composites.7−17 Several approaches have been devoted to enhance zT, including the use of resonant doping,18,19 band convergence,20,21 quantum confinement effects,22 all-scale hierarchical architecture,23,24 etc. As one of the candidate materials investigated for thermoelectric power generation, lead chalcogenide (PbTe) based alloys have demonstrated high zT values of up to 2 in p-type materials.23,24 However, the environmental hazards of Pb in the environment limit the thermoelectric applications of lead chalcogenides. As Pb-free thermoelectric materials, binary tin chalcogenides (SnX; X = Te, Se, and S) have been studied with various types of modifications, including resonant doping, band structure engineering, and nanostructuring.25,26 In addition, SnTe, SnSe, and their solid solution has been investigated intensively as a topological crystalline insulator driven by mirror symme© 2017 American Chemical Society
Received: April 20, 2017 Revised: May 29, 2017 Published: May 31, 2017 5344
DOI: 10.1021/acs.chemmater.7b01612 Chem. Mater. 2017, 29, 5344−5352
Article
Chemistry of Materials suggests that the extremely low lattice thermal conductivity comes from the point defect scattering of phonons.
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EXPERIMENTAL DETAILS
Ag0.01Sn0.99Se1−xSx ingots (x = 0, 0.10, 0.15, 0.20, and 0.35) were prepared by melting, quenching, annealing, and hot press sintering process. Stoichiometric mixtures of high purity elements (99.999%) of Ag, Sn, Se, and S were sealed into evacuated quartz tubes, slowly heated up to 1273 K and kept at that temperature for 24 h, followed by water quenching. The obtained ingots were further annealed at 823 K for 3 days. After hand grinding the annealed ingots into fine powders with a mortar and pestle, the powders were hot pressed in a 12.7 mm diameter graphite mold under a uniaxial pressure of 50 MPa at 943 K for 30 min. All the samples were cut and measured parallel to the press direction. X-ray diffraction (XRD) measurements were carried out by Cu Kα radiation (D8 Advance, Bruker, Germany). The microstructures of the samples were investigated using a high-resolution field-emission scanning electron microscope (HR-FESEM) and transmission electron microscope (TEM). The elemental compositions of each sample were characterized by energy dispersive X-ray spectroscopy (EDX). The thermoelectric properties of Seebeck coefficient S and electrical resistivity ρ were measured by four-probe contact method using a thermoelectric measurement system (ZEM-3 ULVAC, Japan) under helium environment in the temperature range of 300−823 K. The thermal conductivity κ was calculated by the relation κ = ρsλCp, where ρs, λ, and Cp are sample density, thermal diffusivity, and specific heat, respectively. The thermal diffusivity λ was measured by laser flash method (LFA-457, NETZSCH, Germany). The specific heat Cp was obtained from the weighted average of the SnSe and SnS heat capacity obtained in previous research.35,37 The Hall carrier density nH and Hall carrier mobility μH were calculated by the relation of nH = −1/(eRH) and μH = σ/(enH), respectively, where RH is the Hall coefficient measured by the four-probe contact method under sweeping magnetic fields within a range −5−5 T using a physical property measurement system (14 T PPMS Dynacool, Quantum Design, USA).
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RESULTS AND DISCUSSION The X-ray diffraction (XRD) patterns and lattice parameters of the Ag0.01Sn0.99Se1−xSx compounds are shown in Figure 1a,b, respectively. All the samples show a low temperature SnSe phase with an orthorhombic structure with the space group Pnma. A small amount of AgSnSe2 secondary phase, denoted by the purple star, can be seen in the x = 0, 0.10, and 0.15 samples; it was also observed in previous reports on Ag-doped SnSe compounds.35 The inset of Figure 1a is the expanded plot for the (400) and (111) peaks of the compounds. It shows a right shift of peaks with increasing x, indicating a decrease in lattice parameters due to the small size of S (105 pm) compared with Se (120 pm). The lattice parameters follow Vegard’s law, as shown in Figure 1b, and the lattice parameters linearly decrease with the increase in S concentration x. The lattice parameters of x = 1.0 are slightly increased after hot press sintering (closed symbols), which may occur due to the evaporation of a small amount of sulfur during the sintering process. The temperature-dependent Seebeck coefficient S(T) and electrical resistivity ρ(T) of the Ag0.01Sn0.99Se1−xSx compounds are shown in Figure 2a,b, respectively. The temperaturedependent Seebeck coefficients show p-type metallic properties with temperature that increase with increasing temperature up to 600 K. The S(T) of the compounds decreases with increasing temperature in the high temperature region (T ≥ 600 K), and pronounced peaks are subsequently observed near 600 K.
Figure 1. Powder XRD patterns with a small amount of AgSnSe2 (stars) (a) and lattice parameters before (open symbol) and after (closed symbol) hot press sintering (b) of the Ag0.01Sn0.99Se1−xSx compounds.
In the depiction of the Seebeck coefficient of single crystalline SnSe, included for comparison (dashed line), the gradual decrease in the Seebeck coefficient near 525 K can be explained by the bipolar conductivity of carriers accompanied by a structural phase transition, from the Pnma to a Cmcm structure.32 In a previously reported polycrystalline compound of Ag0.01Sn0.99Se (dotted line), the broad peak is observed near 550−575 K.35 This is also thought to be caused by the bipolar transport of carriers, because the band gap decreases from room temperature Pnma (0.61 eV) to high temperature Cmcm (0.39 eV), resulting in the generation of opposite sign carriers, negating the Seebeck coefficient.32 The sulfur substitution in Ag0.01Sn0.99Se1−xSx induces an increase in Seebeck coefficient with increasing sulfur concentration, while the broad peak position is increased to 600 K as compared with the Ag0.01Sn0.99Se compound (550 K), which is insensitive to sulfur concentration x. Assuming that the broad peaks come from electron−hole pairs, the thermal band gap Eg can be estimated from the relation: Smax ≅ Eg/2eTmax.38 The thermal band gap of x = 0 is estimated to be 0.51 eV, which is higher than the previous report by Chen et al. (0.47 eV).35 The thermal band gap increases with increasing x, such that 0.53 eV (x = 0.1), 0.55 eV (x = 0.2), and 0.57 eV (x = 0.35). The 5345
DOI: 10.1021/acs.chemmater.7b01612 Chem. Mater. 2017, 29, 5344−5352
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Chemistry of Materials
Figure 3. Temperature-dependent Hall carrier density nH (a) and room temperature Hall carrier density nH and Hall mobility μH with respect to sulfur substitution concentration (b) of the Ag0.01Sn0.99Se1−xSx compounds.
Figure 2. Temperature-dependent Seebeck coefficient S (a) and electrical resistivity ρ (b) of the Ag0.01Sn0.99Se1−xSx compounds compared with those of the previously reported single crystalline SnSe along the b-axis (dash line) and Ag0.01Sn0.99Se (dot line).32,35
Table 1. Carrier Density nH, Seebeck Coefficient S, and Effective Mass m* of the Ag0.01Sn0.99Se1−xSx Compounds at Room Temperature, Compared with Those of the Previously Reported Single Crystalline SnSe (along the b-Axis) and Ag0.01Sn0.99Se32,35
energy band gaps in the crystalline compounds of SnS and 0.5% Ag-doped SnSe were determined to be 1.21 and 1.18 eV from the optical absorption coefficients.37 The energy band gap measurements obtained from the optical spectroscopy of the polycrystalline SnSe1−xSx compounds also exhibited a monotonic increase in energy band gap with increasing S concentration from 0.9 eV (SnSe) and 1.16 eV (SnS).39 The theoretical band structure calculation shows that the energy band gap of SnS is greater than that of SnSe.29 Therefore, the increase in the thermal energy band gap that occurs with increasing sulfur concentration is caused because the band gap of SnS is larger than that of SnSe. The temperature-dependent electrical resistivities ρ(T) of the compounds are similar to the S(T) behavior, in that they show a broad maximum near 610 K. The electrical resistivity up to 600 K is regarded as the thermal activation of carriers.32 The bipolar transport of carriers reduces the electrical resistivity with increasing temperature due to the significant thermal excitation of electron−hole pairs. The electrical resistivities also increase with increasing sulfur concentration, which is consistent with the increase in Seebeck coefficient and the increase in band gap produced by the sulfur incorporation. It is worth noting that the electrical resistivity observed in the
nominal composition
nH (1018 cm−3)
S (μV K−1)
m* (me)
SnSe (ref 32) Ag0.01Sn0.99Se (ref 35) Ag0.01Sn0.99Se Ag0.01Sn0.99Se0.90S0.10 Ag0.01Sn0.99Se0.85S0.15 Ag0.01Sn0.99Se0.80S0.20
0.264 3.495 10.262 9.200 7.641 6.220
506.98 313.10 291.52 297.58 296.41 311.96
0.711 0.867 1.493 1.458 1.276 1.261
present work is much higher than that reported by Chen et al. in spite of having the same composition for x = 0.35 The cause of the differences can be resolved by measuring the Hall carrier density and Hall mobility of carriers. The Hall carrier densities nH of the compounds are shown in Figure 3a. The carrier density increases with increasing temperature, and it decreases with the increase in x due to the increase in band gap produced by S, as mentioned above. The carrier densities of the samples are in the ranges of several 1018 to 1019 cm−3 as presented in Table 1. 5346
DOI: 10.1021/acs.chemmater.7b01612 Chem. Mater. 2017, 29, 5344−5352
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Chemistry of Materials
Figure 4. (a)Temperature-dependent thermal conductivity κ of the Ag0.01Sn0.99Se1−xSx compounds compared with that of the previously reported single crystalline SnSe along the b-axis (dashed line) and Ag0.01Sn0.99Se (dotted line).32,35 (b) Thermal conductivity κ of the Ag0.01Sn0.99Se with different sintering conditions. (c) Thermal conductivity κ of the Ag0.01Sn0.99Se1−xSx (x = 0 and 0.15) compounds from 2 to 825 K with theoretical Debye-Callaway model fitting (see text).
Table 2. Debye Temperature ΘD, Sound Velocity v, Average Grain Size L, Coefficients of Point Defect Scattering A, and Umklapp Scattering B, Respectively, Obtained from the Fitting of the Experimental Data with eqs 1 and 2 x=0 x = 0.15
ΘD (K)
v (m s−1)
L (μm)
A (10−43 s3)
B (10−18 s K−1)
49 49
1284 1284
3.04 4.19
1590 12676
22.07 37.48
(945 K) and long-time (30 min) hot press sintering (as shown in the Supporting Information). There were few precipitates in the previous compounds, which were synthesized using a low temperature (800 K) and short-time (10 min) rapid hot press sintering process.35 Figure 4a shows the temperature-dependent thermal conductivity κ(T) for the Ag0.01Sn0.99Se1−xSx compounds. The thermal conductivity of the sulfur doped compounds exhibit extremely low values (0.11 W m−1 K−1 at 825 K for x = 0.15). In the case of the Ag0.01Sn0.99Se (x = 0) compound, it is also significantly lower (0.28 W m−1 K−1 at 725 K for x = 0) than the previous report (0.43 W m−1 K−1 at 725 K for x = 0).35 The decrease in κ(T) with increasing temperature up to 550 K can be understood in relation to the contribution of acoustic phonons to the thermal conductivity. The upward curvature of κ(T) at around 550 K may come from the bipolar transport of carriers, which is consistent with the temperature-dependent Seebeck coefficient and electrical resistivity behaviors. The thermal conductivity of the compound x = 0 is about 37% lower at 300 K and 43% lower at 800 K than those of the previous report,35 as well as 21% lower at 300 K and 34% lower at 800 K than those of the single crystal SnSe along the b-axis.32
Figure 3b presents the Hall carrier density (left axis, black square) and Hall mobility (right axis, blue circle) at room temperature with respect to sulfur concentration x. Here, the carrier mobility (
