Realizing High Thermoelectric Performance in BaCu2–xAgxTe2

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Realizing High Thermoelectric Performance in BaCu2−xAgxTe2 through Enhanced Carrier Effective Mass and Point-Defect Scattering Chunhui Yang,† Kai Guo,*,† Xinxin Yang,† Juanjuan Xing,†,‡ Ke Wang,† Jun Luo,*,† and Jing-Tai Zhao†,§ School of Materials Science and Engineering, ‡Materials Genome Institute, and §State Key Laboratory of Advanced Special Steel, Shanghai University, 99 Shangda Road, Shanghai 200444, China

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ABSTRACT: With the aim of realizing mid-temperature thermoelectric materials, the electrical and thermal transport properties of the Zintl phase compound BaCu2Te2 with a channel structure (Pnma) were systematically investigated. BaCu2Te2 exhibits moderate electrical transport properties and low intrinsic thermal conductivity, which contribute to its high thermoelectric figure of merit (zT = 0.72 at 823 K). The partial substitution of Cu with Ag led to a significant enhancement of the Seebeck coefficient, as the carrier effective mass increased from 1.0m0 for BaCu2Te2 to 1.5m0 for BaCu1.9Ag0.1Te2 at room temperature, and reduction of the Hall carrier concentration. In addition, at higher temperature, a lower thermal conductivity of ∼0.5 W m−1 K−1 was achieved for BaCu1.9Ag0.1Te2; this reduced thermal conductivity resulted from the point-defect scattering arising from the Ag/Cu isovalent substitution. Together, these integrated effects led to a significant improvement of the quality factor β with a peak thermoelectric figure of merit zT of 1.08 for BaCu1.9Ag0.1Te2 at 823 K. The average zT of BaCu1.9Ag0.1Te2 over the temperature range of 323−823 K was 0.68, demonstrating its potential as a promising thermoelectric Zintl compound in the mid-temperature range. KEYWORDS: thermoelectric, BaCu2Te2, Zintl phase, quality factor, effective mass, thermal conductivity

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improve the quality factor β as well as the thermoelectric properties, including engineering of the band structure,9 introducing a resonant level,10 forming band convergence,11 and generating a nanoprecipitate phase;12 these approaches have led to the achievement of a high figure of merit zT. Zintl compounds with complex structures, which implies tunable electronic transport properties and relatively low thermal conductivities, are ideal systems for potential applications in thermoelectric power generation.13,14 The intrinsically p-type 122 Zintl compounds15−28 such as CaZn2Sb2,15 YbCd1.6Zn0.4Sb2,18 and Eu0.5Yb0.5Mg2Bi2 28 have been extensively studied because of their unique low thermal conductivities, which originate from their strong lattice anharmonicity and polyanion frameworks that provide an electronic tunnel through the covalently bonded network of complex metal or metalloid. These compounds possess CaAl2Si2-type (P3̅m1) crystal structures, in which cation layers are sandwiched between polyanion layers.29 Unlike the trigonal layered 122 phase, BaCu2Te2 adopts a channel structure with the space group Pnma, which consists of the anionic

INTRODUCTION The current energy crisis and environmental issues have led researchers to search for renewable and clean energy sources. One promising solution involves the conversion of heat into electricity and vice versa, which can be achieved using thermoelectric materials based on the Seebeck and Peltier effects.1−3 The conversion efficiency generally depends on the dimensionless figure of merit zT = S2σT/κtot, where S, σ, κtot, and T are the Seebeck coefficient, electrical conductivity, total thermal conductivity (including the lattice conductivity κL and electronic thermal conductivity κe), and working temperature in kelvin, respectively. In theory, a good thermoelectric material should possess a large Seebeck coefficient, high electrical conductivity, and low thermal conductivity. However, in practice, S, σ, and κ are strongly interrelated, which makes it difficult to optimize a single property without affecting the other parameters.4−7 Usually, the material quality factor β ∝ μ(m*)3/2/κL is used to evaluate the thermoelectric performance at the optimum Hall carrier concentration and a temperature T, where μ is the Hall mobility, m* is the carrier effective mass, and κL is the thermal conductivity.8 The quality factor β is considered as an indicator that helps guide the exploration of high-performance thermoelectric materials. Recently, several effective strategies have been developed to © XXXX American Chemical Society

Received: November 13, 2018 Accepted: December 24, 2018 Published: December 24, 2018 A

DOI: 10.1021/acsaem.8b01977 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 1. (a) PXRD patterns, (b) lattice parameters, (c) room-temperature hole concentrations, and (d) mobility of BaCu2−xAgxTe2 (0 ≤ x ≤ 0.1) samples.

framework [Cu2Te2]2− embedded by cationic Ba2+.30 Every Cu atom is surrounded by four Te atoms, forming a regular tetrahedron. These tetrahedral CuTe4/4 structures are connected to each other through common edges or vertices, thus producing a three-dimensional Cu−Te network. The Ba atoms are located in channels along the b-axis and are weakly bonded to the anionic [Cu2Te2]2− framework. In this work, polycrystalline bulk samples of BaCu2Te2 were fabricated using a direct solid-state reaction and subsequent hot-press sintering. The substitution of Cu by Ag in BaCu2Te2 was performed with the aim of enhancing the carrier effective mass and reducing the lattice thermal conductivity and thus improving the quality factor β and the thermoelectric figure of merit. Peak zT values as high as 1.08 at 823 K and an average zT value of 0.68 in the temperature range of 323−823 K were achieved for BaCu1.9Ag0.1Te2.

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cooled to room temperature by switching off the furnace. The obtained bulk solids were ground into powders, which were then sintered using a vacuum hot-press furnace apparatus under a pressure of 70 MPa at 900 K for 30 min to obtain fully dense bulk samples. The relative densities of the as-treated samples were greater than 95%. Powder X-ray diffraction (PXRD; Rigaku, Cu Kα radiation, λ = 1.541 854 Å, 10° < 2θ < 80°, step width 0.02°) was used to determine the phase purity of the samples at room temperature. The stability and phase transition of the pristine sample BaCu2Te2 were examined by variable temperature X-ray diffraction from room temperature to 673 K (Rigaku SmartLab SE). The morphologies of the polycrystalline samples were examined using scanning electron microscopy (SEM; ZEISS GeminiSEM 300) and energy-dispersive X-ray spectroscopy (EDX; Oxford X-MAX). The Hall coefficients (RH) were determined using an 8400 series HMS system (Lake Shore Cryotronics, Inc.) with a constant magnetic field of 0.9 T. The carrier concentration (p) and carrier mobility (μH) were calculated using the formulas p = 1/(eRH) and μH = σ/(ne), respectively. The electrical transport properties were characterized by synchronously measuring the electrical conductivity and Seebeck coefficient using a ZEM-3 apparatus (Ulvac Riko, Inc.) under a helium atmosphere from 300 to 823 K. The thermal conductivities were calculated using the formula κtot = Cpλρ, where λ is the thermal diffusivity measured in an argon atmosphere using the laser flash diffusivity method (LFA 467; Netzsch). The specific heat capacity (Cp) was determined using the Dulong−Petit approximation. The density of the sample (ρ) was determined by applying the Archimedes method.

EXPERIMENTAL SECTION

Polycrystalline samples of BaCu2−xAgxTe2 (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1) were synthesized using a direct element combination reaction. The raw metal Ba (99.9%) was first melted in an induction melting furnace (Edmund Buehler), which was integrated in a glovebox (M. Braun, O2 < 0.3 ppm, H2O < 0.3 ppm). Then, the surface of the as-obtained ingot was removed by polishing. Cu (99.999%), Ag (99.999%), Te (99.999%), and purified Ba were mixed according to the stoichiometric ratio and then placed in a BN crucible. These assemblies were sealed in an evacuated quartz tube using a Partulab device (MRVS-3002). The mixtures were heated to 1073 K in 12 h, maintained at this temperature for 72 h, and then B

DOI: 10.1021/acsaem.8b01977 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

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THEORETICAL CALCULATIONS The calculations were performed using the CASTEP package provided by Materials Studio 8.0 based on density functional theory (DFT) using the generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) exchange− correlation potential.31−33 The cutoff energy in the plane wave expansion was 440 eV. The tolerance in the self-consistent field (SCF) calculation was set to 5.0 × 10−7 eV/atom. The k-point Monkhorst−Pack grid was 3 × 6 × 3 for the integration in the Brillouin zone.

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RESULTS AND DISCUSSION Room-temperature PXRD patterns of BaCu2−xAgxTe2 (x ≤ 0.1) are presented in Figure 1a. All the diffraction peaks of the as-obtained samples were consistent with those in the simulated patterns (ICSD no. 51444), indicating the high purity of the samples. Compared with the diffraction peaks of pristine BaCu2Te2, those of the Ag-substituted samples slightly shifted toward lower angle with increasing x, indicating lattice expansion. Figure 1b shows the variations of the lattice parameters a, b, and c with Ag content. As expected, the lattice parameters linearly increased with increasing Ag content, following Vegard’s law. The pristine BaCu2Te2 exhibited ptype conductive features attributed to the cation vacancies, which are common in 122 Zintl phases34 and Cu-contained compounds such as Cu2Te.35 When Ag was introduced, the Ag atoms most likely occupied the cation vacancies, thus reducing the Hall carrier concentration (Figure 1c). The Hall mobility essentially remained constant with changing x (Figure 1d). Our measured data are lower than those of the isostructural BaZn2Sb2 and layered 122 phase compounds but show good agreement with previous results for BaCu2Te2 (15 cm2 V−1 s−1).22 In general, atomic substitution should result in reduction of the mobility owing to the formation of scattering centers. Herein, the substitution of Ag at Cu vacancies resulted in a decrease of the hole concentration, which is favorable for improvement of the mobility. However, the effect of mass fluctuation may also cause a reduction of the mobility to a certain degree, resulting in a balance and thus constant mobility. Figure 2a and Figure 2b present backscattered-electron images of typical BaCu2Te2 and BaCu1.9Ag0.1Te2 samples, respectively. EDX mapping of the composition of BaCu1.9Ag0.1Te2 (the sample with the highest zT) was performed (Figure 2c−f). The black regions were identified as pores, and the gray regions were identified as the main phase with a homogeneous distribution of the elements Ba, Cu, Ag, and Te, which is consistent with the high purity of the samples determined from the PXRD analysis. PXRD patterns were obtained at various temperatures for the BaCu2Te2 sample to investigate the thermal stability and potential phase transition (Figure 3). The orthorhombic phase was stable up to 523 K, as no additional diffraction peaks were observed. Upon increasing the temperature to 573 K, a new phase identified as the tetragonal structure appeared. The characteristics of this phase transition were slow, and recovery was analogous to the phase transition behaviors in BaCu2S2 and BaCu2Se2.36 The substitution of Ag for Cu led to a decrease in the hole concentration but had a negligible effect on the mobility, thus resulting in a linear decrease in the electrical conductivity with increasing Ag/Cu ratio (Figure 4a). In contrast, the room-

Figure 2. SEM images of BaCu2Te2 (a) andBaCu1.9Ag0.1Te2 (b) and EDX mapping images corresponding to the rectangular area of BaCu1.9Ag0.1Te2 (c−f).

Figure 3. PXRD patterns of BaCu2Te2 at various temperatures.

temperature Seebeck coefficient was enhanced with increasing Ag content, which can be roughly understood from the relationship S ∼ [π/(3n)]2/3m*T. Figure 4b and Figure 4c show the temperature dependence of the electrical transport properties of the pristine and Ag-doped BaCu2Te2 samples. Pristine BaCu2Te2 exhibits typical semiconductor behavior, with the electrical conductivity decreasing with increasing temperature and then increasing above 570 K. The lowtemperature region results from the alloy scattering inducing a decrease in the mobility with constant hole concentration, whereas the high-temperature region mainly originates from the increased hole concentration derived from the phase transition (Figure 4e and Figure 4f). For BaCu2−xAgxTe2 (x ≥ 0.08), a linear temperature dependence of the electrical conductivity was identified, implying that Ag substitution suppresses the phase transition. The Seebeck coefficients were positive for all the samples, which is in good agreement with the previous Hall coefficient measurements. The structural phase transition also affects the dependence of the Seebeck coefficients on the temperature because Smax moves to lower C

DOI: 10.1021/acsaem.8b01977 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 4. (a) Room-temperature electrical conductivity and Seebeck coefficients as a function of Ag content x for BaCu2−xAgxTe2 (x = 0−0.1). Temperature dependence of the electrical conductivity (b), Seebeck coefficient, (c) power factor (d), hole concentration (e), and mobility (f) of BaCu2−xAgxTe2 (x = 0−0.1).

splitting are observed, indicating that Ag substitution has an insignificant effect on the band gap. The temperature dependence of the total thermal conductivity and lattice thermal conductivity for BaCu2−xAgxTe2 (x ≤ 0.1) is shown in Figure 6. The lattice thermal conductivity (κL) was estimated by subtracting the electronic contribution (κe) using the Wiedemann−Franz law (κe = LσT, where L is the Lorenz factor determined from the above SPB model with acoustic scattering from the total thermal conductivity). With increasing temperature, κL decreased, basically following a function of T−1, indicating that phonon scattering is dominant at higher temperature (Figure 6b). Additionally, κL decreased with increasing Ag concentration, which is attributed to the point defects introduced by the Ag/ Cu isovalent substitution. The measured thermal conductivity ranged from 0.9 to 0.5 W m−1 K−1 for all the samples (Figure 6a); these values are lower than those for typical layered 122 phases and cage-like BaZn2Sb2.35 Combined with the increased effective mass and constant mobility, the reduced thermal conductivity is beneficial for further enhancing the quality factor β and achieving high thermoelectric performance for Agdoped BaCu2Te2.

temperature with increasing Ag content. By integration of these effects, the values of the power factor PF = S2σ for the Ag-doped samples were close to the values of pristine BaCu2Te2 over the measured temperature range of 323−823 K (Figure 4d). The Seebeck coefficient is plotted as a function of hole concentration at room temperature for the BaCu2−xAgxTe2 samples in Figure 5a. Data for Na-doped BaCu2Se2 are provided for comparison. The red and blue curves derived from the single parabolic band (SPB) model represent the best fit to the carrier effective mass (m*) data. For the Na-doped BaCu2Se2 samples, the trend line of the Pisarenko plot indicates an effective mass of 1.5m0, indicating that Na doping has an insignificant effect on the hole effective mass in BaCu2Se2.24 However, for the BaCu2−xAgxTe2 compounds, the effective mass gradually increases with increasing hole concentration, which is beneficial for improving the quality factor β because the Hall mobility is basically constant. The enhanced effective mass most likely originates from the increased density of states near the Fermi level resulting from the Ag doping (Figure 5b). The band structures for BaCu2Te2 and BaCu1.75Ag0.25Te2 are displayed in Figure 5c and Figure 5d, respectively. A similar band gap and slight band D

DOI: 10.1021/acsaem.8b01977 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 5. (a) Pisarenko line for BaCu2−xAgxTe2 (x = 0−0.1) compounds compared with earlier reported data for Ba1−xNaxCu2Se2.24 The solid red and blue lines are the calculated lines based on SPB models with effective masses of 1m0 and 1.5m0, respectively. (b) Integrated density of states for BaCu2Te2 and BaCu1.75Ag0.25Te2. A significant increase is observed for the Ag-doped BaCu2Te2. Band structures of (c) BaCu2Te2 and (d) BaCu1.75Ag0.25Te2.

Figure 6. Temperature dependence of (a) the thermal conductivity and (b) the lattice thermal conductivity for BaCu2−xAgxTe2 (x = 0−0.1).

Figure 7. (a) Quality factor β/A (A = constant) of typical Zintl phase compounds.15,21,24,27,37−39 (b) Temperature dependence of zT values of BaCu2−xAgxTe2 (x = 0−0.1) samples. The inset shows the corresponding average zT values in our measured temperature range.

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DOI: 10.1021/acsaem.8b01977 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 7a shows the simplified quality factor β/A (A = constant) at 300 K for some typical Zintl compounds,21,24,27,37−39 which can be roughly utilized to evaluate the effect of doping on the thermoelectric figure of merit zT. For CaZn2Sb2 and EuZn2Sb2, the alloy and doping effects had limited success in increasing the quality factor and thus could not improve the thermoelectric properties. In contrast, the quality factor increased by more than 3-fold for both Na-doped BaCu2Se2 and Ag-doped BaCu2Te2, implying a fundamental increase in the thermoelectric figure of merit zT. Figure 7b shows that the zT values increase with increasing temperature, and a maximum zT of 1.08 is achieved at 823 K for BaCu1.9Ag0.1Te2, which is higher than that of the undoped BaCu2Te2 sample at the same temperature. A substantial enhancement of zT was achieved for the Ag-doped BaCu2Te2 sample in the temperature range of 323−823 K. This enhancement resulted in average zT values of 0.4, 0.4, 0.43, 0.51, 0.65, and 0.68 in our measured temperature range for the samples with x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1, respectively. Thus, an increase of approximately 70% in the average zT value was achieved for the BaCu1.9Ag0.1Te2 sample, which can be attributed to the remarkable enhancement in the quality factor (Figure 6). It can be further expected from the trend of the curve that the zT value may be even higher above 823 K. Therefore, Ag doping leading to the improvement of the carrier effective mass and reduction of the thermal conductivity can result in significant improvement of the TE properties.

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CONCLUSIONS The Zintl phase compound BaCu2Te2 exhibits moderate electrical transport properties and low thermal conductivity, thus showing great potential as a promising thermoelectric material. In this work, we showed that Ag doping enhances the carrier effective mass, with the Hall mobility remaining essentially unchanged, and reduces the thermal conductivity owing to point-defect scattering. These synergistic effects improve the quality factor β, resulting in an increase of the thermoelectric figure of merit up to 1.08 at 823 K for BaCu1.9Ag0.1Te2.

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Article

AUTHOR INFORMATION

Corresponding Authors

*K.G.: e-mail, [email protected]. *J.L.: e-mail, [email protected] ORCID

Kai Guo: 0000-0002-8486-4185 Jun Luo: 0000-0002-8235-2338 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China under Projects 21501118, 21771123, and 51632005, Shanghai Municipal Science and Technology Commission under Project 15DZ2260300, Key Laboratory of Optoelectronic Materials Chemistry and Physics, Chinese Academy of Sciences (Grant 2008DP173016), and the Programme of Introducing Talents of Discipline to Universities (Grant D16002). K.G. gratefully acknowledges the support of Young Eastern Scholar Project of Shanghai Municipal Education Commission (Grant QD2015031). F

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DOI: 10.1021/acsaem.8b01977 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX