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Thermoelectric properties and chemical potential tuning by Cu-doping in n-type ionic conductors CuxAg2−xSe0.5Te0.5
Accepted Manuscript Thermoelectric properties and chemical potential tuning by Cu-doping in n-type ionic conductors CuxAg2−xSe0.5Te0.5 Min Ho Lee, Jae Hyun Yun, Kyunghan Ahn, Jong-Soo Rhyee PII:
S0022-3697(17)31165-4
DOI:
10.1016/j.jpcs.2017.07.034
Reference:
PCS 8152
To appear in:
Journal of Physics and Chemistry of Solids
Received Date: 26 June 2017 Revised Date:
22 July 2017
Accepted Date: 31 July 2017
Please cite this article as: M.H. Lee, J.H. Yun, K. Ahn, J.-S. Rhyee, Thermoelectric properties and chemical potential tuning by Cu-doping in n-type ionic conductors CuxAg2−xSe0.5Te0.5, Journal of Physics and Chemistry of Solids (2017), doi: 10.1016/j.jpcs.2017.07.034. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Graphical Abstract
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Here, we prepare polycrystalline CuxAg2-xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) samples using a high temperature melting followed by hot-press sintering and characterize their thermoelectric properties. We demonstrate that the Cu substitution for Ag is achieved by < 10% in Cu content for CuxAg2-xSe0.5Te0.5 and the Cu doping is quite effective for a significant enhancement in n-type carrier density, which is one order of magnitude higher than the pristine Ag2Se0.5Te0.5 (4.10 x 1018 cm-3). Impressively, the enhancement in the electrical conductivity with increasing Cu content is more considerable than the decrease in absolute value of Seebeck coefficient in the superionic conduction state, leading to a relatively high power factors ranging between 1.10 and 1.30 mW m-1 K-2 at a broad temperature range of 400 – 560 K for Cu0.1Ag1.99Se0.5Te0.5 and thus its highest ZT of 0.85 at 560 K. Furthermore, ZT values approach to > 0.7 over a wide temperature range of 460 – 560 K for x > 0.05. We suggest that the unusual Cu doping effect in Ag2Se0.5Te0.5 should be attributed to the creation of Cu ion conduction besides Ag ion conduction as well as the optimization of its n-type carrier density.
[Figure] Temperature-dependent (a) power factor PF (= S2σ) and (b) ZT values of the CuxAg2-xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) samples.
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Thermoelectric properties and chemical potential tuning by Cu-doping in n-type ionic conductors CuxAg2-xSe0.5Te0.5 Min Ho Lee, Jae Hyun Yun, Kyunghan Ahn*, Jong-Soo Rhyee*
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Department of Applied Physics and Institute of Natural Sciences, Kyung Hee University, Yong-in 17104, Republic of Korea * Correspondence: E-mail address: [email protected] (JSR), [email protected] (KA)
Abstract
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Copper and silver chalcogenides with superionic conduction behavior have shown impressively high ZT values, but there has been no intensive effort to optimize their carrier
xSe0.5Te0.5 (x
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density to further improve their ZT values. Here, we prepared polycrystalline CuxAg2= 0.01, 0.05, 0.1) samples using high temperature melting followed by hot-press
sintering, and characterized their thermoelectric properties. We demonstrated that Cu substitution for Ag was achieved with < 10% Cu content for CuxAg2-xSe0.5Te0.5 and the Cu doping was quite effective and significantly enhanced the compound’s n-type carrier density, which was one order of magnitude higher than the pristine Ag2Se0.5Te0.5 (4.10 x 1018 cm-3).
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Impressively, the enhancement in electrical conductivity with increasing Cu content was greater than the decrease in absolute value of the Seebeck coefficient in the superionic conduction state. This led to relatively high power factors for Cu0.1Ag1.99Se0.5Te0.5, ranging
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between 1.10 and 1.30 mW m-1 K-2 over the broad temperature range of 400 – 560 K, and resulted in the highest ZT of 0.85 at 560 K. Furthermore, ZT values approached > 0.7 over a wide temperature range of 460 – 560 K for x > 0.05. We suggest that the unusual Cu doping
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effect in Ag2Se0.5Te0.5 can be attributed to the creation of Cu ion conduction in addition to Ag ion conduction, and the optimization of the compound’s n-type carrier density.
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1. Introduction Thermoelectric (TE) devices, which can directly convert waste heat into electricity and vice versa, have attracted interest for some time. The efficiency of a TE device is
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predominantly governed by the performance of the TE materials, and is described by the dimensionless figure of merit ZT. ZT is expressed as ZT = S2σT/κ, where S, σ, T, and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively, and S2σ is often called the power factor PF. Obviously, the ZT value can be
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improved by minimizing κ as well as maximizing σ and S, but these parameters cannot be independently controlled because they are strongly coupled to each other [1-3]. For example,
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σ is inversely proportional to S, and κ is the sum of the lattice thermal conductivity κlat and the electrical thermal conductivity κel, which is described as κel = LσT in the WiedemannFranz law, where L is the Lorenz factor.
As a result, achieving ZT > 2 has been extremely challenging. To obtain a high ZT material, not only must a superior TE material be improved, for example, by optimized doping, band structure engineering, or phonon engineering, but a new TE material must also
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be designed and developed that can provide an intrinsically low κlat , by the introduction of anharmonic bonds, a complex crystal structure, and the rattling of guest species. In addition, a high PF is preferred, which can be obtained by band gap control and band degeneracy. It has recently been reported that silver chalcogenides are promising candidates for TE
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materials because of their relatively high σ and S as well as an intrinsically very low κlat [4-7]. Very interestingly, they are intimately related to the superionic material Cu2-xSe, which
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exhibits a high ZT of 1.5 at 1000 K [8]. In the face-centered cubic structure of Cu2-xSe, the Cu+ ions are mobile while the Se2- ions remain well situated in predetermined crystallographic positions. This extraordinary liquid-like Cu+ state, as well as the highly symmetrical crystal structure of the Cu2-xSe material, is considered to be the origin of the impressively high ZT of the copper selenide. Ag2Se and Ag2Te undergo a structural phase transition from orthorhombic and monoclinic to cubic structure upon heating to around 400 K, which is similar to Cu2-xSe. ntype Ag2Se showed a high ZT of 0.32~0.96 at 300 K below its phase transition temperature 2
ACCEPTED MANUSCRIPT [6]. The orthorhombic β-Ag2Se phase exhibits a degenerate semiconducting behavior, whereas the superionic conduction behavior becomes dominant for the cubic α-Ag2Se phase after the phase transition. The ternary Ag2Se0.5T0.5 (AST) compound is isostructural with Ag2Se below the phase transition temperature of ~400 K, and also becomes superionic above the phase transition
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temperature, resulting in a high ZT of 1.0 at 520 K [9]. It has been proposed that AST could exhibit a lower κlat than binary silver chalcogenides such as Ag2Se and Ag2Te because of the mass contrast and strain field fluctuations that are common in solid solutions of multiple elements [10-11]. However, there have not been many theoretical studies of ternary AST. In
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addition, the n-type carrier density of ~1018 cm-3 at 300 K may not be sufficient for the optimization needed to achieve a high PF.
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We recently reported that doped AST showed a higher ZT than Ag2Se based on the Boltzmann transport calculation, combined with a density functional theory (DFT) analysis of AST and Ag2Se at 300 K [12]. These results need to be verified by experimental investigations, which involves controlling the carrier density of AST by atomic doping. In the present study, we prepared Cu doped AST polycrystalline samples of CuxAg2xSe0.5Te0.5
(x = 0.01, 0.05, 0.1) and characterized their TE properties, including σ, S, κ, and
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carrier concentration. We demonstrated that Cu doping is effective for tuning the n-type carrier density of AST and as a result, the Cu doped samples showed a higher σ than pristine AST. Interestingly, the σ was more significantly affected by the Cu doping than the S, especially above the phase transition temperature. Consequently, the Cu0.1Ag1.9Se0.5Te0.5
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sample exhibited the highest PF of 1.29 mW m-1 K-2 at 463 K, which is a 30 % enhancement
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compared to AST, and a maximum ZT of 0.85 was achieved at 558 K.
2. Experimental details Polycrystalline CuxAg2-xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) compounds were synthesized by high-temperature melting and hot press sintering. Stoichiometric amounts of high-purity elements of Ag (99.999 %), Te (99.999 %), Se (99.999 %), and Cu (99.999 %) were placed in a quartz tube. The quartz tube was evacuated, sealed under a vacuum, and heated at 1273 K for 12 h. Then the ampoule was cooled down to 423 K at a rate of 10 K/h, maintained at 423 3
ACCEPTED MANUSCRIPT K for 12 h, and the furnace was quickly turned off. The solidified ingots were manually pulverized using an agate mortar and pestle. The pulverized powders were sintered by hot pressing at 673 K for 1 h under a uniaxial pressure of 70 MPa. The measured density of the sintered samples was within ± 2% range of 7.8 g/cm3. The phase identification as well as structural characterizations of the samples were
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performed by powder X-ray diffraction (XRD) with a Cu Kα radiation. The S and σ were simultaneously measured via a four point probe method using a thermoelectric measurement system (ZEM-3 ULVAC, Japan). The κ can be calculated by κ = dCpλ, where d, Cp, and λ is the sample density, specific heat measured by the physical property measurement system
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(PPMS, Quantum Design, USA), and thermal diffusivity measured by a laser flash method, respectively. The Hall carrier density was also measured using a four point probe contact
3. Results and discussion
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method with PPMS.
Figure 1 shows powder X-ray diffraction (XRD) patterns of the Cu doped AST samples. All the diffraction peaks of the Cu doped AST samples are consistent with those of Ag2Se
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because they share the same orthorhombic structure, with space group P212121. It has been reported that the RT crystal structure of Ag2Se1-xTex can be transformed from orthorhombic to monoclinic structure by Te doping (x ≥ 0.75) [5]. The lattice parameters a, b, and c, estimated from the XRD data, are presented in Table 1.
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The lattice parameters of the single phase Ag2Se and Ag2Se0.5Te0.5 compounds established in this work are in good agreement with the values previously reported in the reference [6,9].
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The lattice parameters of Ag2Se increased by ~2.5 % when half of the Se was replaced by Te, indicative of the isostructural substitution of Te for Se [9]. For the Cu doped AST samples, the lattice parameters decreased with increasing amounts of Cu doping (x). This can be explained because the atomic radius of Cu (1.35 Å) is smaller than that of Ag (1.60 Å), and it reveals that Cu can be effectively substituted for Ag in the β-AST phase. The temperature-dependent electrical conductivities σ of the CuxAg2-xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) compounds are shown in Fig. 2(a). As can be seen, σ increases with increasing Cu content (x), and is likely related to the higher room temperature (RT) Hall carrier 4
ACCEPTED MANUSCRIPT concentration nH in the higher doping range x (Table 2). For example, the RT nH of x = 0, 0.01, 0.05, and 0.1 is 4.10 x 1018, 6.43 x 1018, 3.11 x 1019, and 8.40 x 1019 cm-3, respectively. It is notable that the Cu doped AST samples exhibit abrupt drops in σ in the temperature range between 375 – 400 K. This phenomenon is the result of a thermally induced structural phase transformation. It has been reported that Ag2Se shows a phase transition from
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orthorhombic to body centered cubic at 408 K, whereas the monoclinic-to-face centered cubic (fcc) transition is observed for Ag2Te at 424 K [5,6]. It is found that the structure phase transition temperature is not changed with respect to Cu substitutional concentration. Furthermore, both the Ag2Se and Ag2Te compounds exhibit superionic conduction behavior
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above a phase transition temperature [8].
While the σ(T) of AST is lower than that of Ag2Se and Ag2Te, mainly due to the reduced
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carrier mobility µH in AST caused by atomic disordering, the Cu doped AST samples have a significantly higher σ than AST because of the considerable increase in nH produced by the Cu doping (Table 2). For example, the RT σ of Cu0.01Ag1.99Se0.5Te0.5, Cu0.05Ag1.95Se0.5Te0.5, and Cu0.1Ag1.9Se0.5Te0.5 is 981, 1169, and 1716 S cm-1, respectively. Cu0.05Ag1.95Se0.5Te0.5 has an RT σ that is similar to AST [9], and the RT σ of both β-Ag2Se (1788 S cm-1) and α-Ag2Te (1666 S cm-1) are comparable to Cu0.1Ag1.9Se0.5Te0.5. [4-6]
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The RT nH of Ag2Te, Ag2Se, and Ag2Se0.5Te0.5 ranges between 1.51 and 5.80 x 1018 cm-3 while those of Cu0.05Ag1.95Se0.5Te0.5 and Cu0.1Ag1.9Se0.5Te0.5 are one order of magnitude higher than the former compounds (Table 2), indicating that the Cu is an effective n-type dopant for AST.
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The increase in nH in AST by Cu doping is similar to that observed in Cu doped Ag2Te [13]. However, the RT carrier mobility µH of CuxAg2-xSe0.5Te0.5 decreases with increasing Cu
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content (x), an effect which can be attributed to the increase in atomic disorder with higher Cu content (Table 2). For example, the RT µH of Cu0.01Ag1.99Se0.5Te0.5, Cu0.05Ag1.95Se0.5Te0.5, and Cu0.1Ag1.9Se0.5Te0.5 is 945, 236, and 128 cm2 V-1 s-1, respectively, in comparison with Ag2Se0.5Te0.5 (2100 cm2 V-1 s-1) [9]. The reduction in µH for the Cu doped samples may be related to their carrier relaxation time. In general, there are two possible ways that carrier scattering can be produced by Cu doping: One is carrier scattering induced by strong electron correlation with increasing carrier density, and the other is carrier scattering caused by atomic disordering, following the 5
ACCEPTED MANUSCRIPT substitution of Cu for Ag. We believe that atomic disordering plays the dominant role in the carrier scattering, because the µH of the Cu doped samples is diminished in proportion to carrier density. The temperature-dependent S of the CuxAg2-xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) samples is shown in Fig. 2(b). The negative sign of the S in the samples indicates that the electron is the
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major charge carrier. Th absolute magnitude of S decreases with increasing Cu content, which is consistent with the higher nH observed with higher Cu content. For example, the RT S of x = 0.01, 0.05, and 0.1 is -100, -83, and -72 µV K-1, respectively.
The RT S of AST has been reported to be around -100 µV K-1 [9], which is close to that
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of the x = 0.01 sample in this work. The temperature variation in S for the Cu doped samples is nearly the same as that reported for AST [9], revealing that the Cu doping negligibly affects
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the electronic band structure of AST. In addition, the superionic conduction behavior remains intact with Cu doping. As a result, the Cu doping plays an effective role in tuning the carrier density of AST. Basically, the Cu-doping in Ag2Se1-xTex compounds are thought to be isoelectronic substitution. From the previous investigation[13], the Cu doping in Ag2-xCuxTe showed p-type substitution by Cu. It indicates that the Selenium substitution may play an important role for n-type conduction of carriers.
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Interestingly, the degree of variation in the σ of the other Cu doped samples in comparison with Cu0.01Ag1.99Se0.5Te0.5 was larger than that observed for S. In particular, the degree of variation in S was the same regardless of phase transition, while the phase transition caused the degree of variation in σ to increase more rapidly, implying that the Cu doping is
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more effective for enhancing σ than reducing S after the phase transition. This may be related to the effect of the liquid-like ions in the copper and silver chalcogenides. [8,9,13] In the
?? = ߪ + ߪ
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superionic state after the phase transition, σ and S can be expressed as follows. ܵ = −ܵ + ܵ
(1) (2)
where σe is the electrical conductivity contributed by electrons, σc is the electrical conductivity contributed by silver and cooper ions, Se is the Seebeck coefficient contributed by electrons, and Sc is the Seebeck coefficient contributed by silver and copper ions. Generally, the contribution of Cu ionic conduction is larger than that of Ag ionic conduction because the Cu ion is smaller and lighter than the Ag ion, and the ionic conduction increases 6
ACCEPTED MANUSCRIPT with increasing temperature [5]. Consequently, the σ can be significantly enhanced by the improvement in σc while the considerable enhancement in S can be prohibited by the positive value of Sc. Fig. 2(c) and 2(d) show the temperature-dependent thermal conductivity κ and lattice thermal conductivity κlat of the CuxAg2-xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) samples, respectively.
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It has been reported that the RT κ of a binary silver chalcogenide such as Ag2Se and Ag2Te ranges between 1.2 and 1.4 W m-1 K-1. The ternary AST shows a relatively lower RT κ value of 0.95 W m-1 K-1 than the binary chalcogenides because of enhanced phonon scattering by the atomic disordering in AST, similar to the PbTe1-xSex and Bi2Te3-xSex systems [9,14,15].
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Here, the κ of the x = 0.01, 0.05, and 0.1 samples is 1.05, 0.94, and 1.44 W m-1 K-1 for room temperature and 0.58, 0.60, and 0.74 W m-1 K-1 near 550 K, respectively. The thermal
near 550 K[13].
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conductivity values are similar to the previous report of (Ag0.985-xCux)2Te: 0.4 ~ 0.6 W m-1 K-1 The temperature-dependent κlat can be estimated by subtracting κel from κ. To accurately estimate the κlat in the Wiedemann-Franz law, it is necessary to calculate reliable L values. In normal metals, the L is written as: గమ ଷ
ଶ
ቀ ಳ ቁ = 2.45 × 10ି଼
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ܮ =
W Ω K-2
However, L is not L0 for correlated metals and degenerated semiconductors. To obtain more reliable L values, we can calculated L by using the following equation [16]: ଶ 7 5 ( ݎ+ ) ܨହ (ߟ) (ݎ + )ܨ ଷ (ߟ) 2 ାଶ 2 ାଶ ݇ ۇ ۊ =ܮ൬ ൰ ۈ −൦ ൪ ۋ 3 3 ݁ ( ݎ+ 2)ܨାଵ (ߟ) ( ݎ+ 2)ܨାଵ (ߟ) ଶ ଶ ۉ ی
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ଶ
where r is the scattering parameter, η = EF/kBT is the reduced Fermi energy, and Fn(η) is the n-th order Fermi integral given by ݔ ܨ (ߟ) = න ݀ݔ ௫ିఎ 1+݁ ஶ
For most cases, the scattering parameter for acoustic phonon scattering is r = −1/2. The reduced Fermi energy η can be obtained by fitting S(T) to the following equation: 7
ACCEPTED MANUSCRIPT 5 ( ݎ+ )ܨାଷ (ߟ) 2 ݇ ଶ ܵ=± ൞ − ߟൢ ݁ ( ݎ+ 3))ߟ( ܨ 2 ାଵଶ
The estimated temperature-dependent L is shown in the inset of Fig. 2(d). The L decreases
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with increasing temperature, ranging between 1.7 and 2.1 x 10-8 W Ω K-2 , which is lower than L0. In Fig. 2(d), the temperature variation of κlat for the x = 0.01 sample is almost the same as that of Ag2Se0.5Te0.5 (open star symbol). The κlat decreases with increasing Cu content (x) mainly due to enhanced phonon scattering by atomic disordering. For example,
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the RT κlat of the x = 0.01, 0.05, and 0.1 sample is 0.47, 0.42, and 0.32 W m-1 K-1, respectively.
From the analysis of the electronic band structure of AST in our previous work [12], the
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conduction bands of AST compound are composed of Ag 5s and either Se 4p or Te 5p states while the corresponding valence bands consist of Se 4p and Te 5p states. The relatively high σ originates with the highly dispersive bands in the n-type β-AST phase. Previous studies of theoretical thermoelectric properties in terms of chemical potential have shown that the electronic contribution to the thermoelectric figure-of-merit ZT is
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maximum near the chemical potential µ = -0.1 eV in the n-type conduction carriers in the AST compound (see Fig. 7 of Ref. [12]). Here we fit the chemical potential with respect to Hall carrier concentration, as shown in Fig. 3. The Hall carrier concentrations for the Cu doping levels in this work are marked with red diamond symbols, revealing that Cu doping
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can effectively enhance the chemical potential on the conduction band side in AST. The chemical potentials of the compounds correspond to 70 meV (x = 1 %), 0.13 eV (x = 5 %)
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and 0.23 eV (x = 10 %), respectively. For the negative chemical potential around 0.1 eV, the theoretical maximum ZT is about 0.3 at room temperature. The temperature-dependent PF and ZT of the Cu doped AST samples are shown in Fig. 4(a) and Fig. 4(b), respectively. The experimental ZT values at room temperature are about 0.28 (x = 1 %), 0.26 (x = 5 %), and 0.19 (x = 10 %), respectively, which are similar to the results of the theoretical calculation. Notably, a rapid enhancement in PF is observed after the phase transition in the Cu doped samples. The highest PF of 1.29 mW m-1 K-2 was achieved at 463 K for the x = 0.1 sample, which is a 30 % enhancement compared to undoped AST [9]. Consequently, the x = 8
ACCEPTED MANUSCRIPT 0.1 sample showed the highest ZT of 0.85 at 558 K, mainly due to the combined effect of a relatively high PF and low κlat through the optimized Cu doping. Conclusions Here we investigated the effect of Cu doping on the TE properties of AST for
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polycrystalline n-type Ag2-xCuxTe0.5Se0.5 (x = 0.01, 0.05, 0.1) samples. Based on the XRD measurements, the effective substitution of Cu for Ag in AST was experimentally confirmed. Furthermore, the TE characterization revealed that the Cu doped AST samples underwent a structural phase transition at around 400 K, accompanied by a semiconducting-to-superionic
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conduction transition. We demonstrated that the n-type carrier density of AST can be delicately controlled by Cu doping content, and verified the effect with both TE
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characterization and theoretical DFT calculation.
Interestingly, we found that σ was more significantly affected than S by Cu doping, and the optimized Cu doping content ranged between 5 and 10% in Cu content. Consequently, the Cu0.1Ag1.9Se0.5Te0.5 sample exhibited the highest PF of 1.29 mW m-1 K-2 at 463 K, which is a 30 % enhancement compared to undoped AST [9], and its peak ZT of 0.85 was achieved at
460 – 560 K for x > 0.05.
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558 K. Also, a relatively high ZT of > 0.7 was persistent over a wide temperature range of
We suggest that optimizing the doping amount as well as the appropriate choice of dopant, without sacrificing the superionic conduction state, is critical to obtain a high ZT for
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silver and cooper chalcogenides.
Acknowledgements
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This research was supported by the Nano-Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0030147) and by the Materials and Components Technology Development Program of MOTIE/KEIT (10063286).
References [1]
H. J. Goldsmid, Introduction to thermoelectricity, Springer, Heidelberg, 2009. 9
ACCEPTED MANUSCRIPT [2]
F. J. Disalvo, Thermoelectric cooling and power generation, Science 285 (1999) 703-
706. [3]
G. J. Snyder, E. S. Toberer, Complex thermoelectric materials, Nat. Mater. 7 (2008)
105-114. [4]
H. Kikuchi, H. Iyetomi, A. Hasegawa, Insight into the origin of superionic
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conductivity from electronic structure theory, J. Phys. Condens. Matter 10 (1998) 1143911448. [5]
S. Kashida, N. Watanabe, T. Hasegawa, H. Iida, M. Mori, Electronic structure of
Ag2Te, band calculation and photoelectron spectroscopy, Solid State Ionics 148 (2002) 193[6]
SC
201.
M. Ferhat, J. Nagao, Thermoelectric and transport properties of β-Ag2Se compounds
[7]
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J. Appl. Phys. 88 (2000) 813-816.
J. Capps, F. Drymiotis, S. Lindsey, T. M. Tritt, Significant enhancement of the
dimensionless thermoelectric figure of merit of the binary Ag2Te, Philos. Mag. Lett. 90 (2010) 677-681. [8]
H. Liu, X. Shi, F. Xu, L. Zhang, W. Zhang, L. Chen, Q. Li, C. Uher, T. Day, G. J.
Snyder, Copper ion liquid-like thermoelectrics, Nature Mater. 11 (2012) 422-425. F. Drymiotis, T. W. Day, D. R. Brown, N. A. Heinz, G. J. Snyder, Enhanced
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[9]
thermoelectric performance in the very low thermal conductivity Ag2Se0.5Te0.5, Appl. Phys. Lett. 103 (2013) 143906. [10]
P. G. Klemens, Thermal resistance due to point defects at high temperatures, Phys.
[11]
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Rev. 119 (1960) 507-509.
B. Abeles, Lattice thermal conductivity of disordered semiconductor alloys at high
[12]
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temperatures, Phys. Rev. 131 (1963)1906-1991. J. H. Yun, M.-H. Lee, J. N. Kim, J. H. Shim, J.-S. Rhyee, Boltzmann transport
calculation of thermoelectric properties in Ag2Se1-xTex (x = 0.0 and 0.5), J. Appl. Phys. 119 (2016) 165101. [13]
H. Zhu, J. Luo, H. Zhao, J. Liang, Enhanced thermoelectric properties of p-type
Ag2Te by Cu substitution, J. Mater. Chem. A 3 (2015) 10303-10308. [14]
Q. Zhang, F. Cao, W. Liu, K. Lukas, B. Yu, S. Chen, C. Opeil, D. Broido, G. Chen,
Zhifeng Ren, Heavy doping and band engineering by potassium to improve the 10
ACCEPTED MANUSCRIPT thermoelectric figure of merit in p-type PbTe, PbSe, and PbTe1−ySey, J. Am. Chem. Soc. 134 (2012) 10031-10038. [15]
L. Hu, T. Zhu, X. Liu, X. Zhao, Point defect engineering of high-performance
Bismuth-Telluride-based thermoelectric materials, Adv. Fuct. Mater. 24 (2014) 5211-5218. [16] C.-C. Lin, D. Ginting, R. Lydia, M. H. Lee, J.-S. Rhyee, Thermoelectric properties and
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PbTe precipitation, J. Alloys & Comp. 671 (2016) 538-544.
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extremely low lattice thermal conductivity in p-type Bismuth Tellurides by Pb-doping and
xSe0.5Te0.5
(x = 0.01, 0.05,
Ag2Se
4.333
7.061
7.766
Ag2Se [6]
4.333
7.062
7.764
Ag2Se0.5Te0.5
4.433
7.237
7.967
Ag2Se0.5Te0.5 [9]
4.433
7.235
7.968
x = 0.01
4.419
7.211
7.936
x = 0.05
4.419
7.143
7.916
x = 0.1
4.394
7.039
7.899
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c (Å Å)
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0.1)
b (Å Å)
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Single phase
CuxAg2-
a (Å Å)
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Table 1. Lattice parameters of Ag2Se and CuxAg2-xSe0.5Te0.5 (x = 0, 0.01, 0.05, 0.1) nH
µH
( cm-3 )
( cm2 V-1 s-1 )
Ag2Se
1.51 x 1018
3191
Ag2Te
5.80 x 1018
2320
Ag2Se0.5Te0.5
4.10 x 1018
2100
Cu0.01Ag1.99Se0.5Te0.5
6.43 x 1018
945
Cu0.05Ag1.95Se0.5Te0.5
3.11 x 1019
236
11
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8.40 x 1019
128
Table 2. The room temperature carrier concentration nH and Hall mobility µ H of the Ag2Se,
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Ag2Te, Ag2Se0.5Te0.5, and CuxAg2-xSe0.5Te0.5 (x = 0.01, 0.05, 0.1).
Fig. 1. Powder X-ray diffraction patterns of Ag2Se and CuxAg2-xSe0.5Te0.5 (x = 0, 0.01, 0.05, 0.1).
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Fig. 2. Temperature-dependent electrical conductivity σ (a), Seebeck coefficient S (b), total thermal conductivity κ (c), and lattice thermal conductivity κlat (d) of the CuxAg2-xSe0.5Te0.5 (x
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= 0.01, 0.05, 0.1) compounds and the inset of (d) shows temperature-dependent Lorenz
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number L, calculated by the parabolic band assumption.
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Fig. 3. Negative chemical potential with respect to Hall carrier density, calculated from the
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Boltzmann transport calculation [12], of the CuxAg2-xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) compounds. Red diamond symbols are the experimental Hall carrier density corresponding to
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the chemical potential calculation result.
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Fig. 4. Temperature-dependent (a) power factor PF (= S2σ) and (b) ZT values of the
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CuxAg2-xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) samples.
15
ACCEPTED MANUSCRIPT Highlights
We investigated thermoelectric properties in n-type CuxAg2-xSe0.5Te0.5.
Cu-doping increases Hall carrier density and chemical potential.
Structure phase transition attributes to the increase ZT value.
We observed high ZT over a wide temperature range (460~560 K).
Maximum ZT reaches about 0.85 at 560 K which is enhanced value.
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S0022-3697(17)31165-4
DOI:
10.1016/j.jpcs.2017.07.034
Reference:
PCS 8152
To appear in:
Journal of Physics and Chemistry of Solids
Received Date: 26 June 2017 Revised Date:
22 July 2017
Accepted Date: 31 July 2017
Please cite this article as: M.H. Lee, J.H. Yun, K. Ahn, J.-S. Rhyee, Thermoelectric properties and chemical potential tuning by Cu-doping in n-type ionic conductors CuxAg2−xSe0.5Te0.5, Journal of Physics and Chemistry of Solids (2017), doi: 10.1016/j.jpcs.2017.07.034. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Graphical Abstract
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Here, we prepare polycrystalline CuxAg2-xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) samples using a high temperature melting followed by hot-press sintering and characterize their thermoelectric properties. We demonstrate that the Cu substitution for Ag is achieved by < 10% in Cu content for CuxAg2-xSe0.5Te0.5 and the Cu doping is quite effective for a significant enhancement in n-type carrier density, which is one order of magnitude higher than the pristine Ag2Se0.5Te0.5 (4.10 x 1018 cm-3). Impressively, the enhancement in the electrical conductivity with increasing Cu content is more considerable than the decrease in absolute value of Seebeck coefficient in the superionic conduction state, leading to a relatively high power factors ranging between 1.10 and 1.30 mW m-1 K-2 at a broad temperature range of 400 – 560 K for Cu0.1Ag1.99Se0.5Te0.5 and thus its highest ZT of 0.85 at 560 K. Furthermore, ZT values approach to > 0.7 over a wide temperature range of 460 – 560 K for x > 0.05. We suggest that the unusual Cu doping effect in Ag2Se0.5Te0.5 should be attributed to the creation of Cu ion conduction besides Ag ion conduction as well as the optimization of its n-type carrier density.
[Figure] Temperature-dependent (a) power factor PF (= S2σ) and (b) ZT values of the CuxAg2-xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) samples.
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Thermoelectric properties and chemical potential tuning by Cu-doping in n-type ionic conductors CuxAg2-xSe0.5Te0.5 Min Ho Lee, Jae Hyun Yun, Kyunghan Ahn*, Jong-Soo Rhyee*
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Department of Applied Physics and Institute of Natural Sciences, Kyung Hee University, Yong-in 17104, Republic of Korea * Correspondence: E-mail address: [email protected] (JSR), [email protected] (KA)
Abstract
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Copper and silver chalcogenides with superionic conduction behavior have shown impressively high ZT values, but there has been no intensive effort to optimize their carrier
xSe0.5Te0.5 (x
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density to further improve their ZT values. Here, we prepared polycrystalline CuxAg2= 0.01, 0.05, 0.1) samples using high temperature melting followed by hot-press
sintering, and characterized their thermoelectric properties. We demonstrated that Cu substitution for Ag was achieved with < 10% Cu content for CuxAg2-xSe0.5Te0.5 and the Cu doping was quite effective and significantly enhanced the compound’s n-type carrier density, which was one order of magnitude higher than the pristine Ag2Se0.5Te0.5 (4.10 x 1018 cm-3).
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Impressively, the enhancement in electrical conductivity with increasing Cu content was greater than the decrease in absolute value of the Seebeck coefficient in the superionic conduction state. This led to relatively high power factors for Cu0.1Ag1.99Se0.5Te0.5, ranging
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between 1.10 and 1.30 mW m-1 K-2 over the broad temperature range of 400 – 560 K, and resulted in the highest ZT of 0.85 at 560 K. Furthermore, ZT values approached > 0.7 over a wide temperature range of 460 – 560 K for x > 0.05. We suggest that the unusual Cu doping
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effect in Ag2Se0.5Te0.5 can be attributed to the creation of Cu ion conduction in addition to Ag ion conduction, and the optimization of the compound’s n-type carrier density.
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1. Introduction Thermoelectric (TE) devices, which can directly convert waste heat into electricity and vice versa, have attracted interest for some time. The efficiency of a TE device is
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predominantly governed by the performance of the TE materials, and is described by the dimensionless figure of merit ZT. ZT is expressed as ZT = S2σT/κ, where S, σ, T, and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively, and S2σ is often called the power factor PF. Obviously, the ZT value can be
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improved by minimizing κ as well as maximizing σ and S, but these parameters cannot be independently controlled because they are strongly coupled to each other [1-3]. For example,
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σ is inversely proportional to S, and κ is the sum of the lattice thermal conductivity κlat and the electrical thermal conductivity κel, which is described as κel = LσT in the WiedemannFranz law, where L is the Lorenz factor.
As a result, achieving ZT > 2 has been extremely challenging. To obtain a high ZT material, not only must a superior TE material be improved, for example, by optimized doping, band structure engineering, or phonon engineering, but a new TE material must also
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be designed and developed that can provide an intrinsically low κlat , by the introduction of anharmonic bonds, a complex crystal structure, and the rattling of guest species. In addition, a high PF is preferred, which can be obtained by band gap control and band degeneracy. It has recently been reported that silver chalcogenides are promising candidates for TE
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materials because of their relatively high σ and S as well as an intrinsically very low κlat [4-7]. Very interestingly, they are intimately related to the superionic material Cu2-xSe, which
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exhibits a high ZT of 1.5 at 1000 K [8]. In the face-centered cubic structure of Cu2-xSe, the Cu+ ions are mobile while the Se2- ions remain well situated in predetermined crystallographic positions. This extraordinary liquid-like Cu+ state, as well as the highly symmetrical crystal structure of the Cu2-xSe material, is considered to be the origin of the impressively high ZT of the copper selenide. Ag2Se and Ag2Te undergo a structural phase transition from orthorhombic and monoclinic to cubic structure upon heating to around 400 K, which is similar to Cu2-xSe. ntype Ag2Se showed a high ZT of 0.32~0.96 at 300 K below its phase transition temperature 2
ACCEPTED MANUSCRIPT [6]. The orthorhombic β-Ag2Se phase exhibits a degenerate semiconducting behavior, whereas the superionic conduction behavior becomes dominant for the cubic α-Ag2Se phase after the phase transition. The ternary Ag2Se0.5T0.5 (AST) compound is isostructural with Ag2Se below the phase transition temperature of ~400 K, and also becomes superionic above the phase transition
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temperature, resulting in a high ZT of 1.0 at 520 K [9]. It has been proposed that AST could exhibit a lower κlat than binary silver chalcogenides such as Ag2Se and Ag2Te because of the mass contrast and strain field fluctuations that are common in solid solutions of multiple elements [10-11]. However, there have not been many theoretical studies of ternary AST. In
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addition, the n-type carrier density of ~1018 cm-3 at 300 K may not be sufficient for the optimization needed to achieve a high PF.
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We recently reported that doped AST showed a higher ZT than Ag2Se based on the Boltzmann transport calculation, combined with a density functional theory (DFT) analysis of AST and Ag2Se at 300 K [12]. These results need to be verified by experimental investigations, which involves controlling the carrier density of AST by atomic doping. In the present study, we prepared Cu doped AST polycrystalline samples of CuxAg2xSe0.5Te0.5
(x = 0.01, 0.05, 0.1) and characterized their TE properties, including σ, S, κ, and
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carrier concentration. We demonstrated that Cu doping is effective for tuning the n-type carrier density of AST and as a result, the Cu doped samples showed a higher σ than pristine AST. Interestingly, the σ was more significantly affected by the Cu doping than the S, especially above the phase transition temperature. Consequently, the Cu0.1Ag1.9Se0.5Te0.5
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sample exhibited the highest PF of 1.29 mW m-1 K-2 at 463 K, which is a 30 % enhancement
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compared to AST, and a maximum ZT of 0.85 was achieved at 558 K.
2. Experimental details Polycrystalline CuxAg2-xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) compounds were synthesized by high-temperature melting and hot press sintering. Stoichiometric amounts of high-purity elements of Ag (99.999 %), Te (99.999 %), Se (99.999 %), and Cu (99.999 %) were placed in a quartz tube. The quartz tube was evacuated, sealed under a vacuum, and heated at 1273 K for 12 h. Then the ampoule was cooled down to 423 K at a rate of 10 K/h, maintained at 423 3
ACCEPTED MANUSCRIPT K for 12 h, and the furnace was quickly turned off. The solidified ingots were manually pulverized using an agate mortar and pestle. The pulverized powders were sintered by hot pressing at 673 K for 1 h under a uniaxial pressure of 70 MPa. The measured density of the sintered samples was within ± 2% range of 7.8 g/cm3. The phase identification as well as structural characterizations of the samples were
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performed by powder X-ray diffraction (XRD) with a Cu Kα radiation. The S and σ were simultaneously measured via a four point probe method using a thermoelectric measurement system (ZEM-3 ULVAC, Japan). The κ can be calculated by κ = dCpλ, where d, Cp, and λ is the sample density, specific heat measured by the physical property measurement system
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(PPMS, Quantum Design, USA), and thermal diffusivity measured by a laser flash method, respectively. The Hall carrier density was also measured using a four point probe contact
3. Results and discussion
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method with PPMS.
Figure 1 shows powder X-ray diffraction (XRD) patterns of the Cu doped AST samples. All the diffraction peaks of the Cu doped AST samples are consistent with those of Ag2Se
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because they share the same orthorhombic structure, with space group P212121. It has been reported that the RT crystal structure of Ag2Se1-xTex can be transformed from orthorhombic to monoclinic structure by Te doping (x ≥ 0.75) [5]. The lattice parameters a, b, and c, estimated from the XRD data, are presented in Table 1.
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The lattice parameters of the single phase Ag2Se and Ag2Se0.5Te0.5 compounds established in this work are in good agreement with the values previously reported in the reference [6,9].
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The lattice parameters of Ag2Se increased by ~2.5 % when half of the Se was replaced by Te, indicative of the isostructural substitution of Te for Se [9]. For the Cu doped AST samples, the lattice parameters decreased with increasing amounts of Cu doping (x). This can be explained because the atomic radius of Cu (1.35 Å) is smaller than that of Ag (1.60 Å), and it reveals that Cu can be effectively substituted for Ag in the β-AST phase. The temperature-dependent electrical conductivities σ of the CuxAg2-xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) compounds are shown in Fig. 2(a). As can be seen, σ increases with increasing Cu content (x), and is likely related to the higher room temperature (RT) Hall carrier 4
ACCEPTED MANUSCRIPT concentration nH in the higher doping range x (Table 2). For example, the RT nH of x = 0, 0.01, 0.05, and 0.1 is 4.10 x 1018, 6.43 x 1018, 3.11 x 1019, and 8.40 x 1019 cm-3, respectively. It is notable that the Cu doped AST samples exhibit abrupt drops in σ in the temperature range between 375 – 400 K. This phenomenon is the result of a thermally induced structural phase transformation. It has been reported that Ag2Se shows a phase transition from
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orthorhombic to body centered cubic at 408 K, whereas the monoclinic-to-face centered cubic (fcc) transition is observed for Ag2Te at 424 K [5,6]. It is found that the structure phase transition temperature is not changed with respect to Cu substitutional concentration. Furthermore, both the Ag2Se and Ag2Te compounds exhibit superionic conduction behavior
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above a phase transition temperature [8].
While the σ(T) of AST is lower than that of Ag2Se and Ag2Te, mainly due to the reduced
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carrier mobility µH in AST caused by atomic disordering, the Cu doped AST samples have a significantly higher σ than AST because of the considerable increase in nH produced by the Cu doping (Table 2). For example, the RT σ of Cu0.01Ag1.99Se0.5Te0.5, Cu0.05Ag1.95Se0.5Te0.5, and Cu0.1Ag1.9Se0.5Te0.5 is 981, 1169, and 1716 S cm-1, respectively. Cu0.05Ag1.95Se0.5Te0.5 has an RT σ that is similar to AST [9], and the RT σ of both β-Ag2Se (1788 S cm-1) and α-Ag2Te (1666 S cm-1) are comparable to Cu0.1Ag1.9Se0.5Te0.5. [4-6]
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The RT nH of Ag2Te, Ag2Se, and Ag2Se0.5Te0.5 ranges between 1.51 and 5.80 x 1018 cm-3 while those of Cu0.05Ag1.95Se0.5Te0.5 and Cu0.1Ag1.9Se0.5Te0.5 are one order of magnitude higher than the former compounds (Table 2), indicating that the Cu is an effective n-type dopant for AST.
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The increase in nH in AST by Cu doping is similar to that observed in Cu doped Ag2Te [13]. However, the RT carrier mobility µH of CuxAg2-xSe0.5Te0.5 decreases with increasing Cu
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content (x), an effect which can be attributed to the increase in atomic disorder with higher Cu content (Table 2). For example, the RT µH of Cu0.01Ag1.99Se0.5Te0.5, Cu0.05Ag1.95Se0.5Te0.5, and Cu0.1Ag1.9Se0.5Te0.5 is 945, 236, and 128 cm2 V-1 s-1, respectively, in comparison with Ag2Se0.5Te0.5 (2100 cm2 V-1 s-1) [9]. The reduction in µH for the Cu doped samples may be related to their carrier relaxation time. In general, there are two possible ways that carrier scattering can be produced by Cu doping: One is carrier scattering induced by strong electron correlation with increasing carrier density, and the other is carrier scattering caused by atomic disordering, following the 5
ACCEPTED MANUSCRIPT substitution of Cu for Ag. We believe that atomic disordering plays the dominant role in the carrier scattering, because the µH of the Cu doped samples is diminished in proportion to carrier density. The temperature-dependent S of the CuxAg2-xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) samples is shown in Fig. 2(b). The negative sign of the S in the samples indicates that the electron is the
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major charge carrier. Th absolute magnitude of S decreases with increasing Cu content, which is consistent with the higher nH observed with higher Cu content. For example, the RT S of x = 0.01, 0.05, and 0.1 is -100, -83, and -72 µV K-1, respectively.
The RT S of AST has been reported to be around -100 µV K-1 [9], which is close to that
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of the x = 0.01 sample in this work. The temperature variation in S for the Cu doped samples is nearly the same as that reported for AST [9], revealing that the Cu doping negligibly affects
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the electronic band structure of AST. In addition, the superionic conduction behavior remains intact with Cu doping. As a result, the Cu doping plays an effective role in tuning the carrier density of AST. Basically, the Cu-doping in Ag2Se1-xTex compounds are thought to be isoelectronic substitution. From the previous investigation[13], the Cu doping in Ag2-xCuxTe showed p-type substitution by Cu. It indicates that the Selenium substitution may play an important role for n-type conduction of carriers.
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Interestingly, the degree of variation in the σ of the other Cu doped samples in comparison with Cu0.01Ag1.99Se0.5Te0.5 was larger than that observed for S. In particular, the degree of variation in S was the same regardless of phase transition, while the phase transition caused the degree of variation in σ to increase more rapidly, implying that the Cu doping is
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more effective for enhancing σ than reducing S after the phase transition. This may be related to the effect of the liquid-like ions in the copper and silver chalcogenides. [8,9,13] In the
?? = ߪ + ߪ
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superionic state after the phase transition, σ and S can be expressed as follows. ܵ = −ܵ + ܵ
(1) (2)
where σe is the electrical conductivity contributed by electrons, σc is the electrical conductivity contributed by silver and cooper ions, Se is the Seebeck coefficient contributed by electrons, and Sc is the Seebeck coefficient contributed by silver and copper ions. Generally, the contribution of Cu ionic conduction is larger than that of Ag ionic conduction because the Cu ion is smaller and lighter than the Ag ion, and the ionic conduction increases 6
ACCEPTED MANUSCRIPT with increasing temperature [5]. Consequently, the σ can be significantly enhanced by the improvement in σc while the considerable enhancement in S can be prohibited by the positive value of Sc. Fig. 2(c) and 2(d) show the temperature-dependent thermal conductivity κ and lattice thermal conductivity κlat of the CuxAg2-xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) samples, respectively.
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It has been reported that the RT κ of a binary silver chalcogenide such as Ag2Se and Ag2Te ranges between 1.2 and 1.4 W m-1 K-1. The ternary AST shows a relatively lower RT κ value of 0.95 W m-1 K-1 than the binary chalcogenides because of enhanced phonon scattering by the atomic disordering in AST, similar to the PbTe1-xSex and Bi2Te3-xSex systems [9,14,15].
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Here, the κ of the x = 0.01, 0.05, and 0.1 samples is 1.05, 0.94, and 1.44 W m-1 K-1 for room temperature and 0.58, 0.60, and 0.74 W m-1 K-1 near 550 K, respectively. The thermal
near 550 K[13].
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conductivity values are similar to the previous report of (Ag0.985-xCux)2Te: 0.4 ~ 0.6 W m-1 K-1 The temperature-dependent κlat can be estimated by subtracting κel from κ. To accurately estimate the κlat in the Wiedemann-Franz law, it is necessary to calculate reliable L values. In normal metals, the L is written as: గమ ଷ
ଶ
ቀ ಳ ቁ = 2.45 × 10ି଼
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ܮ =
W Ω K-2
However, L is not L0 for correlated metals and degenerated semiconductors. To obtain more reliable L values, we can calculated L by using the following equation [16]: ଶ 7 5 ( ݎ+ ) ܨହ (ߟ) (ݎ + )ܨ ଷ (ߟ) 2 ାଶ 2 ାଶ ݇ ۇ ۊ =ܮ൬ ൰ ۈ −൦ ൪ ۋ 3 3 ݁ ( ݎ+ 2)ܨାଵ (ߟ) ( ݎ+ 2)ܨାଵ (ߟ) ଶ ଶ ۉ ی
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ଶ
where r is the scattering parameter, η = EF/kBT is the reduced Fermi energy, and Fn(η) is the n-th order Fermi integral given by ݔ ܨ (ߟ) = න ݀ݔ ௫ିఎ 1+݁ ஶ
For most cases, the scattering parameter for acoustic phonon scattering is r = −1/2. The reduced Fermi energy η can be obtained by fitting S(T) to the following equation: 7
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The estimated temperature-dependent L is shown in the inset of Fig. 2(d). The L decreases
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with increasing temperature, ranging between 1.7 and 2.1 x 10-8 W Ω K-2 , which is lower than L0. In Fig. 2(d), the temperature variation of κlat for the x = 0.01 sample is almost the same as that of Ag2Se0.5Te0.5 (open star symbol). The κlat decreases with increasing Cu content (x) mainly due to enhanced phonon scattering by atomic disordering. For example,
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the RT κlat of the x = 0.01, 0.05, and 0.1 sample is 0.47, 0.42, and 0.32 W m-1 K-1, respectively.
From the analysis of the electronic band structure of AST in our previous work [12], the
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conduction bands of AST compound are composed of Ag 5s and either Se 4p or Te 5p states while the corresponding valence bands consist of Se 4p and Te 5p states. The relatively high σ originates with the highly dispersive bands in the n-type β-AST phase. Previous studies of theoretical thermoelectric properties in terms of chemical potential have shown that the electronic contribution to the thermoelectric figure-of-merit ZT is
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maximum near the chemical potential µ = -0.1 eV in the n-type conduction carriers in the AST compound (see Fig. 7 of Ref. [12]). Here we fit the chemical potential with respect to Hall carrier concentration, as shown in Fig. 3. The Hall carrier concentrations for the Cu doping levels in this work are marked with red diamond symbols, revealing that Cu doping
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can effectively enhance the chemical potential on the conduction band side in AST. The chemical potentials of the compounds correspond to 70 meV (x = 1 %), 0.13 eV (x = 5 %)
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and 0.23 eV (x = 10 %), respectively. For the negative chemical potential around 0.1 eV, the theoretical maximum ZT is about 0.3 at room temperature. The temperature-dependent PF and ZT of the Cu doped AST samples are shown in Fig. 4(a) and Fig. 4(b), respectively. The experimental ZT values at room temperature are about 0.28 (x = 1 %), 0.26 (x = 5 %), and 0.19 (x = 10 %), respectively, which are similar to the results of the theoretical calculation. Notably, a rapid enhancement in PF is observed after the phase transition in the Cu doped samples. The highest PF of 1.29 mW m-1 K-2 was achieved at 463 K for the x = 0.1 sample, which is a 30 % enhancement compared to undoped AST [9]. Consequently, the x = 8
ACCEPTED MANUSCRIPT 0.1 sample showed the highest ZT of 0.85 at 558 K, mainly due to the combined effect of a relatively high PF and low κlat through the optimized Cu doping. Conclusions Here we investigated the effect of Cu doping on the TE properties of AST for
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polycrystalline n-type Ag2-xCuxTe0.5Se0.5 (x = 0.01, 0.05, 0.1) samples. Based on the XRD measurements, the effective substitution of Cu for Ag in AST was experimentally confirmed. Furthermore, the TE characterization revealed that the Cu doped AST samples underwent a structural phase transition at around 400 K, accompanied by a semiconducting-to-superionic
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conduction transition. We demonstrated that the n-type carrier density of AST can be delicately controlled by Cu doping content, and verified the effect with both TE
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characterization and theoretical DFT calculation.
Interestingly, we found that σ was more significantly affected than S by Cu doping, and the optimized Cu doping content ranged between 5 and 10% in Cu content. Consequently, the Cu0.1Ag1.9Se0.5Te0.5 sample exhibited the highest PF of 1.29 mW m-1 K-2 at 463 K, which is a 30 % enhancement compared to undoped AST [9], and its peak ZT of 0.85 was achieved at
460 – 560 K for x > 0.05.
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558 K. Also, a relatively high ZT of > 0.7 was persistent over a wide temperature range of
We suggest that optimizing the doping amount as well as the appropriate choice of dopant, without sacrificing the superionic conduction state, is critical to obtain a high ZT for
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silver and cooper chalcogenides.
Acknowledgements
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This research was supported by the Nano-Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0030147) and by the Materials and Components Technology Development Program of MOTIE/KEIT (10063286).
References [1]
H. J. Goldsmid, Introduction to thermoelectricity, Springer, Heidelberg, 2009. 9
ACCEPTED MANUSCRIPT [2]
F. J. Disalvo, Thermoelectric cooling and power generation, Science 285 (1999) 703-
706. [3]
G. J. Snyder, E. S. Toberer, Complex thermoelectric materials, Nat. Mater. 7 (2008)
105-114. [4]
H. Kikuchi, H. Iyetomi, A. Hasegawa, Insight into the origin of superionic
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conductivity from electronic structure theory, J. Phys. Condens. Matter 10 (1998) 1143911448. [5]
S. Kashida, N. Watanabe, T. Hasegawa, H. Iida, M. Mori, Electronic structure of
Ag2Te, band calculation and photoelectron spectroscopy, Solid State Ionics 148 (2002) 193[6]
SC
201.
M. Ferhat, J. Nagao, Thermoelectric and transport properties of β-Ag2Se compounds
[7]
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J. Appl. Phys. 88 (2000) 813-816.
J. Capps, F. Drymiotis, S. Lindsey, T. M. Tritt, Significant enhancement of the
dimensionless thermoelectric figure of merit of the binary Ag2Te, Philos. Mag. Lett. 90 (2010) 677-681. [8]
H. Liu, X. Shi, F. Xu, L. Zhang, W. Zhang, L. Chen, Q. Li, C. Uher, T. Day, G. J.
Snyder, Copper ion liquid-like thermoelectrics, Nature Mater. 11 (2012) 422-425. F. Drymiotis, T. W. Day, D. R. Brown, N. A. Heinz, G. J. Snyder, Enhanced
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[9]
thermoelectric performance in the very low thermal conductivity Ag2Se0.5Te0.5, Appl. Phys. Lett. 103 (2013) 143906. [10]
P. G. Klemens, Thermal resistance due to point defects at high temperatures, Phys.
[11]
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Rev. 119 (1960) 507-509.
B. Abeles, Lattice thermal conductivity of disordered semiconductor alloys at high
[12]
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temperatures, Phys. Rev. 131 (1963)1906-1991. J. H. Yun, M.-H. Lee, J. N. Kim, J. H. Shim, J.-S. Rhyee, Boltzmann transport
calculation of thermoelectric properties in Ag2Se1-xTex (x = 0.0 and 0.5), J. Appl. Phys. 119 (2016) 165101. [13]
H. Zhu, J. Luo, H. Zhao, J. Liang, Enhanced thermoelectric properties of p-type
Ag2Te by Cu substitution, J. Mater. Chem. A 3 (2015) 10303-10308. [14]
Q. Zhang, F. Cao, W. Liu, K. Lukas, B. Yu, S. Chen, C. Opeil, D. Broido, G. Chen,
Zhifeng Ren, Heavy doping and band engineering by potassium to improve the 10
ACCEPTED MANUSCRIPT thermoelectric figure of merit in p-type PbTe, PbSe, and PbTe1−ySey, J. Am. Chem. Soc. 134 (2012) 10031-10038. [15]
L. Hu, T. Zhu, X. Liu, X. Zhao, Point defect engineering of high-performance
Bismuth-Telluride-based thermoelectric materials, Adv. Fuct. Mater. 24 (2014) 5211-5218. [16] C.-C. Lin, D. Ginting, R. Lydia, M. H. Lee, J.-S. Rhyee, Thermoelectric properties and
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PbTe precipitation, J. Alloys & Comp. 671 (2016) 538-544.
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extremely low lattice thermal conductivity in p-type Bismuth Tellurides by Pb-doping and
xSe0.5Te0.5
(x = 0.01, 0.05,
Ag2Se
4.333
7.061
7.766
Ag2Se [6]
4.333
7.062
7.764
Ag2Se0.5Te0.5
4.433
7.237
7.967
Ag2Se0.5Te0.5 [9]
4.433
7.235
7.968
x = 0.01
4.419
7.211
7.936
x = 0.05
4.419
7.143
7.916
x = 0.1
4.394
7.039
7.899
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c (Å Å)
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0.1)
b (Å Å)
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Single phase
CuxAg2-
a (Å Å)
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Table 1. Lattice parameters of Ag2Se and CuxAg2-xSe0.5Te0.5 (x = 0, 0.01, 0.05, 0.1) nH
µH
( cm-3 )
( cm2 V-1 s-1 )
Ag2Se
1.51 x 1018
3191
Ag2Te
5.80 x 1018
2320
Ag2Se0.5Te0.5
4.10 x 1018
2100
Cu0.01Ag1.99Se0.5Te0.5
6.43 x 1018
945
Cu0.05Ag1.95Se0.5Te0.5
3.11 x 1019
236
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8.40 x 1019
128
Table 2. The room temperature carrier concentration nH and Hall mobility µ H of the Ag2Se,
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Ag2Te, Ag2Se0.5Te0.5, and CuxAg2-xSe0.5Te0.5 (x = 0.01, 0.05, 0.1).
Fig. 1. Powder X-ray diffraction patterns of Ag2Se and CuxAg2-xSe0.5Te0.5 (x = 0, 0.01, 0.05, 0.1).
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Fig. 2. Temperature-dependent electrical conductivity σ (a), Seebeck coefficient S (b), total thermal conductivity κ (c), and lattice thermal conductivity κlat (d) of the CuxAg2-xSe0.5Te0.5 (x
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= 0.01, 0.05, 0.1) compounds and the inset of (d) shows temperature-dependent Lorenz
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number L, calculated by the parabolic band assumption.
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Fig. 3. Negative chemical potential with respect to Hall carrier density, calculated from the
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the chemical potential calculation result.
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Fig. 4. Temperature-dependent (a) power factor PF (= S2σ) and (b) ZT values of the
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CuxAg2-xSe0.5Te0.5 (x = 0.01, 0.05, 0.1) samples.
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We investigated thermoelectric properties in n-type CuxAg2-xSe0.5Te0.5.
Cu-doping increases Hall carrier density and chemical potential.
Structure phase transition attributes to the increase ZT value.
We observed high ZT over a wide temperature range (460~560 K).
Maximum ZT reaches about 0.85 at 560 K which is enhanced value.
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