Structure and Improved Thermoelectric Properties of Ag2xCr2–2xSe3

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Structure and Improved Thermoelectric Properties of Ag2xCr2−2xSe3 Compounds Tingting Zhang, Xianli Su,* Junjie Li, Zhi Li, Yonggao Yan, Wei Liu, and Xinfeng Tang* State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/13/18. For personal use only.

S Supporting Information *

ABSTRACT: This work shows that the thermoelectric properties of Cr2Se3 material are improved by doping with Ag. The influence of doping with Ag on the phase composition, microstructure, and thermoelectric properties of Ag2xCr2−2xSe3 (x = 0−0.02) compounds was thoroughly investigated. Ag atoms prefer to occupy the 6c Wyckoff position of the space group, accompanied expansion of the lattice and distortion of the octahedral structure units. The electrical conductivity increases at elevated temperature, while the lattice thermal conductivity decreases significantly through Ag doping, which is primarily attributed to the distorted structure and enhanced alloy scattering. Therefore, it produces a peak ZT value of 0.27 at 673 K for Ag0.04Cr1.96Se3, which shows an increase of 23% compared with that of the undoped Cr2Se3 compound.

1. INTRODUCTION

Chromium selenide (Cr2Se3) is antiferromagnetic with the Neel temperature TN = 43 K.34,35 In the rhombohedral structure (space group: R3) for the Cr2Se3 material, Cr atoms are located in the hexagonal voids formed by Se atoms.36 Owing to the high hole concentration nH caused by the Cr vacancies and high κl, the peak ZT value for the pure Cr2Se3 compound just reached around 0.22 at 623 K. Previous research proved that alloying Se with S markedly increases α, while it decreases κl, and the ZT values for the Cr2Se3−3xS3x compounds are greatly enhanced.37 It should be pointed out that the outer-shell electron of S is isoelectronic to that of Se, indicating that it is very difficult to optimize the carrier concentration with S substitution on Se. Great efforts have been made on doping on the Cr sites with alien valent elements (Mn, Nb, and Ni), and our recent work demonstrates that elemental doping on Cr sites can significantly enhance the thermoelectric properties of Cr2Se3-based material.38 However, it should be noted that Cr atoms in the rhombohedral structure have three different Wyckoff positions, and different dopants have a distinct preference to occupy those Wyckoff positions. More importantly, doping on the different Cr sites has a different effect on the thermoelectric transport properties. To further distinguish the influence of different transitionmetal dopings on the thermoelectric properties, an attempt at doping Ag on the Cr site is implemented to improve the ZT value of Cr2Se3-based material. In our work, the thermoelectric properties of pristine Cr2Se3 have been enhanced by doping Ag on the Cr sites. Ag atoms

As seen from the energy flowchart, more than two-thirds of energy is discharged as unusable heat. As an environmentally friendly technology, thermoelectric conversion can rapidly realize thermoelectric power generation and solid-state refrigeration, showing great potential in providing a solution to improving the energy utilization efficiency and global energy savings and, in turn, reducing emissions.1−9 The conversion efficiency of thermoelectric devices is evaluated by the figure of merit ZT = α2σT/κ; among them, α is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the total thermal conductivity.10,11 According to this equation, it is clear that an excellent thermoelectric device must have high α2σ and low κ simultaneously.12 However, because of the strongly coupled nature of the above physical transport parameters, improving the conversion efficiency of these thermoelectric devices is indeed challenging. In general, α2σ can be optimized by elemental doping,13−16 band-structure engineering,17−21 and a charge-carrier energy-filtering effect;22−24 meanwhile, a decrease of κ can be realized through the formation of solid solutions 25−27 or creation of nanostructures,28−33 enhancing alloy and interfacial phonon scattering. The above approaches have greatly enhanced the ZT values of, for example, Bi2Te3, PbTe, SnTe, and GeTe. Although these materials have excellent thermoelectric properties, most of them contain the expensive constituent element Te and require complex synthetic processing, which seriously restricts their industrial applications. Therefore, it is of great significance to promote cost-efficient and environmentally friendly thermoelectric materials. © XXXX American Chemical Society

Received: June 21, 2018

A

DOI: 10.1021/acs.inorgchem.8b01704 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry have a preference to occupy the 6c site rather than the 3a and 3b sites in the Wyckoff positions. The increase of σ through doping with Ag is ascribed to the slight enhancement of nH, while the remarkable reduction of κl is caused by the improvement of alloy phonon scattering. The highest ZT value 0.27 at 673 K is attained for Ag2xCr2−2xSe3 (x = 0.01− 0.02), showing an improvement of 23%, compared with the maximum ZT value of pristine Cr2Se3.

2. EXPERIMENTAL SECTION The pure powders of Ag (99.999%), Cr (99.95%), and Se (99.999%) were weighed in their stoichiometric compositions Ag2xCr2−2xSe3 (x = 0−0.02) and then mixed, where x is the relative content of the Ag ions to all of the cations. The preparation method of Ag2xCr2−2xSe3 (x = 0−0.02) samples is the same as that in our previously published paper.37,38 In this paper, without any special specification, the thermoelectric properties of the bulk compounds were measured in the direction perpendicular to the direction of the applied sintering pressure. The phase composition of the bulk samples was identified using powder X-ray diffraction (XRD; Empyrean, PANalytical; Cu Kα). The morphologies of the bulk samples were observed using field emission scanning electron microscopy (FESEM; Hitachi SU8020). The chemical composition of the bulk samples was characterized by electron probe microanalysis (EPMA; JXA-8230/INCAX-ACT). The electrical conductivity (σ) and Seebeck coefficient (α) were measured simultaneously using commercial equipment (ZEM-3, Ulvac Riko, Inc.) under a low pressure of the protect gas (He) from 300 to 723 K. The thermal diffusivity D was measured in an argon atmosphere by the laser flash method (LFA 457; Netzsch). The heat capacity (Cp) was calculated according to the Dulong−Petit law. The bulk density (ρ) was evaluated by the Archimedes method. The thermal conductivity (κ) was then obtained using the formula κ = DCpρ. The sample measurement errors in the electrical conductivity, thermopower, and thermal conductivity are ±3%, ±2%, and ±5%, respectively, leading to ∼12% uncertainty in ZT. The electrical conductivity (σ) and Hall coefficient (RH) at room temperature were measured by a physical property measurement system (PPMS-9; Quantum Design). The carrier concentration (n) and carrier mobility (μH) were calculated from nH = 1/eRH and μH = σ/nHe.

Figure 1. (a) XRD patterns for Ag2xCr2−2xSe3 (x = 0−0.02) after SPS. (b) Lattice constants as a function of x. XRD pattern for (c) pure Cr2Se3 and (d) Ag0.02Cr1.98Se3 via Rietveld refinement.

Table 1. Parameters for Obtaining XRD Patterns and the Quality Factors for Ag0.02Cr1.98Se3 and Structure Parameters for Ag0.02Cr1.98Se3 (a) Parameters parameters for obtaining XRD patterns X-ray monochromator 2θ (deg) step width (deg) counts time (s) temperature (K)

3. RESULTS AND DISCUSSION 3.1. Composition Analysis and Structure Characterization. As displayed in Figure 1a, the total XRD patterns for all samples can be indexed to Cr2Se3, indicating that all compounds are single-phase materials. Figure 1b shows variation of the lattice constants with the Ag content for the bulk samples. Obviously, with rising x (x ≤ 0.01), the lattice parameters of all compounds grow linearly, which obeys Vegard’s law. Because Ag (1.53 Å) ions have bigger radii than Cr (1.27 Å) ions, Ag atoms can entirely occupy the Cr sites without a change of the crystal structure. However, when the Ag content exceeds 0.01, the lattice parameters are almost invariable, indicating that the doping limit of Ag is x = 1%. In the structure of Cr2Se3, Cr atoms have three different sites in the Wyckoff positions (i.e., 3a, 3b, and 6c positions). To further investigate the influence of Ag atoms on the structure, the Rietveld refinement analysis is applied to refine the crystal structure of Ag2xCr2−2xSe3 (x = 0−0.02) using the GSAS software.39,40 Table 1 lists the data collection parameters. Parts c and d of Figure 1 show the observed and calculated XRD patterns and the difference between them for pure Cr2Se3 and Ag0.02Cr1.98Se3, respectively. The quality factors Rwp, Rp, and χ2 for the refinement of pure Cr2Se3 are 0.11, 0.0761, and 2.278, respectively, while those of Ag0.02Cr1.98Se3 are 0.0907, 0.0708,

characterization factors

Cu Kα Rwp graphite 5−135 Rp 0.007 40 χ2 300 (b) Structure Parameters

0.0907 0.0708 2.825

atom

x

y

z

Wyckoff

occupancy

Cr1 Cr2 Cr3 Ag Se

0.000000 0.000000 0.000000 0.000000 0.33742

0.000000 0.000000 0.000000 0.000000 1.00590

0.000000 0.500000 0.331314 0.331314 0.25152

3a 3b 6c 6c 18f

1.0000 1.0000 0.9778 0.0222 1.0000

and 2.825, respectively, proving the high reliability of Rietveld analysis. During the process of refinement, attempts have been made to substitute the Ag atoms on different Wyckoff positions. For the Ag0.02Cr1.98Se3 compound, only if Ag is substituted on the 6c site in the Wyckoff positions is the converged refinement result obtained, demonstrating that Ag atoms prefer to substitute on the 6c site rather than the 3a and 3b sites. Parts a−d of Figure 2 display the rhombohedral structure of Cr2Se3, which contains three kinds of characteristic [CrSe6]9− octahedra, because Cr atoms have three different Wyckoff positions. All of those octahedral structures link together by sharing the corners of Se atoms. All of those [CrSe6]9− octahedra in the structure are not perfect regular octahedra. Despite the equal Cr−Se bond length for [CrSe6]9− octahedral structures with Cr atoms at the Cr1 and Cr2 positions, however, the irregular octahedral structure leads to a B

DOI: 10.1021/acs.inorgchem.8b01704 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. (a) Crystal structure of Cr2Se3. (b−d) Coordinated environments of Cr (Cr1, Cr2, and Cr3) atoms (surrounded by six Se atoms) in Cr2Se3. (e−g) Coordinated environments of Cr (Cr1, Cr2, and Cr3) atoms (surrounded by six Se atoms) in Ag0.02Cr1.98Se3.

Figure 3. (a−d) FESEM and (e−h) corresponding BSE images of Ag2xCr2−2xSe3 (x = 0.005−0.02) observed on the polished surfaces.

Figure 4. FESEM images of the fractured surface for Ag2xCr2−2xSe3 (x = 0, 0.01, and 0.02).

slight deviation of the Se−Cr−Se bond angle from 90° or 180°. The [CrSe6]9− octahedron with a Cr atom at the Cr3 position is slightly different from the above two octahedra. The Cr atom deviates from the center position, and the octahedron with a Cr atom at the Cr3 position is asymmetric because the six Cr−Se bonds split into three longer bonds (2.5274 Å) and three shorter bonds (2.4690 Å). Additionally, off centering of the Cr atom in the octahedron leads to a slight deviation of the Se−Cr3−Se bond angle from 90° or 180°. It should be noted that the bond distances of Cr1−Se and Cr2−Se are not the same and are 2.4679 and 2.5753 Å, respectively. Substitution of the Ag atoms on the Cr3 site changes the bond distance and angle of Cr−Se, and the coordinated environments of Cr (Cr1, Cr2, and Cr3) atoms in the Ag0.02Cr1.98Se3 compound are shown in Figure 2e−g. The bond distance of Cr1−Se increases from 2.4679 to 2.4830 Å, that of Cr2−Se decreases from 2.5753 to 2.5582 Å, and that of Cr3 (Ag)−Se increases from

2.4690 to 2.4953 Å and from 2.5274 to 2.5482 Å, respectively. Doping with Ag modifies the octahedral structure units, which would impact the thermoelectric properties of Cr2Se3. Figure 3 displays FESEM and corresponding backscattering electron (BSE) images observed on the polished surfaces for Ag2xCr2−2xSe3 (x = 0−0.02) compounds. None of the contrast difference is detected for Ag2xCr2−2xSe3 (x ≤ 0.01) samples, which means all compounds are homogeneous single phase, and it is worth pointing out that the black areas in all BSE images are pores formed during the polishing process, which can be seen clearly from their corresponding FESEM images. When the x value exceeds 1%, despite the fact that the secondary phase has not been detected in the XRD patterns, however, there exists an obvious contrast difference in the BSE image. For the Ag0.03Cr1.97Se3 and Ag0.04Cr1.96Se3 samples, in the small dispersed dark-gray areas (circled by the red dotted lines), none of Ag has been detected (taking the EPMA C

DOI: 10.1021/acs.inorgchem.8b01704 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 5. Variations of (a) σ, (b) α, and (c) σ2α for Ag2xCr2−2xSe3 (x = 0−0.02) with temperature.

detection limit into account, the composition is Ag2xCr2−2xSe3 with the x value less than 0.5%), while the compositions of the main matrix (light-gray areas) are Ag0.025Cr1.972Se3 and Ag0.029Cr1.968Se3, respectively. Therefore, we can conclude that the doping limit of Ag in the Cr2Se3 compounds is x = 1%. Figure 4 displays the morphologies of the fractured surfaces of Ag2xCr2−2xSe3 (x = 0−0.02) bulk specimens after spark plasma sintering (SPS). We get fully condensed samples with randomly distributed grains. All samples feature an intergranular fracture regardless of the Ag content. Surprisingly, the size of the grain is very small, around 100−300 nm, despite synthesis via a conventional method. 3.2. Thermoelectric Properties. On the basis of the aforementioned discussion, Ag dopes on the Cr sites with a solid solution limit and prefers to occupy the 6c site in the Wyckoff positions. This would impact the thermoelectric transport properties. In this part, we correlate the impact of Ag doping with the transport properties of the Ag2xCr2−2xSe3 (x = 0−0.02) samples. It can be seen in Figure 5a that the σ values of all compounds become smaller with rising temperature. At room temperature, the σ values of Ag-doped samples decline with rising x; meanwhile, with rising T, the σ values of those samples cross over, and then when there is an inverse trend with increasing Ag content, the σ values of the Ag-doped Ag2xCr2−2xSe3 specimens increase. Figure 5b shows that the positive α values for all Ag2xCr2−2xSe3 specimens manifest typical p-type conduction. We can easily attribute the dominant hole transport to the Cr vacancies in the Cr2Se3 crystal structure. The maximum α for all specimens is obtained at around 650 K; with increasing Ag content, α decreases. The largest α value for a pure Cr2Se3 sample at 623 K is 166 μV K−1, while that of Ag0.04Cr1.96Se3 at 673 K is 154 μV K−1. Figure 5c shows that the peak σ2α for all specimens is attained at around 600 K, showing the same trend as the temperature dependence of α. At a temperature above 623 K, the σ2α values of all doping samples are slightly larger than those of pure Cr2Se3. Owing to the enhancement of σ, the maximum σ2α of the Ag0.02Cr1.98Se3 sample obtained at 623 K is 6.5 × 10−4 W m−1 K−2, which is almost the same value as that of the pristine Cr2Se3 sample obtained at 573 K. For a better understanding of charge transport, nH is measured. Figure 6a shows that, with rising x, nH stays almost constant, ranging from 1.02 × 1020 cm−3 for the pristine Cr2Se3 sample to 1.09 × 1020 cm−3 for the Ag0.04Cr1.96Se3 compound. Meanwhile, the carrier mobility goes down slightly, except for that of the Ag0.01Cr1.99Se3 sample, ascribed to the enhanced alloy scattering on the charge carrier. Therefore, with increasing Ag content, σ decreases at 300 K.

Figure 6. (a) Variation of nH and μH of Ag2xCr2−2xSe3 (x = 0−0.02) with x at 300 K. (b) Pisarenko plots (black and red lines are the calculated lines with m* = 0.9m0 and m* = m0, respectively) of Ag2xCr2−2xSe3 (x = 0−0.02) at 300 K.

To better uncover the influence of Ag doping on α, a single parabolic band model is used, considering that acoustic phonon scattering is dominant. Then we can describe α as

8π 2kB 2 ij π yz2/3 jj zz m*T (1) 3eh2 k 3n { Figure 6b displays room temperature Pisarenko plots based on the above equation. With rising x, the calculated m* of Agdoped compounds remains almost unchanged, satisfying with the parabolic band model. The temperature dependence of κ for the Ag2xCr2−2xSe3 compounds is displayed in Figure 7a. κ first falls with rising T; when T exceeds 573 K, κ becomes greater because of the increasing contribution of the ambipolar thermal conductivity κbi. As x increases, κ decreases. The lowest κ of pristine Cr2Se3 is 1.75 W m−1 K−1 obtained at 523 K; meanwhile, that of Ag0.04Cr1.96Se3 at 523 K falls to 1.48 W m−1 K−1, which decreases by about 15%. Because κ = κe + κl + κbi, calculation of κe = LσT is according to the Wiedemann−Franz law, using eqs 2−441−43 to obtain the Lorenz number, where the scattering factor r = −1/2, assuming that acoustic phonon scattering is dominant, and η is the reduced Fermi level. ÄÅ ÉÑ2 | l 7 o ÅÅ r + 5 F ÑÑ o 2o r F ( ) ( ) + η η o o Å ÑÑ o 3/2 r + 5/2 r + κ o Å i B zy o 2 2 o Å ÑÑ o j Å L = jj zz m − } Å Ñ o o Å Ñ 3 3 o r+ F o Å Ñ ke {o o Å Ñ ( ) r F ( ) η + η o o 1/2 1/2 r + r + Å Ñ 2 2 o o Å Ñ Ç Ö n ~ α=

( (

ÄÅ ÅÅ κB ÅÅÅ r + S = ± ÅÅÅ e ÅÅÅ r + ÅÇ

( (

D

) )

5 2 3 2

ÉÑ ÑÑ Fr + 5/2(η) ÑÑ − ηÑÑÑÑ ÑÑ Fr + 1/2(η) ÑÑÖ

) )

( (

) )

(2)

(3) DOI: 10.1021/acs.inorgchem.8b01704 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 8. Variations of the ZT value with temperature for Ag2xCr2−2xSe3 (x = 0−0.02).

Figure 7. Variations of the thermal-transport properties for Ag2xCr2−2xSe3 (x = 0−0.02) with temperature: (a) κ; (b) L; (c) κ − κe; (d) κb.

Fn(η) =

∫0

∞

χn dχ 1 + e χ−η

4. CONCLUSIONS A series of Ag2xCr2−2xSe3 (x = 0−0.02) samples are synthesized by the solid-state reaction, followed by the SPS process. We investigated the influence of Ag doping on the phase composition, microstructure, and thermoelectric properties of Ag2xCr2−2xSe3 (x = 0−0.02) samples. The doping limit of Ag in Cr2Se3 is x = 1%; Ag doping enhances the electrical transport properties at higher temperature, while it decreases κl dramatically, attributed to the improvement of phonon scattering. The maximum ZT value for Ag0.04Cr1.96Se3 attained at 673 K is 0.27, showing an enhancement of 23%, compared to the pristine Cr2Se3 compound (0.22 at 623 K).

(4)

Figure 7b shows the calculated Lorenz number. Figure 7c shows that when the temperature rises, κl + κbi first declines (the lowest value is reached at 523 K) and then increases. The minimum κl + κbi of pure Cr2Se3 is 1.5 W m−1 K−1 obtained at 523 K; meanwhile, that of Ag0.04Cr1.96Se3 has reduced to 1.18 W m−1 K−1 at 523 K, showing a decrease of about 21%. The trends for the temperature dependence of κ and κl + κbi are the same. The initial decrease with rising T is mainly attributed to Umklapp phonon scattering. Above a certain temperature, the contribution of κbi starts to go up, which is attributed to intrinsic excitation; therefore, κ and κl + κbi increase with rising T. Figure 7d shows that, with increasing temperature, κbi increases. In addition, with increasing Ag content, κbi remains almost unchanged because Ag doping has little influence on the carrier concentration. Obviously, Ag doping decreases κl effectively. As is known, the alloy phonon scattering consists of two aspects, i.e., mass fluctuations ΓM and strain fluctuations ΓS. We have used the Calloway model44,45 to calculate the fluctuations. The calculation detail is shown in the Supporting Information. As shown in Table S2, ΓS is greater than ΓM for the Ag2xCr2−2xSe3 (x = 0.005, 0.01) samples, indicating that ΓS caused by Ag doping is dominate in phonon scattering ascribed to the decrease of κl. Figure 8 displays variations of the ZT value with temperature for the Ag2xCr2−2xSe3 (x = 0−0.02) material. Obviously, substitution of Ag atoms on the Cr sites improves the ZT value, and the peak ZT value of pristine Cr2Se3 is 0.22 at 623 K, while that of Ag0.04Cr1.96Se3 at 673 K is 0.27, increased by 23%. The improvement of the ZT value after Ag doping is primarily attributed to the decreased κl coming from the enhanced phonon scattering and distorted structure.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01704. Density ρ, thermal diffusivity D, and calculation of the fluctuation parameters Γ (Γ = ΓM + ΓS) for the Ag2xCr2−2xSe3 (x = 0−0.02) samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.S.). *E-mail: [email protected] (X.T.). ORCID

Xianli Su: 0000-0003-4428-6461 Wei Liu: 0000-0002-3245-7270 Xinfeng Tang: 0000-0001-7555-919X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the Natural Science Foundation of China (Grants 51521001 and 51632006) and the 111 Project of China (Grant B07040). E

DOI: 10.1021/acs.inorgchem.8b01704 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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