Enhanced Thermoelectric Properties of Codoped Cr2Se3: The Distinct

Subscriber access provided by University of Sussex Library

Functional Inorganic Materials and Devices

Enhanced Thermoelectric Properties of Co-doped CrSe: the Distinct Roles of Transition Metal and S 2

3

Tingting Zhang, Xianli Su, Yonggao Yan, Wei Liu, Tiezheng Hu, Cheng Zhang, Zhengkai Zhang, and Xinfeng Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05080 • Publication Date (Web): 15 Jun 2018 Downloaded from http://pubs.acs.org on June 17, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Enhanced Thermoelectric Properties of Co-doped Cr2Se3: the Distinct Roles of Transition Metal and S Tingting Zhang,a Xianli Su,a,* Yonggao Yan,a Wei Liu,a Tiezheng Hu,a Cheng Zhang,a Zhengkai Zhang,a Xinfeng Tanga,* a

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,

Wuhan University of Technology, Wuhan 430070, China *Correspondence to: X. Su ([email protected]), X. Tang ([email protected]).

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Pristine Cr2Se3 is a narrow band gap semiconductor but with inferior ZT value of 0.22 obtained at 623 K. In this paper, we improve the thermoelectric performance of Cr2Se3 material by optimizing carrier concentration, suppressing the bipolar thermal conductivity and reducing the lattice thermal conductivity simultaneously. Firstly, the effect of different dopants (Nb, Ni, and Mn) on the phase composition, and thermoelectric transport properties of M2xCr2-2xSe3 (M = Nb, Ni, and Mn；x = 0-0.02) compounds are systematically investigated. The role of those dopants is distinct. Mn doped samples show superior thermoelectric properties in comparison with that of other elements doped samples. Doping with Mn significantly increases the carrier concentration, accompanied with a suppression of the intrinsic excitation and reduction of both the bipolar thermal conductivity and the lattice thermal conductivity of Cr2Se3. To further reduce the thermal conductivity, we have synthesized Mn and S co-doped Mn0.04Cr1.96Se3-3xS3x (x = 0-0.1) samples. Alloying with S significantly decreases the lattice thermal conductivity and enlarges the band gap, boosting the Seebeck coefficient. The maximum ZT value of Mn0.04Cr1.96Se2.7S0.3 reaches 0.33 at 823 K. Compared with the pristine Cr2Se3 sample, the maximum ZT value is increased by 50% and the temperature corresponding to the peak value shifts toward higher temperature by 200 K. KEYWORDS: Cr2Se3, transition metal, S, carrier concentration, intrinsic excitation, thermoelectric

2


Page 2 of 34

Page 3 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction In recent decades, the energy crisis is becoming increasingly serious. Thermoelectric material, which is a kind of new energy material that can directly convert thermal energy into electrical energy, has aroused worldwide attention.1-6 But the low conversion efficiency restricts its large-scale application.2 The conversion efficiency of thermoelectric materials depends on the dimensionless figure of merit ZT = α2σT/κ, where α is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the total thermal conductivity. The higher the ZT value of a thermoelectric material, the higher the conversion efficiency.7, 8 According to the above formula, to achieve a high ZT values, it is necessary to increase power factor α2σ and reduce the total thermal conductivity κ. Suppressing the thermal conductivity while preserving excellent electronic transport properties is an effective method to enhance the ZT value of the material.9 It is well known that the total thermal conductivity is a sum of κe which is the electronic thermal conductivity, κl which is the lattice thermal conductivity, and κbi which is the bipolar thermal conductivity, κ = κe+ κl + κbi. In general, enhancing alloying scattering and interfacial phonon scattering by forming solid solutions10-16 or creating nanostructures17-32 can significantly reduce the lattice thermal conductivity of the materials. However, for narrow band gap semiconductor, the bipolar thermal conductivity cannot be avoided attributed to the intrinsic excitation especially at high temperature, which deteriorates the thermoelectric performance. It is found that increasing the 3



Page 4 of 34

dominant extrinsic carrier concentration12, 18, 33-36 and the electronic band gap10, 28, 33, 37 can effectively lower the bipolar thermal conductivity and further reduce the thermal conductivity of the material. Using the above strategies, the ZT values of narrow band gap semiconductors, such as PbTe-based and Bi2Te3-based materials, have been greatly improved and the peak ZT shifts towards the higher temperature. L. D. Zhao et al.28 reported an extraordinary role of MgTe in PbTe. For one thing, alloying with MgTe simultaneously suppresses the bipolar thermal conductivity and the lattice thermal conductivity ascribed to the enlarged the electronic band gap and enhanced alloying phonon scattering respectively. Moreover, Alloying with MgTe facilitates the valence band convergence, boosting the Seebeck coefficient and power factor. As a result, a ZT value of 2.0 at 823 K is achieved. S. Y. Wang et al.33 reported the introduction of Se in Bi2Te3

effectively

increases

the

band

gap,

which

significantly

suppresses

the“turn-over”in Seebeck coefficient and reduces the bipolar thermal conductivity, shifting the corresponding temperature of the optimum ZT value toward higher temperature range with ZTavg = 0.8 of Bi2Te1.5Se1.5 between 400 K and 600 K. Chromium selenide (Cr2Se3) has a defect NiAs-type structure with rhombohedral structure (space group: R3).38 In the structure, Cr atoms occupy octahedral voids which are formed by the hexagonal packing of Se atoms. The structure can be viewed as stacking of an alternating fully occupied“CrSe” layer with a Cr-deficient “Cr1/3Se” layer, in which three possible octahedral void positions are partially occupied by Cr along the c-axis, leading to the Cr2Se3 stoichiometry, and the Cr atoms in the Cr-deficient “Cr1/3Se” 4


Page 5 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


layer are randomly distributed.39 The aforementioned structure features its intrinsically low thermal conductivity, which is highly desirable for thermoelectric materials. Pristine Cr2Se3 is a narrow band gap semiconductor with Eg = 0.21 ev. Due to the intrinsically un-optimized carrier concentration induced by Cr vacancy and large contribution of bipolar thermal conductivity at a relative low temperature, the ZT value of pristine Cr2Se3 is as low as 0.22 at 623 K. Furthermore, at the temperature above 623 K, with increasing temperature, the ZT value decreases more sharply due to the onset of intrinsic excitation where the minority carriers play an important role.40 Therefore, optimizing the carrier concentration and suppressing the bipolar thermal conductivity are of critical importance for enhancing the thermoelectric properties of Cr2Se3-based material which could be achieved through elements doping on the sites of Cr. Besides, previous work has demonstrated that S substituting on the sites of Se can effectively impede the lattice thermal conductivity of Cr2Se3-3xS3x compounds, resulting in an increase of the ZT value. Thus co-doping in the Cr2Se3 system on both sites would produce an enhanced thermoelectric property. In this study, we use two steps to improve the thermoelectric properties of Cr2Se3 material. Firstly, doping the sites of Cr with transition metal (Nb, Ni, and Mn) to investigate the doping effect on the structure, phase composition, and thermoelectric transport properties of M2xCr2-2xSe3-3yS3y (M = Nb, Ni, and Mn；x = 0-0.02；y = 0-0.1) compounds. The role of those dopants is distinct. Doping with Mn significantly suppresses the intrinsic excitation and reduces both the bipolar thermal conductivity and 5



the lattice thermal conductivity of Cr2Se3. The maximum ZT value of 0.28 is attained at 823 K for Mn0.04Cr1.96Se3, demonstrating a 30% enhancement in comparison with that of 0.22 obtained by pure Cr2Se3 at 623 K, while the Nb doping deteriorates the thermoelectric properties. For the Ni doped Cr2Se3, the maximum ZT value of 0.25 is obtained at 723 K for Ni0.01Cr1.99Se3. In order to further reduce the thermal conductivity, we have synthesized Mn and S co-doped Mn0.04Cr1.96Se3-3xS3x (x = 0-0.1) samples. Alloying with S enlarges the electronic band gap which boosts the Seebeck coefficient and significantly reduces the lattice thermal conductivity and bipolar thermal conductivity simultaneously. As a result, the maximum ZT value of Mn0.04Cr1.96Se2.7S0.3 is 0.33 at 823 K. Compared to the pristine Cr2Se3 sample, the maximum ZT value is increased by 50% and the temperature corresponding to the peak value shifts toward higher temperature by 200 K.

2. Experimental Section The starting materials for the synthesis are high-purity powders of Nb (99.99%, under 200 mesh), Ni (99.99%, under 200 mesh), Mn (99.99%, under 200 mesh), Cr (99.95%, under 200 mesh), Se (99.999%, under 200 mesh), and S (99.99%, under 200 mesh). The powders were weighed (~12g) according to the stoichiometry of M2xCr2-2xSe3-3yS3y (M = Nb, Ni, and Mn；x = 0-0.02；y = 0-0.1), and then mixed uniformly. The mixtures were cold-pressed into a pellet which was then sealed in evacuated quartz tubes. The tubes were placed in the furnace heated slowly with the heating rate 120 K/h to 923 K, held at 6


Page 6 of 34

Page 7 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


this temperature for 2 h and subsequently heated to 973 K holding for 24 h. And then the furnace was slowly cooled down to room temperature. The obtained ingots were ground into fine powders and then sintered using the SPS apparatus with a pressure of 40 MPa at 823 K for 10 min under vacuum to obtain fully densified bulk samples (Φ16 mm × 12 mm). In this paper, all the thermoelectric properties of the bulk samples were measured perpendicular to the direction of the applied sintering pressure. The phase compositions of the bulk samples were determined by powder X-ray diffraction (Empyrean, PANalytical; Cu Kα). The back-scattered electron images and element mapping were characterized by electron probe microanalysis (EPMA; JXA-8230/INCAX-ACT). The electrical conductivity (σ) and the Seebeck coefficient (α) were measured simultaneously by using the commercial equipment (ZEM-3, Ulvac Riko, Inc.) under a low pressure of the inert gas (He) from 300 K to 823 K. The thermal conductivity (κ) was calculated using the relationship κ = DCpρ. The thermal diffusivity D was measured in an argon atmosphere by the laser flash diffusivity method (LFA 457; Netzsch). The heat capacity (Cp) was measured by a differential scanning calorimeter (DSC Q20; TA instrument). The sample density (ρ) was measured by the Archimedes method. The carrier concentration (n) and the carrier mobility (µH) were calculated from nH = 1/eRH and µH = σ/nHe. The electrical conductivity (σ) and the Hall coefficient (RH) at room temperature were measured by a Physical Property Measurement System (PPMS-9; Quantum Design).

7



3. Results and Discussion 3.1 Phase Composition and Thermoelectric Properties of M2xCr2-2xSe3 Powder XRD patterns of M2xCr2-2xSe3 (M = Nb, Ni , and Mn；x = 0-0.02) compounds after SPS sintering are shown in Figure 1(a)-(c). All samples are single phase without detection of any secondary phase that all diffraction peaks can be well matched with the standard pattern of Cr2Se3 (JCPDF#98-062-6708). Figure 1(d) shows the lattice parameters (along the a-axis and the c-axis) of all samples as a function of the dopants content. It can be seen, the lattice parameters of the samples along both directions (a-axis and c-axis) vary linearly, which can be well described by the Vegard's law due to the larger covalent radius of Nb (1.37 Å) and Mn (1.39 Å) and the smaller covalent radius of Ni (1.21 Å) in comparison with that of Cr (1.27 Å). Those results have further proved that Nb, Ni, and Mn atoms have successfully substituted on the Cr sites and the crystal structure remains unchanged upon doping. The Rietveld refinement analysis using the GSAS software is employed to refine the crystal structure of M0.04Cr1.96Se3 (M = Nb, Ni, and Mn) compounds.41 The data collection parameters and the refined structure parameters for M0.04Cr1.96Se3 (M = Nb, Ni, and Mn) are listed in Table S1 and the Rietveld refinement results are shown in Figure S1. The results in Table S1 and Figure S1 prove the refinement processes are reliable. During the refinement process, we have tried to set the transition metal on different Wyckoff Positions of Space Groups (ie. 3a, 3b and 6c positions). For Nb0.04Cr1.96Se3 sample, only when Nb substitutes on 6c Wyckoff 8


Page 8 of 34

Page 9 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Positions of Space Groups, the refinement can get converged, while for M0.04Cr1.96Se3 (M = Ni, and Mn) samples, only when Mn or Ni substitutes on 3a Wyckoff Positions of Space Groups, the refinement can get converged, indicating that Nb atoms incline to replace the 6c site while Ni and Mn atoms have the preference for 3a site in the Wyckoff Positions of Space Groups. The different preferences of different dopants in the Wyckoff Positions of Space Groups would result in the different role of transition metal (M = Nb, Ni, and Mn) on the charge and phonon transport properties. Figure 2 displays back-scattered electron images of the polished surfaces for pure Cr2Se3 and M0.04Cr1.96Se3 (M = Nb, Ni, and Mn). No contrast difference is observed in all samples indicating the samples are homogeneous single phase without secondary phase appearing on the micrometer scale, which is consistent with the XRD results. As shown in Figure 3(a)-(c), the electrical conductivity of all Nb-, Ni- and Mn-doped samples decreases with the increasing temperature, which is consistent with the trend of pristine Cr2Se3 sample, exhibiting metallic conduction characteristics. The electrical conductivity of Nb-doped samples decreases with the increase of Nb content, while the electrical conductivity of both Ni- and Mn-doped compounds increases with the increasing dopant content. In order to further quantitatively analyze the effects of different dopants on the thermoelectric properties of M2xCr2-2xSe3 compound, we choose the samples with constant 2% doping fraction for comparison. As shown in Figure 3(d), the electrical conductivity of M2xCr2-2xSe3 has been greatly improved for the samples 9



with 2% doping content of Ni and Mn. For example, the room temperature electrical conductivity of Mn0.04Cr1.96Se3 and Ni0.04Cr1.96Se3 is 8×104 S m-1 and 7.6×104 S m-1 respectively, which is twice as high as that of Cr2Se3 compound (4×104 S m-1), whereas the room temperature electrical conductivity of Nb0.04Cr1.96Se3 sample (2.4×104 S m-1) is much lower than that of Cr2Se3 compound. Figure 4(a)-(c) depicts the temperature dependence of the Seebeck coefficient of M2xCr2-2xSe3 (M = Nb, Ni, and Mn；x = 0-0.02) compounds. The Seebeck coefficient of all those samples is positive, indicating a p-type conduction ascribed to the presence of Cr vacancies in Cr2Se3-based compounds. With increasing dopant content for all samples, the Seebeck coefficient decreases. The Seebeck coefficient of Nb-doped samples first increase with the increasing temperature, reaches its maximum value close to 573 K and then falls，while that of Ni-doped and Mn-doped samples keep increasing with the increasing temperature. The Seebeck coefficient of Nb0.04Cr1.96Se3 is higher than that of Ni0.04Cr1.96Se3 and Mn0.04Cr1.96Se3 from 300 K to 800 K, as shown in Figure 4(d). Figure 5(a)-(c) shows the temperature dependence of the power factor of all samples. For Nb-doped samples, with the increase of doping content, the power factor decreases because of the decrease in the electrical conductivity upon doping. However, for the Niand Mn-doped samples, the power factor is higher than that of pristine Cr2Se3 sample at the temperature above 623 K, and increases with the increase of doping content. The maximum power factor of 0.64 mW.m-1.K-2 is obtained at 823 K for Mn0.04Cr1.96Se3 sample shown in Figure 5(d), while for pristine Cr2Se3 sample the maximum power factor 10


Page 10 of 34

Page 11 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of 0.64 mW.m-1.K-2 is achieved at 573 K. Although the peak power factor is the same as the pristine Cr2Se3 after doping with Mn, the temperature corresponding to the maximum power factor increases by 250 K. To better understand the carrier transport of M2xCr2-2xSe3 (M = Nb, Ni, and Mn；x = 0-0.02) compounds, we measured the carrier concentration of all samples. Figure 6(a) displays the carrier concentration and mobility as a function of doping content at room temperature of all samples. The carrier concentration is greatly improved in Ni- and Mn-doped samples while that in Nb-doped samples is decreased, the carrier concentration of 2% Ni- and Mn-doped samples is 2.92×1020 cm-3 and 3.14 ×1020 cm-3 respectively, while that of 2% Nb-doped samples is 7.2×1019 cm-3. Clearly, Nb atom acts as an electron donor decreasing the hole concentration, whereas Mn and Ni atoms behave as an acceptor increasing the hole concentration. For one thing, the outer shell electron of Cr (3d5 4s1) is different from that of Nb (4d3 5s2), Ni (3d8 4s2), and Mn (3d5 4s2). Moreover, structure refinement results demonstrate that Nb atoms tend to enter into the 6c Wyckoff Positions of Space Groups. In contrast, Mn and Ni atoms prefer to occupy the 3a site instead of 6c site in the Wyckoff Positions of Space Groups. The coordination environments of Cr1 at 3a position and Cr3 at 6c position are also different. All those factors result in the different role upon doping. Additionally, the intensified alloy scattering on the carrier shortens the mean free path of charge carriers (holes), so the carrier mobility of all samples decreases upon doping. The electrical conductivity, σ = enµ, is proportional to the carrier concentration and the carrier mobility. Therefore, for 11



Page 12 of 34

Ni- and Mn-doped samples, the electrical conductivity increases with the increasing carrier concentration, while that of Nb-doped samples decreases with the reduction of carrier concentration and mobility. In order to clarify the influence of different dopants on the Seebeck coefficient, we use a single parabolic band model, assuming that the acoustic phonon scattering is the dominant phonon scattering mechanism. The Seebeck coefficient can then be described as: 2

α=

8π 2 kB2  π  3   m *T 3eh 2  3n 

(1)

where α is the Seebeck coefficient, kB is the Boltzmann constant, h is the Planck constant, e is the elemental electron charge, n is the carrier concentration, and m* is the effective mass. According to formula (1), the Pisarenko plots (the Seebeck coefficient versus the carrier concentration) at room temperature are shown in Figure 6(b). Notably, the derived effective mass m* of Nb-doped samples decreases (from 1.05 m0 to 0.7 m0) with the increasing Nb content, which accounts for the decrease of the Seebeck coefficient; while that of Ni- and Mn-doped samples is in the range of 0.85-1.05 m0 and with the increase of carrier concentration or dopant content, the effective mass slightly increases, indicating the non-parabolic band feature in the vicinity of Fermi level. The decrease in the Seebeck coefficient is mainly caused by the increase of the carrier concentration. Figure 7 displays the temperature dependence of the total thermal conductivity κ for all samples. The total thermal conductivity κ initially decreases with the increasing 12


Page 13 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


temperature, above a certain temperature (573 K for 2% Nb-doped, 673 K for 2% Ni-doped and 723 K for 2% Mn-doped), the trend is interrupted and the κ starts to increase rapidly due to the onset of intrinsic excitations. The total thermal conductivity κ of Nb-doped samples decreases with the increase of dopant content while that of Niand Mn-doped samples increases with the increasing dopant content. Since pristine Cr2Se3 is a narrow gap material with Eg = 0.21 eV, at elevated temperature the minority carriers originating during intrinsic excitations start to participate in the transport process and the ensuing ambipolar diffusion (diffusion of electron-hole pairs) rapidly increases the total thermal conductivity. The total thermal conductivity κ = κe + κl + κbi, where κe is the electronic thermal conductivity, κl is the lattice thermal conductivity, and κbi is the ambipolar thermal conductivity. The electronic thermal conductivity κe = LσT can be calculated via the Wiedemann-Franz law, the detailed calculation process of Lorentz number L is shown in the Supporting Information. The temperature dependence of κl + κbi = κ - κe for all samples is shown in Figure 8(a)-(c). With the increasing temperature, κl + κbi initially decreases, reaches a minimum at a certain temperature (573 K for 2% Nb-doped, 673 K for 2% Ni-doped and 723 K for 2% Mn-doped), and then increases. The initial decrease in κl + κbi is mainly due to the enhanced Umklapp phonon scattering, however, as the temperature keeps increasing, the onset of intrinsic excitations in Cr2Se3-based samples dramatically increases the ambipolar thermal conductivity contribution and thus κl + κbi increases. Assuming that the lattice thermal conductivity is inversely proportional to the temperature, we can easily 13



estimate the bipolar thermal conductivity. The detailed process to extract the ambipolar thermal conductivity κbi is shown in the Supporting Information. As shown in Figure 8(e) and (f), Nb doping cannot effectively reduce the ambipolar thermal conductivity κbi while Mn and Ni doping can significantly reduces the ambipolar thermal conductivity of Cr2Se3-based material and shifts the temperature corresponding to intrinsic excitation to higher temperature. For example, κbi for 2% Mn-doped sample only occurs at the temperature above 723K. The relationship between the carrier concentration and the onset temperature for the ambipolar diffusion is shown in the Figure S5. This remarkable reduction is mainly due to the increase of carrier concentration which effectively shifts the onset of ambipolar diffusion to higher temperatures. In order to distinguish the role of different dopants on the thermal transport properties, the Callaway model42-45 was employed. The alloy scattering has two aspects: it induces mass fluctuations (mass difference between doping elements and Cr), and it also causes strain fluctuations (due to the size difference between atoms of doping elements and Cr). The calculation detail of fluctuation parameters Γ (Γ = ΓM ＋ ΓS) is shown in the Supporting Information. The mass fluctuation parameter ΓM and the strain field fluctuation parameter ΓS caused by 2% Nb doping is much larger than that of 2% Ni and Mn doping, as listed in Table S1. As a result, Nb doping reduces the lattice thermal conductivity more dramatically than that doping with the other two elements. The minimum lattice thermal conductivity of Cr2Se3-based material can be calculated according to Debye model:46 14


Page 14 of 34

Page 15 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1/3

Λ min

T  π  =   k B n 2/3 ∑vi   6 i  Θi 

2 Θ /T i

∫ 0

x 3e x

(e

x

− 1)

2

dx

Θi = vi ( h / κ B ) ( 6π 2 n )

1/3

(2)

(3)

where kB is the Boltzmann constant, n is the number density of atoms, vi is speed of sounds corresponding to the three sound modes (two transverse and longitudinal), ћ is the reduced Planck constant. The minimum lattice thermal conductivity calculated for Cr2Se3 is 0.5 W.m-1.K-1, as shown in Figure 8(d). The lattice thermal conductivity is still very high regardless of which dopant we used. For example, the lowest lattice thermal conductivity Mn0.04Cr1.96Se3 is 1.25 W.m-1.K-1, thus further reduction of the lattice thermal conductivity would boost the thermoelectric properties. Figure 9 shows the temperature dependence of the figure of merit ZT for M2xCr2-2xSe3 (M = Nb, Ni, and Mn；x = 0-0.02) samples. Obviously, the ZT value is greatly enhanced by Mn doping compared with that of the samples doped with Nb and Ni. The maximum ZT value of Mn0.04Cr1.96Se3 is 0.28 at 823 K, while that of pure Cr2Se3 is 0.22 at 623 K manifesting the maximum ZT value increases by 30% and the temperature corresponding to the maximum ZT value shifts to higher temperature by 200 K. The enhancement of ZT value by Mn doping is mainly due to the suppression of intrinsic excitation resulting from the increased carrier concentration and the decrease of lattice thermal conductivity ascribed to enhanced alloy scattering. 3.2 Phase Composition and Thermoelectric Properties of Mn0.04Cr1.96Se3-3xS3x Aforementioned lattice thermal conductivity of those doped samples is still much higher 15



than the amorphous limit (0.5 W.m-1.K-1) in the Cr2Se3 system. Previous work has demonstrated that S substituting on the sites of Se can effectively impede the thermal conductivity of Cr2Se3-3xS3x compounds, resulting in an increase of the ZT value. Therefore, in this section, we choose Mn doped Mn0.04Cr1.96Se3 with optimized carrier concentration and then further substitute S on the sites of Se which plays distinct and complementary role with Mn and further reduces the lattice thermal conductivity, thus increasing the ZT value of Cr2Se3-based material. XRD patterns of SPS-sintered Mn0.04Cr1.96Se3-3xS3x (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) samples are shown in Figure 10(a). Clearly, all Mn0.04Cr1.96Se3-3xS3x samples are single phase materials with all peaks corresponding to the peaks of standard pattern of Cr2Se3 (JCPDF#98-062-6708). Figure 10(b) shows the lattice parameters (along the a-axis and the c-axis) of Mn0.04Cr1.96Se3-3xS3x samples as a function of the S content, with the increase of S content, the lattice parameters of the samples along both directions decrease linearly, which is consistent with the Vegard’s law, as the atomic radius of S (1 Å) is smaller than that of Se (1.15 Å). XRD results and the linear variation of the lattice parameters with the S content indicate that S atoms have successfully entered the sites of Se while maintaining the same crystal structure. Figure 10(c) and (d) displays BSE images of the polished surfaces and their corresponding elemental mapping obtained by EDS for Mn0.04Cr1.96Se2.7S0.3. No contrast difference is observed and all four elements Mn, Cr, Se and S are distributed uniformly, indicating that the sample is homogeneous single phase with no secondary phase appearing on the micrometer scale. 16


Page 16 of 34

Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 11 demonstrates the electrical conductivity, Seebeck coefficient, and power factor of Mn0.04Cr1.96Se3-3xS3x (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) as a function of temperature. With the increasing S content, the electrical conductivity decreases, the Seebeck coefficient increases and the power factor decreases. Figure 6(a) depicts the Hall carrier concentration and the carrier mobility at room temperature as a function of the S content, with the increase of S content, the carrier concentration is almost unchanged, while the carrier mobility decreases resulting from the enhanced alloying scattering. Due to the decrease of carrier mobility, the electrical conductivity of Mn0.04Cr1.96Se3-3xS3x compounds decreases with the increasing content of S. Using the Goldsmid-Sharp formula47-49, Eg=2eαmaxTmax, we can estimate the band gap Eg of Mn0.04Cr1.96Se3-3xS3x. For Mn0.04Cr1.96Se3, the band gap is 0.21 eV. With the increasing S content, the band gap increases and the estimated band gap of Mn0.04Cr1.96Se2.7S0.3 is 0.25 eV. At room temperature, the effective mass of carriers m* increases with the increasing content of S, as shown in Figure 6(b). So the enhancement of Seebeck coefficient for the Mn0.04Cr1.96Se3-3xS3x samples are mainly attributed to the increase of the effective mass and band gap with the increase of S content indicating that may be alloying S modified the valence band in the vicinity of Fermi level. Figure 12 displays the temperature dependence of the thermal conductivity of Mn0.04Cr1.96Se3-3xS3x (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) samples. As shown in Figure 12(b), with the increase of S content, the thermal conductivity decreases due to strengthened alloy phonon scattering, and the bipolar thermal conductivity κbi decreases 17



with the increase of S content, in Figure 12(c). Thus the total thermal conductivity κ decreases with the increasing content of S, as shown in Figure 12(a), the minimum total thermal conductivity has decreased from 1.83 W.m-1.K-1 for Mn0.04Cr1.96Se3 at 700 K to 1.62 W.m-1.K-1 for Mn0.04Cr1.96Se2.7S0.3 at 723 K. Figure 12(d) displays the calculated mass fluctuation parameter and strain field fluctuation parameter as a function of S content, clearly the mass fluctuation caused by S substitution is larger than the strain field fluctuation which means mass fluctuation plays a dominant role in point defects and alloy scattering, which is responsible for the decrease of thermal conductivity of Mn0.04Cr1.96Se3-3xS3x compounds. Figure 13 shows the temperature dependence of ZT value of Mn0.04Cr1.96Se3-3xS3x (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) samples. After Mn and S co-doping, the maximum ZT value 0.33 of Mn0.04Cr1.96Se2.7S0.3 is achieved at 823 K and is increased by 50% in comparison with the pristine Cr2Se3. The increase of ZT value is mainly attributed to the suppression of bipolar thermal conductivity and the decrease of lattice thermal conductivity.

4. Conclusions In this study, we improve the thermoelectric properties of Cr2Se3-based material by the synergistic effect of optimization of carrier concentration and the decrease in the thermal conductivity. Doping the sites of Cr with Mn significantly suppresses the intrinsic excitation and reduces both the bipolar thermal conductivity and the lattice thermal 18


Page 18 of 34

Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


conductivity of Cr2Se3, the maximum ZT value of 0.28 is attained at 823 K for Mn0.04Cr1.96Se3. In order to further reduce the thermal conductivity, we have synthesized Mn and S co-doped Mn0.04Cr1.96Se3-3xS3x samples. Alloying with S significantly decreases the lattice thermal conductivity and enlarges the band gap. The maximum ZT value of Mn0.04Cr1.96Se2.7S0.3 is 0.33 at 823 K, compared with the pristine Cr2Se3 sample, the maximum ZT value was increased by 50% and the temperature corresponding to the peak value shifted toward higher temperature by 200 K. The improved thermoelectric performance indicates that Cr2Se3 is a potential thermoelectric material for intermediate-temperature application.

Associated Content Supporting Information The data collection parameters and the refined structure parameters of XRD patterns for M0.04Cr1.96Se3 (M = Nb, Ni, and Mn); calculations for Lorenz number L, the bipolar thermal conductivity κbi and fluctuation parameters Γ; the relationship between the carrier concentration and the temperature for the bipolar diffusion of M2xCr2-2xSe3 (M = Nb, Ni, and Mn；x = 0-0.02).

Author Information Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. ORCID 19



Xinfeng Tang: 0000-0001-7555-919X Xianli Su: 0000-0003-4428-6461 Notes The authors declare no competing financial interest.

Acknowledgements The authors wish to acknowledge support from the Natural Science Foundation of China (Grant No. 51521001, and 51632006), and the 111 Project of China (Grant No. B07040).

References (1) Bell, L. E. Cooling, Heating, Generating Power, and Recovering Waste Heat with Thermoelectric Systems. Science 2008, 321, 1457-1461. (2) Snyder, G. J.; Toberer, E. S. Complex Thermoelectric Materials. Nat. Mater. 2008, 7, 105-114. (3) Sootsman, J. R.; Chung, D. Y.; Kanatzidis, M. G. New and Old Concepts in Thermoelectric Materials. Angew. Chem., Int. Ed. 2009, 48, 8616-8639. (4) Goldsmid, H. J. Introduction to Thermoelectricity; Springer: Berlin, 2010. (5) Vineis, C. J.; Shakouri, A.; Majumdar, A.; Kanatzidis, M. G. Nanostructured Thermoelectrics: Big Efficiency Gains from Small Features. Adv. Mater. 2010, 22, 3970-3980. (6) Su, X. L.; Wei, P.; Li, H.; Liu, W.; Yan, Y. G.; Li, P.; Su, C. Q.; Xie, C. J.; Zhao, W. Y.; Zhai, P. C.; Zhang, Q. J.; Tang, X. F.; Uher, C. Multi-Scale Microstructural Thermoelectric Materials: Transport Behavior, Non-Equilibrium Preparation, and Applications. Adv. Mater. 2017, 29, 1602013. (7) Kanatzidis, M. G. Chapter 3 The Role of Solid-State Chemistry in the Discovery of New Thermoelectric Materials. Semicond. & Semimetals 2001, 69, 51-100. (8) Rowe, D. M. Thermoelectrics Handbook: Macro to Nano; CRC Press: Boca Raton, 1995. (9) Biswas, K.; He, J. Q.; Zhang, Q. C.; Wang, G. Y.; Uher, C.; Dravid, V. P.; Kanatzidis, M. G. Strained Endotaxial Nanostructures with High Thermoelectric Figure of Merit. Nat. Chem. 2011, 3, 160-166. (10) Xu, Z. J.; Wu, H. J.; Zhu, T. J..; Fu, C. G.; Liu, X. H.; Hu, L. P.; He, J; He, J. Q.; Zhao, X. B. Attaining High Mid-Temperature Performance in (Bi,Sb)2Te3 Thermoelectric Materials via Synergistic Optimization. NPG Asia Mater. 2016, 8, e302. 20


Page 20 of 34

Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(11) Poudeu P. F. P.; D'Angelo J.; Kong H. J.; Downey A.; Short J. L.; Pcionek R.; Hogan T. P.; Uher C.; Kanatzidis M. G. Nanostructures versus Solid Solutions: Low Lattice Thermal Conductivity and Enhanced Thermoelectric Figure of Merit in Pb9.6Sb0.2Te10-xSex Bulk Materials. J. Am. Chem. Soc. 2006, 128, 14347-14355. (12) Pei, Y. Z.; May, A. F.; Snyder, G. J. Self-Tuning the Carrier Concentration of PbTe/Ag2Te Composites with Excess Ag for High Thermoelectric Performance. Adv. Energy Mater. 2011, 1, 291-296. (13) He, J. Q.; Blum I. D.; Wang, H. Q.; Girard, S. N.; Doak, J.; Zhao, L. D.; Zheng, J. C.; Casillas, G.; Wolverton, C.; Jose-Yacaman, M.; Seidman, D. N.; Kanatzidis, M. G.; Dravid, V. P. Morphology Control of Nanostructures: Na-doped PbTe-PbS System. Nano Lett. 2012, 12, 5979-5984. (14) He, J. Q.; Girard, S. N.; Kanatzidis, M. G.; Dravid, V. P. Microstructure-Lattice Thermal Conductivity Correlation in Nanostructured PbTe0.7S0.3Thermoelectric Materials. Adv. Funct. Mater. 2010, 20, 764-772. (15) Zheng, G.; Su, X. L.; Xie, H. Y.; Shu, Y. J.; Liang, T.; She, X. Y.; Liu, W.; Yan, Y. G.; Zhang, Q. J.; Uher, C. High Thermoelectric Performance of p-BiSbTe Compounds Prepared by Ultra-Fast Thermally Induced Reaction. Energ. Environ. Sci. 2017, 10, 2638-2652. (16) Yin, K.; Su, X. L.; Yan, Y. G.; You, Y. H.; Zhang, Q.; Uher, C.; Kanatzidis, M. G.; Tang, X. F. Optimization of the Electronic Band Structure and the Lattice Thermal Conductivity of Solid Solutions According to Simple Calculations: A Canonical Example of the Mg2Si1–x–yGexSny Ternary Solid Solution. Chem. Mater. 2016, 28, 5538-5548. (17) Tan, G. J.; Liu, W.; Wang, S. Y.; Yan, Y. G.; Li, H.; Tang, X. F.; Uher, C. Rapid Preparation of CeFe4Sb12 Skutterudite by Melt Spinning: Rich Nanostructures and High Thermoelectric Performance. J. Mater. Chem. A 2013, 1, 12657-12668. (18) Zheng, Y.; Zhang, Q.; Su, X. L.; Xie, H. Y.; Shu, S. C.; Chen, T. L.; Tan, G. J.; Yan, Y. G.; Tang, X. F.; Uher, C.; Snyder, G. J. Mechanically Robust BiSbTe Alloys with Superior Thermoelectric Performance: A Case Study of Stable Hierarchical Nanostructured Thermoelectric Materials. Adv. Energ. Mater. 2015, 5, 1401391. (19) Wu, H. J.; Zhao, L. D.; Zheng, F. S.; Wu, D.; Pei, Y. L.; Tong, X.; Kanatzidis, M. G.; He, J. Q. Broad Temperature Plateau for Thermoelectric Figure of Merit ZT>2 in Phase-separated PbTe0.7S0.3. Nat. commun. 2014, 5, 4515. (20) Ma, Y.; Hao, Q.; Poudel, B.; Lan, Y. C.; Yu, B.; Wang, D. Z.; Chen, G.; Ren, Z. F. Enhanced Thermoelectric Figure-of-merit in p-Type Nanostructured Bismuth Antimony Tellurium Alloys Made from Elemental Chunks. Nano Lett. 2008, 8, 2580-2584. (21) Fan, S. F.; Zhao, J. N.; Yan, Q. Y.; Ma, J.; Hng, H. H. Influence of Nanoinclusions on Thermoelectric Properties of n-Type Bi2Te3 Nanocomposites. J. Electron. Mater. 2011, 40, 1018-1023. (22) Zhang, Y. C.; Wang, H.; Kräemer, S.; Shi, Y. F.; Zhang, F.; Snedaker, M.; Ding, K.; Moskovits, M.; Snyder, G. J.; Stucky, G. D. Surfactant-Free Synthesis of Bi2Te3-Te Micro-nano Heterostructure with Enhanced Thermoelectric Figure of Merit. Acs Nano 21



2011, 5, 3158-3165. (23) Zhao, W. Y.; Liu, Z. Y.; Wei, P.; Zhang, Q. J.; Zhu, W. T.; Su, X. L.; Tang, X. F.; Yang, J. H.; Liu, Y.; Shi, J.; Chao, Y. M.; Lin, S. Q.; Pei, Y. Z. Magnetoelectric Interaction and Transport Behaviours in Magnetic Nanocomposite Thermoelectric Materials. Nat. Nanotechnol. 2017, 12, 55-60. (24) Han, M. K.; Hoang, K.; Kong, H. J.; Pcionek, R.; Uher, C.; Paraskevopoulos, K. M.; Mahanti, S. D.; Kanatzidis, M. G. Substitution of Bi for Sb and its Role in the Thermoelectric Properties and Nanostructuring in Ag1−xPb18MTe20 (M = Bi, Sb) (x = 0, 0.14, 0.3). Chem. Mater. 2008, 20, 3512-3520. (25) Sootsman, J. R.; Kong, H.; Uher, C.; D'Angelo, J. J.; Wu, C. I.; Hogan, T. P.; Caillat, T.; Kanatzidis, M. G. Large Enhancements in the Thermoelectric Power Factor of Bulk PbTe at High Temperature by Synergistic Nanostructuring †. Angew. Chem., Int. Ed. 2008, 120, 8746–8750. (26) Pei, Y. Z.; Heinz, N. A.; LaLonde, A.; Snyder, G. J. Combination of Large Nanostructures and Complex Band Structure for High Performance Thermoelectric Lead Telluride. Energ. Environ. Sci. 2011, 4, 3640-3645. (27) He, J. Q.; Androulakis, J.; Kanatzidis, M. G.; Dravid, V. P. Seeing is Believing: Weak Phonon Scattering from Nanostructures in Alkali Metal-Doped Lead Telluride. Nano Lett. 2012, 12, 343-347. (28) Zhao, L. D.; Wu, H. J.; Hao, S. Q.; Wu, C. I.; Zhou, X. Y.; Biswas, K.; He, J. Q.; Hogan, T. P.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. All-Scale Hierarchical Thermoelectrics: MgTe in PbTe Facilitates Valence Band Convergence and Suppresses Bipolar Thermal Transport for High Performance. Energ. Environ. Sci. 2013, 6, 3346-3355. (29) Biswas, K.; He, J. Q.; Blum, I. D.; Wu, C. I.; Hogan, T. P.; Seidman, D. N.; Dravid, V. P.; Kanatzidis, M. G. High-Performance Bulk Thermoelectrics with All-Scale Hierarchical Architectures. Nature 2012, 489, 414-418. (30) Yang, S. H.; Zhu, T. J.; Sun, T.; He, J.; Zhang, S. N.; Zhao, X. B. Nanostructures in High-Performance (GeTe)x(AgSbTe2)100-x Thermoelectric Materials. Nanotechnology 2008, 19 , 245707. (31) Yin, K.; Su, X. L.; Yan, Y. G.; Uher, C.; Tang, X. F. In Situ Nanostructure Design Leading to a High Figure of Merit in an Eco-Friendly Stable Mg2Si0.30Sn0.70 Solid Solution. RSC Adv. 2016, 6, 16824-16831. (32) Xie, H. Y.; Su, X. L.; Zheng, G.; Zhu, T.; Yin, K.; Yan, Y. G.; Uher, C.; Kanatzidis, M. G.; Tang, X. F. The Role of Zn in Chalcopyrite CuFeS2: Enhanced Thermoelectric Properties of Cu1-xZnxFeS2 with In Situ Nanoprecipitates. Adv. Energy Mater. 2017, 7, 1601299. (33) Wang, S. Y.; Tan, G. J.; Xie, W. J.; Zheng, G.; Li, H.; Yang, J. H.; Tang, X. F. Enhanced Thermoelectric Properties of Bi2(Te1−xSex)3-Based Compounds as n-Type Legs for Low-Temperature Power Generation. J. Mater. Chem. 2012, 22, 20943-20951. (34) Zhang, C. H.; Ng, H. K.; Li, Z.; Khor, K. A.; Xiong, Q. H. Minority Carrier 22


Page 22 of 34

Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Blocking to Enhance the Thermoelectric Performance of Solution-Processed BixSb2-xTe3 Nanocomposites via a Liquid-Phase Sintering Process. ACS Appl. Mater. Inter. 2017, 9, 12501-12510. (35) Pei, Y. Z.; Gibbs, Z. M.; Gloskovskii, A.; Balke, B.; Zeier, W. G.; Snyder, G. J. Optimum Carrier Concentration in n-Type PbTe Thermoelectrics. Adv. Energy Mater. 2014, 4, 1400486. (36) Su, X. L.; Hao, S. Q.; Bailey, T. P.; Wang, S.; Hadar, I.; Tan, G. J.; Song, T. B.; Zhang, Q. J.; Uher, C.; Wolverton, C.; Tang, X. F.; Kanatzidis, M. G. Weak Electron Phonon Coupling and Deep Level Impurity for High Thermoelectric Performance Pb1−xGaxTe. Adv. Energy Mater. 2018, 1800659. (37) Wang, S. Y.; Yang, J.; Toll, T.; Yang, J. H.; Zhang, W. Q.; Tang, X. F. Conductivity-Limiting Bipolar Thermal Conductivity in Semiconductors. Sci. Rep. 2015, 5, 10136. (38) Adachi, Y.; Ohashi, M.; Kaneko, T.; Yuzuri, M.; Yamaguchi, Y.; Funahashi, S.; Morii, Y. Magnetic Structure of Rhombohedral Cr2Se3. J. Phys. Soc. Jpn. 1999, 63, 1548-1559. (39) Maignan, A.; Bréard, Y.; Guilmeau, E.; Gascoin, F. Transport, Thermoelectric, and Magnetic Properties of a Dense Cr2S3 Ceramic. J. Appl. Phys. 2012, 112, 013716. (40) Zhang, T. T.; Su, X. L.; Yan, Y. G.; Liu, W.; You, Y. H.; Xie, H. Y.; Yang, D. W.; Uher, C.; Tang, X. F. Structure and Thermoelectric Properties of 2D Cr2Se3−3xS3x Solid Solutions. J. Mater. Chem. C 2018, 6, 836-846. (41) Toby, B. H. EXPGUI, A Graphical User Interface for GSAS. J. Appl. Crystallogr. 2001, 34, 210–213 (42) Callaway, J. Model for Lattice Thermal Conductivity at Low Temperatures. Phys. Rev. 1959, 113, 1046-1051. (43) Callaway, J.; von Baeyer, H. C. Effect of Point Imperfections on Lattice Thermal Conductivity. Phys. Rev. 1960, 120, 1149-1154. (44) Abeles, B. Lattice Thermal Conductivity of Disordered Semiconductor Alloys at High Temperatures. Phys. Rev. 1963, 131, 1906-1911. (45) Yang, J.; Meisner, G. P.; Chen, L. Strain Field Fluctuation Effects on Lattice Thermal Conductivity of ZrNiSn-Based Thermoelectric Compounds. Appl. Phys. Lett. 2004, 85, 1140-1142. (46) Cahill, D. G.; Watson, S. K.; Pohl, R. O. Lower Limit to the Thermal Conductivity of Disordered Crystals. Phys. Rev. B 1992, 46, 6131-6140. (47) Goldsmid, H. J.; Sharp, J. W. Estimation of the Thermal Band Gap of a Semiconductor from Seebeck Measurements. J. Elec. Mater., 1999, 28, 869-872. (48) Schmitt, J.; Gibbs, Z. M.; Snyder, G. J.; Felser, C. Resolving the True Band gap of ZrNiSn half-Heusler Thermoelectric Materials. Mater. Horiz., 2015, 2, 68-75. (49) Sommet, J. Thermoelectrics, Basic Principles and New Materials Development; Springer: 2011. 23



Figure 1. XRD patterns of SPS-sintered M2xCr2-2xSe3 (M = Nb, Ni, and Mn；x = 0-0.02)： (a) Nb doped；(b) Ni doped; (c) Mn doped; (d) lattice parameters of M2xCr2-2xSe3 (M = Nb, Ni, and Mn；x = 0-0.02) as a function of the doping content.

24


Page 24 of 34

Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. BSE images of the polished surfaces of (a) Cr2Se3; (b) Nb0.04Cr1.96Se3; (c) Ni0.04Cr1.96Se3; (d) Mn0.04Cr1.96Se3.

25



Figure 3. Temperature dependence of the electrical conductivity of M2xCr2-2xSe3 (M = Nb, Ni, and Mn；x = 0-0.02): (a) Nb doped；(b) Ni doped; (c) Mn doped；(d) pure Cr2Se3 and M0.04Cr1.96Se3 (M = Nb, Ni, and Mn).

Figure 4. Temperature dependence of the Seebeck coefficient of M2xCr2-2xSe3 (M = Nb, Ni, and Mn；x = 0-0.02): (a) Nb doped；(b) Ni doped; (c) Mn doped；(d) pure Cr2Se3 and M0.04Cr1.96Se3 (M = Nb, Ni, and Mn).

26


Page 26 of 34

Page 27 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Temperature dependence of the power factor of M2xCr2-2xSe3 (M = Nb, Ni, and Mn；x = 0-0.02): (a) Nb doped；(b) Ni doped; (c) Mn doped；(d) pure Cr2Se3 and M0.04Cr1.96Se3 (M = Nb, Ni, and Mn).

Figure 6. (a) The hall carrier concentration (black line) and the carrier mobility (red line) of M2xCr2-2xSe3 (M = Nb, Ni, and Mn；x = 0-0.02) and Mn0.04Cr1.96Se3-3xS3x (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) as a function of the dopant content at room temperature; (b) the 27



Pisarenko plots (the Seebeck coefficient vs the carrier concentration) with a fixed density-of-states (DOS) effective mass m*DOS at room temperature of M2xCr2-2xSe3 (M = Nb, Ni, and Mn；x = 0-0.02) and Mn0.04Cr1.96Se3-3xS3x (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1). .

Figure 7. Temperature dependence of the total thermal conductivity of M2xCr2-2xSe3 (M = Nb, Ni, and Mn；x = 0-0.02): (a) Nb doped；(b) Ni doped; (c) Mn doped；(d) pure Cr2Se3 and M0.04Cr1.96Se3 (M = Nb, Ni, and Mn).

28


Page 28 of 34

Page 29 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. Temperature dependence of the difference between the total and electronic thermal conductivity of M2xCr2-2xSe3 (M = Nb, Ni, and Mn；x = 0-0.02): (a) Nb doped； (b) Ni doped; (c) Mn doped；(d) pure Cr2Se3 andM0.04Cr1.96Se3(M = Nb、Ni and Mn); (e) the bipolar thermal conductivity and (f) the temperature corresponding to intrinsic excitation of M2xCr2-2xSe3 (M = Nb, Ni, and Mn；x = 0-0.02) as a function of the dopant content.

29



Figure 9. Temperature dependence of the figure of merit ZT of M2xCr2-2xSe3 (M = Nb, Ni, and Mn；x = 0-0.02): (a) Nb doped；(b) Ni doped; (c) Mn doped；(d) pure Cr2Se3 and M0.04Cr1.96Se3 (M = Nb, Ni, and Mn).

30


Page 30 of 34

Page 31 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. (a) XRD patterns and (b) lattice parameters of SPS-sintered Mn0.04Cr1.96Se3-3xS3x (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1); (c) BSE images of the polished surfaces of Mn0.04Cr1.96Se2.7S0.3 and (d) elemental maps by EDS corresponding to (c).

Figure 11. Temperature dependence of electronic transport properties of 31



Page 32 of 34

Mn0.04Cr1.96Se3-3xS3x (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1): (a) the electrical conductivity; (b) the Seebeck coefficient; (c) the power factor.

Figure 12. Temperature dependence of the thermal transport

properties of

Mn0.04Cr1.96Se3-3xS3x (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1): (a) the total thermal conductivity; (b) the difference between the total and electronic thermal conductivity; (c) the bipolar thermal conductivity and (d) the calculated mass fluctuation scattering parameter ΓM and the strain field fluctuation scattering parameter ΓS as a function of the S content of Mn0.04Cr1.96Se3-3xS3x (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) at room temperature.

32


Page 33 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 13. The figure of merit ZT of Mn0.04Cr1.96Se3-3xS3x (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) as a function of temperature.

33



Table of Contents

34


Page 34 of 34

Enhanced Thermoelectric Properties of Codoped Cr2Se3: The Distinct

Recommend Documents