Extraordinary thermoelectric performance realized in hierarchically

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Extraordinary thermoelectric performance realized in hierarchically structured AgSbSe2 with ultralow thermal conductivity Weihong Gao, Zhenyou Wang, Jin Huang, and Zihang Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03243 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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ACS Applied Materials & Interfaces

Extraordinary thermoelectric performance realized in hierarchically structured AgSbSe2 with ultralow thermal conductivity Weihong Gao1, Zhenyou Wang1*, Jin Huang2 and Zihang Liu3*

1

School of Applied Mathematics，Guangdong University of Technology，Guangzhou，

510006, China 2

School of Materials and Energy, Guangdong University of Technology，Guangzhou,

510006, China 3

Department of Physics and TcSUH, University of Houston, Houston, Texas 77204,

USA

Keywords: thermoelectric materials, AgSbSe2, hierarchical microstructure, phonon engineering, low thermal conductivity, mechanical alloying

* To whom correspondence should be addressed. E-mail: [email protected] and [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 Thermoelectric conversion from low-grade heat to electricity is regarded as the highly reliable and environmentally friendly technology in energy harvesting area. However, how to develop efficient thermoelectric materials using a simple fabrication method is still a critical challenge in thermoelectric community. Here we first fabricate the high thermoelectric performance of Ca doped AgSbSe2 with hierarchical microstructure using a facile approach, namely mechanical alloying (for only 30 minutes) and quick hot pressing method. The hierarchical microstructure, including point defects (atomic-scale), dislocations and nanoprecipitates (nano-scale) as well as grain boundaries (microscale), strongly scatters phonons with comparable size without deterioration of carrier mobility. Due to the higher carrier concentration of nanostructured AgSbSe2 than that of coarse-grain AgSbSe2, power factor can be also improved somewhat after nanostructuring. Ca doping further optimizes the carrier concentration and creates the point-defect scattering of phonons, leading to the ultralow lattice thermal conductivity ~0.27 W m-1 K-1 at 673 K and thus largely improving peak ZT up to 1.2. The high thermoelectric performance in combination of facile fabrication method highlights AgSbSe2 based materials as robust thermoelectric candidates for energy harvesting.

1 Introduction In 2016, more than half (~66.4%) of the primary energy is ultimately rejected to environment in the United States, giving rise to various potential heat sources 1. Based on the Seebeck effect, thermoelectric devices, capable of converting the waste heat into electricity energy, have attracted tremendous attentions for energy harvesting applications in the past decade 2-3. The device’s efficiency is mainly determined by the dimensionless thermoelectric figure of merit, ZT =

S2 T , where S, ρ, κlat, ρ (κ lat + κ ele )

κele and T are the Seebeck coefficient, electrical resistivity, lattice thermal conductivity, 2


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electronic thermal conductivity and absolute temperature, respectively. For the targeted bulk thermoelectric, electrical resistivity and Seebeck coefficient are coupled or even contradictory via carrier concentration, carrier scattering mechanism and also effective mass

4-6

. General strategies to effectively enhance power factor (PF = S2σ)

mainly include carrier concentration optimization and electronic band structure engineering 7-11, but in most cases the corresponding increasing-space is limited. Since the lattice thermal conductivity is the relatively independent material-parameter, most of recent advances and progresses in pursuing high ZT lie in strengthening phonon scattering through designing microstructural defects wide range of mean free paths (MFPs)

21

12-20

. Generally, phonons with a

, spanning from atomic scale to mesoscale,

contribute to heat conduction in bulk semiconductors. Different types of defects are specifically aimed to scatter phonons with comparable size. Therefore, designing hierarchical microstructure

22-24

, including point defects

interfacial and 3-dimensional defects

25-26

, dislocations

15-17

,

12-13, 27

, is able to scatter phonons at all length

scales and thus significantly reduce the lattice thermal conductivity, sometimes even approaching the corresponding amorphous limit. Besides, it has been experimentally demonstrated that the hierarchical microstructure is also effective to further suppress the lattice thermal conductivity for those materials with intrinsically low thermal conductivity, e.g. AgSbTe2

28

, polycrystalline SnSe

29

, α-MgAgSb

18

, BiCuSeO

30

,

Cu2Se 31-32, etc. Recently, AgSbSe2 based materials have attracted considerable interests as 33-41

promising candidates for thermoelectric applications

. Pristine compound shows

the poor thermoelectric performance due to the low intrinsic hole concentration

33

.

After doping, the maximum ZT ~1 can be achieved by optimizing carrier concentration

35-41

. Thus, AgSbSe2 based materials show promising prospect for

mediate-temperature power generation due to their beneficial attributes 35-41, e.g. good thermoelectric performance, earth abundant elements and no phase transition. In addition, Guin et al. observed that formation of second phase endotaxial nanoprecipitates in AgSbSe2-ZnSe led to a 30% reduction of lattice thermal conductivity in comparison with pristine sample

36

. However, previous synthesis

3



methods are traditional long-time vacuum-melting or complex colloidal synthesis that are indeed time-wasting and energy-consuming. Therefore, how to develop new facile fabrication method towards realizing high thermoelectric performance still remains a critical challenge. Besides, Calcium, as the typical alkaline earth metal, has two valence electrons in the outermost s-orbital, which can definitely increase the hole concenrtation when doping on the Sb site. More importantly, compared with other reported alkaline earth metals, Ca (~99 pm) has the similar ionic radius with host atom Sb (~92 pm), probably resulting in a larger solid solubility and higher hole concentration. Herein, we directly adopt mechanical alloying and quick hot pressing method to synthesize Ca doped AgSbSe2 materials with high ZT ~1.2 that is the record-high value in AgSbSe2 system. It should be highlighted that the mechanical alloying process is only 30 minutes using high energy ball milling that greatly shorten both the preparation time and consumed energy. This method, to the best of our knowledge, has not been utilized in the AgSbSe2 system, which can be also applied to other A-B-X2 materials. Ca doping on Sb site largely enhances the power factor by optimizing the carrier concentration, similar with the role of other alkaline-earth metals. More significantly, the hierarchical microstructure, including point defects, dislocations, nanoprecipitates as well as microscale grain boundaries, scatters phonons of broad means free paths (or frequencies) and observably suppresses the lattice thermal conductivity, where the lowest value ~0.27 W m-1 K-1 even beats its corresponding glass limit.

2 Experimental Section Synthesis. Ag, Sb, Se and Ca from Alfa Aesar were weighed according to the targeted composition AgSb1-xCaxSe2 where x = 0, 0.02 and 0.04 and then subjected to high-energy ball milling process using SPEX 8000D (SPEX SamplePrep). For undoped AgSbSe2 sample with coarse grain in comparison, it was synthesized by conventional vacuum melting and hand milling method and the detailed information 4


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could be found in our previous report 40. The ball-milled and hand-crush powder were hot pressed at 723 K and 80 MPa for 2 min. Sample Characterization. X-ray diffraction (XRD) analysis was performed using a PANalytical multipurpose diffractometer with an X'celerator detector (PANalyticalX'Pert Pro). The phases were checked with JADE 6.0 software. The morphologies and microstructure were characterized using a Scanning Electronic Microscope (SEM, Hitachi S4700) and also a High Resolution Transmission Electron Microscope (HRTEM, JEOL 2100F) accompanied with the energy-dispersive X-ray spectroscopy (EDS). The TEM samples were prepared by conventional ion-sputtering method. Transport property measurements. Bar samples with dimension of 2×2×10 mm3 were cut from the hot-pressed disks and used for electrical transport measurement, including the electrical resistivity (ρ) and Seebeck coefficient (S), under Helium atmosphere on a commercial system (ULVAC ZEM-3). The total thermal conductivity was calculated using κtotal = DCpd, where D, Cp, and d are the thermal diffusivity measured on a laser flash system (Netzsch LFA 457), specific heat capacity measured on a differential scanning calorimetry thermal analyzer (Netzsch DSC 404 C) and sample density determined by the Archimedes method, respectively. The uncertainty for the electrical conductivity is 3%, the Seebeck coefficient 5%, the thermal conductivity 7%, so the final uncertainty for ZT value is 20%. To increase the readability of the curves, error bars were not shown in the figures. The room-temperature Hall Coefficient RH was measured using the standard four-probe method on the Physical Properties Measurement System (PPMS, Quantum Design). The Hall carrier concentration (nH) and Hall carrier mobility (µH) was obtained by

nH =

1 and µ H = RH σ , respectively, where e is the electronic charge. eRH

5



3 Results and discussion To check the phase purity for ball-milling AgSbSe2 powders, where the corresponding milling time is about 15 and 30 minutes, respectively, powder X-ray diffraction (XRD) patterns are performed and shown in the Figure 1a. Clearly, ball-milling powder for 30 minutes can be well indexed to AgSbSe2 compound (space −

group Fm 3 m ) whereas some impurity phases can be easily detected for ball-milling powder for 15 minutes

42

. This result demonstrates that 30 minutes is necessary and

enough for synthesis of AgSbSe2 based materials using high-energy ball milling method. Figure 1b presents the XRD patterns of hot-pressed AgSb1-xCaxSe2 disk samples (x = 0, 0.02 and 0.04), where no impurity phases can be observed within the detection limit of the XRD spectrometer based on the Rietveld refinement method, shown in the Figure 1c. Since the ionic radius of Ca ~99 pm is slightly larger than Sb ~92 pm, Figure 1d shows that the calculated lattice parameter first increases up to 0.02 Ca doping concentration and beyond that becomes saturated, which indicates the solid solubility of Ca on the Sb sublattice is somewhere around 0.02. Figure 2a and 2b show the fresh fracture surface morphology of AgSbSe2 samples by melting and ball-milling, respectively. It is obvious that the typical grain size of ball-milling AgSbSe2 sample (~1 µm) is much smaller than that of melting sample (~20 µm). High-density grain boundaries resulted from ball-milling process are basically consistent with other nanostructured thermoelectric materials, e.g. α-MgAgSb 43, BiSbTe 13, 44 and SnTe 45. To more delicately investigate the microstructure of nanostructured AgSbSe2 sample, high resolution transmission electron microscopy (HRTEM) images are performed. Figure 3a shows the clear and straight grain boundary, confirming the highly crystallized character for nanostructured AgSbSe2 sample. Figure 3b presents the fast Fourier transformation (FFT) image along the axis of [110], in accordance with the space group information of AgSbSe2 compound. These in-situ nanoprecipitates around 15 nm can be clearly observed within the grain, shown in the Figure 3c. The compositional analysis of both matrix and nanoprecipitates are 6


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performed using TEM-energy dispersive spectroscopy (EDS), shown in the Figure S1 and Figure S2, respectively. It is obvious that the matrix composition is close to nominal AgSbSe2 while nanoprecipitate is Ag and Sb rich somewhat. Generally, severe plastic deformation is responsible for the observed microscale composition and strain fluctuation

44, 46

. Furthermore, Figure 3d shows that a large quantity of

nanoscale stacking faults with 5-10 nm in length and three or four layers of crystallographic lattice in width can be also found in some grains. The inset figure is the corresponding FFT image that further confirms the existence of stacking fault. Those plentiful grain boundaries and interfaces from several nanometers to hundreds of nanometers with different orientation and atomic spacing phonons act as effective scattering centers for phonons with medium-to-long mean free paths (MFPs). Around stacking faults, high dense dislocations inside the grain could be easily found using inverse FFT (IFFT) analysis method, given in the Figure 3e and 3f. This morphology feature is different with that of BiSbTe and filled skutterudites fabricated by liquid-phase compaction process where dislocation arrays are embedded in the grain boundaries

15-16

. The geometric phase analysis (GPA) method is used to investigate

the concomitant strain of dislocation core in the high-quality HRTEM image, shown in the Figure 3g

47-49

. The vertical scale bar shows the maximum tensile and

compressive stress around dislocation core about 5%. Both dislocation cores and associated strain fields specifically scatter short and medium -wavelength phonons. It should be mentioned that this hierarchical microstructure in our work is distinct with the reported spontaneous nanostructure of AgSbTe2 compound (only nanodomains and associated strains) 50. The highly intricate microstructure would play a vital role in suppressing the thermal conductivity that will be thoroughly studied later. Due to the low intrinsic carrier concentration of AgSbSe2 system

33

, various

dopants have been tried to increase the carrier concentration, mainly including univalent ion (Na+) and divalent ions (Mg2+, Ba2+, Zn2+, Cd2+ and Pb2+) doping on Sb sublattice as well as tuning Sb content

35-40

. The carrier concentration dependent

carrier mobility in the AgSbSe2 system is shown in the Figure 4a, where Ca doping on Sb sublattice plays the identical role in increasing carrier concentration compared 7



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35-40

with other previous reported divalent ions

. Besides, nanostructured Ca doped

AgSbSe2 shows the similar decreased trend, indicating that microstructural defects do not obviously deteriorate the carrier mobility. It should be mentioned that strong plastic deformation process, like high energy ball milling process, generally induces the formation of intrinsic defects, which behaves as the donor-like role, and thus enhances the carrier concentration

51

. In our work, the room-temperature carrier

concentration of nanostructured undoped AgSbSe2 (1.6×1019 cm-3) is nearly 3 times higher than that of coarse-grain AgSbSe2 (0.6×1019 cm-3), resulting in the significantly lower electrical resistivity for nanostructured AgSbSe2 (Figure 4b). Besides, Ca doping further reduces the electrical resistivity to a certain extent, the lowest room-temperature value around 13.2×10-5 Ω m for AgSb0.98Ca0.02Se2. As expected, the Seebeck coefficient concomitantly decreases after Ca doping (Figure 4c) as a result of the increased carrier concentration, but their values still maintain relatively high level for all the samples, above 200 µV K-1 at the whole temperature range. In fact, the large Seebeck coefficient of AgSbSe2 system is caused by the multiple degenerate valence bands associated with flat feature

33

, namely high effective mass (m*) at the

Fermi level. Normally, based on the single parabolic band model and simultaneously assuming the acoustic phonon scattering mechanism for carriers (scattering factor is -1/2)

11, 52

, a rough estimation of the effective mass as well as the Pisarenko plots,

Seebeck coefficient dependence on carrier concentration, can be obtained by the following Eq. 1-5.

S=±

kB 2 F1 (η ) ( −η) e F0 (η )

h 2  n ⋅ rH  m = 2k BT  4π F1/2 (η ) 

(1) 2/3

∗

rH =

(2)

3 F1/2 (η ) F−1/2 (η ) 2 2 F02 (η )

Fn (η ) =

∫

∞

0

χn 1 + e χ −η

(3)

dχ

(4)

8


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η=

Ef

(5)

k BT

where kB is the Boltzmann constant, η the reduced Fermi energy, e the electron charge, Fn(η) the nth order Fermi integral and rH the Hall factor. The obtained Pisarenko plot with m* = 2.0 m0 is present in the Figure 4d, where all the doping data lay around this plot, indicating that current doping methods do not introduce the significant alternation on the band structure, like band convergence or resonant band

10, 53

. The

calculated high effective mass of AgSbSe2 system is even comparable with some typical heavy-band thermoelectric materials, like α-MgAgSb and filled skutterudites 16, 54

. Due to the much lower electrical resistivity, power factor of nanostructured

AgSbSe2 is increased in comparison with that of coarse-grain AgSbSe2 (Figure 4e). Furthermore, Ca doping further increases power factor somewhat due to the balance between the deceased electrical resistivity and also Seebeck coefficient. The achieved power factor by Ca doping is comparable with the previous reported values of AgSbSe2 system (Figure 4f)

35-40

. However, the maximum power factor of

AgSb0.98Ca0.02Se2 sample (6.4 µW cm-1 K-2) is much lower than that of advanced thermoelectric materials. Therefore, how to further significantly increase the power factor still remains a critical open question, probably modulation doping 55-56. Figure 5a shows that the temperature dependent total thermal conductivity κtotal for coarse-grain AgSbSe2 and nanostructured AgSb1-xCaxSe2 samples. Electrical thermal conductivity κele is obtained based on the Wiedemann-Franz (WF) law, κele = LσT. L, the Lorenz number, is roughly calculated using the single parabolic band model 11, 52, as shown in the equation 6:

L=(

k B 2 3F2 (η ) 2 F1 (η ) 2 )[ ) ] −( e F0 (η ) F0 (η )

(6)

Thus, κlat can be obtained by subtracting κele from κtotal, shown in the Figure 5b. The intrinsically low thermal conductivity (0.56 W m-1 K-1 at 300 K and 0.43 W m-1 K-1 at 673 K) of coarse-grain AgSbSe2 is primarily related with the strong anharmonicity of chemical bond, which can be reflected by the high Grüneisen parameter γ. This is 9



originated from the significance of lone pair electrons of Sb atom

Page 10 of 27

34, 57

. According to

the Leibfried and Schlömann expression 58-59, κlat can be depicted as following:

κ lat = B

MV 1/3θ D 3 γ 2T

(7)

where B is a numerical coefficient, M the average atomic mass, V the average atomic volume and ΘD the Debye temperature. Since κlat shows an inverse relationship with the square of Grüneisen parameter, high Grüneisen parameter of AgSbSe2 (γ ~3.4) enables the strong scattering of phonons and results in the ultralow thermal conductivity. More importantly, it should be highlighted that nanostructuring further significantly lowers the κlat of AgSbSe2, e.g. 0.43 W m-1 K-1 at 300 K and 0.34 W m-1 K-1 at 673 K. It is well acknowledged that the presence of strong anharmonic bonding is the principle underlying mechanism for the observed low κlat for A-B-X2 compounds, but the hierarchical microstructure definitely plays a vital role in further minimize the κlat of AgSbSe2 in the present work. Due to the fact that phonons with a wide range of MFPs contribute to heat conduction in bulk semiconductors 21, different types of defects are specifically aimed to scatter phonons with comparable size. To thoroughly study the role of microstructural defects on the phonon transport properties, the cumulative thermal conductivity with respect to MFP for AgSbSe2 is calculated based on the Debye-Callaway formula 60:

κ lat

k k T  = B2  B  2π vs  h 

3

∫

θ D /T

0

x 4e x dx τ C−1 (e x − 1) 2

(8)

where vs is the velocity of sound, ħ the reduced Planck constant, x the reduced frequency (x=ħω/kBT), ω the phonon angular frequency and τC the combined phonon relaxation time. Herein, θD ~143 K and vs ~2454 m/s are used for AgSbSe2 compound 34

. Herein, the τC is calculated under the consideration of both phonon-phonon

scattering and grain-boundary scattering, as following:

τ C−1 = AT ω 2 +

vs L

(9)

10


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where the coefficient A is the parameter indicating the strength of phonon-phonon scattering, coefficient L the average grain size ~20 µm for coarse-grain AgSbSe2. As shown in the Figure 6, the majority of heat energy is transported by the phonons with corresponding MFP below 1 µm, because the strong phonon-phonon interaction preferentially scatters short-wavelength phonons and the remaining phonons with medium-to-long-wavelength act as the significant component. That is why the highly intricate microstructure of nanostructured AgSbSe2, including point defects (atomic-scale), dislocations and nanoprecipitates (nano-scale) as well as grain boundaries

(microscale),

could

realize

effectively

scattering

those

medium-to-long-wavelength phonons. Due to the associated point-defect scattering, Ca doping further suppresses the κlat for nanostructured AgSb0.98Ca0.02Se2, where the lowest value even beats the glass limit κmin of AgSbSe2 (~0.42 W m-1 K-1) 34. Combing the electrical and thermal transport properties of AgSb1-xCaxSe2 samples, the corresponding ZT values is present in the Figure 7a. Thanks to the synergistic realization of the optimized electrical transport properties and the suppressed thermal conduction, the final ZT is largely improved by Ca doping. A high ZT ~1.2 at 673 K can be realized for nanostructured AgSb0.98Ca0.02Se2 sample that is the record-high value in the AgSbSe2 system (Figure 7b), which shows the promising prospect for mediate-temperature power generation.

4 Conclusion In summary, we utilize a facile and efficient method, namely the mechanical alloying process, to fabricate the nanostructured AgSbSe2 with high thermoelectric performance. Nanostructured undoped AgSbSe2 exhibits a higher power factor and significantly lower lattice thermal conductivity in comparison with that of coarse-grain counter-part, due to the increased carrier concentration and strengthened phonon scattering, respectively. More importantly, Ca doping on Sb sublattice is chosen to further increase the carrier concentration and suppress the lattice thermal conductivity. Collectively, a record-high ZT of 1.2 can be finally achieved for 11



nanostructured AgSb0.98Ca0.02Se2. The high thermoelectric performance coupled with facile fabrication process would speed up the real applications of AgSbSe2 system for energy harvesting.

Supporting information Compositional analysis result using TEM-energy dispersive spectroscopy (EDS)

Acknowledgement This work was supported by National Natural Science Foundation of China (Grant No. 51701045 and No. 51476038).

12


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Figure Caption

Figure 1.

(a) XRD patterns of ball-milling AgSbSe2 powders of 15 and 30 minutes, respectively; (b) XRD patterns of hot-pressed AgSb1-xCaxSe2 disk samples (x = 0, 0.02 and 0.04); (c) XRD patterns Rietveld refinement analysis of AgSbSe2 disk sample; (d) Lattice parameters for AgSb1-xCaxSb2 samples. The dash line indicates the Vegard's law for solid solution.

Figure 2.

(a) and (b) SEM images for fresh fracture surface morphology of melting and ball-milling AgSbSe2 samples, respectively.

Figure 3.

Typical hierarchical microstructures for AgSbSe2. (a) High magnification TEM (HRTEM) image of the grain boundary showing good crystallization; (b) The corresponding fast Fourier transformation (FFT) image along the C zone [110] axis direction; (c) HRTEM image showing the existence of nanoprecipitates; (d) HRTEM image showing the specific morphology of nanoscale stacking fault and the insert showing the corresponding FFT image; (e) HRTEM image showing large quantity of dislocations inside the grain; (f) The corresponding inverse FFT (IFFT) image; (g) Geometric phase analysis (GPA) analysis image of dislocation.

Figure 4.

Electrical

transport

properties.

(a)

Room-temperature

carrier

concentration dependent mobility for AgSbSe2 system, including Mg, Ba, Pb, Bi, Zn, Na and Cd doping as well as tuning Sb content [35-40]; (b) and (c) Electrical resistivity and Seebeck coefficient for both coarse-grain AgSbSe2 and nanostructured AgSb1-xCaxSe2 samples, respectively; (d) Room-temperature Pisarenko plot with effective mass m* = 2.0 m0 for AgSbSe2 system; (e) Power factor for coarse-grain AgSbSe2 and nanostructured AgSb1-xCaxSe2 samples; (f) Comparison of power

factor

of

nanostructured

AgSb0.98Ca0.02Se2

1


with

other

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high-performance AgSbSe2 based compounds [35-40]. Here the sample label of BM and VM mean ball milling and vacuum melting, respectively. Figure 5.

(a) and (b) Temperature dependent total thermal conductivity κtotal and lattice thermal conductivity κlat for both coarse-grain AgSbSe2 and nanostructured AgSb1-xCaxSe2 samples, respectively.

Figure 6.

Room-temperature normalized cumulative thermal conductivity as a function of phonon mean free paths for coarse-grain AgSbSe2, where green, orange and blue rectangular areas represent the phonons with short, intermediate and long mean free path, respectively.

Figure 7.

(a) Temperature dependent ZT values for coarse-grain AgSbSe2 and nanostructured AgSb1-xCaxSe2 samples; (b) Comparison of ZT of nanostructured AgSb0.98Ca0.02Se2 with other high-performance AgSbSe2 based compounds [35-40].

2



Figure 1

3


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Figure 2

4



Figure 3 5


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Figure 4

6



Figure 5

7


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Figure 6

8



1.5

(a) Coarse grain-AgSbSe2

1.2

Nanostructured AgSbSe2

Nanostructured AgSb0.98Ca0.02Se2

ZT

Nanostructured AgSb0.96Ca0.04Se2

0.9 0.6 0.3 0.0

300

400

500 600 Temperature (K)

700

1.5

(b) 1.2

Ca doping (BM) Mg doping (VM) Ba doping (VM)

0.9 ZT

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

Page 26 of 27

Bi doping (VM) Pb doping (VM) Zn doping (VM) Na doping (VM) Ca doping (VM) Tuning Sb content (VM)

0.6 0.3 0.0

300

400

500 600 Temperature (K)

Figure 7

9


700

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Table of Contents Graphic

10


Extraordinary thermoelectric performance realized in hierarchically

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