简介概要

Enhanced thermoelectric performance of AgBi₃S₅ by antimony doping

来源期刊：Rare Metals2020年第3期

论文作者：Xiao-Cun Liu Min Yang

文章页码：289 - 295

摘要：AgBi₃S₅ is a promising n-type thermoelectric material with low lattice thermal conductivity.In this paper,polycrystalline bulk samples of n-type Ag_1-xSb_xBi₃S₅（x=0-0.03） were prepared by high-temperature reaction and pressed by spark plasma sintering（SPS）.Electrical conductivity of AgBi3 S5 is enhanced significantly due to the increased carrier concentration.There is a remarkable enhancement of power factor from～2.1 μW·cm^-1·K^-2 for undoped AgBi₃S₅ to～3.3 μW·cm^-1·K^-2 for Ag_0.97Sb_0.03Bi₃S₅.The Sb lone pair electrons,as indicated from density functional theory（DFT） calculation results,contribute to the Fermi energy and enhance the carrier effective mass.In addition,the point defects enhance phonon scattering and decrease the lattice thermal conductivity.Owing to the enhanced power factor and reduced thermal conductivity,the thermoelectric figure of merit（ZT） at 800 K for Ag_0.97Sb_0.03Bi₃S₅ reaches 0.53,which is 70% higher than that of the pristine AgBi₃S₅.

稀有金属(英文版) 2020,39(03),289-295

Enhanced thermoelectric performance of AgBi₃S₅ by antimony doping

Xiao-Cun Liu Min Yang

School of Civil Engineering,Shandong Jiaotong University

School of Chemistry and Chemical Engineering,Qingdao University

作者简介：*Xiao-Cun Liu,e-mail:liuxiaocunde@163.com;

收稿日期：22 June 2019

基金：financially supported by the National Natural Science Foundation of China (No.21601021);Shandong Jiaotong University Start-Up Grant (No.BS2018027);Shandong Younger Scientist Foundation (No.ZR2017BEM030);

Enhanced thermoelectric performance of AgBi₃S₅ by antimony doping

Xiao-Cun Liu Min Yang

School of Civil Engineering,Shandong Jiaotong University

School of Chemistry and Chemical Engineering,Qingdao University

Abstract：

AgBi₃S₅ is a promising n-type thermoelectric material with low lattice thermal conductivity.In this paper,polycrystalline bulk samples of n-type Ag_1-xSb_xBi₃S₅(x=0-0.03) were prepared by high-temperature reaction and pressed by spark plasma sintering(SPS).Electrical conductivity of AgBi3 S5 is enhanced significantly due to the increased carrier concentration.There is a remarkable enhancement of power factor from～2.1 μW·cm^-1·K^-2 for undoped AgBi₃S₅ to～3.3 μW·cm^-1·K^-2 for Ag_0.97Sb_0.03Bi₃S₅.The Sb lone pair electrons,as indicated from density functional theory(DFT) calculation results,contribute to the Fermi energy and enhance the carrier effective mass.In addition,the point defects enhance phonon scattering and decrease the lattice thermal conductivity.Owing to the enhanced power factor and reduced thermal conductivity,the thermoelectric figure of merit(ZT) at 800 K for Ag_0.97Sb_0.03Bi₃S₅ reaches 0.53,which is 70% higher than that of the pristine AgBi₃S₅.

Keyword：

Thermoelectric; Thermal conductivity; Seebeck; Density functional theory(DFT);

Received： 22 June 2019

1 Introduction

Thermoelectric materials,which can convert between electrical energy and heat energy,have attracted great attention over the past several decades [ 1, 2, 3] .The thermoelectric performance can be evaluated by a dimensionless value,figure of merit of ZT=S²σT/(κ_latt+κ_ele),where S,σ,T,κ_latt andκ_ele are the Seebeck coefficient,electrical conductivity,temperature,lattice thermal conductivity and electrical thermal conductivity,respectively.“Phonon-glass electric-crystal”materials [ 4] are expected to be ideal candidates for thermoelectric applications.In recent years,many sulfur compounds with complex crystal structures were recognized as excellent thermoelectric materials [ 5, 6, 7, 8, 9] .For example,SnSe,which crystallizes in the orthorhombic space group Pnma at room temperature,shows a maximum ZT value of 2.6 at 923 K [ 10] .

The interest in I-V-VI (I=Cu,Ag;V=Sb,Bi;VI=S,Se,Te) ternary compounds derives from their low lattice thermal conductivities,environmentally friendly nature and high-power factors [ 11, 12, 13, 14] .P-type AgSbSe₂ features a low thermal conductivity,due to its anharmonic lattice vibrations [ 15] .Substitution of Te for Sb in tetrahedrite(Cu₁₂Sb₄S₁₃) leads to the highest ZT value of 0.92 at 723 K [ 16] .AgBiSe₂ shows interesting phase transition behaviors and is considered as a potential thermoelectric material [ 17] .Density functional theory (DFT) calculations suggest that CuBiS₂ is a promising thermoelectric material with ultra-low lattice thermal conductivity (～0.46 W·m^-1·K^-1at room temperature) [ 18] .Anharmonic lattice vibrations caused by V³⁺lone pairs play an important role in the scattering of phonons [ 19, 20] .

AgBi₃S₅ is a native n-type semiconductor [ 21, 22, 23] ,and its complex structure is shown in Fig.1.Its threedimensional structure can be viewed as two types of alternating slabs.Chlorine-doped AgBi₃S₅ has a maximum ZT value of～1.0 at 800 K [ 21] ,which indicates that aliovalent anion doping is an efficient way to enhance thermoelectric performance.Electronic structure calculations show that the unusual double rattling phonon modes of Ag and Bi atoms in AgBi₃S₅ lead to high Grüneisen parameters and low thermal conductivity.In this work,we show that the thermoelectric properties of AgBi₃S₅ can be enhanced by optimizing Sb doping.Theoretical calculations indicate that the Sb-impurity level has an important contribution to the Fermi energy.The thermoelectric properties of AgBi₃S₅ are enhanced due to the increase in the power factor and a decrease in thermal conductivity.

Fig.1 Ball-and-stick view of crystal structure for AgBi₃S₅,where Ag,Bi and S atoms are shown as black,light blue and yellow spheres,respectively

2 Experimental

All syntheses were performed in an argon-filled glovebox with the oxygen level below 1×10^-6 or under vacuum.High-purity elements of Ag (Alfa,99.99%),Bi (Aladdin,99.99%),Sb (Aladdin,99.999%) and S (Aladdin,99.99%)were weighed out and sealed into evacuated fused silica tubes.The tubes were heated in a programmable furnace up to 773 K at a rate of 30 K·h^-1 and homogenized at this temperature for about 4 h,and then slowly heated up to1273 K at a rate of 50 K·h^-1,dwelled at that temperature for 12 h.These tubes were annealed at 773 K for 3 days and finally cooled down to room temperature at a rate of100 K·h^-1.The obtained ingots were ground into fine powders using agate mortar and subsequently sintered into dense pellets using a spark plasma sintering system (SPS LABOX-325,Sinter Land) at 773 K with a holding time of3 min under a stress of 40 MPa in a vacuum.All the pellets have relative mass densities of above 95%,as estimated from the mass and volume and compared with the theoretical density.The densities of all samples were also confirmed using Archimedes'method with distilled water.

Powder X-ray diffraction (PXRD) patterns were obtained at room temperature on a Bruker AXS X-ray powder diffractometer using Cu Kαradiation.The data were recorded in a 2θmode with a step size of 0.02°.A representative powder X-ray diffraction (XRD) pattern and Rietveld refinement fit for a pristine AgBi₃S₅ are plotted in Fig.S1.PXRD results of all sintered pellets are listed in Fig.2.The micro structure and energy-dispersive spectroscopy (EDS) mapping of Ag_0.97Sb_0.03Bi₃S₅ were carried out by a Zeiss Merlin field emission scanning electron microscopy (SEM).

The S andσdata were collected in flowing argon gas using the standard four-probe method on a Netzsch SB A458 system.Thermal diffusivity was measured using the laser flash method (Netzsch,LFA 457) under the argon atmosphere.The thermal conductivity was calculated from the standard formulaκ=C_pDρ,where C_p is the specific heat,D is thermal diffusivity andρis mass density.The specific heat was estimated according to Dulong-Petit's law,and the mass density was calculated from the mass and volume.The squared samples with a thickness less than 300μm were used in Hall coefficient measurements on a Lakeshore 8400 Hall measurement system.Assuming that the main scattering mechanism in the compounds is acoustic phonon scattering,the Lorenz factor (L) was calculated by a single parabolic band (SPB) model using the following equations [ 24]

Fig.2 Powder XRD patterns of Ag_1-xSb_xBi₃S₅ (x=0,0.01,0.02,0.03) samples,in comparison with reference patterns for AgBi₃S₅(PDF card No.50-1645)

where k is the Boltzmann constant,e is the elemental electric,charge,ηis the chemical potential and F_j is the jth Fermi integral with the reduced electrochemical potential.The electrical thermal conductivity was estimated by usingκ_ele=LσT,where L,σand T are the Lorenz factor,electrical conductivity and absolute temperature,respectively.Theκ_latt was calculated by subtracting the electronic component from the total thermal conductivity.

To better understand the relationship between thermoelectric properties and electronic structures,density functional theory (DFT) calculations of pristine and Sb-doped AgBi₃S₅ were performed with the aid of Wien2K code [ 25] using the full potential linearized augmented plane wave method (FP-LAPW) [ 26] .The wave functions in the interstitial regions are expanded in plane waves up to R_MT×K_max=7,where R_MT is the smallest radius of all muffin-tin (MT) spheres and K_max is the plane wave cutoff.The valence wave functions inside the MT spheres are expanded up to l_max=10,and the charge density was Fourier expanded up to G_max=12 (au)^-1.The MT radii for Ag,Bi,Sb and Se atoms were chosen to be 2.5,2.5,2.3 and2.2 Bohr,respectively.The exchange correlation potential was calculated using the modified Becke-Johnson (mBJ) [ 27] potential jointly to the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) [ 28] with9×2×9 k-point mesh.The Brillouin zone (BZ) integration was performed using the tetrahedron method,and the self-consistent calculations were considered to have converged if the total energy and the charge of the system are stable within 1×10^-4 Ryd and 1×10^-4 e^-,respectively.For Sb-doped sample,a 1×3×1 supercell was selected and one Ag atom in the supercell was replaced by one Sb atom.The Sb doping concentration was～0.08,which was slightly higher than the actual doping concentration.

3 Results and discussion

XRD result indicates that AgBi₃S₅ is the primary phase(Fig.S1).However,there are some low-intensity peaks due to an impurity in the sample,which were identified as Bi₂S_3.Owing to the very low concentrations of Bi₂S₃present in the sample,its contribution to the thermoelectric properties is going to be ignored.As shown in Fig.2,the powder XRD results of all the pellets in this work are consistent with the AgBi₃S₅ characteristic peaks.There are no significant shifts in peak positions for Sb-doped AgBi₃S₅,possibly due to the similar covalent radii of Ag and Sb atoms.EDS elemental distributions for Ag,Sb,Bi and S are presented in Fig.3.There is no clear inhomogeneous distribution for all elements,suggesting that the doping of Sb was successful.

The measured temperature-dependent Seebeck coefficients and electrical conductivities of all samples are shown in Fig.4.The pristine AgBi₃S₅ has a low electrical conductivity of～65 S·cm^-1 at room temperature,which is higher than the previously reported result [ 21] .With the substitution of Sb³⁺in an Ag⁺site,the electrical conductivity (σ) is significantly enhanced,but the absolute value of the Seebeck coefficient (S) decreases.Both of these changes are due to the increased electron concentration with Sb³⁺compared to Ag+.The room-temperature electrical conductivity of Ag_0.97Sb_o.03Bi₃S₅ is～318 S·cm^-1,which is about four times higher than that of the pristine AgBi₃S₅.The negative Seebeck coefficient,～-135μV·K^-1,indicates that pristine AgBi₃S₅ is intrinsic n-type semiconductor.The Seebeck coefficient of pristine AgBi₃S₅ increases gradually from 300 to 700 K and then decreases with temperature increasing.Since AgBi₃S₅ is a semiconductor with a narrow band gap,the decrease in the Seebeck coefficient at high temperatures is attributed to the bipolar effect.The band gap(E_g) can be estimated roughly,using E_g=2eS_maxT_max [ 29] which gives a calculated band gap of～0.36 eV.For all Sbdoped samples,the Seebeck coefficient increases with temperature increasing.As mentioned before,the absolute values of Seebeck coefficient for Sb-doped samples are lower than those of pure AgBi₃S₅.For instance,absolute values of the Seebeck coefficient for 3%antimony doping are reduced by about 20%-50%over the entire measurement temperature range.The reduction in the Seebeck coefficient is counteracted by the enhancement in the electrical conductivity.As shown in Fig.4c,the maximum power factor (S²σ)at 800 K increases from 2.07μW·cm^-1·K^-2 for AgBi₃S₅ to3.28μW·cm^-1·K^-2 for Ag_0.97Sb_0.03Bi₃S₅.

Fig.3 a SEM image and EDS elemental mappings (b Ag,c Sb,d Bi,e S) of Ag_0.97Sb_0.03Bi₃S₅ sample

Fig.4 a Electrical conductivity,b seebeck coefficient,c power factor of Ag_1-xSb_xBi₃S₅ (x=0,0.01,0.02,0.03) samples measured from 300 to800 K (error bars indicating uncertainty in measurements)

The measured carrier concentration (n) and mobility (μ)are shown in Fig.5a.As expected,the electron concentration in Sb-doped samples is enhanced because one substitution of Ag+site by Sb³⁺in AgBi₃S₅ introduces two more free electrons.The carrier concentration of Ag_1-xSb_xBi₃S₅increases from～5.8×10¹⁸ cm^-3 for x=0to～2.8×10¹⁹ cm^-3 for x=0.03.It is noted that the measured carrier concentration is much lower than the expected carrier concentration,according to simple charge counting.For example,the theoretical electron concentration for Ag_0.97Sb_0.03Bi₃S₅ should be close to～4.7×10¹⁹ cm^-3.The difference between the measured and predicted results may be due to the incomplete ionization of Sb atoms.Interestingly,increases in both the carrier concentration and carrier mobility are observed in Ag_0.97Sb_o.03Bi₃S₅.This result indicates that the antimony dopant cannot be simply treated as an electron donor,and its participation at the Fermi energy should be not neglected.A similar phenomenon has been reported in Pb-doped BiCuSeO [ 30] in which Pb 6s states participate in the valence band maximum and enhance the hole mobility.Electronic structure analysis of Sb-doped AgBi₃S₅ was carried out and is discussed later.According to the single parabolic band model and measured results,the calculated effective mass(m^*) at room temperature for AgBi₃S₅ is～0.24 me.Figure 5b shows the measured Seebeck coefficients of Ag_1-xSb_xBi₃S₅ as a function of electron concentration.Assuming that there is no dependence between the energy and the carrier mean-free path,the relationship of the Seebeck coefficient,effective mass and carrier concentration is S～m×n^-2/3.For Sb-doped samples,the measured data perge from the calculated Pisarenko curve,indicating that the carrier effective mass in Sb-doped sample is different from that of pristine AgBi₃S₅.

Fig.5 a Measured carrier concentration and mobility of Ag_1-xSb_xBi₃S₅ (x=0,0.01,0.02,0.03) samples at room temperature.b Seebeck coefficients versus carrier concentration for Ag_1-xSb_xBi₃S₅.The Pisarenko curve is drawn using a single parabolic band model with an effective mass of 0.24 m_e

Fig.6 a Total thermal conductivity,b lattice thermal conductivity of Ag_1-xSb_xBi₃S₅ (x=0,0.01,0.02,0.03) samples in range from 300 to800 K (error bars indicating a 3%uncertainty in thermal conductivity measurements)

Figure 6 shows the temperature-dependent total thermal conductivity (κ_tot) and lattice thermal conductivity (κ_latt) of Ag_1-xSb_xBi₃S₅ (x=0,0.01,0.02,0.03).The electrical thermal conductivity (κ_ele) is estimated usingκ_ele=LσT,where L is the calculated Lorenz factor,as shown in Fig.S2.Theκ_tot of AgBi₃S₅ is～0.68 W·m^-1·K^-1 at room temperature and decreases with temperature increasing.Antimony doping increases the total thermal conductivity in the range from 300 to500 K due to the enhanced contribution of electrons.The calculated lattice thermal conductivity of Ag_1-xSb_xBi₃S₅ based on the SPB model is shown in Fig.5b.Antimony doping has a significant effect on scattering phonons and leads to a remarkable decrease inκ_latt.Taking Ag_0.97Sb_0.03Bi₃S₅ as an example,theκ_latt at room temperature is～0.65 W·m^-1·K^-1,which reduces with temperature increasing andκ_lattreaches～0.36 W·m^-1·K^-1 at 800 K.One obvious reason for the decrease inκ_latt is the enhanced point defect scattering,because of the mismatch of masses from antimony substitution.

To better understand the role of antimony in the thermoelectric properties of Ag_1-xSb_xBi₃S_5,DFT calculations were performed using Wien2k code.The calculated density of states (DOS) of AgBi₃S₅ is shown in Fig.S3.The total DOS for pristine AgBi₃S₅ shows that the band gap is～0.7 eV,which is consistent with the semiconductor character of the material.AgBi₃S₅ is an intrinsic n-type semiconductor,and thus,the conduction band minimum (CBM)plays an important role in determining the electrical transport properties.The conduction bands near the Fermi energy are mostly composed of Bi and S orbitals,while the Ag atoms also have a weak contribution to the CBM.For the Sb-doped sample,two models were selected to evaluate the site preference of the antimony.As shown in Fig.7,one Ag atom at Ag1 and Ag2 sites is replaced by one Sb atom in ModelⅠand ModelⅡ,respectively.The calculated total energy of ModelⅡis～1.7 eV lower than that of ModelⅠ,which indicates that the Sb dopant prefers to occupy the Ag2 site.Figure 8 shows the calculated DOS of the Sb and its surrounding S atoms based on ModelⅡ.Hybridization between antimony s-states and sulfur p-states is observed around-2 eV versus the Fermi energy.In addition,there are sharp Sb p-states at the Fermi energy.Weak hybridization between Sb p-states and S electron states indicates the presence of Sb 5p lone pair electrons.The delocalized 5p lone pair electrons in Sb are believed to enhance the electron mobility.

Fig.7 Ball-and-stick representation of a ModelⅠ,b ModelⅡfor Ag₁₁SbBi₃₆S_60,viewed along b axis,where Ag,Sb,Bi and S atoms are shown as gray,purple,light blue and yellow spheres,respectively

Fig.8 Calculated density of states (DOS) of an Sb atom and its surrounding S atoms (Fermi energy is selected as reference at 0 eV,as indicated by dotted line)

Fig.9 Thermoelectric figure of merit (ZT) of Ag_1-xSb_xBi₃S₅ (x=0,0.01,0.02,0.03) samples (error bars indicating 20%uncertainty)

The temperature-dependent dimensionless ZT values of Ag_1-xSb_xBi₃S₅ are shown in Fig.9.Owing to the combined effect of enhanced power factors and reduced lattice thermal conductivities,the highest ZT value of～0.53 at800 K is achieved for Ag_0.97Sb_0.03Bi₃S₅,which is 70%higher than that of an undoped sample.

4 Conclusion

In conclusion,the thermoelectric properties of Ag_1-xSb_xBi₃S₅ were studied.Antimony doping at the silver site leads to an increase in the carrier concentration,and the electrical conductivity was significantly improved.Although the Seebeck coefficient is decreased upon Sb doping,there is a significant enhancement in the power factor.The decreased lattice thermal conductivity of AgBi₃S₅ with Sb doping originates in the point defects added into the structure.The maximum thermoelectric figure of merit (ZT) at 800 K for Ag_0.97Sb_0.03Bi₃S₅ is enhanced to 0.53.