简介概要

Enhanced thermoelectric performance of CoSbS_0.85Se_0.15 by point defect

来源期刊：Rare Metals2018年第4期

论文作者：Shan-Shan Zhang Ding-Feng Yang Nusrat Shaheen Xing-Chen Shen Dan-Dan Xie Yan-Ci Yan Xu Lu Xiao-Yuan Zhou

文章页码：326 - 332

摘要：In this study, we report the effect of Zn doping on the thermoelectric properties of Co_1-xZn_xSbS_0.85Se_0.15solid solutions（x = 0, 0.02, 0.05, 0.08）. The results show the dimensionless figure of merit（zT） increases from 0.17 to 0.34 at 875 K for Co_0.95Zn_0.05SbS_0.85Se_0.15 sample, due to the noticeable decrease in the lattice thermal conductivity by introducing point defect, which is further confirmed by an analysis based on the Debye-CallawayKlemens model. Meanwhile, the thermoelectric power factor is maintained at high temperatures. This work highlights the important role of point defect in improving the thermoelectric performance of CoSbS-based compounds.

稀有金属(英文版) 2018,37(04),326-332

Enhanced thermoelectric performance of CoSbS_0.85Se_0.15 by point defect

Shan-Shan Zhang Ding-Feng Yang Nusrat Shaheen Xing-Chen Shen Dan-Dan Xie Yan-Ci Yan Xu Lu Xiao-Yuan Zhou

Department of Applied Physics, Chongqing University

Institute of Microstructure and Properties of Advanced Materials,Beijing University of Technology

收稿日期：10 September 2017

基金：financially supported by the National Natural Science Foundation of China (Nos. 11344010. 11404044 and 51472036);the Fundamental Research Funds for the Central Universities (No. 106112016CDJZR308808);

Enhanced thermoelectric performance of CoSbS_0.85Se_0.15 by point defect

Shan-Shan Zhang Ding-Feng Yang Nusrat Shaheen Xing-Chen Shen Dan-Dan Xie Yan-Ci Yan Xu Lu Xiao-Yuan Zhou

Department of Applied Physics, Chongqing University

Institute of Microstructure and Properties of Advanced Materials,Beijing University of Technology

Abstract：

In this study, we report the effect of Zn doping on the thermoelectric properties of Co_1-xZn_xSbS_0.85Se_0.15solid solutions(x = 0, 0.02, 0.05, 0.08). The results show the dimensionless figure of merit(zT) increases from 0.17 to 0.34 at 875 K for Co_0.95Zn_0.05SbS_0.85Se_0.15 sample, due to the noticeable decrease in the lattice thermal conductivity by introducing point defect, which is further confirmed by an analysis based on the Debye-CallawayKlemens model. Meanwhile, the thermoelectric power factor is maintained at high temperatures. This work highlights the important role of point defect in improving the thermoelectric performance of CoSbS-based compounds.

Keyword：

CoSbS; Point defect; Thermal conductivity; Thermoelectric performance;

Author： Xu Lu e-mail: luxu@cqu.edu.cn;

Received： 10 September 2017

1 Introduction

The worldwide energy crisis and environmental pollution problems lead to a pressing need for the environmentfriendly energy and new technology for improving the energy efficiency [ 1, 2] .Thermoelectric conversion technology has drawn intensive attentions in that it can easily convert thermal energy into electrical energy,holding the promise of waste heat recovery [ 3, 4] .The key factor to improve the efficiency of thermoelectric conversion is to find thermoelectric materials with high performance.The thermoelectric performance of a material is evaluated by the dimensionless figure of merit,zT=S²σT/κ,where S is the Seebeck coefficient,σis the electrical conductivity,κis the thermal conductivity (κ=κ_e+κ_L,whereκ_e andκ_Lare the carrier and lattice contributions,respectively) and T is the absolute temperature [ 5] .The strong correlation between the three transport parameters,S,σandκ_e,makes it difficult to optimize zT values by tuning one of these parameters alone without degrading the other two parameters [ 6] .For enhancing the thermoelectric power factor(S²σ),the most frequently used strategy is to tuning the carrier concentration [ 7, 8, 9] ,while other methods involving the modification of electronic band structure [ 10, 11, 12, 13] have recently been developed and proved successful for certain but not all thermoelectric compounds.The reduction in the lattice thermal conductivity,which is to a large extent independent of other transport parameters,is another conventional method for enhancing zT.In fact,the most progress in thermoelectrics in the last decade relies on pushing the lattice thermal conductivity close to the glass limit while keeping the minimum influence on the electrical transport properties [ 14, 15] .As known,the heat is transported in solids through phonons with different frequencies,and thus,there are a variety of ways to scatter different phonons.For example,boundary interfaces are used for scattering the phonons with low frequency [ 16, 17] ,dislocations for the phonons with mid-range frequency [ 18] and point defect for the phonons with high frequency [ 19, 20] .

People have discovered a number of high-performance thermoelectric materials from intermetallic compounds,such as skutterudites and Heusler compounds.Those compounds are mainly composed of the elements with small difference in electronegativity [ 21, 22] .Recently,it has been identified that the natural mineral-based intermetallic compound CoSbS is a potential high-temperature thermoelectric material with a suitable band gap of 0.5 eV.It has a rather high power factor of 1.5 mW·m^-1·K^-2 and a high thermal conductivity of 3.9 mW·m^-1·K^-1 at 773 K [ 21] .The electronic structure calculation shows that the high power factor of CoSbS is derived from its favorable characteristics near the bottom of the conduction band [ 23] .In order to fully utilize the band structure features,a series of experiments involving different doping elements have been conducted.Ni doping on Co site [ 24] can further improve the carrier concentration and the density of state effective mass,leading to the increase in power factor to2 mW·m-1·K-2 at 873 K.Te doping on Sb site [25] can simultaneously increase carrier concentration and mobility of the compound,and the resultant power factor can reach 2.7 mW·m-1·K-2 at 730 K.

However,the lattice thermal conductivity of the material is still relatively high,which greatly limits the thermal power generation efficiency of CoSbS.The reason for the high thermal conductivity of CoSbS can be attributed to the smaller Gruneisen parameter,the high Debye temperature and the strong chemical bond strength (ionic bond) [25,26],such as skutterudite.The attempt to reduce the lattice thermal conductivity was carried out by Yao et al. [26] through CoSbS1-xSex solid solutions using the traditional solid-state reaction method.It was found that the thermoelectric performance was optimized when the Se content x was 0.15,which introduced influential mass fluctuations and stress fluctuations in CoSbS compound. Nevertheless,the lattice thermal conductivity after solid solution is still much higher than the theoretical minimum value,leaving much room for the further reduction in the lattice thermal conductivity of CoSbS compounds. Based on the previous discussion,in this work,we aim to further reduce the lattice thermal conductivity on the basis of solid solution CoSbS0.85Se0.15 while maintaining the power factor,by Zn doping on Co site.By introducing more point defects into the compounds,it is expected that the point defect phonon scattering will be enhanced, leading to the reduction in lattice thermal conductivity [27,28].Our results demonstrate that the thermoelectric performance of CoSbS0.85Se0.15 could be optimized through the substitution of Zn for Co and an improved zT of 0.34 at 875 K for Co0.95Zn0.05SbS0.85Se0.15 sample is attained.

2 Experimental

Polycrystalline Co_1-xZn_xSbS_0.85Se_0.15 (x=0,0.02,0.05,0.08) samples were synthesized by traditional solid phase sintering method.Firstly,high-purity elements Co (pieces99.999%),Sb (granular 99.9999%),S (pieces 99.9999%),Se (granular 99.999%) and Zn (granular 99.9999%) were weighed in stoichiometric quantities and put into a quartz tube,which was then sealed under high vacuum of1×10^-4 Pa.Secondly,all the tubes were put into the muffle furnace,slowly heated from room temperature to1273 K for 15 h and kept at 1273 K for 20 h,and then slowly cooled down to room temperature.Afterward,the obtained reaction products were reground into powders and sintered at 1053 K for 30 h to achieve the pure phase of CoSbS.Finally,these powders were consolidated by spark plasma sintering (SPS) at 953 K for 5 min under the pressure of 70 MPa in an Ar flow atmosphere.The densities of the sample measured by the Archimedes method were about 97%of the theoretical density.

X-ray diffractometer (XRD,PANalytical X'Pert) was used to perform the phase purity analysis with Cu Kαradiation.The thermoelectric properties of the samples were measured from 295 to 873 K in helium gas atmosphere.The obtained pellets were cut into about8 mm×2.5 mm×2.5 mm rectangular bars for electrical property measurements and disks with 10 mm in diameter and about 1 mm in thickness for thermal conductivity measurement.Seebeck coefficient and electrical conductivity were measured using LSR-3 (Linseis,Germany)system under a static helium atmosphere.The thermal diffusivity (λ) was measured by a laser flash technique(Netzsch LFA457).The thermal conductivity was calculated through the formulaκ=λdC_p,where d is the density estimated by the Archimedes method and the heat capacity value of 0.34 J·g^-1·K^-1 based on the Dulong-Petit approximation was used in this study.The lattice thermal conductivity was obtained (κ_L) byκ_L=κ-κ_e,whereκ_eis estimated by Wiedemann-Franz lawκ_e=σLT,whereσis electrical conductivity,L is Lorenz constant with a number of 2.0×10^-8 V²·K^-2 [ 26] and T is the temperature.The room-temperature Hall coefficient of the Co_1-xZn_xSbS_0.85Se_0.15 (x=0,0.02,0.05,0.08) samples was obtained by a homemade Hall system with magnetic field of±1 T.Then the Hall carrier concentration (n_H)and Hall mobility (μ_H) were calculated from Hall coefficient (R_H) at room temperature based on n_H=1/eR_H and u_H=R_Hσ.The errors in the measurements of electrical conductivity,Seebeck coefficient,thermal conductivity and zT are about 5%,5%,5%and 15%,respectively.The microstructure of CoSbS_0.85Se_0.15 was investigated by a field emission scanning electron microscope (FESEM,JSM-7800F,JEOL).

3 Results and discussion

3.1 Structural characterization

CoSbS belonging to the orthorhombic family crystallizes in the space group Pbca with unit cell parameters of a=0.5842 nm,b=0.5951 nm,c=1.1666 nm,respectively [ 22] .In the structure,each S atom is closely next to a Sb atom,and the distance between them is 0.2481 nm,basically the sum of the radius of the two atoms,suggesting that they are bonded by covalent bonds to form[SbS]atomic pairs [ 29] .Similarly,the Sb atom is also coordinated with three Co atoms and one pair of Sb atoms.Each Co atom together with three Sb atoms and three S atoms collaboratively forms an octahedron.It should be noted that the coordination of the Co octahedron is irregular,which can be analyzed from the bond length and angles of the octahedron [ 24] ,as shown in Fig.1a.

Figure 1b shows the room-temperature XRD patterns of all Co_1-xZn_xSbS_0.85Se_0.15 (x=0,0.02,0.05,0.08) samples after SPS.All major reflections peaks are indexed to the CoSbS phase,while a small amount of secondary phases ZnS is found in the samples with x≧0.05,indicating that the solubility of Zn is between 2 and 5%.It is considered that that the appearance of ZnS secondary phase may influence the transport properties of the samples.Figure 1c,d shows the morphology of the fractured surface of CoSbS_0.85Se_0.15 sample.As a result,crystalline particles are packed closely,consistent with the high density of our bulk samples.

3.2 Electrical transport properties

The electrical conductivity,Seebeck coefficient,the calculated power factors,carrier concentration and mobility are shown in Fig.2a-c.Figure 2a displays the temperature dependence of the electrical conductivity of C0_1-xZn_xSbS_0.85Se_0.15 with nominal stoichiometric component (x=0,0.02,0.05,0.08).As can be seen,the electrical conductivity for all samples increases with temperature increasing due to the increasing number of activated carriers,an indicative of a typical non-degenerate semiconductor.However,the electrical conductivity decreases dramatically with respect to the increasing Zn content from room temperature to 875 K,which can be ascribed to the decrease in carrier concentration caused by Zn doping.This is further confirmed by the Hall measurement shown in Fig.2d.

Figure 2b displays the temperature dependence of the Seebeck coefficient of Co_1-xZn_xSbS_0.85Se_0.15 (x=0,0.02,0.05,0.08).For all these samples,the negative values of the Seebeck coefficients are maintained from 300 to 875 K,suggesting that the main carriers of Co_1-xZn_xSbS_0.85Se_0.15samples are electrons.At room temperature,the absolute values of Seebeck coefficient decrease from 430 to250μV·K^-1 with Zn concentration increasing.The absolute values of Seebeck coefficient of all samples monotonously decrease with temperature rising,which is typical for nondegenerate semiconductors with a low carrier concentration,indicating that Zn doping does not cause any additional electrons.However,we do observe the abnormal phenomenon that the Seebeck coefficient drops with the carrier concentration decreasing.It is speculated that the Zn doping probably creates a shallow acceptor level in the compounds,namely p-type doping,which leads to the mixing of two types of carriers in the electrical transport.For this specific case,the Seebeck coefficient is evaluated by the formula [ 30, 31, 32] :

Fig.1 a Crystal structure of CoSbS (blue,red and yellow balls are Co,Sb and S atoms,respectively);b XRD patterns of Co_1-xZn_xSbS_0.85Se_0.15(x=0,0.02,0.05,0.08);c-d SEM images of fracture surface of CoSbS_0.85Se_0.15

Fig.2 Electrical transport properties of Co_1-xZn_xSbS_0.85Se_0.15 (x=0,0.02,0.05,0.08) solid solutions:a temperature-dependent electrical conductivity,b temperature-dependent Seebeck coefficient,c temperature-dependent power factor (PF) and d relationship between carrier concentration (n)/mobility (μ) and Zn concentration

where s₁ and s₂ are the Seebeck coefficients for the electrons induced by intrinsic defect and the holes induced by Zn acceptor level,σ₁ andσ₂ are the electrical conductivities for the electrons and holes,respectively.It is then easy to understand that the decline of the Seebeck coefficient is caused by the offset effect between two types of carriers.As a result,the absolute value of the Seebeck coefficient reaches the minimum,while x equals to 0.08.The power factor of all samples is calculated and shown in Fig.2c.Although both the Seebeck coefficient and electrical conductivity decrease upon Zn doping,the power factor of Zndoped samples is well maintained at high temperature in spite of the dramatic drop at room temperature.

Figure 2d displays the variation of carrier concentration and mobility with respect to Zn doping content.The carrier concentration of all the samples at room temperature drops with respect to Zn content,except for x=0.08 samples.For instance,the carrier concentration drops from7.6×10¹⁸ cm^-3 for pristine CoSbS_0.85Se_0.15 to 0.8×10¹⁸cm^-3 for x=0.05 sample.The decline in the carrier concentration may result from the shallow acceptor level caused by Zn doping,as mentioned earlier.Then turning to the carrier mobility,as shown in Fig.2d,the carrier mobility generally drops first,which can be attributed to the enhanced point defect scattering caused by Zn doping,largely shortening the mean free path of the carriers.Then it increases with Zn concentration,probably due to that the carriers are facilitated by the ZnS second phase with high mobility.After that,the mobility drops when x=0.08since the point defect scattering for the electrons dominates again.

3.3 Thermal transport properties

The temperature-dependent thermal conductivities are depicted in Fig.3.As shown in Fig.3a,the CoSbS_0.85Se_0.15shows a relatively high thermal total conductivity in the whole temperature range and the value descends rapidly with temperature increasing.We should note that the lattice thermal conductivity of CoSbS_0.85Se_0.15 compound in this work is indeed higher than that in Ref. [ 26] ,which is ascribed to the fact they are from different batches.Meanwhile,the total thermal conductivity of the Zn-doped samples shows a significant decrease as a function of Zn content expect x=0.08 sample,in which the ZnS secondary phase also makes considerable contribution to the thermal transport.In order to investigate the intrinsic lattice thermal conductivity,the electronic contribution is subtracted from the total thermal conductivity,following the method described in the experimental procedure part.Figure 3b shows that the electronic conductivity increases with temperature increasing,which is consistent with the variation of electrical conductivity of the samples.As displayed in Fig.3c,the lattice thermal conductivity of the Zn-doped s amples is much lower than that of the pure sample over the whole measured temperature range.This can be attributed to the enhanced phonon scattering due to the increased point defect induced by Zn doping,making the lattice thermal conductivity closer to the glass limit value of 1.1 W·m^-1·K^-1given by the Cahill model [ 22, 33] :

where k_B is the Boltzmann constant and n is the number density of atoms.The sum is taken over the three acoustic modes (two transverse and one longitudinal).v_i is the phonon velocity,andΘ_i is the Debye temperature.The phonon velocities are the slope of the acoustic phonon dispersions around theΓpoint.

To get better understanding of the influence of point defect on the lattice thermal conductivity of the Co_1-xZn_xSbS_0.85Se_0.15 compounds,we are capable of reproducing the lattice thermal conductivity using the Debye-Callaway-Klemens model [ 29, 33, 34, 35] .In this model,only the Umklapp process and point defect scattering are taken into account for phonon transport.As a result,the following relationship should be satisfied [ 19, 36, 37, 38, 39, 40] .

Fig.3 Thermal transport properties of Co_1-xZn_xSbS_0.85Se_0.15 (x=0,0.02,0.05,0.08) compounds as a function of temperature:a electronic thermal conductivity,b total thermal conductivity,c lattice thermal conductivity and d zT values

whereκ_L andκ_L0 are the lattice thermal conductivity for doped samples and pure CoSbS_0.85Se_0.15,respectively,Γis the disorder parameter including the mass (AM/M) and strain (ΔS/S) components,S_x and S₀ are the lattice parameters for doped samples and pure CoSbS_0.85Se_0.15,respectively.In more detail,v (4021 m·s^-1) is the average speed of sound,θ_D (470 K) is the Debye temperature,v_p(0.235) is the Poisson ratio,γ(1.431) is the Gruneisen parameter,h is the Plank constant,Q is the volume per atom,x is the guest compound content,M is the atomic mass,ΔM is the atomic mass difference,S is the atomic radius,ΔS is the atomic radiuses difference,εis defined as the strain field factor.All the values for the parameters in the brackets are chosen from the literature [ 20] .Then,the room-temperature lattice thermal conductivity of Zn-doped samples can be reproduced based on the experimental lattice thermal conductivity of CoSbS_0.85Se_0.15,while the data for x=0.08 sample are not included due to the appearance of ZnS second phase.The results are in good agreement with the experimental ones.As listed in Table 1,the impurity scattering parameter (Γ) increases with the increase in Zn doping content,which is consistent with the experimental observation for the change in the lattice thermal conductivity with respect to Zn doping content.The contributions of mass fluctuation (Γ_M) and stress fluctuation (Γ_s) to impurity scattering parameter (Γ) are also given in Table 1.For Co_1-xZn_xSbS_0.85Se_0.15 compounds,the influence of the stress field fluctuations on the lattice thermal conductivity is greater than that of the mass field fluctuations,perhaps because of the negligibledifference in atomic mass between Zn and Co.In all,this model provides us quantitative analysis to show that point defect scattering plays a crucial role in reducing the lattice thermal conductivity of CoSbS compounds.

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Table 1 Disorder scaling parameter (u),imperfection scaling parameter (Γ),lattice thermal conductivity from experiment (κL,expt) and lattice thermal conductivity from calculation (κL,calc) of Co1-xZnxSbS0.85Se0.15 (x=0.02,0.05) compounds

Combining the electrical transport and the total thermal conductivity data,we are able to calculate the temperature dependence of the dimensionless thermoelectric figure of merit of the Co_1-xZn_xSbS_0.85Se_0.15 (x=0,0.02,0.05,0.08) samples,as shown in Fig.3d.Apparently,for all the samples,the zT values monotonously increase with the temperature increasing in the measured temperature range.Also,it is found that the zT value in the doped samples with single phase shows a significant increase with Zn content increasing due to the significantly reduced lattice thermal conductivity while maintaining the power factor at high temperature.The maximum zT value achieved in this study is 0.34 at 875 K for x=0.05 sample,which is 100%enhancement compared with that of the sample CoSbS_0.85Se_0.15.Judging from the results,Zn doping does not promote the power factor,but the resultant thermal conductivity decreases considerably,leading to the great enhancement in thermoelectric performance of CoSbS compounds.

4 Conclusion

In sum,the n-type Co_1-xZn_xSbS_0.85Se_0.15 (x=0,0.02,0.05,0.08) bulks were prepared via the conventional solidstate reaction method.It is an effective way to markedly reduce the lattice thermal conductivity in Co_1-xZn_xSbS_0.85Se_0.15 compounds by the substitution of Zn for Co.The mechanism of the point defect scattering,due to the strain field fluctuation induced by Zn doping,to decrease the lattice thermal conductivity is identified successfully by the Debye-Callaway-Klemens model.Such reduction in the thermal conductivity leads to a maximum zT up to 0.34 at 875 K in Co_0.95Zn_0.05SbS_0.85Se_0.15,100%enhancement compared to the CoSbS_0.85Se_0.15.Further enhancement of the thermoelectric performance of CoSbS compounds is plausible if appropriate doping elements can be adopted to optimizing the carrier concentration while keeping the low lattice thermal conductivity through point defect.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos.11344010,11404044 and 51472036) and the Fundamental Research Funds for the Central Universities (No.106112016CDJZR308808).