稀有金属(英文版) 2020,39(12),1374-1382

Optimizing thermoelectric properties of BiSe through Cu additive enhanced effective mass and phonon scattering

Xing-Chen Shen Xiao Zhang Bin Zhang Guo-Yu Wang Jian He Xiao-Yuan Zhou

Chongqing Key Laboratory of Soft Condensed Matter Physics and Smart Materials,College of Physics,Chongqing University

Chongqing Institute of Green and Intelligent Technology,Chinese Academy of Science

Department of Physics and Astronomy,Clemson University

Analytical and Testing Center of Chongqing University

University of Chinese Academy of Sciences

作者简介：*Xiao-Yuan Zhou,e-mail:xiaoyuan2013@cqu.edu.cn;

收稿日期：27 February 2020

基金：financially supported by the Graduate Scientific Research and Innovation Foundation of Chongqing,China (No.CYB 19064);the National Natural Science Foundation of China (Nos.51772035,11674040,51472036 and 51672270);the Fundamental Research Funds for the Central Universities (No.106112017CDJQJ308821);the Key Research Program of Frontier Sciences,CAS (No.QYZDB-SSW-SLH016);the CSC Scholarship (No.201806050180);2019 ITS Summer Fellowship,the Natural Science Foundation of Chongqing,China (No.cstc2019jcyj-msxmX0554);the Starting Research Fund from Chongqing University;

Optimizing thermoelectric properties of BiSe through Cu additive enhanced effective mass and phonon scattering

Xing-Chen Shen Xiao Zhang Bin Zhang Guo-Yu Wang Jian He Xiao-Yuan Zhou

Chongqing Key Laboratory of Soft Condensed Matter Physics and Smart Materials,College of Physics,Chongqing University

Chongqing Institute of Green and Intelligent Technology,Chinese Academy of Science

Department of Physics and Astronomy,Clemson University

Analytical and Testing Center of Chongqing University

University of Chinese Academy of Sciences

Abstract：

Known as a weak topological insulator(TI),BiSe structurally exhibits alternating stacks of quantum spin Hall bilayer(“Bi₂”) and three-dimensional TI layer(“Bl₂ Se₃”).The low lattice thermal conductivity of BiSe due to the presence of Bi₂ bilayers promises potentially good thermoelectric performance.Herein,the thermoelectric properties of nominal Bi_1-xCu_xSe samples were studied as the functions of the content of Cu additive and temperature.It is found that Cu additives in BiSe(1)profoundly affect the texture of densified polycrystalline samples by inclining the crystallographic c-axis parallel toward the pressure direction in the densification process,(2) increase considerably the effective mass and thus the Seebeck coefficient,and(3) yield point defects and Cu-Se secondary phases that effectively scatter heat-carrying phonons.As a result,the optimized electrical and thermal properties yield a thermoelectric figure of merit of zT ～0.29 in Bi_1-xCu_xSe(x=0.03) sample at 467 K in parallel to the pressure direction and a zT～0.20 at 468 K in the perpendicular direction.

Keyword：

Cu additives; Phonon scattering; Effective mass; Texture; Thermoelectric;

Received： 27 February 2020

1 Introduction

Developing novel energy materials and green energy technologies constitutes a key part of the concept of sustainable development.Accordingly,there is always an urgent need for developing higher performance thermoelectric (TE)materials,directly converting heat and electricity with no moving parts or greenhouse emissions.The material performance in thermoelectrics is gauged by the figure of merit,zT=σS²T/(k_e+k_L),whereσ,S,K_e,k_L,and T are the electrical conductivity,Seebeck coefficient,electronic thermal conductivity,lattice thermal conductivity,and absolute temperature,respectively.The termσS²,also known as the power factor (PF),gauges the electrical performance of a TE material [ 1, 2] .Owing to the adverse interdependence among the{σ,S,k_e}parameters,it is still a challenge to decouple these transport parameters in TE materials toward superior TE performance upon electronic and phonon band engineering [ 3, 4] .On the microscopic level,the quality factor (β) of material is defined asμ(m*)^3/2/K_L,whereμ,m*and k_L are carrier mobility,carrier effective mass,and lattice thermal conductivity,respectively.Theβfactor is employed to evaluate the TE potential of material and serves as a prescreen indicator for discovering novel TE materials [ 5] .In this vein,several effective approaches have been performed to increase the value ofβ,such as band convergence [ 6, 7] ,resonant level [ 8] ,nanoprecipitate [ 9] ,and defect engineering [ 10] for boosting the thermoelectric performance in thermoelectric field.

The V-VI binary compound Bi₂Se₃ with a narrow band gap of～0.3 eV has been received extensive attraction as a three-dimensional (3D) topological insulator [ 11, 12] as well as a potential TE material [ 13, 14] .Notably,Bi₂Se₃ is a member of (Bi₂Se₃)_m(Bi₂)_n family,which displays a variety of phases with varying values of m and n [ 15] .Recently,BiSe,a member of the (Bi₂Se₃)_m(Bi₂)_n family with m:n=2:1,has been unearthed as a weak topological insulator,and the low thermal conductivity makes BiSe also interesting in thermoelectrics.The topologically"trivial"Bi₂ layers are regarded as playing a surprisingly critical role in hindering phonon propagation.It is thought that the weak bonding in the Bi₂ layers results in significantly low sound velocity,and the localized low-frequency vibrations of the Bi₂ layers cause strong acoustic-optical interactions and lattice anharmonicity [ 16, 17] .The crystal structure of BiSe material is peculiar,each crystallographic unit cell consists of five layers Se-Bi-Se-Bi-Se (Bi₂Se₃)and bilayer slab Bi-Bi (Bi₂),and the ultralow lattice thermal conductivity of～0.6 W·m^-1·K^-1 at 300 K is intimately related to the Bi₂ bilayer [ 16] .

In this work,the effect of Cu additives on the TE properties of BiSe was studied.Specifically,polycrystalline bulk samples with nominal compositions Bi_1-xCu_xSe (x=0,0.01,0.03,0.05) were fabricated using a growth-frommelting-hot-press technique.The textures,electrical,and thermal properties of the nominal Bi_1-xCu_xSe samples were systematically investigated.The Cu additives tend to promote textures by aligning the crystallographic c-axis parallel to the pressure direction in the sample densification process(hereafter called the parallel direction or parallel sample,and the perpendicular direction or perpendicular sample is likewise defined) and enhance the effective mass and Seebeck coefficients over the whole temperature range.The Cu additives yield point defects and Cu-Se nanoprecipitates that effectively scatter heat-carrying phonons over a rather wide phonon frequency range,hence lowering the lattice thermal conductivity in both parallel and perpendicular direction.

2 Experimental

2.1 Synthesis

Bulk polycrystalline samples with nominal compositions Bi_1-xCu_xSe (x=0,0.01,0.03,0.05) were synthesized by growth-from-melting and hot-pressing (HP) sintering method.High-purity elemental Bi (granular 99.999%),Se(granular 99.999%),and Cu (granular 99.99%) were weighted and mixed according to the stoichiometry,sealed in quartz tubes under vacuum (～1×10^-3 Pa),heated to1123 K in 15 h,dwelt for 24 h,slowly cooled to 893 K in4 h,dwelt for another 6 h,and then furnace-cooled to room temperature.The obtained products were crushed,ground into fine powders,loaded into graphite die (12.7 mm in diameter),and hot pressed at 723 K under a uniaxial pressure of 45 MPa for 30 min into cylinder pellets.The packing density of Bi_1-xCu_xSe samples is～97%of the theoretical crystalline density.

2.2 Phase and transport properties characterizations

The room-temperature powder X-ray diffraction (PXRD)patterns of all samples and X-ray diffraction (XRD) patterns of all HP pelletized samples were collected by a Bruker D8 diffractometer equipped with a Cu Kαradiation source (λ=0.15406 nm).The morphology images of assynthesis samples were taken via the field emission scanning electron microscope (SEM,JSM-7800F,JEOL ).The microstructure and chemical elemental characterization of HP pelletized BiSe and Bi_1-xCu_xSe (x=0.03) samples were performed by transmission electron microscope(TEM,Talos F200S G2 ,Thermoscientific,Czech)equipped with an energy-dispersive spectrometer (EDS) at200 kV.

The high-temperature electrical conductivity and Seebeck coefficient were concurrently measured by an LSR-3 system under a helium atmosphere from room temperature to 623 K.The total thermal conductivity (k)was estimated from k=ρC_pD,where D is the thermal diffusivity obtained on a Netzsch LFA 457 laser flash system under a nitrogen atmosphere from room temperature to 623 K,C_p is the Dulong-Petit limit,andρis the measured density from the Archimedes method.The Hall coefficients (R_H) of HP pelletized products were carried out by a homemade apparatus from room temperature to 623 K under a reversible magnetic field of 1.0 T.

3 Results and discussion

3.1 Phase composition and microstructures

BiSe,as a member of a general series of stacks(Bi₂Se₃)_m(Bi₂)_n with m:n=2:1,consists of five-layer SeBi-Se-Bi-Se (Bi₂Se₃) and two-layer Bi-Bi (Bi₂)(Fig.1a) and crystallizes in the trigonal structure in space group 164:P-3ml with unit cell parameters of a=b=0.4180 nm,c=2.280 nm at room temperature.The room-temperature PXRD patterns of Bi_1-xCu_xSe(x=0,0.01,0.03,0.05) samples are displayed in Fig.1b,showing that the main phases of all samples are in accord with the standard BiSe pattern (PDF-29-246),and minor phases are recognized as Cu-Se secondary phases (PDF-02-487).As depicted in Fig.S1,the refined lattice constants of the Bi_1-xCu_xSe samples show a systematic decrease with increase in Cu contents,which is ascribed to the fact that the ionic radius of Cu(0.077 nm for Cu⁺or 0.073 nm for Cu²⁺) is smaller than that of Bi (0.103 nm),indicating that the Cu dopants enter the lattice in Bi sites.Generally,the smaller the difference is in ionic radius between the host and the substituted atoms,the more easily the substituted atoms replace the corresponding atomic position.For the Bi atoms with six coordinates in BiSe-based materials,the difference in ionic radius between Cu⁺(0.077 nm)and Bi³⁺(0.103 nm) is slightly smaller than that between Cu²⁺(0.073 nm) and Bi³⁺(0.103 nm) with six coordinates,suggesting that the substituted Cu atoms are prone to present+1 valence at the Bi site [ 18] .

SEM images of freshly fractured surface of Bi_1-xCu_xSe(x=0,0.03) samples are drawn in Fig.S2,suggesting that BiSe-based materials are layered structural materials,consistent with the above structural analysis.Furthermore,the high-resolution transmission electron microscopy(HRTEM) images and EDS results of Bi_1-xCu_xSe(x=0.03) sample are depicted in Fig.2 to identify the distribution of Cu in the host matrix.As shown in Fig.2a,Bi_1-xCu_xSe (x=0.03) matrix exhibits a uniform elemental(Bi,Se,Cu) distribution,corroborating that some Cu atoms enter the crystal lattice.Meanwhile,as shown in the nanoparticle region of Fig.2b,it is obvious that Cu and Se are enriched in the yellow region,whereas Bi is less distributive than Cu and Se,implying that Cu-Se nanoprecipitates form in the matrix of Bi_1-xCu_xSe (x=0.01,0.03,0.05).Combining room-temperature XRD patterns with the results of HRTEM and EDS measurements,it is clear that Cu exists as dopants in crystal lattice and Cu-Se nanoprecipitates in Bi_1-xCu_xSe samples.Notably,the point defects and Cu-Se nanoprecipitates,as phonon scattering sources,tend to enhance the phonon scattering and lower the lattice thermal conductivity in Bi_1-xCu_xSe samples,which will be discussed in the following thermal transport properties section.

Fig.1 a Room-temperature crystal structure of BiSe;b room-temperature PXRD patterns of Bi_1-xCu_xSe (x=0,0.01,0.03,0.05) samples;c room-temperature XRD patterns of HP pelletized Bi_1-xCu_xSe (x=0.00,0.01,0.03,0.05) measured on parallel (‖) samples;d room-temperature XRD patterns of HP pelletized Bi_1-xCu_xSe (x=0,0.01,0.03,0.05) measured on perpendicular (⊥) samples

Fig.2 HRTEM images and EDS color mappings for Bi (blue),Se (green),Cu (red) of a Bi_1-xCu_xSe (x=0.03) matrix and b nanoprecipitates of Bi_1-xCu_xSe (x=0.03) matrix

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Table 1 Lotgering factor (LF) in parallel (‖) and perpendicular (⊥) samples and room-temperature electrical transport properties of Bi_1-xCu_xSe(x=0,0.01,0.03,0.05) samples

Figure 1c,d plots XRD patterns of HP pelletized Bi_1-xCu_xSe (x=0,0.01,0.03,0.05) measured on the parallel (‖) and perpendicular (⊥) samples,respectively.It is clear that the (00l) diffraction intensity in the parallel (‖)sample is pronouncedly stronger than that in the perpendicular (⊥) sample,suggesting the preferred (00l) direction in the parallel (‖) sample.As expected,the (hk0) diffraction intensity in the parallel (‖) sample is significantly weaker than that in the perpendicular (⊥) sample,manifesting the preferred (hk0) direction in the parallel (‖)sample.Therefore,it is expected that the BiSe-based materials possess the anisotropic electrical and thermal transport properties.Additionally,the Lotgering factor(LF) is calculated from measured XRD data to quantify the extent of texture of HP pelletized Bi_1-xCu_xSe (x=0,0.01,0.03,0.05) samples.The LF values of the (00l) and (hk0)diffraction peaks are derived from the following equations [ 19, 20] :

where P is the ratio of the integrated intensity of (00l) or(hk0) peaks in PXRD to that of all (hkl) peaks from a HP sample,P₀ is from a randomly oriented sample as given by the standard XRD card (PDF-29-246),and I and I₀ are the peak integral intensities of the measured HP sample and randomly oriented sample,respectively.Basically,a higher LF value means a stronger preferred orientation in that direction.As shown in Table 1,with increase in Cu contents,the LF (0012) and (005) values of all HP pelletized Bi_1-xCu_xSe (x=0,0.01,0.03,0.05) in the parallel (‖)samples are rising,in contrast,the LF (110) values of all the perpendicular (⊥) samples are decreasing.The Cu additive might promote the growth of (00l) oriented grains due to the lower (00l) formation energy in the Cu additive samples.With increase in Cu content,the proportion and size of (00l) oriented grains increase.Therefore,it is observed the enhanced texturation with increase in Cu content in the samples [ 21] .As for the pristine BiSe,the lattice constant of c～2.280 nm is longer than the lattice of a=b～0.4180 nm,suggesting that the probability of the Umklapp process is higher in c direction than a and b directions.Hence,the lattice thermal conductivity along caxis is expected to be lower than that along a-axis and baxis.Therefore,the increased LF values in the parallel (‖)sample are associated with the decreased LF values in the perpendicular (⊥) sample.Accordingly,all HP pelletized Bi_1-xCu_xSe (x=0.01,0.03,0.05) samples tend to exhibit lower lattice thermal conductivities than pristine BiSe in both parallel (‖) and perpendicular (⊥) directions.

3.2 Thermal transport properties

As displayed in Fig.3a,the k of the Bi_1-xCu_xSe samples in the parallel (‖) direction is decreasing with increase in Cu content.At room temperature,the k of Bi_0.95Cu_0.05Se is0.94 W·m^-1·K^-1,exhibiting a 30%reduction compared with 1.22 W·m^-1·K^-1 of the pristine BiSe sample.The lattice thermal conductivities (k₁) of all Bi_1-xCu_xSe samples are calculated from k₁=k-k_e,where electronic thermal conductivity (k_e) is electronic thermal conductivity obtained from k_e=LσT in the Wiedemann-Franz relationship.The Lorenz number (L) can be estimated from Eq.(4) in the context of single parabolic band model [ 22, 23] :

where k_B and e are the Boltzmann constant and elementary charge,respectively.The reduced Fermi energy (η) and Fermi integral (F_n(η)) are derived from the Seebeck coefficient in Eqs.(5,6).It is assumed that scattering factor(r) is-1/2 in our calculation since the acoustic phonon scattering plays a dominant role in BiSe-based materials [ 23, 24] .

All Bi_1-xCu_xSe samples in the parallel (‖) direction present decreased k₁ (Fig.3b),which is mainly due to the higher probability of the Umklapp process and enhanced phonon scattering from the point defects and Cu-Se nanometersized precipitates.Based on the spectral lattice thermal conductivity (k_s) relation [ 25, 26] ,the phonon scattering mechanisms include Umklapp phonon scattering,point defects scattering,grain boundary,and nanoprecipitates scattering,and each plays a dominant role in phonon scattering at specific phonon frequency [ 27] .Through experimental and theoretical results,the high-frequency phonons are primarily scattered by point defects,and the mid-frequency phonons are mainly scattered by nanoprecipitates,while the low-frequency phonons are scattered by grain boundary [ 28] .Therefore,it is thought that the combined scattering sources would enhance phonon scattering in a rather wide phonon frequency regime and effectively reduce k₁ since k_s is related to k₁.In Bi_1-x Cu_xSe samples,the decreased lattice thermal conductivity is attributed to the enhanced phonon scattering from the combination effect of the Umklapp process,point defects,and Cu-Se nanoprecipitates over a wide phonon frequency range.As displayed in Fig.3c,the k_e of all samples in the parallel (‖) direction are monotonic ally rising with temperature increasing and are decreasing upon Cu contents increasing.Benefiting from the decreased k_e and the reduced k₁,the k values of all Bi_1-xCu_xSe samples exhibit a remarkable declining trend over the entire temperature range,implying that the Cu additives method is an effective approach to lower thermal conductivity in BiSe-based materials.Similarly,as shown in Fig.S3,the total and lattice thermal conductivities of all Bi_1-xCu_xSe samples in the perpendicular (⊥) direction are decreasing as expected compared with those of the pristine BiSe samples,especially,the k of Bi_0.95Cu_0.05Se is～1.30 W·m^-1·K^-1 at room temperature,presenting a 27%reduction compared with that of～1.77 W·m^-1·K^-1 in the pristine BiSe sample,further testifying the role of the intensified phonon scattering induced by Cu additives in inhibiting heat transport.The electronic thermal conductivities of all HP pelletized Bi_1-xCu_xSe (x=0,0.01,0.03,0.05) samples in the perpendicular (⊥) direction can be found in Fig.S3.

Fig.3 Temperature dependence of a total thermal conductivities,b lattice thermal conductivities,and c electronic thermal conductivities for Bi_1-xCu_xSe (x=0,0.01,0.03,0.05) samples in parallel (‖) direction

Fig.4 Temperature dependence of a electrical conductivities (σ),b carrier concentrations (n_H),c electron mobilities (μ_H),d Seebeck coefficients (S) for Bi_1-xCu_xSe (x=0,0.01,0.03,0.05) samples in parallel (‖) direction;e n_H dependence of Seebeck coefficients at distinct temperature for Bi_1-xCu_xSe (x=0,0.01,0.03,0.05) samples in parallel (‖) direction;f estimated temperature dependence of m^*for Bi_1-xCu_xSe(x=0,0.01,0.03,0.05) samples in parallel (‖) direction

3.3 Electrical transport properties

The temperature-dependent electrical conductivities of all Bi_1-xCu_xSe (x=0,0.01,0.03,0.05) samples in the parallel(‖) direction are shown in Fig.4a.The electrical conductivities (σ) of Bi_1-xCu_xSe (x=0,0.01) samples are decreasing with temperature rising,suggesting a typical degenerate transport behavior,while Bi_1-xCu_xSe (x=0.03,0.05) samples show a trend that first decrease and then increase with temperature rising,due to that the minority carriers start to play a dominant role in band conduction at high temperatures [ 29] .It is considered that Cu additives in BiSe will introduce extra holes in the matrix since the valence state of substituted Cu is+1 that is less than the valence state of Bi (+2),and thus,the Cu additives produce more holes to decrease electron carrier concentrations.The characteristic temperature of Bi_1-xCu_xSe samples,representing the onset of bipolar conduction region,is shifting to lower temperature from 589 K at x=0.01 to 442 K at x=0.05,and the decreasing characteristic temperature in Bi_1-xCu_xSe samples would help for achieving high electrical performance at low temperatures.The electrical conductivity is decreasing as Cu contents increasing from 0to 0.05,particularly,theσof Bi_1-xCu_xSe (x=0.05) is about 5.9×10⁴ S·m^-1,decreasing by about 38%compared with 9.5×10⁴ S·m^-1 of the pristine BiSe at room temperature (Table 1),due to the decreasing carrier concentration after Cu additives.For further understanding the electrical transport behaviors of all samples,the hightemperature carrier concentrations are probed in our experiment.Figure 4b shows the high-temperature carrier concentrations of all Bi_1-xCu_xSe (x=0,0.01,0.03,0.05)samples in the parallel (‖) direction from room temperature to 623 K,and the carrier concentration is reducing from2.9×10²⁰ cm^-3 of BiSe to 2.4×10²⁰ cm^-3 of Bi_0.95Cu_0.05Se at room temperature (Table 1).The high electron carrier concentrations are observed in all samples,which is attributed to the defects in BiSe-based materials.In the absence of known formation energy value for each specific type of defect in BiSe-based materials,hence,it is thought that both Se vacancies and Te_Bi antisite defects contribute to the high electron concentrations in n-type BiSe-based materials [ 30] .The observed temperature-independent carrier concentrations of the pristine BiSe sample at T<523 K confirms a degenerated conduction that is in agreement with the analysis of electrical conductivity behavior,while above 523 K,the carrier concentration rises with temperature increasing on account of the thermally excited minority carriers.For the Bi_1-xCu_xSe samples,it is clear that the carrier concentrations of all Bi_1-xCu_xSe samples are sharply rising at high temperatures,affirming that the bipolar effects play a dominant role in conduction.Compared with the pristine sample,all Bi_1-xCu_xSe samples display a sharply rising trend,uncovering the enhanced bipolar effect associated with increased minority carrier.As depicted in Fig.4c,the carrier mobilities of all Bi_1-xCu_xSe (x=0,0.01,0.03,0.05) samples in the parallel (‖) direction show a monotonously declining trend from room temperature to 623 K and roughly follow aμ T^-3/2 law at high temperatures,manifesting that the acoustic phonon scattering is dominant in BiSe-based materials [ 31] .As Cu contents increases,the carrier mobilities are decreasing compared with those of the pristine BiSe sample over the whole experimental temperature region,which is ascribed to the enhanced phonon-defect,phonon-carrier scattering in the Bi_1-xCu_xSe samples.

Fig.5 a Power factors (PF),b quality factorsβ()or Bi_1-xCu_xSe (x=0,0.01,0.03,0.05) samples and c figure of merits (zT) for Bi_1-xCu_xSe(x=0,0.01,0.03,0.05) samples in parallel (‖) direction

Figure 4d shows temperature dependence of Seebeck coefficients of all samples in the parallel (‖) direction.All samples display negative Seebeck values,indicating that the major carriers are electrons in BiSe-based materials.The absolute Seebeck coefficients of all samples first increase and then decrease at high temperatures,further affirming that the bipolar conduction behaviors are apparent at high temperatures in BiSe-based materials.At room temperature,the absolute Seebeck coefficient increases from 76μV·K^-1 for BiSe to 96μV·K^-1 for Bi_0.95Cu_0.05Se(Table 1),following the opposite trend of variation as electrical conductivity.The band gap for all samples can be estimated by using Goldsmid-Sharp relationship [ 31] ,and the calculated values for Bi_1-xCu_xSe (x=0,0.01,0.03,0.05) samples in the parallel (‖) direction are 0.09,0.11,0.13,0.13 eV,respectively.Cu additives can enlarge the band gap compared with the pristine BiSe sample,and the enhanced bipolar effect in Cu-added samples might mainly be attributed to the decreased electron concentrations because Cu⁺is an acceptor.

Additionally,to better comprehend the Seebeck coefficient behavior,a single parabolic band model (SPB) with acoustic phonon scattering is utilized to probe the temperature-dependent density of states (DOS) effective mass(m^*).It is worth noting that the two-band model should be used for BiSe-based materials with dominant bipolar effect at high temperatures;therefore,the derived transport parameters based on the single-band model should be taken with caution.Figure 4e shows the n_H dependence of Seebeck coefficients at 300 and 623 K with estimated m^*(m_e^*is the electron rest mass).The m^*increases significantly from～1.70 to～2.60 with temperature of the pristine BiSe increasing,reveling the increased density of states.Note the estimated m^*value of Bi_1-xCu_xSe samples shows a slight deviation from the Pisarenko curve at 300 K and a large deviation at 623 K,indicating the increased DOS effective mass and thus enhanced Seebeck coefficients after Cu additives.In addition,the estimated temperature dependence of m^*of all samples is shown in Fig.4f,uncovering that the increased DOS effective mass occurs at the whole experimental temperature range.The increased DOS effective mass might be attributed to the enhanced band effective mass through Cu additives,and the increment of Seebeck coefficient could be attributed to the combined effect of the increased DOS effective mass and decreased electrons carrier concentrations.For conciseness,the electrical transport properties including electrical conductivities,Seebeck coefficients,room-temperature carrier concentrations,and carrier mobilities of all HP-ed pelletized Bi_1-xCu_xSe (x=0,0.01,0.03,0.05) samples in the perpendicular (⊥) are covered in Figs.S4,S5.

3.4 Thermoelectric performance

Combining the electrical conductivity with Seebeck coefficient,the Bi_1-xCu_xSe (x=0.03) sample in the parallel (‖)direction attains the highest power factor of 0.71mW·m^-1·K^-2 at 442 K (Fig.5a).Figure 5b shows the temperature dependence of quality factors of Bi_1-xCu_xSe(x=0,0.01,0.03,0.05) samples in the parallel (‖),and the quality factors are slightly increased except for Bi_0.95Cu_0.05Se sample,pointing toward that the electricalthermal properties are optimized after Cu additives.Consequently,the highest zT value～0.29 is obtained for Bi_1-xCu_xSe (x=0.03) sample in the parallel (‖) direction at around 467 K (Fig.5c),and all the Bi_1-xCu_xSe(x=0.01,003,0.05) samples display improved zT values compared with the pristine BiSe.Besides,the highest zT value～0.20 is obtained for Bi_1-xCu_xSe (x=0.03) sample in the perpendicular (⊥) direction at around 468 K,and the power factors and figure of merits of all HP pelletized Bi_1-xCu_xSe (x=0,0.01,0.03,0.05) samples in the perpendicular (⊥) direction can be found in Fig.S6.

4 Conclusion

In summary,polycrystalline bulk samples Bi_1-xCu_xSe(x=0,0.01,0.03,0.05) were prepared by a growth-frommelting followed by the hot-pressing method.The textures,electrical,and thermal properties of the nominal Bi_1-xCu_xSe samples were systematically studied.The Cu additives tend to facilitate textures by aligning the crystallographic c-axis parallel to the pressure direction and increase the effective mass and Seebeck coefficients over the whole temperature range.Besides,the point defects and Cu-Se nanoprecipitates introduced by Cu additives intensify phonon scattering,hence lowering the lattice thermal conductivities in both parallel and perpendicular directions.Combing the optimized electrical properties and decreased thermal conductivities,the quality factorsβand zT have been improved in Bi_1-xCu_xSe samples.As a result,due to the optimized electrical properties and decreased thermal conductivities in Bi_1-xCu_xSe samples,the highest thermoelectric figure of merit of zT～0.29 for Bi_1-xCu_xSe (x=0.03) sample in the parallel direction at 467 K is attained (zT～0.20 at 468 K for Bi_1-xCu_xSe (x=0.03) sample in the perpendicular direction).This study uncovers that the Cu additives method is an effective way to enhance the thermoelectric properties

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