稀有金属(英文版) 2021,40(01),40-47

Enhanced thermoelectric performance of n-type TiCoSb halfHeusler by Ta doping and Hf alloying

Rui-Fang Wang Shan Li Wen-Hua Xue Chen Chen Yu-Mei Wang Xing-Jun Liu Qian Zhang

School of Materials Science and Engineering,Institute of Materials Genome and Big Data,Harbin Institute of Technology

Beijing National Laboratory for Condensed Matter Physics,Institute of Physics,Chinese Academy of Science

作者简介：Xing-Jun Liu e-mail:xjliu@hit.edu.cn;Qian Zhang e-mail:zhangqf@hit.edu.cn;

收稿日期：20 June 2020

基金：financially supported by the National Natural Science Foundation of China (Nos.51971081 and 11674078);the Natural Science Foundation for Distinguished Young Scholars of Guangdong Province of China (No.202031515020023);Shenzhen Science and Technology Innovation Plan (No. KQJSCX20180328165435202);

Enhanced thermoelectric performance of n-type TiCoSb halfHeusler by Ta doping and Hf alloying

Rui-Fang Wang Shan Li Wen-Hua Xue Chen Chen Yu-Mei Wang Xing-Jun Liu Qian Zhang

School of Materials Science and Engineering,Institute of Materials Genome and Big Data,Harbin Institute of Technology

Beijing National Laboratory for Condensed Matter Physics,Institute of Physics,Chinese Academy of Science

Abstract：

The p-type TiCoSb-based half-Heuslers are widely studied due to the good electrical transport properties after hole doping,while the pristine TiCoSb is intrinsically n-type.It is thus desired to obtain a comparable n-type counterpart through optimization of electron concentration.In this work,n-type Ti_0.9-xHf_xTa_0.1CoSb half-Heuslers were fabricated by arc melting,ball milling,and spark plasma sintering.An optimized carrier concentration,together with a decreased lattice thermal conductivity,was obtained by Ta doping at the Ti site,leading to a peak figure of merit(ZT) of 0.7 at 973 K in Ti_0.9Ta_0.1-CoSb.By further alloying Hf at the Ti site,the lattice thermal conductivity was significantly reduced without deteriorating the power factor.As a result,a peak ZT of 0.9 at 973 K and an average ZT of 0.54 in the temperature range of 300-973 K were achieved in Ti_0.6Hf_0.3Ti_0.1CoSb.This work demonstrates that n-type TiCoSb-based halfHeuslers are promising thermoelectric materials.

Keyword：

Thermoelectric; Half-Heusler; N-type; TiCoSb;

Received： 20 June 2020

1 Introduction

The overexploitation of traditional fossil fuels has caused energy crisis and serious environmental pollution problems,which pushes the exploration of high-efficiency green renewable energy sources [ 1, 2, 3, 4] .Thermoelectric(TE) materials can realize the direct conversion between thermal energy and electrical energy through the transport process of electrons and phonons within the material for the special applications in the waste heat recovery or accurate refrigeration [ 5, 6, 7] .The conversion efficiency is determined by the dimensionless figure of merit (ZT),which is defined as ZT=S2σT/k,where S,σ,T,andκare Seebeck coefficient,electrical conductivity,absolute temperature,and total thermal conductivity,respectively [ 8] .A high ZT material should possess a high Seebeck coefficient,a high electrical conductivity,and a low thermal conductivity.However,all these physical parameters are interdependent via the carrier concentration [ 9, 10, 11] .

Half-Heusler (HH) compounds have received significantly attention due to the merits of outstanding electrical properties,robust mechanical properties,and good thermal stabilities,which are very promising for power generation at higher working temperatures [ 12, 13, 14, 15, 16] .In recent years,the progress on HH materials has mainly focused on MNiSn (M=Ti,Zr,and Hf),MCoSb,and RFeSb (R=V,Nb,and Ta)-based alloys [ 17, 18, 19, 20, 21, 22, 23, 24] .High ZT values of 1.4and 1.5 have been achieved in the best n-type Hf_0.6Zr_0.4NiSn₀.99Sb_0.01+5 wt%W and p-type FeNb_0.88Hf_0.12Sb [ 13, 25] ,respectively.However,the poor TE properties of p-type MNiSn and n-type RFeSb lagged the device preparation,which expects a pair of n-and p-type legs using the same parent materials to minimize the thermal stress [ 26, 27] .Typically,p-type MCoSb compounds were mostly studied due to their good electrical properties after doping Sn at the Sb site,and a highest ZT value of 1.2 at 983 K was reported in Ti_0.25Hf_0.75-CoSb_0.85Sn_0.15 [ 18, 22, 28, 29] .But the undoped MCoSb is intrinsically n-type,and hence a comparable n-type counterpart is desired through optimization of the electron concentration.The doping effect of many n-type dopants,including V,Nb,Ta,or Mn doping at the Ti site,Ni or Pt doping at the Co site,and Te doping at the Sb site has been investigated;however,the corresponding study on TE properties at higher temperature is lacking [ 30] .Subsequently,Zhou et al. [ 31] reported a peak ZT of 0.3 at 900 K in Ti_0.92Ta_0.08CoSb with relatively high thermal conductivity.By the combination of alloying and doping,a higher peak ZT of 0.7 at 900 K was achieved in Ti_0.6Hf_0.4-Co_0.87Ni_0.13Sb [ 32] .Recently,a recorded high peak ZT exceeding 1.0 at 1073 K for Zr_0.5Hf_0.5Co_0.9Ni_0.1Sb was developed by He et al. [ 33] .Although the ZTs of n-type MCoSb have been significantly boosted,their thermoelectric performances are not competitive as compared with that of n-type MNiSn,especially average ZTs [ 25, 34] .

In this work,we report noticeably enhanced thermoelectric performance of n-type TiCoSb-based half-Heuslers via Ta doping and Hf alloying.Series of Ti_0.9-xHf_xTa_0.1-CoSb (x=0,0.1,0.2,0.3,and 0.4) samples were fabricated by arc melting,ball milling,and spark plasma sintering(SPS).Doping Ta at the Ti site effectively increases the carrier concentration,thus significantly increases the electrical conductivity and power factor,as well as suppresses the thermal conductivity.Importantly,alloying HfCoSb can further decrease the lattice thermal conductivity while maintaining high power factor.Consequently,a peak ZT of0.9 at 973 K and an average ZT of 0.54 between 300 and973 K were achieved in Ti_0.7Hf_0.2Ta_0.1CoSb.

2 Experimental

Ti_1-xTa_xCoSb (x=0.1,and 0.14) and Ti_0.9-xHf_xTa_0.1CoSb(x=0,0.1,0.2,0.3,and 0.4) samples were prepared by arc melting of stoichiometric amount of the elements (Ti,99.995%;Hf,99.9%;Co,99.97%;Ta,99.5%;Sb,99.999%) under Ar protection.The ingots were melted several times to guarantee uniformity.Excess 5 wt%Sb was added to compensate the evaporation of Sb during arcmelting process.The ingot was crushed into powders by high energy ball milling (SPEX 8000 M Mixer/Mill) for2 h in an Ar-protected environment.The obtained powders were loaded into a graphite die with an inner diameter of12.7 mm and sintered by spark plasma sintering (SPS) at1373 K for 3 min under 60 MPa.

The crystal structures of all the samples were verified by X-ray diffraction (XRD,Rigaku D/max2500 PC).The microstructures were investigated by a spherical aberration-corrected (C_s-corrected) electron microscope (JEM-ARM200F).

Thermoelectric properties were measured from room temperature to 873 K.The thermal conductivity (κ) was calculated byκ=DαC_p,where D is the room-temperature mass density obtained via Archimedes method,αis thermal diffusivity and C_p is the specific heat which was measured on a differential scanning calorimetry thermal analyzer(DSC 404 F3,Netzsch).The densities of the Ti_0.9-xHf_xTa_0.1CoSb samples are 7.8,8.1,8.5,8.8,and 9.1 g·cm^-3for x=0,0.1,0.2,0.3,and 0.4,respectively.The thermal diffusivity (x) was measured using a laser flash apparatus(LFA 457,Netzsch).Then the samples were cut into a bar with the dimension of～2 mm×2 mm×10 mm for the simultaneously measurements of Seebeck coefficient(S) and electrical conductivity (σ) on a commercial apparatus (CTA-3,Cryoall).The room-temperature Hall coefficients (R_H) were measured using the van der Pauw technique under a reversible magnetic field of 1.5 T.The Hall carrier concentration (n_H) and Hall mobility (μH) were calculated using n_H=1/(eR_H)(e is electron charge) andμ_H=σR_H,respectively.The uncertainties for the electrical conductivity,Seebeck coefficient,and thermal conductivity were 3%,5%,and 7%,respectively,leading to a 12%uncertainty for the ZT value.

3 Results and discussion

We optimized the carrier concentration of TiCoSb by doping Ta at the Ti site based on the previous report [ 31] .As shown in Fig.1,the best composition is Ti_0.9Ta_0.1CoSb(we will discuss it later).The power factor (PF) of Ti_0.84Ta_0.14CoSb is lower than that of Ti_0.9Ta_0.1CoSb probably because the solubility limit of Ta in TiCoSb is lower than x=0.14.Therefore,Ti_0.9Ta_0.1CoSb was chosen for the further improvement by suppressing the lattice thermal conductivity via alloying Hf at the Ti site.XRD patterns of Ti_0.9-xHf_xTa_0.1CoSb (x=0,0.1,0.2,0.3,and0.4) bulk samples are shown in Fig.2a.All of the main diffraction peaks are well-indexed to the cubic half-Heusler structure with the space group F 43m without any impurities.It is noteworthy that the (200) diffraction peak becomes stronger,while the (111) peak becomes weaker with Hf content increasing,and such a change in the peak intensity is also observed in p-type double half-Heusler alloys Ti_2-yHf_yFeNiSb_2-xSn_x [ 35] .Additionally,the peak positions shift to lower angles with the increase in Hf content because of the substitution of the smaller Ti atoms(r=0.140 nm) by the larger Hf atoms (r=0.155 nm).Figure 2b shows the calculated lattice parameters of Ti_0.9-xHf_xTa_0.1CoSb based on XRD peaks,where the lattice parameter increases linearly from 0.5893 nm for Ti_0.9Ta_0.1CoSb to 0.5958 nm for Ti_0.5Hf_0.4Ta_0.1CoSb,further indicating the complete incorporation of Hf in the halfHeusler matrix.

Fig.1 Temperature-dependent a electrical conductivity,seebeck coefficient (inset),and b power factor for Ti_1-xTa_xCoSb (x=0.08 [31] ,0.10,and 0.14)

Fig.2 a XRD patterns and b lattice parameters of Ti_0.9-xHf_xTa_0.1CoSb (x=0,0.1,0.2,0.3,and 0.4) bulk samples

Figure 3a-c shows the temperature-dependent electrical conductivity,Seebeck coefficient,and power factor of Ti_0.9-xHf_xTa_0.1CoSb (x=0,0.1,0.2,0.3,and 0.4) samples,respectively.As shown in Fig.3a,the electrical conductivity of undoped TiCoSb is very low and increases with increase in temperature,exhibiting a typical semiconducting behavior [ 31] .When doping 10 at%Ta at the Ti site,the electrical conductivity is significantly improved and shows a metallic behavior.For example,the room-temperature electrical conductivity is 2×10³ S·m^-1 for the undoped TiCoSb [ 31] ,and it increases to 1.15×10⁵ S·m^-1for Ti_0.9Ta_0.1CoSb.Such an enhancement in electrical conductivity should be attributed to the increased Hall carrier concentration because Ta provides one more electron as a donor when it substitutes for Ti.As shown in Table 1,the carrier concentration is 1.5×10²¹ cm^-3 for Ti_0.9Ta_0.1CoSb,and that is only 4.5×10¹⁸ cm^-3 for undoped TiCoSb [ 31] .The carrier concentration decreases after alloying Hf content up to x=0.3 probably due to the smaller electronegativity of Hf than that of Ti,while the unusual increase in carrier concentration for x=0.4 sample may be related to the large numbers of disorders [ 36] .Together with the reduced carrier mobility,the roomtemperature electrical conductivity decreases with Hf content increasing.The negative signs of the Seebeck coefficients for all the samples indicate n-type transport behavior,as shown in Fig.3b.The Seebeck coefficient of the undoped TiCoSb first increases with increase in temperature and then decreases when T>600 K due to the bipolar effect of minority carriers'activation [ 31] .In contrast,the bipolar effect in Ti_0.9-xHf_xTa_0.1CoSb is suppressed over the entire temperature range due to the increased Hall carrier concentration.

Fig.3 Temperature-dependent a electrical conductivity,b Seebeck coefficient,and c power factor for Ti_0.9xHf_xTa_0.1CoSb (x=0,0.1,0.2,0.3,and 0.4)(data for reported TiCoSb [31] ,Ti_0.92Ta_0.08CoSb [31] ,Ti_0.6Hf_0.4Co_0.87Ni_0.13Sb [32] ,TiCo_0.95Ni_0.05Sb [37] ,and Ti_0.5Zr_0.25Hf_0.25Co_0.95-Ni_0.05Sb [38] plotted for comparison);d room-temperature power factor as a function of Hall carrier concentration for Ti0.9-xHf_xTa_0.1CoSb(x=0,0.1,0.2,and 0.3).(data for reported Ti₁-xTa_xCoSb [31] ,TiCo₁-xNi_xSb [37] ,and Ti_0.5Zr_0.25Hf_0.25Co_1-xNi_xSb [38] included for comparison)(solid curve calculated by single parabolic band (SPB) model)

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Table 1 Room-temperature electrical conductivity (σ),Seebeck coefficient (S),Hall carrier concentration (nH) and carrier mobility(μH) for Ti0.9-xHfxTa0.1CoSb (x=0,0.1,0.2,0.3,and 0.4)

Benefiting from the enhanced electrical conductivity and optimized Seebeck coefficient,the power factor of Ti_0.9g-xHf_xTa_0.1CoSb is substantially improved as compared with the undoped TiCoSb,as shown in Fig.3c.With the change of Hf content,the power factor has only slight fluctuation with the exception of Ti_0.5Hf_0.4Ta_0.1CoSb.The room-temperature power factor is 20μW·cm^-1·K^-2 for Ti_0.8Hf_0.1Ta_0.1CoSb and its peak power factor reaches as high as 28μW·cm^-1·K^-2.Such a high power factor is much higher than those of reported TiCo_0.95Ni_0.05Sb [ 37] ,Ti₀.92Ta_0.08CoSb [ 31] ,Ti_0.5Zr_0.25Hf_0.25Co_0.95Ni_0.05Sb [ 38] ,and Ti_0.6Hf_0.4Co_0.87Ni_0.13Sb [ 32] over the whole temperature range of 300-973 K.To better illustrate the origin of the higher factor power,Hall carrier concentration-dependent power factor is plotted in Fig.3d.The solid line is calculated based on the single parabolic band (SPB) model coupled with acoustic phonon scattering using density of states (DOS) effective mass of m_d*=9.1me (me is free elctron mass).With carrier concentration increasing,the power factor increases.Clearly,the noticeably high power factors of the Ti_0.9-xHf_xTa_0.1CoSb samples should be mainly ascribed to the optimization of the carrier concentration.An optimized carrier concentration is successfully obtained by 10 at%Ta doping in this study.Hf alloying does not change the carrier concentration and the carrier mobility that much.However,the reported power factors of Ti_0.5Zr0.25Hf_0.25Co_1-xNi_xSb samples are much lower than the theoretical prediction at the corresponding carrier concentration,which results from the low Hall carrier mobility induced by much more disorders with Zr and Hf double alloying at Ti site.For example,the room-temperature carrier mobility is 0.52 cm²·V^-1·s^-1 for T_i0.5Zr_0.25Hf_0.25Co_0.95Ni_0.05Sb [ 38] ,much lower than that of our samples,as shown in Table 1.

Fig.4 Temperature-dependent a thermal conductivity and b lattice thermal conductivity for Ti_0.9-xHf_xTa_0.1CoSb (x=0,0.1,0.2,0.3,and 0.4);inset in a showing temperature-dependent specific heat (C_p)(data for reported TiCoSb plotted for comparison [31] )

Fig.5 Typical microstructures of Ti_0.6Hf0.3Ta_0.1CoSb:a,b Low-magnification TEM images,c SAED pattern of Region I viewed along[111]zone axis;d high-resolution TEM image of white boxed region in b (insets being corresponding FFTs for Region I and II,respectively)

Figure 4a shows the temperature-dependent thermal conductivity and specific heat (inset) of the Ti_0.9-xHfxTa_0.1CoSb (x=0,0.1,0.2,0.3,and 0.4) samples.The specific heat of all the samples increases roughly with increase in temperature,and it decreases as the Hf content increases since Hf has a higher atomic mass than Ti.The thermal conductivity decreases with increase in temperature.Compared with the undoped TiCoSb,the thermal conductivity shows a noticeable reduction after Ta doping.The room-temperature thermal conductivity of undoped TiCoSb is 18.8 W·m^-1·K^-1,and it is only 5.9 W·m^-1·K^-1for Ti_0.9Ta_0.1CoSb,showing a reduction of 69%.To better understand the phonon scattering effect,we estimated the lattice thermal conductivity (κL) by subtracting the electronic thermal conductivity (κ_e) from the total thermal conductivity,K_L=κ-κ_e,and the electronic thermal conductivity was obtained using the Wiedemann-Franz lawκe=LσT,where L is the Lorenz number calculated by a single parabolic band (SPB) model with measured Seebeck coefficient [ 39] .As shown in Fig.4b,the roomtemperature lattice thermal conductivity is 18.7 W·m^-1-K^-1 for the pristine TiCoSb [ 31] ,and it is only5.3 W·m^-1·K^-1 for Ti_0.9Ta_0.1CoSb,showing a reduction of72%.The reduction in lattice thermal conductivity should be mainly attributed to the enhanced point defect scattering induced by Ta doping at the Sn site.In addition to Ta doping,the lattice thermal conductivity was further reduced by alloying Ti_0.9Ta_0.1CoSb with different contents of HfCoSb.The room-temperature lattice thermal conductivity is 3.4 W·m^-1·K^-1 for Ti_0.6Hf_0.3Ta_0.1CoSb sample,showing a 36%reduction compared with that of Ti_0.9Ta_0.1CoSb.The lowest lattice thermal conductivity is2 W·m^-1.K^-1 at 973 K for Ti_0.6Hf_0.3Ta_0.1CoSb.It should be noted that the lattice thermal conductivity has a slight change with respect to Hf content,especially at evaluated temperature.Similar phenomenon was also observed in Ti1-xZrxCoSb alloys [ 32] .

Fig.6 a Temperature-dependent ZT values of Ti_0.9-xHf_xTa_0.1CoSb (x=0,0.1,0.2,0.3,and 0.4)(data for reported TiCoSb plotted for comparison (dashed line) [31] );b average figure of merit (ZT_avg) in temperature range between 300 and 973 K for Ti_0.9-xHf_xTa_0.1CoSb;comparison of c ZT and d ZT_avg (300-973 K) between Ti_0.6Hf_0.3Ta_0.1CoSb and other reported n-type TiCoSb-and ZrCoSb-based half-Heusler compounds [31-34,38]

To further study the microstructural features of the nanostructured Ti_0.6Hf_0.3Ta_0.1CoSb,we conducted a detailed microstructural investigation using high-resolution transmission electron microscopy (HRTEM).The lowmagnification TEM images in Fig.5a,b show that the sample is highly dense with an average grain size of500 nm.Figure 5c shows the selected area electron diffraction (SEAD) pattern of Region I in Fig.5b viewed along[111]direction,which demonstrates F 43m cubic structure.In Fig.5d,the high-resolution TEM reveals the existence of lattice distortion at interface between Grains I andⅢ,which could significantly enhance the phonon scattering and contribute to the lower lattice thermal conductivity in Ti_0.6Hf_0.3Ta_0.1CoSb sample.

Figure 6a shows ZT values of n-type Ti_0.9-xHf_xTa_0.1-CoSb (x=0,0.1,0.2,0.3,and 0.4).Owing to the simultaneously increased power factor and reduced thermal conductivity,the ZT value of TiCoSb is significantly improved by Ta doping.A peak ZT of 0.7 is achieved in Ti_0.9Ta_0.1CoSb at 973 K.By further alloying Ti_0.9Ta_0.1CoSb with HfCoSb,the thermal conductivity is further reduced,thus enhancing the thermoelectric performance across a broad temperature range.As a result,a maximum ZT of 0.9 at 973 K is obtained in Ti_0.6Hf_0.3Ta_0.1CoSb.We farther calculated the average figure of merit (ZT_avg)between 300 and 973 K using the integration method,as shown in Fig.6b.Ti_0.6Hf_0.3Ta_0.1CoSb possesses the highest average ZT of 0.54,about 42%higher than that of Ti_0.9Ta_0.1CoSb.To demonstrate the high thermoelectric performance of TiCoSb by Ta doping and Hf alloying,a comparison between the ZTs of Ti_0.6Hf_0.3Ta_0.1CoSb and other n-type TiCoSb-and ZrCoSb-based half-Heuslers is presented in Fig.6c.It is clear that Ti_0.6Hf_0.3Ta_0.1CoSb outperforms all other n-type half-Heuslers within the studied temperature range.Specifically,the average ZTs(300-973 K) are 0.17 for Ti_0.92Ta_0.08CoSb [ 31] ,0.32 for Ti_0.Zr_0.25Hf_0.25Co0.95Ni_0.05Sb [ 38] ,0.39 for Ti_0.6Hf_0.4-Co_o.87Ni_o.13Sb [ 32] ,0.40 for (Zr_0.4Hf_0.6)_0.88Nb_0.12CoSb [ 34] ,and 0.42 for Zr_0.5Hf_0.5Co_0.9Ni_0.1Sb [ 33] ,all of which are lower than～0.54 for Ti_0.6Hf_0.3Ta_0.1CoSb (Fig.6d).

4 Conclusion

In summary,we have successfully synthesized single phase n-type TiCoSb-based half-Heusler alloys by arc melting,ball milling,and SPS.The effect of Ta doping and Hf alloying on the thermoelectric properties of Ti_0.9-xHf_xTa_0.1CoSb has been investigated.Firstly,Ta doping optimizes the Hall carrier concentration from 4.5×10¹⁸ to 1.5×10²¹ cm^-3 and hence improves the electrical conductivity and power factor.Together with the reduced thermal conductivity,a peak ZT of 0.7 at 973 K is obtained in Ta_0.9Ta_0.1CoSb.Secondly,it is found that alloying with HfCoSb could further suppress the lattice thermal conductivity because of the enhanced alloying scattering without deteriorating the power factor.As a result,a peak ZT of 0.9 at 973 K and an average ZT of 0.54 between 300and 973 K are achieved in Ti_0.6Hf_0.3Ta_0.1CoSb,which is the highest value for the reported n-type TiCoSb-based half-Heuslers.

参考文献

[1] Zhu TJ,Fu CG,Xie HH,Liu YT,Zhao XB.High efficiency half-Heusler thermoelectric materials for energy harvesting.Adv Energy Mater.2015;5(19):1500588.

[2] Zhao LD,Kanatzidis MG.An overview of advanced thermoelectric materials.J Materiomics.2016;2(2):101.

[3] Bell LE.Cooling,heating,generating power,and recovering waste heat with thermoelectric systems.Science.2008;321(5895):1457.

[4] Liu WS,Hu JZ,Zhang SM,Deng MJ,Han CG,Liu Y.New trends,strategies and opportunities in thermoelectric materials:a perspective.Mater Today Phys.2017;1:50.

[5] DiSalvo FJ.Thermoelectric cooling and power generation.Science.1999;285(5428):703.

[6] Snyder GJ,Toberer ES.Complex thermoelectric materials.Nat Mater.2008;7(2):105.

[7] Mao J,Liu ZH,Zhou JW,Zhu HT,Zhang Q,Chen G,Ren ZF.Advances in thermoelectrics.Adv Phys.2018;67(2):69.

[8] Rowe DM.CRC Handbook of Thermoelectrics.Boca Raton:CRC-Press;1995.2.

[9] Zhu TJ,Liu YT,Fu CG,Heremans JP,Snyder JG,Zhao XB.Compromise and synergy in high-efficiency thermoelectric materials.Adv Mater.2017;29(14):1605884.

[10] Zhang Q,Song QC,Wang XY,Sun JY,Zhu Q,Dahal K,Lin X,Cao F,Zhou JW,Chen S,Chen G,Mao J,Ren ZF.Deep defect level engineering:a strategy of optimizing the carrier concentration for high thermoelectric performance.Energy Environ Sci.2018;11(4):933.

[11] Zhang ZW,Wang XY,Liu YJ,Chen C,Yao HH,Yin L,Li XF,Li S,Zhang F,Bai FX,Sui JH,Yu B,Cao F,Liu XJ,Mao J,Xie GQ,Zhang Q.Balancing the anionic framework polarity for enhanced thermoelectric performance in YbMg_2Sb_2 Zintl compounds.J Mater.2019;5(4):583.

[12] Chen S,Ren ZF.Recent progress of half-Heusler for moderate temperature thermoelectric applications.Mater Today.2013;16(10):387.

[13] Fu CG,Bai S,Liu YT,Tang Y,Chen L,Zhao XB,Zhu TJ.Realizing high figure of merit in heavy-band p-type half-Heusler thermoelectric materials.Nat Commun.2015;6:8144.

[14] Rogl G,Grytsiv A,G(u|")rth M,Tavassoli A,Ebner C,W(u|")nschek A,Puchegger S,Soprunyuk V,Schranz W,Bauer E,M(u|")ller H,Zehetbauer M,Rogl P.Mechanical properties of half-Heusler alloys.Acta Mater.2016;107(1):178.

[15] Li S,Zhu HT,Mao J,Feng ZZ,Li XF,Chen C,Cao F,Liu XJ,Singh DJ,Ren ZF,Zhang Q.N-Type TaCoSn-based half--Heuslers as promising thermoelectric materials.ACS Appl Mater Interfaces.2019;11(44):41321.

[16] Lei Y,Li Y,Wan RD,Chen W,Zhou HW.Microwave synthesis and enhancement of thermoelectric performance in Hf_xTi_((1-x))NiSn_(0.97)Sb_(0.03)half-Heusler bulks.Rare Met.2019.https://doi.org/10.1007/s12598-019-01290-7.

[17] Yu C,Zhu TJ,Shi RZ,Zhang Y,Zhao XB,He J.High-performance half-Heusler thermoelectric materials Hf_(1-x)Zr_xNiSn_(1-y)Sb_y prepared by levitation melting and spark plasma sintering.Acta Mater.2009;57(9):2757.

[18] Yan X,Liu WS,Wang H,Chen S,Shiomi J,Esfarjani K,Wang HZ,Wang DZ,Chen G,Ren ZF.Stronger phonon scattering by larger differences in atomic mass and size in p-type half--Heuslers Hf_(1-x)Ti_xCoSb_(0.8)Sn_(0.2).Energy Environ Sci.2012;5(6):7543.

[19] Zhu HT,Mao J,Li YW,Sun JF,Wang M,Zhu Q,Li GN,Song QC,Zhou JW,Fu YH,He R,Tong T,Liu ZH,Ren WY,You L,Wang ZM,Luo J,Sotnikov A,Bao JM,Nielsch K,Chen G,Singh DJ,Ren ZF.Discovery of TaFeSb-based half-Heuslers with high thermoelectric performance.Nat Commun.2019;10(1):270.

[20] Fu CG,Zhu TJ,Liu YT,Xie HH,Zhao XB.Band engineering of high performance p-type FeNbSb based half-Heusler thermoelectric materials for figure of merit ZT> 1.Energy Environ Sci.2015;8(1):216.

[21] He R,Kraemer D,Mao J,Zeng LP,Jie Q,Lan YC,Li CH,Shuai J,Kim HS,Yuan Liu,Broido D,Chu CW,Chen G,Ren ZF.Achieving high power factor and output power density in p-type half-Heuslers Nb_(1-x)Ti_xFeSb.Proc Natl Acad Sci USA.2016;113(48):13576.

[22] Hu C,Xia KY,Chen X,Zhao XB,Zhu TJ.Transport mechanisms and property optimization of p-type(Zr,Hf)CoSb half--Heusler thermoelectric materials.Mater Today Phys.2018;7:69.

[23] Al Malki MM,Qiu Q,Zhu TJ,Snyder GJ,Dunand DC.Creep behavior and postcreep thermoelectric performance of the n-type half-Heusler alloy Hf_(0.3)Zr_(0.7)NiSn_(0.98)Sb_(0.02).Mater Today Phys.2019;9:100134.

[24] Rahnamaye AHA,Nodehi Z,Maleki B,Abareshi A.Electronical and thermoelectric properties of half-Heusler ZrNiPb under pressure in bulk and nanosheet structures for energy conversion.Rare Met.2019;38(11):1015.

[25] Kang HB,Poudel B,Li WJ,Lee H,Saparamadu U,Nozariasbmarz A,Kang MG,Gupta A,Heremans JJ,Priya S.Decoupled phononic-electronic transport in multi-phase n-type half-Heusler nanocomposites enabling efficient high temperature power generation.Mater Today.2020;36:63.

[26] Shen J,Fan L,Hu C,Zhu TJ,Xin J,Fu T,Zhao D,Zhao XB.Enhanced thermoelectric performance in the n-type NbFeSb half-Heusler compound with heavy element Ir doping.Mater Today Phys.2019;8:62.

[27] Zhu HT,Mao J,Feng ZZ,Sun JF,Zhu Q,Liu ZH,Singh DJ,Wang YM,Ren ZF.Understanding the asymmetrical thermoelectric performance for discovering promising thermoelectric materials.Sci Adv.2019;5(6):5813.

[28] Yan X,Liu WS,Chen S,Wang H,Zhang Q,Chen G,Ren ZF.Thermoelectric property study of nanostructured p-type half--Heuslers(Hf,Zr,Ti)CoSb_(0.8)Sn_(0.2).Adv Energy Mater.2013;3(9):1195.

[29] Rausch E,Balke B,Stahlhofen JM,Ouardi S,Burkhardt U,Felser C.Fine tuning of thermoelectric performance in phase-separated half-Heusler compounds.J Mater Chem C.2015;3(40):10409.

[30] Xia Y,Ponnambalam V,Bhattacharya S,Pope AL,Poon SJ,Tritt TM.Electrical transport properties of TiCoSb half-Heusler phases that exhibit high resistivity.J Phys:Condens Matter.2001;13(1):77.

[31] Zhou M,Chen LD,Feng CD,Wang DL,Li JF.Moderate-temperature thermoelectric properties of TiCoSb-based half-Heusler compounds Ti_(1-x)Ta_xCoSb.J Appl Phys.2007;101(11):113714.

[32] Qiu PF,Huang XY,Chen XH,Chen LD.Enhanced thermoelectric performance by the combination of alloying and doping in TiCoSb-based half-Heusler compounds.J Appl Phys.2009;106(10):103703.

[33] He R,Zhu HT,Sun JY,Mao J,Reith H,Chen S,Schierning G,Nielsch K,Ren ZF.Improved thermoelectric performance of n-type half-Heusler MCo_(1-x)Ni_xSb(M=Hf,Zr).Mater Today Phys.2017;1:24.

[34] Liu YT,Fu CG,Xia KY,Yu JJ,Zhao XB,Pan HG,Felser C,Zhu TJ.Lanthanide contraction as a design factor for high-performance half-Heusler thermoelectric materials.Adv Mater.2018;30(32):1800881.

[35] Wang QM,Li XF,Chen C,Xue WH,Xie XD,Cao F,Sui JH,Wang YM,Liu XJ,Zhang Q.Enhanced thermoelectric properties in p-type double half-Heusler Ti_(2-y)Hf_yFeNiSb_(2-x)Sn_x compounds.Phys Status Solidi A.2020;217(11):2000096.

[36] Xie HH,Wang H,Pei YZ,Fu CG,Liu XH,Snyder GJ,Zhao XB,Zhu TJ.Beneficial contribution of alloy disorder to electron and phonon transport in half-Heusler thermoelectric materials.Adv Funct Mater.2013;23(41):5123.

[37] Zhou M,Feng CD,Chen LD,Huang XY.Effects of partial substitution of Co by Ni on the high-temperature thermoelectric properties of TiCoSb-based half-Heusler compounds.J Alloys Compd.2005;391(1):194.

[38] Xie WJ,Jin Q,Tang XF.The preparation and thermoelectric properties of Ti_(0.5)Zr_(0.25)Hf_(0.25)Co_(1-x)Ni_xSb half-Heusler compounds.J Appl Phys.2008;103(4):043711.

[39] Shen JW,Chen ZW,Lin SQ,Zheng LL,Li W,Pei YZ.Single parabolic band behavior of thermoelectric p-type CuGaTe2.J Mater Chem C.2016;4(1):209.