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1 Introduction▲
2 Experimental▲
3 Results and discussion▲
4 Conclusion▲
图表▲

Rare Metals2018年第4期

Thermoelectric performance of p-type zone-melted Se-doped Bi_0.5Sb_1.5Te₃ alloys

Ren-Shuang Zhai Ye-Hao Wu Tie-Jun Zhu Xin-Bing Zhao

State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University

收稿日期：30 November 2017

基金：supported by the National Natural Science Foundation of China (Nos. 61534001 and 11574267);the National Science Fund for Distinguished Young Scholars (No.51725102);

Thermoelectric performance of p-type zone-melted Se-doped Bi_0.5Sb_1.5Te₃ alloys

Ren-Shuang Zhai Ye-Hao Wu Tie-Jun Zhu Xin-Bing Zhao

State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University

Abstract：

For zone-melted(ZM) bismuth telluride-based alloys, which are widely commercially available for solidstate cooling and low-temperature power generation around room temperature, introducing point defects is the chief approach to improve their thermoelectric performance. Herein, we report the multiple effects of Se doping on thermoelectric performance of p-type Bi_0.5Sb_1.5Te_3-xSe_x+3 wt% Te ZM ingots, which increases carrier concentration, reduces lattice thermal conductivity and deteriorates the carrier mobility. As a result, the peak figure of merit(ZT) is shifted to a higher temperature and a high ZT～1.2 at 350 K is obtained, due to the reduced thermal conductivity and suppressed intrinsic conduction. Further,decreasing Sb content is followed to optimize the room temperature performance and a ZT ～1.1 at 300 K is obtained. These results are significant for designing and optimizing the thermoelectric performance of commercial Bi_0.5Sb_1.5Te₃+3 wt% Te ZM alloys.

Keyword：

Thermoelectric materials; Bismuth telluride; Zone melting; Se doping; Bi_0.5Sb_1.5Te₃;

Author： Tie-Jun Zhu e-mail:zhutj@zju.edu.cn;

Received： 30 November 2017

1 Introduction

Thermoelectric (TE) technology has attracted extensive attention due to the promising applications in direct interconversion between thermal and electrical energy over the past decades.The conversion efficiency of a TE device strongly depends on the materials'dimensionless figure of merit,ZT=(α²σ/κ) T,whereα,σandκare Seebeck coefficient,electrical conductivity and thermal conductivity (including carrier contributionκ_e and lattice contribution K_l),respectively,and T is the operating temperature.In recent years,multi-sc ale micro structure [ 1, 2, 3, 4] and band engineering [ 4, 5, 6, 7] have proven two effective approaches to enhance TE performance,aiming to reduce the lattice thermal conductivity (κ₁) and boost the power factor (PF),respectively.

Bismuth telluride-based solid solutions are the unique commercially available TE materials for solid-state cooling and low-temperature power generation around room temperature,and their commercial ingots with preferred orientation are mainly prepared via zone-melting (ZM)process.Zone melting is a common approach for unidirectional solidification,and it is favorable for a better TE performance due to the anisotropy of bismuth telluridebased solid solutions [ 8] .The optimization compositions of commercial p-type and n-type ZM ingots are Bi_0.5Sb_1.5Te₃with excess tellurium and Bi₂Te_2.7Se_0.3 doped with iodine,respectively,for which the ZT has been remained around unity for decades [ 9] .The relatively high lattice thermal conductivity (κ₁～1.0 W·m^-1·K^-1 at 300 K for both p-type and n-type bismuth telluride-based ZM ingots) [ 10, 11] ,with less suppressed intrinsic conduction caused by the relatively low carrier concentration (n) and narrow band gap (E_g),limits the further enhancement of ZT in zone-melted bismuth telluride-based alloys.

To decrease the relatively highκ₁,much work has been focused on their polycrystalline alloys due to the enhanced phonon scattering at grain boundaries,together with improved mechanical properties.Nanostructuring is at present the primary strategy for bismuth telluride-based alloys.There are two main approaches to achieve nanostructure,namely“bottom-up”and“top-down”approaches.The former approach refers to the procedure that the nanoscale precursors,synthesized by ball milling (BM) [ 2] ,melt spinning (MS) [ 12] or hydrothermal method [ 13] ,were sintered into bulks by hot pressing (HP) or spark plasma sintering (SPS).For p-type bismuth telluride-based solid solutions,Poudel et al. [ 2] reducedκ₁ remarkably via high-energy BM and enhanced ZT>1.3 at 375 K;Xie et al. [ 12] induced unique nanostructures to reduceκ₁through combining MS with SPS to enhance the ZT significantly.In addition,Li et al. [ 14] dispersed nano-SiC into Bi_0.3Sb_1.7Te₃ to reduceκ₁.However,for n-type counterparts,the ZT was not significantly enhanced through such“bottom-up”approach due to the obvious texture loss [ 15, 16] .

Recently,some“top-down”nanostructuring approaches have been proposed.For example,hot deformation (HD) is applied to form in situ nanostructures in bismuth telluridebased alloys [ 17] .Hu et al. [ 1] directly hot deformed n-type Bi₂Te_2.79Se_0.21 ZM ingot and induced multi-scale microstructures including microscale texture,nanoscale defects and atomic point defects to scatter phonons in a wide frequency range,and a state-of-the-art ZT～1.2 at 357 K was obtained.Hot-deformed p-type Bi_2-xSb_xTe₃ HP bulks [ 18] and directly hot-deformed Bi_2-xSb_xTe₃ ZM ingots [ 19, 20] were also reported to exhibit high ZT with lowκ₁～0.6 W·m^-1·K^-1.In particular,hot deformation is a common texturing approach,which has been demonstrated to effectively enhance the ZT of polycrystalline n-type counterparts.Hu et al. [ 21] performed a three-time HD on n-type Bi₂Te_2.3Se_0.7 bulk to enhance the texture and manipulate intrinsic points,and a high ZT～1.2 was obtained at 450 K;Pan and Li [ 22] combined the texturing and in situ nanostructuring effects to enhance the ZT of>1.1 at 473 K for n-type hot-deformed Bi₂(TeSe)₃ SPS bulks.Induced multiscale microstructure generally reducesκ₁ but deteriorates carrier mobility (μ)simultaneously,and the ability to enhance ZT depends on the trade-off between the reduction ofκ₁ and the deterioration ofμ.Via nanostructuring,the ZT is often enhanced in p-type polycrystalline bismuth telluridebased solid solutions and combined with texturing that n-type counterparts can be enhanced to some extent.However,for unidirectionally solidified ZM ingots,tuning multiscale microstructure is not applicable and introducing point defects by alloying is the chief approach to reduce the lattice thermal conductivityκ₁.

Se doping was adopted to decrease n in p-type Bi_0.6Sb_1.4Te₃ without excess Te in 1960 s,since it would suppress the formation of antisite defects [ 23] .Later,excess Te was found appropriate to decrease n without obvious deterioration of carrier mobility (μ) [ 9] .In this work,for a disparate purpose,Se doping was adopted to induce extra point defects to scatter phonons and reduceκ₁,and the multiple effects of Se doping in p-type Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te ZM alloys were investigated.The high ZT～1.2 at 350 K is obtained due to the reducedκ₁ and the suppressed intrinsic conduction.To enhance the TE properties at room temperature,decreasing Sb content slightly was applied sequentially in the ZM ingot with the lowestκ₁ (x=0.09) to further optimize the carrier concentration n,and a ZT～1.1 is obtained at300 K.

2 Experimental

Highly pure (5 N) element chunks of Bi,Sb,Te and Se were weighed according to the nominal compositions Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te (x=0,0.02,0.05,0.08,0.09and 0.10) and Bi_2-ySb_yTe_2.91Se_0.09+3 wt%Te(y=1.50,1.42 and 1.38).The mixtures were sealed into well-cleaned quartz tubes with about 4-cm-long tapers,placed into a rocking furnace and melted at 1073 K for 10 h to ensure the homogeneity of the initial ingots.Subsequently,the initial ingots were placed into a vertical zone-melting furnace at 923 K with a growth rate of 8 mm·h^-1 and a roughly estimated temperature gradient at liquid-solid surface～25 K·cm^-1.The obtained zone-melted ingots with a diameter of 12 mm and a total length of 12 cm including a 4-cm-long taper were diced from the very middle part into a 2-mm-thick disk and a 12 mm×3 mm×2 mm bar,which are for the measurements of thermal and electrical properties,respectively.

Phase structures were examined by X-ray diffractometer(XRD,Rigaku D/MAX-2550P.The Se deficiency was determined by electron probe microanalysis (EPMA,JEOL JEA-8100) with a wave-dispersive spectrometer.A Netzsch LFA-467 flash laser apparatus with a Pyroceram standard was applied to measure the thermal diffusivity(D) with the uncertainty of 3%.An ordinary dimensionand-weight measurement was applied to measure the density of samples (ρ).Dulong-Petit value was adopted as the specific heat (C_p),and the thermal conductivity was calculated according toκ=DρC_p.A commercial Linseis LSR-3 was applied to measure the Seebeck coefficient (α)and electrical conductivity (σ) simultaneously,and the uncertainties ofαandσare within 5%and 3%,respectively.The room temperature Hall coefficient (R_H) was determined on a Quantum Design PPMS-9T instrument using a four-probe configuration,and then the Hall carrier concentration (n) and the Hall mobility (μ) were computed according to n=1/eR_H andμ=σR_H,respectively,where e is carrier charge.It is necessary to stress that all the measurements were performed along the in-plane direction,which is parallel to the zone-melting direction.

3 Results and discussion

No impurity phases are found in XRD patterns of all the Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te powders,as shown in Fig.1a.The peaks around 28.1°are shifted to higher degrees in Fig.1b,indicating that the Se atoms enter the lattice.As Fig.1c displays,the lattice parameters show a climb at the samples with low Se doping (x<0.02) and a decrease at x>0.02,which is similar to the previous report [ 24] .The inconsistence with the Vegard's law might be attributed to the various point defects induced via Se doping,which will be discussed in detail later.The consistent out-of-plane XRD patterns of Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te bulk samples indicate the similar orientation of these zone-melted ingots,as shown in Fig.1d.

Fig.1 a XRD patterns of Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te powders,b local XRD patterns of Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te powders,c lattice parameters of Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te powders and d out-of-plane XRD patterns of Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te ZM bulk samples

Fig.2 a Dependences of Se deficiency and n on nominal Se content (x) in Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te ZM ingots and n of Bi₂Te₃ ZM ingots doped with Se;b dependences ofμon nominal Se content (x)in Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te ZM ingots

Fig.3 Temperature dependences of a electrical conductivity (σ) and b Seebeck coefficient (α) in Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te ZM ingots

Fig.4 a Dependences of room temperatureσandαon nominal Se content (x) in Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te ZM ingots and b temperature dependence of PF in Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te ZM ingots

The variations of n and Se deficiency with x increasing are displayed in Fig.2a.The n first increases and then decreases with x increasing,and the highest n is obtained at x=0.08.The similar phenomenon was reported in Sedoped Bi₂Te₃ single crystals [ 24] .Lost'ak et al. [ 24] supposed that the high vapor pressure of Se enhanced offstoichiometry and induced extra antisite defects.When x<0.08,the Se deficiency increases and n increases due to the increase in antisite defects Bi'_Se;when x>0.08,the increased formation energy of antisite defects becomes dominant due to the larger differences of covalent radius and electronegativity between Se and Bi/Sb,and n is decreased due to the decrease in antisite defects Bi'_Se [ 21, 25] .The variation of point defects with Se doping content increasing is similar to the previous work [ 26] .The carrier mobility (μ) decreases with x increasing,as displayed in Fig.2b.

Fig.5 Temperature dependences of aκand bκ-κ_e in Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te ZM ingots

Fig.6 Room temperature PF andκ_l with nominal Se content x in Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te ZM ingots

With x increasing,the electrical conductivity (σ)increases when x<0.08 and decreases subsequently when x>0.08,which is consistent with n,as displayed in Fig.3a.With Se doping,the temperature of peakαis shifted from 350 to 400 K due to the increased n,as shown in Fig.3b.Variation ofαwith x increasing is normally opposite to that ofσ,as displayed in Fig.4a.The density of states effective mass (m^*) is calculated to be～1.1m₀(m₀is the free electron mass) by the single parabolic band(SPB) model,close to the previous result [ 27] .The power factor (PF) at T>400 K is boosted with Se doping,and a high PF～5.1×10^-3 W·m^-1·K^-2 at x=0.08 is obtained at room temperature,which is comparable to that at x=0,as shown in Fig.4d.

Fig.7 Temperature dependences of a electrical conductivity (σ),b Seebeck coefficient (α),c thermal conductivity(κ) and d ZT in different batches of Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te ZM ingots with x=0.08,0.09 and 0.10

Fig.8 a Temperature dependence of ZT in Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te ZM ingots.b ZT at 300 K,peak ZT and average ZT during 325～425 K with nominal Se content x in Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te ZM ingots

Attributed to the increased n,the intrinsic conduction is suppressed and henceκis reduced around room temperature,as displayed in Fig.5a.At x=0.09,the lowestκat room temperature is obtained mainly due to the reduced lattice contributionκ_l,as indicated in Fig.5b.κ₁ is calculated according to the relationshipκ₁=κ-LσT,and L is estimated via SPB model [ 22, 28, 29] .Room temperature K₁ decreases with x increasing when x<0.09 due to the increased point defects which scatter phonons effectively.The sharp decrease ofκ₁ is consistent with the situation of Se alloying in n-type Bi₂Te_3-ySe_y ZM counterparts (y<0.3) [ 30] .When x>0.09,the increase in room temperatureκ₁ can be attributed to the less suppressed intrinsic conduction caused by the decreased n and the decrease in point defects,which is similar to the Se-deficient n-type Bi₂Te_2.3Se_0.7 HD poly crystalline bulks [ 26] .

Extra point defects induced via Se doping reduceκ₁,and the PF fluctuates within 10%(4.5±0.5)×10^-3W·m^-1·K^-2,as displayed in Fig.6.Turnover of the thermal conductivity is found among the samples with x=0.08,0.09 and 0.10.To confirm the variations,several batches of the ZM ingots were prepared with the same compositions and the TE parameters were proved reproducible,as shown in Fig.7.The enhanced TE performance at>350 K is mainly attributed to the suppressed intrinsic conduction.The highest ZT～1.2 at 350 K is obtained at x=0.08 and0.09 for Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te ZM ingots,as displayed in Fig.8a,benefiting from the increased n and reducedκ₁.Note that the ZT at room temperature is not significantly enhanced due to the trade-off between the reducedκ_l and the deterioratedμ,and a limited enhancement within 10%of the peak ZT and the average ZT during325-425 K is obtained,as exhibited in Fig.8b.

Since the room temperature TE performance is not enhanced by Se doping and the increased high n contributes to the enhanced ZT at>300 K,decreasing Sb content slightly was applied for Bi_0.5Sb_1.5Te_2.91Se_0.09+3wt%Te ZM ingot to obtain optimized n with retained lowκ₁ to further optimize the ZT at 300 K.XRD patterns of Bi_2-ySb_yTe_2.91Se_0.09 (y=1.50,1.42 and 1.38)+3 wt%Te powders show no impurity phase,as displayed in Fig.9.With Sb content y decreasing,the hole concentration decreases due to the increased formation energy of antisite defects [ 21, 25] .Theσslightly decreases,as shown in Fig.10a.Theαincreases correspondingly,and PF remains the same as displayed in Fig.10b,c.Theκis reduced due to the decreased carrier contribution (κ_e) and the retained relatively lowκ₁,as shown in Fig.10d,e.As a result,with Sb content (y) decreasing,the peak ZT at 300 K is enhanced to 1.1,as exhibited in Fig.10f.

Fig.9 XRD patterns of Bi_2-ySb_yTe_2.91Se_0.09+3 wt%Te powders

4 Conclusion

The Se doping in p-type Bi_0.5Sb_1.5Te_3-xSe_x+3 wt%Te ZM ingots increases carrier concentration (n),reduces lattice thermal conductivity (κ₁) while deterioratesμsimultaneously.The increased carrier concentration suppresses the intrinsic conduction,and the peak ZT is shifted to a higher temperature.High ZT～1.2 at 350 K is obtained.Owing to the trade-off between the reducedκ₁and the deterioratedμ,no significant enhancement in room temperature TE performance is obtained by Se doping.Followed by decreasing Sb content slightly,the peak ZT at300 K is enhanced to 1.1.

Fig.10 Temperature dependences of a electrical conductivity (σ),b Seebeck coefficient (α),c power factor (PF) and d thermal conductivity(κ)of Bi_2-ySb_yTe_2.91Se_0.09+3 wt%Te ZM ingots;e variations of carrier contribution (κ_e) and lattice contribution (κ-κ_e) with Sb content (y);f temperature dependence of ZT of Bi_2-ySb_yTe_2.91Se_0.09+3 wt%Te ZM ingots

Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos.61534001 and 11574267) and the National Science Fund for Distinguished Young Scholars(No.51725102).