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Intermetallics 25 (2012) 131e135

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Intermetallics

journal homepage: www.elsevier .com/locate/ intermet

Thermoelectric properties of CuyBixSb2�x�yTe3 alloys fabricated by mechanicalalloying and spark plasma sintering

Chen Chen a, Boping Zhang a,*, Dawei Liu b, Zhenhua Ge a

aBeijing Key Lab of New Energy Materials and Technology, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, Chinab State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

a r t i c l e i n f o

Article history:Received 5 December 2011Accepted 28 February 2012Available online 20 March 2012

Keywords:A. Intermetallics, miscellaneousB. Thermoelectric propertiesC. Mechanical alloying and millingE. Physical properties, miscellaneous

* Corresponding author. Tel.: þ86 10 62334195.E-mail address: bpzhang@ustb.edu.cn (B. Zhang).

0966-9795/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.intermet.2012.02.018

a b s t r a c t

In this work, CuyBixSb2�x�yTe3 alloys containing a small amount (0.1 vol%) of SiC nano-particles wereprepared by spark plasma sintering using mechanical alloying derived powders. It is found that both theelectrical resistivity and Seebeck coefficient of (Bi,Sb)2Te3 alloy are significantly decreased by dopinga trace Cu and increasing Sb-to-Bi ratio, but the thermal conductivity of (Bi,Sb)2Te3 alloy is also obviouslyincreased by doping a trace Cu, especially at low temperature. A peak ZT value about 1.23 is obtained at423 K for Bi0.3Sb1.7Te3, in which all ZT values at the whole measuring temperature range are above 1.0.Such ZT characteristics are more attractive for commercial applications.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Thermoelectricmaterials are increasingly being seen as a kind ofenergy materials used in thermoelectric refrigeration and powergeneration due to their ability to make a direct conversion betweenelectricity and heat [1,2]. (Bi,Sb)2Te3 alloys such as Bi2Te3 (n-type)and Bi0.5Sb1.5Te3 (p-type) are one of the most important thermo-electric materials used near room temperature [3,4]. The perfor-mance of thermoelectric materials is determined by thedimensionless figure of merit (ZT), defined as ZT ¼ (a2/rk)T, wherea, r, k, and T are the Seebeck coefficient, electrical resistivity,thermal conductivity, and absolute temperature, respectively [5,6].

The laminated structure of Bi0.5Sb1.5Te3 composed of five atomiclayers (eTe(1)eBieTe(2)eSbeTe(1)e), which are stacked by van derWaals interactions along c-axis, makes it having a remarkableanisotropy on thermoelectric properties [5e7]. For this reason,single crystal (Bi,Sb)2Te3 alloys are usually prepared by unidirec-tional growth methods such as zone melting, Bridgman and Czo-chralski techniques [8,9]. But the poormechanical properties due tothe weak van der Waals bonding between Te(1)eTe(1) causecleavage in fabrication process and potential problems in reliabilityfor the applications. Consequently, many methods such as hotpressing, shear extrusion, hydrothermal synthesis and melt spin-ning have been used to prepare polycrystalline (Bi,Sb)2Te3 alloys

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[10e13]. In our previous work, the combination of mechanicalalloying (MA) and spark plasma sintering (SPS) was confirmed to bea simple and effective process to fabricate (Bi,Sb)2Te3 thermoelec-tric materials with superior performance [14]. MA has severaladvantages in preparing (Bi,Sb)2Te3 alloys compared withconventional melting technique. The MA derived powders alwayshave fine grain size and homogeneous microstructure, which arebeneficial to enhance the mechanical properties and to lower thethermal conductivity of (Bi,Sb)2Te3 alloys. The SPS process is per-formed by introducing a high-density electric current through thespecimen under pressure during sintering process, which canachieve the rapid densification in a relative low temperature andshort sintering period. Compared to the conventional sinteringmethods, the materials sintered by SPS have homogeneous grainstructure, high density and better mechanical properties.

The properties of thermoelectric materials depend strongly onthe Fermi level, which is determined by the carrier density [5]. Thecomposition of Bi0.5Sb1.5Te3 is thought to have the optimized carrierdensity for more than 50 years, but as new methods are used toprepare (Bi,Sb)2Te3 alloys, the carrier density should be furtheroptimized. The main carrier of Bi0.5Sb1.5Te3 alloys comes from theantisite defect SbTe, so changing the Sb-to-Bi ratio is one way tooptimize the carrier density, and the composition of Bi0.3Sb1.7Te3 isproved to have better thermoelectric performance in our previ-ously work. It is reported that doping a small amount of metalelement, like Au and Ag, can significantly increase the carrierdensity of Bi0.5Sb1.5Te3 alloys [15,16]. In this work, in order tooptimize the carrier density, a small amount of Cu was doped into

Fig. 1. XRD patterns of sintered bulks CuyBixSb2�x�yTe3 from a section perpendicular tothe pressing direction.

C. Chen et al. / Intermetallics 25 (2012) 131e135132

Bi0.5Sb1.5Te3 and Bi0.3Sb1.7Te3 composition. The electrical andthermal transport properties of the resultant bulk samples wereinvestigated with an emphasis on the dual effects of Cu-dopingcontent and Sb-to-Bi ratio.

2. Material and methods

Commercial high purity powders of 99.99% Bi (under 100mesh),99.99% Te (under 200 mesh), 99.99% Sb (under 200 mesh), 99% Cu(under 200 mesh) and 99% SiC (average diameter is about 100 nm)were used as raw materials. Since the dispersion of SiC nano-particles in Bi2Te3 can reduce the thermal conductivity andimprove themechanical properties as found in our previous studies[17], a small amount (0.1 vol%) of SiC nano-particles was alsodispersed into CuyBixSb2�x�yTe3 in the present study. Thesepowders with the chemical compositions ofCuyBixSb2�x�yTe3 þ 0.1 vol%SiC (x ¼ 0.3, 0.5 and y ¼ 0, 0.025) wereprepared after MA under 450 rpm for 3 h in a purified argonatmosphere using a planetary ball mill (QM-3SP2, NanjingUniversity, China). Stainless steel vessel and balls were used, andthe weight ratio of ball to powder was kept at 20:1. Subsequently,the as-milled powders were sintered using a SPS system (Sumi-moto SPS 1050, Japan) under an axial compressive stress of 50 MPaat 673 K in vacuum [14,17,18].

Phase identifications of the bulk samples were analyzed with X-ray diffraction (XRD, Cu-Ka, Rigaku, D/Max-2500, Japan). Thefractographs were observed by scanning electronmicroscopy (SEM,JSM-6460, Japan). The Seebeck coefficient and electrical resistivitywere evaluated along the sample section perpendicular to thepressing direction of SPS, and the thermal diffusivity was evaluatedalong the sample section parallel to the pressing direction of SPS.The Seebeck coefficient and electrical resistivity were measuredusing a Seebeck Coefficient/Electrical Resistivity Measuring System(ZEM-2, Ulvac-Riko, Japan) at 323e473 K under a helium atmo-sphere. The thermal diffusivity (D) was measured by the laser flashmethod (TC-9000, Japan). The specific heat (Cp) was measuredusing thermal analyzing apparatus (DSC-60, Japan). The density (d)of the samples was measured by the Archimedes method. Theresulting thermal conductivity (k) was calculated from the density(d), specific heat (Cp) and thermal diffusivity (D) using the rela-tionship k ¼ D$Cp$d.

3. Results and discussion

Fig. 1 shows the XRD patterns of sintered bulkCuyBixSb2�x�yTe3 þ 0.1 vol%SiC (x ¼ 0.3, 0.5 and y ¼ 0, 0.025)samples. All the diffraction peaks of the samples were obtainedfrom the section perpendicular to the pressing direction. Thestandard diffraction peaks cited from the Bi0.5Sb1.5Te3 (PDF#49-1713) are indicated with vertical lines for comparison. Regardless ofthe different compositions, the characteristic peaks of all the bulkswith and without Cu match up well with the standard pattern(PDF#49-1713) and no obvious preferential orientation is observed,which mean that the polycrystalline materials prepared by SPS aresupposed to be isotropy. No characteristic peaks of SiC were foundin the XRD patterns owing to the low SiC content (0.1 vol.%). It hasbeen reported that Cu4Te3 is observed in CuxBi0.5Sb1.5�xTe3 alloys asx � 0.05 [15,16]. Due to the low Cu content (y ¼ 0.025), nodiffraction peaks of Cu4Te3 are observed in the XRD patterns of oursamples, which implies that the Cu atoms have dissolved into the(Bi,Sb)2Te3 alloys. Since ionic radius of Cu2þ (w0.072 nm) is smallerthan that of Sb3þ (w0.092 nm), the crystal cell of (Bi,Sb)2Te3 willbecome smaller if the Cu2þ displaces the Sb3þ. The lattice param-eters of CuyBixSb2�x�yTe3 alloys analyzed by XRDwere summarizedin Table 1. The both lattice constants a and c of Bi0.5Sb1.5Te3 and

Bi0.3Sb1.7Te3 decrease by doping Cu, which also proves that the Cuatoms has dissolved into the (Bi,Sb)2Te3 alloys. The SEM fracto-graphs of sintered bulk CuyBixSb2�x�yTe3 þ 0.1 vol%SiC (x ¼ 0.3, 0.5and y ¼ 0, 0.025) samples are shown in Fig. 2. All the samples showdense microstructures, which coincide with the high measureddensities (Table 1). The average grain size of (Bi,Sb)2Te3 alloys isabout 3 mm and obviously increases by Cu-doping. We suppose thatwhen the content of Cu is beyond the substitution limit, part ofthem will get into an interstitial site of (Bi,Sb)2Te3 and form a kindof interstitial solid solution, which is undetected in the XRD resultsbecause of the low concentration. Consequently, the large diffusionactivation energy of the interstitial solid solution contributes to theslight growth of grain.

Fig. 3 shows the electrical transport properties of sintered bulkCuyBixSb2�x�yTe3 þ 0.1 vol%SiC (x ¼ 0.3, 0.5 and y ¼ 0, 0.025)samples. The electrical resistivity (r) of all the samples in Fig. 3(a)gets their minimums at 323 K and increase with increasingtemperature from 323 to 473 K, which indicates a metallic con-ducting behavior. The minimum resistivity of Cu0.025Bi0.5Sb1.475Te3(w5.2� 10�6 Um) and Cu0.025Bi0.3Sb1.675Te3 (w3.8� 10�6 Um) getsa decrease about 77% and 55%, respectively, as comparedwith thoseof Bi0.5Sb1.5Te3 (w2.3 � 10�5 Um) and Bi0.3Sb1.7Te3(w8.5 � 10�6 Um). The remarkably reduced electrical resistivityupon Cu-doping might be attributed to a noticeable increase in thecharge carrier density, which can be explained by the equation:

CuTe/Sb2Te3 Cu0

Sb þ ð1=2ÞV�TeþTeTe þ h� (1)

where h� denotes the produced hole. The minimum resistivity ofBi0.3Sb1.7Te3 shows a big decrease about 63% than that ofBi0.5Sb1.5Te3, which indicates that the resistivity could also bereduced by optimizing the compositions of (Bi,Sb)2Te3 alloys. It iswell known that the major charge carrier in Bi0.5Sb1.5Te3 is createdby the antisite defects where Sb atoms occupy the Te sites, whichcan be described as:

Sb2Te3¼ 2Sb0TeþV�Teþ2V�

Sb þ ð3=2ÞTe2ðgÞ[þ 2h� (2)

where h� denotes the produced hole and [ denotes the volatiliza-tion of Te [8]. The increase of Sb-to-Bi ratio in (Bi,Sb)2Te3 alloys willincrease the SbTe defects, which can enhance the charge carrierdensity and reduce the electrical resistivity [19]. Therefore, the dual

Table 1Lattice parameters and density of CuyBixSb2�x�yTe3 alloys.

Samples Bi0.5Sb1.5Te3 Cu0.025Bi0.5Sb1.475Te3 Bi0.3Sb1.7Te3 Cu0.025Bi0.3Sb1.675Te3

Lattice parameters (nm) a 0.42999 0.42964 0.42869 0.42837c 3.05975 3.04951 3.04795 3.04756

Measured density (g/cm3) 6.86 6.77 6.73 6.77

C. Chen et al. / Intermetallics 25 (2012) 131e135 133

action of the Cu-doping and the Sb-to-Bi ratio optimizing contrib-utes to the increased carrier density and the lowest electricalresistivity of the Cu0.025Bi0.3Sb1.675Te3.

The temperature dependence of Seebeck coefficient (a) ofCuyBixSb2�x�yTe3 þ 0.1 vol%SiC (x ¼ 0.3, 0.5 and y ¼ 0, 0.025) isshown in Fig. 3(b). The positive Seebeck coefficient reveals that allthe samples are p-type in the whole temperature range. The See-beck coefficient shows a maximum value about 254 mV/K at 323 Kand decreases with increasing temperature for the Bi0.5Sb1.5Te3. Onthe other hand, the Seebeck coefficient increases with increasingtemperature and gets the maximum value at 473 K for theCu0.025Bi0.5Sb1.475Te3, Bi0.3Sb1.7Te3 and Cu0.025Bi0.3Sb1.675Te3. Whenthe extrinsic conduction transits to the intrinsic conduction, whichmainly caused by the increase of carrier concentration, the Seebeckcoefficient will significant decrease [20]. The low transitiontemperature (near room temperature) for the Bi0.5Sb1.5Te3 may beattributed to the narrow forbidden band. Increasing Sb-to-Bi ratioor doping Cu will increase the carrier concentration and result inthat the transition from extrinsic to intrinsic conduction shifts tohigher temperature [5]. Due the intrinsic conduction behavior, theCu0.025Bi0.5Sb1.475Te3, Bi0.3Sb1.7Te3 and Cu0.025Bi0.3Sb1.675Te3samples show the similar temperature dependences of Seebeckcoefficient and the electrical resistivity, which are different fromthose of the Bi0.5Sb1.5Te3 sample.

Fig. 3(c) shows the temperature dependence of power factor(PF ¼ a2/r) for the CuyBixSb2�x�yTe3 þ 0.1 vol%SiC (x ¼ 0.3, 0.5 andy ¼ 0, 0.025). All samples get their maximum PFs at 323 K, anddecrease with increasing measuring temperature. The maximumPFs at 323 K are about 2.8 and 3.6 mW/mK2 for the Cu-free

Fig. 2. SEM fractographs of the sintered bulks Bi0.5Sb1.5Te3 (a), Cu0.02

Bi0.5Sb1.5Te3 and Bi0.3Sb1.7Te3 as well as 3.0 and 2.6 mW/mK2 forthe Cu-doped counterparts, respectively. Since the decreasedelectrical resistivity by optimizing Sb-to-Bi ratio dominantlycontributes to the enhancement of the PF, the Bi0.3Sb1.7Te3 showsa high PF during the whole temperature range, as comparing withthe Bi0.5Sb1.5Te3. On the other hand, the increasing trend of r withtemperature leads the reduction of the PF. When the reduction ofa plays the dominant role in a2/r, the reducing trend of the PF willbe accelerated for the (Bi,Sb)2Te3. The reducing trend of the PFseems to be relaxed after doping Cu, which may be contributed tothe shift of the transition from extrinsic to intrinsic conduction tohigher temperature.

Fig. 4 shows the temperature dependence of thermal conduc-tivity (k) of CuyBixSb2�x�yTe3 þ 0.1 vol%SiC (x ¼ 0.3, 0.5 and y ¼ 0,0.025). The k of Bi0.5Sb1.5Te3 increases with increasing measuringtemperature but decreases with increasing measuring temperaturein the case of the Cu0.025Bi0.5Sb1.475Te3, Bi0.3Sb1.7Te3 andCu0.025Bi0.3Sb1.675Te3. The minor carriers generate from intrinsicexcitation not only decrease the a but also increase the k due to thebipolar diffusion kbipolar in the intrinsic region, especially at hightemperature [20]. But we should take into account the contributionof kbipolar near room temperature for Bi0.5Sb1.5Te3, as discussed inFig. 3(b) since the transition from extrinsic to intrinsic excitation isnear room temperature. For Cu0.025Bi0.5Sb1.475Te3, Bi0.3Sb1.7Te3 andCu0.025Bi0.3Sb1.675Te3, the transition from extrinsic to intrinsicconduction shifts to higher temperature which means that the k ismainly composed of a lattice component klat and a carrier compo-nent kel at low temperatures. And the decreased k with increasingtemperature is mainly caused by the decrease of klat, which is due to

5Bi0.5Sb1.475Te3 (b), Bi0.3Sb1.7Te3 (c) and Cu0.025Bi0.3Sb1.675Te3 (d).

Fig. 4. Temperature dependence of thermal conductivity k (a), carrier thermalconductivity kel (b) and k�kel (c) for the sintered bulks CuyBixSb2�x�yTe3.

Fig. 5. Temperature dependence of dimensionless ZT values for the sintered bulksCuyBixSb2�x�yTe3.

Fig. 3. Temperature dependence of resistivity (a), Seebeck coefficient (b) and powerfactor (c) for the sintered bulks CuyBixSb2�x�yTe3.

C. Chen et al. / Intermetallics 25 (2012) 131e135134

the improved phonon scattering [10,13]. The kel was estimated bytheWiedemaneFranz law (kel ¼ LT/r, where L is the Lorenz number(L ¼ 2.0 � 10�8 V2/K2 for a heavily doped semiconductor [13]), T isthe temperature in K and r is the electrical resistivity) and shown inFig. 4(b) [5,13]. We can observe that the kel of Bi0.5Sb1.5Te3 is obvi-ously increased by the trace Cu-doping, which results the increaseof k, especially at low temperature, as shown in Fig. 4(a).

Fig. 4(c) shows the temperature dependence of k�kel ofCuyBixSb2�x�yTe3 þ 0.1 vol%SiC (x ¼ 0.3, 0.5 and y ¼ 0, 0.025). Wecould neglect the effect of kbipolar on k and consider the k-kel as klat atlow temperature. As klat is proportional to T�1,klat ¼ 3:5ðkB=hÞMV1=3q3D=g

2T , where the M is the average massper atom, V is the average atomic volume, qD is the Debyetemperature, and g is the Gruneisen parameter, the klat decreaseswith increasing temperature [20]. For the Cu0.025Bi0.5Sb1.475Te3 andCu0.025Bi0.3Sb1.675Te3, this decrease trend of klat can be obviouslyobserved at low temperature (323e423 K), but the k�kel tends toincrease when the temperature is above 423 K, which means thek�kel cannot be considered as klat for the kbipolar caused byintrinsic excitation has affected the k. The k�kel of Bi0.5Sb1.5Te3increases with increasing temperature, which means that theinfluence of kbipolar should be considered even at lowtemperature. This result coincides with the discussion in Fig. 3(b)that the transition from extrinsic excitation to intrinsic excitation isnear room temperature for Bi0.5Sb1.5Te3.

Fig. 5 shows the temperature dependence of ZT values (ZT ¼ a2/rk ¼ PF/k) of CuyBixSb2�x�yTe3 þ 0.1 vol%SiC (x ¼ 0.3, 0.5 and y ¼ 0,0.025). The Bi0.3Sb1.7Te3 shows a better thermoelectric performance

than those of the Bi0.5Sb1.5Te3, especially at high temperature, inwhich all the ZT values are above 1.0 in the whole measuringtemperature range and get the highest value about 1.23 at 423 K. Asthe PF and k of Bi0.3Sb1.7Te3 both decreased with increasingtemperature, the ZT value arrived at peak value (1.23) at 423 Kbecause the decrease of k playing a main role according to the

C. Chen et al. / Intermetallics 25 (2012) 131e135 135

equation ZT ¼ a2/rk ¼ PF/k. Due to the same coupling effect, thesimilar variation tendency of ZT values is also observed in the Cu-doped Cu0.025Bi0.5Sb1.475Te3 and Cu0.025Bi0.3Sb1.675Te3 samples.Although doping Cu slightly increase the PF (Fig. 3c), the significantincreased k at low temperature (Fig. 4a) makes that the ZT value ofCu-doped samples reduces at low temperature and slightlyincreases at high temperature. The decrease of PF and significantincrease of k make the ZT value of Cu-doped Bi0.3Sb1.7Te3 seriouslyreduced in the whole measuring temperature range. This resultindicates that the thermoelectric properties of (Bi,Sb)2Te3 alloys canbe effectively improved by optimizing the composition of the alloysrather than by doping Cu.

4. Conclusions

The p-type CuyBixSb2�x�yTe3 þ 0.1 vol%SiC (x ¼ 0.3, 0.5 andy ¼ 0, 0.025) samples were synthesized via MA, followed by SPSmethod. A peak ZT value about 1.23 is obtained at 423 K forBi0.3Sb1.7Te3, in which all ZT values at the whole measuringtemperature range are above 1.0. Compare with the well-knowncomposition Bi0.5Sb1.5Te3, which has already been used in refrig-eration field, such ZT characteristics are more attractive for coolingand low temperature waste heat recovery applications. Doping Cuto (Bi,Sb)2Te3 alloys decreases not only the electrical resistivity butalso the Seebeck coefficient, which finally degrades the thermo-electric performance. Optimizing the Sb-to-Bi ratio is a moresignificant method to improve the thermoelectric performance of(Bi,Sb)2Te3 alloys than Cu-doping.

Acknowledgments

This work was supported by the National Basic ResearchProgram of China (Grant No. 2007CB607500) and National NatureScience Foundation (Grant No. 50820145203).

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