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Intermetallics 19 (2011) 1024e1031

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Intermetallics

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Enhanced performances of melt spun Bi2(Te,Se)3 for n-type thermoelectric legs

Shanyu Wang, Wenjie Xie, Han Li, Xinfeng Tang*

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China

a r t i c l e i n f o

Article history:Received 31 December 2010Received in revised form22 February 2011Accepted 5 March 2011Available online 29 March 2011

Keywords:A. Ternary alloy systemsC. Rapid solidification processingB. Thermoelectric propertiesG. Thermoelectric power generation

* Corresponding author. Tel.: þ86 27 87662832; faE-mail address: tangxf@whut.edu.cn (X. Tang).

0966-9795/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.intermet.2011.03.006

a b s t r a c t

In this article, a rapid and cost-effective melt spinning (MS) subsequently combined with a spark plasmasintering (SPS) process was utilized to prepared n-type Bi2(Te1�xSex)3 (x ¼ 0.0e1.0) solid solutions fromhigh purity single elemental chunks. The substitution of tellurium by selenium has significant impacts onthe electrical and thermal transport properties of the Bi2(SexTe1�x)3 compounds in a manner which canbe well understood using a valence bond rule and the corresponding change in band gap. Furthermore,the selenium substitution effectively adjusts the carrier density allowing an optimum value of w5 � 10�19 cm�3. As a result, a maximum ZT of 1.05 at 420 K was achieved for the Bi2(Se0.2Te0.8)3 sample whichalso shows an improved average ZT of w0.97 in the entire measurement temperature range. By adoptingthe same p-type legs, the module fabricated by the MS-SPS Bi2(Se0.2Te0.8)3 material which acts as n-typelegs shows w10% enhancement in thermoelectric conversion efficiency compared with the modulefabricated by n-type zone melted ingots.

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1. Introduction

With increasing concerns on environmental protection andgrowing energy crisis, thermoelectricity based on solid-statephysics theory draws comprehensive attention owing to itsimmense advantages, such as environmental friendliness, silentoperation, absence of moving parts and high reliability [1e3]. In thelast decade, what restrict the comprehensive applications of ther-moelectricity are the low thermoelectric performances of currentapplied materials. Among the current thermoelectric materials,Bi2Te3 compound and its alloys are one of the most important andwidespread applied materials used in the vicinity of roomtemperature for cooling applications [4e6]. Besides, Bi2Te3-basedalloys also can be good candidates for power generation applica-tions at low temperature range from 300 K to 500 K [7,8]. However,due to poor thermoelectric and mechanical performances ofcommercial zonemelted Bi2Te3-basedmaterials, the correspondingpower generation modules show very low thermoelectric conver-sion efficiency (<5%) which restricts their further applications.Therefore, it is important to simultaneously improve the thermo-electric and mechanical performances of Bi2Te3-based materials,especially at elevated temperature (w500 K).

Recently, a lot of experimental results indicate that the refine-ments of Bi2Te3-based materials can significantly enhance the

x: þ86 27 87860863.

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thermoelectric performances [9e13]. To yield refined Bi2Te3-basedmaterials for large scale applications, Cao et al. [9] had previouslyutilized a hydrothermal method to create nano-scale Bi2Te3/Sb2Te3starting powders and then hot-pressed the powders into a bulknanocomposite which shows a maximum ZT of 1.47 at about 420 K.Besides, Poudel et al. [10] and Ma et al. [12] reported an extensiveball milling technique in an inert atmosphere followed by a hotpressing process yielding a p-type Bi2Te3-based nanocrystallinematerial with enhanced ZT of w1.4 at approximately 380 K.Furthermore, Xie et al. [11,13] also successfully employed a meltspinning combined with a subsequent spark plasma sinteringtechnique (MS-SPS) to fabricate nanostructured p-type (Bi,Sb)2Te3-based materials. They found that the MS-SPS technique cangenerate multi-scale nano-inclusions in the bulk matrix whichsignificantly reduce the lattice contribution to thermal conduc-tivity, and the as-prepared materials show a highest ZT of w1.50around 300 K. Compared with the p-type Bi2Te3-based materialsexhibiting excellent thermoelectric performances, n-type Bi2Te3-basedmaterials which are essential for module fabrication have notshown an improvement from their ZT < 1 for several decades.Hence, it is of great importance to optimize the thermoelectricperformances of n-type Bi2Te3-based materials.

In particular, due to the large consumptions (particularly in CdTephotoelectric industry [14,15]) and a low reserves of telluriumelement, it is vital and necessary to explore tellurium-absence orlow tellurium-bearing thermoelectric materials to replace thetraditional Bi2Te3-based materials [16]. For n-type Bi2Te3-basedmaterials, owing to the same crystal structure and similar electronic

S. Wang et al. / Intermetallics 19 (2011) 1024e1031 1025

structure between Bi2Te3 and Bi2Se3, selenium substitution oftellurium in Bi2Te3 alloy seems to be an effective approach to reducethe usage of tellurium element. Furthermore, selenium substitutionis also an operative choice to avoid intrinsic conduction and adjustcarrier concentration, along with the resultant enhancement ofthermoelectric performances [17]. It has been reported that withincreasing the selenium content in ternary Bi2(Te1�xSex)3compounds, the band gap and carrier concentration of the solidsolution are both increased gradually. Simultaneously, carriermobility and lattice thermal conductivity can be reduced owing tothe intensified alloy scattering for electrons and phonons [17,18]. Inthe last decade, though the alloying effects of selenium on thethermoelectric properties of Bi2(SexTe1�x)3 solid solutions had beeninvestigated, it can be noted that the reported results mainly focuson the low selenium content range (x � 0.2), as well as the ther-moelectric properties of compounds with high selenium content(x > 0.5) are seldom researched or reported [17e19].

In this work, we report a characterization of thermoelectrictransport properties for the ternary Bi2(SexTe1�x)3 (x ¼ 0.0e1.0)solid solutions prepared by a rapid and cost-effective MS-SPStechnique. We found that the selenium content (namely x value)has significant impact on the thermoelectric transport properties ina manner that corporately modulates the transport properties ofboth electrons and phonons. As a result, the compoundBi2(Se0.2Te0.8)3 shows the highest ZT of 1.05 at 420 K, and a rela-tively high average ZT of 0.97 between 300 K and 500 K whichimproves by w30% compared with the traditional n-type zonemelted ingot.

2. Experiments

High purity elemental chunks of bismuth (5N), tellurium (5N)and selenium (4N) were weighed according to stoichiometry ofBi2(Te1�xSex)3.06 (x ¼ 0.0e1.0) where the additions of excess tellu-rium and selenium are used to compensate their volatilization, andthen sealed into glass tubes under high vacuum. The glass tubeswith elemental chunks were put into a furnace and heated to1123 K for 60 min, then quenched to room temperature to getstarting ingots for melt spinning. The obtained ingots were put intoquartz tubes with a 0.35mmdiameter nozzle, and placed under theprotection of an argon atmosphere, the ingots were melted andejected under a pressure of 0.02 MPa Ar onto copper roller rotatingwith linear speed of 30 m/s, respectively. The obtained ribbonswere pulverized and sintered by the SPS method at 723 K for 3 minwith a pressure of 15 MPa to get compact bulk Bi2(Te1�xSex)3.06samples which are designated as MS-SPS-x for convenience.

Fig. 1. (a) The XRD patterns and (b) the enlarged XRD patterns of (

The phase compositions of bulk samples were determined bypowder X-ray diffraction (XRD) (PANalytical X’Pert Pro X-raydiffraction) using Cu Ka radiation (l ¼ 1.5406 Å). The morphologiesand compositions of bulkmaterials were analyzed by field emissionscanning electronmicroscopy (FESEM) and energy-dispersive X-rayspectroscopy (EDS) (Sirion 200 FESEM). Electrical conductivity (s)and Seebeck coefficient (a) were simultaneously measured bycommercial equipment (ZEM-1, Ulvac Riko, Inc.) under a lowpressure inert gas (He) atmosphere from 300 K to 500 K. Thethermal conductivity (k) was calculated from themeasured thermaldiffusivity (D), specific heat (cp), and density (d) using the rela-tionship k ¼ Dcpd. The thermal diffusivity was tested by the laserflash diffusivity method using a Netzsch LFA 457 system, and thespecific heat (cp) was measured by TA instrument: DSC Q20. All themeasurements were performed in the temperature range from300 K to 500 K. The densities of the bulk samples weremeasured bythe Archimedes method and decrease from 7.82 to 7.1 g/cm3 withincreasing the selenium content, which indicates that the relativedensities of obtained bulk samples are higher than 99.0%. The Hallcoefficient (RH) and electrical conductivity (sH) at room tempera-ture weremeasured by Accent HL5500PC Hall system using the vander Pauw method with the magnetic field strength of 0.513 T, andthe corresponding carrier concentration (n) and carrier mobility(mH) were calculated by the followed equations: n ¼ 1=ejRH j andmH ¼ s=ne. The compressive strength of the MS-SPS samples andZM ingots were measured by DEBEN Microtest (MT 10121) system,and the size of measured samples is 6 � 3 � 3 mm3. The thermo-electric conversion efficiency and maximum output power ofa thermoelectric module were simultaneously measured bycommercial power conversion efficiency measuring apparatus(Model PEM-1S Ulvac Riko, Inc.). The size of the measured moduleis 30 � 30 � 4 mm3 and each module includes 71 p-n legs.

3. Results and discussion

3.1. Phase compositions and microstructure characterizations forbulk samples

The phase compositions of bulk samples were characterized bypowder XRD shown in Fig. 1. All the peaks visible in the XRDpatterns can be indexed to the rhombohedral lattice (space groupR3m), which indicates that all the as-prepared specimens are singlephase as well as possess the same crystal structure as the binaryBi2Te3 compound [20]. The enlarged (205) peaks of the XRDpatterns shown in Fig. 1(b) display an apparent shift to high anglewith increasing the selenium content, which is mainly due to the

205) peak for the MS ribbons with different selenium content.

Table 1The nominal and actual composition results measured by EDS for Bi2(SexTe1�x)3(x ¼ 0.0, 0.2, 0.5, 1.0) samples.

Sample Nominal composition Actual composition (EDS)

x ¼ 0.0 Bi2Te3.06 Bi2Te3.05x ¼ 0.2 Bi2Te2.448Se0.612 Bi2Te2.44Se0.56x ¼ 0.5 Bi2Te1.53Se1.53 Bi2Te1.52Se1.41x ¼ 1.0 Bi2Se3.06 Bi2Se2.93

S. Wang et al. / Intermetallics 19 (2011) 1024e10311026

smaller atomic radius of selenium (1.15 Å) compared with that oftellurium (1.4 Å). This also presumably verifies that the seleniumatoms successfully enter into the crystal lattices of Bi2Te3 to formternary solid solutions.

Fig. 2 presents the FESEM photos of the free crack surface for thebulk samples with selenium content x equaling to 0.0, 0.2 and 0.5. Itcan be noted that the as-prepared compounds show similarmorphologies and well preserve the laminated structure of Bi2Te3-based materials due to the existence of van der Waal gaps [21]. Thegrains of all the samples are placed tightly and show no preferredorientations, which are in accordance with the high relativedensities and the isotropic transport properties for the MS-SPSsamples of our previous studies [13,22]. In addition, the averagesize of grains apparently decreases via the selenium substitution,which indicates that selenium alloying is an effective approach forcrystalline refinement. Compared with the traditional zone meltedingot or other melting ingots, the MS-SPS samples display muchmore refined crystals and possess numerous fine layered struc-tures. These structure refinements are probably originated from thecoexistences of a large amount of amorphous components and highdensity strain fields in the ribbons associated with the ultrahighcooling rate of the non-equilibrium MS process [11,13].

Fig. 2. The FESEM figures and the corresponding high resolution figures

In practice, bismuth, tellurium and selenium are easy tovaporize during the fabrication processes, whereas the thermo-electric properties of Bi2(Te,Se)3-based materials are very sensitiveto their chemical composition, especially the ratio of anionic andcationic components [4]. In order to well understand the variationsof transport properties for ternary Bi2(SexTe1�x)3 with varyingselenium content, it is necessary to make certain what is the actualcomposition of as-prepared bulk samples. Table 1 shows thenominal compositions and actual compositions measured by EDSfor bulk Bi2(SexTe1�x)3 (x ¼ 0.0, 0.2, 0.5 and 1.0) samples. It is clearthat when the value of x is small, the corresponding samples ownexcess cationic elements and thus the ratios of anionic and cationic

of the MS-SPS bulk Bi2(SexTe1�x)3 (x ¼ 0.0, 0.2 and 0.5) compounds.

S. Wang et al. / Intermetallics 19 (2011) 1024e1031 1027

components (which can be defined as g for convenience) are largerthan the stoichiometric formula 3:2. However, with furtherincreasing the selenium content, g decreases gradually and finallythe samples with higher x value would show excess cationiccomponent (namely g < 3/2). These different composition varia-tions are mainly due to the different vaporization capabilities ofselenium and tellurium elements (for example, at 640 K, thesaturated vapor pressures of bismuth, tellurium and seleniumelements arew10�5, 1 and 100 Pa, respectively [23]). Furthermore,these fluctuations of chemical compositions would presumablyresult in the changes in carrier concentration as well as the ther-moelectric properties of bulk materials.

3.2. Electrical transport properties

Fig. 3 shows the temperature dependences of electrical trans-port properties for the prepared bulk materials. The electricalconductivities decrease monotonically with increasing thetemperature which indicates a degenerate semiconductor charac-teristic. Moreover, with increasing the selenium content, the elec-trical conductivity shows a nonmonotonic variation which firstdecreases and then increases. Conversely, the Seebeck coefficientexhibits an opposite variation tendency and theMS-SPS-0.2 samplepossesses the highest value in the entire temperature range.Nevertheless, it is worthwhile to note that the samples with highselenium content exhibit relatively lower Seebeck coefficient. Inorder to further investigate the detailed transport properties of theprepared bulk samples, the room temperature Hall coefficientsweremeasured, as well as the corresponding carrier concentrationsand mobilities were calculated.

As plotted in Fig. 4(a), the carrier concentration first decreasesand then increases with increasing the selenium content. Thisvariation can be accounted for the conduct mechanisms of Bi2Te3-based materials along with the inconsistent changes of actualchemical composition of the prepared ternary solid solutions. InBi2Te3-based materials, anion vacancies and anti-site defects aretwo dominating factors for carrier concentration and also itstransport properties, even for the determination of the type ofmajority carriers [4,19]. With regards to anti-site defects, takebinary Bi2Te3 compound for example, excess bismuth atoms (wheng is lower than 3/2) would occupy the sites of tellurium to generateBiTe anti-site defects acting as electron acceptors. Whereas, wheng is larger than 3/2, excess tellurium atoms could enter the sites of

Fig. 3. Temperature dependences of (a) the Seebeck coefficient a and (b) e

bismuth to generate TeBi anti-site defects and each of TeBi woulddonate one electron into the material [24]. Furthermore, in thecourses of material preparation-especially in the MS process,selenium or tellurium element would considerably vaporize andleave a large number of lattice vacancies, namely VSe or VTe, andeach of the lattice vacancy produces two electrons. At the sametime, bismuth cations which own similar ionic radius and physicalproperties with tellurium or selenium anion would occupy theanionic lattice vacancies to generate anti-site defects BiTe or BiSeproviding holes into the material. As mentioned in our previousworks [22], VSe and BiTe are more easily formed than VTe and BiSe inBi2(SexTe1�x)3 compounds, respectively. Therefore, due to theexcess additions of tellurium and selenium, the samples with smallx value possess excess anion components. These excess tellurium orseleniumwould occupy the sites of bismuth to generate TeBi or SeBi,and the corresponding samples show high n value. When x � 0.4and with increasing x value, n decreases gradually due to thereduction of the number of TeBi which is the dominant point defectin the lower selenium compounds. However, with furtherincreasing the selenium content, the number of TeBi would reducesharply, whereas the increase of the number of VSe along with thesimultaneous reduction of the content of BiTe predominatelycontributes to the increase of carrier concentration.

The composition dependence of carrier mobility is plotted inFig. 4(b) and shows a nonmonotonic variation. Besides the carrierscattering mechanisms, the properties of chemical bonds shouldsignificantly influence the value of carrier mobility. Generally,covalent bond is more favorable for the transport of carrierscompared with ionic component [25]. Based on the traditionalPauling empirical formula and the electronegativities of bothbonding atoms, the proportion of ionic component of the AeB bondcan be estimated by the following equation: 1� exp[�(cA� cB)2/4],where cA and cB are electronegativities of A and B atoms, respec-tively [26]. According to the electronegativities of three atomsbismuth (2.02), tellurium (2.10) and selenium (2.55), it can becalculated and noticed that BieSe bond (6.78%) displaysmuchmoreionic component than BieTe bond (0.16%), therefore binary Bi2Te3possessing more covalent component shows much higher carriermobility than binary Bi2Se3. Moreover, with the incorporation ofselenium into the binary Bi2Te3 compound, carrier mobility firstdecreases due to intensifying alloying scattering and gradualtransformation of bond property. However, carrier mobility thenslightly increases because of subdued alloying scattering with

lectrical conductivity s for bulk Bi2(SexTe1�x)3 (x ¼ 0.0e1.0) samples.

Fig. 4. Room temperature (a) carrier concentrations n and (b) carrier motilities mH for the bulk Bi2(SexTe1�x)3 (x ¼ 0.0e1.0) samples which were measured by the room temperatureHall effect.

S. Wang et al. / Intermetallics 19 (2011) 1024e10311028

further increasing the selenium content (x > 0.5). Theoretically, thesamples with intermediate selenium content show lowest m valuewhich is in accordance with previous researches [4,18]. Based onequation s¼ nme, it is not difficult to understand that the variationsof n and m co-affect the variation of electrical conductivity.

To further understand the mechanisms of the variation of See-beck coefficient, by assuming a charge carrier scattering distanceindependent of energy and degenerate approximation, the Seebeckcoefficient can be written as Eq. (1) [27,28]:

a ¼ p

3k2BTe

dlnsðEÞdE

����E¼EF¼ 8p2k2B

3eh2m*T

� p

3n

�2=3(1)

where kB, EF, h,m*, and T are the Boltzmann constant, Fermi energy,Planck constant, effect electron mass and absolute temperature,respectively. The relationship of room temperature a-n is presentedin Fig. 5(a) and the solid line demonstrates an n�2/3 dependencewhich indicates the dominant scattering mechanism in MS-SPSBi2(SexTe1�x)3 bulk samples is acoustic phonon scattering [27].Hence, the change of the Seebeck coefficient is predominated bythe variation of carrier concentration. Therefore, though alloyingscattering has significant impact on the transport properties ofelectrons and thus decreases the electron mobility, it can be

Fig. 5. (a) The room temperature Seebeck coefficients as a function of carrier concentratdependences of power factors (a2s) for bulk Bi2(SexTe1�x)3 (x ¼ 0.0e1.0) samples.

inferred that the bond property and phononeelectron interactionsexclusively dominate the transport of electrons.

The power factor (a2s) of Bi2(SexTe1�x)3 compounds are pre-sented in Fig. 5(b). It can be noted that the compounds with higherselenium content (x > 0.5) show very low power factors. On thecontrary, due to the high proportion of covalent bond componentand thus high carrier mobility, the compoundswith lower seleniumexhibits relatively better electrical properties and may be prom-ising candidates for thermoelectric applications. As a result, theMS-SPS-0.2 sample possesses highest power factor in the entiremeasured temperature range.

3.3. Thermal transport properties

The temperature dependences of thermal transport propertiesfor Bi2(SexTe1�x)3 compounds are presented in Fig. 6. As shown inFig. 6(a), with increasing temperature, the thermal conductivities ofthe compounds with lower selenium (x < 0.5) first decrease due tothe intensified phononephonon coupling, and then increase owingto the bipolar diffusion carriers (namely electron-hole pairs) con-ducting much heat. Interestingly, the thermal conductivities of thecompounds with high selenium (x � 0.5) decrease monotonically

ion and the solid line demonstrates such an n�2/3 dependence, (b) the temperature

Fig. 6. (a) The temperature dependences of the thermal conductivities k and (b) lattice thermal conductivities (kL) for bulk Bi2(SexTe1�x)3 (x ¼ 0.0e1.0) samples. The inset in (a)shows the composition dependence of room temperature thermal conductivity.

S. Wang et al. / Intermetallics 19 (2011) 1024e1031 1029

with increasing temperature. These apparent distinctions may bedue to the differences in energy gaps (Eg) and carrier concentrationswhich decide the excitation temperature of intrinsic conduction[29]. With increasing the selenium content (x value), the roomtemperature thermal conductivity first decreases and thenincreases, as shown in the inset map of Fig. 6(a). The sample whoseratio of selenium and tellurium equals to 1:1 (MS-SPS-0.5 sample)shows the lowest k value which is in conformity with previousstudies [4,19]. These tendencies are partially attributed to thesimilar variations of electrical contributions to thermal conduc-tivity, whereas the most important factor is the different phononscattering effects of selenium alloying.

The electronic contribution to thermal conductivity was calcu-lated according to theWiedemanneFranz equation: ke¼ LsT, whereL is Lorenz constant and can be taken to be 1.5 � 10�8 V2 K�2 fornear-degenerate or degenerate semiconductor [30,31]. The latticecontributions were then calculated by subtraction and are shown inFig. 6(b). With increasing temperature, the lattice thermalconductivities show the same variation trends as those of thermalconductivities, where the samples with high selenium contentdecrease monotonically and the low selenium-bearing compoundsdecrease firstly and then increase. These changes are consistent

Fig. 7. (a) The temperature dependences of ZT and (b) the composition dependen

with the variations of Eg and nwith the incorporation of selenium.Furthermore, the alloying of selenium largely reduces the latticethermal conductivity which is mainly attributed to the strongphonons scattering by atom mass fluctuations. In particular, it canbe noticed that the samples, especially the ternary solid solutionsprepared by the MS-SPS technique, exhibit much lower latticethermal conductivities compared with the materials fabricated byother techniques [17,19,32]. The ultra-low lattice thermal conduc-tivities should be partially attributed to the compositionalhomogenization in atomic size associated with the rapid frozenprocess, which largely intensifies the alloy scattering of shot-wavephonons. Moreover, the grain refinements, a large number ofnanostructures, point defects and strain fields generated by thenon-equilibrium MS technique also contribute much to the signif-icant reduction of lattice thermal conductivity [11,13].

3.4. The dimensionless figure of merit ZT and the thermoelectricconversion efficiencies of modules

The dimensionless figure of merit ZT was calculated by themeasured values of a, s and k and is shown in Fig. 7(a). Due to thedetrimental bipolar effects, the ZTs of the samples with lower

ce of room temperature ZT for the bulk Bi2(SexTe1�x)3 (x ¼ 0.0e1.0) samples.

Fig. 8. (a) The temperature dependence of capability factors (s) for ZM ingot and the MS-SPS-0.2 samples, (b) the schematic photo of the fabricated thermoelectric module forthermoelectric conversion efficiency measurements.

S. Wang et al. / Intermetallics 19 (2011) 1024e10311030

selenium content first increase and then decrease with increasingtemperature, reaching peaks around 450 K. The MS-SPS-0.2 sampleshows a highest ZT of 1.05 at 420 K. More importantly, comparedwith the ZM ingot provided by Guangdong Fuxin Electronic Tech-nology Co., Ltd whose ZT drops rapidly with temperature down toabout 0.5 at 500 K, the ZT value of the MS-SPS-0.2 sample showsmuch lower temperature dependence which exhibits a relativelyhigh average ZT of 0.97 between 300 and 500 K. This average ZTvalue is about 30% improvement compared with the ZM ingot, andit can be inferred that the MS-SPS-0.2 material should be morebeneficial for low temperature power generation and also cooling.Furthermore, the samples with intermediate selenium(x ¼ 0.3e0.5) also show comparable average ZT with the ZM ingotbetween 300 K and 500 K, and their thermoelectric properties canbe further optimized by proper carrier concentration and chemicalcomposition modifications. With increasing the x value, the roomtemperature ZT presented in Fig. 7(b) enhances previously and thendecreases, which is consistent with the hot-press results and otherstudies [19]. In practice, excellent mechanical properties arenecessary for material processing and module fabrications. Thecompressive strength measurements reveal that the MS-SPS-0.2sample (w220 MPa) shows more than 300% improvement incompressive strength in comparison with that of ZM ingots(w50 MPa).

The compatibility factor s is also important for thermoelectricmaterials in practice and defined as [33,34]:

s ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ ZT

p � 1aT

(2)

Fig. 8(a) shows the temperature dependences of s for the MS-SPS-0.2 sample and ZM ingot. It is obvious that the MS-SPS-0.2sample shows lower temperature dependence, particularly atelevated temperature, than that of ZM ingot, which indicates that

Table 2The output powers and conversion efficiencies of modules fabricated by differentn-type materials.

Modules Th (K) Tc (K) U (V) I (A) Pmax (W) h (%)

ZM 476 321 2.112 0.973 2.053 4.1MS-SPS-0.2 474 321 2.915 0.776 2.26 4.5

the MS-SPS-0.2 sample may be more favorable for n-type ther-moelectric legs. In order to ascertain and further identify thethermoelectric performances of these two kinds of n-type mate-rials, with the same zone melted p-type legs and fabricating tech-niques, two modules were fabricated by these two n-typematerials, respectively. Fig. 8(b) presents the schematic photo ofa fabricatedmodule which includes 71 p-n pairs, two Al2O3 ceramicplates and two leg wires. By fixing the Th (the temperature of hotside) and Tc (the temperature of cold side) which determine theinput power, the thermoelectric conversion efficiencies (h) andmaximum output powers (Pmax) of the two modules weremeasured and are presented in Table 2. It can be noted the MS-SPS-0.2 module shows w10% enhancement in conversion efficiencyequaling to 4.5% in comparison with that of the ZM module (4.1%).This result also confirms that the n-type MS-SPS-0.2 samplepossesses better thermoelectric performances as well as promisingapplication potentials in comparison with the n-type ZM ingot.

4. Conclusions

In this work, a melt spinning combined with a subsequent sparkplasma sintering technique (MS-SPS) was successfully utilized tosynthesize n-type selenium substituted ternary Bi2(Te1�xSex)3(x ¼ 0.0e1.0) solid solutions. By significantly influencing thechemical bond component and the transport properties of elec-trons and phonons, selenium substitution and its content appar-ently impact the microstructures and thermoelectric properties ofn-type Bi2Te3-based compounds. As a result of better electricalproperties and lower lattice thermal conductivity, the samples withintermediate selenium show improved thermoelectric perfor-mances. The maximum ZT reaches 1.05 at 420 K for the MS-SPS-0.2sample, and the sample also shows a high average ZT of 0.97between 300 and 500 K which is about 30% enhancementcompared with the n-type ZM ingots. Furthermore, the MS-SPS-0.2module shows w10% improvement in thermoelectric conversionefficiency compared with that of ZM module. Therefore, it isworthwhile to note that the substitution of tellurium by seleniumas well as the utilization of time-saving and cost-efficient MS-SPStechnique, which results in bulk n-type Bi2(Te1�xSex)3 basedmaterials with high thermoelectric and mechanical performances,are both of great significances for the commercial applications ofBi2Te3-based materials.

S. Wang et al. / Intermetallics 19 (2011) 1024e1031 1031

Acknowledgments

We thank the supports of the National Basic Research Programof China (Grant No. 2007CB607501), the Natural Science Founda-tion of China (Grant Nos. 50672118 and 50731006), and 111 Project(Grant No. B07040) alongwith the Fundamental Research Funds forthe Central Universities (Grant Nos. 2010-IV-046 and 2010-ZY-CL-040). We also acknowledge Jianzhong Zhang, Guoliang Yu andWeiqiang Cao of Guangdong Fuxin Electronic Technology Co. Ltdfor the supports in module fabrications and material supplies.

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