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Characterization and thermoelectric properties of p-type 25%Bi2Te3–75%Sb2Te3 prepared

via mechanical alloying and plasma activated sintering

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2006 J. Phys. D: Appl. Phys. 39 740

(http://iopscience.iop.org/0022-3727/39/4/021)

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INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 39 (2006) 740–745 doi:10.1088/0022-3727/39/4/021

Characterization and thermoelectricproperties of p-type25%Bi2Te3–75%Sb2Te3 preparedvia mechanical alloying and plasmaactivated sinteringX A Fan1, J Y Yang1, R G Chen1, H S Yun2, W Zhu1, S Q Bao1

and X K Duan1

1 State Key Laboratory of Plastic Forming Simulation and Dies Technology,Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074,People’s Republic of China2 Materials Research and Test Center, Wuhan University of Science and Technology,122 Luoshi Road, Wuhan 430070, People’s Republic of China

E-mail: jyyang@public.wh.hb.cn

Received 14 August 2005, in final form 7 November 2005Published 3 February 2006Online at stacks.iop.org/JPhysD/39/740

AbstractIn the present work, starting from elemental bismuth, antimony andtellurium powders, p-type 25%Bi2Te3–75%Sb2Te3 thermoelectric materialswith high density were prepared by mechanical alloying (MA) and plasmaactivated sintering (PAS). The single phase 25%Bi2Te3–75%Sb2Te3 alloyswere obtained after MA for 12 h. The effect of sintering temperatures onmicrostructure and thermoelectric properties of the as-PASed samples wasresearched. Highly compact samples with relative density over 99% couldbe obtained when sintering temperature was over 653 K. A preferentiallyorientated microstructure with the (1 1 0) plane parallel to and the basalplanes (0 0 l) perpendicular to the pressing direction was formed, and theorientation factors of the (0 0 l) planes changed from 0.11 to 0.12 at differentsintering temperatures. The maximum power factor and figures of merit (Z)at room temperature were 3.10 × 10−3 W m−1 K−2 and 2.85 × 10−3 K−1,respectively. The Vickers microhardness reached 112.7 Hv, which was twicethat of the single crystal samples prepared by zone-melting.

1. Introduction

The Bi2Te3–Sb2Te3 compounds are known as one of thebest thermoelectric materials near room temperature [1].They are extensively used for Peltier refrigeration suchas cooling laser diodes, infrared detectors, microprocessorchips, etc [2]. These alloys have a remarkable anisotropy,which originates from the rhombohedral structure composedof quintuple atomic layer series in the order of Te(1)–Bi–Te(2)–Bi–Te(1) along the c-axis [3]. Currently, the Bi2Te3–Sb2Te3 alloys are usually prepared by unidirectional crystal

growth methods such as zone melting or Bridgman technology[4, 5]. Although the resulting single crystal materials present

excellent thermoelectric properties, they have poor mechanicalproperties due to weak Van der Waals bonding betweenTe(1)–Te(1) layers and coarse grain size. With the requirementof a good combination of thermoelectric and mechanicalproperties, some powder metallurgical methods, whichproduce randomly oriented polycrystalline microstructure andthus good mechanical properties, such as PIES (pulverizedand intermixed elements sintering), BMA (bulk mechanicalalloying), HP (hot pressing), HE (hot extrusion), etc [6–10],

0022-3727/06/040740+06$30.00 © 2006 IOP Publishing Ltd Printed in the UK 740

Characterization and thermoelectric properties of p-type 25%Bi2Te3–75%Sb2Te3

are extensively studied. The PIES method consists of platenball milling, moulding and sintering. It requires a long processtime and is not favourable for industrial application. Thusimprovement should be made to shorten the powder processtime and to reduce the requirements for starting materials. Asfor BMA, it requires expensive apparatus and this may hinderits application. In addition, melting and then pulverizationwith subsequent sintering process cause unavoidable waste andcontamination of the materials. Efficiency and high productioncost somewhat offset the above-mentioned advantages [11,12].High energy planetary ball milling is a good substitute, whichcan prepare the object alloy powders in quite a short time andwith no special particle size requirement of the raw materials.

Plasma activated sintering (PAS) is a novel rapidconsolidation technology, in which plasma generates dueto discharge between powders; the surface of powders isactivated and purified; a self-heating phenomenon is achievedbetween the particles; heat-transfer and mass-transfer canbe completed instantaneously [13]. Therefore, it can obtainmaterials with very fine microstructure and high density ata relatively lower sintering temperature in a very short time[14–16], which should be helpful to enhance both mechanicaland thermoelectric properties. In addition, it can saveenergy sources and increase production efficiency. So, p-type25%Bi2Te3–75%Sb2Te3 alloys were prepared via mechanicalalloying (MA) and PAS in the present work, and the effect ofprocessing parameters on microstructure and thermoelectricproperties was reported.

2. Experiments

Element Bi (99.9 wt.%, 300 mesh), Sb (99.99 wt.%, 100mesh) and Te (99.99 wt.%, 100 mesh) were subjected toMA according to the nominal composition of 25%Bi2Te3–75%Sb2Te3 in a QM-4H planetary ball mill for different time(3, 6, 9 and 12 h) in purified argon atmosphere. Stainlesssteel vessels and balls were used and the weight ratio ofball to powder was kept at 10 : 1. Subsequently, the as-MAed powders were sintered under axial compressive stressof 40 MPa in a PAS system. The heating rate was 40 K min−1

and the sintering process was held at different temperatures(593, 623, 653, 683 and 713 K) for 10 min. The sinteredspecimens were columns with dimensionsφ21×10 mm. Phaseidentification and crystal orientation were analysed with XRD(x-ray diffraction) in a Philips X’Pert PRO diffractometerusing Cu Kα radiation (λ = 1.5406 Å). The fractographswere observed in a Sirion 200 FE-SEM (field emissionscanning electron microscope). Density was measured bythe Archimedes method. In order to evaluate the mechanicalproperties of the alloys, the alloy bars were machined andVickers microhardness testing was performed using 10 and25 g load. A 10 K temperature difference was applied betweentwo ends of a bar specimen (3×3×15 mm) to evaluate Seebeckcoefficient (α). Electrical resistivity (ρ) was measured by astandard two-probe method. A TC-7000 laser flash apparatuswas used for thermal conductivity (κ) measurement. Thepower factor (PF) was calculated from PF = α2/ρ, andthe figures of merit (Z) was determined by the equationZ = α2/ρκ .

Figure 1. XRD patterns of the as-MAed powders for differentmilling time.

3. Results and discussion

Figure 1 shows the XRD patterns of the powders milled fordifferent time. After milling for 3 h, the peaks of Te andSb remain while that of Bi disappears, and Bi2Te3 can beobserved in the XRD patterns, indicating that Bi and Te atomsfirstly reacted into Bi2Te3 during the MA process. With MAproceeding, the peaks of Te and Sb become weaker and thesolid solution (Bi,Sb)2Te3 is formed. The peaks of solidsolution phase move towards a higher degree gradually andbecome more and more intensified with prolonged MA time,indicating that the mechanically assisted reaction proceededand the solution concentration increased with more millingtime [17]. A single phase 25%Bi2Te3–75%Sb2Te3 solidsolution was obtained after MA for 12 h.

Figure 2 shows the XRD patterns of the as-PASed samplesfrom the sections (a) parallel and (b) perpendicular to thepressing direction at different sintering temperatures. Therelative intensities of (1 1 0) plane reflections in figure 2(a)are stronger than that of the as-milled powders. It indicatesthat (1 1 0) plane was preferentially orientated along thepressing direction during the PAS process. Furthermore, therelative intensities of (0 0 l) planes including (0 0 6), (0 0 15)and (0 0 18) in figure 2(b) are slightly stronger than thoseof the as-milled powders, indicating that the basal planeswere preferentially orientated perpendicular to the pressingdirection. The orientation degree of the (0 0 l) planes can bedetermined by the orientation factor F , which can be calculatedusing the Lotgering method [18]:

F = (P − P0)/(1 − P0), (1)

P = I (0 0 l)/�I (h k l), (2)

P0 = I0(0 0 l)/�I0(h k l), (3)

where P and P0 are the ratios of the integrated intensities of all(0 0 l) planes to those of all (h k l) planes for the preferentiallyorientated and randomly orientated samples, respectively. Theorientation factors F of the (0 0 l) planes of the samples withdifferent sintering temperatures range slightly from 0.11 to

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X A Fan et al

Figure 2. XRD patterns of the as-PASed samples from sections(a) parallel and (b) perpendicular to the pressing direction atdifferent sintering temperatures.

0.12, indicating that the influence of sintering temperature isnot obvious.

Figure 3 shows SEM fractographs of the sample sinteredat 593 K from the fractures parallel and perpendicular to thepressing direction. The longitudinal fractograph presents asomewhat orientated microstructure (figure 3(a)); in contrast,a very fine and equiaxed microstructure is observed in thetransverse fractograph (figure 3(b). It is in good agreementwith the XRD results shown in figure 2. The preferentialorientation formation is mainly attributed to the lamellarcrystal structure and the weak Van der Waals bonding betweenTe(1)–Te(1). The particles with basal planes bonded by Vander Waals force would rotate perpendicular to the pressingdirection. Furthermore, the pulse current induced a highelectric field in samples and might promote the preferentialorientation [5, 15].

Figure 4 shows the variation of the relative densities ofthe samples sintered at different temperatures. It can be foundthat fully dense products are obtained and the relative densitiesof all samples are over 96%. It is not surprising that therelative densities increase with sintering temperature, and themaximum is obtained as 99.4% when sintered at 683 K.

Figure 3. SEM fractographs of the as-PASed samples at sinteringtemperature 593 K from sections (a) parallel and (b) perpendicularto the pressing direction.

The mechanical strength of the as-PASed samples isshown in table 1. The zone-melting samples were boughtfrom the market. The Vickers microhardness testing wasperformed using 10 and 25 g load to evaluate the mechanicalstrength. The load direction was along the pressing directionfor the as-PASed samples. As for the zone-melting samples,the longitudinal direction indicated that the load direction wasparallel to the growth direction and the transverse directionindicated that the load direction was perpendicular to thegrowth direction of the single crystal samples (table 1). TheVickers microhardness of the as-PASed samples was twice thatof the single crystal samples, which should be attributed to thedecrease in grain size and dense bonding between powders.

Seebeck coefficient (α) and electrical resistivity (ρ) ofthe samples sintered at different temperatures are shown infigure 5. Electrical resistivity (ρ) decreases with increasingsintering temperature when sintered below 683 K, whileit shows little change when sintered at 683 and 713 K.As we know, electrical resistivity can be expressed as

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Characterization and thermoelectric properties of p-type 25%Bi2Te3–75%Sb2Te3

Figure 4. Relative densities of the samples sintered at differenttemperatures.

Table 1. The mechanical properties of 25%Bi2Te3–75%Sb2Te3

alloys prepared by this work and zone melting.

Vickers micro-Samples Load (g) hardness (Hv)

This work 10 112.725 99.4

Zone-melting 10 (longitudinal 47.5direction)

10 (transverse 52.9direction)

ρ = 1/(nceµ) [19], where e is the charge, nc is thecarrier concentration and µ is the carrier mobility. As aconsequence, two competing factors, carrier concentration (nc)and mobility (µ), determine the electrical resistivity (ρ). Withincreasing sintering temperature, the concentration of latticedefects generated during the MA process decreased, grainsize increased and the density of the samples increased too(figure 4), which resulted in a decrease in the scattering oflattice defects to carrier. So the carrier mobility (µ) increased.On the other hand, high sintering temperature and long holdingtime might cause a small quantity of compositions deviationand induce the decrease in carrier concentration. But inthe PAS process, the sintering temperature was 80–100 Klower than that of the conventional sintering process (suchas HP) and the sintering time was also very short (thewhole PAS process was greatly shortened to about 30 min),so the carrier concentration experienced only a very slightdecrease. Therefore, with increasing sintering temperature,when sintered below 683 K, the decrease in electrical resistivity(ρ) would result from an increase in mobility and a slightdecrease in carrier concentration. When sintered at 683 and713 K, the concentration of the lattice defects experiencedlittle change, and the scattering of the lattice defects to carrierremained stable. Therefore the electrical resistivity (ρ) showsthe trend as given in figure 5.

Seebeck coefficient increases with sintering temperaturefrom 593 to 683 K. Unfortunately, when sintering temperatureis higher than 683 K, Seebeck coefficient decreases slightly.Seebeck coefficient (α) of p-type thermoelectric materials in

Figure 5. Sintering temperature dependence of Seebeck coefficient(α) and electrical resistivity (ρ) of the as-PASed samples.

ph

el

Figure 6. Sintering temperature dependence of thermal conductivity(κ) of the as-PASed samples.

Figure 7. Sintering temperature dependence of figures of merit (Z)and PF of the as-PASed samples.

the extrinsic conduction region can be expressed as [15]:

α = kB

e

[δ + 2 + ln

2(2πm∗kBT )3/2

nch3

], (4)

where κB is Boltzmann constant, e is electronic charge, δ

is scattering parameter, m∗ is effective mass, nc is carrierconcentration and h is the Planck constant. At a constant

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X A Fan et al

Table 2. Thermoelectric properties of 25%Bi2Te3–75%Sb2Te3 prepared by different routes.

Routes Doping α (µv K−1) ρ (10−5 m) κ (W mK−1) PF (10−3 W mK−2) Z (10−3 K−1)

This work No doping 241.5 1.88 1.09 3.10 2.85Bridgman–Annealing 8%Te 225 0.915 1.21 5.53 4.57HP No doping 278 2.65 0.975 2.92 2.99HE 4%Te 225.5 1.48 1.27 3.43 2.7

temperature T , the Seebeck coefficient can be simplified as

α = kB

e[δ + C − ln nc] , (5)

where C is a constant. With increasing sintering temperature,the solid state reaction was much more complete and theremainder of the metal unpurified phase (although they couldnot be detected) decreased. In addition, a small quantity ofcompositions deviation might also induce the weak decreasein carrier concentration. Therefore Seebeck coefficient (α)slightly increased from 218.7 to 241.5 µv K−1 with increasingsintering temperature when sintered below 683 K. Whensintered at a temperature higher than 683 K, the decreasein Seebeck coefficient might be attributed to the increase inhole concentration, which resulted from the formation of anti-structure defects of BiTe and SbTe due to a small quantity of Teevaporation. The hole concentration was created by the anti-structure defects generated by the occupation of Te sites by Biand Sb atoms, which could be described as

Bi2Te3 = 2Bi′Te + 2VBi + TeTe + Te2(g) + 2h., (6)

Sb2Te3 = 2Sb′Te + 2VSb + TeTe + Te2(g) + 2h., (7)

where VBi and VSb are Bi and Sb vacancy, respectively [20].Figure 6 displays the dependence of thermal conductivity

(κ) of the sintered samples on PAS temperature. It increaseswith increasing sintering temperature. As we know, thermalconductivity (κ) of semiconductor can be expressed as κ =κel + κph [8], where κel and κph correspond to carrier andphonon contribution to thermal conductivity. The carrier-related thermal conductivity κel can be calculated from theelectrical resistivity (ρ) according to the commonly knownWiedemann–Franz law: κel = LT/ρ, where L is Lorentzconstant (L = 2.45 × 10−8 W K−2). Thus a decrease inρ would result in an increase in κel. While for the phononcontribution to thermal conductivity κph, the increase in grainsize and density due to higher sintering temperature wouldresult in a decrease in phonon–grain boundary scattering,thus κph would increase accordingly. Due to the two factorsdiscussed above, thermal conductivity (κ) increases withincreasing sintering temperature.

The variation of the PF and figures of merit (Z) ofthe as-PASed samples with sintering temperature is shownin figure 7. PF and Z increase with increasing sinteringtemperature when holding temperature is below 683 K. Themaximum PF and Z are 3.10 × 10−3 W m−1 K−2 and 2.85 ×10−3 K−1, respectively, when sintered at 683 K. However, theydecrease slightly when the holding temperature is over 683 K.

The thermoelectric properties for 25%Bi2Te3–75%Sb2Te3

prepared by the MA–PAS process are shown in table 2 in com-parison with those of materials prepared by other routes, suchas the Bridgman method [5], HP [8] and HE [21], although the

thermoelectric properties are still lower than the reported bestvalue for single crystal p-type bismuth telluride-based materi-als. Further work on microstructure control and PAS processoptimization is underway in our lab. Several approaches canbe suggested to further increase the Z value. The optimiza-tion of the carrier concentration will be an effective method,which can be obtained by adjusting the nominal compositionof compounds (Bi2Te3)x(Sb2Te3)1−x and appropriate doping.On the other hand, controlling the microstructure is anothergood approach too. By optimizing the PAS process, obtain-ing stronger preferentially orientated microstructure shouldenhance the thermoelectric properties. In addition, control-ling the particle size and obtaining finer microstructure willincrease phonon–grain boundary scattering and result in a fur-ther decrease in thermal conductivity.

4. Conclusions

The p-type 25%Bi2Te3–75%Sb2Te3 compounds with highdensity were obtained by MA and PAS in this work. Singlephase 25%Bi2Te3–75%Sb2Te3 solid solution was obtainedafter MA for 12 h. A preferentially orientated microstructurewith the (1 1 0) plane orientated parallel to and the basal planes(0 0 l) perpendicular to the pressing direction was formedin the PASed samples, and the orientation degree showedless variation with sintering temperature. The maximumorientation factor of the (0 0 l) planes was obtained as 0.12when sintered at 593 K. The maximum PF and figures of merit(Z) at room temperature were 3.10 × 10−3 W m−1 K−2 and2.85 × 10−3 K−1, respectively. The Vickers microhardnessreached 112.7 Hv, which is twice that of the single crystalsamples prepared by zone-melting.

Acknowledgment

This work is co-financed by the National Basic ResearchProject (2004CCA03200) and Natural Science Foundation ofChina (50401008).

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