Effect of Silicon and Sodium on Thermoelectric Properties ofThallium-Doped Lead Telluride-Based MaterialsQinyong Zhang,†,‡ Hengzhi Wang,‡ Qian Zhang,‡ Weishu Liu,‡ Bo Yu,‡ Hui Wang,‡ Dezhi Wang,‡

George Ni,§ Gang Chen,§ and Zhifeng Ren*,‡

†School of Material Science and Engineering, Xihua University, Chengdu, Sichuan 610039, P. R. China‡Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, United States§Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States

ABSTRACT: Thall ium (Tl)-doped lead tel lur ide(Tl0.02Pb0.98Te) thermoelectric materials fabricated by ballmilling and hot pressing have decent thermoelectric propertiesbut weak mechanical strength. Addition of silicon (Si)nanoparticles strengthened the mechanical property byreducing the grain size and defect density but resulted inlow electrical conductivity that was not desired for anythermoelectric materials. Fortunately, doping of sodium (Na)into the Si added Tl0.02Pb0.98Te brings back the high electricalconductivity and yields higher figure-of-merit ZT values of ∼1.7 at 770 K. The ZT improvement by Si addition and Na doping inTl0.02Pb0.98Te sample is the direct result of concurrent electron and phonon engineering by improving the power factor andlowering the thermal conductivity, respectively.

KEYWORDS: Thermoelectric materials, lead telluride, mechanical strength, silicon, sodium

Thermoelectric (TE) materials and devices have demon-strated great potentials in solid state cooling andconversion of heat to electricity.1,2 For TE device, higherefficiency comes from TE materials’ higher dimensionlessfigure-of-merit (ZT), ZT = (S2σ/κ)T, where S and σ areSeebeck coefficient and electrical conductivity, respectively, κ isthe thermal conductivity (κ = κL + κe), the sum of electronthermal conductivity κe and lattice thermal conductivity κL, andT is the absolute temperature. In the past a few years, mostprogress has been made on enhancing ZT to above 1 throughdecreasing κL via phonon scattering by nanostructures,

3−5 whilesome advance also been achieved through power factor (S2σ)improvement.6−9 Our previous work showed that ball millingplus hot pressing is a simple and scalable method to fabricatenanostructures with high performance bulk TE materials.10,11

As one of the most studied intermediate temperature TEmaterials system, lead telluride (PbTe)-based materials retainthe highest ZT that has ever been obtained in bulk TEmaterials. For example, by band engineering,12 resonant statesdoping,13 nanostructuring,14 endotaxial precipitating,15 andquantum nanodots,16 peak ZTs of ∼1.5−1.8 have been reachedin p- and n-type, which shows good potentials in moderatetemperature applications and attract much attention. Mostrecently, through combining the effects of resonant doping byTl and alloy scattering by S partial substitution of Te, peak ZTof 1.6 at 700 K was achieved in the Na0.01Tl0.02Pb0.97Te0.92S0.08sample with higher average ZT.17

However, materials based on PbTe suffer from weakmechanical strength. With an 8 at. % Si addition in PbTe,the sample was significantly strengthened, but the ZT was also

degraded significantly to ∼0.9 at 660 K by PbI2 doping.18 In

our previous work on Tl0.02Pb0.98Te by ball milling plus hotpressing,11 the sample softens above 673 K as well. We reporthere, by addition of a smaller amount of Si and doping of Na,very strong samples of p-type Tl0.02Pb0.98TeSi0.02Na0.02 can beobtained through ball milling plus hot pressing, with peak ZT of∼1.7 at 770 K.Si (99.99%, Alfa Aesar) chunk was first loaded into a ball mill

jar with stainless steel balls and ball milled in a high energy ballmill SPEX 8000D (SPEX SamplePrep) for 30 h into Sinanopowder. Then, the obtained Si nanopowder, element Tl(granules, 99.999%, Alfa Aesar), Pb (granules, 99.99%, AlfaAesar), Te (chunks, 99.999%, EZMetals Corp.), and Na2Te(powder, 99.99%, CERAC Inc.) were weighed according to thestiochiometry of Tl0.02Pb0.98TeSixNay (x = 0 or 0.02, while y =0, 0.015, 0.02, and 0.025) and loaded into ball mill jar with ballsfor mechanical alloying by SPEX 8000D. The alloyednanopowders were then compacted into dense bulk disks of12.7 mm in diameter in a graphite die through direct current(dc)-induced hot pressing. An argon gas-filled glovebox wasused in materials handling process to minimize contaminations.A laser flash system (NETZSCH LFA-457) and a DSC

system (NETZSCH DSC 200-F3) were used to measure thethermal diffusivity and the specific heat of the disk samples,respectively. Thermal conductivity was then calculated as the

Received: January 18, 2012Revised: March 17, 2012Published: April 11, 2012

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product of the thermal diffusivity, specific heat, and volumetricdensity that was determined by the Archimedes’ method. Afour-point probe system (ULVAC ZEM-3) was used tomeasure the Seebeck coefficient and electrical conductivity ofthe bar samples with dimensions of about 2 × 2 × 12 mm3. Hallmeasurements were carried out on a Lakeshore system (HallEffect System7712A) for thin disk samples of around 0.5 mm inthickness. The structures of the as-pressed samples werecharacterized by X-ray diffraction (XRD Bruker-AXS, G8GAADS) using Cu radiation (Kα: 1.54 Å), field emissionscanning electron microscopy (SEM, JEOL-6340F), andtransmission electron microscopy (TEM, JEOL-2010F).Through bal l mil l ing and hot press ing, dense

Tl0.02Pb0.98TeSixNay samples (∼97% relative to theoreticaldensity) with different values of x and y were prepared, andall were crystallized in rock salt structure evidenced by X-raydiffraction patterns, shown in Figure 1a. Within the detectionlimit of XRD, there is no impurity found.One interesting phenomena is that after 770 K measure-

ments of Seebeck coefficient and resistivity in ZEM-3 the barsamples without Si bend like a bow, shown in Figure 1b, whichclearly shows the weak mechanical strength at that temperature.In our previous TEM work on PbTe-based materials, we foundhigh density of defects, such as Pb-depleted disks,19 after highenergy ball milling and hot pressing. The bending of the samplewithout Si may be due to the high residual stress and strain inthe sample made by high energy ball milling process, which willbe detailed later.For mechanical strengthening, Si was added in the materials,

inspired by its effects in the PbTe:I material to overcomebrittleness.18 With sufficient Si addition, the sample is strongenough to experience high temperature measuring, as shown inthe photograph of Figure 1c. However, the electricalconductivity was degraded significantly. To bring back theelectrical conductivity, we added Na to further dope the Siadded Tl0.02Pb0.98Te samples, as suggested by Singh’scalculation20 and evidenced by Pei’s experimental results21

that heavy doping would help to improve thermoelectricproperties of PbTe. Recent work by Wang et al. on reducingthe thermal conductivity using TlSbTe2 addition inTl0.02Pb0.98Te also found the decrease of electrical conductivity,resulting in a lower ZT than the pure Tl0.02Pb0.98Te sample.

22

To understand the effects of the addition of Si and doping ofNa in Tl0.02Pb0.98Te samples, extensive microscopy study wasconducted by SEM, TEM, and HRTEM. The main results areshown in Figure 2. Analyzed by the linear intercept particlemethod23 from the SEM image of Tl0.02Pb0.98Te sample(without Si and Na) shown in Figure 2a, the average grainsize of this sample is ∼7 μm, much bigger than the ball milledstarting nanopowder with particle size less than 50 nm, clearlyindicating significant grain growth happened during hotpressing as shown in our previous work.11 It is interesting tonote that after a 2 at. % Si addition the grain size dramaticallydecreased to ∼200 nm, shown in Figure 2b. Although themechanism of refinement by Si addition is not clear now, thisdramatic decrease in grain size, certainly causes much strongerphonon scattering than that without Si addition, and leads tothe reduction of lattice thermal conductivity. Like our previousresults on PbTe,19 there are Pb-depleted disks, and the straincaused by it, shown in Figure 2d,f. These features withdimensions of a few nanometers will also contribute to thelattice thermal conductivity reduction. By doping Na, the grainsgrow to an average of ∼500 nm, shown in Figure 2c.

Another interesting structure character discovered by TEMstudies is that the defect density is significantly decreased byaddition of Si. Our previous report on PbTe19 showed that thesample without Si have many Pb-depleted disks lying in the⟨001⟩ planes (the inset of Figure 2d shows the plane

Figure 1. (a) XRD spectra of various samples: (1) Tl0.02Pb0.98Te, (2)Tl0 . 0 2Pb0 . 9 8TeSi0 . 0 2 , (3) Tl0 . 0 2Pb0 . 9 8TeSi 0 . 0 2Na0 . 0 1 5 , (4)Tl0.02Pb0.98TeSi0.02Na0.02, and (5) Tl0.02Pb0.98TeSi0.02Na0.025, (b) Photo-graph of the softened sample Tl0.02Pb0.98Te after measurement up to673 K. (c) Photograph of the strong sample Tl0.02Pb0.98TeSi0.02Na0.02up to 770 K.

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orientation) with a volume density of 9 × 1017 cm−3, diameterof 2−5 nm, and thickness less than 0.5 nm. It is believed thatthese defects cause strain and stress near the disks leading tothe sample bending at temperature higher than 673 K. It issurprising that with Si addition and Na doping the volumedensity of the Pb-depleted disks is greatly decreased, as shownin Figure 2e,f, which is believed to be one of the main reasonsof the much higher mechanical strength in the sample with Siaddition and leads the samples to endure temperature up to770 K without bending.The Hall effect measurements show that the carrier

concentration and mobility of Tl0.02Pb0.98Te samples preparedby ball milling and hot pressing are 2.8 × 1019 cm−3 and 72.7cm2 V−1 s−1, respectively, which are different from 5.3 × 1019

cm−3 and 50 cm2 V−1 s−1 in the samples prepared by meltingmethod reported by Heremans,13 primarily due to the different

fabricating processes. After a 2 at. % Si addition, these valuesturn into 1.9 × 1019 cm−3 and 44.4 cm2 V−1 s−1, resulting in amuch lower electrical conductivity (further discussed later).The decreasing of carrier concentration and mobility from Siaddition is consistent with the results reported by Nemov.24 Bydoping of 1.5, 2, and 2.5 at. % Na, the carrier concentrationincreased to 5.7, 8.9, and 12 × 1019 cm−3, respectively, while themobility stayed as 42, 46, and 45.7 cm2 V−1 s−1, respectively.The room temperature (RT) Seebeck coefficient dependence

on carrier concentration (Pisarenko plot25) of the samples isillustrated in Figure 3a, compared with the reported Tl13 andNa21,26−29 doped PbTe samples, as well as the recentlyreported PbTeS:Tl and PbTeSe:Tl data.8 When the carrierconcentration is higher than 5 × 1019 cm−3, the deviation of thereported Na-doped samples from the single parabolic band(SPB) model30 (solid line in Figure 3a) clearly shows the

Figure 2. (a) SEM image of sample Tl0.02Pb0.98Te, (b) SEM image of sample Tl0.02Pb0.98TeSi0.02, (c) SEM image of sample Tl0.02Pb0.98TeSi0.02Na0.02,(d) TEM image of sample Tl0.02Pb0.98Te showing the very high density of Pb-depleted disks (the inset is the FFT of this image), (e) TEM image ofsample Tl0.02Pb0.98TeSi0.02Na0.02 with low density of Pb-depleted disk, with the inset of FFT of this image, and (f) HRTEM of sampleTl0.02Pb0.98TeSi0.02Na0.02.

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failure of SPB model in PbTe, where there are actually twononparabolic valence bands: a “light” one whose band edge liesat ∼0.3 eV under the bottom of conduction band and another“heavy” one whose band edge lies at ∼0.2 eV under the “light”one, as discussed in the reported theory20,31−33 andexperimental work.26,34−39 When the carrier concentration ishigher than 5 × 1019 cm−3, the Fermi level enters in the secondband, and the “heavy” carrier contributes to higher thermo-power, which leads to the deviation from the SPBmodel.21,26−29 The Tl-doped PbTe samples, with the help ofdistortion of density of states (DOS) caused by resonant statesfrom Tl, have much higher Seebeck coefficient than thatwithout resonance enhancement13 at the same carrierconcentration. The research on the Tl effects was extendedto PbTexS1−x and PbTeySe1−y in recent report,

8 which yielded apeak ZT ∼ 1.6 at 700 K, due to the Seebeck enhancement ofresonant states created by Tl, illustrated also in Figure 3a. TheRT Seebeck coefficient of our Tl0.02Pb0.98TeSi0.02Nax (x = 0,0.015, 0.02, 0.025) samples, shown in Figure 3a, confirms theresonant states created by Tl at even higher carrierconcentration than that reported by Heremans.13 One moreinteresting thing can be observed in Figure 3a is that in the Tl-doped samples where resonant states exist in our study andother reports,8,13 the Seebeck coefficients at RT decreased from180 to 150 μV K−1 (about a 20%) when carrier concentrationincreased from 2 × 1019 cm−3 to 12 × 1019 cm−3, wherease thedecrease is about 50% for the samples without resonant states,

which agrees well with the recent theory prediction includedresonant states effects.17

The temperature dependence of Seebeck coefficient shownin Figure 3b clearly demonstrate that the sample of the lowestcarrier concentration, Tl0.02Pb0.98TeSi0.02, has the highestSeebeck coefficient, which may be possibly due to the carrierscattering from grain boundary and Si impurity. From Figure 2and the discussions above, the Tl0.02Pb0.98TeSi0.02 sample hasthe smallest grain size in the studied samples that would causestrongest carrier scattering by grain boundaries, resulting inhigher Seebeck coefficient. This also can explain higher Seebeckcoefficient than Tl-doped PbTe/Se/S samples8 and theoreticalprediction17 in the same carrier concentration. In the range of570−670 K, the Seebeck coefficient of this sample showsindependent relationship with temperature and begins todecrease with temperature at above 670 K, due to the presenceof thermally excited minor carriers. In the three samples withNa doping, the sample of moderate Na doping and carrierconcentration, 2 at. % and 8.9 × 1019 cm−3, respectively, has thehighest Seebeck coefficient in the measured temperature, whichmay originate from the good alignment of Fermi level with theenergy of the Tl impurity created resonant states, that wouldlead to higher Seebeck coefficient enhancement.40,41 Attemperature higher than 570 K, Seebeck coefficient staysalmost the same for samples with 1.5 and 2 at. % Na dopingprimarily due to the bipolar effect, leading to lower Seebeckcoefficient than the reference data.The electrical conductivity and power factor are demon-

strated in Figures 4a and 4b, respectively, compared withreference data (dashed line) from Heremans.13 The 2 at. % Siadded Tl0.02Pb0.98Te sample has electrical conductivity of 18000 S m−1 at 300 K and 9000 S m−1 at 767 K, respectively,about half of the reference data, which is due to the low carrierconcentration and high grain boundary scattering of carriers

Figure 3. (a) RT Seebeck coefficient dependence of carrierconcentration (Pisarenko plot). The references are Jaworski,8

Heremans,13 Airapetyant,26 Pei,21 Crocker,27 Alekseeva,28 andAndroulakis.29 The solid line is calculated from single parabolicband model (SPB)30 where acoustic scattering is assumedpredominant and m* = 0.36m0 as that adopted by Airapetyant.

26 (b)Seebeck coefficient dependence of temperature of the studied samples;the dashed line is reference data from Heremans.13

Figure 4. Electrical conductivities (a) and power factor (b) of thestudied samples. The reference data (dashed line) is from Heremans.13

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caused by Si nanoparticles. To make up the loss of electricalconductivity by Si, Na as an effective p-type dopant in PbX (X= Te, Se, and S)12,15,21,42,43 was doped into Tl0.02Pb0.98TeSi0.02samples. The results indicate that, first, the electricalconductivity is greatly improved and is even higher than thereference data when enough Na is used. Second, higherconcentration of Na leads to higher electrical conductivity. The2 at. % Na-doped Tl0.02Pb0.98TeSi0.02 sample has electricalconductivity of 18 000 S m−1 at 770 K, a little bit higher thanthat of reference data. This improvement of electricalconductivity is due to the increased carrier concentration byNa doping.The power factors of the measured samples, calculated from

S2σ, are shown in Figure 4b. It can be seen that theTl0.02Pb0.98TeSi0.02 sample has the lowest power factor due tothe lowest electrical conductivity, but Na doping increased thepower factors significantly. The power factors are even higherthan the reference data at temperatures lower than 525 Kthough lower than the reference data at temperatures higherthan 525 K. The highest power factor at 770 K, 16.3 μWcm−1 K−2, of Tl0.02Pb0.98TeSi0.02Na0.02 sample is about ∼13%lower than that of the reference sample.13

The total thermal conductivity was calculated using κ =ρDCp, where ρ is the volumetric density, D the diffusivity(Figure 5a), and Cp the specific heat (Figure 5b). In calculatingκ, the Cp values of 2 at. % Na-doped sample were used for allNa-doped samples in a conservative way. The total (Figure 5c)and lattice thermal conductivity (Figure 5d) of our studiedsamples are plotted together with the Si added PbTe samples,18

the only data we can find in the literature of Si added PbTe, asreference as well as that of samples PbTe:Na21 and PbTe:Tl.13

The lattice thermal conductivity is calculated by subtracting theelectronic contribution from the total thermal conductivity (κL= κ − κe = κ − LσT, where L is the Lorenz number). Becausethe difficulty of accurate determination of Lorenz number dueto the complex band structure and nonparabolicity of the lighthole band, especially when presented with Tl created resonantstates which has strong impact on the carrier transport, anassumption of one parabolic band, predominant acousticscattering of phonons, and elastic mechanism of carriers

scattering was used to estimate L,44 where the reduced Fermienergy were deduced from the Seebeck coefficient in SPBmodel.30 Despite the roughness in the estimation, the result ofL ∼ 1.52 × 10−8 V2 K−2 at ∼770 K is consistent with therigorous calculation based on multiband model havingconsidered the nonparabolicity of the band,44 quite lowerthan the widely used metal values of 2.44 × 10−8 V2 K−2. Also,the RT Lorenz number ∼1.7 × 10−8 V2 K−2 is about 11% offthe results of Kaidanov,45 who ascribed the reducing factor of1.65 from full degenerate value to the resonant scattering by Tlin PbTe. This method of estimation was used to recalculate theLorenz number of the PbTe:Si sample.18

From Figure 5d, the lattice thermal conductivity of thestudied samples is well below the reference data. The κL ofTl0.02Pb0.98TeSi0.02Na0.02 sample at 770 K, 0.54 W m

−1 K−1, isabout 27% lower than that of PbTe:Tl sample,13 and 45% lowerthan that of PbTe:Na sample,21 which shows big increase ofphonon scattering by the grain boundary and the point defects,directly from the fine Si particles. The sample without Nadoping, having smallest grain size in this study (shown inFigure 2b), demonstrates the lowest κL, confirming the sizeeffects on lattice thermal conductivity reduction. It is interestingthat the κL of our 2 at. % Si added samples from RT to 580 K isabout 40%−22% lower than the PbTeSi0.08 sample fabricated bymelting and quenching method,18 which again suggests aneffective way of κL reduction by ball milling and hot pressingmethod.Because of the quite low lattice thermal conductivity by

greatly increased grain boundary scattering of phonons, the Siadded Na doped samples have much lower total thermalconductivity, shown in Figure 5c, leading to ZTs higher thanthe reference data shown in Figure 6. We achieved peak ZTvalue ∼1.7 at 770 K in samples Tl0.02Pb0.98TeSi0.02Na0.02, closeto the most recently reported 1.8 in samples PbSeTe:Na12 andPbTe:Sr,Na.15

In summary, Si was found to have dramatically increased themechanical strength of samples Tl0.02Pb0.98TeSi0.02 made by ballmilling and hot pressing due to probably the dramaticallydecreased defect density of Pb-depleted disks and much smallergrains of ∼200 nm, but also much lower electrical conductivity

Figure 5. Thermal diffusivity (a), Cp (b), thermal conductivity (c), and lattice thermal conductivity (d) of the studied samples, with the referencesamples of Tl0.02Pb0.98Te from Heremans,

13 PbTe:Na from Pei,21 and PbTe:Si from Sootsman.18

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for lower ZTs. However, a small amount of Na doping forsamples Tl0.02Pb0.98TeSi0.02Na0.02 brings back the high electricalconductivity while keeping the enhanced Seebeck coefficient byTl created resonant states and also produces low thermalconductivity, with the lowest lattice thermal conductivity of∼0.54 W m−1 K−1 at 770 K. The highest ZT value reaches ∼1.7a t 7 7 0 K i n m e c h a n i c a l l y s t r o n g s am p l e sTl0.02Pb0.98TeSi0.02Na0.02 involving resonant doping, nanograins,and high carrier concentration.

■ AUTHOR INFORMATIONNotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work is funded by the “Solid State Solar-Thermal EnergyConversion Center (S3TEC)”, an Energy Frontier ResearchCenter funded by the U.S. Department of Energy, Office ofScience, Office of Basic Energy Science, under Award DE-SC0001299/DE-FG02-09ER46577 (G.C. and Z.F.R.).

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Figure 6. Dimensionless figure-of-merit (ZT) dependence of temper-ature of the studied samples.

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