High performance thermoelectrics from earth-abundant materials: Enhanced figure of merit in PbS through nanostructuring grain size

Accepted Manuscript High performance thermoelectrics from earth-abundant materials: Enhanced figure of merit in PbS through nanostructuring grain size Cheng Chang, Yu Xiao, Xiao Zhang, Yanling Pei, Fu Li, Shulan Ma, Bifei Yuan, Yong Liu, Shengkai Gong, Li-Dong Zhao PII:

S0925-8388(15)31312-8

DOI:

10.1016/j.jallcom.2015.10.052

Reference:

JALCOM 35607

To appear in:

Journal of Alloys and Compounds

Received Date: 30 August 2015 Revised Date:

25 September 2015

Accepted Date: 6 October 2015

Please cite this article as: C. Chang, Y. Xiao, X. Zhang, Y. Pei, F. Li, S. Ma, B. Yuan, Y. Liu, S. Gong, L.-D. Zhao, High performance thermoelectrics from earth-abundant materials: Enhanced figure of merit in PbS through nanostructuring grain size, Journal of Alloys and Compounds (2015), doi: 10.1016/ j.jallcom.2015.10.052. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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High

performance

thermoelectrics

from

earth-abundant materials: Enhanced figure of merit

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in PbS through nanostructuring grain size Cheng Chang,1 Yu Xiao,1 Xiao Zhang,1 Yanling Pei,1 Fu Li,2Shulan Ma,3 Bifei Yuan,4 Yong Liu,5 Shengkai Gong,1 Li-Dong Zhao1,* 1

School of Materials Science and Engineering, Beihang University, Beijing 100191, China Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China 3 Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, China. 4 CNPC Greatwall Drilling Company (GWDC), Beijing, 100101, China 5 AVIC Beijing Inst Aeronaut Mat, Beijing 100095, China

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*Corresponding author: [email protected]

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ABSTRACT As an earth abundant material, PbS, has been paid extensive attention in the thermoelectric community. The high lattice thermal conductivity of PbS indicates there is room left to further improve thermoelectric performance of PbS. In this system we aimed to

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reduce the lattice thermal conductivity through nanostructuring grain sizes, which was processed by mechanical alloying followed by spark plasma sintering. We found that the lattice thermal conductivity can be reduced from ~ 1.5 Wm-1K-1 at 723K for PbS ingot to as low as ~ 0.50 Wm-1K-1 for the PbS with nano-scale grain sizes, as a result, a ZT value of 0.80

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at 723 K was achieved for 1.0% PbCl2 doped PbS with the second phases of Bi2S3 (Sb2S3, SrS, CaS). These results indicate that PbS is a robust alternative of PbTe and PbSe for

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medium temperature thermoelectric applications.

Keywords: PbS; Thermoelectric materials; Electrical conductivity; Seebeck coefficient;

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Thermal conductivity

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Introduction Thermoelectric power generation technology is a type of solid-state ‘heat engine’ that is capable of converting heat to electricity.1,2 The efficiency of thermoelectric materials is determined by the dimensionless figure of merit, ZT=(S2σ/к)T, where S, σ, к, and T are the

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Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively.3,4 Therefore, excellent thermoelectric materials require a perfect combination of high power factor (S2σ) and low thermal conductivity (κ).5 To date, several classes of bulk materials with high ZT values have been discovered, including nanostructured BiSbTe

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alloys,6 AgPbmSbTem+2(LAST),7 Tl doped PbTe,8 Na doped PbTe1-xSex,9 Na doped PbTe-SrTe.10 These materials show high performance at room and middle temperature range

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(600-850K), however, the common feature of these materials is that they contain significant amount of Te, which is a scarce element in the crust of the earth. Hence the Te price is likely to rise sharply if Te-based thermoelectric materials reach mass markets. A broad search for more inexpensive alternatives is therefore warranted.

PbS could potentially be such a highly alternative because it has many attractive features.

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For example, PbS shares the same highly symmetric NaCl-type cubic structure as its heavier congeners, it is composed of abundant elements, it has an even higher melting point (1391K) and energy band gap (0.41 eV). By comparison PbTe has 1197K and 0.32 eV

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respectively.11-13 Recently, we investigated the thermoelectric properties of Te-free PbS and reported that significant thermoelectric enhancements can be achieved through introducing

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nanostructures,11-13 and then several groups reconfirmed the potential thermoelectric properties of PbS.14-17 These results imply that PbS-based materials may in fact be promising for high temperature application with continued improvements. However, one can find that the lowest lattice thermal conductivity for the best performing samples has been observed so far is ~0.70 Wm-1K-1,13 which is still much higher than the “minimal lattice thermal conductivity” value of ~0.36 Wm-1K-1 for lead chalcogenides as suggested by Cahill et al.18 Wu et al reported that the lattice thermal conductivity of PbS could be reduced as low as ~0.50 Wm-1K-1 at 923K by strong phonon scattering using nano-scale grains,15 which indicated an even higher ZT value could be expected in PbS through further reducing lattice 3

ACCEPTED MANUSCRIPT thermal conductivity. Usually, reducing the thermal conductivity is an effective way to enhance ZT, and it can be reduced significantly by introducing nano-grain boundaries,6 nano-particles,19 in situ nanoprecipitates,11-13,20,21 endotaxial nanometer-sized inclusions10,22 etc. In this paper, we

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focus on the high-performance PbCl2 doped PbS with second phases Bi2S3 (Sb2S3, SrS, CaS),13 a very stable and simple system which contains highly abundant elements. In this system we aimed to reduce the lattice thermal conductivity through nanostructuring grain sizes, which was processed by mechanical alloying (MA) followed by spark plasma sintering

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(SPS). We found that the lattice thermal conductivity of PbS can be reduced from ~ 1.5 Wm-1K-1 at 723K for PbS ingot to as low as ~ 0.50 Wm-1K-1 for the MA+SPS processed PbS,

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as a result, a ZT value of 0.8 at 723 K was achieved for 1.0% PbCl2 doped PbS with second phases of Bi2S3 (Sb2S3, SrS, CaS). These results indicate that PbS is a boost potential thermoelectric material for medium temperature thermoelectric applications.

1) Sample preparation

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Experimental

Starting materials: Reagents chemicals were used as obtained: Pb wire (99.99%, American Elements, US), S shot or chunk (99.999%, Inc., Canada), Bi chunk (99.999%,

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Noranda, Canada), Sb shot form (99.999%, Noranda, Canada), Sr chunk (99.9%, Cerac, US), Ca redistilled granule (99.5%, Alfa Aesar, US) and PbCl2 powders (99.999%, Aldrich, US).

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PbS: PbS was prepared by a melting reaction method using mixing elemental Pb and S inside carbon-coated fused silica tubes. The tubes were then evacuated to a base pressure of ~10-4 torr, flame-sealed, slowly heated to 723 K in 12 h, and then heated to 1423 K in 7 h, soaked at this temperature for 6 h and subsequently air quenched to room temperature. Bi2S3 (Sb2S3): Bi2S3 (Sb2S3) was prepared by mixing elemental Bi (Sb) and S inside fused silica tubes. The tubes were subsequently evacuated to a base pressure of ~10-4 torr, fused, slowly heated to 723 K in 12 h, then heated to 1223 K in 5 h, soaked at this temperature for 6 h and subsequently cooled to room temperature.

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ACCEPTED MANUSCRIPT SrS (CaS): SrS (CaS) was synthesized using elemental Sr (Ca) and S inside an Al2O3 crucible and then charged in a fused silica tube. The manipulations and preparative steps for the SrS (CaS) starting powders were carried out within a purified Ar-atmosphere glove box, with total O2 and H2O level < 0.1 ppm. The silica tubes were subsequently evacuated to a

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base pressure of ~10-4 torr, fused, slowly heated to 723 K in 12 h, then heated to 1153 K in 4 h, soaked at this temperature for 20 h and subsequently cooled to room temperature.

PbS0.9Cl0.1+1% Bi2S3 (Sb2S3, SrS and CaS): PbS ingots (~10 g) with 1 mol % doping PbCl2 and/or with 1 mol% second phases (Bi2S3, Sb2S3, SrS and CaS) were synthesized by

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melting reaction method through mixing appropriate ratios of starting materials in carbon-coated silica tubes (Ø ~ 8 mm) under an Ar-filled glove box. The tubes were sealed

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under high vacuum (~10-4 torr) and slowly heated to 723 K in 12 h, then heated to 1423 K in 7 h, soaked at this temperature for 6 h. The furnace was rocked for good mixing of the molten sample, which tends to reduce ingot bubbles and ensures homogenous composition. The tubes were subsequently air quenched to room temperature.

Nano-polycrystalline PbS0.9Cl0.1+1%Bi2S3(Sb2S3, SrS and CaS): The obtained PbS

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ingots were reduced into pieces by hand using mortar and pestle. The PbS pieces were subjected to mechanical alloying (MA) process in a planetary ball mill (QM-4F, Nanjing University, China) at 450 rpm for 20 h in a purified argon atmosphere. Stainless steel vessels

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and balls were used. The weight ratio of ball to powder was kept at 20:1. The powders subsequently were densified using a spark plasma sintering (SPS) system (SPS-211Lx) at 823 K with holding time of 10 min in a Ф 20 mm graphite die under an axial compressive stress

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of 60 MPa, highly dense samples can achieve >97% of theoretical density. Highly dense disk-shaped pellets with dimensions of 20 mm (diameter) × 9 mm (thickness) were obtained. 2) Properties characterization Electrical properties: The obtained SPS processed pellets were cut into bars with dimensions of 18 mm × 3 mm × 3 mm that were used for simultaneous measurement of the Seebeck coefficient and the electrical conductivity using an Ulvac Riko ZEM-3 instrument under a helium atmosphere from room temperature to 723 K. The samples were coated with a thin layer (0.1-0.2 mm) of boron nitride (BN) to protect instruments. Heating and cooling

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ACCEPTED MANUSCRIPT cycles gave repeatable electrical properties to verify the thermal stability. The uncertainty of the Seebeck coefficient and electrical conductivity measurements is 5%. Hall measurements: Room Hall coefficients were measured on a home-built system in magnetic fields ranging from 0 to 1.25 T, utilizing simple four-contact Hall-bar geometry, in

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both negative and positive polarity to eliminate Joule resistive errors. Thermal conductivity: High density SPS processed pellets were cut and polished into coins of Ø ~8 mm and 1-2 mm thickness for thermal diffusivity measurements. The samples were coated with a thin layer of graphite to minimize errors from the emissivity of the

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material. The thermal conductivity was calculated from κ=D·Cp·ρ, where the thermal diffusivity coefficient (D) was measured using the laser flash diffusivity method in a Netzsch

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LFA457, the specific heat capacity (Cp) was indirectly derived using a representative sample (Pyroceram 9606) in the range 300-723K. The density (ρ) was determined using the dimensions and mass of the sample, which was then reconfirmed using a gas pycnometer (Micromeritics AccuPyc1340) measurements. The thermal diffusivity data were analyzed using a Cowan model with pulse correction and heating and cooling cycles give reproducible

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values for each sample. The uncertainty of the thermal conductivity is estimated to be within 5%, considering the uncertainties for D, Cp and ρ. The combined uncertainty for all measurements involved in the calculation of ZT is about 20 %.

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Electron microscopy and X-ray diffraction: Scanning electron microscopy (SEM) studies were performed using a Hitachi S-3400N VP-SEM equipped with an Oxford detector for energy dispersive X-ray spectroscopy (EDS). The samples used for SEM and EDS were

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the coins used for thermal diffusivity, and they were polished using a suspension of 50 nm Al2O3 particles. Samples pulverized with an agate mortar were used for powder X-ray diffraction (XRD). The powder diffraction patterns were obtained with Cu Kα (λ=1.5418Å) radiation in a reflection geometry on an Inel diffractometer operating at 40 kV and 20 mA and equipped with a position-sensitive detector.

Results and discussion Figure 1(a) shows the powder XRD patterns of PbS ingot and nano-polycrystalline PbS, 6

ACCEPTED MANUSCRIPT they show single phase that can be indexed to the NaCl structure type. It can be seen that the XRD peaks of PbS processed by mechanical alloying (MA) are wider than those of PbS ingot, which indicates that the grain sizes are significantly decreased by MA. Figure 1(b) shows the XRD patterns of PbCl2 doped PbS with different second phases of Bi2S3 (Sb2S3, SrS and

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CaS), no PbCl2 or Bi2S3 (Sb2S3, SrS or CaS) or other phases were observed within the detection limits of the measurements.

Figure 2(a) indicates that MA method could process PbS powders into nano-scale, the particle size is around several tens of nanometer. Figure 2(b) is SEM image of the PbS bulk

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densified by spark plasma sintering (SPS), the grain experiences slightly growth after SPS, grain size still maintains nano-scale. The scanning electron microscope (SEM) images of the

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PbS bulk confirm that MA+SPS is an effective and facile method to introduce nano-scale grains, which lead to the reduction of lattice thermal conductivity. Figures from (c) to (h) show the element distributions determined by energy dispersive X-ray spectroscopy (EDX) in the lower images, the elemental distributions determined by EDX indicate that all the elements (Pb, S, Bi, Sb, Sr and Ca) are homogeneously distributed throughout the samples.

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As shown in Figure 3(a), the electrical conductivity of the PbS samples increases with rising temperature. After PbCl2 doping, the room temperature electrical conductivity significantly increases from ~25 Scm-1 for the undoped PbS to ~190 Scm-1 for the sample with 1.0 % PbCl2 doping. The enhanced electrical conductivity by PbCl2 doping is consistent

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with the room temperature carrier concentration, which increases from 4×1018cm-3 for undoped PbS to 5.5×1019cm-3 for PbCl2 doped PbS. The electrical conductivity is lowered

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after adding the second phases Bi2S3 (Sb2S3, SrS and CaS) in the entire temperature range, which is due to the carrier scattering as indicated by the carrier mobility since the carrier concentration is fixed by PbCl2 doping, as shown in Figure 3(b). Figure 3 (c) shows that the Seebeck coefficients are negative, indicating all samples are n-type conductors. At room temperature, the Seebeck coefficient decreases by adding PbCl2. Namely, it varies from about −230 µV/K Scm-1 for the undoped pristine PbS to about −75 µV/K for the sample after 1.0 % PbCl2 doping, which is consistent with an increase of the carrier concentrations induced by the substitution of S2- by Cl-. It also can be found that the Seebeck coefficients are independent of the second phases, which was also observed in lead chalcogenides with 7

ACCEPTED MANUSCRIPT second phases.11-13,20,21 The power factor of undoped PbS shows a maximum of ~5.8 µWcm-1K-2 at 723 K, Figure 3 (d). The power factor for the PbCl2 doped sample reaches about 8.9 µWcm-1K-2 at 723 K, and then decreases to 8.3 µWcm-1K-2 at 723 K for the samples with second phases Bi2S3 (Sb2S3, SrS and CaS).

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Figure 4(a) shows the thermal diffusivity of PbS system, the thermal diffusivity for all the samples decrease with rising temperature. The heat capacity increases with adding second phases and rising temperature, Figure 4(b). The total thermal conductivity (κtot) includes a sum of the electronic (κele) and lattice thermal conductivity (κlat). As shown in Figure 4(d), the

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increase of the total thermal conductivity of PbCl2 doped PbS is the result of the increasing fraction of the electronic thermal conductivity (κele). The electronic part κele is directly

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proportional to the electrical conductivity σ through the Wiedemann-Franz relation, κele = LσT, where L is the Lorenz number. Since nanostructuring has been proved to be an effective method of reducing the lattice thermal conductivity κlat, separating the electronic and lattice parts is important in studying the effect of nanostructures. In this work, Lorenz numbers were extracted on the basis of fitting the respective Seebeck coefficient values that estimate the

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reduced chemical potential, the calculation details were explained previously.11-13,20,21 The Lorenz numbers are shown in Figure 4(d). After proper calculation of κele, Figure 4(e), we observe that the lattice thermal conductivity is nearly independent of doping, see Figure 4(f),

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but the lattice thermal conductivity was significantly reduced by introducing second phases Bi2S3 (Sb2S3, SrS and CaS). Namely, the lattice thermal conductivity at room temperature was reduced from 1.7 Wm-1K-1 for undoped PbS to 1.2 Wm-1K-1 for the PbS samples with

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second phases Bi2S3 (Sb2S3, SrS and CaS), correspondingly, lattice thermal conductivity at 723K was reduced from 0.71 Wm-1K-1 for undoped PbS to 0.50 Wm-1K-1 for the PbS samples with second phases.

Figure 5 (a) shows the lattice thermal conductivity comparisons for PbS ingot,13 micro-scale polycrystalline PbS13 and nano-scale polycrystalline PbS. It can be seen that the lattice thermal conductivity was significantly reduced through fining grain size, and further decreased by introducing second phases. The lattice thermal conductivity could be reduced as low as 0.50 Wm-1K-1 at 723K, which is the lowest value ever reported in the PbS system. As shown in Figure 5 (b), all samples exhibit high ZT values at 723 K, and exceed the value of 8

ACCEPTED MANUSCRIPT 0.31 of the PbS ingot.13 Comparing with the latter sample, the present higher ZT values are attributed to a lower thermal conductivity. The ZT value of 0.8 at 723 K has been obtained for PbCl2 doped PbS with second phases.

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Concluding remarks We demonstrated that mechanical alloying followed by spark plasma sintering is an effective method to synthesize nano-crystalline PbS materials. The present results reported a Te-free and inexpensive material, n-type PbS, can achieve high ZT value of 0.8 at 723 K. The

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high performance of PbS was accomplished by significantly reducing lattice thermal conductivity through fabricating nano-scale grain size. The promising thermoelectric

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properties indicate that PbS could be a robust alternative for PbTe and PbSe thermoelectric materials.3 Higher thermoelectric performance from PbS would be expected by increasing the Seebeck coefficient and the power factor through the incorporation of elements forming

Acknowledgments

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resonance states such as electronic band structure modifications.8,23,24

This work was also supported by the “Zhuoyue” program of Beihang University, the Recruitment Program for Young Professionals, and NSFC under Grant No. 51571007 (L.D.Z),

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by NSFC under Grant No. 51202008 and Postdoctoral Science Foundation of China

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(2013M540037) (Y.L.P), and partly supported by NSFC under Grant No. 51302140 (F.L).

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Figure captions Figure 1. Powder XRD patterns for (a) undoped PbS ingot and nano-polycrystal, (b) doped PbS with different second phases of Bi2S3 (Sb2S3, SrS and CaS). Figure 2. Microstructures of (a) PbS powders prepared by mechanical alloying and (b)

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the PbS bulk prepared by mechanical alloying and spark plasma sintering. Figures from (c) to (h) show the element distributions determined by energy dispersive X-ray spectroscopy (EDX) in the lower images: (c) Pb, (d) S, (e) Bi, (f) Sb, (g) Sr, and (h) Ca.

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Figure 3. Electrical transport properties as a function of temperature for PbCl2 doped PbS with different second phases of Bi2S3 (Sb2S3, SrS and CaS): (a) Electrical

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conductivity. (b) Carrier concentration and carrier mobility. (c) Seebeck coefficient. (d) Power factor.

Figure 4. Thermal transport properties as a function of temperature for PbCl2 doped PbS with different second phases of Bi2S3 (Sb2S3, SrS and CaS): (a) Thermal diffusivity. (b) Heat capacity. (c) Total thermal conductivity. (d) Lorenz number. (e)

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Electronic thermal conductivity. (f) Lattice thermal conductivity. Figure 5. (a) Lattice thermal conductivity comparisons for PbS ingot,13 micro-polycrystalline PbS,13 nano-polycrystalline PbS, nano-polycrystalline PbS with

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second phases of Bi2S3 (Sb2S3, SrS and CaS). (b) Dimensionless figure of merit (ZT) as a function of temperature for PbCl2 doped PbS with different second phases of

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Bi2S3 (Sb2S3, SrS and CaS).

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Figure 1. Powder XRD patterns for (a) undoped PbS ingot and nano-polycrystal, (b)

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doped PbS with different second phases of Bi2S3 (Sb2S3, SrS and CaS).

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Figure 2. Microstructures of (a) PbS powders prepared by mechanical alloying and (b) the PbS bulk prepared by mechanical alloying and spark plasma sintering. Figures from (c) to (h) show the element distributions determined by energy dispersive X-ray spectroscopy (EDX) in the lower images: (c) Pb, (d) S, (e) Bi, (f) Sb, (g) Sr, and (h) Ca. 3

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Figure 3. Electrical transport properties as a function of temperature for PbCl2 doped PbS with different second phases of Bi2S3 (Sb2S3, SrS and CaS): (a) Electrical conductivity. (b) Carrier concentration and carrier mobility. (c) Seebeck coefficient. (d)

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Power factor.

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Figure 4. Thermal transport properties as a function of temperature for PbCl2 doped PbS with different second phases of Bi2S3 (Sb2S3, SrS and CaS): (a) Thermal diffusivity. (b) Heat capacity. (c) Total thermal conductivity. (d) Lorenz number. (e) Electronic thermal conductivity. (f) Lattice thermal conductivity.

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Figure 5. (a) Lattice thermal conductivity comparisons for PbS ingot,13 micro-polycrystalline PbS,13 nano-polycrystalline PbS, nano-polycrystalline PbS with

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second phases of Bi2S3 (Sb2S3, SrS and CaS). (b) Dimensionless figure of merit (ZT) as a function of temperature for PbS ingot,13 and PbCl2 doped PbS with different

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second phases of Bi2S3 (Sb2S3, SrS and CaS).

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Highlights ●Nanocrystalline PbS was synthesized by mechanical alloying and spark plasma sintering.

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●The lattice thermal conductivity at 723K of PbS was reduced from 1.5 Wm-1K-1 to as low as ~0.50 Wm-1K-1.

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●A high ZT value of 0.80 at 723 K was achieved in PbS system.