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Enhancing thermoelectric performance of n-type PbTe through separately optimizing phonon and charge transport properties
Journal Pre-proof Enhancing thermoelectric performance of n-type PbTe through separately optimizing phonon and charge transport properties Zhi Yang, Siqi Wang, Yuejun Sun, Yu Xiao, Li-Dong Zhao PII:
S0925-8388(20)30740-4
DOI:
https://doi.org/10.1016/j.jallcom.2020.154377
Reference:
JALCOM 154377
To appear in:
Journal of Alloys and Compounds
Received Date: 14 January 2020 Revised Date:
13 February 2020
Accepted Date: 14 February 2020
Please cite this article as: Z. Yang, S. Wang, Y. Sun, Y. Xiao, L.-D. Zhao, Enhancing thermoelectric performance of n-type PbTe through separately optimizing phonon and charge transport properties, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154377. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.
Author Contributions Section Z. Y., S. Q. W. and Y. J. S. synthesized the samples. S. Q. W., Y. J. S. and Y. X. did the measurements work. Z. Y., S. Q. W., Y. J. S., Y. X. and L. D. Z conceived the experiments, analysed the results and co-edited the manuscript.
Enhancing thermoelectric performance of n-type PbTe through separately optimizing phonon and charge transport properties
Zhi Yang1, Siqi Wang1,2, Yuejun Sun1, Yu Xiao2,*, Li-Dong Zhao2,*
1
School of Materials Science and Engineering, Liaoning Technical University, Fuxin
123000, China 2
School of Materials Science and Engineering, Beihang University, Beijing 100191,
China E-mail: [email protected]; [email protected]
1
ABSTRACT: The n-type PbTe still maintains relatively low thermoelectric
performance compared with its p-type counterpart. Thus, to promote its performance in n-type PbTe, we conduct successive strategies that firstly suppressing the lattice thermal conductivity (κlat) through alloying with ternary compound AgSbSe2, and then optimizing carrier density with Sb, Bi or I doping. In AgSbSe2-alloyed PbTe samples, the κlat can be evidently reduced, and the minimum κlat decreases from ~ 1.1 Wm-1K-1 in undoped PbTe to ~ 0.8 Wm-1K-1 in PbTe-5%AgSbSe2. Based on PbTe-5%AgSbSe2 matrix, several dopants (Sb, Bi or I) are used to optimize its carrier density, and found that I element is verified to be the most effective dopant to enhance its electrical transport properties. In I-doped PbTe-5%AgSbSe2 samples, the carrier density raises from ~ 2.49 × 1016 cm-3 in PbTe-5%AgSbSe2 to ~ 1.15 × 1019 cm-3 in PbTe0.992I0.008-5%AgSbSe2, which contributes to the maximum power factor ~ 14.8 µWcm-1K-2. Combining the reduced lattice thermal conductivity and improved power factor, the maximum ZTmax and average ZTave values in PbTe0.992I0.008-5%AgSbSe2 are boosted to ~ 1.0 at 723 K and ~ 0.77 at 300-823 K, respectively. This work points out a valid avenue to improve thermoelectric transport property in n-type PbTe with separately successive strategies to adjust its thermal and electrical performance. Keywords:
Thermoelectric;
n-type
PbTe;
conductivity; ZT value.
2
Electrical
conductivity;
Thermal
1. Introduction In recent years, the research and development of green new energy materials have received much attention. Thermoelectric materials, as a kind of eco-friendly material, enable to directly and reversely convert thermal energy into electric power, therefore it possesses wide application prospects in mitigating energy loss and environmental issue[1-5]. The level of thermoelectric efficiency is mainly determined by the dimensionless figure of merit, ZT, ZT = S2σT/(κlat+κele), where S, σ, T, κlat and κele are the Seebeck coefficient, electrical conductivity, absolute temperature in Kelvin, the lattice thermal conductivity and electronic thermal conductivity, respectively[6-11]. It is obvious that the acquisition of eximious thermoelectric materials can be achieved by concurrently enhancing its power factor (S2σ) and depressing its thermal conductivity. But the three thermoelectric parameters are not independently controllable, making it difficult to pursue high thermoelectric performance. PbTe is a prominent thermoelectric material suitable for the medium temperature region (500-900 K)[12-14]. In order to bring about high efficiency in PbTe-based thermoelectric device, we need both p- and n-type PbTe with excellent thermoelectric transport performance. However, the thermoelectric transport proprety of n-type PbTe relatively falls behind that of p-type PbTe, thus it attracts broad attention to heighten the thermoelectric property of n-type PbTe in thermoelectric field. In term of n-type PbTe, it is difficult to improve the effective mass (m*) of electrons with band convergence[15] due to large energy difference (ΔEL-Σ = 0.45 eV) between the light band (L) and heavy band (Σ) in conduction bands[16]. Therefore, recent progresses for n-type PbTe universally perform nanostructures to reduce thermal conductivity after well optimizing electrical transport properties, such as introducing ZnTe nanostructures into I-doped PbTe to lower its thermal conductivity[17]; Using Ag2Te nanoparticle distribution in Bi-doped PbTe to prominently minish the lattice thermal conductivity[18]; Sb-rich
nano-precipitates in PbTe to cause an obvious diminution
of lattice thermal conductivity[19]. However, the nanostructures in above samples indeed lower the thermal conductivity, but deteriorate the electrical transport 3
properties simultaneously, finally obtain a limit enhancement of the net ZT. For n-type PbTe, well balancing the phonon and carrier transport property is vital to harvest good thermoelectric performance. In this work, we firstly introduce AgSbSe2 alloying into PbTe to lower its thermal conductivity, and then based on the sample with minimum lattice thermal conductivity, the donor dopants (Sb, Bi or I) are used to optimize its carrier transport properties. The roadmap of this work is schematically presented in Fig. 1. Results shows that AgSbSe2 alloying can largely reduce the thermal conductivity, and the minimum lattice thermal conductivity can be impeded from ~ 1.1 Wm-1K-1 in undoped PbTe to ~ 0.8 Wm-1K-1 in PbTe-5%AgSbSe2. Based on the matrix of PbTe-5%AgSbSe2, several dopants (Sb, Bi or I) are tentatively introduced to optimize its carrier density. Among these three dopants, I element shows the most effective doping behavior and largely boosts the power factor from ~ 3.4 µWcm-1K-2 in PbTe-5%AgSbSe2 to ~ 14.8 µWcm-1K-2 in PbTe0.992I0.008-5%AgSbSe2, contributing to a high ZT values in a broad temperature zone. Finally, the maximum ZTmax and average ZTave value in PbTe0.992I0.008-5%AgSbSe2 are boosted to ~ 1.0 at 723 K and ~ 0.77 at 300-823 K, respectively.
2. Experimental section 2.1 Sample preparation Raw materials included Pb pieces (99.99%, Aladdin element, China), Ag wires (99.9985%, Alfa Aesar, China), Sb pieces (99.99%, Aladdin element, China), Te pieces (99.999%, Aladdin element, China), Se pieces (99.999%, Aladdin element, China), Bi pieces (99.99%, Aladdin element, China), PbI2 granules (99.99%, Alfa Aesar, China). The quartz tubes filled with nominal compositions were sealed up under atmospheric pressure with 10-4 Pa, gradually heated from ambient temperature to 1323 K for 24 h, kept at 1323 K for 10 h, and ultimately cooled down to room temperature. The synthetic ingots were ground into fine powder (＜ 200 mesh), and subsequently loaded into graphite mold for spark plasma sintering (SPS-211Lx, Dr. Sinter). The sintered block was cut into a cuboid of 10 × 3 × 3 mm3 for testing electrical performance and a slice of 6 × 6 × 1.5 mm3 for testing thermal transport 4
performance. 2.2 Phase analysis The phase composition was tested at room temperature by X-ray diffraction (XRD, D/max 2200pc, Japen) using the Cu Kα radiation (λ = 1.5418 Å). The scanning range (2θ) and speed were 20-80° and 6°/min, respectively. 2.3 Electrical transport properties At first, the cuboid samples were sprayed with a thin layer of boron nitride to prevent sample evaporation. The electrical conductivity and Seebeck coefficient were tested under He condition by Cryoall CTA. 2.4 Thermal transport properties The total thermal conductivity (κ) was measured by κ = D × Cp × ρ, where D was the thermal diffusivity detected by Netzsch LFA 457 instrument based on the laser flash method, Cp was the heat capacity figured out by Debye model[20], and ρ was the sample density. 2.5 Hall measurement Using the Lake Shore 8400 Series equipment on the basis of Van der Pauw test principle, the room-temperature carrier density (n) was tested at a magnetic field ~ 0.9 T and direct current of 10 mA. The carrier mobility (µ) was calculated from µ = σ /(e × n), where σ was electrical conductivity and e was unit charge.
3. Results and discussion 3.1 Suppressing lattice thermal conductivity in n-type PbTe by introducing ternary compound AgSbSe2 To depress the thermal conductivity of n-type PbTe, we conduct the ternary compound AgSbSe2 into the PbTe matrix. In Fig. 2(a), the PXRD patterns of PbTe-x%AgSbSe2 (x = 2-6) samples reveal that every peaks coincide with cubic structure (Fm3m) and no extra peaks display. In Fig. 2(b), the lattice parameter gradually shrinks with ascending AgSbSe2 alloying content because of the substitution of smaller Ag+ (~ 1.15 Å) and Sb3+ (~ 0.92 Å) for larger Pb2+ (~ 1.2 Å), and smaller Se2- (~ 1.98 Å) for larger Te2- (~ 2.11 Å). The PXRD patterns confirm that Ag, Sb and 5
Se elements successfully dissolve in PbTe matrix. In Fig. 3, the thermal transport property of PbTe-x%AgSbSe2 (x = 2-6) samples is presented. In Fig. 3 (a), the total thermal conductivity is evidently depressed in the AgSbSe2-alloyed samples, decreasing from ~ 1.2 Wm-1K-1 in PbTe-2%AgSbSe2 to ~ 0.8 Wm-1K-1 in PbTe-5%AgSbSe2 at 300 K. In general, the total thermal conductivity (κtot) is contributed from charge carriers (κele) and lattice phonons (κlat). The electronic thermal conductivity (κele) is estimated by κele = L × σ × T based on the Wiedemann-Franz law, where L is Lorenz number, which is estimated by the reduced chemical potential, σ is electrical conductivity and T is the absolute temperature in Kelvin[21, 22]. Accordingly, the lattice thermal conductivity is acquired from the formula κlat = κtot-κele, and the calculated lattice thermal conductivity dramatically drops after AgSbSe2 alloying caused by atomic-scale defect scattering[23], as shown in Fig. 3(b). The difference of lattice parameters between AgSbSe2 (~5.79 Å) and PbTe (~6.44 Å) leads to intensive lattice distortion and phonon scattering, thereby a reduced lattice thermal conductivity. The minimum lattice thermal conductivity is impeded from ~ 1.0 Wm-1K-1 in PbTe-2%AgSbSe2 to ~ 0.8 Wm-1K-1 in PbTe-5%AgSbSe2. In Fig. 3(c), the relatively low electrical conductivity of PbTe-x%AgSbSe2 slightly declines from ~ 4.7 Scm-1 in PbTe-2%AgSbSe2 to ~ 0.7 Scm-1 in PbTe-6%AgSbSe2 at room temperature. In Fig. 3(d), the Seebeck coefficients present p-n transition, ultimately identified as n-type doping. Owing to the slight influence of AgSbSe2 alloying on electrical transport property, the samples remain a nearly constant power factor. Combining reduced lattice thermal conductivity with maintained power factor, a maximum ZT ~ 0.45 at 873 K is obtained in PbTe-3%AgSbSe2. Other thermoelectric diagrams related to PbTe-x%AgSbSe2 are displayed in Fig. 4.
3.2 Optimizing carrier density in n-type PbTe with Sb, Bi and I doping Because the low electrical transport property in PbTe-x%AgSbSe2 (x = 2-6) limits its final ZT value, we further utilize Sb, Bi and I as donor dopants to promote the carrier density based on PbTe-5%AgSbSe2 because it possesses relatively low 6
thermal conductivity. The powder X-ray diffraction patterns of Sb, Bi or I-doped PbTe-5%AgSbSe2 are shown in Fig. 5 (a), (c) and (e), respectively. All samples crystallize the cubic phase with NaCl crystal structure (space group: Fm3m) and no extra peaks of the secondary phases appear. In Fig. 5 (b), (d) and (f), the decrease of lattice parameter with Sb, Bi or I doping suggests the substitution of smaller Sb3+ (~ 0.76 Å) and Bi3+ (~ 0.96 Å) for larger Pb2+ (~ 1.2 Å), and smaller I- (~ 2.06 Å) for larger Te2- (~ 2.11 Å).
Thermoelectric
performance
of
Sb-doped
PbTe-5%AgSbSe2.
Temperature
dependent thermoelectric transport properties of Pb1-xSbxTe-5%AgSbSe2(x = 0.005-0.02) samples are shown in Fig. 6. The electrical conductivity is shown in Fig. 6(a) and has a large increase at 300 K from ~ 1.2 Scm-1 in PbTe-5%AgSbSe2 to ~ 899.6 Scm-1 in Sb-doped with x = 0.02. The negative Seebeck coefficients suggest n-type semiconducting feature. In Fig. 6(b), as Sb content increases, the absolute values of Seebeck coefficient decrease from ~ 334.8 µVK-1 in PbTe-5%AgSbSe2 to ~ 87.9 µVK-1 in Pb0.98Sb0.02Te-5%AgSbSe2 at 300 K. The improved electrical conductivity and decreased Seebeck coefficient indicate that Sb element in PbTe-5%AgSbSe2 can significantly boost the carrier density in the matrix, which is consistent with Hall measurements data. Because of the compensation between electrical conductivity and Seebeck coefficient, the maximum power factor in Fig. 6(c) is amplified from ~ 3.4 µWcm-1K-2 in PbTe-5%AgSbSe2 to ~ 13.8 µWcm-1K-2 in Pb0.98Sb0.02Te-5%AgSbSe2. In Fig. 6(d), the total thermal conductivity of PbTe-5%AgSbSe2 with Sb doping shows a significant increase with mounting Sb content originated from the rising electronic thermal conductivity after Sb doping. Correspondingly, the calculated lattice thermal conductivity decreases, and the minimum value is diminished from 0.80 Wm-1K-1 in PbTe-5%AgSbSe2 to ~ 0.57 Wm-1K-1 in Pb0.99Sb0.01Te-5%AgSbSe2. As shown in Fig. 6(f), a maximum ZT ~ 0.82 at 723 K is achieved in Pb0.99Sb0.01Te-5%AgSbSe2. Other thermoelectric diagrams related to Pb1-xSbxTe-5%AgSbSe2 are displayed in Fig. 7. 7
Thermoelectric
performance
of
Bi-doped
PbTe-5%AgSbSe2.
Temperature
dependent thermoelectric transport property of Pb1-xBixTe-5%AgSbSe2 (x = 0.005-0.02) samples is presented in Fig. 8. The electrical conductivity at 300 K increases largely from ~ 1.15 Scm-1 in PbTe-5%AgSbSe2 to ~ 885.4 Scm-1 in Pb0.98Bi0.02Te-5%AgSbSe2 as the carrier density is magnified from ~ 2.49 × 1016 cm-3 to ~ 5.57 × 1019 cm-3 in Table 1. The electrical conductivity through Bi doping is also altered visibly, exhibiting semiconductor characteristics, as shown in Fig. 8(a). The negative Seebeck coefficients for the Bi-doped PbTe-5%AgSbSe2 shown in Fig. 8(b) result in n-type doping. The absolute values of Seebeck coefficients at 300 K decrease from ~ 334.8 µVK-1 in PbTe-5%AgSbTe2 to ~ 71.9 µVK-1 in Bi-doping with x=0.015 as a result of improved carrier density. By virtue of the elevated electrical conductivity
and
reduced
Seebeck
coefficient,
the
power
factor
of
Pb0.99Bi0.01Te-5%AgSbSe2 achieves the maximum value ~ 10.4 µWm-1K-2 at 473 K. The total thermal conductivity rises after Bi doping in Fig. 8(d) due to increasing electronic thermal conductivity. After subtracting the contribution from carriers, the lattice thermal conductivity is the lowest one of ~ 0.55 Wm-1K-1 at 673 K in the Pb0.99Bi0.01Te-5%AgSbSe2 sample in Fig. 8(e). The maximum ZT ~ 0.82 is finally acquired in Pb0.99Bi0.01Te-5%AgSbSe2. Other thermoelectric diagrams related to Pb1-xBixTe-5%AgSbSe2 are displayed in Fig. 9.
Thermoelectric performance of I-doped PbTe-5%AgSbSe2. Temperature dependent thermoelectric transport property of PbTe1-xIx-5%AgSbSe2 (x = 0.002-0.012) samples is presented in Fig. 10. The dopant I in PbTe can effectively improve electrical conductivity, increasing from ~ 1.2 Scm-1 in PbTe-5%AgSbSe2 to ~ 251.2 Scm-1 in I-doping with x=0.012 at room temperature, as shown in Fig. 10(a). It can be clearly noticed in Fig. 10(b) that all the compositions confirm n-type conducting, which is supported by the negative Seebeck coefficient values. The absolute values of Seebeck coefficients at 300 K decrease from ~ 334.8 µVK-1 in PbTe-5%AgSbSe2 to ~ 134.8 µVK-1 in I-doping with x=0.012. The rise in electrical conductivity and the accompanied descent in Seebeck coefficient is ascribed to the ascending carrier 8
density, which is related to the Hall measurements. Finally, the maximum power factor shows a remarkable enhancement in Fig. 10(c) from ~ 3.4 µmWcm-1K-2 in PbTe-5%AgSbSe2 to ~ 14.8 µWcm-1K-2 in PbTe0.992I0.008-5%AgSbSe2. Fig. 10(d) shows that a significant ascension of total thermal conductivity with mounting I amount is associated with the raised electronic thermal conductivity. By contrary, the falling tendency is noticed in the lattice thermal conductivity at the same time. The minimum
lattice
thermal
conductivity
reaches
~
0.53
Wm-1K-1
in
PbTe0.992I0.008-5%AgSbSe2 at 673 K, as shown in Fig. 10(e). Overall, a high ZT ~ 1.0 is observed in PbTe0.992I0.008-5%AgSbSe2 at 723 K. Other thermoelectric diagrams related to Pb1-xIxTe-5%AgSbSe2 are displayed in Fig. 11.
Comparison of thermoelectric properties in n-type PbTe-5%AgSbSe2 with Sb, Bi and I doping. In conclusion, the ternary compound AgSbSe2 alloying aims to depress the lattice thermal conductivity and following with Sb, Bi or I doping to optimize the carrier density, respectively. To distinctly present the different effects of Sb, Bi or I dopants on n-type PbTe, the comparison in 5%AgSbSe2 with Pb0.99Sb0.01Te, Pb0.99Bi0.01Te, and PbTe0.992I0.008 are discussed in Fig. 12. Among the three samples, I doping is the most effective element to tune the carrier density in PbTe-5%AgSbSe2 and then well balances the electrical conductivity and Seebeck coefficient, ultimately causing a prominent power factor ~ 14.8 µWcm-1K-2 at 423 K and an average power factor ~ 8.84 µWcm-1K-2 at 300-873 K in PbTe0.992I0.008-5%AgSbSe2, as shown in Fig. 12(c) and (d). The average PFave value is calculated by:
PFave
∫ =
Th
Tc
PF (T )dT Th − Tc
(1)
where Tc is the cold-end temperature, Th is the hot-end temperature. The power factors of n-type PbTe with I doping are larger than those in n-type PbTe with Sb or Bi doping, indicating that I doping on anion site is more effective to produce electrons than Sb or Bi substitutions on cation sites. Notably, all samples can maintain a relatively low total thermal conductivity and lattice thermal conductivity resulting 9
from different lattice parameters (lattice parameter: PbTe ~ 6.44 Å, AgSbSe2 ~ 5.79 Å). The strong lattice distortion enable to availably scatter phonon, and the minimum lattice thermal conductivity shown in Fig. 12(f) respectively reaches ~ 0.55 Wm-1K-1, ~ 0.57 Wm-1K-1, ~ 0.53 Wm-1K-1 in PbTe-5%AgSbSe2 with Sb, Bi or I doping. Moreover, the total thermal conductivity of doped PbTe-5%AgSbSe2 samples (Sb ~ 0.70 Wm-1K-1, Bi ~ 0.72 Wm-1K-1, I ~ 0.69 Wm-1K-1) are all obviously lower than those of reported n-type PbTe (Sb ~ 0.85 Wm-1K-1[24], Bi ~ 1.71 Wm-1K-1[25], I ~ 1.31 Wm-1K-1[15]), which derives from low lattice thermal conductivity by the initial AgSbSe2-alloying, as shown in Fig. 12(e) and (f). Combining suppressed thermal conductivity and improved electrical properties, the peak ZT values reach ~ 0.82 in Pb0.99Sb0.01Te-5%AgSbSe2, ~ 0.83 in Pb0.99Bi0.01Te-5%AgSbSe2, and ~ 1.0 in PbTe0.992I0.008-5%AgSbSe2, as shown in Fig. 13(a). When it comes to practical applications of thermoelectric devices, the conversion efficiency depends on the average ZT rather than the maximum ZT, so it is necessary to promote the average ZT. In this work, the average ZTave value is calculated by[11-13, 26]:
ZTave
∫ =
Th
Tc
Z (T )dT
Th − Tc
(2)
where Tc is the cold-end temperature, Th is the hot-end temperature. Importantly, the average ZTave can reach ~ 0.62 in Pb0.99Bi0.01Te-5%AgSbSe2, ~ 0.65 in Pb0.99Sb0.01Te-5%AgSbSe2 and ~ 0.77 in PbTe0.992I0.008-5%AgSbSe2, which is 61.3%, 44.6%, 18.2% higher than the average ZTave ~ 0.24, ~ 0.36 and ~ 0.63 in PbTe with Bi[25], Sb[24] and I[15] doping, respectively.
4. Conclusions In this work, the ternary compound AgSbSe2 is reported as one inclusion that can obviously scatter phonon to suppress the lattice thermal conductivity of n-type PbTe. Based on low lattice thermal conductivity in PbTe-5%AgSbSe2, Sb, Bi or I dopants can further enhance its power factor by well optimizing carrier density. Results show 10
that I element is the most effective dopant in n-type PbTe-5%AgSbSe2, and a maximum power factor ~ 14.8 µWcm-1K-2 is obtained. Finally, a maximum ZT ~ 1.0 at 723 K and high ZTave ~ 0.77 at 300-873 K are gained in PbTe0.992I0.008+5%AgSbSe2. This work extends a valid avenue to optimize thermoelectric transport property in n-type PbTe-based materials through successively alloying to reduce thermal conductivity and effective doping to enhance carrier transport properties.
Acknowledgements This work was supported by the National Key Research and Development Program of China (2018YFA0702100 and 2018YFB0703600), National Natural Science Foundation of China (51772012 and 51671015), Beijing Natural Science Foundation (JQ18004), Shenzhen Peacock Plan team (KQTD2016022619565991), 111 Project (B17002), National Postdoctoral Program for Innovative Talents (BX20190028), and Postdoctoral Science Foundation of China (2019M660399). L.D.Z. thanks for the support from the National Science Fund for Distinguished Young Scholars (51925101).
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properties of chalcogenides, J. Mater. Chem. C 7 (2019) 3787-3794. [24] H.S. Dow, M.W. Oh, B.S. Kim, S.D. Park, B.K. Min, H.W. Lee, D.M. Wee, Effect of Ag or Sb addition on the thermoelectric properties of PbTe, J. Appl. Phys. 108 (2010) 113709. [25] T. Chen, H. Wang, W. Su, F. Mehmood, T. Wang, J. Zhai, X. Wang, C. Wang, Low thermal conductivity and high figure of merit for rapidly synthesized n-type Pb1-xBixTe alloys, Dalton T 47 (2018) 15957-15966. [26] J. Zhang, D. Wu, D. He, D. Feng, M. Yin, X. Qin, J. He, Extraordinary Thermoelectric Performance Realized in n-Type PbTe through Multiphase Nanostructure Engineering, Adv. Mater. 29 (2017) 1703148.
13
Table 1 Electrical transport properties of n-type PbTe at 300 K in this work. Sample
Carrier
Carrier
density
density
mobility
(gcm-3)
(cm-3)
(cm2V-1s-1)
(Scm-1)
(µVK-1)
PbTe-5%AgSbSe2
7.88
2.49×1016
291.14
1.16
-334.76
0.13
Pb0.995Sb0.005Te-5%AgSbSe2
7.70
1.52×1016
1961.35
4.77
-310.47
0.46
Pb0.99Sb0.01Te-5%AgSbSe2
7.85
1.19×1019
202.22
385.03
-133.43
6.86
Pb0.985Sb0.015Te-5%AgSbSe2
7.70
1.52×1019
260.78
634.22
-111.82
7.93
Pb0.98Sb0.02Te-5%AgSbSe2
7.80
1.83×1019
307.24
899.59
-87.95
6.96
Pb0.995Bi0.005Te-5%AgSbSe2
7.90
2.24×1019
129.20
463.04
-176.25
5.06
Pb0.99Bi0.01Te-5%AgSbSe2
7.92
2.29×1019
147.72
514.23
-99.39
5.08
Pb0.985Bi0.015Te-5%AgSbSe2
8.0
3.39×1019
136.98
742.96
-76.37
4.33
Pb0.98Bi0.02Te-5%AgSbSe2
7.91
5.57×1019
100.47
885.39
-71.93
4.58
PbTe0.998I0.002-5%AgSbSe2
7.92
1.19×1017
66.70
1.27
-378.80
0.18
PbTe0.996I0.004-5%AgSbSe2
8.06
4.13×1018
23.76
1.57
-457.17
0.33
PbTe0.994I0.006-5%AgSbSe2
8.06
9.74×1018
10.17
15.85
-342.50
1.86
PbTe0.992I0.008-5%AgSbSe2
7.99
1.15×1019
126.97
233.63
-134.11
4.20
PbTe0.99I0.01-5%AgSbSe2
7.93
1.00×1019
99.45
159.12
-201.36
6.45
PbTe0.988I0.012-5%AgSbSe2
7.84
9.36×1018
167.71
251.17
-134.78
4.56
Sample name
1
Electrical
Seebeck
Power factor
conductivity coefficient (µWcm-1K-2)
Figure captions Fig. 1 A roadmap towards high thermoelectric performance of n-type PbTe samples via combining point defects with increased carrier concentration. Fig. 2 Phase identification of PbTe-x%AgSbSe2 (x = 2-6) system: (a) Powder XRD pattern; (b) Lattice parameter. Fig. 3 Thermoelectric transport properties of PbTe-x%AgSbSe2(x = 2-6): (a) Total thermal conductivity; (b) Lattice thermal conductivity; (c) Electrical conductivity; (d) Seebeck coefficient; (e) Power factor; (f) ZT value. Fig. 4 Thermoelectric transport properties as a function of temperature for PbTe-x%AgSbSe2 (x = 2-6): (a) Lorenz number; (b) Electronic thermal conductivity; (c) Thermal diffusivity; (d) Heat capacity. Fig. 5 Phase identification of n-type doped PbTe-5%AgSbSe2: (a) Powder XRD pattern with Sb doping; (b) Lattice parameter with Sb doping; (c) Powder XRD pattern with Bi doping; (d) Lattice parameter with Bi doping; (e) Powder XRD pattern with I doping; (f) Lattice parameter with I doping. Fig. 6 Thermoelectric transport properties of Pb1-xSbxTe-5%AgSbSe2(x = 0.005-0.02): (a) Electrical conductivity; (b) Seebeck coefficient; (c) Power factor; (d) Total thermal conductivity; (e) Lattice thermal conductivity; (f) ZT value. Fig. 7 Thermoelectric transport properties as a function of temperature for Pb1-xSbxTe-5%AgSbSe2 (x = 0-0.02): (a) Lorenz number; (b) Electronic thermal conductivity; (c) Thermal diffusivity; (d) Heat capacity. Fig. 8 Thermoelectric transport properties of Pb1-xBixTe-5%AgSbSe2(x = 0.005-0.02): (a) Electrical conductivity; (b) Seebeck coefficient; (c) Power factor; (d) Total thermal conductivity; (e) Lattice thermal conductivity; (f) ZT value. Fig. 9 Thermoelectric transport properties as a function of temperature for Pb1-xBixTe-5%AgSbSe2 (x = 0-0.02): (a) Lorenz number; (b) Electronic thermal
1
conductivity; (c) Thermal diffusivity; (d) Heat capacity. Fig. 10 Thermoelectric of PbTe1-xIx-5%AgSbSe2(x = 0.002-0.012): (a) Electrical conductivity; (b) Seebeck coefficient; (c) Power factor; (d) Total thermal conductivity; (e) Lattice thermal conductivity; (f) ZT value. Fig. 11 Thermoelectric transport properties as a function of temperature for PbTe1-xIx-5%AgSbSe2 (x = 0-0.012): (a) Lorenz number; (b) Electronic thermal conductivity; (c) Thermal diffusivity; (d) Heat capacity. Fig. 12 Comparison of the most prominent electrical transport properties of PbTe-5%AgSbSe2 with Sb, Bi and I doping in this work: (a) Electrical conductivity; (b) Seebeck coefficient; (c) Power factor; (d) Average power factor; (e) Total thermal conductivity comparisons; (f) Lattice thermal conductivity comparisons. The examples are PbTe-0.003I, PbTe-0.02Bi, PbTe-0.1Sb. Fig. 13 PbTe-5%AgSbSe2 with Sb, Bi and I doping and other n-type PbTe: (a) ZT values; (b) ZTave values.
2
Fig. 1
3
Fig. 2
4
Fig. 3
5
Fig. 4
6
Fig. 5
7
Fig. 6
8
Fig. 7
9
Fig. 8
10
Fig. 9
11
Fig. 10
12
Fig. 11
13
Fig. 12
14
Fig. 13
15
Highlights 1．κmin ~ 0.8 Wm-1K-1 was achieved in n-type PbTe through introducing AgSbSe2.
2. PFmax of n-type PbTe was significantly enhanced from ~ 3.4 µWcm-1K-2 to ~ 14.8 µWcm-1K-2 by I doping.
3. ZTmax of n-type PbTe was enhanced from ~ 0.45 to ~ 1.00.
Declaration of Interest Statement Authors declare no any interests in this work.
S0925-8388(20)30740-4
DOI:
https://doi.org/10.1016/j.jallcom.2020.154377
Reference:
JALCOM 154377
To appear in:
Journal of Alloys and Compounds
Received Date: 14 January 2020 Revised Date:
13 February 2020
Accepted Date: 14 February 2020
Please cite this article as: Z. Yang, S. Wang, Y. Sun, Y. Xiao, L.-D. Zhao, Enhancing thermoelectric performance of n-type PbTe through separately optimizing phonon and charge transport properties, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154377. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.
Author Contributions Section Z. Y., S. Q. W. and Y. J. S. synthesized the samples. S. Q. W., Y. J. S. and Y. X. did the measurements work. Z. Y., S. Q. W., Y. J. S., Y. X. and L. D. Z conceived the experiments, analysed the results and co-edited the manuscript.
Enhancing thermoelectric performance of n-type PbTe through separately optimizing phonon and charge transport properties
Zhi Yang1, Siqi Wang1,2, Yuejun Sun1, Yu Xiao2,*, Li-Dong Zhao2,*
1
School of Materials Science and Engineering, Liaoning Technical University, Fuxin
123000, China 2
School of Materials Science and Engineering, Beihang University, Beijing 100191,
China E-mail: [email protected]; [email protected]
1
ABSTRACT: The n-type PbTe still maintains relatively low thermoelectric
performance compared with its p-type counterpart. Thus, to promote its performance in n-type PbTe, we conduct successive strategies that firstly suppressing the lattice thermal conductivity (κlat) through alloying with ternary compound AgSbSe2, and then optimizing carrier density with Sb, Bi or I doping. In AgSbSe2-alloyed PbTe samples, the κlat can be evidently reduced, and the minimum κlat decreases from ~ 1.1 Wm-1K-1 in undoped PbTe to ~ 0.8 Wm-1K-1 in PbTe-5%AgSbSe2. Based on PbTe-5%AgSbSe2 matrix, several dopants (Sb, Bi or I) are used to optimize its carrier density, and found that I element is verified to be the most effective dopant to enhance its electrical transport properties. In I-doped PbTe-5%AgSbSe2 samples, the carrier density raises from ~ 2.49 × 1016 cm-3 in PbTe-5%AgSbSe2 to ~ 1.15 × 1019 cm-3 in PbTe0.992I0.008-5%AgSbSe2, which contributes to the maximum power factor ~ 14.8 µWcm-1K-2. Combining the reduced lattice thermal conductivity and improved power factor, the maximum ZTmax and average ZTave values in PbTe0.992I0.008-5%AgSbSe2 are boosted to ~ 1.0 at 723 K and ~ 0.77 at 300-823 K, respectively. This work points out a valid avenue to improve thermoelectric transport property in n-type PbTe with separately successive strategies to adjust its thermal and electrical performance. Keywords:
Thermoelectric;
n-type
PbTe;
conductivity; ZT value.
2
Electrical
conductivity;
Thermal
1. Introduction In recent years, the research and development of green new energy materials have received much attention. Thermoelectric materials, as a kind of eco-friendly material, enable to directly and reversely convert thermal energy into electric power, therefore it possesses wide application prospects in mitigating energy loss and environmental issue[1-5]. The level of thermoelectric efficiency is mainly determined by the dimensionless figure of merit, ZT, ZT = S2σT/(κlat+κele), where S, σ, T, κlat and κele are the Seebeck coefficient, electrical conductivity, absolute temperature in Kelvin, the lattice thermal conductivity and electronic thermal conductivity, respectively[6-11]. It is obvious that the acquisition of eximious thermoelectric materials can be achieved by concurrently enhancing its power factor (S2σ) and depressing its thermal conductivity. But the three thermoelectric parameters are not independently controllable, making it difficult to pursue high thermoelectric performance. PbTe is a prominent thermoelectric material suitable for the medium temperature region (500-900 K)[12-14]. In order to bring about high efficiency in PbTe-based thermoelectric device, we need both p- and n-type PbTe with excellent thermoelectric transport performance. However, the thermoelectric transport proprety of n-type PbTe relatively falls behind that of p-type PbTe, thus it attracts broad attention to heighten the thermoelectric property of n-type PbTe in thermoelectric field. In term of n-type PbTe, it is difficult to improve the effective mass (m*) of electrons with band convergence[15] due to large energy difference (ΔEL-Σ = 0.45 eV) between the light band (L) and heavy band (Σ) in conduction bands[16]. Therefore, recent progresses for n-type PbTe universally perform nanostructures to reduce thermal conductivity after well optimizing electrical transport properties, such as introducing ZnTe nanostructures into I-doped PbTe to lower its thermal conductivity[17]; Using Ag2Te nanoparticle distribution in Bi-doped PbTe to prominently minish the lattice thermal conductivity[18]; Sb-rich
nano-precipitates in PbTe to cause an obvious diminution
of lattice thermal conductivity[19]. However, the nanostructures in above samples indeed lower the thermal conductivity, but deteriorate the electrical transport 3
properties simultaneously, finally obtain a limit enhancement of the net ZT. For n-type PbTe, well balancing the phonon and carrier transport property is vital to harvest good thermoelectric performance. In this work, we firstly introduce AgSbSe2 alloying into PbTe to lower its thermal conductivity, and then based on the sample with minimum lattice thermal conductivity, the donor dopants (Sb, Bi or I) are used to optimize its carrier transport properties. The roadmap of this work is schematically presented in Fig. 1. Results shows that AgSbSe2 alloying can largely reduce the thermal conductivity, and the minimum lattice thermal conductivity can be impeded from ~ 1.1 Wm-1K-1 in undoped PbTe to ~ 0.8 Wm-1K-1 in PbTe-5%AgSbSe2. Based on the matrix of PbTe-5%AgSbSe2, several dopants (Sb, Bi or I) are tentatively introduced to optimize its carrier density. Among these three dopants, I element shows the most effective doping behavior and largely boosts the power factor from ~ 3.4 µWcm-1K-2 in PbTe-5%AgSbSe2 to ~ 14.8 µWcm-1K-2 in PbTe0.992I0.008-5%AgSbSe2, contributing to a high ZT values in a broad temperature zone. Finally, the maximum ZTmax and average ZTave value in PbTe0.992I0.008-5%AgSbSe2 are boosted to ~ 1.0 at 723 K and ~ 0.77 at 300-823 K, respectively.
2. Experimental section 2.1 Sample preparation Raw materials included Pb pieces (99.99%, Aladdin element, China), Ag wires (99.9985%, Alfa Aesar, China), Sb pieces (99.99%, Aladdin element, China), Te pieces (99.999%, Aladdin element, China), Se pieces (99.999%, Aladdin element, China), Bi pieces (99.99%, Aladdin element, China), PbI2 granules (99.99%, Alfa Aesar, China). The quartz tubes filled with nominal compositions were sealed up under atmospheric pressure with 10-4 Pa, gradually heated from ambient temperature to 1323 K for 24 h, kept at 1323 K for 10 h, and ultimately cooled down to room temperature. The synthetic ingots were ground into fine powder (＜ 200 mesh), and subsequently loaded into graphite mold for spark plasma sintering (SPS-211Lx, Dr. Sinter). The sintered block was cut into a cuboid of 10 × 3 × 3 mm3 for testing electrical performance and a slice of 6 × 6 × 1.5 mm3 for testing thermal transport 4
performance. 2.2 Phase analysis The phase composition was tested at room temperature by X-ray diffraction (XRD, D/max 2200pc, Japen) using the Cu Kα radiation (λ = 1.5418 Å). The scanning range (2θ) and speed were 20-80° and 6°/min, respectively. 2.3 Electrical transport properties At first, the cuboid samples were sprayed with a thin layer of boron nitride to prevent sample evaporation. The electrical conductivity and Seebeck coefficient were tested under He condition by Cryoall CTA. 2.4 Thermal transport properties The total thermal conductivity (κ) was measured by κ = D × Cp × ρ, where D was the thermal diffusivity detected by Netzsch LFA 457 instrument based on the laser flash method, Cp was the heat capacity figured out by Debye model[20], and ρ was the sample density. 2.5 Hall measurement Using the Lake Shore 8400 Series equipment on the basis of Van der Pauw test principle, the room-temperature carrier density (n) was tested at a magnetic field ~ 0.9 T and direct current of 10 mA. The carrier mobility (µ) was calculated from µ = σ /(e × n), where σ was electrical conductivity and e was unit charge.
3. Results and discussion 3.1 Suppressing lattice thermal conductivity in n-type PbTe by introducing ternary compound AgSbSe2 To depress the thermal conductivity of n-type PbTe, we conduct the ternary compound AgSbSe2 into the PbTe matrix. In Fig. 2(a), the PXRD patterns of PbTe-x%AgSbSe2 (x = 2-6) samples reveal that every peaks coincide with cubic structure (Fm3m) and no extra peaks display. In Fig. 2(b), the lattice parameter gradually shrinks with ascending AgSbSe2 alloying content because of the substitution of smaller Ag+ (~ 1.15 Å) and Sb3+ (~ 0.92 Å) for larger Pb2+ (~ 1.2 Å), and smaller Se2- (~ 1.98 Å) for larger Te2- (~ 2.11 Å). The PXRD patterns confirm that Ag, Sb and 5
Se elements successfully dissolve in PbTe matrix. In Fig. 3, the thermal transport property of PbTe-x%AgSbSe2 (x = 2-6) samples is presented. In Fig. 3 (a), the total thermal conductivity is evidently depressed in the AgSbSe2-alloyed samples, decreasing from ~ 1.2 Wm-1K-1 in PbTe-2%AgSbSe2 to ~ 0.8 Wm-1K-1 in PbTe-5%AgSbSe2 at 300 K. In general, the total thermal conductivity (κtot) is contributed from charge carriers (κele) and lattice phonons (κlat). The electronic thermal conductivity (κele) is estimated by κele = L × σ × T based on the Wiedemann-Franz law, where L is Lorenz number, which is estimated by the reduced chemical potential, σ is electrical conductivity and T is the absolute temperature in Kelvin[21, 22]. Accordingly, the lattice thermal conductivity is acquired from the formula κlat = κtot-κele, and the calculated lattice thermal conductivity dramatically drops after AgSbSe2 alloying caused by atomic-scale defect scattering[23], as shown in Fig. 3(b). The difference of lattice parameters between AgSbSe2 (~5.79 Å) and PbTe (~6.44 Å) leads to intensive lattice distortion and phonon scattering, thereby a reduced lattice thermal conductivity. The minimum lattice thermal conductivity is impeded from ~ 1.0 Wm-1K-1 in PbTe-2%AgSbSe2 to ~ 0.8 Wm-1K-1 in PbTe-5%AgSbSe2. In Fig. 3(c), the relatively low electrical conductivity of PbTe-x%AgSbSe2 slightly declines from ~ 4.7 Scm-1 in PbTe-2%AgSbSe2 to ~ 0.7 Scm-1 in PbTe-6%AgSbSe2 at room temperature. In Fig. 3(d), the Seebeck coefficients present p-n transition, ultimately identified as n-type doping. Owing to the slight influence of AgSbSe2 alloying on electrical transport property, the samples remain a nearly constant power factor. Combining reduced lattice thermal conductivity with maintained power factor, a maximum ZT ~ 0.45 at 873 K is obtained in PbTe-3%AgSbSe2. Other thermoelectric diagrams related to PbTe-x%AgSbSe2 are displayed in Fig. 4.
3.2 Optimizing carrier density in n-type PbTe with Sb, Bi and I doping Because the low electrical transport property in PbTe-x%AgSbSe2 (x = 2-6) limits its final ZT value, we further utilize Sb, Bi and I as donor dopants to promote the carrier density based on PbTe-5%AgSbSe2 because it possesses relatively low 6
thermal conductivity. The powder X-ray diffraction patterns of Sb, Bi or I-doped PbTe-5%AgSbSe2 are shown in Fig. 5 (a), (c) and (e), respectively. All samples crystallize the cubic phase with NaCl crystal structure (space group: Fm3m) and no extra peaks of the secondary phases appear. In Fig. 5 (b), (d) and (f), the decrease of lattice parameter with Sb, Bi or I doping suggests the substitution of smaller Sb3+ (~ 0.76 Å) and Bi3+ (~ 0.96 Å) for larger Pb2+ (~ 1.2 Å), and smaller I- (~ 2.06 Å) for larger Te2- (~ 2.11 Å).
Thermoelectric
performance
of
Sb-doped
PbTe-5%AgSbSe2.
Temperature
dependent thermoelectric transport properties of Pb1-xSbxTe-5%AgSbSe2(x = 0.005-0.02) samples are shown in Fig. 6. The electrical conductivity is shown in Fig. 6(a) and has a large increase at 300 K from ~ 1.2 Scm-1 in PbTe-5%AgSbSe2 to ~ 899.6 Scm-1 in Sb-doped with x = 0.02. The negative Seebeck coefficients suggest n-type semiconducting feature. In Fig. 6(b), as Sb content increases, the absolute values of Seebeck coefficient decrease from ~ 334.8 µVK-1 in PbTe-5%AgSbSe2 to ~ 87.9 µVK-1 in Pb0.98Sb0.02Te-5%AgSbSe2 at 300 K. The improved electrical conductivity and decreased Seebeck coefficient indicate that Sb element in PbTe-5%AgSbSe2 can significantly boost the carrier density in the matrix, which is consistent with Hall measurements data. Because of the compensation between electrical conductivity and Seebeck coefficient, the maximum power factor in Fig. 6(c) is amplified from ~ 3.4 µWcm-1K-2 in PbTe-5%AgSbSe2 to ~ 13.8 µWcm-1K-2 in Pb0.98Sb0.02Te-5%AgSbSe2. In Fig. 6(d), the total thermal conductivity of PbTe-5%AgSbSe2 with Sb doping shows a significant increase with mounting Sb content originated from the rising electronic thermal conductivity after Sb doping. Correspondingly, the calculated lattice thermal conductivity decreases, and the minimum value is diminished from 0.80 Wm-1K-1 in PbTe-5%AgSbSe2 to ~ 0.57 Wm-1K-1 in Pb0.99Sb0.01Te-5%AgSbSe2. As shown in Fig. 6(f), a maximum ZT ~ 0.82 at 723 K is achieved in Pb0.99Sb0.01Te-5%AgSbSe2. Other thermoelectric diagrams related to Pb1-xSbxTe-5%AgSbSe2 are displayed in Fig. 7. 7
Thermoelectric
performance
of
Bi-doped
PbTe-5%AgSbSe2.
Temperature
dependent thermoelectric transport property of Pb1-xBixTe-5%AgSbSe2 (x = 0.005-0.02) samples is presented in Fig. 8. The electrical conductivity at 300 K increases largely from ~ 1.15 Scm-1 in PbTe-5%AgSbSe2 to ~ 885.4 Scm-1 in Pb0.98Bi0.02Te-5%AgSbSe2 as the carrier density is magnified from ~ 2.49 × 1016 cm-3 to ~ 5.57 × 1019 cm-3 in Table 1. The electrical conductivity through Bi doping is also altered visibly, exhibiting semiconductor characteristics, as shown in Fig. 8(a). The negative Seebeck coefficients for the Bi-doped PbTe-5%AgSbSe2 shown in Fig. 8(b) result in n-type doping. The absolute values of Seebeck coefficients at 300 K decrease from ~ 334.8 µVK-1 in PbTe-5%AgSbTe2 to ~ 71.9 µVK-1 in Bi-doping with x=0.015 as a result of improved carrier density. By virtue of the elevated electrical conductivity
and
reduced
Seebeck
coefficient,
the
power
factor
of
Pb0.99Bi0.01Te-5%AgSbSe2 achieves the maximum value ~ 10.4 µWm-1K-2 at 473 K. The total thermal conductivity rises after Bi doping in Fig. 8(d) due to increasing electronic thermal conductivity. After subtracting the contribution from carriers, the lattice thermal conductivity is the lowest one of ~ 0.55 Wm-1K-1 at 673 K in the Pb0.99Bi0.01Te-5%AgSbSe2 sample in Fig. 8(e). The maximum ZT ~ 0.82 is finally acquired in Pb0.99Bi0.01Te-5%AgSbSe2. Other thermoelectric diagrams related to Pb1-xBixTe-5%AgSbSe2 are displayed in Fig. 9.
Thermoelectric performance of I-doped PbTe-5%AgSbSe2. Temperature dependent thermoelectric transport property of PbTe1-xIx-5%AgSbSe2 (x = 0.002-0.012) samples is presented in Fig. 10. The dopant I in PbTe can effectively improve electrical conductivity, increasing from ~ 1.2 Scm-1 in PbTe-5%AgSbSe2 to ~ 251.2 Scm-1 in I-doping with x=0.012 at room temperature, as shown in Fig. 10(a). It can be clearly noticed in Fig. 10(b) that all the compositions confirm n-type conducting, which is supported by the negative Seebeck coefficient values. The absolute values of Seebeck coefficients at 300 K decrease from ~ 334.8 µVK-1 in PbTe-5%AgSbSe2 to ~ 134.8 µVK-1 in I-doping with x=0.012. The rise in electrical conductivity and the accompanied descent in Seebeck coefficient is ascribed to the ascending carrier 8
density, which is related to the Hall measurements. Finally, the maximum power factor shows a remarkable enhancement in Fig. 10(c) from ~ 3.4 µmWcm-1K-2 in PbTe-5%AgSbSe2 to ~ 14.8 µWcm-1K-2 in PbTe0.992I0.008-5%AgSbSe2. Fig. 10(d) shows that a significant ascension of total thermal conductivity with mounting I amount is associated with the raised electronic thermal conductivity. By contrary, the falling tendency is noticed in the lattice thermal conductivity at the same time. The minimum
lattice
thermal
conductivity
reaches
~
0.53
Wm-1K-1
in
PbTe0.992I0.008-5%AgSbSe2 at 673 K, as shown in Fig. 10(e). Overall, a high ZT ~ 1.0 is observed in PbTe0.992I0.008-5%AgSbSe2 at 723 K. Other thermoelectric diagrams related to Pb1-xIxTe-5%AgSbSe2 are displayed in Fig. 11.
Comparison of thermoelectric properties in n-type PbTe-5%AgSbSe2 with Sb, Bi and I doping. In conclusion, the ternary compound AgSbSe2 alloying aims to depress the lattice thermal conductivity and following with Sb, Bi or I doping to optimize the carrier density, respectively. To distinctly present the different effects of Sb, Bi or I dopants on n-type PbTe, the comparison in 5%AgSbSe2 with Pb0.99Sb0.01Te, Pb0.99Bi0.01Te, and PbTe0.992I0.008 are discussed in Fig. 12. Among the three samples, I doping is the most effective element to tune the carrier density in PbTe-5%AgSbSe2 and then well balances the electrical conductivity and Seebeck coefficient, ultimately causing a prominent power factor ~ 14.8 µWcm-1K-2 at 423 K and an average power factor ~ 8.84 µWcm-1K-2 at 300-873 K in PbTe0.992I0.008-5%AgSbSe2, as shown in Fig. 12(c) and (d). The average PFave value is calculated by:
PFave
∫ =
Th
Tc
PF (T )dT Th − Tc
(1)
where Tc is the cold-end temperature, Th is the hot-end temperature. The power factors of n-type PbTe with I doping are larger than those in n-type PbTe with Sb or Bi doping, indicating that I doping on anion site is more effective to produce electrons than Sb or Bi substitutions on cation sites. Notably, all samples can maintain a relatively low total thermal conductivity and lattice thermal conductivity resulting 9
from different lattice parameters (lattice parameter: PbTe ~ 6.44 Å, AgSbSe2 ~ 5.79 Å). The strong lattice distortion enable to availably scatter phonon, and the minimum lattice thermal conductivity shown in Fig. 12(f) respectively reaches ~ 0.55 Wm-1K-1, ~ 0.57 Wm-1K-1, ~ 0.53 Wm-1K-1 in PbTe-5%AgSbSe2 with Sb, Bi or I doping. Moreover, the total thermal conductivity of doped PbTe-5%AgSbSe2 samples (Sb ~ 0.70 Wm-1K-1, Bi ~ 0.72 Wm-1K-1, I ~ 0.69 Wm-1K-1) are all obviously lower than those of reported n-type PbTe (Sb ~ 0.85 Wm-1K-1[24], Bi ~ 1.71 Wm-1K-1[25], I ~ 1.31 Wm-1K-1[15]), which derives from low lattice thermal conductivity by the initial AgSbSe2-alloying, as shown in Fig. 12(e) and (f). Combining suppressed thermal conductivity and improved electrical properties, the peak ZT values reach ~ 0.82 in Pb0.99Sb0.01Te-5%AgSbSe2, ~ 0.83 in Pb0.99Bi0.01Te-5%AgSbSe2, and ~ 1.0 in PbTe0.992I0.008-5%AgSbSe2, as shown in Fig. 13(a). When it comes to practical applications of thermoelectric devices, the conversion efficiency depends on the average ZT rather than the maximum ZT, so it is necessary to promote the average ZT. In this work, the average ZTave value is calculated by[11-13, 26]:
ZTave
∫ =
Th
Tc
Z (T )dT
Th − Tc
(2)
where Tc is the cold-end temperature, Th is the hot-end temperature. Importantly, the average ZTave can reach ~ 0.62 in Pb0.99Bi0.01Te-5%AgSbSe2, ~ 0.65 in Pb0.99Sb0.01Te-5%AgSbSe2 and ~ 0.77 in PbTe0.992I0.008-5%AgSbSe2, which is 61.3%, 44.6%, 18.2% higher than the average ZTave ~ 0.24, ~ 0.36 and ~ 0.63 in PbTe with Bi[25], Sb[24] and I[15] doping, respectively.
4. Conclusions In this work, the ternary compound AgSbSe2 is reported as one inclusion that can obviously scatter phonon to suppress the lattice thermal conductivity of n-type PbTe. Based on low lattice thermal conductivity in PbTe-5%AgSbSe2, Sb, Bi or I dopants can further enhance its power factor by well optimizing carrier density. Results show 10
that I element is the most effective dopant in n-type PbTe-5%AgSbSe2, and a maximum power factor ~ 14.8 µWcm-1K-2 is obtained. Finally, a maximum ZT ~ 1.0 at 723 K and high ZTave ~ 0.77 at 300-873 K are gained in PbTe0.992I0.008+5%AgSbSe2. This work extends a valid avenue to optimize thermoelectric transport property in n-type PbTe-based materials through successively alloying to reduce thermal conductivity and effective doping to enhance carrier transport properties.
Acknowledgements This work was supported by the National Key Research and Development Program of China (2018YFA0702100 and 2018YFB0703600), National Natural Science Foundation of China (51772012 and 51671015), Beijing Natural Science Foundation (JQ18004), Shenzhen Peacock Plan team (KQTD2016022619565991), 111 Project (B17002), National Postdoctoral Program for Innovative Talents (BX20190028), and Postdoctoral Science Foundation of China (2019M660399). L.D.Z. thanks for the support from the National Science Fund for Distinguished Young Scholars (51925101).
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13
Table 1 Electrical transport properties of n-type PbTe at 300 K in this work. Sample
Carrier
Carrier
density
density
mobility
(gcm-3)
(cm-3)
(cm2V-1s-1)
(Scm-1)
(µVK-1)
PbTe-5%AgSbSe2
7.88
2.49×1016
291.14
1.16
-334.76
0.13
Pb0.995Sb0.005Te-5%AgSbSe2
7.70
1.52×1016
1961.35
4.77
-310.47
0.46
Pb0.99Sb0.01Te-5%AgSbSe2
7.85
1.19×1019
202.22
385.03
-133.43
6.86
Pb0.985Sb0.015Te-5%AgSbSe2
7.70
1.52×1019
260.78
634.22
-111.82
7.93
Pb0.98Sb0.02Te-5%AgSbSe2
7.80
1.83×1019
307.24
899.59
-87.95
6.96
Pb0.995Bi0.005Te-5%AgSbSe2
7.90
2.24×1019
129.20
463.04
-176.25
5.06
Pb0.99Bi0.01Te-5%AgSbSe2
7.92
2.29×1019
147.72
514.23
-99.39
5.08
Pb0.985Bi0.015Te-5%AgSbSe2
8.0
3.39×1019
136.98
742.96
-76.37
4.33
Pb0.98Bi0.02Te-5%AgSbSe2
7.91
5.57×1019
100.47
885.39
-71.93
4.58
PbTe0.998I0.002-5%AgSbSe2
7.92
1.19×1017
66.70
1.27
-378.80
0.18
PbTe0.996I0.004-5%AgSbSe2
8.06
4.13×1018
23.76
1.57
-457.17
0.33
PbTe0.994I0.006-5%AgSbSe2
8.06
9.74×1018
10.17
15.85
-342.50
1.86
PbTe0.992I0.008-5%AgSbSe2
7.99
1.15×1019
126.97
233.63
-134.11
4.20
PbTe0.99I0.01-5%AgSbSe2
7.93
1.00×1019
99.45
159.12
-201.36
6.45
PbTe0.988I0.012-5%AgSbSe2
7.84
9.36×1018
167.71
251.17
-134.78
4.56
Sample name
1
Electrical
Seebeck
Power factor
conductivity coefficient (µWcm-1K-2)
Figure captions Fig. 1 A roadmap towards high thermoelectric performance of n-type PbTe samples via combining point defects with increased carrier concentration. Fig. 2 Phase identification of PbTe-x%AgSbSe2 (x = 2-6) system: (a) Powder XRD pattern; (b) Lattice parameter. Fig. 3 Thermoelectric transport properties of PbTe-x%AgSbSe2(x = 2-6): (a) Total thermal conductivity; (b) Lattice thermal conductivity; (c) Electrical conductivity; (d) Seebeck coefficient; (e) Power factor; (f) ZT value. Fig. 4 Thermoelectric transport properties as a function of temperature for PbTe-x%AgSbSe2 (x = 2-6): (a) Lorenz number; (b) Electronic thermal conductivity; (c) Thermal diffusivity; (d) Heat capacity. Fig. 5 Phase identification of n-type doped PbTe-5%AgSbSe2: (a) Powder XRD pattern with Sb doping; (b) Lattice parameter with Sb doping; (c) Powder XRD pattern with Bi doping; (d) Lattice parameter with Bi doping; (e) Powder XRD pattern with I doping; (f) Lattice parameter with I doping. Fig. 6 Thermoelectric transport properties of Pb1-xSbxTe-5%AgSbSe2(x = 0.005-0.02): (a) Electrical conductivity; (b) Seebeck coefficient; (c) Power factor; (d) Total thermal conductivity; (e) Lattice thermal conductivity; (f) ZT value. Fig. 7 Thermoelectric transport properties as a function of temperature for Pb1-xSbxTe-5%AgSbSe2 (x = 0-0.02): (a) Lorenz number; (b) Electronic thermal conductivity; (c) Thermal diffusivity; (d) Heat capacity. Fig. 8 Thermoelectric transport properties of Pb1-xBixTe-5%AgSbSe2(x = 0.005-0.02): (a) Electrical conductivity; (b) Seebeck coefficient; (c) Power factor; (d) Total thermal conductivity; (e) Lattice thermal conductivity; (f) ZT value. Fig. 9 Thermoelectric transport properties as a function of temperature for Pb1-xBixTe-5%AgSbSe2 (x = 0-0.02): (a) Lorenz number; (b) Electronic thermal
1
conductivity; (c) Thermal diffusivity; (d) Heat capacity. Fig. 10 Thermoelectric of PbTe1-xIx-5%AgSbSe2(x = 0.002-0.012): (a) Electrical conductivity; (b) Seebeck coefficient; (c) Power factor; (d) Total thermal conductivity; (e) Lattice thermal conductivity; (f) ZT value. Fig. 11 Thermoelectric transport properties as a function of temperature for PbTe1-xIx-5%AgSbSe2 (x = 0-0.012): (a) Lorenz number; (b) Electronic thermal conductivity; (c) Thermal diffusivity; (d) Heat capacity. Fig. 12 Comparison of the most prominent electrical transport properties of PbTe-5%AgSbSe2 with Sb, Bi and I doping in this work: (a) Electrical conductivity; (b) Seebeck coefficient; (c) Power factor; (d) Average power factor; (e) Total thermal conductivity comparisons; (f) Lattice thermal conductivity comparisons. The examples are PbTe-0.003I, PbTe-0.02Bi, PbTe-0.1Sb. Fig. 13 PbTe-5%AgSbSe2 with Sb, Bi and I doping and other n-type PbTe: (a) ZT values; (b) ZTave values.
2
Fig. 1
3
Fig. 2
4
Fig. 3
5
Fig. 4
6
Fig. 5
7
Fig. 6
8
Fig. 7
9
Fig. 8
10
Fig. 9
11
Fig. 10
12
Fig. 11
13
Fig. 12
14
Fig. 13
15
Highlights 1．κmin ~ 0.8 Wm-1K-1 was achieved in n-type PbTe through introducing AgSbSe2.
2. PFmax of n-type PbTe was significantly enhanced from ~ 3.4 µWcm-1K-2 to ~ 14.8 µWcm-1K-2 by I doping.
3. ZTmax of n-type PbTe was enhanced from ~ 0.45 to ~ 1.00.
Declaration of Interest Statement Authors declare no any interests in this work.