High-Performance PbTe Thermoelectric Films by Scalable and Low

Feb 23, 2018 - By employing Pb0.98Na0.02Te-4 mol % SrTe powder, which is one group of superior thermoelectric materials, this work explores the possib...
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High Performance PbTe Thermoelectric Films by Screen Printing Chao Han, Gangjian Tan, Tony Varghese, Mercouri G. Kanatzidis, and Yanliang Zhang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00041 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 24, 2018

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High Performance PbTe Thermoelectric Films by Screen Printing Chao Han, Gangjian Tan, Tony Varghese, Mercouri G. Kanatzidis*, Yanliang Zhang* Dr. Chao Han Department of Mechanical and Biomedical Engineering, Boise State University, Boise, Idaho 83725, United States Dr. Gangjian Tan, Prof. Mercouri G. Kanatzidis Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States Mr. Tony Varghese Micron School of Material Science and Engineering, Boise State University, Boise, Idaho 83725, United States Prof. Yanliang Zhang Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame Indiana 46556. United States Corresponding Author * Prof. Mercouri G. Kanatzidis Email: [email protected] * Prof. Yanliang Zhang

Email: [email protected]

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ABSTRACT: Thermoelectric (TE) energy conversion has a significant impact on low-grade waste heat recovery, increasing the efficiency of energy usage and reducing carbon emissions. Two of the big obstacles for wide applications of this technology, however, are the high fabrication cost and low efficiency of thermoelectric modules. This paper explores the possibility of converting PbTe-SrTe particles into high-power-factor thermoelectric films via a scalable and low-cost screen printing method, and investigates ways of further enhancing its performance. Addition of tellurium powder can significantly increase the thermoelectric performance of the printed PbTe-SrTe films through doping and enhancement of inter-particle connection. The conservative estimate of thermoelectric figure of merit, ZT, is above 1.0 at 350 °C for PbTe-SrTe film with 2 wt.% Te powders. This opens up a new, low cost direction for manufacturing high performance thermoelectric devices.

TOC GRAPHICS

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Thermoelectrics, a technology that can directly convert heat into electricity or vice versa, has a significant impact on increasing energy efficiency and reducing carbon emissions via low-grade waste heat recovery. Moreover, it is a solid-state, reliable, quiet, maintenance-free, and zeroemission technology.1-4 The key parameter that describes thermoelectric properties for materials is the “figure of merit” expressed in the form of ܼܶ =

௦మ ఙ ఑

ܶ; where S, σ, κ, and T are the

Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively. The difficulty in obtaining a high ZT arises from the fact that S, σ, and κ are interlocked with each other and are hard to optimize simultaneously.3, 5 Therefore, for a long period, low ZT values, which would lead to extremely low energy conversion efficiency compared with the traditional heat engine, have limited the wide application of this green technology. Fortunately, thermoelectric materials have undergone tremendous improvement of their figure of merit (ZT) over the past decade. For example, a ZT of 2.6 in single crystal SnSe6 and ZT of 2.5 for the PbTe-SrTe system7 has been realized. He et al.8 reported a maximum ZT of 2.1 in a mosaic crystal of Cu2S0.52Te0.48, and the AgSbPbmTem+2 (LAST) system showed a maximum ZT of 1.6 due to the nanostructures originating from spinodal decomposition.9 Recently, TE modules fabricated from PbTe-4%MgTe bulk material have been demonstrated to achieve 9% efficiencies and >11 % in a segmented configuration.10 Compared to conventional bulk thermoelectric modules, the film based thermoelectric generator (f-TEG) has many merits.12, 11-20

These include (1) the cost and materials consumptions are low; (2) f-TEGs do not need

extra dicing to make the TE legs, which further minimize materials waste and fabrication cost; (3) films can be readily fabricated by screen printing, inkjet printing, roll-to-roll manufacturing, etc., which makes the scalable and low-cost TEG fabrication possible; and (4) it is easy to achieve microscale manufacturing, which opens up the opportunity for f-TEGs to be used as

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micro power sources for electronics. Because of the superior features of f-TEGs, researchers have increasingly focused on this subject.1-2, 11-20 Nevertheless, the ZT of f-TEGs fabricated by assembling TE nanoparticles is much lower than rigid bulks due to the poor connections between particles. In addition, most of the existing reports aimed at room-temperature applications of fTEGs using Bi-Te system and polymers.21-28 Although there are some reports on the thermoelectric performance of PbTe-SrTe superlattices, their fabrication procedures mainly involve growth by molecular beam epitaxy which is difficult to scale up and too expensive to be widely used.29-31 In this paper, we employ the all-scale hierarchical structured PbTe-SrTe system,32 for the first time, to explore the possibility of screen printing PbTe-SrTe based ink into films and consider the effects of adding extra tellurium powder on its performance. The 2 wt.% Te added Pb0.98Na0.02Te-4 mol.%SrTe composite film demonstrates the best performance, with an estimated peak ZT of above 1.0 at 350 °C, which is the highest reported value for a printed thermoelectric film.

Figure 1. (a) XRD patterns of the ball milled PbTe-SrTe powder, the blue vertical line indicates

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the standard peak position of pure fcc PbTe (JCPDS 65-0881); (b) SEM image of the ball milled PbTe-SrTe powder; (c) schematic of the experimental process. Diffraction peaks of the ball-milled Pb0.98Na0.02Te-4 mol.% SrTe powder sample could be indexed to face-centered-cubic (fcc) PbTe (JCPDS 65-0881) with slightly right shift due to the ball milling and element doping. The particle size is below 5 µm [Figure 1(a-b)]. Then, as illustrated by Figure 1(c), the powder was made into a printable paste and screen printed on an alumina substrate.

Figure 2. Temperature-dependent thermoelectric properties of three printed PbTe-SrTe films with 0%, 2% and 4% tellurium additions prepared under the same condition: (a) Electrical conductivity; (b) Seebeck coefficient; (c) Power factor. The control Pb0.98Na0.02Te-4%SrTe film, and the 2 wt.% Te- and 4 wt.% Te-added films are simply denoted as samples PbTe-SrTe, PbTe-SrTe-Te2%, and PbTe-SrTe-Te4%, respectively. As presented by Figure 2 (a), with the addition of Te (from PbTe-SrTe to PbTe-SrTe-Te4%), the electrical conductivity at room temperature is enhanced from 47 S/cm to 84 S/cm and 132 S/cm, respectively. Also, the electrical conductivity of the PbTe-SrTe-Te2% sample above 250 °C is obviously higher than the values of PbTe-SrTe and PbTe-SrTe-Te4%. Figure 2(b) shows the Seebeck coefficients (S) of all the three samples exhibiting an increasing trend with increasing

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temperature, which coincides with the performance of their bulk counterpart.32 It is well known that pure PbTe has two valence bands that are close in energy.7 At relatively low temperature or doping levels, the p-type electrical transport properties are dominated by the light effective mass (m*) valence band lying on top (with extrema at the L point of the Brillouin zone). As the carrier concentration or temperature increases, the contribution from the low-lying heavy effective mass band (with extrema along the Σ line of the Brillouin zone) increases, and simultaneously, the band gap increases. This phenomenon is defined as the band convergence and if both bands are contributing to transport they can enhance the Seebeck coefficient.33-38 With simultaneously enhanced electrical conductivity and Seebeck coefficient, the PbTe-SrTe-Te2% sample shows the highest peak power factor (σS2) of 1.38 mW/(m·K2) at 350 °C [Figure 2(c)]. Figure S1(a) presents the calculated thermal conductivity (κ) of the three film samples using the corresponding bulk lattice thermal conductivity [κlb, value from Ref. [32] via the following equation: ߢ = ‫ ܶߪܮ‬+ ߢ୪ୠ , in which L is the Lorenz number (calculated according to Ref. [32], ranging between 1.6- 2.1×10-8 W.Ω.K-2), σ is the electrical conductivity of the film sample, and T is the temperature. Because of the porosity of the film sample, its actual lattice thermal conductivity is usually lower than the corresponding bulk value reported elsewhere.32 Thus the evaluated ZT values of the printed films presented in Figure S1(b) are actually lower than the real values. Hence, it is safe for us to conclude that a maximum ZT above 1.0 is obtained for the PbTe-SrTe-Te2% sample at 350 °C. Table 1 listed some reported ZT for thermoelectric films, which indicates that our printed film is among the highest state of the art. Table 1. Thermoelectric performance comparison between our work and previous reported films.

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Composition

Temperature

ZT

Method

Reference This work

PbTe-SrTe-Te2%

623 K

>1.0

Screen printing

Bi0.3Sb1.7Te3

300 K

0.80

Screen printing

39

Bi0.4Sb1.6Te3

448 K

0.43

Screen printing

40

Cu2Se

300 K

0.40

Sputtering

41

CuI

300 K

0.21

Sputtering

42

PEDOT:PSS

300 K

0.32

Filtration

43

CNT

300 K

0.40

Spray

44

Comparing with its corresponding bulk counterpart (+81 µV/K, 1465 S/cm),32 at room temperature, the printed film has a higher Seebeck coefficient (S) (+100 µV/K) but a significantly lower electrical conductivity (47 S/cm) due to the reduced carrier concentration and the porosity in the printed film. Compared with the bulk counterpart, the film was made by printing and sintering the paste containing ball milled powder, solvent and binder, during which oxidation may induce decreased Na dopant and carrier concentration. In addition, the evaporation of solvent and binder during the film printing and sintering process can lead to poor densification and higher porosity than the dense bulk sample, resulting in reduced carrier mobility and electrical conductivity. Interestingly, after adding 2 wt.% and 4 wt.% tellurium, however, the electrical conductivity of samples PbTe-SrTe-Te2% and PbTe-SrTe -Te4% increased significantly without deterioration of the Seebeck coefficient. To explain this phenomenon, we have to consider the status and effect of Te in the film. According to the phase diagram of the Te-Pb system,45-46 the solubility of Te in PbTe at room temperature is extremely low − usually no more than 0.1 wt.%. Because of the low

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melting point of Te (450 °C) and the high sintering temperature (550 °C), the excess Te would melt and coat the outsides of the PbTe-SrTe particles. This makes the PbTe- SrTe-Te2% and PbTe-SrTe-Te4% samples into Te/PbTe-SrTe composites. The XRD and SEM results confirm the existence of this extra Te. As shown in Figure 3(a), a rather weak peak of Te (JCPDS 270871) appears at 36.9° [(101) plane] in samples PbTe-SrTe-Te2% and PbTe-SrTe-Te4% after sintering, in addition to the peaks of the main phase of fcc PbTe (JCPDS 65-0881) and the alumina (Al2O3) substrate (JCPDS 46-1212).

Figure 3. (a) XRD patterns of sintered samples PbTe-SrTe, PbTe-SrTe-Te2%, and PbTe-SrTeTe4%; surface morphology of samples (b) PbTe-SrTe, (c) PbTe-SrTe-Te2%, and (d) PbTe-SrTeTe4%, respectively. All the scale bars in the SEM images are 5 µm.

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Table 2. EDX results corresponding to points in Figure 3(b) (c) and (d).[a]

Point

Pb (at.%)

Te (at.%)

Na (at.%)

Sr (at.%)

A

46.43

46.84

3.84

2.89

B

47.61

46.97

2.41

3.01

C

0.52

97.10

1.27

1.11

D

47.06

47.85

2.32

2.77

E

2.57

91.99

2.43

3.01

[a] The other peaks including C, O elements also appeared in the EDX spectra. However, they come from the carbon tape substrate; therefore in order to compare the difference between Te and PbTe-SrTe phases, only Pb, Te, Na and Sr elements were shown in the table. Figure 3(b-d) presents high magnification surface images of samples PbTe-SrTe, PbTe-SrTeTe2% and PbTe-SrTe-Te4%, respectively, in which a gray color phase is clearly seen besides the matrix in Figure 3(c-d). The surface SEM images of these samples also confirmed higher density (less pores) of Te added samples than pure PbTe-SrTe film. The compositions at different points shown in Figure 3(b-d) were analyzed by energy-dispersive X-ray spectroscopy (EDX), and the results are shown in Table 2. The matrix areas (points A, B, and D) show an atomic ratio of Pb to Te close to 1:1, while the gray areas (points C and E) show extremely high Te concentration and almost no Pb element. Hence, it is safe to conclude that samples PbTe-SrTe-Te2% and PbTe-SrTe-Te4% are actually Te/PbTe-SrTe composites, where the Te is coated on the outside of the PbTe-SrTe particles. Cross section views of the three samples presented in Figure 4 also demonstrated the increased connection between PbTe-SrTe particles and less porosity in Te added film than pure PbTe-SrTe film. As illustrated by Figure 4, the thicknesses of the printed films are all in 20-30 µm range. The thickness variation directly measured from different places

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in SEM images is within ±2 µm, implying the uniformity of the printed films. The value obtained from the profilometer also coincides well with the SEM measurement.

Figure 4. Cross-section view of sample (a) PbTe-SrTe, (b) PbTe-SrTe-Te2% and (c) PbTe-SrTeTe4%. It is clearly shown that for the pure PbTe-SrTe sample, lots of pores and cracks could be observed in the cross-section SEM image. While for Te added samples, the porosity is significantly decreased. All the scale bars are 10 µm. Besides the effect of improved particle connection during sintering, the addition of extra Te serves other functions to enhance the thermoelectric performance of the printed films. First of all, the Te coating can suppress the oxidation of the PbTe-SrTe matrix and thereby may diminish the number of trapped charge carriers.47 Meanwhile, although only a small amount of Te can dissolve into the PbTe-SrTe matrix, each excess Te atom can lead to the formation of two extra holes, and thus, the hole concentration could be increased via Te doping.46, 48-49. To confirm the increase of carrier concentration via Te doping, room temperature Hall measurement on these samples was conducted on physical property measurement system (PPMS) via a simple four wire connection. At room temperature, the Hall carrier concentration (n) of the PbTe-SrTe film is 2.09×1019 cm-3, significantly lower than corresponding bulk counterpart (9.2×1019 cm-3).32 After adding extra Te, carrier concentrations of PbTe-SrTe-Te2% and PbTe-SrTe-Te4% significantly increase to 7.50×1019 cm-3 and 6.01×1019 cm-3, respectively.

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The carrier mobility of PbTe-SrTe, PbTe-SrTe-Te2% and PbTe-SrTe-Te4% could also be calculated to 12, 6.6, and 13.5 cm2/(V.s), respectively. The increased carrier concentration plays important roles in electrical conductivity and Seebeck coefficient. It is well known that the band structure of p-type PbTe is a two-band model. Thus the increased carrier concentration leads to band degeneracy which could cause the Seebeck coefficient to increase.33-38 A similar phenomenon was also reported for heavily doped p-type SnTe, which has similar band structure to PbTe.50-51 According to the reference, at room temperature, heavy band starts to contribute at carrier concentration (n) exceeding 3×1019 cm-3.38 In this work we explored the possibility of making PbTe-SrTe based thermoelectric films via a simple, scalable, and low-cost screen-printing method. We showed that the addition of extra tellurium powder can significantly increase the thermoelectric performance of the printed PbTeSrTe films via a synergistic doping effect and enhancement of particle binding. The highest ZT achieved is above 1.0 at 350 °C for PbTe-SrTe-2wt.% Te composite, which is the highest reported value for printed thermoelectric films. These results open up a new, low-cost direction for developing and utilizing high performance thermoelectric materials for a broad range of applications. ASSOCIATED CONTENT The Experimental Section, evaluated thermal conductivity and ZT values are well elaborated in the Supporting Information. AUTHOR INFORMATION * Prof. Mercouri G. Kanatzidis Email: [email protected]

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* Prof. Yanliang Zhang

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Email: [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is partially funded by the US Department of Energy, Office of Nuclear Energy, under Award number DE-NE0008255 (screen printing and characterization). At Northwestern University (G. T. and M. G. K.), the work was supported by DARPA Grant HR0011-16-C-0035.

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