Ga-doping induced carrier tuning and multiphase engineering in n

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Functional Inorganic Materials and Devices

Ga-doping induced carrier tuning and multiphase engineering in n-type PbTe with enhanced thermoelectric performance Zhengshang Wang, Guoyu Wang, Ruifeng Wang, Xiaoyuan Zhou, Zhiyu Chen, Cong Yin, Mingjing Tang, Qing Hu, Jun Tang, and Ran Ang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05117 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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ACS Applied Materials & Interfaces

Ga-doping induced carrier tuning and multiphase engineering in n-type PbTe with enhanced thermoelectric performance Zhengshang Wanga, Guoyu Wangb,c, Ruifeng Wangb,c, Xiaoyuan Zhoud, Zhiyu Chena, Cong Yina, Mingjing Tanga, Qing Hua, Jun Tanga,*, Ran Anga,e,*

a

Key Laboratory of Radiation Physics and Technology, Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, China

b

Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China c

d

e

University of Chinese Academy of Sciences, Beijing 100190, China

College of Physics, Chongqing University, Chongqing 401331, China

Institute of New Energy and Low-Carbon Technology, Sichuan University, Chengdu 610065, China

ACS Paragon Plus Environment

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KEYWORDS: Thermoelectric materials; n-type PbTe; Ga doping; Carrier tuning; Multiphase engineering

ABSTRACT: P-type lead telluride (PbTe) emerged as a promising thermoelectric material for intermediate temperature waste heat energy harvesting. However, n-type PbTe still confronted with a considerable challenge owing to its relatively low figure of merit ZT and conversion efficiency η, limiting widespread thermoelectric applications. Here, we report that Ga-doping in n-type PbTe can optimize carrier concentration and thus improve power factor. Moreover, further experimental and theoretical evidences reveal that Ga-doping induced multiphase structures with nano- to micrometer size can simultaneously modulate phonon transport, leading to dramatic reduction of lattice thermal conductivity. As a consequence, a tremendous enhancement of ZT value at 823 K reaches ~1.3 for n-type Pb0.97Ga0.03Te. In particular, in a wide temperature range from 323 to 823 K, the average ZTave value of ~0.9 and calculated conversion efficiency η of ~13% are achieved by Ga doping. The present findings demonstrate the great potential in Ga-doped PbTe thermoelectric materials through a synergetic carrier tuning and multiphase engineering strategy.

1. Introduction Thermoelectric materials have been recognized as an ideal candidate in power generation and solid-state cooling, which can directly convert waste heat into electricity and vice versa.1-4 The efficiency of thermoelectric material is governed by the dimensionless figure of merit ZT =


2

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S2σT/κtot, where S is the Seebeck coefficient, σ is the electrical conductivity, and κtot is the total thermal conductivity which comprises the lattice and electronic components κlat and κele.5,6 It is apparent that a high ZT requires a high S, high σ, and low κtot at a certain temperature. Unfortunately, these parameters S, σ, and κtot are closely interrelated with each other.7-9 Thus, how to modulate harmoniously the electrical and thermal properties of high performance thermoelectric materials is a crucial issue to be resolved.10-12 As one of the state-of-the-art thermoelectric materials for power generation at the intermediate temperature (500-900 K),13 lead telluride (PbTe) has an inherent ZT value of 0.8.10 For p-type PbTe, the electrical properties (S and σ) have been governed by carrier concentration tuning,14 band convergence,15 and band energy alignment.16 On the other hand, the thermal properties (κlat) of p-type PbTe have also been determined by introducing point defects,17 nanostructuring,18,19 high density of dislocations,20 interfaces,21 and all-scale hierarchical architecturing.22 Consequently, the thermoelectric performance exhibits ultrahigh ZT values for p-type PbTe materials. However, the ZT values for n-type PbTe materials are relatively lower than that of ptype PbTe materials.23-29 From the viewpoint of commercial applications, the implementation of thermoelectric energy conversion requires an identical thermoelectric performance between nand p-type PbTe materials.30 Therefore, it is urgent for n-type PbTe materials to enhance ZT merit to break through the limitation of large-scale application. Current strategies to improve thermoelectric performance of n-type PbTe, which is focused on enhancing power factor PF (PF = S2σ) and suppressing κlat, can be mainly summarized in the following aspects: (1) self-tuning carrier concentration with temperature-dependent solubility of impurity in the matrix,31 (2) increasing effective mass m* by producing impurity levels which locally distort density of states (DOS) of conduction band and form so-called resonant states,32


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(3) scattering low-energy carriers with multiphase potential barriers resulting in a record-high ZT for PbTe-4%InSb,29 (4) decreasing κlat by synergistic alloying and nanostructuring.33-37 However, few novel methods for n-type PbTe have been developed by multiphase engineering to suppress κlat. Particularly, it is not yet known that individual contribution of various phases to reduce κlat and to what extent the multiphase can play a role in manipulating transport properties of carriers and phonons. This motivates us to seek for a possible avenue by multiphase engineering to enhancing ZT values, as well as relatively high average ZT (ZTave) and conversion efficiency (η) over a wide temperature range. In this work, we report that Ga-doping-driven n-type PbTe not only promotes a noticeable enhancement of power factor by carrier tuning, but also induces multiphase structures with nanoto micrometer size which simultaneously manipulates phonon transport. Ultimately, the maximum of ZT value for n-type Pb0.97Ga0.03Te reaches ~1.3 at 823 K, as well as the average ZTave value of ~0.9 and calculated conversion efficiency η of ~13% in a wide temperature region from 323 to 823 K. 2. Experimental Polycrystal Pb1-xGaxTe (x = 0.01, 0.02, 0.03, 0.04) samples were fabricated by convention solid-state reaction technique. Stoichiometric quantities of high purity elements [lead (99.99%), tellurium (99.999%), and gallium (99.999%)] were loaded into quartz tubes. The tubes then were evacuated, flame-sealed, slowly heated to 773 K, soaked at this temperature for 6 h, followed by melting at 1273 K for 6 h, finally cooled to room temperature rapidly. The obtained ingots were crushed into powders and densified by spark plasma sintering (SPS) method (LABOX-325, Japan) at 823 K for 10 min with a 60 MPa uniaxial pressure. All the disk shaped samples


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obtained were 10 mm in diameter with density no less than 97% of theoretical density (Table S1). Powder X-ray diffraction (PXRD) measurements with Cu-Kα radiation of powder samples after SPS process were performed on an X-ray diffractometer (DX-2700). Structural refinements were carried out by Rietveld analysis. The surface morphology was characterized by a FEI inspect F50 scanning electron microscope (SEM). The temperature dependence of electrical conductivity σ and Seebeck coefficient S were carried out by using apparatus (LSR-3). The temperature dependence of thermal diffusivity D and thermal conductivity κ were performed by the laser flash method (LFA 457, Netzsch). The heat capacity Cp was determined from the measurements

of

Blachnik

by Cp(kB/atom) = 3.07

+

0.00047(T/K-300) for

lead

chalcogenides.14,15 To verify the accuracy of Cp from the calculation, the experimental Cp was also measured using a TA Q20 differential scanning calorimeter (DSC). The uncertainties in ZT are nearly ±10%. The Hall coefficient RH and carrier concentration nH were determined by Hall measurements using a standard five-probe method and conducted by the Quantum Design Physical Property Measurement System (PPMS). 3. Results and discussion Figure 1a shows the PXRD patterns for Pb1-xGaxTe (x = 0.01, 0.02, 0.03, 0.04) samples, in which the main diffraction peaks in all samples are well indexed to face-centre-cubic PbTe structure. Interestingly, for the samples with x = 0.03 and 0.04, the diffraction peaks of impurity marked by the asterisks are observed at 26°, 29° and 45°, verifying the appearance of Ga6PbTe10 phase (inset of Figure 1a). In addition, we also carried out structural Rietveld refinements to reveal the variation of impurity phase (Figure S1), and the detailed phase fraction of Ga6PbTe10


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is plotted in Figure S2. It is noted that no diffraction peak of Ga6PbTe10 appears in XRD patterns when x < 0.03, since the phase fraction of Ga6PbTe10 is much lower than the limited resolution of XRD technology (about 1%), which can also be verified by the following SEM results. While for x ≥ 0.03, the phase fraction of Ga6PbTe10 increases from 1.1% to 2.8%, triggering the emergence of impurity peaks as shown in the inset of Figure 1a. To probe thermoelectric performance of pristine and Ga-doped PbTe, we first measured electrical properties as a function of temperature. Evidently, Ga doping improves the electrical conductivity σ due to the increase of carrier density nH (Figure 1b and Figure 1c). Therefore, the optimal carrier concentration can be achieved by manipulating Ga content. With rising temperature, σ for each Ga-doped sample decreases (Figure 1c), exhibiting a typical characteristic of degenerate semiconductor. One might have noticed that temperature-dependent σ increases with increasing of Ga content from x = 0 to 0.03, while it drops for x = 0.04, taking into account the lower mobility µH (Figure 1b). In particular, at low temperatures, Seebeck coefficient S for Ga-doped PbTe presents a rising feature in Figure 2d, further supporting the degenerate behavior. The slow-down characteristic of S at high temperatures originates from the thermal excitation of minority carriers. It is worth noting that with increasing of Ga content, the absolute value of S declines because of the increased nH (Figure 1b). In addition, the S for pristine PbTe shows an apparent transition from p- to n-type occurring around 550 K (inset of Figure 3d), which is accordant with previous results,38 indicating the comparable carrier concentrations between electrons and holes. Especially for the sample with x = 0.03, carrier tuning by Ga doping induces a remarkable improvement of power factor PF (Figure 1e). While for the sample with x = 0.04, it has a lower carrier mobility µH (Figure 1b), resulting in the depression of PF in the entire temperature range. Theoretical Pisarenko relation based on single


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Kane band model (SKB) and acoustic phonon scattering can explain experimental data well.39,40 The S at room temperature as a function of nH (Figure 1f) demonstrates that the experimental data matches well with the theoretical predictions of n-type PbTe (effective mass m* = 0.25 me). Such results reveal that Ga doping do not significantly affect the conduction band structure. Note that the magnitudes of nH for all samples reach the order of ~1019 cm-3. However, the low values of nH do not follow with the theoretical assumption, i.e. each Ga atom occupying the Pb site would offer one electron. Hence, a large number of Ga atoms can be favorable to the formation of precipitates when Ga point defects exceed the limit of solubility, which is also supported by the diffraction peaks of impurity marked by the asterisks (Figure 1a). Thus, it is necessary to examine the evolution of microstructures by Ga doping. The characterization of SEM and energy dispersive spectroscopy (EDS) for all of samples are shown in Figure 2. As for x = 0.01, the matrix can be clearly distinguished due to the distinct grains and boundaries (Figure 2a). The black particles marked by white circles labeled as phase 1 (P1), are different from that of pristine PbTe. Additionally, another phase distributes in the grain boundaries of PbTe with nano-scale size denoted as phase 2 (P2). As for x = 0.02, the content of P2 precipitates visibly increase (Figure 2b). In contrast, the black particles P1 start to assemble (marked by the white arrowheads in Figure 2b). Nevertheless, the atomic ratio is very close between P1 and matrix (i.e. Pb : Ga : Te = 41.15 : 52.57 : 6.28 for P1 and Pb : Ga : Te = 43.25 : 52.40 : 4.35 for matrix). Therefore, it is reasonable that both P1 and matrix should have the same contribution to scattering of phonons and carriers, as further verified in Figure S3. As for x = 0.03 and 0.04, a new kind of phase with micrometer scale emerges, denoted as P3, which is surrounded by grains and P1 (Figure 2c and Figure 2d). More details about P1, P2, and P3 are described in Figure S3. In comparison with matrix, Ga element is excessive in P2, while


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both Ga and Te elements are excessive in P3. These multiple phases with scales from nanometer to micrometer provide a new route to modulate thermoelectric properties, especially for phonon transport. In order to understand the intrinsic relationship between phonon scattering and multiphase, we measured the thermal transport properties. The detailed thermal diffusivity and heat capacity are listed in Figure S4. Figure 3a shows the temperature dependence of total thermal conductivity κtot. Due to intense Umklapp scattering, the calculated lattice thermal conductivity κlat rapidly decreases in a wide temperature range (Figure 3b). To gain more insight into the contribution of κlat, we firstly simulated the values of κlat at room temperature (Figure 3c) in the framework of modified Klemen’s model,41,42 which only includes Umklapp process and point defects as the scattering sources in thermal transport. At relatively low Ga content (x ≤ 0.01), the experimental values of κlat follow fairly well with the theoretical prediction, revealing that point defects dominate phonon scattering for the reduction of κlat. While for x ≥ 0.02, the experimental values of κlat gradually deviate from the calculated lines (Figure 3c), demonstrating the strong phonon scattering arising from various multiple phases. Herein, we utilized a well-accepted model based on Debye approximation43 to simulate the experimental values of κlat. The theoretical κlat can be expressed as:

κ lat

k k T  = B2  B  2π ν  h 

3

∫

θ D /T

0

τc ( x)

x 4e x

(e

x

− 1)

2

dx

(1)

where kB is the Boltzmann constant, ћ is Plank’s reduced constant, ν is the average sound velocity, θD is the Debye temperature, τc is combined relaxation time, and x is defined as x = ћω/kBT. As mentioned above, both matrix and P1 can be regarded as an identical scattering


8

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source. More theoretical details, including the equations for different relaxation times and the relevant parameters for calculation are described in SI. To validate the accuracy of simulation, we compare the calculated and experimental values of κlat for the sample with x = 0.04 (Figure 3b). It is apparent that the simulated κlat agrees quite well with the experimental ones, which reinforces our assumption, i.e. the multiphase plays an important role in scattering heat-carrying phonon. In addition, we also developed the Debye model to examine the individual contribution of these different phases to κlat reduction. Since all the Pb1-xGaxTe samples reach the maximum value of ZT at 823 K, it is crucial to simulate κlat at this temperature (Figure 3d). As for x = 0.01, the experimental value of κlat falls exactly on the simulated red dot line, which takes into account Umklapp process, normal process and point defects (UN + PD) as the dominant phononscattering source, confirming that point defects are the most effective scatters. As for x = 0.02, the deviation between the experimental value of κlat and the calculated line (UN + PD) emerges, demonstrating the intense phonon scattering arising from nanoprecipitates of P2. With the rising of P2 content (x = 0.03), the simulated values (UN + PD + P2) of κlat decrease and match the experimental value of κlat, indicating that the importance of P2 for phonon scattering. Since abundant P3 only appear for the sample with x = 0.04, we just subtracted a fixed value (κlat reduction by P3 at 823 K) from the green dash dot line (UN + PD + P2). Based on the consistency between experimental results and theoretical line (UN + PD + P2 + P3), we reached an unexpected conclusion that the multiple phases (P2 + P3) play a key role in scattering heatcarrying phonon. All of the input parameters based on above theoretical simulation of κlat are listed in Table S2. Such calculated simulations provide a comprehensive account for the intrinsic contribution of each phase and the fundamental role of multiphase.


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As a result, we achieved a noticeable enhancement of ZT merit through manipulating Ga content. Figure 4a plots the figure of merit ZT as a function of temperature. Because of the increasing contents of P2 and P3, multiphase leads to the lowest values of κlat for the sample with x = 0.04. On the other hand, the carrier mobility µH decreases, leading to the suppression of PF. Therefore, it is not the highest value of ZT for x = 0.04. While for the sample with x = 0.03, it has the highest value of ZT ~1.3 at 823 K. Such enhanced value of ZT driven by carrier tuning and multiphase engineering is much larger than most of previous outstanding results (Figure S5a), especially compared with PbSe-0.5%Ga.37 Additionally, the high performance of Pb0.97Ga0.03Te indicates negligible variation of thermoelectric properties during the experimental heating and cooling cycles, revealing an excellent thermal stability (Figure S6). And also, it is found a good experimental repeatability for high value of ZT, as evidenced by three different samples fabricated independently for the sample with x = 0.03 (Figure S6f). Meanwhile, we should not ignore the importance of average ZTave from the viewpoint of thermoelectric application. The calculated ZTave in the temperature range from 323 to 823 K are shown in Figure 4b, as well as the thermoelectric conversion efficiency η (Figure S5b). It is emphasized that the enhanced PF and reduced κlat for x = 0.03 triggers a large ZTave of ~0.9 in a wide temperature range. Moreover, it is well known that the value of η can be calculated from ZTave, hot temperature Th, and cold side temperature Tc: 44-46

η=

1 + ZTave − 1 Th − Tc Th 1 + ZTave + Tc Th

(2)

Distinctly, from the viewpoint of practical application for thermoelectric, a great η is determined by large ZTave and broad temperature range. However, the previous results for the ntype PbTe, to the best of our knowledge, cannot reach the above requirement due to narrow


10

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temperature range (e.g. PbTe-5%InSb29), or relatively low values of ZTave (e.g. PbTe-0.5%Cr,36 Pb0.99Cd0.01Te0.25Se0.7538) as shown in Figure 4c. In comparison, our present work of Pb0.97Ga0.03Te exhibits an appropriately high value of ZTave in a wide temperature region (Th - Tc = 500 K), reflecting its potential application as an ideal candidate of thermoelectric materials. Moreover, the conversion efficiency η for the sample with x = 0.03 for Tc = 323 K and Th = 823 K is ~13%, which exceeds that of many other n-type PbT (T = Te, Se, S) materials (Figure 4d). 4. Conclusions In summary, the introduction of Ga into PbTe matrix enhances the PF through the modulation of carrier concentration. Furthermore, the Ga-doping triggers the emergence of multiple phases (i.e., P1, P2, and P3) responsible for the scattering of heat-carrying phonon, which have been demonstrated by a series of theoretical simulations. In particular, both P2 and P3 dominate the phonon transport, leading to the reduction of κlat. As a result, the ZT value at 823 K reaches ~1.3 for n-type Pb0.97Ga0.03Te, as well as the average ZTave value of ~0.9 and conversion efficiency η of ~13% from 323 to 823 K. These findings open up a novel avenue for manipulating thermoelectric properties in Ga-doped PbTe via synergetic carrier tuning and multiphase engineering.

ASSOCIATED CONTENT Supporting Information. Calculation of lattice thermal conductivity; Rietveld refinement for Ga-doped samples; phase fraction of Ga6PbTe10 in Ga-doped samples; SEM and EDX elemental mapping; thermal


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diffusivity, heat capacity, Lorenz number, and electronic thermal conductivity for the Pb1-xGaxTe samples; temperature dependence of ZT and calculated conversion efficiency η; stability and reproducibility evaluation for Pb0.97Ga0.03Te; tables with density of all samples for Pb1-xGaxTe and input parameters based on the theoretical simulation of lattice thermal conductivity. AUTHOR INFORMATION Corresponding Author [email protected] (R. Ang), [email protected] (J. Tang) ACKNOWLEDGMENT R. Ang thanks the financial supports from National Natural Science Foundation of China (NSFC) (grant no. 51771126), Youth Foundation of Science & Technology Department of Sichuan Province (grant no. 2016JQ0051), Sichuan University Outstanding Young Scholars Research Funding (grant no. 2015SCU04A20), Sichuan University Talent Introduction Research Funding (grant no. YJ201537), World First-Class University Construction Funding. J. Tang is grateful to the financial support from NSFC (grant no. 11274234). G. Y. Wang thanks the financial supports from Key Research Program of Frontier Sciences, Chinese Academy of Sciences (grant no. QYZDB-SSW-SLH016).


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Figure captions:

Figure 1. (a) Powder XRD patterns for Pb1-xGaxTe (x = 0.01, 0.02, 0.03, 0.04); (b) carrier concentration nH and mobility µH at 300 K. Temperature dependence of (c) electrical conductivity σ; (d) Seebeck coefficient S; (e) power factor PF; (f) Seebeck coefficient at room temperature as a function of carrier concentration for this work and refs 35, 38. The solid curve is the theoretical Pisarenko plot for n-type PbTe with effective mass of electrons of m* = 0.25 me.

Figure 2. (a) Surface morphology and corresponding EDS mapping for x = 0.01; (b) surface morphology for x = 0.02, along with the EDS spectrum and the corresponding quantitative results of matrix and P1, respectively; (c) surface morphology and corresponding EDS mapping for x = 0.03; (d) surface morphology for x = 0.04, along with the EDS spectrum and the corresponding quantitative results of P2 and P3, respectively.

Figure 3. Thermal transport properties and simulation of Pb1-xGaxTe. Temperature dependence of (a) total thermal conductivity κtot; and (b) lattice thermal conductivity κlat; the orange dot line represents the simulated κlat for x = 0.04 based on Debye approximation; (c) κlat at room temperature as a function of Ga content, and the solid line stands for κlat in the framework of Klemens-Debye model; (d) Ga content dependence of κlat at 823 K, where the dashed curves express the theoretical predictions based on a Debye approximation with various types of phonon scattering, the solid dots stand for the experimental results.


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Figure 4. Temperature dependence of (a) ZT; and (b) average ZTave for Pb1-xGaxTe, compared with ref 38; (c) comparison of ZTave with previous n-type lead chalcogenides refs 14, 26, 28, 29, 36-38, 47, 48; (d) corresponding ZTave values and calculated conversion efficiency η in a wide temperature range from 323 to 823 K.


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Table of Contents


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Ga-doping induced carrier tuning and multiphase engineering in n

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