ZnTe Alloying Effect on Enhanced Thermoelectric Properties of p-Type

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ZnTe Alloying Effect on Enhanced Thermoelectric Properties of p-type PbTe Kyunghan Ahn, Hocheol Shin, Jino Im, Sang Hyun Park, and In Chung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15295 • Publication Date (Web): 04 Jan 2017 Downloaded from http://pubs.acs.org on January 5, 2017

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ACS Applied Materials & Interfaces

ZnTe Alloying Effect on Enhanced Thermoelectric Properties of ptype PbTe

Kyunghan Ahn‡, Hocheol Shin†, ‡, Jino Im§, Sang Hyun Park∥,*, In Chung †, ‡*

†

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic

of Korea. ‡

School of Chemical and Biological Engineering and Institute of Chemical Processes, Seoul

National University, Seoul 08826, Republic of Korea. §

Thin Film Materials Research Center, Korea Research Institute of Chemical Technology,

Daejeon 34114, Republic of Korea. ∥

Advanced Materials and Devices Laboratory, Korea Institute of Energy Research, Daejeon

34129, Republic of Korea.

* To whom correspondence should be addressed: [email protected], [email protected]

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Abstract We investigate the effect of ZnTe incorporation on PbTe to enhance thermoelectric performance. We report structural, microscopic, and spectroscopic characterizations, ab initio theoretical calculations, and thermoelectric transport properties of Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4). We find that the solid solubility limit of ZnTe in PbTe is less than 1 mol%. The introduction of 2% ZnTe in p-type Pb0.985Na0.015Te reduces the lattice thermal conductivity through the ZnTe precipitates at the microscale. Consequently, a maximum thermoelectric figure of merit (ZT) of 1.73 at 700 K is achieved for the spark plasma sintered Pb0.985Na0.015Te-2% ZnTe, which arises from a decreased lattice thermal conductivity of ~0.69 W m-1 K-1 at ~700 K in comparison with Pb0.985Na0.015Te.

Keywords: thermoelectric; PbTe; ZnTe; alloying; renewable energy; hole doping;


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Introduction Thermoelectric (TE) energy conversion of heat to electricity is an important renewable energy technology because it can generate electric power using waste heat and consequently improve energy efficiency significantly in various fields of applications.1-3 The efficiency of TE materials is defined by the dimensionless TE figure of merit ZT = (σS2T)/κ, where σ is the electrical conductivity, S is the Seebeck coefficient, T is the absolute temperature, and κ is the thermal conductivity. PbTe-based compounds are a top TE material operating in the mid-temperature range of 500 – 900 K.4-14 Recently, their ZT values have been significantly improved by many innovative strategies such as nanostructuring,15-17 hierarchical architecturing,18 and electronic band structure engineering19-20. Such advances are mainly attributed to their availability of facile doping or alloying. For example, 4 mole% SrTe alloying on PbTe coupled with spark plasma sintering (SPS) results in a surprisingly high ZT value of ~2.2 at 915 K due to multiscale hierarchical architecture for maximally reducing lattice thermal conductivity.18 Further enhancement of ZT is achieved by higher degree of alloying with SrTe that enables the significant improvement of Seebeck coefficient because of valence band convergence.21-24 The group 12 elements of Zn, Cd, and Hg are also known as effective dopants to PbTe.25-28 For CdTe- and HgTe-doped PbTe, the specimens prepared by SPS shows higher ZT than those by melt-processed ingot.29-31 For ZnTe-doped PbTe, only melt-processed n-type materials were reported.32 Therefore, it is worth studying the stabilization of the p-type ZnTe-doped PbTe and characterization of its TE properties. Here we report the TE properties of the Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4) system processed by SPS. We present the detailed investigations of structural, microscopic, spectroscopic, electrical, and thermal transport properties as well as the results of ab initio density functional theory calculations to understand the effect of ZnTe on TE properties. 3 ACS Paragon Plus Environment


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Unlike the PbTe-SrTe system, we find that the solubility limit of ZnTe in PbTe is less than 1% and consequently the effect of valence band convergence is negligible. However, the reduction in lattice thermal conductivity occurs upon 2% ZnTe incorporation. Consequently, the Pb0.985Na0.015Te-2% ZnTe shows the improved ZT value of ~1.73 at ~700 K compared to that of Pb0.985Na0.015Te without the incorporation of ZnTe (ZT ~ 1.53).

Experimental Synthesis of bulk ingot and powder processing. Stoichiometric amounts of elemental Pb, Na, Zn, and Te were weighed for the synthesis of Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4) and loaded into a carbon-coated fused silica tube. The tube was flame-sealed under a vacuum of ~1.33 x 10-3 Pa. The tube was then heated to 1323 K for 6 h, held at the same temperature for 12 h, and then water quenched followed by vacuum annealing at 873 K for 3 days. The cast ingot samples were manually ground with a mortar and pestle in an Ar-filled glove box, and the ground powder was finally sieved under 45 µm. Spark plasma sintering. The ground bulk powders of the samples of Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4) were loaded into a graphite die in an Ar-filled glove box and coldcompacted manually. The graphite die was taken out of the glove box and placed in a spark plasma sintering system (SPS-211Lx, Fuji Electronic Industrial Co., Japan). The sample chamber was evacuated to a vacuum of ~1.4 × 10−2 Torr, and the powder samples in a graphite die were consolidated at ~823 K for 5 min by spark plasma sintering under an axial pressure of 50 MPa in vacuum. Powder X-ray diffraction (XRD) and infrared spectroscopy. Powder XRD analysis was performed using a SmartLab Rigaku powder X-ray diffractometer (Cu Kα radiation) operating at 40 kV and 30 mA. Fourier transform infrared spectroscopy (FT-IR) spectra were


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recorded using a Vertex 70 Bruker FT-IR spectrometer equipped with a DLATGS detector and a KBr substrate with a multilayer coating beam splitter. Scanning electron microscopy. Cross-sectional samples for scanning electron microscopy (SEM) were prepared by cutting the SPS pellet followed by polishing to make its mirror surface. The microstructural image, chemical composition, and elemental mapping were examined using a Schottky field emission scanning electron microscope (JSM-7600F, JEOL) equipped with a silicon drift detector (SDD) type energy dispersive X-ray spectroscopy (EDS) detector (X-MaxN, Oxford Instruments) and a EDS software (AZtec, Oxford Instruments). First-principles electronic structure calculations. To figure out the effect of ZnTe substitution for Pb in PbTe, we carried out first-principles electronic structure calculations within the density functional theory formalism. We utilized the projector augmented wave method33 with the plane wave basis set, implemented in Vienna Ab-initio Simulation Package (VASP).34-35 For exchange correlation functional, we employed the generalized gradient approximation within the Perdew-Burke-Ernzerhof formalism.36 Spin-orbit interaction (SOI) was included to accurately describe the effect of large SOI of Pb. To describe ~1% ZnTe substitution and ~2% Na doping in PbTe, we used 3 × 3 × 3 supercell of the conventional unit cell which accommodates 216 atoms. For ZnTe substitution, one Zn atom was substituted for one Pb atom, and for Na doping two Pb atoms was replaced by two Na atoms. Totally, four configurations (pristine, ZnTe substitution, Na doping, and ZnTe substitution with Na doping) were considered, and their internal coordinates were optimized with force criterion of 0.01 eV/Å. Experimental lattice parameters were used in the calculations. Electrical transport properties. The samples after SPS were cut for electrical and thermal property measurements using a diamond saw and polished with ethanol using a polishing machine under N2 atmosphere. The polished samples are rectangular-shaped with dimensions 5 ACS Paragon Plus Environment


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of ~3 mm × 3 mm × 9 mm. The longer direction coincides with the direction in which the electrical conductivity is measured. The electrical conductivity σ and the Seebeck coefficient S were measured simultaneously under a He atmosphere from room temperature to ~700 K on a ULVAC-RIKO ZEM-3 instrument system. Thermal conductivity. Thermal diffusivity (D) was directly measured under an Ar atmosphere, and the specific heat (CP) was indirectly derived using a standard sample (Pyroceram) as a function of temperature from room temperature to ~700 K using a flash diffusivity method in a Netzsch LFA 457 MicroFlash instrument. In a flash diffusivity method, the front face of a disk-shaped sample is irradiated by a short laser burst, and the resulting rear face temperature rise is recorded and analyzed by an IR detector. The thermal conductivity (κ) was calculated from the equation κ = DCPρ, where ρ is the density of the sample. Hall measurement. The Hall coefficient (RH) were measured in the temperature range of ~300 K to ~673 K using a LakeShore 8407 Hall effect measurement system in a magnetic field of 1.5 T under Ar flowing.

Results and discussion Powder X-ray diffraction (PXRD) patterns of Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4) samples are shown in Figure 1(a). The samples of x = 0 and 1 are a single phase crystallizing in the NaCl-type structure whereas those of x = 2 and 4 show reflection peaks originating from ZnTe at 25°, 42°, and 50°. Their intensities increase with increasing ZnTe content. This observation indicates that a maximum solubility limit of ZnTe in PbTe is less than 1%, consistent with a previous study.25-26 Indeed, the lattice hardly changes upon ZnTe addition. 6 ACS Paragon Plus Environment

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The refined lattice parameters of the samples of x = 0, 1, 2, and 4 are 6.453(1), 6.454(1), 6.453(1), and 6.455(1) Å, respectively. If ZnTe is soluble enough in the PbTe matrix, the lattice would contract under the same Na concentration because the effective ionic radius of Zn2+ (0.74 Å) in octahedral geometry37 is much smaller than that of Pb2+ (1.19 Å) and Na+ (1.02 Å). Infrared absorption spectra of the Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4) support the PXRD results (Figure 1b). Doping Na into pure PbTe makes the band gap shifted from ~0.26 eV to 1020 cm-3) in these samples. In comparison, we measured infrared absorption spectra of the samples of PbTe-x% ZnTe (x = 1, 2, 4) without Na doping and all samples exhibit spectroscopically observable energy band gaps between 0.25 and 0.26 eV, which is nearly same as that of pure PbTe. Microstructural images reveal that the higher number densities of precipitates are present with the higher nominal concentration of ZnTe (Figure 2). The size of precipitates ranges from 1 to 10 µm, which is comparatively larger than a nanoscale phonon mean free path of PbTe-based materials and may not be effective for phonon scattering. This will be discussed below on thermoelectric property section. In addition, we do EDS elemental mapping analyses to assign exact positions and determine actual chemical compositions of precipitates at the micro-scale (Figure 3). It is worthwhile to note that ZnTe-doped samples really show only ZnTe precipitates at the micro-scale whereas the undoped sample does not exhibit any precipitates. Furthermore, the chemical analyses of the samples based on EDS indicate that the actual Zn content at the whole area having precipitates increases with 7 ACS Paragon Plus Environment


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increasing nominal Zn concentration, the matrix without precipitates does not show any Zn content, and the precipitates are determined to be only ZnTe (Table 1). However, EDS does not detect any Na element for all samples though the nominal content of 0.75 at% Na is present in the samples. To understand the effect of ZnTe substitution on TE properties of PbTe system, we performed first principles electronic structure calculations within the density functional theory (DFT) formalism for PbTe and its Na and Zn-doped phases. The DFT electronic band structures with spin-orbit coupling of PbTe and PbTe-1% ZnTe show that Zn substitution does not induce the localized defect level between the valence band maximum (VBM) and conduction band minimum (CBM) (Figures 4a and 4b). Instead, the localized states of Zn emerge in the high energy level of E - EF at 0.6 eV, far above CBM. The incorporation of Zn also results in small distortions at the band edge of the VB (Figure 4c) to give a relatively large band splitting at VBM compared to little modification at the CBM. Recent reports show that a high mole fraction of SrTe (>5 %) alloying with PbTe create convergence of a primary valence band along the L line and a secondary band along the Σ line and consequently enhance TE performance.21 Similar band distortion occurs with the Zn substitution. Our calculations for PbTe and PbTe-1% ZnTe also show the presence of the corresponding bands that having a maximum at R point and one between Г and R points, indicated by a black arrow. The energy difference between the first and the second highest valence bands (∆Eval) is 0.11 eV, which is close to that of the lightly doped PbTe-SrTe but larger than that of the heavily-alloyed PbTe-SrTe.21 When Zn substitutes Pb, the maximum of the second highest valence band is shifted to Г point, and its energy level is elevated. As a result, ∆Eval is reduced by 0.01 eV by 1% ZnTe alloying. This band convergence is consistent with that of the previous study21 with taking account of the spin-orbit interaction. We further considered a hole doping effect by incorporation of Na along with ZnTe. Figures 4(d) and 4(e) show that 8 ACS Paragon Plus Environment

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electronic band structures of PbTe-2% Na and PbTe-1% ZnTe-2% Na, respectively. Doping Na introduces hole carriers into PbTe, but significant band distortions do not occur, especially at the first and second highest valence bands. When Zn is additionally introduced, the similar change on the second highest valence band to the case of PbTe-1% ZnTe is observed. As a result, ∆Eval decreases by 0.014 eV for the Pb0.990Na0.010Te-2% ZnTe. Co-doping of Na and Zn causes the larger decrease in ∆Eval, possibly due to additional interaction between Zn and Na impurities in PbTe. The temperature-dependent electrical conductivity (σ) of the Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4) decreases with increasing temperature, indicative of heavily degenerate doping (Figure 5a). The σ value decreases with increasing ZnTe content mainly due to the reduction in hole mobility, as explained below: ~2700, ~1900, ~1700, and ~1600 S cm-1 for x = 0, 1, 2, and 4 at 300K, respectively. Then, it follows a nearly quadratic temperaturedependent power law of σ ~ T-δ (δ = 2.4 – 2.6), which is a typical behavior for p-type PbTe materials because of

dominating acoustic phonon scattering at high temperature.41 The

values of δ are -2.48(5), -2.50(4), -2.51(6), and -2.54(5) for x = 0, 1, 2, and 4, respectively. The temperature-dependent Seebeck coefficient (S) is positive over the entire temperature range of the measurement, indicative of p-type conduction. The S value at 300 K are ~68, ~67, ~70, and ~72 µV K-1 for x = 0, 1, 2, and 4, respectively, with nearly linear temperature dependence. The values of the Hall coefficients (RH) are positive over the entire temperature range of the measurement, consistent with the positive sign of S and confirming p-type nature. The RH value is temperature dependent: for x = 2, it increases from ~0.054 cm3 C-1 at 300 K to a maximum of ~0.065 cm3 C-1 at 400 K and subsequently decreases to ~0.036 cm3 C-1 at 673 K. This behavior is common for p-type PbTe and SnTe materials with multi-valence 9 ACS Paragon Plus Environment


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bands. In these compounds, the light L band shifts to lower energy whereas the heavy Σ band nearly remains with increasing temperature. As a result, the energy difference between L and Σ band (∆EL-Σ) becomes smaller and hole carriers between the two bands are repopulated, causing the strong temperature dependence of the RH.42-44 In contrast, a typical semiconductor with only a single band contributing to the charge transport exhibits the nearly temperatureindependent RH. The peak temperature of the RH (TH) corresponds to a critical temperature at which the contribution from the two valence bands of L and Σ to hole transport is comparable, thereby implying that the lower TH correlates with the smaller ∆EL-Σ (Figure 5b).42, 44-44 The TH decreases from 423 K for x = 0 to 400 K for x = 1, indicative of the reduction in ∆EL-Σ supported by theoretical calculation results mentioned above, but negligibly changes with further addition of ZnTe (x ≥ 2) probably due to the low solubility limit of ZnTe in PbTe. Note that pure p-type PbTe typically shows TH ~ 425 K regardless of hole concentration. A similar behavior was observed in the PbTe-SrTe system, in which SrTe shows the very low solubility in PbTe.18 When SrTe is heavily alloyed with PbTe up to 8% via non-equilibrium synthesis, a decreasing trend of TH is seen because of valence band convergence caused by the high solubility of SrTe with PbTe.21 The temperature-dependent hole concentration (Np) was determined from the relationship Np = 1/(e·RH), where e is the electronic charge, considering a simple parabolic band and single carrier conduction (Figure 5c). The Np values for x = 0, 1, 2, and 4 at 300 K are ~1.56 × 1020, ~1.15 × 1020, ~1.15 × 1020, and ~1.06 × 1020 cm-3, respectively, indicating that the incorporation of Zn cation in PbTe affect the Np value negligibly because of the same valence of Zn and Pb cations. We estimated the effective mass of hole carriers (m*) for the samples from the measured S and Np based on a simple single parabolic band. The m* of x = 0, 1, 2, and 4 is 0.85, 0.80, 0.83, and 0.81 me (me is the free electron mass) in comparison with 0.34 me of PbTe-2% CdTe-1.5% Na31 and 0.33 me of PbTe-2% HgTe-1% Na31. 10 ACS Paragon Plus Environment

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Relatively higher m* values of ZnTe alloyed samples should result from their roughly one order of magnitude higher hole carrier concentrations than CdTe and HgTe alloyed samples. For instance, the hole carrier concentration of PbTe-2% CdTe-1.5% Na28 and PbTe-2% HgTe1% Na28 is ~3.0 × 1019 and ~4.0 × 1019 cm-3, respectively. The Pisarenko plot21, 43 shows that correlation between S and Np of Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4) falls near the theoretical Pisarenko line of p-type PbTe with the consideration of two valence-band structure (Fig. 5d). This observation suggests that the valence band convergence caused by the incorporation of ZnTe is insufficient to enhance the Seebeck coefficient because of the low solubility of ZnTe in PbTe, as in the case of the lightly alloyed PbTe-SrTe samples.18 The temperature-dependent hole mobility µH was estimated from the equation µH = σ/(Np·e) (Figure 6a). The µH values for x = 0, 1, 2, and 4 at 300 K are ~94, ~81, ~68, and ~59 cm2 V-1 s-1, respectively. They decrease with increasing ZnTe content because ZnTe precipitates in the PbTe play a role as the scatterers of hole carrier. The temperaturedependence of µH follows a power law of µH ~ T-λ (λ = 3.3 – 3.9) in the range from 400 K to 673 K, which is similar to µH ~ T-4 of p-type PbTe-SrTe materials.21 Rapid drop of the µH at high temperature is typical for p-type PbTe-based materials and is attributed to contribution of the heavy holes in the Σ valence band.21 N-type PbTe materials tend to show different dependence of µH ~ T-2.46 Figure 6(b) shows the temperature-dependent power factors (PF, σS2) of the samples. The PF values for x = 0, 1, 2, and 4 at 300 K are ~12.4, ~8.5, ~7.7, and ~8.7 µW cm-1 K-2, respectively. The PF increases with temperature to reach a maximum, followed by decrease. For example, the PF value for x = 2 increases from ~7.7 µW cm-1 K-2 at 300 K to a maximum of ~31.6 µW cm-1 K-2 at ~610 K, and subsequently decreases to ~27.0 µW cm-1 K-2 at ~700 K. The PF values for x = 0, 1, 2, and 4 reach to ~29.9, ~27.1, ~27.0, and ~26.4 µW cm-1 K-2 at ~700 K, respectively, which are higher than ~21 µW cm-1 K-2 for PbTe-2% CdTe-1.5% Na 11 ACS Paragon Plus Environment


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and ~22 µW cm-1 K-2 at ~700 K for PbTe-2% HgTe-1% Na.31 The temperature dependence of the total thermal conductivity κtot and the lattice thermal conductivity κlat of the samples are shown in Figures 6(c) and 6(d), respectively. The κtot of the samples follows a power law of κtot ~ T-α (α = 1.4 – 1.5). For x = 2, the κtot value is ~3.76 W m-1 K-1 at 300 K and decreases to ~1.10 W m-1 K-1 at ~700 K. Those for x = 0, 1, 2, and 4 at 300 K are ~4.34, ~3.87, ~3.76, and ~3.98 W m-1 K-1, respectively. The electrical thermal conductivity κel was estimated from κel = LσT, where L is the Lorenz number. The L values as a function of temperature were obtained from temperature-dependent Seebeck coefficient data on the assumption of a single parabolic band (SPB) model.47 The calculated room temperature κel value for x = 0, 1, 2, and 4 is ~1.72, ~1.22, ~1.02, and ~1.08 W m-1 K-1, respectively. The respective κel value at ~700 K is ~0.58, ~0.45, ~0.40, and ~0.43 W m-1 K-1. The lattice thermal conductivity κlat value was estimated by subtracting the κel value from the κtot value: κlat = κtot – κel (= LσT). The temperature dependence of the κlat follows a power law of ~ T-β (β = 1.6 – 1.8) and is significantly different from the typical dependence of T-1 observed for normal semiconductors with Umklapp scattering as the major mechanism for phonon scattering. Such deviation from T-1 results from bipolar contribution in p-type PbTe materials.44, 48 The lowest κlat of ~0.69 W m-1 K-1 was observed at ~700 K for x = 2 and the corresponding values for x = 0, 1, and 4 are ~0.79, ~0.82, and ~0.84 W m-1 K-1, respectively, which are 25 – 50% higher than that of Pb0.98Na0.02Te-10% SrTe (0.5 – 0.6 W m-1 K-1 at ~700 K) with the hierarchical architecturing.21 In comparison, the κlat value at ~700 K of PbTe-2% CdTe-1.5% Na31 and PbTe-2% HgTe-1% Na31 is ~1.0 and ~0.6 W m-1 K-1, respectively. Consequently, the reduction of room-temperature κtot value with ZnTe incorporation is mainly due to the reduction of room-temperature κel value whereas the reduction of κtot value at ~700 K, especially for x = 2, results from both the reduction of the κlat value and the κel value, compared to undoped sample (Table 2). The ZnTe precipitates at the microscale, shown 12 ACS Paragon Plus Environment

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earlier in SEM section (Figures 2 and 3), could not considerably prevent phonon propagation because the size distribution of ZnTe precipitates is not smaller than the phonon mean free path of PbTe (< 100 nm).49 Formation of ZnTe nano-precipitates by either rapid solidified process or ball-milling is necessary for further significant reduction in the κlat value, resulting in a considerably higher ZT. The TE figure of merit ZT as a function of temperature increases with increasing temperature (Figure 7a), which is typical behavior of PbTe-based materials. The ZT for x = 0, 1, 2, and 4 at ~700 K is ~1.53, ~1.51, ~1.73, and ~1.47, respectively. Pb0.985Na0.015Te-2% ZnTe exhibits the highest ZT of ~1.73 at ~700 K, mainly due to a combination of relatively higher PF and lower κlat, compared to ZT ~1.53 at ~700 K for Na-doped PbTe with no incorporation of ZnTe. In comparison, the ZT value at ~700 K for Pb0.985Na0.015Te-2% ZnTe is higher than ZT ~1.17 for PbTe-2% CdTe-1.5% Na31 and ZT ~ 1.59 for PbTe-2% HgTe-1% Na31, but is relatively lower than ZT ~ 1.9 for Pb0.98Na0.02Te-8% SrTe (Figure 7b).21 The relatively higher PF and lower or comparable κlat by ZnTe alloying are attributable for the enhanced ZT compared to CdTe and HgTe alloying.

Conclusions We synthesized p-type Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4) system to study the effect of alloying ZnTe with PbTe on TE properties. The Hall coefficient data of Na-doped PbTe-ZnTe materials reveal that the energy separation ∆EL-Σ between the light-hole L band and the heavy-hole Σ band become narrower even at a low solubility (< 1 %) of ZnTe in PbTe. However, there is no considerable enhancement in the Seebeck coefficient, suggesting that the higher solubility of ZnTe with PbTe is required for valence band convergence. Their scanning electron microscopic images demonstrate that the microscale ZnTe precipitates are prevalent in the PbTe matrix. The enhanced ZT of ~1.73 for the spark plasma sintered 13 ACS Paragon Plus Environment


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Pb0.985Na0.015Te-2% ZnTe is mainly attributed to reduction in thermal conductivity both from lattice and electrical contribution in comparison with Pb0.985Na0.015Te (ZT ~ 1.53). Studies on how to realize the higher solubility of ZnTe in PbTe and how to stabilize ZnTe precipitates at the nanoscale are necessary to merit the effect of valence band convergence and nanostructuring for achieving higher ZT.

Acknowledgements This work was supported by the National Research Foundation of Korea(NRF) Grant funded by the Korean Government(MSIP) (NRF-2015R1A5A1036133) and the Research and Development Program of the Korea Institute of Energy Research (KIER, B6-2448).

References (1) Bell, L. E. Cooling, Heating, Generating Power, and Recovering Waste Heat with Thermoelectric Systems. Science 2008, 321, 1457-1461. (2) Zeier, W. G.; Zevalkink, A.; Gibbs, Z. M.; Hautier, G.; Kanatzidis, M. G.; Snyder, G. J Thinking Like a Chemist: Intuition in Thermoelectric Materials. Angew. Chem., Int. Ed. 2016, 55, 6826-6841. (3) Tan, G.; Zhao, L. D.; Kanatzidis, M. G. Rationally Designing High-Performance Bulk Thermoelectric Materials. Chem. Rev. 2016, 116, 12123-12149. (4) Sootsman, J. R.; Kong, H.; Uher, C.; D'Angelo, J. J.; Wu, C. I.; Hogan, T. P.; Caillat, T.; Kanatzidis, M. G. Large Enhancements in the Thermoelectric Power Factor of Bulk PbTe at High Temperature by Synergistic Nanostructuring. Angew. Chem., Int. Ed. 2008, 47, 86188622. 14 ACS Paragon Plus Environment

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List of Tables Table 1. The chemical composition of Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4) analyzed by EDS. Table 2. The κtot, κel, and κlat values at 300 K and 700 K for Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4)


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Figure captions Figure 1. (a) Powder X-ray diffraction patterns of samples of Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4) and the peaks of ZnTe phase are marked by asterisk, and (b) infrared absorption spectra of Pb1-xZnxTe (x = 0, 0.01, 0.02, 0.04) and Pb0.985Na0.015Te-x% ZnTe (x = 0, 2). Figure 2. Scanning electron microscopic images for (a) Pb0.985Na0.015Te, (b) Pb0.985Na0.015Te1% ZnTe, (c) Pb0.985Na0.015Te-2% ZnTe, and (d) Pb0.985Na0.015Te-4% ZnTe. Figure 3. EDS elemental mapping for (a) Pb0.985Na0.015Te, (b) Pb0.985Na0.015Te-1% ZnTe, (c) Pb0.985Na0.015Te-2% ZnTe, and (d) Pb0.985Na0.015Te-4% ZnTe. Pb (green), Zn (yellow), and Te (red) elements are defined in color. Figure 4. Electronic band structures are depicted for (a) pristine PbTe, (b) Zn-substituted PbTe, (d) Na-doped PbTe, and (e) Na-doped and Zn-substituted PbTe. (c) and (f) show the band structures in the area marked by red rectangles in enlarged scale for undoped and Nadoped cases, respectively. Figure 5. Temperature dependence of (a) electrical conductivity σ (left) and Seebeck coefficient S (right), (b) Hall coefficient RH, and (c) hole concentration Np of samples of Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4), and (d) Room temperature Seebeck coefficient S as a function of hole concentration (Np) of samples of Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4). The solid line is the theoretical Pisarenko plot for p-type PbTe.20, 39 Figure 6. Temperature dependence of (a) hole mobility µH, (b) power factor PF (= σS2), (c) total thermal conductivity κtot, and (d) lattice thermal conductivity κlat of samples of Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4). Figure 7. (a) Temperature dependence of the TE figure of merit ZT of samples of Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4) and (b) ZT comparison of PbTe-2% CdTe-1.5% 21 ACS Paragon Plus Environment


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Na,31 PbTe-2% HgTe-1% Na,31 Pb0.98Na0.02Te-8% SrTe,21 and Pb0.985Na0.015Te-x% ZnTe (x = 0, 2).


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Table 1. The chemical composition of Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4) analyzed by EDS

Nominal composition Na0.015Pb0.985Te Na0.015Pb0.985Te -1% ZnTe Na0.015Pb0.985Te -2% ZnTe Na0.015Pb0.985Te -4% ZnTe

Composition analyzed by EDS Whole area w/ precipitate

Matrix w/o precipitate

Precipitate

PbTe0.939

N/A

N/A

PbZn0.015Te0.971

PbTe0.963

ZnTe1.022

PbZn0.031Te0.980

PbTe0.952

ZnTe0.992

PbZn0.054Te0.978

PbTe0.961

ZnTe1.03

Table 2. The κtot, κel, and κlat values at 300 K and 700 K for Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4)

Composition

κtot(300 K)/ κtot(700 K)

κel(300 K)/ κel(700 K)

κlat(300 K)/ κlat(700 K)

Na0.015Pb0.985Te

4.34/1.37

1.72/0.58

2.62/0.79

3.87/1.27

1.22/0.45

2.65/0.82

3.76/1.10

1.02/0.40

2.75/0.69

3.98/1.27

1.08/0.43

2.90/0.84

Na0.015Pb0.985Te -1% ZnTe Na0.015Pb0.985Te -2% ZnTe Na0.015Pb0.985Te -4% ZnTe



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Figure 1. (a) Powder X-ray diffraction patterns of samples of Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4) and the peaks of ZnTe phase are marked by asterisk, and (b) infrared absorption spectra of Pb1-xZnxTe (x = 0, 0.01, 0.02, 0.04) and Pb0.985Na0.015Te-x% ZnTe (x = 0, 2).


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Figure 2. Scanning electron microscopic images for (a) Pb0.985Na0.015Te, (b) Pb0.985Na0.015Te1% ZnTe, (c) Pb0.985Na0.015Te-2% ZnTe, and (d) Pb0.985Na0.015Te-4% ZnTe.



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Figure 3. EDS elemental mapping for (a) Pb0.985Na0.015Te, (b) Pb0.985Na0.015Te-1% ZnTe, (c) Pb0.985Na0.015Te-2% ZnTe, and (d) Pb0.985Na0.015Te-4% ZnTe. Pb (green), Zn (yellow), and Te (red) elements are defined in color.


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Figure 4. Electronic band structures are depicted for (a) pristine PbTe, (b) Zn-substituted PbTe, (d) Na-doped PbTe, and (e) Na-doped and Zn-substituted PbTe. (c) and (f) show the band structures in the area marked by red rectangles in enlarged scale for undoped and Nadoped cases, respectively.



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Figure 5. Temperature dependence of (a) electrical conductivity σ (left) and Seebeck coefficient S (right), (b) Hall coefficient RH, and (c) hole concentration Np of samples of Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4), and (d) Room temperature Seebeck coefficient S as a function of hole concentration (Np) of samples of Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4). The solid line is the theoretical Pisarenko plot for p-type PbTe.20, 39


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Figure 6. Temperature dependence of (a) hole mobility µH, (b) power factor PF (= σS2), (c) total thermal conductivity κtot, and (d) lattice thermal conductivity κlat of samples of Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4).



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Figure 7. (a) Temperature dependence of the TE figure of merit ZT of samples of Pb0.985Na0.015Te-x% ZnTe (x = 0, 1, 2, 4) and (b) ZT comparison of PbTe-2% CdTe-1.5% Na,31 PbTe-2% HgTe-1% Na,31 Pb0.98Na0.02Te-8% SrTe,21 and Pb0.985Na0.015Te-x% ZnTe (x = 0, 2).


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TOC:


ZnTe Alloying Effect on Enhanced Thermoelectric Properties of p-Type

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