Improved Thermoelectric Performance of Tellurium by Alloying with a

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Improved Thermoelectric Performance of Tellurium by Alloying with a Small Concentration of Selenium to Decrease Lattice Thermal Conductivity Udara Saparamadu, Chunhua Li, Ran He, Hangtian Zhu, Zhensong Ren, Jun Mao, Shaowei Song, Jingying Sun, Shuo Chen, Qinyong Zhang, Kornelius Nielsch, David Broido, and Zhifeng Ren ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13121 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 10, 2018

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ACS Applied Materials & Interfaces

Improved Thermoelectric Performance of Tellurium by Alloying with a Small Concentration of Selenium to Decrease Lattice Thermal Conductivity

Udara Saparamadua, Chunhua Lib, Ran Hec, Hangtian Zhua, Zhensong Rena, Jun Maoa,d, Shaowei Songa,e, Jingying Suna, Shuo Chena, Qinyong Zhangf, Kornelius Nielschc, David Broidob, and Zhifeng Rena*

a

Department of Physics and Texas Center for Superconductivity, University of Houston, Houston, TX 77204, USA

b

Department of Physics, Boston College, Chestnut Hill, MA 02467, USA

c

Leibniz Institute for Solid State and Materials Research, IFW-Dresden, Helmholtzstr. 20, 01069

Dresden, Germany d

Department of Mechanical Engineering, University of Houston, Houston, TX 77204, USA

e

Program of Material Science and Engineering, University of Houston, Houston, TX 77204, USA

f

Key Laboratory of Fluid and Power Machinery of Ministry of Education & Center for Advanced Materials and Energy, Xihua University, Chengdu, Sichuan 610039, China

* Corresponding author, email address: [email protected]

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Phonon scattering through alloying is a highly effective way to reduce lattice thermal conductivity due to the mass difference between the host and alloyed atoms and strains caused by the different atoms. In this work we investigate the thermoelectric properties of Te between 323 K and 623 K. By varying the alloying concentration of Se, a minimum lattice thermal conductivity was achieved with ~10% (by stoichiometry) alloying of Te by Se. Additionally, Sb has been used as a dopant to increase the carrier concentration of the system. With reduced lattice thermal conductivity by Se alloying and increased carrier concentration by Sb doping, the room-temperature figure of merit (ZT) increased by 60%, leading to an average 𝑍𝑇 of ~0.8 in Te0.88Se0.10Sb0.02, which corresponds to an engineering figure of merit (𝑍𝑇)eng ~0.5 between 323 K and 623 K and an efficiency of ~8% in the same temperature range. The results indicate that the combination of Se alloying and Sb doping is successful in improving the thermoelectric properties of Te.

Keywords: Te, average ZT; Thermoelectric; alloying; Elemental Te, Ball milling, Hot pressing

1. Introduction Thermoelectric materials could play a key role in converting waste heat into useful electricity.1-6 Researchers are making great efforts to build high-performance thermoelectric generators for large-scale applications.7-12 The efficiency of a thermoelectric material is determined by its figure of merit, 𝑍𝑇 = [𝑆2/𝜌(𝜅𝑒𝑙𝑒 + 𝜅𝑙𝑎𝑡𝑡)]𝑇, where 𝑆, 𝜌, 𝜅𝑒𝑙𝑒, 𝜅𝑙𝑎𝑡𝑡, and 𝑇 are the Seebeck coefficient, electrical resistivity, electronic thermal conductivity, lattice thermal conductivity, and absolute temperature, respectively. 𝑆, 𝜌, and 𝜅𝑒𝑙𝑒 are strongly coupled with one another, which makes


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independent property manipulation challenging. Since carrier concentration is a factor common to all of these parameters, it is important to achieve an optimized carrier concentration.13,

14

To

achieve a high 𝑍𝑇, two strategies can be implemented, either improving the power factor (𝑆2/𝜌) by band engineering,15-19 or reducing 𝜅𝑙𝑎𝑡𝑡 by increasing the phonon scattering.20-22 Reducing 𝜅𝑙𝑎𝑡𝑡 has been employed in silicon germanium alloys to improve their overall performance. Based on the theoretical work by Abeles et al., it has been shown that alloying Ge to elemental Si reduced 𝜅𝑙𝑎𝑡𝑡 due to mass defect scattering.23 Yamashita et al. reported that alloying 30% Ge to elemental Si reduced the total thermal conductivity significantly over the whole temperature range.24 The investigation on Bi-Sb alloys conducted by Yim et al. showed that Sb alloying decreased the total thermal conductivity due to alloying scattering.25 Lin et al. recently reported the thermoelectric properties of pristine Te, including a peak 𝑍𝑇 of ~ 1 at 650 K.26 This high 𝑍𝑇 in an element was attributed to the particular band structure of Te that provides additional conduction channels for better electrical transport properties.26, 27 There have been no other reports about further efforts to reduce the lattice thermal conductivity of pristine Te for possible 𝑍𝑇 improvement. In this work, we investigated the effect on the thermoelectric properties of alloying Se to Te and doping with Sb (Te0.98-xSexSb0.02; x = 0.05, 0.075, 0.1, and 0.125) by ball milling and hot pressing. Crystal and microscopic structural, electrical, and thermal transport properties were studied, with the electronic band structure obtained from density functional theory calculations to understand the results. We showed that by alloying Se to Te, point defects are introduced into the structure, which significantly reduces 𝜅𝑙𝑎𝑡𝑡, resulting in an improved average 𝑍𝑇 in the temperature range between 300 and 623 K. In addition, phonon velocity is a very important parameter to consider where it can be approximated by the low frequency sound velocity. Sound velocity



depends on the crystal structure, bonding, and chemical composition and it can play a significant role in the phonon contribution to 𝜅𝑙𝑎𝑡𝑡. The measured transverse and longitudinal sound velocities are 1974 m s-1 and 1289 m s-1, respectively, for the optimized composition Te0.88Se0.10Sb0.02. Ballmilled undoped Te exhibits a 𝜅𝑙𝑎𝑡𝑡 of ~1.3 W m-1 K-1 at room temperature, which is lower than the reported value of 1.7 W m-1 K-1 by Lin et al.26 With Se alloying and Sb doping, Te0.88Se0.10Sb0.02 shows a significantly reduced 𝜅𝑙𝑎𝑡𝑡 of ~0.64 W m-1 K-1, which enhances the 𝑍𝑇 to ~0.4 at room temperature, an ~150% increase compared to that reported by Lin et al., and achieves a similar peak 𝑍𝑇 of 1.0, resulting in an average 𝑍𝑇 of ~0.8 in the temperature range between 323 K and 623 K, about ~40% higher than that reported by Lin et al.,26 corresponding to an (ZT)eng ~0.5 and a conversion efficiency of 8% in the temperature range between 323 and 623 K.

2. Experiment Te0.98-xSexSb0.02 (x = 0, 0.05, 0.075, 0.1, and 0.125) samples were prepared by ball-milling highpurity elements (Te chunks, 99.999%; Se granules, 99.99%; and Sb chunks, 99.9%) weighed according to the planned stoichiometry in a stainless-steel ball milling jar. The contents of the sealed jar were ball-milled by a high-energy ball mill (SPEX 8000D) for 10 hours. The ball-milled powders were then loaded into a graphite die with an inner diameter of 12.7 mm inside a glove box and consolidated by hot pressing at ~673 K for 2 minutes, obtaining disk-shaped samples with dimensions of 12.7 mm diameter x 3 mm width. The hot-pressed disks were cut and polished into bars of 12 x 2 x 2 mm3 for simultaneous resistivity and Seebeck coefficient measurements on a ZEM-3 (ULVAC Riko). The thermal conductivity 𝑘 = 𝑑𝐷𝑐𝑝 was calculated using the measured density (𝑑) by the Archimedean method,


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the thermal diffusivity (𝐷) by a laser flash method (LFA 457; Netzsch), and the specific heat (𝑐𝑝) by differential scanning calorimetry (DSC 404 C; Netzsch). The Hall coefficient 𝑅𝐻 was measured at room temperature on a commercial system (PPMS Dynacool; Quantum Design) using a fourprobe configuration, with a magnetic field of 3 T and an electrical current of 8 mA. The Hall carrier concentration (𝑛𝐻) was calculated from the relation 𝑛𝐻 = 1/(𝑒𝑅𝐻). Phase compositions of the samples were characterized by X-ray diffraction (PANalytical X’pert PRO). The lattice parameters were calculated by Rietveld refinement using the Fullprof software. The microstructure was analyzed by a JEOL 6304F scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) was used for elemental analysis of the samples as shown in Fig. S1. The microstructure of the sample was analyzed by a JEOL 2100F field emission electron microscope, as shown in Fig. S2. We calculated the band structures of Te and Se in the plane-wave projector augmented wave basis28 as implemented in the Vienna ab initio simulation package (VASP)29-32. To accurately capture the band gaps, we included the spin-orbit interaction (SOI) and used the modified Becke-Johnson (mBJ) function33, 34, with the exchange-correlation energy taken from the Perdew-Becke-Ernzerhof (PBE) generalized gradient approximation35. It is known that the mBJ band gap depends on the lattice constant used in the calculation. For certainty, we used the experimental crystal structure for both Te and Se (a = 4.456 Å and c = 5.921 Å for Te36; a = 4.368 Å and c = 4.958 Å for Se37). An 18 × 18 × 12 Γ centered k grid was used for the band structure calculation and the k grid for the density of states (DOS) calculation was set at 28 × 28 × 18. For Te, the planewave cutoff was 290 eV, while that for Se was set at 250 eV. The sound velocities of the longitudinal and transverse phonon branches were measured using the RITEC Advanced Ultrasonic Measurement System RAM-5000. The system realizes the



pulse-echo method of time propagation measurements with an accuracy of about 10-3 µs. To generate longitudinal (𝐿) and transverse (𝑇) ultrasonic bulk waves, Olympus transducers V129RM (10 MHz) and V157-RM (5 MHz) were used. Propylene glycol and SWC (both from Olympus) were used as couplant materials for 𝐿 and 𝑇 modes, respectively. Thickness measurements were carried out using a Mitutoyo ID-HO530 digital indicator. All data were obtained at room temperature.

3. Results and discussion

The SEM image of the optimized composition of Te0.88Se0.10Sb0.02 is shown in Fig. S1 (a) and the elemental analysis is shown in Fig. S1 (b). The X-ray diffraction (XRD) patterns of all of the Te0.98xSexSb0.02 (x

= 0, 0.05, 0.075, 0.1, and 0.125) compositions are shown in Fig. 1(a). All of the XRD

patterns were indexed into the trigonal structure (space group P3121) and no impurity phases were observed. In Fig. 1(b), the lattice parameters were calculated by Rietveld refinement. The decrease in the lattice parameter c agrees with Vegard’s law, which indicates that Te (atomic radius, ~0.97 Å) atoms are partially replaced by Se (atomic radius, ~0.5 Å) atoms. The calculated band structure along some high symmetry directions and the density of states (DOS) are shown in Fig. 2. For Te, the valence band maximum (VBM) and the conduction band minimum (CBM) are both located around the H point, resulting in a slightly direct gap of ~0.26 eV, which agrees with the experimental value of 0.33 eV38 and is also consistent with the previously reported calculations by first principles27. The VBM of Se is located slightly away from the L point and its CBM is in the vicinity of the H point, giving an indirect gap of 1.7 eV, in agreement of the experimental value of


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1.85 eV39. We also observed the camelback shape around the H point27 in the VBM of Te (not shown in the plot), while this feature is absent in Se due to its smaller spin-orbit interaction (SOI). Fig. 3 displays the temperature-dependent thermoelectric properties of Te0.98-xSexSb0.02 samples. Here Sb is used to tune the carrier concentration (𝑛𝐻). Upon 2% Sb doping to Te, the electrical resistivity (𝜌) decreases from 292 μΩ m to ~22 μΩ m at room temperature, a factor of more than 93% (Fig. 3a and its inset). This is consistent with the 𝑛𝐻 increase upon Sb doping to Te as shown in Table 1. Increasing the Se concentration increased the electrical resistivity over the entire temperature as shown in Fig. 3(a) due to the much larger band gap of Se. Specifically, the room-temperature 𝜌 increased from ~45 μΩ m for Te0.93Se0.05Sb0.02 to ~89 μΩ m for Te0.855Se0.125Sb0.02. With 5% Se alloying the Hall mobility (𝜇) decreases significantly from that of pristine Te, as shown in Table 1. Interestingly, for samples with higher Se concentrations, the 𝜇 increases slightly. As shown in Fig. 3(b), the Seebeck coefficients (𝑆) of Sb-doped samples are positive, which indicates their p-type semiconducting properties. For pristine Te, the 𝑆 peak at 376 K is an indication of onset bipolar diffusion (inset of Fig. 3b). Upon 2% Sb doping, the 𝑆 is decreased over the whole temperature range due to the increase in 𝑛𝐻, which is consistent with the Hall measurement data as shown in Table 1. With increasing Se concentration, the 𝑛𝐻 decreases due to the increased band gap (shown in Table 1), which is calculated using the relationships of the band gap (𝐸𝑔), the maximum Seebeck coefficient (𝑆𝑚𝑎𝑥), and the temperature (𝑇𝑚𝑎𝑥): 𝐸𝑔 = 2𝑒 𝑆𝑚𝑎𝑥𝑇𝑚𝑎𝑥, where 𝑒 is the electron charge.40 The decreasing 𝑛𝐻 data is consistent with the increasing 𝑆. For higher Se concentration the onset bipolar diffusion appears at higher temperature. The effective mass is obtained for the Se-alloyed samples based on the single parabolic band model.41, 42

All of the relevant equations are shown in the supplementary information.



The DOS effective mass (𝑚 ∗ ) can be calculated by the measured 𝑛𝐻 and the measured 𝑆. As shown in Fig. 4, all of the Te0.98-xSexSb0.02 samples show good agreement with the wellestablished Pisarenko plot assuming 𝑚 ∗ = 0.6 at room temperature. Combining the 𝜌 and the 𝑆, the power factor (𝑃𝐹) is calculated, as shown in Fig. 3(c). The reduction of the 𝑃𝐹 with increasing Se concentration is due to the increasing defect density.43 The temperature dependence of the total thermal conductivity (𝜅𝑡𝑜𝑡) is plotted in Fig. 3(d). Compared to pristine Te, the total thermal conductivity is significantly reduced due to the formation of solid solutions. The lattice thermal conductivity (𝜅𝑙𝑎𝑡𝑡) was determined by subtracting the electronic thermal conductivity (𝜅𝑒𝑙𝑒 = 𝐿𝑇/𝜌) from the total thermal conductivity. The Lorenz number (𝐿) was calculated by the single parabolic band model for simplicity.44 The reduction of the total 𝜅 is mainly caused by the reduction of the 𝜅𝑙𝑎𝑡𝑡 due to phonon scattering by mass fluctuation and strain.44 As shown in the inset of Fig. 3(d), at room-temperature pristine Te shows a total 𝜅 of ~1.3 W m-1 K-1, which is lower as compared with the reported data.45, 46 The best composition Te0.88Se0.10Sb0.02 shows a low 𝜅𝑙𝑎𝑡𝑡 of ~0.5 W m-1 K-1 at 476 K, which is the lowest among all elemental thermoelectric materials as shown in Fig. 3(e).45-48 The 𝜅𝑙𝑎𝑡𝑡 at room temperature is reduced by 50% due to Se alloying. Here, the effect of the point defect scattering caused by Sb doping is weak due to the low solubility of Sb in the lattice and the small mass difference.45 To gain more insight regarding the 𝜅𝑙𝑎𝑡𝑡 of Te0.88Se0.10Sb0.02, the transverse (𝑣𝑡) and longitudinal (𝑣𝑙) sound velocities were measured and the average sound velocity (𝑣𝑚) was calculated by:


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𝑣𝑚 =

([

1 2 3 𝑣𝑡3

1

])

+ 𝑣3 𝑙

1

―3

(1)

The measured transverse and the longitudinal sound velocities are 1974 m s-1 and 1289 m s-1, respectively. The calculated average velocity using Eq. (1) is 1413 m s-1 for Te0.88Se0.10Sb0.02 at room temperature, which is lower than that reported for pristine Te.45 The lower sound velocity contributes to the lower 𝜅𝑙𝑎𝑡𝑡. As compared with the well-known elemental thermoelectric material Si, the sound velocity of Te is significantly lower, and it is among the lowest of the elemental thermoelectric materials.49 Young’s modulus (𝐸) and the Debye temperature (𝜃𝐷) were calculated from the sound velocities by: 𝐸=

𝜌𝑣2𝑠 (3𝑣2𝑙 ― 4𝑣2𝑡 )

(2)

(𝑣2𝑙 ― 𝑣2𝑡 )

and 𝜃𝐷 =

1 3

[ ] 𝑣𝑚,

ℎ 3𝑁 𝑘𝐵 4𝜋𝑉

(3)

where 𝜌 is the sample density, ℎ is the Plank’s constant, 𝑘𝐵 is the Boltzmann constant, 𝑁 is the number of atoms in a unit cell, and 𝑉 is the unit-cell volume. The calculated Young’s modulus and Debye temperature are 26 GPa and 128 K, respectively. The low Debye temperature and Young’s modulus are attributed to weak interatomic bonding.50, 51 Low sound velocity and a small mean 1

free path (𝑙) are beneficial for low 𝜅𝑙𝑎𝑡𝑡 according to: 𝑘𝑙𝑎𝑡𝑡 = 3𝐶𝑣𝑣𝑚𝑙. It is commonly believed that lower sound velocities will lead to lower lattice thermal conductivities.50-54 The measured mean sound velocity for Te0.88Se0.1Sb0.02 is ~1413 m s-1. The relationships between the room-temperature lattice thermal conductivities and sound velocities of the well-known thermoelectric materials are shown in Fig. 5. Assuming that Te0.88Se0.10Sb0.02 has glass-like thermal conductivity, the temperature-independent glassy limit can be estimated by:



𝑘𝐺𝑙𝑎𝑠𝑠 =

3𝑘𝑏𝑣𝑔 𝜋 2

2𝑉

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1

( ), 3

(4)

6

3

where 𝑣𝑔 is the phonon group velocity approximated by the average sound velocity (𝑣𝑚~𝑣𝑔) with the atomic volume 𝑉. The estimated 𝑘𝐺𝑙𝑎𝑠𝑠 ~0.12 W m-1 K-1 is mainly due to the optical phonons. The thermoelectric figure of merit, 𝑍𝑇, is shown in Fig. 3(f). The 𝑍𝑇 of Te0.98Sb0.02 is comparable with that reported previously.26, 46 Sb shows low solubility in the lattice compared to the As doping reported by Lin et al.26 The reduction of 𝜅𝑙𝑎𝑡𝑡 upon Se alloying increased the average 𝑍𝑇 to ~0.8 for Te0.88Se0.10Sb0.02 in the temperature range between 323 K and 623 K, an improvement of ~40% over previous reports. The average 𝑍𝑇 for all of the compositions is shown in Fig. 6. The measured carrier concentration for the optimized sample is lower compared to the reported data by Lin et al.26 This might be due to the lower solubility of Sb in the lattice.46 To increase the carrier concentration, we further increased the Sb concentration, as shown in Fig. S3. Increasing the Sb concentration resulted in a decrease in the average 𝑍𝑇, although it is still higher than that reported in the literature. The engineering figure of merit (𝑍𝑇)eng and the efficiency were calculated assuming a hot-side temperature (T) at 623 K and a cold-side temperature (Tc) at 323 K for all of the samples.55 Fig. 7(a) and (b) show the dependence of the calculated (𝑍𝑇)eng and efficiency, respectively, on the hot side temperature. At 623 K, the optimized sample, Te0.88Se0.10Sb0.02, shows an (𝑍𝑇)eng and efficiency of ~0.5 and 8%, respectively, representing increases of ~37% and ~33%, respectively, compared to those of Te0.98Sb0.02. The calculated (𝑃𝐹)eng and power output density are shown in Fig. S4.


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4. Conclusion

Due to enhanced alloy scattering of the phonons, the lattice thermal conductivity of Te0.98xSexSb0.02

(x = 0, 0.05, 0.075, 0.1, and 0.125) is significantly decreased and the average 𝑍𝑇 is

increased over a wide temperature range. Sb was used as a dopant to optimize the carrier concentration. The low sound velocity of Te0.88Se0.10Sb0.02 results in lower lattice thermal conductivity. The significantly reduced lattice thermal conductivity doubles the room temperature 𝑍𝑇 to ~0.4 and improves the average 𝑍𝑇 to ~0.8 in the temperature range between 323 and 623 K, and achieves an (𝑍𝑇)eng of ~0.5 with conversion efficiency of ~8% from 323 to 623 K.

5. Acknowledgment The work performed at the University of Houston was funded by the U.S. Department of Energy’s Basic Energy Science Program under Grant DE-SC0010831. 6. Supporting Information SPB model, SEM, EDS, TEM, (𝑃𝐹)eng, Output power

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Figures

a)

b)

c)

Figure 1. (a) The XRD patterns and (b) Rietveld refinement of Te0.98-xSexSb0.02 (x = 0, 0.05, 0.075, 0.1, and 0.125).



Figure 2. The calculated electronic band structure for Te (blue) and Se (red). The Fermi level was set at 0 eV.


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a)

b)

c)

d)

e)

f)

Figure 3. The temperature-dependent thermoelectric properties of Te0.98-xSexSb0.02 (x = 0, 0.05, 0.075, 0.1, and 0.125). (a) Electrical resistivity (b) Seebeck coefficient (c) power factor (d) total thermal conductivity (e) lattice thermal conductivity and (f) ZT values. The respective transport properties of pristine Te are shown in the insets of the figures.



Figure 4. The Pisarenko plot of Te0.98-xSexSb0.02 (x = 0.05, 0.075, 0.1, and 0.125).


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Figure 5. Lattice thermal conductivity vs. sound velocity measured experimentally for wellstudied thermoelectric materials.51,56-61

Figure 6. The average 𝑍𝑇 of Te0.98-xSexSb0.02 (x = 0, 0.05, 0.075, 0.1, and 0.125).



a)

b)

Figure 7. (a) Calculated (𝑍𝑇)eng and (b) calculated efficiency of Te0.98-xSexSb0.02 (x = 0 and 0.1).


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1 2 3 4 5 6 7 Table 1. Important parameters of Te0.98-xSexSby (x = 0, 0.05, 0.075, 0.1, and 0.125; y = 0 and 0.02). 8 9 10 11 𝑛𝐻 𝑚∗ 𝑆 (μV K-1) 𝜇 (cm2 V-1 s-1) 𝐸𝑔 (eV) 12 Composition 19 -3 (x 10 cm ) 13 14 15 Te 0.03 426 762.3 0.39 0.33 16 17 18 Te Se Sb 1.19 174 116.1 0.6 0.37 0.93 0.05 0.02 19 20 21 0.78 194 154.7 0.54 0.35 22 Te0.905Se0.075Sb0.02 23 24 25 Te0.88Se0.10Sb0.02 0.62 233 156.6 0.67 0.35 26 27 28 Te Se Sb 0.44 254 159.7 0.62 0.37 29 0.855 0.125 0.02 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 ACS Paragon Plus Environment 60

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Alloying effect

Short wavelength phonons Mid wavelength phonons


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Improved Thermoelectric Performance of Tellurium by Alloying with a

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