Promising Thermoelectric Ag5−δTe3 with Intrinsic Low Lattice Thermal

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Promising Thermoelectric Ag Te with Intrinsic Low Lattice Thermal Conductivity Xinyue Zhang, Zhiwei Chen, Siqi Lin, Binqiang Zhou, Bo Gao, and Yanzhong Pei ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00813 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 27, 2017

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Promising

Thermoelectric

Ag5-δTe3

with

Intrinsic Low Lattice Thermal Conductivity Xinyue Zhang1, Zhiwei Chen1, Siqi Lin1, Binqiang Zhou1, Bo Gao1 and Yanzhong Pei1,* 1

Interdisciplinary Materials Research Center, School of Materials Science and

Engineering, Tongji Univ., 4800 Caoan Rd., Shanghai 201804, China. *

Email: [email protected]

1

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ABSTRACT New materials have been playing a continuously important role on advancing thermoelectric technology. Many known novel thermoelectric materials share the similarity of an intrinsic low lattice thermal conductivity (κL) due to various mechanisms. Because heat are generally conducted by acoustic phonons due to the much higher velocities as compared to those of optical phonons, many known low-κL thermoelectrics rely on complexity of crystal structure leading the fraction of acoustic phonons to be small. In addition to structural complexity, an overall low sound velocity is found helpful for realizing an extremely low κL. In this work, a new thermoelectric compound Ag5-δTe3, having both a complex crystal structure and a low sound velocity (~1300 m/s), is shown to be one of the least thermally conductive dense solids (κL~0.2 W/m-K). The resultant high thermoelectric figure of merit zT of unity, lead this material to be superior to known silver tellurides, with the help of its large band gap of ~0.6 eV. These preliminary results demonstrate Ag5-δTe3 as a promising thermoelectric material, with possibilities for further improvements particularly through enhancements focusing on electronic properties. TOC GRAPHICS 1.2

1.2

Ag5-δTe3

1.0

1.0

Ag

0.8

Te

zT

0.6 0.4

0.4

κL~ 0.2 W/m-K

0.2 0.0

0.6

zT

0.8

κ L (W/m-K)

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0.2

300

350

400

450

500

T (K)

550

600

650

0.0

2

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Without emissions or moving parts, thermoelectrics, which can directly convert waste heat to electricity1, have commonly been considered as a clean and sustainable technology for energy crisis. The biggest limitation for large-scale applications is their relatively low efficiency, which is largely determined by the materials’ dimensionless figure of merit zT, zT= S2T/ρ(κE+κL), where S, T, ρ, κE and κL are the Seebeck coefficient, the absolute temperature, the electrical conductivity, the electronic and lattice component of the thermal conductivity, respectively. Due to the strong coupling effect between S, ρ and κE, it is difficult to achieve high zT by simply improving one of these parameters. Providing the carrier concentration can be optimized2, proven strategies for improving zT of existing thermoelectrics are typified either by an enhancement of the power factor S2/ρ via band engineering such as band degeneracy3-8, nestification9, distortion10 or by a reduction of the lattice thermal conductivity (κL) through phonon scattering due to various sources including 0D substitutional11-14, interstitial15-17 and vacancy18-20 point defects, 1D dislocations21-23 and 2D boundary interfaces24-27. The above-mentioned strategies have been effectively guiding the discovery and development of

novel

thermoelectrics

as

well.

Importantly,

many

novel

thermoelectrics additionally have advantages, such as liquid-like ions28-29, intrinsically strong lattice anharmonicity30 and low sound velocity31-32, for an extremely low inherent lattice thermal conductivity. These are likely enabled in materials with a complex crystal structure, with either local or overall weak chemical bonds. These mechanisms successfully lead to an intrinsic lattice thermal conductivity of 3

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450 K,

α-Ag2Te transits to β-Ag2Te (a transition temperature of ~420 K44). The absence of β-Ag2Te impurity peaks suggests a single phase of the material at these high temperatures. When the samples are cooled back from high temperatures (T>~500 K), the X-ray diffraction peak of Ag2Te impurities appears again at 300 K. as indicated for Ag5.04Te3 in Figure 2b. This suggests that the Ag2Te impurities dissolve into the Ag5-δTe3 matrix when T>450 K and gradually precipitate out when T400 K for all the materials. Therefore, it is unlikely to have a strong bipolar in the materials here, which significantly distinguishes Ag5-δTe3 from known Ag2Te with a much smaller band gap (≤0.2 eV) and a much stronger bipolar conduction43-45, 48-49. Therefore, the decreases in both Seebeck coefficient and resistivity with increasing temperature at T>400 K is presumably due to an increase in the concentration of charged defects inherent to this material, the reason of which deserves further investigations. A similar temperature induced increase in carrier concentration, leading to a decrease in Seebeck coefficient and resistivity with increasing temperature, has been observed in PbTe with metallic Ag as an impurity phase59. Approximating Ag5-δTe3 as a Ag-deficient analogue to Ag2Te, the deficiency of Ag would lead to a hole concentration to be as high as 5×1021 cm-3, leading a reliable Hall measurement, on these bulk materials with relatively a high resistivity (>10 mΩ·cm), to be challenging. The total thermal conductivity (κ) and its lattice component (κL) are shown in 9

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Figure 4. The thermal conductivity was calculated via κ=dCpλ, where d is the density measured by a mass/volume method, λ is the thermal diffusivity and Cp is the specific heat at constant pressure. Temperature dependent Cp measurement by DSC for Ag5Te3 shows multiple endothermic peaks (Figure S1b), which is consistent with the richness of phase transition in the Ag-Te system (Figure S1a). Due to the existence of various phase transitions, it is challenging to calculate thermal conductivity using the measured Cp. We therefore use the Dulong-Petit limit for estimating the thermal conductivity. κL is estimated by subtracting the electronic contribution (κE=LT/ρ) from κ via the Wiedemann-Franz law, where the Lorenz factor (L; Figure S3) is determined by the single parabolic band model with acoustic scattering60. The lattice thermal conductivity of Ag5-δTe3 is found to be extremely low in the whole temperature range. A κL of ~0.2 W/m-K actually makes this material one of the least thermally conductive dense solids. The extremely low κL observed here partially stems from its low sound velocity (vs) of only ~1300 m/s. Room temperature elastic properties of Ag5-δTe3 are listed in Table 1. The details on the calculation of elastic properties from sound velocity measurement can be found elsewhere61-62. The sound velocity of Ag5-δTe3 is actually one of the lowest among known semiconductors31, 38. Propagation of a sound wave in gas, liquid and solid, its velocity gets higher as the chemical bonds get stronger, roughly indicating that weak bonding helps reduce sound velocity. Similar to Ag2Te, Ag5-δTe3 contains heavy elements and highly diffusive Ag atoms enabled by the partial occupancy of certain crystallographic sites52-55. This indicates that Ag atoms 10

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are loosely bonded in both compounds, and therefore the sound velocities are quite comparable (vs~1360 m/s for Ag2Te42 and vs~1300 m/s for Ag5-δTe3). The κL in Ag5-δTe3 is found to be even lower than that of Ag2Te in the whole temperature range. This can be ascribed to the much larger number of atoms in the primitive cell (n) of Ag5-δTe3. This leads to smaller contribution to lattice thermal conductivity due to acoustic phonons, because most of phonons are optical in a material with a large n37. Further due to nearly zero velocity of optical phonons, their contribution to lattice thermal conductivity are usually negligible, leading to an important measure of κL by acoustic Debye temperature (ΘD,a) according to Klemens and Slack37,

62

. Acoustic Debye temperature approximately measures the highest

energy of acoustic phonons with a Debye dispersion, and is linearly proportional to the length of the first Brillouin zone at a given sound velocity vs. Since the sound velocity in both Ag2Te and Ag5-δTe3 are comparable, the much larger n of 112 in Ag5-δTe3 would lead to a first Brillouin zone length to be only about 1/2 of that in α-Ag2Te, with n=1263. The resulting lower ΘD,a in Ag5-δTe3 could explain its lower κL at T99.99%) sealed in vacuum quartz ampoule at 1123 K for 7 hours, quenching in cold water and then annealing at 693 K for 3 days. The obtained ingots were then hand-ground into fine powders for optical measurements and hot press. Pellet samples were obtained by an induction heating hot press system69 at 673 K for 20 mins under a uniaxial pressure of ~60 MPa. The obtained dense samples (>97% of the theoretical density) were about 12 mm in diameter and ~1.5 mm in thickness. X-ray diffraction (XRD) was carried out on pellet samples to identify the phase composition. Resistivity and Seebeck coefficient were simultaneously measured on the pellet samples under helium. The Seebeck coefficient was obtained from the slope of the thermopower vs. temperature difference within 0~5 K, where the hot and cold side temperatures were measured by two K-type thermocouples attached to the opposite edges of the pellet sample70. The resistivity was measured using the van der Pauw technique. Thermal diffusivity (λ) was measured using laser flash technique with the Netzsch LFA457 system. A Differential Scanning Calorimetry apparatus (DSC 200F3, Netzsch, Shanghai, China) was used to measure the specific heat at constant pressure (Cp). The longitudinal (vL) and transverse (vT) sound velocities were measured on the pellet

samples

at

room

temperature,

using

an

ultrasonic

pulse-receiver

(Olympus-NDT) equipped with an oscilloscope (Keysight). The optical reflectance 16

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was measured by Fourier Transform Infrared Spectroscopy (Bruker Tensor II equipped with a Diffuse Reflectance attachment). The band gap of all samples are estimated based on these optical reflectance measurements, the measurement details have been given elsewhere71. The microstructure was characterized by a Scanning Electron Microscope (SEM, Phenom Pro) equipped with Energy Dispersive Spectrometer (EDS).

ASSOCIATED CONTENT Supporting Information Phase diagram, temperature dependent Cp, cyclic electrical properties, Lorenz factor, isotropic thermoelectric properties.

AUTHOR INFORMATION Corresponding Author *

Email: [email protected]

ORCID Yanzhong Pei: 0000-0003-1612-3294 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT 17

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This work is supported by the National Natural Science Foundation of China (Grant No. 51422208, 11474219 and 51772215). The authors thank Professor Jie Xiao and Xun Shi from the Shanghai Institute of Ceramics, CAS for their supports on DSC measurements.

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