Elsevier

Current Applied Physics

Volume 20, Issue 9, September 2020, Pages 1036-1040
Current Applied Physics

Effective phonon scattering and enhancement of thermoelectric performance in Ga-excess Bi0.4Sb1.6Te3 compounds

https://doi.org/10.1016/j.cap.2020.06.024Get rights and content

Highlights

  • We investigate the thermoelectric properties on Ga-excess p-type Ga-excess Bi0.4Sb1.6Te3 compounds.

  • Significant decrease of lattice thermal conductivity and high ZT value (1.13 at 350 K) for the Ga x = 0.03 doped compound.

  • We observed Sb and Ga phase separation leading to the phonon scattering.

  • The synergetic phonon scatterings from various scattering sources enhances thermoelectric performance.

Abstract

We investigate the thermoelectric properties on Ga-excess p-type GaxBi0.4Sb1.6Te3 compounds. Even though the random distribution of Ga-doping increases electrical resistivity giving rise to the decrease of power factor, the significant decrease of lattice thermal conductivity by the excess Ga-doping induces significant enhancement of ZT value (1.13 at 350 K) for the Ga x = 0.03 doped compound. From the X-ray diffraction and elemental mapping by energy dispersive X-ray spectroscopy, we observed Sb and Ga phase separation leading to the phonon scattering. The Sb precipitation implies atomic defect in the matrix which can induce short wavelength phonon scattering. The synergetic phonon scatterings from various scattering sources such as point defect, alloy scattering, and grain boundary phonon scattering have an important role in the enhancement of thermoelectric performance.

Introduction

Thermoelectric (TE) materials can directly convert waste heat into electricity and vice versa, which can be applied to solid state cooling and power generation. The TE performance can be expressed by the dimensionless figure-of-merit, ZT=S2T/(κe+κL)ρ, where S, T, ρ, κe, and κL are the Seebeck coefficient, absolute temperature, electrical resistivity, electronic thermal conductivity, and lattice thermal conductivity, respectively. In order to achieve high ZT value, the large power factor (PF = S2σ) and the low thermal conductivity are required simultaneously.

Considerable investigations have been studied to improve thermoelectric performance such as nano-structuring by process engineering [1]. For example, the hot-deformation process can modify the microstructure and decrease the lattice thermal conductivity [[2], [3], [4], [5]]. Various substitutional doping elements such as Cu [6], Ag [4,7], rare-earth elements [8], and alkaline-earth doping [9] on Bi2Te3 based alloys also significantly reduced the lattice thermal conductivity by the mass fluctuation and defect scatterings.

Previous report suggested that grain boundary dislocation by Te segregation in Bi0.5Sb1.5Te3 effectively scatters phonon, resulting in the significantly enhanced ZT value 1.86 at 320 K [10]. Besides, it was argued that the large ZT value was an incorrect combination of thermal conductivity measured along the press direction while charge transport measured along the perpendicular direction. It showed that the meaningful ZT value on the Te-excess BST is limited by 1.2 at 350 K [11]. We tried liquid-phase sintering of the Te-excess Bi0.4Sb1.6Te3+x compound to identify the excess Te effect on the compound [12]. Interestingly, we observed the Sb phase separation, instead of Te-liquid phase near the grain boundary with enhanced ZT value 1.41 at 417 K [12]. In this work, we study the thermoelectric effect on the excess Ga addition in p-type Bi0.4Sb1.6Te3 (BST) alloy to check the possible grain boundary phonon scattering.

Section snippets

Experimental procedures

In order to make a Ga phase separation near grain boundaries, the Ga-excess BST compounds GaxBi0.4Sb1.6Te3 (x = 0.0, 0.01, and 0.03) were synthesized by two step process. The pristine BST ingots were prepared by melting and quenching. We evacuated quartz ampoules with stoichiometric mixture of high purity elements of Bi (99.999%), Sb (99.999%), and Te (99.999%) and heated 650 °C for 20 h with following quenching in cold water. The ingots were pulverized by hand grinding and mixed with high

Results and discussion

X-ray diffraction patterns of the Ga-excess Bi0.4Sb1.6Te3 compounds are shown in Fig. 1(a). All the compounds are well indexed to the rhombohedral Bi2Te3 structure with R-3m space group. As listed in Table 1, the c-axis lattice parameters are slightly decreased from c = 30.479 Å (pristine BST) to c = 30.464 Å for x = 0.03 Ga excess BST, while the a-axis lattice parameters are not changed significantly, resulting in the lattice volumes of the compounds are not sensitive with Ga-excess doping.

Conclusions

We synthesize the GaxBi0.4Sb1.6Te3 alloys by melting and hot press sintering. We found the homogeneous Ga substitution and Sb precipitation in the matrix. Sb precipitation implies the atomic point defects of the matrix. Even though the decrease of power factor owing to the increase of electrical resistivity by Ga doping, we observed significantly enhanced ZT value (1.13 at 350 K) for the Ga x = 0.03 doped compound, attributed from the significant decrease of thermal conductivity. The synergetic

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

J.S.R. was supported by the Materials and Components Tech-nology Development Program of MOTIE (Ministry of Trade, Industry, and Energy)/KEIT (Korea Evaluation Institute of Industrial Technology) (10063286) and by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2020R1A2C2009353).

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