Recent Developments in β-Zn4Sb3 Based Thermoelectric Compounds

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2015, Article ID 642909, 15 pages http://dx.doi.org/10.1155/2015/642909 Review Article Recent Developments in 𝛽-Zn4Sb3 Based Thermoelectric Compounds Tianhua Zou,1 Wenjie Xie,1 Jian Feng,1 Xiaoying Qin,2 and Anke Weidenkaff1 1 Institute of Materials Science, University of Stuttgart, 70569 Stuttgart, Germany Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China 2 Correspondence should be addressed to Wenjie Xie; and Anke Weidenkaff; Received 9 April 2015; Revised 6 August 2015; Accepted 9 August 2015 Academic Editor: Matteo Ferroni Copyright © 2015 Tianhua Zou et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Thermoelectricity has been recognized as an environmentally friendly energy conversion technology due to its ability to directly achieve conversion between heat and electricity for a long time. 𝛽-Zn4 Sb3 has attracted considerable interest as promising thermoelectric material in the moderate temperature range (500 K–900 K), which is the temperature range of most industrial waste heat sources. In this paper, first we present the structure of 𝛽-Zn4 Sb3 and the traditional doping strategy used to enhance its performance. Next, we review the details of some new methods utilized for improving the thermoelectric properties of 𝛽-Zn4 Sb3 and its thermal stability as well as reliability. Finally, the review finishes with highlighting some promising strategies for future research directions in the material. 1. Introduction During the last decade, due to their ability to convert heat into electricity directly as well as without moving parts or greenhouse emissions, thermoelectric materials have drawn much attention because of the energy crisis and the environmental concerns of fossil fuel use [1, 2]. When a temperature gradient is applied to a thermoelectric couple consisting of n-type and p-type materials, the charge carriers at the hot side will diffuse to the cold side, producing a thermoelectric voltage. This characteristic is the basis of thermoelectric power generation, known as the Seebeck effect. Conversely, when a voltage is applied to a thermoelectric couple, a temperature difference will be created. This characteristic is the basis of thermoelectric electronic refrigeration, known as the Peltier effect. The conversion efficiency of a thermoelectric material is qualified by the dimensionless figure of merit, 𝑍𝑇, defined as 𝑍𝑇 = (𝑆2 /𝜌𝜅)𝑇, where 𝑆, 𝜌, 𝜅, and 𝑇 are the Seebeck coefficient, electrical resistivity, thermal conductivity (including the lattice thermal conductivity 𝜅𝐿 , and the carrier thermal conductivity 𝜅𝑒 ), and absolute temperature, respectively [3, 4]. A good thermoelectric material should possess a high power factor PF (=𝑆2 /𝜌) and low thermal conductivity. Typically, there are two strategies to improve the 𝑍𝑇 of thermoelectric materials: one is to lower the thermal conductivity 𝜅 and the other is to boost the PF. Thermoelectric materials are normally classified into four categories depending on their temperature range of application: (1) cryogenic temperature range: from 4 K to 250 K; (2) near room-temperature range: from 250 K to 500 K; (3) intermediate temperature range: from 500 K to 900 K; and (4) high temperature range: beyond 900 K. As intermediate temperature range is just the temperature range of most industrial waste heat sources, it is very important to research high 𝑍𝑇 intermediate thermoelectrics. Among the wide variety of intermediate temperature materials, 𝛽-Zn4 Sb3 compounds with low thermal conductivity and made of relatively cheap and nontoxic elements are pointed out as one kind of most promising thermoelectric materials [5–7]. The ideal thermoelectric material should be a “phononglass and electron crystal” material, which possesses electronic properties similar to a good semiconductor single crystal but has thermal properties associated with amorphous materials [8–12]. 𝛽-Zn4 Sb3 , the p-type intermetallic Journal of Nanomaterials Thermal conductivity (10−3 W cm−1 K−1 ) 2 1.4 Zn4 Sb3 1.2 Figure of merit ZT TAGS 1.0 Bi2 Te 3 0.8 CeFe4 Sb12 0.6 PbTe 0.4 ZnSb 0.2 0.0 0 100 200 300 400 Temperature (∘ C) 500 600 (a) 30 CeFe4 Sb12 25 ZnSb (Sb,Bi)2 Te 3 20 TAGS 15 PbTe 10 Zn4 Sb3 5 0 0 100 200 300 400 Temperature (∘ C) 500 600 (b) Figure 1: (a) Thermoelectric figure of merit and (b) thermal conductivity of 𝛽-Zn4 Sb3 compared with other materials. Reproduced with permission from [16]. Copyright 2004, Nature Publishing Group. compound which is most suitable for use as a state-of-theart material at moderate temperatures, is one of the three modifications of Zn4 Sb3 . Zn4 Sb3 is known to have three structural phases, namely, 𝛼-, 𝛽-, and 𝛾-Zn4 Sb3 , which are stable below 263 K, between 263 K and 765 K, and above 765 K, respectively [13]. The highest 𝑍𝑇 value reported for 𝛽Zn4 Sb3 is 1.40 at 675 K [13–15]. The power factor in 𝛽-Zn4 Sb3 is reasonably high (∼13 W m−1 K−2 at 675 K) while it possesses a remarkable “phonon-glass” behavior, characterized by an unusually low thermal conductivity of ∼0.9 Wm−1 K−1 at 300 K, comparable to that of a glass [13], as shown in Figure 1. The organization of the review is as follows. First, we would like to provide some important backgrounds by introducing the structure of 𝛽-Zn4 Sb3 and highlight traditional doping method previously used in order to enhance the thermoelectric properties of 𝛽-Zn4 Sb3 . We then will review the recent progress in 𝛽-Zn4 Sb3 in details, including the energy filtering effect, distortion of the electronic density of states, in situ nanostructures, and its thermal stability. Finally, we identify strategies and research directions which could lead to further research in the material. 2. Structure of 𝛽-Zn4 Sb3 and Traditional Doping Strategy to Enhance ZT for 𝛽-Zn4 Sb3 The detailed crystal structure of 𝛽-Zn4 Sb3 has been determined by using both single-crystal and powder X-ray diffraction methods coupled with maximum entropy analysis [16, 27]. The 𝛽-Zn4 Sb3 has the hexagonal rhombohedral crystal structure and lattice constants are 𝑎 = 12.231 Å and 𝑐 = 12.428 Å with a R-3c space group [28]. To date, there are mainly two different models (Mayer model [29] and threeinterstitial model [16]) to explain the crystal structure of 𝛽Zn4 Sb3 unit cell. In the three-interstitial model, the mass density and composition of the crystal structure are reported to be in agreement with measurements [30, 31]. In the experimentally determined 𝛽-Zn4 Sb3 unit cell, it contains 30 Sb atoms, but there are four in equilibrium Zn positions with partial occupations: a deficiency of Zn1 site with ∼90% occupancy and interstitial Zn2, Zn3, and Zn4 sites with ∼5% occupancy to (...truncated)