Thermoelectric materials: Energy conversion between heat and electricity Xiao Zhang, Li-Dong Zhao* School of Materials Science and Engineering, Beihang University, Beijing 100191, China Received 10 January 2015; revised 19 January 2015; accepted 20 February 2015 Available online 24 April 2015 Abstract Thermoelectric materials have drawn vast attentions for centuries, because thermoelectric effects enable direct conversion between thermal and electrical energy, thus providing an alternative for power generation and refrigeration. This review summaries the thermoelectric phenomena, applications and parameter relationships. The approaches used for thermoelectric performance enhancement are outlined, including: modifications of electronic band structures and band convergence to enhance Seebeck coefficients; nanostructuring and all-scale hierarchical architecturing to reduce the lattice thermal conductivity. Several promising thermoelectric materials with intrinsically low thermal conductivities are introduced. The low thermal conductivities may arise from large molecular weights, complex crystal structures, liquid like transports or high anharmonicity of chemical bonds. At the end, a discussion of future possible strategies is proposed, aiming at further thermoelectric performance enhancements. © 2015 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Thermoelectric; Electrical conductivity; Seebeck coefficient; Thermal conductivity 1. Introduction Statistical results show that more than 60% of energy is lost in vain worldwide, most in the form of waste heat. High performance thermoelectric (TE) materials that can directly and reversibly convert heat to electrical energy have thus draw growing attentions of governments and research in- stitutes [1]. Thermoelectric system is an environment-friendly energy conversion technology with the advantages of small size, high reliability, no pollutants and feasibility in a wide temperature range. However, the efficiency of thermoelectric devices is not high enough to rival the Carnot efficiency [2,3]. A dimensionless figure of merit (ZT ) is defined as a symbol of the thermoelectric performance, ZT¼(a 2 s/к)T . Conceptu- ally, to obtain a high ZT , both Seebeck coefficient (a) and electrical conductivity (s) must be large, while thermal con- ductivity (k) must be minimized so that the temperature difference producing Seebeck coefficient (a) can be main- tained [4,5]. Historically, in 1821, the German scientist Thomas Johann Seebeck (Fig. 1(a)) noticed an interesting experimental result that a compass needle was deflected by a nearby closed cycle jointed by two different metals, with a temperature difference between junctions. This phenomenon is called the Seebeck effect, which can be simply schematized by Fig. 1(b), where an applied temperature difference drives charge carriers in the material (electrons and/or holes) to diffuse from hot side to cold side, resulting in a current flow through the circuit [6]. Fig. 1(c) shows the power generation efficiency as a function of average ZT ave , and the relationship can be given by Refs. [7,8]: h p ¼ T h  T c T h  ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ ZT ave p  1 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ ZT ave p þ T c =T h  ð1Þ where ZT ave is the average value of both n-type and p-type two legs, the ZT ave per leg is averaged over the temperature dependent ZT curve between T h and T c , T h and T c are the hot and cold ends temperature, respectively [7,8]: * Corresponding author. E-mail address: zhaolidong@buaa.edu.cn (L.-D. Zhao). Peer review under responsibility of The Chinese Ceramic Society. Available online at www.sciencedirect.com ScienceDirect Journal of Materiomics 1 (2015) 92e105 www.ceramsoc.com/en/ www.journals.elsevier.com/journal-of-materiomics/ http://dx.doi.org/10.1016/j.jmat.2015.01.001 2352-8478/© 2015 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-NDlicense (http:// creativecommons.org/licenses/by-nc-nd/4.0/).