Utilizing the phonon glass electron crystal concept to improve the thermoelectric properties of combined Yb-stuffed and Te-substituted CoSb 3 Jibran Khaliq, a Qinghui Jiang, a Junyou Yang, b Kevin Simpson, c Haixue Yan a,d and Michael J. Reece a,d,⇑ a School of Engineering and Material Science, Queen Mary University of London, London E1 4NS, UK b State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China c European Thermodynamics Limited, Leicester LE8 0RX, UK d Nanoforce Technology Limited, London E1 4NS, UK Received 19 September 2013; revised 23 October 2013; accepted 26 October 2013 Available online 1 November 2013 A cost-effective and rapid mechanical alloying method was used to prepare Yb-stuffed and Te-substituted CoSb 3 to study the thermoelectric properties of this material. Yb rattles inside the cage-like structure of CoSb 3 which effectively reduces the phonon mean free path and results in a lattice thermal conductivity comparable to those of costly nanostructured CoSb 3 materials. A very low lattice thermal conductivity of 1.17 W m 1 K 1 at 550 K was obtained. A zT value of 0.7 was reported at 600 K due to the very low lattice thermal conductivity. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: CoSb 3 ; Thermoelectric material; Mechanical alloying; Rare earth stuffing; Thermal conductivity Thermoelectric materials are of interest because of their ability to directly convert waste heat into elec- tricity by the Seebeck effect, or electricity into cooling by the Peltier effect. They do not use any moving parts, which makes them highly reliable. Their low efficiency limits their practical use on a large scale, but on a small scale they are comparable with existing batteries or cool- ing devices [1,2]. The efficiency of thermoelectric materi- als is represented by a dimensionless figure of merit zT = T(S 2 r)/j, where S is the Seebeck coefficient, r is the electrical conductivity and j is the thermal conduc- tivity. Higher values of the Seebeck coefficient and the electrical conductivity and lower values of the thermal conductivity give higher values of zT. Increasing zT is difficult due to the interdependency of all the relevant properties. Increasing electrical conductivity has an ad- verse effect on the Seebeck coefficient and the thermal conductivity. Bismuth telluride (Bi 2 Te 3 ) is a widely used thermo- electric material in the near-room-temperature range be- cause of its high zT value at these temperatures [3]. For higher-temperature applications, new materials must be found with zT values optimized at high temperatures. Skutterudites have been the focus of research for inter- mediate-temperature applications (298–850 K) [4,5], such as cobalt antimonide (CoSb 3 ) [6]. It has a melting point of 873 °C [7] with a high carrier concentration and Seebeck coefficient (60 lVK 1 ) [8]. However, due to strong covalent bonding, its intrinsic thermal conductivity is 10 W m 1 K 1 , which is a high value for thermoelectric applications [9]. This high thermal conductivity limits the zT of CoSb 3 -based materials. Nanostructuring is one way to reduce the thermal con- ductivity of CoSb 3 , but this also decreases its electrical conductivity by creating more scattering centres for elec- trons [10]. Moreover, making nanostructured materials is not a cost-effective processing route. Slack suggested the idea of phonon glass electron crystals (PGECs), in which electrical properties are separated from thermal properties [11]. In PGECs, optimization of the electrical and thermal properties can be done at the same time. Substitution of Te for Sb is effective way of improving 1359-6462/$ - see front matter Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2013.10.021 ⇑ Corresponding author at: School of Engineering and Material Science, Queen Mary University of London, London E1 4NS, UK. Tel.: +44 20 7882 8872; e-mail: m.j.reece@qmul.ac.uk Available online at www.sciencedirect.com ScienceDirect Scripta Materialia 72–73 (2014) 63–66 www.elsevier.com/locate/scriptamat