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Enhancing thermoelectric properties of Cu1.81xSe compounds Liang Zou, Bo-Ping Zhang,a) Zhen-Hua Ge, and Li-Juan Zhang Beijing Key Lab of New Energy Materials and Technology, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China (Received 8 January 2014; accepted 20 March 2014)

P-type Cu1.81xSe (x 5 0, 0.16, 0.20) compounds were synthesized by mechanical alloying and spark plasma sintering technique. A 100% enhancement of the Seebeck coefficient was achieved in the whole temperature interval for x 5 0.16 and x 5 0.20 bulks compared with that of the x 5 0 bulk. The thermal conductivity was all below 1.6 W m
1 K
1 in the whole temperature interval for x 5 0.16 and x 5 0.20 bulks, showing a pronounced reduction compared with that of the x 5 0 bulk. The lowest thermal conductivity 0.69 W m
1 K
1 was achieved in the x 5 0.16 sample at 773 K, whereby a maximum ZT value of 1.23 was obtained, revealing that optimizing Cu content in Cu1.81xSe system is an effective method to improve the thermoelectric (TE) merit and indicating a great potential for TE application along with their nontoxicity and low cost.

I. INTRODUCTION

Thermoelectric (TE) materials are capable of driving a direct conversion between heat and electricity, showing a huge advantage of improving the efficiency of energy. The efficiency of TE devices depends on material properties through the dimensionless figure of merit ZT, where T is the working temperature. Z is defined as ra2/j, where r is the electrical conductivity expressed by r 5 nel in which n is the carrier concentration and l is the carrier mobility, a is the Seebeck coefficient expressed by a  c-ln n in which c is the scattering factor, j 5 jl 1 je is the thermal conductivity composed of lattice and electronic parts. je increases with r, which can be expressed by the Wiedemann–Franz law, je 5 LrT 5 LnelT, where L is the Lorenz constant. Recent leading TE materials such as PbTe,1 BiSbTe,2 skutterudites,3 AgPbSbTe (LAST)4 exhibit outstanding TE properties (ZT . 1). However, they are confronted with the common problem of high costs and toxic raw materials, especially Pb and Te. Cu1.8Se as a potential TE material with abundant source has been studied earlier because of its interesting properties and potential application in solar cells, window material, optical filter, electron-optical devices, and TE converter.5 Its crystal structure is cubic with a high symmetry. Based on the rigid fcc sublattice of Se, the Cu ions are concentrated at the tetrahedral 8(c) (1/4, 1/4, 1/4) sites at room temperature and gradually become mobile by raising temperature, showing a superionic behavior.6 According to Danilkin’s report,7 low-energy phonons could be strongly scattered by mobile Cu ions, which contribute to reducing jl. However, Cu1.8Se still a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.90 J. Mater. Res., Vol. 29, No. 9, May 14, 2014

possesses a high j 5 5.7 W m
1 K
1 (Ref. 8) and a rather low a 5 8 lV K
1