Stability and Thermoelectric Property of Cu 9 Fe 9 S 16 : Sulfide Mineral as a Promising Thermoelectric Material
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Stability and Thermoelectric Property of Cu9Fe9S16: Sulfide Mineral as a Promising Thermoelectric Material Naohito Tsujii1 and Takao Mori2 1 National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Ibaraki, Japan, 2 International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Ibaraki, Japan. ABSTRACT Thermoelectric materials based on non-toxic and earth-abundant elements are important from the viewpoint of energy harvesting from the widely-spread waste heat. We have investigated the thermoelectric properties of Cu9Fe9S16, known as the natural mineral mooihoekite or talnakhite. Seebeck coefficient shows a large negative value of about -140 μV/K around room temperature. Thermal conductivity is found to be as small as 2.0 W/Km above 100 K, which is attributed to the large unit cell and the complicated crystal structure. Our results indicate that Cu9Fe9S16 can make a high-performance and environmentally-friendly thermoelectric material if the concentration of carriers, especially holes, is reduced. INTRODUCTION Thermoelectric conversion has a great advantage to reduce the consumption of energy resources, since it can directly convert waste heat to electricity [1]. The key parameter is the conversion efficiency η of thermoelectric materials. The rate η is an increasing function of the dimensionless figure of merit, ZT = S2T/ρκ, where S, T, ρ, κ are the Seebeck coefficient, temperature, electrical resistivity, and thermal conductivity, respectively. So far, great effort has been paid to increase the value ZT in the last few decades. As a result, several compounds have been discovered with ZT ~ 1 or higher. However, the challenge is that thermoelectric materials must meet the environmental friendliness and economical requirements in addition to achieving high efficiency. Therefore, designing thermoelectric device from non-toxic and earth-abundant materials will also be a key strategy. Recently, we have found that chalcopyrite CuFeS2, can turn into good thermoelectric materials by carrier doping [2,3]. The electron doped sample Zn0.03Cu0.97FeS2 and Cu0.97Fe1.03S2 show large negative Seebeck coefficient and relatively low electrical resistivity, resulting in a high power factor of 1 mW/K2m around room temperature. Since the chalcopyrite-based materials are made up of non-toxic and earth-abundant elements, much more works are desired to improve the thermoelectric performance. The problem of this chalcopyrite-based system is its high thermal conductivity, κ = 5 W/Km around room temperature. To overcome this problem, we have focused on Cu9Fe9S16. This compound exists as natural minerals with two polymorphs, mooihoekite [4] and talnakhite [5]. For both of the cases, the crystal unit cell of Cu9Fe9S16 is larger than that of CuFeS2, involving 10 to 11 inequivalent crystallographic sites. The complex crystal structure is expected to have a great effect in reducing lattice thermal conductivity [6]. We have synthesized Cu9Fe9S16 samples and meas
