Jpn. J. Appl. Phys. 46 (2007) pp. 5226-5229  |Previous Article| |Next Article|  |Table of Contents|
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Structure and High Temperature Thermoelectric Properties of Delafossite-Type Oxide CuFe_1-xNi_xO₂ (0 ≤x ≤0.05)

Kei Hayashi^1,2, Tomohiro Nozaki^1,2, and Tsuyoshi Kajitani^1,2

(Received March 7, 2007; accepted May 18, 2007; published online August 6, 2007)

We have investigated crystal structure of delafossite-type oxide CuFe_1-xNi_xO₂ (0 ≤x ≤0.05) and measured its thermoelectric properties at high temperatures ranging from 300 and 1100 K. The lattice parameter a of the Ni²⁺-doped samples is nearly equal to that of CuFeO₂, while the lattice parameter c of the Ni²⁺-doped samples increases. Nearly constant a-axis is due to the decrease of (Fe/Ni)–O distance and simultaneous increase of O–(Fe/Ni)–O angle. Increase of the c-axis is due to the increase of Cu–O distance in the Ni²⁺-doped samples. The valence states of the Fe- and Cu-sites are calculated from bond valence summation. The valence state of the Fe-site in the Ni²⁺-doped samples is larger than that of CuFeO₂, an indication of hole doping in the Fe-site. This increase of hole carriers enhances the electrical conductivity σ. The highest electrical conductivity is 18 S/cm. Although the Seebeck coefficient S decreased by Ni²⁺ doping, the S is still high value (S>250 µV/K). The thermal conductivity κ of CuFe_1-xNi_xO₂ is relatively high (κ>4 W/mK). The maximum dimensionless figure of merit ZT=σS²T/κ=0.14 is obtained with the sample of x=0.01 at 1100 K, being higher than that of the polycrystalline γ-Na_0.7CoO₂. There is no significant evaporation of the constituent elements after the heat cycles.

URL: http://jjap.jsap.jp/link?JJAP/46/5226/
DOI: 10.1143/JJAP.46.5226

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