Improvement in thermoelectric properties of (ZnO) 5 In 2 O 3 through partial substitution of yttrium for indium
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Improvement in thermoelectric properties of (ZnO)5 In2 O3 through partial substitution of yttrium for indium Masaki Kazeoka, Hidenori Hiramatsu, Won-Seon Seo, and Kunihito Koumoto Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan (Received 7 April 1997; accepted 17 June 1997)
We first measured the thermoelectric properties of layer-structured homologous compounds, (ZnO)m In2 O3 (m integer), and reported that they would become candidate materials for high-temperature thermoelectric energy conversion.1–4 We further tried to improve their thermoelectric properties by partially substituting yttrium for indium in (ZnO)5 In2 O3 . Though the ionic radius of Y 31 is larger than that of In31 , the a-axis (hexagonal system) elongated and c-axis shrank as Y was substituted for In. The thermoelectric properties were found to vary with a varying amount of Y substitution; 3% Y substitution gave rise to the largest thermoelectric figure of merit, i.e., 1.121.3 3 1024 K21 at 960 – 1100 K. The abnormal change in the lattice structure by Y substitution was responsible for the unusual behavior of the thermoelectric properties.
Thermoelectric semiconductors with high thermoelectric conversion efficiencies, such as PbTe, Bi2 Te3 , etc., are easily oxidized, and decompose or melt at high temperatures in air (melting points of PbTe and Bi2 Te3 are 1190 K5 and 858 K,6,7 respectively). SiGe alloys are comparatively stable to quite high temperatures and are currently used above 1000 K temperature range, though the environment for their use is usually limited to the vacuum or an inert atmosphere.8 Doped FeSi2 is another material which is oxidation resistant even at 1000 K in air, but its thermoelectric properties are still unsatisfactory9 and it decomposes above about 1260 K. Refractory ceramic semiconductors, on the other hand, can be applied to high-temperature thermoelectric energy conversion because of their high thermal stability and oxidation resistance. Recently, metallic oxides having rather high electrical conductivity, such as sZn12x Alx dO,10,11 In2 O3 –SnO2 ,12 sCa12x Bix dMnO3 ,13 CdIn2 O4 ,14 and Nd22x Cex CuO4 15 have been investigated for possible thermoelectric applications. Generally, thermoelectric materials are evaluated by their thermoelectric figure of merit, Z (sa 2yk; s: electrical conductivity, a: Seebeck coefficient, k: thermal conductivity). The values of Z of thermoelectric semiconductors developed so far, such as Bi2 Te3 ,16 PbTe,17 SiGe alloys,8 and doped FeSi2 ,9 are of the order of 1024 – 1023 K21 , while those of refractory oxides are usually far less than 1024 K21 . In order to utilize their superior hightemperature performance, the thermoelectric properties of refractory oxides ought to be greatly improved. Homologous compounds, (ZnO)m In2 O3 (m integer), have unique crystal structures with space groups of J. Mater. Res., Vol. 13, No. 3, Mar 1998
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