Thermoelectricity of Al-doped ZnO at different carrier concentrations
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Tomohiro Aoki Sinto V-Cerax Ltd., 3-1 Honohara, Toyokawa 442-8505, Japan
Koji Watari National Institute of Advanced Industrial Science and Technology (AIST), Advanced Manufacturing Research Institute, Shimoshidami, Moriyama, Nagoya 463-8560, Japan (Received 22 December 2006; accepted 28 March 2007)
Optimization of the carrier concentration is a key to improve the power factor of thermoelectricity. The carrier concentration of sintered zinc oxides was primarily controlled by impurity doping of aluminum and secondarily adjusted by defect concentration by varying the oxygen partial pressure in the range of 101 to 104 Pa. The resultant carrier concentration measured at room temperature ranged from 1 to 1.8 × 1020 cm−3, which drastically modified the thermoelectricity. The Jonker plot of the measured Seebeck coefficient and conductivity revealed deviation of the slope from k/e (where k is the Boltzmann constant and e is the elemental electric charge), which was attributed to a mobility variation with respect to the carrier concentration. The approach to estimating the optimum conductivity taking into account mobility variation is discussed. Finally, the optimum conductivity is estimated to be 1800 to 2000 S/cm for high-temperature operation (500 to 800 °C).
I. INTRODUCTION
Thermoelectricity is considered as one of the electric power generation systems complementary to the present energy-generating system, which is based on the ability of thermoelectricity to reuse the wasted heat discharged from industrial facilities and consumer products such as furnaces and automobiles. Among the thermoelectric materials, oxides are regarded as materials potentially used at high temperatures, i.e., higher than 500 °C. The faculty of thermoelectric materials is evaluated by the figure of merit or power factor. The figure of merit comprises electric conductivity, Seebeck coefficient, and thermal conductivity, which are suitable factors for achieving highly efficient energy conversion, while power factor consists of electric conductivity and Seebeck coefficient, which are proper factors for realizing high-power generators. Because of its high electric conductivity, zinc oxide is a candidate for an n-type thermoelectric component of oxide-based power generators. The potential of zinc ox-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0244 1942 J. Mater. Res., Vol. 22, No. 7, Jul 2007 http://journals.cambridge.org Downloaded: 28 Aug 2015
ide as a thermoelectric generator was first pointed out by Ohtaki in 1996.1 Its power factor exceeds 1 × 10−3 W/m·K2, which is still the highest power factor among n-type bulk oxides prepared by sintering. However, its thermal conductivity is relatively high (10 to 40 W/m·K),2 resulting in low conversion efficiency (ZT ⳱ 0.2 at 800 °C, where Z is the figure of merit and T the absolute temperature). Since thermoelectricity is a phenomenon of carrier transport under temperature gradient, carrier concentration primarily governs the thermoelectric prop
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