Reduction of Contact Resistance between Na x CoO 2 Thermoelectric Chip and Ag Electrode by Using Spark Plasma Sintering
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Reduction of Contact Resistance between NaxCoO2 Thermoelectric Chip and Ag Electrode by Using Spark Plasma Sintering Method. Haruna Hachiya1, Koya Arai1, Tomoyuki Nakamura2, Masahiro Minowa2, Yuki Yamaguchi3, Kenjiro Fujimoto3 and Keishi Nishio1 1
Department of Materials Science and Technology, Tokyo University of Science, 3-6-1 Niijuku, Katsushika-ku, Tokyo 125-8585, Japan 2 SWCC Showa Cable Systems Co., Ltd., LTD, 4-1-1 Minami-Hashimoto Chuo-Ku Sagamihara, Kanagawa, Japan 3 Department of Pure and Applied Chemistry, Tokyo University of Science, 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan
ABSTRACT NaxCoO2 has a particularly high contact resistance because it forms an insulated layer of NaHCO3 and Na2CO3, which are produced in a chemical reaction with carbon dioxide and water in air on the surface. In this study, we tried to improve the interface resistance between Na xCoO2 and Ag sheet electrodes by connecting these materials with the spark plasma sintering (SPS) technique. The interface resistance between NaxCoO2 and Ag sheet electrodes connected by SPS is compared with that connected with Ag paste. In an experiment, the interface resistance of a sample treated by decrease to less than 1/600 of the former value. It is thought that the NaHCO3 and Na2CO3 insulated layer is decomposed through the application of a large value of applied DC current by using the SPS technique.
INTRODUCTION It was reported that the thermoelectric oxide material NaxCoO2 had high thermoelectric power at high temperatures by Terasaki et al. [1]. NaxCoO2 has an intercalation compound and a layered crystal structure with sodium ions in the Co oxide between layers. The inter-lamellar sodium ions suffer losses at random [2-4]. However, these sodium ions react to carbon dioxide and water in air and form insulated layer; NaHCO3 and Na2CO3 on the surface of NaxCoO2. Na2CO3 need 1073K for decomposition and form immediately [5]. The contact resistance increases due to NaHCO3 and Na2CO3 between the NaxCoO2 surface and Ag electrode. The spark plasma sintering method has been suggested as a way to solve this problem. Aluminum powder is usually difficult to sinter due to an oxide layer, alumina, with a high melting point existing on the surface of aluminum particles. However, the spark plasma sintering (SPS) method can break down this oxide layer and sinter aluminum powder [6]. SPS method that generates joule heat between ceramic powders by the pulsed DC current can advance sintering lower temperature and shorter time than hot press method. Additionally, the SPS method facilitates adhesion between an element and an electrode to enable sintering by adding additional uni-axis pressure. In this study, we tried to break down the insulated layer between Ag electrode and NaxCoO2 by SPS
method and reduce the contact resistance. Moreover, we tried to establish an easier way to combine Ag electrodes and NaxCoO2.
EXPERIMENT The NaxCoO2 polycrystalline powders were synthesized by thermal decomposition in a metal-citric acid compound [7]. First, citric acid
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