Highly textured Na_xCoO_2-d ceramicsfabricated by the templated grain growth method

H. Itahara¹, K. Fujita²,J. Sugiyama¹, T. Tsukada², T. Tani¹

¹ ToyotaCentral Research and Development Labs. Inc., 41-1, Nagakute, Aichi 480-1192,Japan

² FrontierResearch Institute, Tokyo Gas Co., Ltd., 1-7-7, Suehiro-Cho, Tsurumi-ku,Yokohama 230-0045, Japan

Textured Na_xCoO_2-d (NCO)ceramics with high thermoelectric properties have been prepared using atemplated grain growth (TGG) technique. At first, NCO platelets with 5 - 10 mmin diameter and ~ 5 in aspect ratio were synthesized as templates by aco-precipitation method. Then, the NCO platelets were well mixed with Na₂CO₃particles and binder, tape-cast by a doctor-blade technique, dewaxed at673 K and then fired in a hot press (HP) furnace in air. In particular, weinvestigated the effects of preparation conditions on thermoelectricproperties; that is, 1) the mixing ratio of NCO and Na₂CO₃particles and 2) the temperature of the HP furnace. As a result, highly c-axis alignedsamples were obtained; the maximum c-axis orientation degree(f)was estimated around 94% by a Lotgering’s method. Also, the NCO samplewith the maximum f exhibited the highest thermoelectric powerfactor in the a-b plane, i.e., 1.44 mWm^-1K^-2at 1050 K.