Highly textured Na/sub x/CoO/sub 2-δ/ ceramics fabricated by tape casting

Highly textured NaCoO/sub 2-/spl delta// (NCO) ceramics with improved thermoelectric properties have been prepared by tape casting. NCO platelets with 5 - 10 /spl mu/m in diameter and 5 in aspect ratio were synthesized by a co-precipitation method and used as starting materials. The NCO platelets were well mixed with Na/sub 2/CO/sub 3/ particles and binder, tape-cast by a doctor-blade technique, dewaxed at 1073 K and then sintered with a uniaxial pressure in air. In particular, we investigated the effects of preparation conditions on thermoelectric properties; that is, 1) the mixing ratio of NCO and Na/sub 2/CO/sub 3/ particle, 2) composition of NCO platelets, and 3) the sintering temperature. As a result, highly c-axis aligned specimens were obtained; the maximum c-axis orientation degree (f) was estimated as 0.942 by a Lotgering's method. Also, the NCO specimen with the maximum Na content exhibited the highest power factor (1.44 mWm/sup -1/K/sup -2/ at 1050 K) in the direction of the preferred {001} plane.