Thermoelectric properties of layered Co-based oxides

Bi/sub 2/Sr/sub 2/Co/sub 2/O/sub x/ (BC-2202) polycrystalline materials with a layered structure have been prepared using a partial melting method. In this phase, insulating Bi-O layers and conducting Co-O layers are piled up alternately. The compositions of samples are Bi/sub 2/Sr/sub 2/Co/sub 2/O/sub x/ (stoichiometric sample), Bi/sub 1.8/Sr/sub 2/Co/sub 2/O/sub x/ (Bi-less sample), Bi/sub 2/S/sub 1.8/PCo/sub 2/O/sub x/ (Sr-less sample), and Bi/sub 2/Sr/sub 2/Co/sub 1.8/O/sub x/ (Co-less sample). All samples are p-type conductors. Seebeck coefficient (S) depends on cationic composition. The highest S is 150 /spl mu/VK/sup -1/ for the Sr-less sample at 700/spl deg/C. Electric resistivity (p) for the samples is 9.8-22 m/spl Omega/ cm at 700/spl deg/C. Thermal conductivity (/spl kappa/) at 700/spl deg/C is 1.3 and 0.93 Wm/sup -1/ K/sup -1/ for the stoichiometric and Sr-less samples, respectively. These values are lower than those for other thermoelectric oxides. Figure of merit (Z) for the Sr-less sample is 2.0/spl times/10/sup -4/ K/sup -1/ and ZT value, in consequent, is 0.19 at 700/spl deg/C. It is clear that the layered structure is effective to increase thermoelectric performance.