TEXTURED STRUCTURE AND ANISOTROPIC THERMOELECTRIC PROPERTIES OF Ca_(2,7)Bi_(0.3)Co_4O_9 OXIDE PREPARED BY CONVENTIONAL SOLID-STATE REACTION

摘要

Ca_(3-x)Bi_xCo_4O_9 (x = 0.0,0.3) samples have been prepared at 1223 K by conventional solid-state reaction method. XRD and SEM investigations reveal that c-axis-oriented structure could be formed in Ca_(2,7)Bi_(0,3)Co_4O_9 samples, whereas grains in Ca_3Co_4O_9 samples distribute randomly. Moreover, Bi doping increases the grain size and relative density of Ca_(2.7)Bi_(0.3)Co_4O_9. The electrical conductivity along the ab plane for Ca_(2.7)Bi_(0.3)Co_4O_9 is about four times as large as that along the c-axis, but the Seebeck coefficient is almost isotropic, which leads to a remarkable rise of the power factor in ab plane for Ca_(2.7)Bi_(0.3_Co_4O_9 compared with untextured Ca_3Co_4O_9. The textured structure in Ca_(2,7)Bi_(0.3)Co_4O_9 sample should be attributed to the effect of Bi doping.