Thermoelectric properties of Ca3-x Bi (x) Co4O9+delta (0.0 a (c) 1/2 x a (c) 1/2 1.5) ceramics

摘要

We have studied the thermal, electrical, and thermoelectric properties of Ca3-x Bi (x) Co4O9 + delta (0.0 a parts per thousand currency sign x a parts per thousand currency sign 1.5) ceramics prepared by solid-state reactions. Phase-pure Ca3-x Bi (x) Co4O9 + delta solid solutions were obtained at x a parts per thousand currency sign 0.3. The samples with 0.3 < x < 1.5 consisted of three phases: Ca2.7Bi0.3Co4O9 + delta, Bi2Ca2Co1.7O (y) , and DD3/4(3)D(4). The sample with the composition Ca1.5Bi1.5Co4O9 + delta consisted of two phases: Bi2Ca2Co1.7O (y) and Co3O4. The materials are p-type semiconductors with linear thermal expansion coefficients in the range (10.6-12.8) x 10(-6) De(-1). Their electrical and thermal conductivities are nonmonotonic functions of x, and their thermoelectric power increases with increasing x. The mixed-phase ceramics have been shown to have better thermoelectric properties: the highest thermoelectric power factor, a parts per thousand 0.24 mW/(m K-2) at T = 1100 K, is offered by the ceramics with x = 0.9-1.0, which contain nearly equimolar amounts of Ca2.7Bi0.3Co4O9 + delta and Bi2Ca2Co1.7O (y) . This power factor is 2.5 times that of the layered calcium cobaltite Ca3Co4O9 + delta and solid solutions based on it: 0.094-0.098 mW/(m K-2) at the same temperature.