Synthesis, structure, and properties of Ca3Co3.85M0.15O9+δ (M = Ti-Zn, Mo, W, Pb, Bi) layered thermoelectrics

We have prepared Ca3Co3.85M0.15O9 + delta (M = Ti-Zn, Mo, W, Pb, Bi) solid solutions, investigated their crystal structure and microstructure, assessed their thermal stability in air, and measured their thermal expansion, electrical conductivity, and thermoelectric power in air at temperatures from 300 to 1100 K. The results demonstrate that the Ca3Co3.85M0.15O9 + delta cobaltites are p-type semiconductors and that their unitcell parameters decrease with an increase in the number of d electrons in the 3d transition metal ion and with an increase in the average oxidation state of the cobalt. Their thermoelectric power increases with increasing temperature, reaching the highest value in the Ca3Co3.85Pb0.15O9 + delta solid solution: 380 mu V/K at a temperature of 1100 K. The Ca3Co3.85Bi0.15O9 + delta solid solution has the largest thermoelectric power factor among the materials studied, 206 mu W/(m K-2) at a temperature of 1100 K, which is twice the power factor of the unsubstituted calcium cobaltite Ca3Co4O9 + delta.