Optimization Of Thermoelectric Properties Of Ca₃Co₄O₉ Composite

Misfit layered cobaltites have attracted more and more attention because of their high temperature-resistance, anti-oxidation, non-toxic, non-pollution, low cost and simple synthesis process. Ca3Co4O9 is a typical cobaltite, should be sintered below its decomposition temperature 926℃, which is not convenient as the high pressure is always nessary. Ca3Co4O9-based composite materials were prepared by liquid-phase reaction sintering method in this paper, and the thermoelectric performance of Ca3Co4O9 was optimized by ion doping and improving of composite process.Ca3Co4O9 powder, as well as Ce-doped, Ag-doped and Cu-doped Ca3Co4O9 powder were synthesized by solvent method and bulk samples were prepared by ordinary sintering method in this paper. Micro-structure and thermoelectric properties of as-prepared samples were characterized and the doping content was optimized. The results show that thermoelectric properties of materials can be improved by doping Ce, Ag and Cu, and the best is by doping Ag. The power factor of Ca3.8Ag0.2Co4O9 sample reached 2.45×10-4 W/(m·K2) at 973K.In oder to enhance the relative density of samples sintered with conventional methods, Ca3Co4O9-based composite materials were prepared by liquid-phase reaction sintering method with Bi2O3 as sintering agent and with Ca3.9Ce0.1Co4O9, Ca3.8Ag0.2Co4O9 and Ca3Co3.9Cu0.1O9 as the starting power, with the increase of Bi2O3 addicontention, thermoelectric properties of Ca3Co4O9-based composite materials were improved, its density and compactness increased, resistivity and thermal conductivity reduced significantly. The minimum of resistivity is 9.34mΩ·cm, and the maximum of power factor is 3.62×10-4 W/(m·K2) at 973K.Ca3Co4O9/Bi2Ca2Co2Oy composite is also directly prepared with mixed powers of synthized by solvent method and Bi2Ca2Co2Oy synthized by solid reaction method The resistivity of as-prepared samples decreases to 9.22 mΩ·cm, and the power factor increases to 4.41×10-4 W/(m·K2), which is close to that of hot-press sintered samples and of SPS sintered ones, and that of composite materials prepared by electric field induced floating zone method. It is showed that it was an effective method to combine doping with composite for improving the thermoelectric performance of Ca3Co4O9.