Study On The Synthesis And Thermoelectric Properties Of Ag Doped And Ag, Cu Doped Ca₃Co₄O₉

The Ca3Co4O9 samples were synthesized by solid state reaction and citrate acid sol-gel method. The microstructure and thermoelectric properties were compared for the samples prepared by the two methods. The (Ca1-xAgx)3Co4O9(x=0～0.15) samples were prepared by citrate acid sol-gel method. X-ray diffraction and scanning electron microscope were applied to analyze the phase distributions and microstructure. The electrical resistivity, Seebeck coefficient and thermal conductivity were measured to compare thermoelectric properties of samples with different amount of doped Ag. We also studied the substitution for Ca-site with Ag and Co-site with Cu in Ca3Co4O9 prepared by sol-gel method. The effects of doping on the thermoelectric properties and transport mechanism were analysed systematically. The results indicate:The well-crystallized Ca3Co4O9 samples were successfully synthesized by solid state reaction and citrate acid sol-gel method. Compared with solid state reaction method, sol-gel method could prepare Ca3Co4O9 sample which has higher diffraction peak,finer grain size,evenly distributed grains and the material density is enhanced. The results show that the electrical resistivity of the sample synthesized by sol-gel method decreases and the Seebeck coefficient increases slightly, so the thermoelectric properties could be improved. The power factor of the sample prepared by sol-gel method is 108μW·m-1·K-2 at 873 K, which is 19% larger than that of the sample synthesized by solid state reaction method.The XRD patterns of the (Ca1-xAgx)3Co4O9(x=0～0.15) samples prepared by citrate acid sol-gel method are in accordance with the standard card of Ca3Co4O9 except for the x=0.15 sample in which Ag phase exists; the electrical resistivity decreases while the Seebeck coefficient increases obviously with doping Ag, and the thermal conductivity is found to decrease apparently; x=0.1 is the best substitution of Ag, the merit ZT of the (Ca0.gAg0.1)3CoO9 reaches 0.13 at 673 K, which is 5.8 times as that of the undoped sample. Analysises to the experimental results demonstrate that doping Ag could improve the thermoelectric performance of Ca3Co4O9 remarkably, while the doping amount, the position which Ag occupies and the existence form of doping element all have unnegligible influences on the thermoelectric properties of materials.The Ca3Co4O9 samples with partial Ag and Cu substitution for Ca and Co atoms were synthesized by sol-gel method. We studied the thermoelectric properties of the samples with different Cu doping amount while keeping the same Ag substitution(0.1). The results show that Ag and Cu substitution for Ca and Co simutaneously results in better thermoelectric properties of the material comparing to the material with only Ag doping. The electrical resistivity decreases initially, then increases with the increasing doping. The sample of x=0.05 has the lowest electrical resistivity. The electrical resistivity of the sample with x=0.05 reaches 8.04 mΩ·cm at room temperature, which is 36.2% lower than that of the sample with Ag doping and almostly half of that of non-doped Ca3Co4O9. The Seebeck coefficient is reduced slowly by Ag and Cu doping and the thermal conductivity also decreases. The thermoelectric properties are improved as a whole. The merit ZT is 0.18 for the x=0.05 sample at 673 K, which is 38.4% higher than the doping Ag sample. The electrical conductivity behaviors of all the samples fit the small polaron hopping conduction mechanism above 650 K. The results indicate that doping Cu doesn't change the conduction mechanism and Cu may substitute for Co in the Ca2CoO3 layer.