Fabrication And Characterization Of Doped Ca₃Co₄O₉ Thermoelectric Oxide Ceramic

Recently, people pay much attention to Ca₃Co₄O₉ based thermoelectric oxides due to their thermal stability,anti-oxidation and pollution-free.However,thermoelectric performances of this kind of materials are still very low,which urges people to find new ways to improve the properties of the materials. In this dissertation,doped Na or La and Cu were used to enhance the thermoelectric propertiesof Ca₃Co₄O₉.Thermoelectric powders Ca_3-xR_xCo_4-yCu_yO₉ (R=Na, La, x=0.1, 0.2, 0.3, y=0.0, 0.2) with different doping of Cu, La, and Na were synthesized using sol-gel method, and consolidated into bulk ceramics by spark plasma sintering.The crystalline structures of different doping powders were confirmed using X-ray diffraction (XRD), the morphologys of the powders before and after SPS were examined using a scanning electron microscopy (SEM). Then we investigated thermoelectric properties of bulk ceramics at the temperature range from 373 K to 873 K, including the Seebeck coefficient, the electric conductivity, the thermal conductivity and the ZT values. The detailed contents of this thesis are as following:Sol-gel method was used to synthesize Ca₃Co₄O₉ powders with different doping. In the progress of preparation xerogel, the pH value of precursors should be controlled around 1², then calcination at 1023 K for 4 h in air atmosphere. The grain size of sol-gel synthesized Ca₃Co₄O₉ powders is in the range of 1²μm.The results of thermoelectric properties show that single substitution of Cu²⁺ for Co³⁺ resulted in the increase of electrical conductivity, thermal resistivity and thermoelectric figure of merit ZT, the value of ZT was 0.253 for Ca₃Co_3.8Cu_0.2O₉ at 873 K; Compared with Cu single substitution, La and Cu double substitution further reduced thermal conductivity of Ca₃Co₄O₉, improved the ZT at high temperature, the value of ZT reached 0.28 for Ca_2.9La_0.1Co_3.8Cu_0.2O₉ at 873 K; However, compared with Ca₃Co₄O₉ without doping, Na and Cu co-substitution improved electric conductivity and decreased seebeck coefficient at low temperature, increased thermal conductivity simultaneously, resulting in substantial reduction of ZT, the value of ZT only reach 0.23 at 873 K.In conclusion,selecting the appropriate elements and doping concentration, especially, co-doped in Ca site and Co site is an more efficient way to improve thermoelectric performance of Ca₃Co₄O₉ base ceramic.