Microstructrue And Thermoelectric Properties Of Oxide Ceramics With Low Thermal Conductivity

With the rapid development of the society and aggravation of urban population, theproblem of the energy poverty and environmental pollution attract more and moreattentions nowadays. Thermoelectric conversion is a promising technology of friendlyenvironment for both electrical power generation in harvesting waste heat and electroniccooling. Among all the thermoelectric materials, metal oxides have received increasingattention because of their lower cost, high thermal and chemical stability, andenvironmental compatibility. However, the thermoelectric property of these materials islower compared to the traditional materials because of the lower electrical conductivityand higher thermal conductivity. Therefore, more efforts are required to further enhanceits thermoelectric performances and also to find such new oxide materials with lowerthermal conductivity and high thermoelectric property. In view of the importance of thecrystal structural and microstructure of polycrystalline materials on the thermoelectricproperty, this dissert is focused on one kind of cobalt oxides LaCoO3with perovskitecrystal structure and relatively lower thermal conductivity, to improve its thermoelectricproperty with an emphasis on the influence of the microstructure. Meanwhile, newoxide thermoelectric materials with lower thermal conductivity are explored combinedwith crystal structure.The effect of the Ni and Cu substitution on the microstructure and thermoelectricproperty of the LaCoO3were investigated in the first part of this dissert. It is found thatNi doping is effective to suppress the grain growth of LaCoO3ceramics which havereduced the thermal conductivity around room temperature. The electrical conductivityhas been enhanced due to the increased carrier concentration. Therefore the ZT valuehas improved which is nearly3.5times higher than that of the undoped one.Nevertheless, nano second phase was aggregated around the grain boundaries after Cudoped of LaCoO3ceramics and the grain growth was also suppressed which caused aneffective reduction of the thermal conductivity in the whole range of measuredtemperature. The thermal conductivity is only1.21W/mK around room temperature forthe sample with25%doped content. Combining the increased electrical conductivity,the ZT value has been enhanced to0.043for the sample with15%Cu doped content at 323K, which higher approximately by40%than the peak ZT value of the Ni-dopedone.According to the different transport mechanism of the carrier and phonon in thematerial, a composite structure was introduced into LaCoO3ceramics under themotivation to decouple the electrical and thermal transport properties. It shows that thethermoelectric property of the matrix materials can be effectively improved viaintroducing the composite microstructure design. The electrical conductivity of theresultant sintered materials was enhanced and an apparent increase in thermalconductivity and decrease in Seebeck coefficient were suppressed. The ZT value ofnearly0.06was obtained at323K, which is about twice higher than that withoutcoating and30%than that of the Cu doped one.New oxide materials of layered BiCuSeO and defect pyrochlore AgSbO3aspotential thermoelectric materials with lower thermal conductivity and high property areexplored. It was revealed that both of the two oxide materials have lower thermalconductivity. However, the BiCuSeO shows much more excellent thermoelectricproperty. For an oxide thermoelectric material, the thermal conductivity of BiCuSeO isvery low originating from the weak bonding between layers; in addition, the uniquenatural supperlattice structure characteristic make the phonon scatter within the layers.It is only1.1~0.5W/mK in the range of300K to800K. The electrical conductivityincreased after inducing the high energy ball milling. The ZT value of BiCuSeO withoutdoping is nearly0.7at773K which has improved to0.8after Ca doping because of theenhanced carrier concentration. The ZT value around room temperature also improvedto0.3after Ca substitution duo to the greatly increased electrical conductivity andreduced lattice thermal conductivity.