Thermoelectric Performance Optimization Of Ca₃Co_3.9Cu_0.1O₉/Bi₂Ca₂Co_1.9Cu_0.1O_y Composite

Co-based oxides thermoelectric have been got much attention for its chemical stability and high figure of merit at 600 K-1000 K.Bi₂Ca₂Co₂O_y and Ca₃Co₄O₉ are misfit-layered structures,with conductive layer and insulating layer stacking alternately in the direction of c-axis.Due to the high electrical conductivity of Ca₃Co₄O₉ and the high Seebeck coefficient of Bi₂Ca₂Co₂O_y,Ca₃Co₄O₉/Bi₂Ca₂Co₂O_y composites have better thermoelectric performance.The Ca₃Co_3.9Cu_0.1O₉/Bi₂Ca₂Co_1.9Cu_0.1O_y composites,which have been doped with Cu,make a further improvement of thermoelectric performance.In this thesis,we aimed to further reduce the electrical resistivity of Ca₃Co_3.9Cu_0.1O₉/Bi₂Ca₂Co_1.9Cu_0.1O_y composite by doping and recombination,in order to enhance the thermoelectric performances of composite.Firstly,the effects of La doping on thermoelectric properties of Ca₃Co_3.9Cu_0.1O₉/Bi₂Ca₂Co_1.9Cu_0.1O_y composite have been studied.Ca_3-xLa_xCo_3.9Cu_0.1O₉synthesized in molten salt and Bi₂Ca₂Co_1.9Cu_0.1O_y synthesized by solid reaction method,were made into composites by hot press sintering.The results show that La occupation of Ca-site induces a lower electrical resistivity and a higher Seebeck coefficient,and improve the thermoelectric performances of the composite.At 973 K,the maximum power factor of Ca_2.92La_0.08Co_3.9Cu_0.1O₉/Bi₂Ca₂Co_1.9Cu_0.1O_y composite is about 4.48×10^-44 W?m^-1?K^-2.Secondly,the effects of Ag combination on thermoelectric properties of Ca₃Co_3.9Cu_0.1O₉/Bi₂Ca₂Co_1.9Cu_0.1O_y composite have been studied,too.The mixed powder of Ca_2.92La_0.08Co_3.9Cu_0.1O₉ and Bi₂Ca₂Co_1.9Cu_0.1O_y was added into AgNO₃ aqueous solution,then AgNO₃ decomposed by calcining,and the composites were prepared by hot press sintering.It turned out that Ag elemental locates at the particle boundary of Ca₃Co_3.9Cu_0.1O₉/Bi₂Ca₂Co_1.9Cu_0.1O_y,and the resistivity of composite is decreased after Ag combination.The lowest resistivity is 7.76×10^-5??m at 973 K when 9wt.%Ag is added,about 15%reducing to that of Ca₃Co_3.9Cu_0.1O₉/Bi₂Ca₂Co_1.9Cu_0.1O_y composite.What's more,both Ca₃Co_3.9Cu_0.1O₉ and Bi₂Ca₂Co_1.9Cu_0.1O_y are lamellar particles.Lamellar particles are easy to grow preferential orientation when subjected to uniaxial pressure,and hence leading to these materials' anisotropy of the thermoelectric properties.Therefore,the anisotropic properties of Ca₃Co_3.9Cu_0.1O₉/Bi₂Ca₂Co_1.9Cu_0.1O_y composite have been investigated.It's shown that Ca₃Co_3.9Cu_0.1O₉/Bi₂Ca₂Co_1.9Cu_0.1O_y composite has strong anisotropic property.The power factor,PF,and figure-of-merit,ZT,were hence anisotropic,exhibiting respective ratios PF_?/PF_//?5.7 and ZT_?/ZT_//?3.0.Finally,we also investigated the effects of preparation process on thermoelectric propertiesofCa₃Co_3.9Cu_0.1O₉/Bi₂Ca₂Co_1.9Cu_0.1O_ycomposite.The Ca₃Co_3.9Cu_0.1O₉/Bi₂Ca₂Co_1.9Cu_0.1O_y composite was prepared by multi-sheet hot pressed technology,in which a thin slices green compact was firstly prepared.Consequently,the thin slices were hot pressed into a sample.The results show that the sample prepared by multi-sheet hot pressed method with lower resistivity about 6%,higher than that prepared by traditional method,but a little change of Seebeck coefficient,so thermoelectric properties of composite is improved further more.