Investigation On Thermoelectric Properties Of Filled Strontium Barium Niobate Ceralmics

Thermoelectric materials can realize the direct conversion of heat flow and electrical power,which have wide application foregrounds in the fields of electronic refrigeration and power generation.Thermoelectric performance is evaluated by the figure of merit ZT,and excellent performance of thermoelectric materials not only needs to posses high value of Seebeck coefficient and conductivity but also to show low value of thermal conductivity.Tungsten bronze-structured Sr_xBa_1-xNb₂O_6-?materials have a cage-like structure,so pristine Sr_xBa_1-xNb₂O₆ materials show intrinsic low thermal conductivity.Elements can be filled into unfilled A₂ sites to influence thermoelectric properties.In this thesis,compact Sr_0.70Ba_0.30M_xNb₂O₆ ceramics were prepared through filling alkali metal elements,rare earth elements.And we investigated the influences of the sintering methods,filling elements,filling contents on the thermoelectric properties.The main conclusions of this thesis are as following:1.Alkali metal elements were filled into Sr_0.70Ba_0.30Nb₂O_6-? materials:?A?the Sr_0.70Ba_0.30Li_xNb_2O6 samples were sintered in an air atmosphere and were annealed in a forming gas by solid state reaction,and the filling contents are x=0.01,0.03,0.05,0.08,0.10,0.15,and 0.20 respectively.The samples prepared are of single phase and with compact microstructures.After Li filling,electrical resistivity decreases,so does Seebeck coefficient.The lowest resistivity value is obtained in the Sr_0.70Ba_0.30Li_0.05Nb₂O₆ sample and so is the highest PF value?486?W/K2m at 1073 K?.However,thermal conductivity increases due to Li filling,so the value of ZT does not increase obviously.?B?The Sr_0.70Ba_0.30Li_xNb₂O₆ samples were directly sintered in a forming gas,however the PF value is lower than the samples,which sintered in an air atmosphere and annealed in a forming gas,so thermoelectric properties were not optimized.2.Rare earth elements were filled into Sr_0.70Ba_0.30Nb₂O_6-? materials:?A?After Yb filling,electrical resistivity is lower than thos of the Sr_0.70Ba_0.30Nb₂O₆ sample,and the highest PF value?426?W/K2m?is obtained in the Sr_0.70Ba_0.30Yb_0.10Nb₂O₆ sample.Both of the total thermal conductivity and lattice thermal conductivity decrease after Yb filling.Moreover,the total thermal conductivity decreases monotonically with filling contents.The values of ZT of Yb-filled samples are higher than those of the Sr_0.70Ba_0.30Nb₂O₆ sample,and the highest value of ZT?0.21 at 1073 K?is obtained in the Sr_0.70Ba_0.30Yb_0.05Nb₂O₆ sample.?B?After Y filling,the electrical resistivity is lower than that of the Sr_0.70Ba_0.30Nb₂O₆ sample,and decreases monotonically with filling contents.The highest PF value?420?W/K2m?is obtained in the Sr_0.70Ba_0.30Y_0.10Nb₂O₆ sample and the highest ZT value?0.21 at 1073 K?is obtained in the Sr_0.70Ba_0.30Y_0.03Nb₂O₆ sample.?C?Properties of samples filled with Yb and Y are very similar when filled the same filling content.3.Alkali metal element K was filled into Sr_0.70Ba_0.30Nb₂O_6-?materials:according to the results of experiments,more ions could be filled into Sr_0.70Ba_0.30Nb₂O_6-?materials when the samples are calcined in a forming gas.The electrical performances of samples calcined and sintered in a forming gas were superior to those of samples sintered in an air atmosphere and annealed in a forming gas.The highest PF value?528?W/K2m?is obtained in the Sr_0.70Ba_0.30K_0.10Nb₂O₆ sample and thus the highest ZT value is 0.23.In this thesis,the influences of element-filling on thermoelectric properties of the Sr_xBa_1-xNb₂O_6-? materials have been systematically investigated.The results indicate that the resistivity and the Seebeck coefficient decrease while the PF value increases after alkali metal elements or rare earth elements are filled into unfilled A-sites.The PF value of samples filled by alkali metal elements could be enhanced considerably as compared with pristine SBN ceramics and exceed 500?W/K2m.Meanwhile,the highest PF value of samples filled by rare earth elements could only achieve 420?W/K2m.When the filling content is small,the total thermal conductivity increases,which would decrease when the filling content increases.Thermoelectric performances of samples filled by alkali metal elements were superior to those of samples filled by rare earth elements,with the highest ZT value of 0.23.