Influences Of Vacancies On Thermoelectric Properties Of SrTiO₃ And Sr_xBa_1-xNb₂O₆ Ceramics

The thermoelectric material can utilize the carrier movement inside the solid to realize direct conversion between electric energy and thermal energy,and can be widely applied to the fields of semiconductor electric refrigeration and thermoelectric power generation.In recent years,it has also been considered to have potential in the field of waste heat recovery and has received attention from the academic community.Oxide thermoelectric materials have the advantages of no pollution,low cost and high temperature stability,and become promising environment-friendly materials.The performance of thermoelectric materials depends on the dimensionless thermoelectric figure of merit ZT.The larger ZT value gives higher efficiency of the thermoelectric energy conversion.The ZT depends on the Seebeck coefficient(S),electrical conductivity(?)and thermal conductivity(?)parameters.To achieve a high ZT value,it is necessary to have a high Seebeck coefficient,high electrical conductivity and low thermal conductivity.In this thesis,environment-friendly oxide thermoelectric materials were studied,such as SrTiO3 and(Sr,Ba)Nb2O6 based thermoelectric materials,and the vacancies regulation was attempted to analyze the changes of thermoelectric properties.Cubic perovskite-structured SrTiO3 material can optimize thermoelectric properties by vacancies regulation.(Sr,Ba)Nb2O6 ceramics have a tungsten bronze structure,and their intrinsic thermal conductivity is low.Ions can be filled into unfilled A2 sites to regulate electrical performance,which can optimize thermoelectric performance.In this paper,introducing Sr vacancies into SrTiO3 ceramics or multi-doped elements were into SrTiO3 ceramics,and sintered in a reduced atmosphere.The effects of different Sr vacancies concentrations,doped elements and other factors on the thermoelectric properties of the materials were investigated.For(Sr,Ba)Nb2O6 based thermoelectric materials,they were filled with lanthanide and achieved that all the vacancies are filled,finally sintered in a reduced atmosphere.The main conclusions of this thesis are as following:(1)The effect of Sr vacancies on the thermoelectric properties of SrTiO3 ceramics:(A)Sr1-xTi0.8Nb0.2O3 samples with certain Sr vacancies were prepared by solid state reaction and sintered under reduced atmosphere.The vacancies concentrations were x=0,0.01,0.02,0.03,0.04,respectively.And the ceramics with good single-phase denseness were obtained.By introducing Sr vacancies,the absolute Seebeck coefficient and the electrical conductivity are enhanced simultaneously as compared with those of the sample without Sr vacancy,and thus the values of power factor are improved obviously.Although thermal conductivity increases after introducing Sr vacancies,thermoelectric performance of Sr1-xTi0.8Nb0.2O3 ceramic is enhanced notably because of the significantly enhanced power factor,and the figure of merit zT at 1073 K increases from 0.017(of the sample x = 0)to 0.236(of the sample x = 0.03).(B)Sr0.9-xNd0.1Ti0.9Nb0.1O3 samples with certain Sr vacancies were prepared by solid state reaction and sintered under reduced atmosphere.The vacancies concentrations were x = 0,0.02,0.04,0.06,0.10,0.14,respectively.The samples prepared are of single phase and with compact microstructures.By introducing Sr vacancies,both of electrical conductivity and absolute Seebeck coefficient are enhanced,resulting in the improvement of power factor.Meanwhile,the lattice thermal conductivity decreases with the increasing content of Sr vacancies,indicating that Sr vacancies act as point defects and scatter phonons.Consequently,thermoelectric performance of Sr0.9-xNd0.1Ti0.9Nb0.1O3 ceramics is enhanced obviously by introducing Sr vacancies,and the figure of merit ZT reaches a maximum of 0.32 at 1073 K in the sample x = 0.04.(C)By introducing Sr vacancies into Sr-site and Ti-site doped SrTi03 ceramics,the mobility is lowered,resulting in a decrease in electrical conductivity.However,introducing Sr vacancies into the Ti-site doped samples,the mobility and carrier concentration increase,which promotes the increase of electrical conductivity.(2)Multi-elements were doped into SrTiO3 ceramics:Multiple elements doped SrTiO3 ceramics were prepared by solid state reaction method and sintered under reduced atmosphere,and Sr vacancies were reserved for comparison.In the case of the most doping type,the Sr0.86Nd0.05Y0.05Ti0.9Nb0.05Ta0.05O3 sample not only has a large electrical conductivity and Seebeck coefficient,but also has a low thermal conductivity.At 927 K,the ZT reaches the maximum of 0.24.(3)La element was filled into the vacancies of Sr0.70Ba0.30Nb2O6 ceramics:La-filled Sr0.7Ba0.3LaxNb2O6 samples were prepared by solid state reaction and sintered under reduced atmosphere.The filled concentrations were x=0.05,0.10,0.15 and 0.20,respectively.The sample achieved full filling of the vacancies when x=0.20.The samples prepared are of single phase and with compact microstructures.The oxygen vacancies were generated by the reducied atmosphere sintering,resulting in the increase of the carrier concentration.As the filling concentration increases,the absolute Seebeck coefficient decreases,while the electrical conductivity increases firstand then decreases.The power factor is 473 mWm-1K-2(x=0.15).