Microstructure And Thermoelectric Properties Of Rare Earth And Niobium Co-doped SrTiO₃

Due to the over-exploitation of the Earth's shared resources,we are faced with many problems such as oil shortage,environmental pollution,climate anomalies,etc.The research and development and utilization of clean new energy materials have become a hot spot in scientific research.Thermoelectric material is a functional material that can directly convert electrical energy and thermal energy.It has a series of remarkable features such as high temperature stability,no pollution and simple preparation process.At present,thermoelectric materials with good performance are almost all composed of rare earth elements.The price is expensive and they have certain toxicity.In the oxide thermoelectric material system,high-temperature n-type oxide barium titanate?SrTiO₃?is a thermoelectric material with great research potential,However,it is difficult to be commercialized on a large scale,and the research on the distribution and evolution of the microstructure of materials is not systematic enough.The law affecting the transport properties of thermoelectrics is still unclear.Therefore,the systematic study of these laws is of guiding significance for improving the thermoelectric properties of oxides.In this thesis,high temperature n-type barium titanate-based oxide thermoelectric materials are used as the main research object to study the co-doping of SrTiO₃?STO?with rare earth elements and lanthanum elements,and to analyze the multi-element doped SrTiO₃?LaCeNb-STO?and the multi-element doped SrTiO₃?LaYbNb-STO?of microstructure and thermoelectric transport properties,study the microstructure distribution of multi-element doped SrTiO₃ and the evolution of high-temperature sintering process,and use multiple testing methods to study multi-element doping The effects of heterogeneity on conductivity,Seebeck coefficient and thermal conductivity are as follows:1)La,Ce and Nb co-doped SrTiO₃ powders and La,Yb and Nb co-doped SrTiO₃powders were successfully synthesized by hydrothermal method.There were no impurity phases in the powders of the two systems,and the average particle size of the two powders was 20nm.The doped elements entered the strontium titanate lattice.2)The LaCeNb-STO powder was densely formed by cold isostatic pressing at 200 MPa,and then sintered at a high temperature of 1300°C for 5h to obtain a LaCeNb-STO bulk material.The La,Ce and Nb elements were successfully incorporated into the matrix,and there was a uniformly distributed TiO₂ phase in the bulk.The high doping concentration of the sample formed a large difference with the low doping concentration of the microstructure,high doping concentration.The sample has a larger average grain size and a wider grain size.Sr vacancies are generated during high-temperature sintering,and Sr vacancies contribute to the improvement of conductivity.At the same time,the formation of multi-element doping and nano-scale grain boundaries effectively reduces the thermal conductivity of LaCeNb-STO blocks and improves the ZT value of materials.In the LaCeNb-STO system,the power factor of La3Ce3Nb3 sample reached 1.62mWm^-1K^-2 at 530K in the middle temperature range,and the highest ZT value was 0.38 at 1000K in La5Ce5Nb5 sample.3)In the LaYbNb-STO system,the effects of multi-element doping on microstructure and thermoelectric transport law are studied by cold-press immersion method.The high-temperature sintered bulk precipitates Yb₂Ti₂O₇ and TiO₂ second phase,and a large number of grain boundaries are formed.The thermal conductivity of the lattice is reduced.At the same time,the influence of the doping of each element of La,Yb and Nb on the electrical properties of the system is studied systematically.The conductivity is improved by doping regulation and reducing atmosphere,and the power factor is optimized.The La8Yb2Nb15sample is 1000K.The ZT value is up to 0.30.In order to study the influence of sintering process on the microstructure and thermoelectric properties of multi-element doping,micro-nano pores appeared in the vicinity of the second phase in the vacuum hot-pressing sintering system,and there were more TiO₂ precipitated than the cold-press immersion method,and the La8Yb2Nb10 sample The ZT value was 0.37 at 1000K,and the ZT value of the La8Yb5Nb10 sample was 0.38 at 1000K.