Research On Preparation And Thermoelectric Properties Of Nb-La Co-doped SrTiO₃ Ceramics

With the rapid development of industrialization,the energy crisis and environment problems are increasingly more and more serious.Clean and effective using of energy sources has attracted more public attention.Thermoelectric material has been regarded as a potential new clean energy material which can be used to convert waste heat into electrical energy directly.In addition,thermoelectric material has the advantage of being nontoxic,environmental friendly,low cost,without introduction of moving parts.SrTiO₃ is one of the n-type oxide thermoelectric material that is expected to wide applications.However,due to it’s low thermoelectric performance Sr TiO₃-based thermoelectric material is far away from application.Therefore,to increase the thermoelectric performance of SrTiO₃-based has become a hot topic.In this thesis,SrTiO₃-based thermoelectric oxide are chosen to as research object.The energy bands and density of energy states were calculated to explore the influence of doping concentration.In order to improve the thermoelectric system,sintering technology and doping concentration are analyzed to find the possibility of enhancing the thermoelectric performance of SrTiO₃.Main research results are as follows:1.By using first principles calculation,we found that the greater the Nb doping concentration,the bigger the formation energy was,and then more difficult material synthesis was.We can adjust the proportion of Nb and La to achieve different doping concentrations.By analyzing the band structure and density of energy states,we found that the Fermi energy of doped SrTiO₃ moved to the conduction band and all doped SrTiO₃ became degenerate semiconductor,simultaneously,that allow the electric conductivity improved.2.Experimental results show that the shape of nano-powders prepared by hydrothermal method was irregular flake structure.Average particle size was 15²5 nm.3.We research the sintering process including sintering temperature,time and pressure of bulk preparation by hot pressing sintering.The final result shows that the thermoelectric properties was best when samples were sintered at 1300℃ for 2 hours under the pressure of 25 MPa.4.Research of thermoelectric properties for Nb-La co-doped SrTiO₃ ceramics sintered by hot pressing shows that the La10Nb10 sample has the higher power factor than others and it’s PF can reach 1.3 mWm-1K-2 at 560 K.The La10Nb10 sample has a high ZT 0.3 compared with other samples due to the lowest thermal conductivity of 1.9 Wm-1 K-1 at 1050 K.5.Study on comparison test of different sintering methods shows that doped SrTiO₃ sintered by CIPSC（Cold Isostatic Pressing and sintering in carbon powder）has high ZT values than by HPS（Hot Pressing Sintering）and the ZT of La10Nb10 sample can resch 0.6 at 1050 K which is the highest value in SrTiO₃ bulk.The ZT of doped SrTiO₃ sintered by HPS is lower than CIPSC.The grain size of SrTiO₃ ceramic by CIPSC is smaller than HPS so it help to reducing the thermal conductivity.