Design,Preparation,and Electrical Properties Of SrTiO₃-based Thermoelectric Textured Ceramics

With the energy and environment issues becoming more prominent,the fossil fuels,such as oil and coal,will gradually be replaced by clean and renewable energy.Large amounts of the utilized energy are released into environment as exhaust heat,including industrial waste heat,and waste heat generated from incineration of municipal solid waste.Therefore,research on the recovery of exhaust heat contributes to alleviation of the environmental problems associated with this release.Nowadays,thermoelectric（TE）power generation technology offers unique advantages for exhaust heat recovery,and TE materials which can convert waste heat to electricity through Seebeck effect,attract many researchers.Oxide thermoelectric materials have many potential advantages,such as oxidation resistance,good chemical and thermal robustness,non-toxic,simple preparation process,and etc.,having great application potential in high temperature field.In this thesis,Sr_0.9La_0.1TiO₃ based textured ceramics with cubic perovskite structure,are prepared by the reactive template grain growth（RTGG）method combined with the tape casting process,using plate-like Sr₃Ti₂O₇ seeds as templates.The influences of metal particles type and addition amounts,and preparation techniques on the microstructure and TE properties are investigated.And Sr_0.9La_0.1TiO₃ based ceramics with ZT value higher than 0.5are obtained.Sr_0.9La_0.1TiO₃ ceramics added with nano-sized silver（Ag）particles,micro-sized titanium（Ti）particles,or nano-sized Ti particles are prepared by the conventional solid-state reaction method.The effects of metal particles’content on the microstructure and thermoelectric properties are investigated.The results show that,after sintering in Ar atmosphere and annealing in Ar+C atmosphere,Ag particles reserve in the form of metallic Ag,but Ti particles are oxidized to rutile TiO₂.The added sintering aid Bi₂O₃ is partly reduced to metallic Bi.Metal Ag and Bi particles can form electrical connections between the neighboring Sr_0.9La_0.1TiO₃ and/or TiO₂ grains.Hence,carrier scattering at grain boundaries is further reduced,leading to enhanced carrier mobility.It is consistent with the phenomenon that electrical resistivity decreases with increasing addition content of nano-sized metal Ag or Ti particles.Besides,the addition of nano-sized Ag or Ti particles reduces the thermal conductivity from 3.2 W/m/K to 1.3～1.4 W/m/K at 1073 K.On the one hand,nanoscale grain boundaries and phase interfaces block phonons transmission;on the other hand,Bi and Bi₂O₃nanoparticles embedded in larger particles or located at the grain boundaries can serve as phonon scattering centers and are effective in reducing the lattice thermal conductivity.Under the synergistic effect of these factors,the thermal conductivity decreases significantly.At last,Sr_0.9La_0.1TiO₃ added with 20wt%nano-sized Ti particles shows the best TE properties with a relatively high PF value（0.61 m W/m/K²）and lowκvalue（1.31 W/m/K）at 1073 K,leading to a high ZT value of 0.50.Nano-structured ceramics are prepared by spark plasma sintering（SPS）technology,using Sr_0.9La_0.1TiO₃ added with 20wt%nano-sized Ti particles as matrix material.The influences of sintering temperature and annealing heat treatment time on the TE properties are studied.The results show that,SPS technology can effectively promote the densification of ceramics and inhibit grain growth.Relative densities of sintered ceramics are all higher than97%,and the grain size is smaller than 1μm.A very low electrical resistivity(1.5×10^-3Ω·cm at 1120 K)is obtained,which is one order of magnitude lower than that prepared by the conventional solid-state reaction method.The annealing mechanism of Sr_0.9La_0.1TiO₃ based ceramics using graphite powders is studied.It is not through carbothermal reduction but through the reducing agents,H₂ and CO,which are generated from graphite powders in the annealing process.H₂ is produced in carbon gasification reaction,which also generates CO.Additional CO is produced in carbon incomplete combustion,and/or comes from decomposition of phenol and carbonyl groups.In the annealing process of Sr_0.9La_0.1TiO₃ based ceramics with graphite powders,about 67%of the oxygen vacancies are created by CO reduction while about 33%by H₂ reduction.Then the oxygen vacancies ionize and produce more electrons,which increases the carrier concentration,thus reducing the electrical resistivity and improving the TE performance.Sr_0.9La_0.1TiO₃ based textured ceramics are successfully prepared by the reactive template grain growth method combined with tape casting,using Sr_0.9La_0.1TiO₃ added with 20wt%nano-sized Ti particles as matrix material and 10wt%plate-like Sr₃Ti₂O₇ seeds as templates.The results show that,grains grow with a preferred orientation along（h00）.A maximum texture fraction of 0.81 is achieved for the ceramic samples sintered at 1400℃/2h in Ar atmosphere and annealed at 1350℃/8h in Ar+C atmosphere.Thermoelectric properties measurement results indicate that the values of electrical resistivity,Seebeck coefficient,and power factor are isotropic,but the thermal conductivity shows anisotropic,resulting to the anisotropic figure of merit ZT.In the direction perpendicular to the tape casting direction,lower thermal conductivity and higher ZT values are obtained.Compared with the Sr_0.9La_0.1TiO₃/20wt%Ti non-textured ceramics,the textured ceramics have higher ZT values in the temperature range of 400 K～1000 K.