Synthesis And Thermophysical Properties Of Rare-Earth Oxide Ceramic Materials For Thermal Barrier Coatings

With the development of high performance gas turbines,the increasing inlet temperature of high pressure turbine makes the hot components such as combustion chamber,blade and nozzle,etc.facing a more harsh service environment.Ordinary superalloys cannot meet these use requirements.In recent years,thermal barrier coatings have been widely used.However,further application of the most widely used yttria stabilized zirconia?YSZ?ceramic material is restricted by the problems of phase transformation,accelerated sintering,pronounced radiation heat transfer and molten salt corrosion at high temperature.In this research work,we focus on the problems of the temperature-dependent phase transformation,radiation heat transfer,and mismatch of thermal expansion coefficient in active thermal barrier coatings.The research contents are as follows:Rare earth niobate thermal barrier coating ceramic materials with chemical composition of Ln₃NbO₇?Ln=rare earth?were prepared by solid state method.Thermal physical analysis shows that the intrinsic thermal conductivity of rare earth niobate bulk ceramics are significantly decreased as compared to YSZ.According to the results of molecular dynamics simulation,defects and oxygen vacancies lead to the decrease of the acoustic slope in the phonon spectra and enhance the Umklapp scattering rate,which reduce the thermal conductivity of the materials.It is also shown that rare earth niobate material has excellent oxygen barrier capacity and chemical stability.Fluorite crystal structure of rare earth niobate maintain phase stability from room temperature to 1600°C,and it can be a potential thermal barrier coating materials.Radiation heat transfer at high temperature was studied by using the rare earth zirconate materials.According to the different physical meanings of calculation models of thermal conductivity test by Laser Flash Method,the values of radiation heat transfer of samples are obtained.The relationship between radiation heat transfer and crystal structures was analyzed preliminary.In order to reduce the radiation heat transfer,high quality La₂Zr₂O₇/LaPO₄ multi-phase ceramic materials were designed and prepared.It is shown that effective scattering and absorption of radiation heat transfer was achieved by controlling both the concent and the grain size of second phase LaPO₄,and radiation heat transfer was significantly reduced.Considering of effective radiation heat transfer shielding characteristics of rare earth stannates,?La₂Zr₂O₇?_1-x?Yb₂Sn₂O₇?_x quaternary pyrochlore solid solutions were designed and prepared.It is found that both the radiation heat transfer and phonon thermal conductivity are suppressed.The solid solutions have very low thermal conductivity in the whole testing temperature range,and it can further enhance heat insulation of coating materials.Considering of low thermal expansion coefficient of rare earth zirconate with great application prospects,the Yb₂(Zr_1-xCe_x)₂O₇ solid solutions were designed and prepared to improve the thermal expansion coefficient by the relatively larger ionic radius of Ce⁴⁺and the weak bond energy of Ce-O.The results show that the thermal expansion coefficient of the solid solutions increase with the increasing of the content of Ce⁴⁺,while the thermal conductivity are reduced due to the solid solution defects with low Ce⁴⁺contents.