Optimization Of Thermal And Mechanical Properties Of Rare Earth Tantalum/niobate (RETa/NbO₄) Ceramics

As the most effective thermal protection method,thermal barrier coatings（TBCs）are widely used in the field of aero-engines.However,with the development of aero-engines in the direction of high thrust-to-weight ratio and high gas efficiency,the traditional TBCs material yttria-stabilized zirconia（YSZ）has been unable to meet the new generation of aviation the technical requirements of engines,the search for a new type of TBCs materials has become a bottleneck problem that limits the current technological advancement of aero-engines.Currently,rare earth tantalite（RETaO₄）ceramics are considered to be the most promising new generation of TBCs material due to their excellent thermophysical and mechanical properties,such as extremely high melting points,low thermal conductivity,high thermal expansion coefficient,and ferroelastic toughness.Based on RETaO₄,this thesis mainly optimizes its thermal and mechanical properties through doping,substitution,alloying,etc.Besides,this thesis explores a new type of TBCs material system-rare earth niobate（RENbO₄）ceramics,to enrich the large family of TBCs materials.The main contents of this paper include:The single-phase solid solution(Y_0.2Ce_0.2Sm_0.2Gd_0.2Dy_0.2)TaO₄ high-entropy ceramic was synthesized and to investigate the effect of multiple rare earth element doping at the A site on the structure and properties.The results show that the high-entropy effect causes strong phonon scattering,which leads to thermal conductivity as low as 1.59 Wm^-1.K^-1（700℃）and thermal expansion coefficient increased to 10.3×10^-6K^-1（1200℃）.The comprehensive properties of(Y_0.2Ce_0.2Sm_0.2Gd_0.2Dy_0.2)TaO₄ high entropy ceramics have been significantly improved compared to the single component RETaO₄ ceramics.The Y(Ta_1-xNb_x)O₄ ceramic system was prepared by replacing the Ta⁵⁺ion with Nb⁵⁺ion,to explore the effect of B position substitution on the crystal structure and performance.With the gradual replacement of Ta⁵⁺ions by Nb⁵⁺ions,the thermal conductivity of Y(Ta_1-xNb_x)O₄ ceramic first decreases and then increases,and showing obvious symmetry.The minimum value of Y(Ta_1-xNb_x)O₄ ceramic is obtained at a substitution amount of 50%.mol.Nb and Ta have the same valence state and ionic radius,but there is a large difference in relative atomic mass and electronegativity,which resulting phonon-point defect scattering becomes the main factor affecting the thermal conductivity of Y(Ta_1-xNb_x)O₄ ceramic system.The ZrO₂-YTaO₄ ceramic system was synthesized by alloying and to explore the solid solution mechanism and thermophysical-mechanical properties of ZrO₂ in YTaO₄.The results show that the Zr⁴⁺ions are simultaneously substituted at the A and B positions of the crystal structure,and the resulting lattice distortion effect significantly changes the properties of YTaO₄ ceramics.With the increase of Zr⁴⁺ion content,its thermal conductivity decreases.When the Zr⁴⁺ion content is 20%.mol,the thermal conductivity of the ZrO₂-YTaO₄ ceramic system is reduced to 1.33 W.m^-1.K^-1（900℃）,and its thermal conductivity is reduced by 40%relative to YSZ.A density bulk rare earth niobate RENbO₄ ceramics were synthesized by a high-temperature solid-state reaction method to investigate the thermophysical and mechanical properties of RENbO₄ ceramics.The results show that the RENbO₄ceramics possess lower thermal conductivity(1.42 Wm^-1.K^-1)and higher thermal expansion coefficient(11.6×10^-6 K^-1),and the ferroelastic phase transition similar to the RETaO₄ ceramics.Besides,the rare earth niobate RENbO₄ possesses a lower density（5.33-7.40 g/cm³）,so the RENbO₄ coating will be subjected to less centrifugal force to reduce the risk of coating peeling.