First-Principles Study On The Thermo-Mechanical Properties Of Novel Tantalate Materials

Thermal barrier coatings（TBCs）are refractory ceramic coatings that laid on the surface of the superalloy parts to protecte the hot sections of modern turbine engines in aerospace as well as utility applications.High melting point,ultra-low thermal conductivity,large thermal expansion coefficient,excellent damage tolerance and excellent mechanical properties are the main requirements of TBCs materials.Currently,6-8wt.%Y₂O₃-stabilized Zr O₂（YSZ）is the most successful TBCs material.However,YSZ can hardly be operated for long-term service application above 1200℃due to the sintering,phase transformation and environmental sediment corrosion.They would result in thermophysical properties deterioration,the formation of cracks,and eventually accelerate the failure of TBCs.Consequently,it is necessary to seek novel materials to meet the increasing demands of advanced TBCs.However,YSZ is still irreplaceable for its outstanding fracture toughness which comes from its ferroelasticity.Recently,many tantalates have have attracted considerable attention because of their low thermal conductivities,high thermal expansion coefficient,high melting points,and good high-temperature phase stabilities.Theoretically,first-principles calculations combined with the the empirical or semi-emprical models have been widely used in discovering promising structures as well as investigating the intrinsic properties of TBC candidates,which is a useful tool in predicting the elastic,mechanical and thermal properties of the materials for future applications in high and ultra-high temperature area.The main conclusion of this thesis can be summarized as following parts.1.Using ab inito calculations,we investigate the structure,thermodynamic stability,elastic,mechanical and thermal properties of ternary oxide A₆B₂O₁₇（A=Zr,Hf;B=V,Nb,Ta）.The first-principles calculations prove that the orthorhombic A₆B₂O₁₇compound is mechanically stable along with proper elastic modulus.Moreover,the hardness of A₆B₂O₁₇ is 9.757～12.56GPa with a fracture toughness of1.609～2.040MPa·m^1/2 and brittleness index of 6.065～6.193μm^-1/2.Besides,the melting point of A₆B₂O₁₇is higher than 2200K,while the thermal conductivity is1.306～1.631W/m·K（Cahill model）and 1.170～1.463W/m·K（Clarke model）.The thermal expansion coefficient of A₆B₂O₁₇ is 8.215～9.027×10^-6K^-1,indicating its great potential as candidate of thermal barrier coatings.2.The structure,thermodynamic stability,mechanical and thermal properties of rare-earth tantalate LnTa₂O₆（Ln=Ce,Pr,Nd,Pm,Sm,Eu,Gd,Tb）are calculated based on the first-principles calculations.The results show that orthorhombic LnTa₂O₆has better thermal insulation properties than YSZ and A₆B₂O₁₇ with lower thermal conductivities as 1.152～1.194（Cahill model）and 1.032～1.073W/m·K（Clarke model）,which originates from its complex crystal structure as well as the introduction of rare earth elements.The thermal expansion coefficient of LnTa₂O₆ is 8.78～9.65×10^-6K^-1while the melting points is above 2000K.As for the mechanical properties,LnTa₂O₆compound has hardness ranging from 10.268 to 10.953GPa,which is comparable to the YSZ coatings.The fracture toughness and brittleness index of LnTa₂O₆are1.712?1.817MPa·m^1/2 and 5.977～6.035μm^-1/2,respectively.The thermo-mechanical properties of LnTa₂O₆ surpass many known tantalates.3.We investigate the structure,thermodynamic stability,elastic,mechanical and thermal properties of rare-earth tantalates Ln₃Ta₅O₁₅（Ln=Ce,Pr,Nd,Sm,Eu,Gd,Tb）.The results reveal that the tetragonal tantalate Ln₃Ta₅O₁₅ has good mechanical properties with hardness ranging in 10.472～11.813GPa,a fracture toughness of1.761～1.958MPa·m^1/2 and brittleness index of 5.931～6.032μm^-1/2.In addition,Ln₃Ta₅O₁₅ exhibits melting temperature of 1887.39～2024.10K with a minimum thermal conductivity of 1.143～1.225 W/m?K（Cahill model）and 1.013～1.102W/m?K（Clarke model）.Besides,the thermal expansion coefficient of Ln₃Ta₅O₁₅ is9.881～10.597×10^-6K^-1,indicating that Ln₃Ta₅O₁₅compounds are promising candidates for the next generation TBCs.