First-Principles Investigations Of Mechanical And Thermal Properties Of Rare Earth Molybdates

Aeroengines and gas turbines are widely used in large ships,energy,aviation,aerospace and other fields,and are the epitome of a national high-end manufacturing industry and advanced science and technology.In order to develop a new generation of engines with higher thrust weight ratio and energy efficiency,the development and design of new high-performance thermal barrier coatings have become a hot research topic in recent years.At the same time,rare earth molybdates are expected to become one of the potential candidates for thermal barrier coatings due to its high melting point,good mechanical and chemical stability and excellent high-temperature thermal properties.However,there is little research on rare earth molybdates in thermal barrier coatings and other fields,and there is still a lack of systematic research and in-depth understanding of the relationship between its intrinsic structure and performance.Therefore,in order to explore the possibility of rare earth molybdates material system for thermal barrier coating application,this work focused on six RE₂MoO₆（RE=Dy,Ho,Er,Tm,Yb and Lu）,based on the first principle calculation method,respectively studied the influence of intrinsic（composition and structure）and extrinsic（pressure and temperature）factors on their key mechanical and thermal properties,and mainly carried out the following two aspects of research work:Firstly,the first principle calculation method based on density functional theory is used to systematically predict the equilibrium crystal structure,bonding characteristics,mechanical properties,elastic anisotropy and thermal conductivity of six rare earth molybdates RE₂MoO₆（RE=Dy,Ho,Er,Tm,Yb,and Lu）.The results show that the lattice thermal conductivity of rare earth molybdate decreases with increasing temperature,and increases with increasing rare earth atomic number.It is believed that the inhomogeneity of chemical bonds in RE₂MoO₆（RE=Dy-Lu）and the distortion of local structure will jointly enhance the phonon scattering,thereby reducing the thermal conductivity.In addition,its theoretical minimum thermal conductivity range is 0.769～0.805 W·m^-1·K^-1,and may be caused by the mass of larger atoms and complex crystal structure.RE₂MoO₆ is a kind of high temperature thermal insulation ceramics with great research significance and application prospects.Research results predicted the great potential of rare earth molybdates RE₂MoO₆ as a thermal barrier coating material,and provided some insights for the development of high-performance ceramics for thermal insulation applications.Subsequently,we further selected Dy₂MoO₆ and Lu₂MoO₆ with the lowest thermal conductivity,and further studied the effect of external strain conditions on the structure and properties of rare earth molybdates.First,within the studied strain range,the structures of Dy₂MoO₆ and Lu₂MoO₆ can remain stable.The results show that a certain compressive strain can increase the elastic constants and moduli of both,and the anisotropy is also enhanced.In addition,the applied strain can reduce the minimum thermal conductivity of the material to a certain extent.When the tensile strain is 10%,the minimum thermal conductivity of Dy₂MoO₆（Lu₂MoO₆）is reduced by～23%（40%）compared with that under zero strain.Similarly,with the increase of compressive strain or tensile strain,the minimum thermal conductivity of both decreases monotonously.Finally,it is concluded that the effect of strain on the anharmonicity of lattice vibration to regulate and reduce the thermal conductivity is greater than that on the intrinsic properties such as crystal structure and mass density.In addition,it also reveals that the internal relationship between the change of thermal conductivity of RE₂MoO₆（RE=Dy,Lu）with strain and its crystal structure,which provides practical information for further exploring the research of low thermal conductivity materials in strain engineering.