| In the future of gas turbine technology,silicon based ceramic matrix composite(CMC),such as silicon carbide fiber reinforced CMC(SiCf/SiC CMC)has important applications as structural materials for the hot-section components of gas turbines.In the combustion environment,thermal/environmental barrier coating(T/EBC)should be applied on the surface of SiCf/SiC CMC to improve its environmental durability.Promising materials for T/EBC system should satisfy a series of requirements,including high-temperature stability,low thermal conductivity,good resistance to oxidation and calcium-magnesium-aluminosilicate(CMAS),good match of thermal expansion with substrates,and good chemical compatibility,etc..In fact,low thermal conductivity and proper coefficient of thermal expansion are the threshold considerations for T/EBC design.Rare-earth silicates have attracted great attention as the diffuson layer in the state-of-art multilayer T/EBC system,which protect the SiCf/SiC CMC substrate from water vapor corrosion.Rare-earth silicates exhibit complex crystal structures and various polymorphs;however,little has been known about the mechanisms of thermal conduction and thermal expansions.Therefore,comprehensive investigations on the phonon behaviors and thermal properties of rare-earth silicates,as well as strategies for phonon engineering are of vital significance,which would promote the optimization and design of advanced T/EBC systems for SiCf/SiC CMC.The works in this dissertation are developed as follow.Using first-principles calculations,we investigate the basic structural units and the distortions,atomic bonding,elastic properties,and intrinsic lattice thermal conductivity(KL)for β-,γ-,δ-Y2Si2O7 and X2-Y2SiO5.These materials exhibit obvious bonding heterogeneity,elastic anisotropy and strong anharmonicity of lattice vibrations.Based on these results,the intrinsic relationship between microscopic structural and bonding characteristics and the macroscopic elastic and thermal properties are established for rare-earth silicates,which provides a theoretical foundation for the prediction and screening of complex-structure oxide ceramics showing low KL.Using lattice dynamics calculations,we investigate the phonon density of states,phonon dispersions,mode Gruneisen parameters,and atomic vibration patterns of X2-Y2SiO5 and y-Y2Si2O7.It is found that the low-frequency optical phonons are obviously coupled with acoustic phonon,yielding high level of phonon anharmonicity.Empirical models are used to predict KL,where we find that longitudinal acoustic phonons contribute higher to the total KL as compared with transverse acoustic phonon for the two materials.Coefficients of thermal expansions of X2-Y2SiO5 versus γ-Y2Si2O7 are studied using quasi-harmonic approximations,and the difference in between are found to be originated from the magnitude as well as the positive or negative nature of anharmonicity for low frequency phonons,together with the strength of structural units.The mechanisms of thermal conduction and thermal expansion for rare-earth silicates are summarized,which provide theoretical guidelines for the coordinated optimization of thermal properties based on the concept of "phonon engineering".Using first-principles calculations,we investigate the dependence of crystal structure,atomic bonding,elastic modulus,phonon dispersions,and KL of y-Y2Si2O7 under hydrostatic pressure.Interestingly,KL shows an abnormal decrement upon increased hydrostatic pressure,which is originated from the increased anharmonicity and intensified scatterings of acoustic phonon.These results enlighten new ways to tune the KL of rare-earth silicates.The thermal expansion behaviors of several P-,γ-,and 8-RE2Si2O7(RE=Y,Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu)are studied and compared.The "gene" that controls the thermal expansions of rare-earth disilicates are found to be the linear or bended configuration of Si-O-Si bridge in the corner-shared pyrosilicate units as well as the weak or strong interaction between the bridging O atom and the surrounding rare-earth atoms,which determine the vibration patterns and thus the positive or negative anharmonicity for low-frequency phonons,as well as the strength of structural units.Powder(Dy0.15Y0.85)2Si2O7 solid solution is synthesized using solid-state reactions under 1550 °C,which exhibits imperfect polymorph of y.It shows increased coefficient of thermal expansion as compared with y-Y2Sl2O7,which could presumably be attributed to altered anharmonicity of low-frequency phonons and overall bonding strength,brought by the local lattice distortions around the pyrosilicate units due to ionic substitution.These results cast light on tuning and optimizing the thermal expansion behaviors of rare-earth disilicates,which promotes improved compatibility of thermal expansions between different layers in advanced multilayer T/EBC systems. |
