Qu, L; (2016) Processing and Characterisation of Novel High Temperature Ceramic Materials. Doctoral thesis , (UCL) University College London.

Abstract

Ceramic oxides that have high-temperature capabilities can be deposited as thermal barrier coatings (TBCs) on superalloy components in high-temperature applications such as gas turbines, to advance thermal efficiency and reduce fuel consumption. This thesis is aimed at developing novel compositions of high-temperature ceramic materials that have lower thermal conductivity and better thermodynamic properties than conventional 6-8 wt.% Y2O3 stabilised ZrO2. Experimental methods and molecular dynamics (MD) simulation were performed and the doping effects on the microstructure and thermodynamic properties were investigated at the atomistic level. The phonon-defect scattering and phonon-phonon scattering which significantly influence the thermal conductivity have been analysed via theoretical calculation, providing an effective way to study the thermal transport properties of ionic oxides. Dy3+, Al3+, Ce4+ and Y3+ doped ZrO2 were synthesised via solid-state reaction methods and sol-gel routes. It was discovered that with the increase in the concentration of Dy3+, the tetragonal structure transforms into the cubic structure and the oxygen displacement in the tetragonal phase decreases. In contrast with the incorporation of Ce4+ ions, the tetragonal structure can be maintained and the oxygen displacement in the tetragonal structure increases. The thermal conductivity decreases due to the incorporation of Dy3+ ions, Y3+ ions and Ce4+ ions to ZrO2. The phonon-defect scattering coefficient increases with the incorporation of Dy3+, whereas it reaches a maximum value because of the Ce4+ substitution. In particular, multi-doping heavy and light elements to ZrO2 can enhance phonon scattering significantly and decrease thermal conductivity. The coefficient of thermal expansion (CTE) increases because of doping Dy3+, Al3+, Y3+ and Ce4+ ions to ZrO2. 3%Dy3+ incorporation results in the largest increase in CTE among (Dy, Y)-ZrO2, whereas for (Dy, Ce, Al, Y)-ZrO2, Dy0.03Al0.03Y0.068Ce0.15Zr0.722O1.936 shows the lowest thermal conductivity (1.7019 0.3597 W/m·K). With respect to the sol-gel routes synthesised (DyY)Zr2O7 (DYZ), the thermochemical compatibility with Al2O3 could be maintained up to 1300°C, whereas at 1350°C DYZ reacts with Al2O3 forming a small number of new phases, Dy3Al5O12 and Al5Y3O12. The thermodynamic compatibility between Dy0.06Y0.072Zr0.868O1.934 and Al2O3, and Dy0.03Al0.03Y0.068Ce0.15Zr0.722O1.936 and Al2O3, has been studied and no reaction has been identified from the powder mixtures that were sintered at 1500°C. Theoretical thermodynamic properties have been successfully calculated via the MD simulation. It was discovered that the Dy3+, Al3+, Y3+ and Ce4+ substitution to ZrO2 can result in a decrease in intrinsic thermal conductivity. The influence of doping these cations on the decrease of the predicted thermal conductivity becomes less significant with the increase in temperature. DYZ shows the lowest thermal conductivity, whereas 3Dy-YSZ exhibits the highest CTE. Based on experimental measurement and theoretical calculation, the decrease in thermal conductivity attributed to a doping trivalent cation such as Dy3+ is higher than that for doping a tetravalent cation such as Ce4+.

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