| The continual increase in gas turbine temperatures necessitates an advanced TBC system with the ability to reduce both thermal conduction and thermal radiation through the coatings. New TBC materials and structures are required in order to simultaneously lower the thermal conduction and thermal radiation to the metal substrate. To achieve these objectives, thermal conductivities of various metal oxides doped yttria stabilized zirconia bulk materials were studied by systematically doping metal oxides to 7YSZ material in order to find the optimal doping material. Additionally, based on the experimental results a model to predict thermal conductivity of the doped zirconia based ceramics was established, incorporating integrated defect scattering cluster consisted of substitutional atoms and oxygen vacancies. The calculated thermal conductivity of doped material obtained using this model was found to be very consistent with the experimental results. Furthermore, a novel multiple layered coating structure was designed with more than 80% hemispherical reflectance to radiation within a wavelength range of 0.3--5 mum. The structure consists of sets of high reflectance multiple layered stacks and a single layer ceramic material with low thermal conductivity. The simulated temperature distribution results showed that the this new structure, containing high reflectance multiple layered stacks, can achieve as much as 46°C temperature reduction on the metal surface when compared to a monolayered coating structure of the same thickness. The outcome of this study provides directions to future coating composition and microstructure design in combining the doped zirconia possessing the lowest thermal conductivity with the multiple layered coating structures. |
