Defect Chemistry And Thermophysical Properties Of Rare-earth Oxide Ceramics For Thermal Barrier Coating Applications

With the development of heat engines, there is a growing demand for a better performance of the ceramic top-coat in thermal barrier coatings (TBCs) whereas conventional yttria-stabilized zirconia (YSZ) ceramics can not meet the demand any more. Thus it is integral to identify novel materials with better combination property for the next generation thermal barrier coating applications. In this dissertation, several rare-earth oxides series ceramics were identified and the effect of crystal structure and point defects on the thermophysical properties of materials was investigated by means of doping, nonstoichiometry, substitution of rare earth ions and composition design, which is crucial to the materials selection and properties tailoring.Alkaline earth ions doping was carried out to tailor the thermophysical properties of rare-earth zirconate Sm₂Zr₂O₇. The results show that the thermal expansion coefficient of material is remarkably increased through MgO doping with a maximum value around 11.94×10^-6 K^-1 (RT¹000℃) though there is a slight increase in thermal conductivity. A new solid-solution model that is different from previous research was proposed and confirmed by analyzing the variation of the Raman spectroscopy, lattice parameters, X-ray photoelectron spectroscopy, and experimental density of samples. It involves various kinds of point defects and a turning point in solid solution mechanism, which may be responsible for the variation of the thermophysical properties.The solid solutions of SmO_1.5-ZrO₂ system in pyrochlore/fluorite phase region were identified and the effect of nonstoichiometry on the crystal structure, defect species and thermlphysical properties was investigated. It has been shown that the thermal conductivity and thermal expansion coefficient decrease simultaneously with the nonstoichiometry. Sample with composition of 0.55 SmO_1.5·0.45ZrO₂ turns out to be the one with best combination property, that is, low thermal conductivity and relative high thermal expansion coefficient. Moreover, the Raman spectra of this system provide some important information of fluorite/pyrochlore phase transformation and experimental evidence for the assignments of vibration modes.The crystal structure and thermophysical properties of rare-earth stannate pyrochlores were also investigated. The experimental data of thermal conductivity and average thermal expansion coefficient of rare-earth stannates are 1.9².3 W/(m·K) and 8.3⁹.3×10-6 K^-1 (30¹000℃), respectively. The minimum thermal conductivity calculated by Clarke model shows that there is a possibility to reduce the thermal conductivity further.The thermal conductivities of rare-earth silicates with apatite structure which shows a good composition tolerance were studied for the first time. Various kinds of point defects were introduced into the crystal through composition design, and the effect of defect species and concentration on the thermal conductivity was investigated. These compounds show a very low and almost temperature-independent thermal conductivity. The increase of cation and anion vacancies in crystal further reduces the thermal conductivity while oxygen interstitial ions exhibit no remarkable influence.