| 其他摘要 | Y2Si2O7 and Y2SiO5 ceramics are the two most important compounds in the system of Y2O3-SiO2. They are also frequently identified at the grain boundaries of Si3N4 when the sintering aid additives Y2O3 or Y2O3+SiO2 were used. It is proved that these intergranular phases play a crucial role in the mechanical and dielectric properties of Si3N4 ceramic. Therefore, Y2Si2O7 and Y2SiO5 ceramics are attractive not only on the scientific understand of these two novel ceramics but also on the application of Si3N4 ceramic.
In the diagram of Y2O3-SiO2, Y2Si2O7 and Y2SiO5 compounds are presented as strait lines. It means that the synthesis of phase-pure materials is very difficult and a strict stoicheometric ratio for the starting powders is needed. Moreover, Y2Si2O7 and Y2SiO5 compounds possess very complex polymorphs: there are seven polymorphs (y, and ) for Y2Si2O7 and two polymorphs (X1 and X2) for Y2SiO5. These polymorphs will transform from one to another with temperature during heating process. The strict stoicheometric ratio for the starting powders and complex polymorphs during heating encumber the preparation of phase-pure Y2Si2O7 and Y2SiO5 powders and bulk materials, and subsequently hinder the discovery and research on these two ceramics.
In this dissertation, a novel solid-liquid reaction method was developed for the synthesis of Y-Si-O ceramics. The starting materials are Y2O3 and SiO2, with the addition of LiYO2 additive, a transit liquid-phase forms in the heating process. The formation of liquid phase significantly increases the transport of ions, and thus improves the reaction velocity and lowers the reaction temperature between Y2O3 and SiO2. For the powders without LiYO2 additive, no chemical reaction was detected in the sample during heating to 1450 oC, while a major phase of -Y2Si2O7 or Y2SiO5 was detected in the sample with 3 mol% LiYO2 addition under the same heating conditions. Via the solid-liquid reaction method, phase-pure -Y2Si2O7 and Y2SiO5 powers and fully dense bulk materials were obtained.
Based on the purity -Y2Si2O7 and Y2SiO5 powders and fully dense bulk materials, we studied the crystal structure and crystal chemistry of -Y2Si2O7 and Y2SiO5 via X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopies. The mechanical properties, tribological properties and environmental durability of -Y2Si2O7 and Y2SiO5 were also investigated. The mechanical properties of -Y2Si2O7 and Y2SiO5 demonstrated that these ceramics have low shear-deformation resistance and possess the ability of “microductility”, and thus can be machined readily by conventional cemented carbide tools. Therefore, Y-Si-O ceramics are defined to be a kind of “machinable ceramics”. From the perspective of thermal properties, -Y2Si2O7 has a very low thermal expansion coefficient (3.90×10-6/K) and a low thermal conductivity (< 3.0 W/mK above 600 K), and Y2SiO5 has thermal expansion coefficient of 8.36 ×10-6/K as well as a thermal conductivity of 1.34 W/m•K. Moreover, Y-Si-O ceramics have been proved that they present excellent resistance to hot-corrosion environments. It is concluded that Y-Si-O ceramics are promising candidates for oxidation-resistance/thermal barrier/environmental barrier coatings.
Attributing to the unique properties, Y-Si-O ceramics are anticipated to be applied as both structural and functional materials. Thus, the wet-chemistry forming technique was investigated using -Y2Si2O7 as model material. Based on the well-dispersed aqueous-based and ethanol-based suspensions, highly textured (Lotgering factor up to 0.90) -Y2Si2O7 bulk materials with a controllable grain size were prepared via slip casting in a 12 T strong magnetic field and the following two-step sintering method. A material with a tailored microstructure is useful for applications taking advantage of its grain-size- or preferred-orientation-dependant mechanical or physical properties.
In summary, this dissertation systematically investigated the preparation, microstructure, property, forming technique and the possible engineering applications of Y-Si-O ceramics. Based on the preparation-structure-property relationship of Y-Si-O ceramics, a guideline of machinability of ceramics by tailoring the crystal structure was proposed. The work of this dissertation benefits further developments and technological applications of Y-Si-O ceramics. |
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