Preparation, Characterization And Properties Of Rare Earth Doped Oxynitride Glass And Its Glass-ceramics

Abstract:Oxynitride glasses are a branch of high performance glasses, obtained by incorporating nitrogen atoms into oxide glass networks of e.g. silicates, borates and phosphates. Compared to oxide glasses, oxynitride glasses present higher glass transition temperature, microhardness, bending strength, elastic modulus, fracture toughness, refractive index and lower thermal expansion coefficient. Due to the unique properties, oxynitride glasses have led to some potential areas of application:high elastic modulus glasses for computer hard discs, ceramic seals, coatings on metals, encapsulation of nuclear waste for long term storage, high electrical resistivity coatings for use at high temperatures, glass fibers and bullet-proof glass for transparent armor. Meanwhile, oxynitride glass-ceramics with some selected crystalline phases prepared after appropriate heat treatments will be a kind of promising oxide-nitride composites materials. Due to exciting comprehensive high-temperature mechanical properties, such as a good combination of greater high-temperature strength and toughness, superior resistant to thermal-shock, in-situ grown β-Si3N4reinforced glass-ceramic composites has been a hot topic in materials research in recent years.Oxynitride glasses, oxynitride glass-ceramics and Si3N4/glass-ceramics composites were chosen as research object in the present thesis. Differential scanning calorimeter (DSC), X-ray diffraction (XRD), Scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDS) and IR absorption spectra were used to investigate the formation of these oxynitride glasses, the influences of rare-earth and nitrogen content on the structures, thermal properties and mechanical properties of the oxynitride glasses, the influences of composition and heat-treatment process on the microstructures and mechanical properties oxynitride glass-ceramics and Si3N4/glass-ceramics composites. The main results are listed as follows:Y2O3, La2O3, CaCO3, A12O3, SiO2, AlF3-3H2O and Si3N4powders were used as raw materials powders to prepare RE-Ca-Si-Al-O-N (RE=Y, La) oxynitride glasses by melting batches under N2atmosphere, in order to explore the glass forming region of these compositions. Fine appearance, homogeneous, translucent and defect-free bulk oxynitride glasses were prepared successfully at a relatively lower temperature (1550-1650℃).The amorphous nature of the Y-Ca-Si-Al-O-N oxynitride glasses was verified by an X-ray diffractometer. Fourier-transform IR spectroscopy was done to find out the basic structural units in these glasses. The influences of nitrogen and yttrium content on the properties such as density, glass transition temperature, thermal expansion coefficients, the crystallization temperature, chemical durability and Vickers hardness were investigated. The physical properties were found to change linearly with the concentration of nitrogen or yttrium content, respectively.For La-Ca-Si-Al-O-N oxynitride glasses, the effects of N/O ratios and La/Ca ratios on the properties such as density, thermal expansion coefficient, glass transition temperature, the crystallization temperature, Vickers hardness, bending strength and leaching rate were investigated. The effects of nitrogen and lanthanum on the properties are independent and additive. At the same time, the relationship between these properties and the structures of the glasses were discussed.The influences of La or Y on the properties such as thermal expansion coefficients, glass transition temperature, the crystallization temperature, Vickers hardness and bending strength were comparatively studied. The physical properties are found to change linearly with the concentration of lanthanum or yttrium content, respectively. In general, the structure and properties of glasses show a larger dependence on the content of yttrium than on the content of lanthanum. At the same time, the relationship between these properties and the structures of the glasses were discussed by using cationic field strength (CFS).The effect of nitrogen substitution on the crystallisation of oxynitride glass in the Y-Ca-Si-Al-O-N system has been studied. The appropriate heat treatment temperatures were selected according to the information provided by the differential scanning calorimeter (DSC) measurement. There is a significant increase in Tg and Tc with increasing nitrogen content. XRD and SEM analysis demonstrated that, N content has significant effect on crystallized products, microstructure, crystalline size and crystalline contribution of the oxynitride glass-ceramics, resulting different mechanical properties.Y-Al-Si-O-N-F oxyfluoronitride glass-ceramics were prepared successfully. Crystalline phases in the oxyfluoronitride glass-ceramics were identified by X-ray diffraction and energy dispersion spectrometer (EDS). Microstructures of the glass-ceramics were observed by scanning electron microscopy (SEM). The results show that, addition of fluorine lowers the Tg and Tc of oxyfluoronitride glasses and influences the microstructures, including crystal size and morphology. Nitrogen influences the crystallization products and microstructures. The glass-ceramics all exhibit higher micro-hardness and bending strength compared with their corresponding glasses at different fluorine or nitrogen contents.In-situ grown β-Si3N4reinforced Y2O3-La2O3-Al2O3-SiO2(YLAS) glass-ceramic matrix composites were obtained by a two-step sintering process at a relatively lower temperature. Analyses of resultant products were carried out using X-ray diffraction and scanning electron microscope. The results showed that adding30-50wt%YLAS glass could promote effectively densification of Si3N4composites and a-Si3N4to (3-Si3N4phase transformation. The crystallized rare-earth disilicate phases with a high melting point significantly benefited the high-temperature mechanical properties of the composites. The YLAS glass-ceramic matrix Si3N4composites exhibit excellent mechanical properties compared to unreinforced glass-ceramic matrix, which is undoubtedly attributed to the high concentration of rod-like β-Si3N4grains.In-situ grown (3-Si3N4and a-SiC co-reinforced La2O3-CaO-Al2O3-SiO2(LCAS) glass-ceramic matrix composites were obtained by a two-step sintering process. Densities of samples were measured and analyses of result products were carried out using X-ray diffraction and scanning electron microscope. The results show that LCAS glass-ceramic could effectively promote a-Si3N4to β-Si3N4phase transformation. However, β-Si3N4grain growth was hindered by the existent a-SiC to a certain extent, resulting an adverse effect on mechanical properties of the composites. The mechanical properties of the composites were found to enhance with increasing sintering temperature.