||Effects of additives on crystallization of ZnO-Al2O3-SiO2 glasses
Glass ceramics are used worldwide nowadays and transparent glass ceramics are especially desired due to their excellent characteristics which is the combination of the good formability of glass and excellent optical properties of ceramics. To produce transparent glass-ceramics, smaller crystal sizes than the wavelength of visible light is required for preventing light scattering. For this purpose, glasses showing volume crystallization are desired. For such glasses, the size of crystals can be controlled by the number density of nuclei because the growth of crystals is hindered by the other crystals around them. For this purpose, some nucleating agents (such as TiO2, ZrO2, etc.) are used to increase the number density of nuclei. High processing temperature (>1550 ℃) is required to melt ZnO-Al2O3-SiO2 (ZAS) glasses; therefore, it is desired to lower the processing temperature by using some additives as a flux. In this research, 20 mol% of ZnO, 20 mol% of Al2O3, and 60 mol% of SiO2 was chosen as a base glass, and the effects of additives (such as B2O3, GeO2, alkaline metal oxides, alkaline-earth metal oxides) on the crystallization behavior of ZAS glasses after nucleation heat-treatment (NHT) at various temperatures for 24 hours were investigated by DTA and XRD measurements. There were three main studies as following; First study is about the effect of glass-formers addition on the crystallization behavior of this glass system. This study is composed of two chapters; In chapter 2, the effect of B2O3 addition was discussed. The change of appearance of glass, dependence of crystallization peak temperature on nucleation heat-treatment (NHT) temperatures, the effect of B2O3 on crystallization behavior, were investigated. In chapter 3, the effect of GeO2 addition on the crystallization behavior was studied and the production of transparent glass-ceramics was tried. In addition, the results of this research will compare with B2O3-added glass (Chapter 2) Second study is about the effect of HfO2 on the nucleation of ZAS glass. This study has one chapter. In chapter 4, we investigated the effect of HfO2 as a nucleating agent which is the same chemical group as Ti, Zr. Third study is about the effect of alkaline-earth metal oxides (CaO, SrO, BaO) on the crystallization behavior of this system. This study is composed of two chapters as following; In chapter 5, the effect of alkaline and alkaline-earth oxide addition on the glass-formation and the change of crystallization mechanism of this glass system were studied. In chapter 6, the crystallization mechanism of ZAS glasses containing alkaline-earth metal oxides was investigated. In conclusions, in ZAS glasses, the addition of Li2O affected to the precipitation of the main crystalline phase and it made the lowest crystal growth rate among all of alkaline metal oxide addition; however, glasses could be formed when alkaline-earth metal oxides were added. To produce the transparent glass, the addition of GeO2 with TiO2 was the most effective to accelerate the number densities of nuclei, so the sizes of crystal became lower than 20 nm. It was found that the addition of GeO2 is the best for the production of transparent glass-ceramics in the ZnO-Al2O3-SiO2 glass system.