| Sodium tellurite glasses are considered as potential candidates for non-linear optical applications because of high refractive index values and wide infrared transmittance. On the other hand, because the glasses have relatively low melting temperature and low viscosity after melting, they are often chosen to study the melting, evaporation and solidification of glasses in low gravity. In the present dissertation, some properties of sodium tellurite glasses, including density, chemical durability, stability against crystallization and fragility of glass forming liquid are studied. The melting, evaporation and solidification of sodium tellurite glasses in low gravity during drop shaft experiments are observed. Except this, the phase-separation of lead borate glasses under different gravity condition is also studied. The main results and conclusions are as follows:1. A new crystalline phase with the composition of Na2O-8Te02 is found and the formation and decomposition of the crystalline phase are explored. Na2O-8TeO2 crystal is stable at temperatures below 330 and will decomposed into two crystalline phase including Na2O4TeO2 and TeO2 at 340 or a higher temperature. The phase diagram of sodium tellurite is modified according to the results obtained. The transformation of Na2O-8TeO2 into Na2O4TeO2 and TeO2 includes two steps: Na2O-8TeO2 is first transformed into an intermediate amorphous phase and then the intermediate amorphous phase crystallizes to form Na2O-4TeO2 and TeO2- The intermediate amorphous phase formed during the transformation is amorphous powder.2. The factors that affect the chemical durability of Na2O4TeO2 glass in air are identified. It is found that the chemical durability of Na2O4TeO2 glass is dominated by the humidity of the surrounding air during the processes of glass melting and the following cooling. The mystery that Na2O4TeO2 glass crystallizes after drop shaft experiment, which has puzzled glass researchers for many years, is solved. This work indicates a way for making Na2O4TeO2 glass with good chemical durability.3. The effect of melting temperature and time on the properties of Na2O4TeO2 glass from different raw materials is studied and proper melting temperature and time for making Na2O4TeO2 glass with composition close to the designed one are determined. It is found that when melted at low temperature for short time, the Na2O4TeO2 glass from different raw materials will show different thermal properties. When the glass is melted at 800 for one hour, the glasses from different raw materials can have similar thermal properties.4. The phase separation of a PbO-B2O3 glass under different gravity conditions is explored. Results show that the composition and microstructure at different locations of the samples from drop shaft experiment are different with that from ground experiment. The possible mechanisms of a secondary phase separation for PbO-B2O3 glass is proposed and the shortage of the current phase diagram is pointed out.5. The evaporation of sodium tellurite glass in low gravity is explored and the formation and growth processes of TeO2 microspheres are explained. The size of TeO2 microspheres from low gravity is much larger than that from ground experiments and it is affected mainly by the temperature of melt in the heating coil and the duration time of vapor cloud during drop experiments.6. The density, chemical durability in water, and stability against crystallization of glasses in the Na2O-TeO2 system are explored and the fragility of glass forming liquid is studied. Results show that both the density and the chemical durability of glasses decrease with the increase of Na2O content.On the other hand, the stability against crystallization of glasses increases gradually with the increase of Na2O content. Glasses containing 20~25mol% Na2O have the strongest stability against crystallization. The glass forming liquid in the Na2O-TeO2 system is moderate fragile and the fragility of glass forming liquid increases with t... |
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