Study And Process Optimization Towards The Fabrication Of High-purity Silica Glass

Silica glass is a special type of fuctional glass possessing extrodinary properties,which serves as an important basic material in information electronics,bio-medical and national defense and military industries,etc.With the rapid development of these fileds in China,the demands for large size and high purity silica glass become more and more urgent.A two-step chemical vapor deposition(CVD)method is recently developed in industries for fabricating high purity silica glass ingot.Advantages of this method include that the raw materials are easy to obtain and silica glass with low hydroxyl or controllable hydroxyl concentrations can be produced,which bring it a broad development prospect.However,this method is still facing many technical challenges,including immature preparation technique and insufficient understandings of the key scientific problems involved in the preparation process,which restrict it from being the mainstream technology of industrial silica glass preparation.In addition,bubble defects and difficult control of thermal reshaping of silca glass products are also typical problems in silica glass industries.To handle these problems,it is necessary to fully understand the physical and chemical phenomena as well as heat and mass transport related to the preparation process,and on this basis,establish the relationship between equipment and process parameters with product quality to provide guidance for the actual preparation.According to above guidelines,typical problems in the preparation of high purity silica glass are studied in this thesis by means of theoretical analysis and numerical simulation assisted by experimental verification,which are summarized as follows.(1)Multiphysics model is built for the dehydroxylation and sintering process of porous silica preform in two-step CVD method.Numerical simulations are preformed in the configuration of an actual furnace and the variations of key parameters,including temperature,porosity,dehydroxylation reaction rate,hydroxyl concentration,gas pressure and gas velocity,are analyzed in detail to reveal the heat and mass transport characteristics involved in this process.Influences of process parameters on the dehydroxylation effect are explored,which shows that reducing the preform size or increasing heating time can not only reduce the hydroxyl concentration,but also increase the hydroxyl uniformity.(2)The escape process of bubble group in highly viscous molten quartz is studied numerically based on Euler-Lagrange multiphase model.Flow characteristics of molten quartz and bubble transport behaviour are analyzed,and the influences of temperature and flow pattern of molten quartz on bubble escaping time are studied.For the typical bubble group distribution in molten quartz,an optimized scheme is proposed to accelerate bubble escaping using forced stirring followed by short-time temperature increase,in which the bubble escaping efficiency increases by 78.8% compared to the natural escape process.(3)Induction heating model is built for the thermal reshaping process of silica glass rod,and the electromagnetic and temperature fields in the pulling-down furnace for silica glass rod are analyzed based on simulation results.The relationship between softening zone of silica glass rod and its quality is established and the influencing mechanisms of key components in the furnace on the softening zone are explored.The optimal combinations of structural parameters of key components in the pulling-down furnace are determined,and the optimization of temperature field for silica glass rod fabrication is realized.The above conclusions cover different fields in the preparation of high purity silica glass,which can provide useful guidance for the improvement and optimization of its preparation techniques.