Effects Of Thermal Environment On Dehydroxylation Of Porous Silica Preform In Two-Step Cvd Synthesis

Two-step chemical vapor deposition (CVD) method is a promising technique for industrial fabrication of low-hydroxyl silica glass ingot, in which a porous silica preform is first synthesized by flame hydrolysis deposition, which is subsequently sintered and vitrified to form silica glass. During sintering hydroxyls can be easily removed through the small pores in porous silica preform, which is called dehydroxylation. A deep understanding of the heat and mass transport characteristics involved in the dehydroxylation process is prerequisite to its controlling. In our previous work, a numerical model was developed to simulate the sintering and dehydroxylation of porous silica preform considering heat and mass transfer in porous media at high temperatures, chemical reaction of dehydroxylation, and volume shrinkage of preform. The results revealed that the dehydroxylation effect largely depends on the temperature fields in the furnace. In present work, the influences of thermal environments, including heating curve, heater position, and crucible structure on the dehydroxylation process of porous silica preform are systematically studied by numerical methods. Based on the results, increasing temperature at either stage is helpful for hydroxyl removal as well as increasing the hydroxyl distribution uniformity. However, long-time exposure to 1500 degrees C will instead worsen the dehydroxylation effect. Compared to top or bottom heating, side heating is more beneficial to hydroxyl removal. To improve the dehydroxylation condition at the bottom of porous silica preform, a novel crucible structure is designed which shows better performance than the ordinary crucible. At last, an optimized heating scheme is proposed in this study, which obtains much better dehydroxylation effect than other cases.