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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°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. [ABSTRACT FROM AUTHOR]

Dehydroxylation of porous silica preform plays a key role in two-step chemical vapor deposition (CVD) method for low-hydroxyl silica glass fabrication, the good control of which requires a clear understanding of the heat and mass transport characteristics involved in this process. For this purpose, a coupled numerical model is developed in this paper to simulate the sintering and dehydroxylation of porous silica preform, which considers the main phenomena including dehydroxylation reaction, heat and mass transfer in porous silica preform at high temperatures and shrinkage of porous silica preform. Numerical simulations are performed with this model in the configuration of a practical industrial heating furnace. Model verification is carried out by comparing the simulated hydroxyl concentration with experimental measurements at the end of sintering, which shows reasonable agreements. Based on the simulation results, variations of key parameters during heating, including temperature, porosity, hydroxyl concentration, gas pressure and gas velocity are analyzed in detail. Different transport modes of water vapor in porous silica preform are compared which shows that diffusion is the dominant way for its expelling. According to the sensitivity analysis, activation energy and frequency factor of dehydroxylation reaction have significant influences on the simulated hydroxyl concentration. Heat transfer coefficient and intrinsic permeability has little influences on the variation history of hydroxyl concentration, but has certain influences on the final hydroxyl distribution. Finally, the effects of preform size and heating curve on the dehydroxylation effect are determined, which could provide useful guidance for better controlling of the dehydroxylation of porous silica preform. [ABSTRACT FROM AUTHOR]