Numerical modeling of sintering and dehydroxylation of porous silica preform for low-hydroxyl silica glass fabrication

Abstract

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.