Ultra-Low-Bubble-Density Quartz Glass Enabled by Stepwise Calcination of High-Purity Synthetic Quartz Powder

High-purity synthetic quartz powders are widely used for quartz crucibles and quartzware in semiconductor processes. However, sol-gel-derived synthetic quartz powders contain hydrogen bonds on pore interiors and surfaces, inducing bubble formation during quartz glass fusion. In this study, the removal behavior of hydrogen bonds depending on calcination temperature was investigated by mass reduction, Brunauer-Emmett-Teller (BET) specific surface area, tap density, and Fourier transform infrared (FT-IR) absorbance. Physisorbed and weakly hydrogen-bonded water (~3350 cm^-1), vicinal/geminal silanol (~3650 cm^-1), and isolated silanol (~3745 cm^-1) were removed in the distinct temperature ranges of 200-600 °C, 700-1000 °C, and above 1100 °C, respectively. Based on this removal behavior, a stepwise calcination process at 300 °C for 5 h, 700 °C for 5 h, and 1200 °C for 10 h was designed. This process reduced OH concentration to 3.6 ppm and decreased bubble density to 0.6 bubbles cm^-3 in fused quartz glass.