Ultrasonic microspectroscopy characterization of silica glass

Ultrasonic microspectroscopy technology, using the line-focus-beam and plane-wave ultrasonic material characterization system, is applied to characterization of silica glass. Eight types of commercial silica glasses fabricated by different production methods and conditions are used as specimens to measure their acoustic properties accurately, viz., acoustic velocity, density, elastic constant. The variations in those acoustic properties resulting from small amounts of impurities, such as hydroxyl (OH) and chlorine, incorporated during the production processes were quantitatively obtained. The longitudinal velocity, density, and elastic constant c₁₁ of silica glass with 860 ppm OH ions were less by 0.28%, 0.06%, and 0.62%, respectively, than those for OH-free silica glass, while the temperature coefficient of the longitudinal velocity was greater by 7.4%. In contrast, the leaky surface acoustic wave velocity, shear velocity and elastic constant C₄₄ of silica glass containing 1500 ppm residual chlorine were less by 0.23%, 0.30%, and 0.59%, respectively, than those of OH-free and chlorine-free silica glass, while the longitudinal velocity, elastic constant C₁₁, and density increased slightly by 0.04%, 0.10%, and 0.02%, respectively. Further, it was found that the decrease in acoustic velocity due to OH or chlorine is mainly related to the decrease in the elastic constant, which also corresponds to the decrease in the viscosity of the SiO₂ material. These results demonstrate that the ultrasonic method is very useful and effective for analyzing and evaluating both the glass properties and the production processes.