Experimental study for evaluating striae structure of TiO ₂-SiO₂ glasses using the line-focus-beam ultrasonic material characterization system

Striae configurations in TiO₂-SiO₂ ultra-low-expansion glasses caused by variations in TiO₂ concentration were investigated through measurement of leaky surface acoustic wave (LSAW) velocities by the line-focus-beam (LFB) ultrasonic material characterization system. LSAW velocity distributions were measured on the surface of a specimen substrate prepared from a large circular plate glass ingot produced by the direct method using a flame hydrolysis process. A subsequent procedure that included polishing the surface of the specimen to reduce the specimen thickness by 40 μm and measuring the LSAW velocity distributions on the surface was repeated five times in order to examine at least one period of the striae. TiO₂ concentration variations were clearly observed in the deposit direction of the glass ingot and in its radial direction. The maximum LSAW velocity difference over the whole examined region was 11.7 m/s, corresponding to 0.70 wt % TiO₂ concentration. The three-dimensional striae structure revealed that the striae plane in the examined region was almost parallel to the substrate surface but gently curved down in the radial direction. The plane had a slightly convex-shaped cross section layered with a striae periodicity of about 0.16 mm and a curvature radius of about 440 mm, and also existed in a circular form with a curvature radius of about 450 mm. The ultrasonic method will contribute to improvement of characteristics and homogeneity of glass associated with production-process conditions.