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Super-Accurate Velocity Measurement for Evaluating TiO2–SiO2 Ultra-Low-Expansion Glass Using the Line-Focus-Beam Ultrasonic Material Characterization System

A method of improving the measurement accuracy of leaky surface acoustic wave (LSAW) velocity for TiO2-doped SiO2 ultra-low-expansion glass using the line-focus-beam ultrasonic material characterization (LFB-UMC) system was investigated theoretically and experimentally. The frequency dependence of the interference waveform attenuation in a V(z) curve obtained for the glass was calculated by considering the propagation attenuation of LSAWs. The theoretical results revealed that the waveform attenuation depends primarily on acoustic energy loss due to the water-loading effect on the specimen surface, and that the waveform attenuation decreases with decreasing frequency. Significant improvement of the measurement accuracy was successfully demonstrated by using an LFB ultrasonic device with a larger curvature radius R of the cylindrical sapphire acoustic lens, R=2.0 mm, yielding an improved value of ±0.0020% obtained at 75 MHz, as compared to ±0.0053% obtained at 225 MHz with a cylindrical lens of R=1.0 mm.