Optimization of Ultrasonic Parameters and Model Development for Nondestructive CTE Measurement of LAS Ultra-Low-Expansion Glass Ceramics.

Lithium aluminosilicate (LAS) ultra-low-expansion glass ceramics are core materials for precision optical systems, whose quality and dimensional stability are critically constrained by the uniformity of the coefficient of thermal expansion (CTE). This work proposes a nondestructive ultrasonic immersion pulse reflection (UIPR) method for rapid and low-cost characterization of the CTE in LAS glass ceramics. Key parameters of the ultrasonic measurement system are optimized via finite element method (FEM) simulations and experimental validation. Employing the correlation method, the ultrasonic longitudinal wave velocity is measured in LAS glass ceramic samples with distinctly different CTE values. The proposed method achieves an ultrasonic longitudinal wave velocity measurement uncertainty of 0.49 m/s, contributing 4.78 ppb/°C to the overall uncertainty of ultrasonic CTE determination. Within the investigated sample set, a negative relationship is observed between ultrasonic longitudinal wave velocity and the mean CTE (0-50 °C). The linear fit yields a slope of -9.75736 (ppb/°C)/(m/s), with a Pearson correlation coefficient of -0.84303. Featuring noncontact and nondestructive capabilities, this method lays a solid methodological foundation for CTE evaluation and efficient iterative optimization of material fabrication processes. Meanwhile, it shows great potential for rapid full-aperture characterization of CTE uniformity in large-size LAS glass ceramics.