Comparison of two high-precision nondestructive measurement methods for evaluating thermal expansion differences in the 8.3-m ultralow-expansion Subaru primary mirror blank.
The building blocks for fabricating the 8.3-m Subaru primary mirror blank are ultralow-expansin solid hexagonal units that were subjected to rigorous ultrasonic examination to establish thermal expansion characterization as reported by Hagy [Appl. Opt. 12, 1440 (1973)]. Following assembly of the mirror by fusion and fine annealing, photoelastic analyses at hex-to-hex seals were used to calculate thermal expansion differences. These differences are found to be in excellent agreement with the ultrasonically established differences.
High precision photoelastic and ultrasonic techniques for determining absolute and differential thermal expansion of titania-silica glasses.
Photoelastic analysis of a large fused sandwich seal, composed of two glasses with slight composition differences, permits reliable measurement of the thermal expansion difference to 0.0002 ppm/ degrees C. If the expansion of one of the glasses is known, the method becomes absolute. For the titania-silica system, it is shown that for any two compositions with small titania differences the expansion coefficient differential remains constant from -195 degrees C to 925 degrees C. A direct correlation between ultrasonic velocity and thermal expansion has been established for this glass system and has led to successful nondestructive measurements. With an experimentally defined relationship, the measurement of the ultrasonic velocities yields absolute or differential expansions. Excellent agreement with seal testing data is shown to exist with differential data taken by both methods.
Rheological Behavior of Glass
The viscosity of glass receives much attention in both academic and manufacturing circles of endeavor. It is a property that is very sensitive to composition differences and hence helps the theoretical scientist understand glass structure and, at the same time, gives the manufacturing team a property suitable for quality control. However, quality control is only one aspect of importance that viscosity has in glass manufacture and applications. It is used in melting, forming, annealing, tempering, ionexchange, and sealing.