Henry E. Hagy

Determining absolute thermal expansion of titania–silica glasses: a refined ultrasonic method

Specific test conditions for determining ultrasonic velocities in titania-silica glasses are shown to improve the experimentally established linear correlation between velocity and thermal expansion. Finer surface finish, normalized thermal history (annealing), and controlled temperature for the test specimen improve the relationship. A statistical analysis shows that 5-35 degrees C average expansion coefficients can be accurately predicted from ultrasonic velocities with a standard deviation of 1 ppb/ degrees C. Excellent agreement is established between the ultrasonic predicted expansion and test results supplied by the University of Arizona Optical Sciences Center.

Precision thermal expansion measurements on low expansion optical materials.

Recent critical applications for ultralow expansion materials have required thermal expansion measurements with very high precision and accuracy. A Fizeau interferometer employing a helium-neon laser has given a precision of +/-0.1 x 10(-6) cm/cm. A second, more rapid method consists of a rod type vitreous silica dilatometer. A stirred water bath is used for specimen temperature control and a high output, linear variable differential transformer serves as the extensometer. Precision of this method is +/-0.5 x 10(-6) cm/cm. Calibration procedures and results for several low expansion materials are discussed.

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.

Fine Annealing of Optical Glass for Low Residual Stress and Refractive Index Homogeneity

A discussion is presented on both the theoretical and practical aspects of fine annealing. The role of the glass transformation range in determining refractive index through thermal history is discussed. Necessary conditions of thermal history during annealing which lead to low residual thermal stress and high homogeneity of refractive index are outlined. Two types of annealing, rate and soak, are described and the advantages of rate annealing in achieving better homogeneity and economy of time are emphasized. A practical scheme for rate annealing of large optical glass disks is described. The use of high conductivity metal plates and peripheral heaters reduces troublesome temperature gradients. For mixed alkali glasses, a low temperature ionic structural reorganization suggests the use of an intermediate cooling rate between the glass transformation range and 150 degrees C.

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.

Review of measurement systems for evaluating thermal expansion homogeneity of Corning Code 7971 ULE

Early on in the development of glass Code 7971 ULETM, Corning recognized that applications demanded thermal expansion measurements with higher precision and accuracy than that available. Hence a program was launched starting with 10cm path laser Fizeau interferometry to establish an absolute base. Differential methods followed with large sandwich seals and a high precision vitreous silica superdilatometer. Destructive in nature, these tests led to a nondestructive ultrasonic procedure that permits absolute and differential measurements with an uncertainty of +/- 2 ppb/ degree(s)C and reliably performed at the manufacturing plant. Details of these measurement systems are described along with optional experimental variations. A principle, established with seal testing, is reviewed showing that expansion differentials in ULETM are independent of temperature. Finally, recent plant measurements of both axial and radial thermal expansion variations in ULETM boules are presented.