Abstract

Ultraprecise thermal expansion differentials are determined through photoelastic analysis of a symmetrical sandwich seal.¹ Seal fabrication normally employs fusion or fritting, requiring temperature cycles that can modify initial expansions of interest. Optical wringing of flat, polished surfaces at room temperature gives strong, elastic bonds for joining seal components. Observation of optical retardation in the central seal member with temperature leads to the determination of expansion differentials with a precision of ±2 ppb/°C over the −195–100°C range. Test advantages are (a) inexpensive test specimens and experimental apparatus, (b) easy, quick data collection and reduction, and (c) seals may be disassembled and components used for other combinations. If the expansion of one component is known, the absolute expansion of the other member is obtained. Test results are presented for Corning code 7971 ULE glass seals in which component parts are given different thermal histories. The absolute expansion of a low-expansion transparent glass ceramic is derived from a seal made against a sample of ULE glass with known expansion.

© 1986 Optical Society of America