Chemistry and structure of glass-ceramic materials for high precision optical applications

Abstract

Li2O–Al2O3–SiO2-based glass-ceramics are materials that are used in large volumes in applications in which low thermal expansion is the property of prime importance. Although these glass-ceramics are low-expansion materials in a broad sense, the very low-expansion characteristic for which they are famous is achieved only in certain temperature ranges. The deviation of the coefficient of thermal expansion from the ideal zero-value and the temperature ranges in which close to zero-values are achieved are dependent on the chemical composition of the glass-ceramic base glass and the ceramization conditions. Some principal rules by which the desired expansion characteristics can be adjusted are illustrated. For materials that are used in precision optical applications representative data of commercially available products are given. These data include the expansion characteristics as well as homogeneity. Recently the development of the microstructure in glass-ceramics with Zerodur-type composition has been further elucidated. It has been shown that phase separation precedes nucleation. In addition, some specially developed microstructures suggest the existence of an epitaxial relationship between the nuclei and the crystalline phase of the glass-ceramic. Examples of applications of Zerodur-type glass-ceramics together with some of the requirements for these applications are discussed.