Design of High Thermal Expansion Glass-Ceramics Through Microstructural Control

摘要

High thermal expansion glass-ceramics can be made from lithium silicate glasses that contain small amounts of P sub 2 O sub 5 . The essential step involves heating the glass to a temperature near 1000 deg C to grow well-faceted 0.1 to 0.5 mu m Li sub 3 PO sub 4 crystallites in the molten glass matrix. When the temperature is lowered to the 750 to 850 deg C range, a mixture of high expansion SiO sub 2 polymorphs and lithium silicate phases crystallize. TEM observations on one composition reveal the crystallization is by epitaxial growth on specific faces of the Li sub 3 PO sub 4 crystallites. Heat treatments that do not form faceted Li sub 3 PO sub 4 crystallites give glass-ceramics with much lower expansion coefficients. Our understanding and control of this epitaxial crystallization and growth mechanism has led to the development of a family of high thermal expansion glass ceramics useful for making hermetic seals with a number of stainless steels and superalloys. (ERA citation 12:013777)