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How to Seal Using Glass-Ceramics.

Glass-Ceramic: Hard, strong, nucleated glass with a non-porous crystalline structure. It has a high flexural strength and shock resistance; used for coatings, molded mechanical and electrical parts, and heat-exchanger tubes.

The glass-ceramic method is a process by which crystalline ceramics are produced by the controlled crystallization of glasses. To accomplish this, a component that acts as a nucleating agent is added to the glass composition. Upon proper cooling, nuclei are formed in the glass melt and subsequent crystal growth on these nuclei will result in the formation of fine-grained, high-strength ceramics.

The crystalline phases produced and the resulting microstructure can be controlled by varying the nucleation and growth temperatures during the cooling cycle, making it possible to produce glass-ceramics containing different crystalline phases from the same initial glass composition.

The most important property in sealing applications is the coefficient of thermal expansion (CTE). In order to form hermetic seals to metals, the sealing material's CTE should fall between that of the outer metal housing and the conductor pin so that the seal interfaces are in compression. By designing the time-temperature profile of the sealing cycle, the CTE of the glass-ceramic can be controlled so that the initial glass composition can be used to seal to a variety of materials.

For glass-ceramic-to-metal sealing, the process involves assembling the components--metal conductor pin and housing and a sintered glass bead or preform--then firing the assembly in an inert atmosphere at temperatures near 1000 [degrees] C. By controlling the time-temperature profile of the sealing cycle, the desired CTE of the glass-ceramic can be obtained and an hermetic seal will result.

Glass-ceramic-to-metal seals have several advantages over glass-to-metal seals. Because of their crystallinity, glass-ceramics have higher strengths than glasses, making these seals more reliable. Also, due to their crystallinity, glass-ceramics are more refractory and can be used at higher temperatures. Their crystallinity also makes them more chemically durable

WEB RESOURCES FOR GLASS-CERAMICS:
www.ceramaseol.com
www.imca.aps.anl.gov
www.avs.org

Sarno is an R&D engineer at Ceramaseal.

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Author:Sarno, Richard D.
Publication:R & D
Article Type:Brief Article
Geographic Code:1USA
Date:Jun 1, 2001
Words:336
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