Development of Steatite Based Machinable Ceramic Insulators for Ultra High Vacuum System

Abstract

Developmental work on machinable ceramics was carried out with compositions containing steatite and pyrophyllite with minor additions such as barium carbonate and calcium oxide. Cold pressed compacts of the mixtures of selected constitutents, after seasoning with a phospho-organic binder and curing at 800°C resulted in a machinable character. After machining, the samples had the desired electrical breakdown strength (13 kV/cm) and degassing rate of 1 x to-• torr.lit.sec-1 .cm-•. The roles of raw materials, the type of binder used and curing temperature on the development of machinable character of the steatite based ceramic compositions have been discussed in the present paper. IntroductionCeramic insulators play a very important role in electrical and electronic equipment including large scale op:!rations like transmission as well as efficient working of electronic devices like hybrid circuits which depend on the quality of ceramic substrates. When ceramic insulators are to be used in an ultra high vacuum (UHV) system, they have to satisfy stringent specifications, e.g. dimensional tolerance and degassing characteristics. Degassing is directly related to the efficiency of operation of UHV systems, because, in general, ceramic components used as insulators are seldom impervious. Insulating parts used in UHV systems are of intricate shape and need to have accurate dimensions within the given tolerance. Insulators made from machinable ceramics can fulfill the stringent requirements of UHV systems provided the composition and processing conditions are suitable to develop machinable character.Machinable ceramics based on various ceramic systems, such as Al 2 0 3 -Si0 2 (porcelain) and complex glass ceramic &ystems like Mg0-AI 2 0 3 -Si0 2 are selected on the basis of property requirements and application. Machinable ceramics based on alumina are preferred particularly for high temperature, high vacuum equipment. Looking into the requirements of UHV systems, an in-house development, it was felt necessary

Phase Analysis and Densification of Steatite-based Ceramics

“…For industrial use, SBC usually is in the form of the enstatite phase, whose density is 3.21 g/cc and melting point is 1557C (Chao, 1994). Some studies have shown that densification could be improved by doping steatite with boron oxide, aluminum oxide, antimony oxide or by substituting magnesium ions (Tripathi et al, 1978;Kulkarni et al, 1990;Soykan, 2007). In all these studies, stabilization of the protoenstatite phase was achieved but frequently resulted in grain coarsening and low mechanical strength.…”