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Mixed Alkali Suppression of Deleterious Alkali Effects In Ceramics And Glasses

IP.com Disclosure Number: IPCOM000048878D
Original Publication Date: 1982-Apr-01
Included in the Prior Art Database: 2005-Feb-09
Document File: 3 page(s) / 16K

Publishing Venue

IBM

Related People

Foster, LM: AUTHOR [+2]

Abstract

Ceramics and glasses are employed in a number of critical applications. Generally, the ceramics are used as substrates, and glasses for passivating structures. Ceramic structures can contain a deliberately added glassy phase to aid in sintering, increase strength, etc. "Pure" ceramics, to which no deliberate glass additions have been made, can have traces of a glass phase because of the presence of unintentional impurities, and, moreover, truly pure polycrystalline bodies exhibit "glass-like" behavior in some properties because of the highly defective and disordered structure at grain and crystalline boundaries.

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Mixed Alkali Suppression of Deleterious Alkali Effects In Ceramics And Glasses

Ceramics and glasses are employed in a number of critical applications. Generally, the ceramics are used as substrates, and glasses for passivating structures. Ceramic structures can contain a deliberately added glassy phase to aid in sintering, increase strength, etc. "Pure" ceramics, to which no deliberate glass additions have been made, can have traces of a glass phase because of the presence of unintentional impurities, and, moreover, truly pure polycrystalline bodies exhibit "glass-like" behavior in some properties because of the highly defective and disordered structure at grain and crystalline boundaries.

In essentially all uses of ceramics and glasses, it is assumed that they are stable, insulating structures that provide isolation for passive electrical circuits that are applied to them. As regards electronic conductivity, there is a high level of assurance that very long term electrical isolation is achieved in presently used materials. The migration of alkali ions to constitute an ionic current is not excluded, however, and it is to be expected that ion drift between two points of different electrical potential will occur over long periods, and that this drift could have deleterious effects on some circuits. The resulting capacitance effect would be potentially serious, for example.

Sodium is the alkali most likely to be encountered as an impurity in alumina based ceramics, and is a common constituent of glasses. All commercial grade A1(2)O(3) comes from the Bayer process for refinement of bauxite. In that process, the ore is dissolved in caustic soda to form sodium aluminate. This is hydrolyzed to form a precipitate of A1(2)O(3)xH(2)O and a supernatent caustic solution which is recycled. It is impossible to exclude all soda from the A1(2)O(3) in this process. Alternative processes of making high purity A1(2)O(3), for example, via metal-organic intermediates or by chlorine treatment to extract soda, are too expensive for large scale commercial use.

During high temperature sintering of A1(2)O(3)-based ceramics, the soda can segregate to the grain and crystallite boundaries where its concentration can reach significant levels. In ceramic bodies to which soda containing glass has been deliberately added, the glass and soda are, of course, also situated at those boundaries. The ionic conductivity of such ceramic bodies is attributed almost entirely to migration of this sodium and other alkali ions in an electrical field.

A "mixed alkali" or "poly-alkali" effect has been recognized in glasses and non-crystalline solids for many years. The effect consists of pronounced non- additivity of properties that depend on ion movement when one kind of mobile alkali ion is partially replaced by another. In the case of ionic conductivity, the mixed alkali effect is manifest as a pronounced minimum in conductivity at some intermediate composition. The conducti...