Synthesis and properties of BaO{center_dot}AL{sub 2}O{sub 3}{center_dot}2SiO{sub 2}-SrO{center_dot}Al{sub 2}O{sub 3}{center_dot}2SiO{sub 2}(BAS-SAS) ceramics

Conference · Tue Aug 01 04:00:00 EDT 1995

OSTI ID:86594

Talmy, I G; Haught, D A; Zaykoski, J A ^[1]

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1. Naval Surface Warfare Center, Silver Spring, MD (United States); and others

Ceramics based on BaO{center_dot}AL{sub 2}O{sub 3}{center_dot}2SiO{sub 2} (celsian, BAS) and SrO{center_dot}Al{sub 2}O{sub 3}{center_dot}2SiO{sub 2} (SAS) are attracting considerable interest for a variety of applications due to their unique combination of high refractoriness, low thermal expansion, low dielectric constant and loss tangent both stable over a broad range of temperatures and frequencies. Dielectric ceramics, electronic packaging, and structural ceramics are all possible applications for BAS and SAS. Celsian is the only ternary compound in the BaO-AL{sub 2}O{sub 3}{center_dot}SiO{sub 2} system. It exists in two main crystalline modifications: monoclinic, stable up to 1590{degrees}C and hexagonal, stable from 1590{degrees}C to the melting temperature (1760{degrees}C). Even though the hexagonal modification (hexacelsian) is the high temperature modification, it tends to be the first product of synthesis (solid- and gaseous state reactions, melt crystallization, sol-gel process, and oxidation of metals) and persists metastably throughout the whole temperature range. Hexacelsian reversibly transforms at 300{degrees}C into the low temperature orthorhombic modification. Transformation of hexagonal celsian into the desirable monoclinic form is promoted by prolonged high-temperature heating, hydrothermal treatment, and by the presence of impurities or the addition of certain additives (such as B{sub 2}O{sub 3}LiF, Cr{sub 2}O{sub 3}, ZrSiO{sub 3}). The additives or impurities can adversely affect all properties of the ceramics, particularly their dielectric behavior and high-temperature mechanical properties. Strontium aluminosilicate (SAS) with a melting point of 1710{degrees}C exhibits similar polymorphism. This article summarizes the results of research conducted at the Naval Surface Warfare Center (Dahlgren Division) on synthesis, processing and characterization of ceramics based on the BAS-SAS system.

• OSTI ID: 86594
• Report Number(s): CONF-940416--
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ALUMINIUM OXIDES
BARIUM OXIDES
DIELECTRIC PROPERTIES
MECHANICAL PROPERTIES
MICROSTRUCTURE
SILICA
SILICATES
SILICON NITRIDES
STRONTIUM OXIDES
THERMAL EXPANSION
WHISKERS