Sintering of Mullite-Silica Ceramics and Some Properties for Insulating Substrate Material

Abstract

The sinterability of SiO2-Al2O3 powders containing 20, 30, 40wt% Al2O3, synthesized by hydrolysis of metal alkoxide, was examined. The relationship between the chemical composition and the properties, such as thermal, electrical and mechanical properties, of the sintered bodies has been studied. These powders densified to almost full density at 1300°-1350°C for 4h without forming cristobalite. When heated above 1350°C, however, crystallization of cristobalite was recognized which resulted in the increase in thermal expansion of the sintered body due to α-β phase transformation of cristobalite. The densification rate increased with increasing SiO2 content in the powder. The sintered bodies consisted of very fine acicular mullite particles and glassy matrix. The aspect ratio of the mullite particle increased with the decrease in Al2O3 content. The dielectric constants, thermal expansion coefficients and flexural strength of the sintered bodies containing no cristobalite phase were 4.3-5.3 (at 1MHz), 1.8-2.9×10-6/C (30°-400°C) and 124-201MPa, respectively. These values seem to be more advantageous in application for the insulating substrate material than those of conventional alumina ceramics. The above mentioned values increased with the increase in Al2O3 content which determines the amount of mullite phase in the sintered body. Consequently, the optimum property values can be designed by adjusting Al2O3/SiO2 ratio in the starting powder.