LOW-DIELECTRIC COMPOSITIONS WITH CONTROLLED CRYSTALLIZATION AND THERMAL-EXPANSION

Cristobalite is formed in an initially amorphous low-dielectric lead b orosilicate glass at 700-900 degrees C. With added alumina content gre ater than a critical value, however, the cristobalite formation in the lead borosilicate glass is completely inhibited. The critical alumina content is found to decrease with decreasing alumina particle size bu t with increasing sintering temperature. This result is attributed to a strong coupling reaction between Al3+ from alumina and Na+ and Pb2from lead borosilicate glass. The resulting cristobalite-free glass co mposite has a dielectric constant of 5.0 at 1 MHz and a thermal expans ion coefficient of (2.5 - 4.0) x 10(-6) K-1 in the temperature range o f 25-200 degrees C.