Aluminum nitride filled flexible silicone rubber composites for microwave substrate applications

Mechanically flexible aluminum nitride-silicone rubber composites (SAN) were prepared by hot pressing technique for different filling fractions. The effects of filler content on the dielectric, thermal and mechanical properties as well as on moisture absorption were investigated. The relative permittivity and dielectric loss of the composite were found to vary linearly with filler content. The variation in relative permittivity of SAN composites with temperature was also investigated at a frequency of 1 MHz. Theoretical modelling of relative permittivity of the composites was performed and the results were correlated with the experimental data. Among the theoretical models effective medium theory is in good agreement with experimental values of relative permittivity. The coefficient of thermal expansion and specific heat capacity of the composite were found to decrease with filler content and thermal conductivity, thermal diffusivity and the moisture absorption increased with filler loading. The SAN composite is found to be a good candidate for a thermally conductive flexible microwave substrate application.