Influence of SiO2 Addition on Properties of PTFE/TiO2 Microwave Composites

Composite substrates for microwave circuit applications have been fabricated by filling polytetrafluoroethylene (PTFE) polymer matrix with ceramic powder consisting of rutile TiO2 (D-50 approximate to 5 mu m) partially substituted with fused amorphous SiO2 (D-50 approximate to 8 mu m) with composition x vol.% SiO2 + (50 - x) vol.% TiO2 (x = 0, 3, 6, 9, 12), and the effects of SiO2 addition on characteristics such as the density, moisture absorption, microwave dielectric properties, and thermal properties systematically investigated. The results show that the filler was well distributed throughout the matrix. High dielectric constant (epsilon(r) > 7.19) and extremely low moisture absorption (< 0.02%) were obtained, resulting from the relatively high density of the composites. The ceramic particles served as barriers and improved the thermal stability of the PTFE polymer, retarding its decomposition. The temperature coefficient of dielectric constant (tau (epsilon) ) of the composites shifted toward the positive direction (from - 309 ppm/degrees C to - 179 ppm/degrees C) as the SiO2 content was increased, while the coefficient of thermal expansion remained almost unchanged (similar to 35 ppm/degrees C).