Microwave dielectric properties of low temperature sintered ZnWO4–TiO2 composite ceramics

The (1−x)ZnWO4–xTiO2 composite ceramics were synthesized by the conventional solid-state method. The phase formation, microstructures, element composition and microwave dielectric properties of (1−x)ZnWO4–xTiO2 composite ceramics and 0.75ZnWO4–0.25TiO2 ceramics with Li2CO3–H3BO3 addition were investigated. Biphase structure of (1−x)ZnWO4–xTiO2 composite ceramics sintered at 1160 °C/2 h was confirmed by using X-ray diffraction and the back scattering electron images. The dielectric constant εr and the temperature coefficient of resonant frequency τf values of composite ceramics increased linearly with the increase of TiO2 phase. The τf value of (1−x)ZnWO4–xTiO2 was found to be dependent on phase composition, which is related to the amount of TiO2. When x is 0.25, the τf value of (1−x)ZnWO4–xTiO2 was near 0 ppm/°C. With small amount of Li2CO3–H3BO3 addition, the sintering temperature of composite ceramics was further reduced to below 950 °C from about 1100 °C without obviously deteriorating the microwave dielectric properties, and the τf values could be adjusted with the Li2CO3–H3BO3 content. The 0.75ZnWO4–0.25TiO2 ceramics with 5.0 wt% Li2CO3–H3BO3 sintered at 950 °C/2 h exhibited good microwave dielectric properties of εr=18.99, Q×fQ×f=13,517 GHz and τf=−11.44 ppm/°C.