Dependence of dielectric properties on the crystallinity of ceramic/polymer composites

Effects of ZnNb₂O₆ content and crystallinity of polymers on the dielectric properties of ZnNb₂O₆/polytetrafluoroethylene (PTFE), polypropylene (PP), and polystyrene (PS) composites were investigated at microwave frequencies. With increasing ZnNb₂O₆ content, the dielectric constant (K) of the composites increased, whereas the dielectric loss (tanδ) and temperature coefficient of resonant frequency (TCF) decreased. The tanδ of the composites with amorphous PS was lower than those of the composites with semi-crystalline PP and PTFE. For the composites with semi-crystalline PTFE and PP, the tanδ was strongly dependent on the degree of crystallinity of composites. Several types of theoretical models were applied to predict the effective dielectric properties of the composites. Typically, K of 5.73, tanδ of 1.45 x 10^-3, and TCF of 2.66 ppm/°C were obtained for the PP composites with 0.5 volume fraction V_f of ZnNb₂O₆.