Dielectric Droperties Of Polypropylene/SiO₂Composites And Polyvinylidene Fluoried/Modified BaTiO₃Composites

Polypropylene(PP)/SiO2composites were prepared through hot pressing, where the critobalite, quartz and amorphous SiO2phases were used as the inorganic fillers, and their surface morphology, relative density, thermal properties and microwave dielectric properties were characterized. With increasing the volume faction of SiO2from0to0.6, the properties of all the three composites show the similar tendency—the relative density and coefficient of thermal expansion decrease, the dielectric constant and dielectric loss increase, and the temperature coefficient of resonant frequency decreases. Comparing with the PP/amorphous SiO2composite, the other two composites show better comprehensive properties. This is mainly because of low relative density caused by large quantity of air holes in raw amorphous SiO2powder. Good comprehensive properties can be obtained for the composites with cristobalite and quartz as the filler at the filler volume fraction of0.5as listed below:εr=2.85,δ=0.0024(at15GHz), τf=16ppm/℃, CTE=75ppm/℃for PP/cristobalite composites, and εr=3.11,tan δ=0.0026(at15GHz), τf=19ppm/℃, CTE=96ppm/℃for PP/quartz composites. Modified conventional parallel model which took porosity into consideration has been identified as the best model to predict the dielectric constant of polypropylene/SiO2composites.Polyvinylidene fluoride (PVDF)/modified BaTiO3(BTO) composites were prepared by hot pressing, and the dielectric properties were investigated. The relative density decreases with increasing of volume fraction of BTO. The dielectric constant of the PVDF-BTO composites at1kHz increases significantly with increasing the volume fraction of BTO up to0.5, while it decreases for higher volume fraction of BTO due to the low relative density. Good temperature stability of dielectric constant is obtained for the composites, which benefits from the temperature-stable dielectric properties of the modified BaTiO3. With increasing of temperature, two peaks for dielectric loss are observed. The peak at lower temperature is caused by β-transition of PVDF, and a typical dielectric relaxation is observed, which fits the Vogel-Fulcher fitting; while, the dielectric relaxation peak at higher temperature becomes less obvious with increasing the volume fraction of BTO. For PVDF-BTO composites with volume fraction of BTO less than0.4, the dielectric relaxation of the peak at higher temperature fits the Arrhenius fitting.