Nano- and microsize effect of CCTO fillers on the dielectric behavior of CCTO/PVDF composites

The microstructure and dielectric properties of composites comprising polyvinylidene fluoride (PVDF) and calcium copper titanate (CCTO) particles have been investigated. Nano- and microsized CCTO were employed separately and investigated comparatively. The effective dielectric constant (εr) of the composite containing 40 vol.% nanosized CCTO filler is over 106 at 102 Hz and room temperature, which is substantially higher than that of the composite containing microsized CCTO, of which the εr value is 35.7 (with 40 vol.%). The εr and loss tangent (tan δ) decrease with temperature for the composite containing nanosized CCTO, while the one with microsized CCTO shows the opposite tendency. For the composite with nanosized CCTO, the conductivity decreases sharply with increasing temperature in the low frequency range (100–104 Hz) and slightly increases in the high frequency range, while the conductivity of the composite with microsized CCTO is nearly independent of temperature. The theoretical calculations demonstrate that the activation energies of the composites containing nano- or microsized CCTO are −0.52 and 0.051 eV, indicating active interfaces and insulated grain boundaries in these two composites, respectively. Theoretical analysis also shows that the dielectric performance of the composite with nanosized CCTO does not follow the conventional mixing rules and the giant dielectric constant comes mainly from the interfacial polarization. The dielectric property of the composite containing microsized CCTO matches well with the Maxwell–Garnett and effective medium theory models, indicating insulate interfaces between the fillers and the matrix. The results obtained in this study indicate that the composite containing microsized CCTO may be suitable for embedded device applications, while the one with nanosized CCTO may find a new application in the temperature sensor field.