Existence of negative permittivity in carbon coated iron nanoparticle - PDMS composites

We prepared polydimethylsiloxane (PDMS)-carbon coated iron nanoparticle (CCFeNP) composites (varying filler content) to investigate the dielectric behaviour, in the frequency range 100 Hz-100 MHz. The sample structures were studied by means of Transmission Electron Microscopy (TEM), Scanning electron microscopy (SEM) and X-ray diffraction (XRD). The ac conductivity, impedance, permittivity and dielectric loss of PDMS-CCFeNP composites were analysed. The real part of permittivity/epsilon (epsilon') for the cured composites varied inversely with frequency and switched from positive to negative. The 10 wt% composites exhibit negative permittivity at higher frequency, whereas for 50 wt% permittivity switches from a highly positive to negative at very low frequency and remained negative thereafter. The shifting of switching frequency towards lower frequencies with increasing filler loading is attributed to the increased inductive nature of the composites. The unique frequency dispersion of permittivity is explained by interband transition and Drude model (multiband nature of the electronic excitations). The well dispersed filler particles in PDMS matrix gave the high permittivity of 833 for 30 wt% composite at 1 kHz. The frequency dependence of real part of ac conductivity (sigma(ac)') and impedance studies were carried out to elucidate the capacitive-inductive transition and dielectric resonance in the composites. (C) 2017 Elsevier B.V. All rights reserved.