Polydimethylsiloxane-Multiwalled Carbon Nanotube Nanocomposites as Dielectric Materials: Frequency, Concentration, and Temperature-Dependence Studies

Abstract

Polydimethylsiloxane/multiwalled carbon nanotube (PDMS/MWCNT) nanocomposites with different amounts of MWCNT nano-filler (1 wt.%, 2 wt.%, 4 wt.%, 5 wt.%, 6 wt.%) were prepared by mechanical mixing using a Brabender and two roll mixer. The surface and bulk properties, and dispersion of the nanofillers in the nanocomposite matrix were studied using scanning and transmission electron microscopy (SEM and TEM); the average diameter of the MWCNTs in the PDMS matrix was ∼ 25 nm. Variable-temperature complex impedance analysis (CIA) showed that the impedance decreased with the frequency, MWCNT concentration, and temperature from 10⁸ Ω to 10⁵ Ω, demonstrating the possibility of increasing the electrical conductivity of the nanocomposites. The dielectric permittivity ($\varepsilon^{{\prime }}$) decreased with frequency from 800 to 52 (6% MWCNT), and increased from 52 to 430 (at 1 Hz) with MWCNT doping and from 430 to 1870 (at 1 Hz) with temperature, attributed to interaction of the nanofillers inside the PDMS matrix and the positive temperature coefficient (PTC) effect. Electrical conductivity was observed in both the direct current (DC) and alternating current (AC) region, and increased from 10⁻⁴ S cm⁻¹ to 10⁻² S cm⁻¹ with frequency (6% MWCNT) and from 10⁻⁷ S cm⁻¹ to 10⁻⁴ S cm⁻¹ with the MWCNT concentration due to the hopping or tunneling mechanism. The PTC increased with temperature given the conductive nature of the filler and positive temperature coefficient effect. Percolation studies of the dielectric permittivity and electrical conductivity as a function of frequency at different temperatures (30°C, 50°C, 70°C, 100°C) showed that the threshold limit of the MWCNTs in the PDMS matrix was 4%.