Double Dielectric Relaxation of Oil Nanofluids with Graphite and Carbon Nanotubes

Nanofluids generated by the addition of various fractions of multi-walled carbon nanotubes (MWCNTs) and 0.5 wt% of graphite (Gt) in commercial engine oil have been electrically studied at frequencies ranging from 100 Hz to 1 MHz and temperatures from 300 to 400 K. The analysis of the electrical resistivity of the prepared nanofluids has shown a percolation threshold of 0.6 wt% and revealed a positive temperature coefficient in resistance (PTCR) and negative temperature coefficient in resistance (NTCR) effects, in temperature ranges below and above the critical temperature point, T c ≈350 K, respectively. The electric modulus has shown two dielectric relaxations for the nanofluids containing MWCNTs, the first one at low frequency related to the interfacial polarization known as MWS-relaxation, while the second one appearing at high frequency is attributed to the interaction between MWCNTs and dipolar molecules of additives. The data of real and imaginary parts of the complex electric modulus were fitted using the Havriliak-Negami model.