Investigation of dielectric relaxation phenomena and AC electrical conductivity in graphite/carbon nanotubes/engine oil nanofluids

In this paper, we report an investigation on electrical conduction mechanisms of nanofluids based on commercial engine oil loaded with graphite (Gt) and multiwalled carbon nanotubes, at different concentrations. The impedance spectroscopy technique was used to measure the resistance and capacity characterizing each sample in a frequency range 100 Hz–1 MHz and a temperature range 300–400 K. Two formalisms were used to analyze the data: (a) the electrical conductivity which has found to follow the Jonscher’s law with single and double exponents for carbon nanotube concentrations below and above the percolation threshold, respectively, and (b) the complex impedance that has permitted to identify the relaxation peaks according to the Cole–Cole model. Both the two formalisms showed that when the carbon nanotube concentration is higher than the percolation threshold, a positive temperature coefficient and a remarkable change in conductivity were observed, suggesting that the presence of the carbon nanotube greatly affect the electrical properties of the engine oil as a result of additional polarization effect induced by these nanoparticles. Furthermore, the analysis of the temperature dependence of dc conductivity and relaxation time using the Arrhenius equation indicated the addition of carbon nanotubes into engine oil increase the activation energies.