Electrical and Thermal Experimental Characterization and Modeling of Carbon Nanotube/Epoxy Composites

The present work investigates the effect of carbon nanotube (CNT) inclusions on the electrical and thermal conductivity of a thermoset epoxy resin. The characterization of electrical and thermal conductivity of CNT/epoxy composites is presented. Pristine, oxidized, and fluorine-functionalized unpurified CNT mixtures ("XD grade") were dispersed in an epoxy matrix, and the effect of stirring rate and pre-curing of the epoxy on the dispersion of the CNTs was evaluated. In order to characterize the dispersion of the CNTs at different length scales, Optical Microscopy (OM), Raman Spectroscopy, and Scanning Electron Microscopy (SEM) was performed. Samples of varying CNT weight fractions were fabricated in order to find the effect of CNT weight fraction on thermal and electrical conductivity. Electrical conductivity was measured using a dielectric spectrometer, and thermal conductivity was determined by a transient plane source thermal analyzer. It was found that electrical conductivity increases by orders of magnitude for the pristine and oxidized XD CNT composites relative to the neat epoxy matrix, while fluorinated XD CNT composites remain electrically non-conductive. A small, but significant, increase in thermal conductivity was observed for pristine, oxidized, and fluorinated XD CNT composites, showing a linear increase in thermal conductivity with increasing CNT weight fraction. Pristine XD CNTs were ball-milled for different times in order to reduce the degree of agglomeration and entanglement of CNTs, and composites were fabricated using the same technique as with non-milled XD CNTs. Using ball-milled CNTs shows improved dispersion but results in an electrically non-conductive composite at the CNT weight fractions tested. The thermal conductivity of the ball-milled CNT samples shows an initial increase higher than that of non-milled pristine, oxidized, and fluorinated XD CNTs, but remains constant with increasing CNT weight fraction. A micromechanics model based on the composite cylinders method was implemented to model the electrical and thermal conductivity of the CNT/epoxy composites. Nanoscale effects in electrical and thermal conduction, such as electron hopping and interface thermal resistance, respectively, were incorporated into the model in order to accurately predict the acquired results. Modeling results show good agreement with acquired experimental results