Thermal and electrical conductivity of carbon nanotube materials

The thermal conductivity (kappa) and electrical conductivity (sigma) of three types of carbon nanotube (CNT) materials, CNT/polymer composites, CNT films, and CNT arrays, were studied over temperature ranges spanning from 2 to 390 K.;Covalent CNT-matrix bonding was explored to reduce RK. Composites with the highest degree of direct SWCNT-epoxy bonding, achieved by reaction of charged SWCNTs with epoxy, showed small kappa-enhancements (2 to 3%), limited by low loading (0.13 mass%). The estimated reduction in RK was somewhat larger than predicted and suggests that significant enhancements could be achieved in a system with higher loading. The composites had large sigma-enhancements and CNT-epoxy bonding did not necessarily reduce sigma. As a function of temperature and voltage, sigma was consistent with the model of fluctuation-assisted tunneling.;A novel approach was developed to produce SWCNT films by filtration and transfer them to arbitrary substrates. The sheet resistances (e.g. , 400 O/□ at 85% transmittance) were competitive with films produced by more cumbersome procedures, and varied with HNO3 treatments. Thermal characterization suggests that these materials have low kappa.;For tall (2 to 6 mm) MWCNT arrays, the measured kappa (1.2 to 0.5 W m-1 K-1) and sigma (14 to 7 S cm-1 ) values at room temperature decreased with array height. These values are higher than CNT/polymer composites but, even considering the low fill fraction (< 3%), suggest relatively modest kappaCNT.;For composites of multi- or single-walled CNTs (MWCNTs or SWCNTs) in epoxy, small kappa-enhancements (a few percent) were observed at room temperature. The enhancements decreased with decreasing temperature, and at low temperature (2 to 10 K) kappa can be less than that of the neat epoxy. This was attributed to interfacial thermal resistance (RK). For MWCNT/epoxy with a range of loading, RK was found to increase with decreasing temperature. Larger kappa-enhancements (50% at 300 K) were observed for 1 mass% SWCNT/polystyrene and a similar trend of decreasing kappa-enhancement was evident from 360 to 140 K.;The overall results indicate that high kappa is difficult to achieve in CNT materials. The materials studied here suggest directions for design of electrically conductive thermal insulators.