Limiting Mechanisms of Thermal Transport in Carbon Nanotube-Based Heterogeneous Media

The outstanding thermal properties of carbon nanotubes (CNTs) have generated considerable interest and research activity. A very promising application is the use of vertically aligned arrays of CNTs as thermal interface materials (TIMs) for electronic systems. TIMs require a high thermal conductivity, a low thermal interface resistance with the adjacent microprocessor and heat sink, and significant mechanical compliance to help minimize the impact of mismatched thermal expansion coefficients. However, due to high thermal interface resistances of CNT films, more detailed measurements and improved fabrication methods are needed. This review paper presents current patents and last experimental/ computational techniques investigating heat transfer and limiting mechanisms of CNTs, CNT-thermal interface structures, CNT-polymer nanocomposites (PNCs), and thermal boundary resistance (TBR) of the CNTs and different surrounding matrices (metals, nanofluids and polymers). Effects on directional thermal conductivities of aligned CNTs in PNCs/TIMs are predicted using a random walk simulation. The TBR of CNT-matrix and inter-CNT contact significantly affect the effective thermal transport properties including anisotropy ratios. When CNT-CNT contact is significant or CNT-CNT TBR is low (relative to the CNT-matrix TBR), then heat transport is dominated by CNT-CNT contact effects, rather than CNT-matrix interfacial effects. The effects of CNT orientation, type (single-versus multi-wall), inter-CNT contact, volume fraction and TBR on the effective thermal conductivities of CNT-PNCs/ TIMs are quantified. The simulation results agreed well with reported experimental data for randomly-oriented SWNT-epoxy and polymethyl methacrylate nanocomposites. These simulation results can be very useful for developing techniques to enhance the effective thermal conductivity of composites using conductive nanomaterials embedded in matrices, and assist experimentalists in interpreting heat conduction measurements.