Thermal conductivity from hierarchical heat sinks using carbon nanotubes and graphene nanosheets

The in-plane (k_ip) and through-plane (k_tp) thermal conductivities of heat sinks using carbon nanotubes (CNTs), graphene nanosheets (GNs), and CNT/GN composites are extracted from two experimental setups within the 323-373 K temperature range. Hierarchical three-dimensional CNT/GN frameworks display higher k_ip and k_tp values, as compared to the CNT- and GN-based heat sinks. The k_ip and k_tp values of the CNT/GN-based heat sink reach as high as 1991 and 76 W m^-1 K^-1 at 323 K, respectively. This improved thermal conductivity is attributed to the fact that the hierarchical heat sink offers a stereo thermal conductive network that combines point, line, and plane contact, leading to better heat transport. Furthermore, the compression treatment provided an efficient route to increase both k_ip and k_tp values. This result reveals that the hierarchical carbon structures become denser, inducing more thermal conductive area and less thermal resistivity, i.e., a reduced possibility of phonon-boundary scattering. The correlation between thermal and electrical conductivity (ε) can be well described by two empirical equations: k_ip = 567 ln(ε) + 1120 and k_tp = 20.6 ln(ε) + 36.1. The experimental results are obtained within the temperature range of 323-373 K, suitably complementing the thermal management of chips for consumer electronics.