Phonon resonance effect and defect scattering in covalently bonded carbon nanotube networks

Covalently bonded carbon nanotube (CNT) networks offer promising potential for heat-dissipation applications due to their low interfacial thermal resistivity between connected CNTs. In this work, the thermal-transport properties of covalently bonded CNT networks were simulated by molecular dynamics. It was found that the thermal conductivity (TC) of the networked CNTs is periodic dependent. Although the TCs are reduced compared to those of pristine CNTs, they are considerably larger than the values in thermal interface materials. The TC reduction with respect to the pristine CNTs primarily stems from defect scattering at junctions and resonant scattering generated by CNTs oriented perpendicular to the transport direction. The two mechanisms operate over different frequency ranges and collectively contribute to a reduction in both phonon group velocity and relaxation time across the entire frequency range. Moreover, we demonstrate that increasing the network period in a specific direction increases the TC along this direction, while reduces TC in the perpendicular direction due to intensified resonant coupling. Such directional-dependent TC variations with period facilitate the regulation of thermal-transport anisotropy within CNT networks. Overall, our findings elucidate the underlying phonon transport mechanisms in CNT networks and offer valuable insights into the design of thermal interface materials, thermal insulation materials, and materials with tailored thermal anisotropy. By leveraging the identified mechanisms, it becomes possible to develop CNT-based materials with enhanced heat-dissipation capabilities and engineered TC profiles.