Ultralow and anisotropic thermal conductivity in graphene phononic metamaterials

Abstract

Phononic metamaterials based on the idea of phonon coherent resonance attract increasing research attention because of their unique properties. In this paper, we have studied the heat transport in graphene phononic metamaterials (GPMs, pillared graphene nanoribbons) using molecular dynamics simulations. The results show that, GPMs have lower thermal conductivities in the different in-plane directions and stronger anisotropy in thermal conductance, compared with graphene nanoribbons (GNRs). As the temperature decreases or the height and width of pillar increases, the GPM thermal conductivity in both directions decreases and the anisotropic ratio increases. The pillar-driven suppression of thermal conductivity along the periodic direction is attributed to the phonon localization and the local resonance leading to the flat bands and band gaps, while the reduction in thermal conductivity along the aperiodic direction is due to the phonon boundary scattering. The thermal anisotropy results from the anisotropic phonon relaxation times and the anisotropic contributions of low-frequency-phonons. Our findings reveal the phonon transport mechanisms of GPMs in different directions, and shed some light on the GPM potential for thermoelectric and directional heat management applications.