Ultralow Thermal Conductivity and Thermal Diffusivity of Graphene/Metal Heterostructures through Scarcity of Low-Energy Modes in Graphene
Weidong Zheng, Bin Huang, Yee Kan Koh
Abstract
In many ultralow thermal conductivity materials, interfaces of dissimilar materials are employed to impede heat flow perpendicular to the interfaces. However, when packed within a distance comparable to the phonon wavelengths, these interfaces are coupled and thus ineffective to scatter low-energy phonons, due to either coherent phonon transmission across the closely packed interfaces or weak coupling of the low-energy phonons and the interfaces. Here, we propose to block the propagation of these low-energy phonons by periodically distributed scarcity of available low-energy phonon modes using graphene/metal heterostructures of transferred graphene and ultrathin metal films. We demonstrate the effectiveness of graphene in blocking propagation of low-energy phonons by comparing the effective transmission probabilities of phonons in a wide range of multilayered structures; we find that interfaces in our graphene/metal heterostructures remain decoupled even when the spacing between interfaces is <2 nm. With the proposed strategy, we successfully achieve an ultralow thermal conductivity of Λ = 0.06 W m–1 K–1 and a world-record lowest thermal diffusivity of α = 2.6 × 10–4 cm2 s–1 suitable for thermal insulation. Moreover, we demonstrate the capability to tune the electronic heat transport across the new materials by creating atomic-scale pinholes on graphene through magnetron sputtering, with electrons carrying ≈50% of heat when Λ is ≈0.15 W m–1 K–1. With the ultralow Λ and substantial electronic transport, the new graphene/metal heterostructures could be explored for thermoelectric applications.