Sub-μm c -axis Structural Domain Size of Graphene Paper Uncovered by Low-momentum Phonon Scattering

Abstract The c -axis thermal conductivity ( k c ) of graphene paper (GP) is measured from 295 K to 12.3 K and extrapolation is used to identify the residual thermal reffusivity at the 0 K limit. To uncover the structural domain size based on the residual thermal reffusivity, we develop an anisotropic specific heat model to identify the phonons that sustain the heat conduction along the c -axis. This model predicts a c -mean free path (MFP) of 165 nm for graphite at room temperature (RT), very close to the value of 146 nm by molecular dynamics (MD) modeling. For widely studied normal graphite materials, this model predicts a structure domain size of 375 nm, close to the 404 nm grain size uncovered by transmission electron microscopy. Using our model, the c -MFP induced by defect in the GP is evaluated at 234 nm based on the low-momentum phonon scattering. This structural domain size significantly exceeds the graphene flake thickness (1.68–2.01 nm) in our GP, uncovering excellent c -direction atomic structure order. By subtracting the residual thermal reffusivity, the defect-free k c and c -MFP of GP are obtained. At RT, the defect-free k c is 9.67 Wm −1 K −1 , close to 11.6 Wm −1 K −1 of graphite from the recent MD simulations.