Experimental Reexamination of Thickness-Dependent Phonon Transport in Graphite

In recent research, hydrodynamic phonon transport in graphitic materials has been demonstrated unexpectedly higher thermal conductivities compared to bulk materials through theoretical and experimental approaches. Phonon hydrodynamics, which considers the collective motion of phonons akin to a viscous fluid (Poiseuille flow), results in extremely high thermal conductivity. Graphitic materials are promising candidates for observing hydrodynamic phonon transport due to a large density of states of low-frequency flexural phonons, which results in the predominance of momentum-conserved scattering processes (normal scattering). In this study, a comprehensive experimental investigation was conducted to explore the thickness-dependent thermal conductivity of suspended graphite ribbons over a range of temperatures. The measurements, which utilized frequency-domain thermoreflectance (FDTR), were performed on samples varying from bulk to a few tens of nanometers in thickness. Taking into account the anisotropic nature and size, the measured results are fitted using finite element method. Additionally, a residue-free fabrication technique for graphite samples was employed to minimize undesirable defect scattering processes. The existence of phonon Poiseuille flow within a specific thickness range of the samples was confirmed through the calculations based on the phonon Boltzmann transport equation, incorporating Callaway’s dual relaxation model and first-principles calculation results. This provides not only experimental validation of phonon hydrodynamics, but also a deeper understanding of the mechanisms underlying this behavior. Our findings highlight the crucial role of boundary and defect scattering processes in phonon hydrodynamics.