Yangyu Guo

The classical Fourier's law fails in extremely small and ultrafast heat conduction even at ordinary temperatures due to strong thermodynamic nonequilibrium effects. In this work, a macroscopic phonon hydrodynamic equation beyond Fourier's law with a relaxation term and nonlocal terms is derived through a perturbation expansion to the phonon Boltzmann equation around a four-moment nonequilibrium solution. The temperature jump and heat flux tangential retardant boundary conditions are developed ba

The single mode relaxation time approximation has been demonstrated to greatly underestimate the lattice thermal conductivity of two-dimensional materials due to the collective effect of phonon normal scattering. Callaway's dual relaxation model represents a good approximation to the otherwise ab initio solution of the phonon Boltzmann equation. In this work we develop a discrete-ordinate-method (DOM) scheme for the numerical solution of the phonon Boltzmann equation under Callaway's model. Heat

The coherent quantum effect has become increasingly important in the heat dissipation bottleneck of semiconductor nanoelectronics with the characteristic size recently shrinking down to a few nanometers scale. However, the quantum mechanical model remains elusive for anharmonic phonon-phonon scattering in extremely small nanostructures with broken translational symmetry. It is a long-term challenging task to correctly simulate quantum heat transport including anharmonic scattering at a scale rel

The understanding and modeling of heat transport across nanometer and subnanometer gaps, where the distinction between thermal radiation and conduction becomes blurred, remains an open question. In this work, we present a three-dimensional atomistic simulation framework by combining the molecular dynamics (MD) and phonon nonequilibrium Green's function (NEGF) methods. The relaxed atomic configuration and interaction force constants of metallic vacuum nanogaps are generated from MD as inputs into

A phonon hydrodynamic equation has been recently derived from the kinetic theory of phonons for nanoscale heat transport at ordinary temperatures. The classical irreversible thermodynamics is no longer valid due to the failure of the local equilibrium hypothesis from temporal and spatial strong nonequilibrium effects. In the present paper, we investigate the thermodynamic consistency of the phonon hydrodynamic equation for heat transport based on the phonon kinetic theory. The macroscopic expres

Based on the Boltzmann transport equation, we demonstrate that the thermal conductance per unit width of a sufficiently thin polar nanofilm supporting the propagation of surface-phonon polaritons along its surfaces is independent of the material properties and is given by $12z(3){k}_{B}^{3}{T}^{2}/c{h}^{2}$, where ${k}_{B}$ and $h$ are the respective Boltzmann and Planck constants, while $c$ is the light speed in vacuum, $T$ is the temperature, and $z(3)$ is the Riemann zeta function. The huge p

The understanding of hydrodynamic heat transport in finite-sized graphitic materials remains elusive due to the lack of an efficient methodology. In this paper, we develop a computational framework enabling an accurate description of heat transport in anisotropic graphite ribbons by a kinetic theory approach with full quantum mechanical first-principles input. A unified analysis of the size scaling of the thermal conductivity in the longitudinal and transverse directions of the system is made wi