  In the hydrodynamic regime, phonons drift with a nonzero collective velocity
under a temperature gradient, reminiscent of viscous gas and fluid flow. The
study of hydrodynamic phonon transport has spanned over half a century but has
been mostly limited to cryogenic temperatures (~1 K) and more recently to
low-dimensional materials. Here, we identify graphite as a three-dimensional
material that supports phonon hydrodynamics at significantly higher
temperatures (~100 K) based on first-principles calculations. In particular, by
solving the Boltzmann equation for phonon transport in graphite ribbons, we
predict that phonon Poiseuille flow and Knudsen minimum can be experimentally
observed above liquid nitrogen temperature. Further, we reveal the microscopic
origin of these intriguing phenomena in terms of the dependence of the
effective boundary scattering rate on momentum-conserving phonon-phonon
scattering processes and the collective motion of phonons. The significant
hydrodynamic nature of phonon transport in graphite is attributed to its strong
intralayer sp2 hybrid bonding and weak van der Waals interlayer interactions.
As a boundary-sensitive transport regime, phonon hydrodynamics opens up new
possibilities for thermal management and energy conversion.
