  Phonons in graphitic materials exhibit strong normal scattering
(N-scattering) compared to umklapp scattering (U-scattering). The strong
N-scattering cause collective phonon flow, unlike the relatively common cases
where U-scattering is dominant. If graphitic materials have finite size and
contact with hot and cold reservoirs emitting phonons with non-collective
distribution, N-scattering change the non-collective phonon flow to the
collective phonon flow near the interface between graphitic material and a heat
reservoir. We study the thermal resistance by N-scattering during the
transition between non-collective and collective phonon flows. Our Monte Carlo
solution of Peierls-Boltzmann transport equation shows that the N-scattering in
graphitic materials reduce heat flux from the ballistic case by around 15%,
30%, and 40% at 100, 200, and 300 K, respectively. This is significantly larger
than ~ 5% reduction of Debye crystal with similar Debye temperature (~ 2300 K).
We associate the large reduction of heat flux by N-scattering with the
non-linear dispersion and multiple phonon branches with different group
velocities of graphitic materials.
