  The authors proposed a simple model for the lattice thermal conductivity of
graphene in the framework of Klemens approximation. The Gruneisen parameters
were introduced separately for the longitudinal and transverse phonon branches
through averaging over phonon modes obtained from the first-principles. The
calculations show that Umklapp-limited thermal conductivity of graphene grows
with the increasing linear dimensions of graphene flakes and can exceed that of
the basal planes of bulk graphite when the flake size is on the order of few
micrometers. The obtained results are in agreement with experimental data and
reflect the two-dimensional nature of phonon transport in graphene.
