  Grain boundaries in graphene are inherent in wafer-scale samples prepared by
chemical vapor deposition. They can strongly influence the mechanical
properties and electronic and heat transport in graphene. In this work, we
employ extensive molecular dynamics simulations to study thermal transport in
large suspended polycrystalline graphene samples. Samples of different
controlled grain sizes are prepared by a recently developed efficient
multiscale approach based on the phase field crystal model. In contrast to
previous works, our results show that the scaling of the thermal conductivity
with the grain size implies bimodal behaviour with two effective Kapitza
lengths. The scaling is dominated by the out-of-plane (flexural) phonons with a
Kapitza length that is an order of magnitude larger than that of the in-plane
phonons. We also show that in order to get quantitative agreement with the most
recent experiments, quantum corrections need to be applied to both the Kapitza
conductance of grain boundaries and the thermal conductivity of pristine
graphene and the corresponding Kapitza lengths must be renormalized
accordingly.
