  Thermal rectifiers are devices that have different thermal conductivities in
opposing directions of heat flow. The realization of practical thermal
rectifiers relies significantly on a sound understanding of the underlying
mechanisms of asymmetric heat transport, and two-dimensional materials offer a
promising opportunity in this regard owing to their simplistic structures
together with a vast possibility of tunable imperfections. However, the
in-plane thermal rectification mechanisms in 2D materials like graphene having
directional gradients of grain sizes have remained elusive. In fact,
understanding the heat transport mechanisms in polycrystalline graphene, which
are more practical to synthesize than large-scale single-crystal graphene,
could potentially allow a unique opportunity, in principle, to combine with
other defects and designs for effective optimization of thermal rectification.
In this work, we investigate the thermal rectification behavior in periodic
atomistic models of polycrystalline graphene whose grain arrangements were
generated semi-stochastically to have different gradient grain-density
distributions along the in-plane heat flow direction. We employ the centroidal
Voronoi tessellation technique to generate realistic grain boundary structures
for graphene, and the non-equilibrium molecular dynamics simulations method is
used to calculate the thermal conductivities and rectification values.
Additionally, detailed phonon characteristics and propagating phonon spatial
energy densities are analyzed based on the fluctuation-dissipation theory to
elucidate the competitive interplay between two underlying mechanisms, namely,
(1) propagating phonon coupling and (2) temperature-dependence of thermal
conductivity that determine the degree of asymmetric heat flow in graded
polycrystalline graphene.
