Figure 7. Effect of temperature on the thermal conductivity of the
monolayer graphene nanoribbon with different transverse edge termination
states.
The effect of the number of layers on the thermal conductivity of the
monolayer graphene nanoribbon is illustrated in Figure 8 with different
transverse edge termination states. Graphene typically refers to a
material having less than about 10 graphitic layers. The graphitic
layers are characterized by an infinite two-dimensional basal plane
having a hexagonal lattice structure and various edge functionalities,
which may include, for example, carboxylic acid groups, hydroxyl groups,
epoxide groups and ketone groups. Graphene nanoribbons are a special
class of graphene, which are similarly characterized by a
two-dimensional basal plane, but with a large aspect ratio of their
length to their width. In this regard, graphene nanoribbons bear
similarity to carbon nanotubes, which have a comparable large aspect
ratio defined by one or more layers of graphene sheets rolled up to form
a cylinder. The graphene nanoribbons can include various layers. For
instance, the graphene nanoribbons may include a single layer. In some
cases, the graphene nanoribbons may include a plurality of layers. In
some cases, the graphene nanoribbons include from about one layer to
about eight layers. In some cases, the graphene nanoribbons include from
about second layers to about ten layers. In some cases, the graphene
nanoribbon layers have interlayer spacings of more than about 0.2
nanometers. In some cases, the graphene nanoribbon layers have
interlayer spacings of 0.34 nanometers or larger. The graphene
nanoribbons may be derived from various carbon sources. For instance,
the graphene nanoribbons may be derived from carbon nanotubes, such as
multi-walled carbon nanotubes. In some cases, the graphene nanoribbons
are derived through the longitudinal splitting of carbon nanotubes.
Various methods may be used to split carbon nanotubes to form graphene
nanoribbons. Carbon nanotubes may be split by exposure to potassium,
sodium, lithium, alloys thereof, metals thereof, salts thereof, and
combinations thereof. For instance, the splitting may occur by exposure
of the carbon nanotubes to a mixture of sodium and potassium alloys, a
mixture of potassium and naphthalene solutions, and combinations
thereof. In some cases, the graphene nanoribbons are made by the
longitudinal splitting of carbon nanotubes using oxidizing agents or by
the longitudinal opening of carbon nanotubes. The thermally conductive
materials may also utilize various graphene nanoribbons. For instance,
the graphene nanoribbons include, without limitation, functionalized
graphene nanoribbons, pristine graphene nanoribbons, doped graphene
nanoribbons, graphene oxide nanoribbons, reduced graphene oxide
nanoribbons, reduced graphene oxide flakes, and combinations thereof.