1. Introduction
Elemental
carbon exists in several forms, each of which has its own physical
characteristics. Two of its well-defined forms, diamond and graphite,
are crystalline in structure, but they differ in physical properties
because the arrangements of the atoms in their structures are
dissimilar. A third form, called fullerene, consists of a variety of
molecules composed entirely of carbon. Spheroidal, closed-cage
fullerenes are called buckerminsterfullerenes, or buckyballs, and
cylindrical fullerenes are called carbon nanotubes. Carbon has five
unique crystalline structures, namely diamond, fullerene, carbon
nanotubes or carbon nanofibers, graphene, and graphite [1, 2].
Fullerene is a zero-dimensional nano-graphitic material [3]. Carbon
nanotubes and carbon nanofibers are a one-dimensional nanocarbon or
nano-graphitic material [4]. Carbon nanotubes and carbon nanofibers
have a diameter on the order of a few nanometers to a few hundred
nanometers. Their longitudinal, hollow structures impart unique
chemical, electrical, and mechanical properties to the material [5,
6]. Graphene is a two-dimensional nano-graphitic material and graphite
is a three-dimensional graphitic material.
Bulk natural graphite is a three-dimensional graphitic material, and
each graphite particle is composed of multiple grains with grain
boundaries that demarcate adjacent graphite single crystals. Each grain
is composed of multiple graphene planes that are oriented parallel to
each other. A graphene plane in a graphite crystallite is composed of
carbon atoms occupying a two-dimensional, hexagonal lattice [7, 8].
In a given grain or single crystal, the graphene planes are stacked and
bonded through van der Waal forces in a direction perpendicular to the
graphene plane. Although all the graphene planes in one grain are
parallel to each other, typically the graphene planes in one grain and
the graphene planes in an adjacent grain are inclined at different
orientations [9, 10]. This means that the orientations of the
various grains in a graphite particle typically differ from each other.
A graphite single crystal per se is anisotropic with a property measured
along a direction in the basal plane being dramatically different than
if measured along the crystallographic c-axis direction [11, 12].
For instance, the thermal conductivity of a graphite single crystal can
be up to approximately 1,920 W/ (m·K) or approximately 1,800
W/ (m·K) in the basal plane [13, 14], but that along the
crystallographic c-axis direction is less than approximately 10
W/ (m·K) [15, 16]. Further, the multiple grains or
crystallites in a graphite particle are typically all oriented along
different and random directions [17, 18]. Consequently, a natural
graphite particle composed of multiple grains of different orientations
exhibits an average property between these two extremes.
It would be highly desirable in many applications to produce a graphitic
film containing single or multiple graphene grains, having sufficiently
large dimensions and having all graphene planes being essentially
parallel to one another along one desired direction [19, 20]. In
other words, it is highly desirable to have one large-size graphitic
film having the c-axis directions of all the graphene planes being
substantially parallel to one another and having a sufficiently large
film length and width for a particular application [21, 22]. It has
not been possible to produce such a highly oriented graphitic film
[23, 24]. Even though some attempts have been made to produce the
so-called highly oriented pyrolytic graphite through tedious, energy
intensive, and expensive chemical vapor deposition followed by
ultra-high temperature graphitization [25, 26], the graphitic
structure of the highly oriented pyrolytic graphite remains inadequately
aligned and laden with defects and, hence, exhibits properties that are
significantly lower than what are theoretically predicted.
The constituent graphene planes of a graphite crystallite in a natural
or artificial graphite particle can be exfoliated and extracted or
isolated to obtain individual graphene sheets of carbon atoms provided
the inter-planar van der Waals forces can be overcome [27, 28]. An
isolated, individual graphene sheet of carbon atoms is commonly referred
to as single-layer graphene [29, 30]. A stack of multiple graphene
planes bonded through van der Waals forces in the thickness direction
with an inter-graphene plane spacing of approximately 0.3354 nanometers
is commonly referred to as a multi-layer graphene [31, 32]. A
multi-layer graphene platelet has up to 300 layers of graphene planes,
but more typically up to 30 graphene planes, even more typically up to
20 graphene planes, and most typically up to 10 graphene planes [33,
34]. Single-layer graphene and multi-layer graphene sheets are
collectively called nanographene platelets [35, 36]. Graphene or
graphene oxide sheets or platelets are a new class of carbon
nanomaterial that is distinct from the zero-dimensional fullerene, the
one-dimensional carbon nanotubes, and the three-dimensional graphite.
Graphene is a two-dimensional form of crystalline carbon, either a
single layer of carbon atoms forming a hexagonal lattice or several
coupled layers of this honeycomb structure. Graphene is a parent form of
all graphitic structures of carbon. However, the field of graphene
science and technology is relatively new. Progress depends not only on
the basic science but also on the development of new ways to produce
graphene on an industrial scale. The present study is focused primarily
upon the thermal transport characteristics of graphene nanoribbons. The
thermal transport characteristics of graphene nanoribbons are studied
using molecular dynamics simulations and by experimental measurements. A
specific heat flux is imposed through the graphene nanoribbon. The
graphene nanoribbon is considered as a single layer of carbon atoms with
each atom bound to three neighbors in a honeycomb structure. The thermal
conductivity is determined from the temperature gradient obtained and
the heat flux imposed. The present study aims to provide a fundamental
understanding of the thermal transport properties of graphene
nanoribbons. Particular emphasis is placed upon the effect of various
factors on the thermal conductivity of graphene nanoribbons under
different conditions.