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.