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Properties of Carbon Bulk Materials: Graphite and Diamond

Kamatchi Jothiramalingam Sankaranand Ken Haenen

Institute for Materials Research (IMO), Hasselt University & IMOMEC, IMEC vzw, Diepenbeek, Belgium

1.1 Introduction

Carbon is unique and without doubt one of the most multipurpose elements well known to man, as can be realized by the fact that it is the source of life on Earth. As the fourth most ample chemical element in the universe by mass, carbon is the second most abundant element by mass in the human body. Its name originates from Latin carbo and French charbon meaning charcoal [1,2]. It is broadly dispersed in nature and found in the Earth's crust mostly in the form of compounds [3,4]. Since 5000 BCE, charcoal and soot, well-known carbon materials, have been typically acquired from wood and employed for several potential applications, for example, in the production of iron. Several synthetic carbon materials are also produced from the natural form of carbon compounds such as coals, hydrocarbon complexes, and gaseous hydrocarbons.

Carbon has directed the general science movement, supporting as models of structural perfection that have motivated the synthesis of other materials with related structures and symmetries [5]. Combining strong bonds with light mass and high melting point, the condensed carbon phases have several exclusive properties making them technologically essential and scientifically interesting. Modern research on carbon nanomaterials has contributed to two Nobel Prizes for fullerenes and graphene materials and two Kavli Prizes in Nanoscience on carbon nanotubes. Carbon is imperative for countless technological applications, extending from drugs to synthetic materials. Amongst the applications as summarized in Figure 1.1 [6] are, but not restricted to, aerospace, automobile, medical, defense components, energy, and household associated applications, etc. [7-12]. The use of carbon materials in the global energy scene is a subject that has fascinated many in modern times.

Basically, the element carbon is denoted by symbol C with atomic number 6. It has four electrons in its valence shell (outer shell). The electron configuration of carbon is 1s22s22p2. Each carbon atom can share electrons with up to four different atoms because the energy shell of carbon can hold eight electrons. Carbon atoms can contribute to many different crystal structures, all of which reveal diverse properties [13]. Classical examples of carbon allotropes are "hard" diamond with sp3-hybridized carbon and "soft" graphite with sp2-hybridized carbon. These two carbon allotropes have their own unique physical and chemical properties in terms of hardness, thermal and electrical conductivities, and lubrication behavior. Conceptually, many other carbon allotropes are also constructed by altering the periodic arrangements of sp3-, sp2-, and sp-hybridized carbon atoms [14]. However, diamond and graphite were the only well-known carbon allotropes for a long time. The first chapter of this book briefly discusses the properties of these two allotropes.

Figure 1.1 Various types of applications of carbon nanomaterials in relation to their properties [6].

1.2 Graphite

1.2.1 History

At standard conditions, graphite is known as the most stable form of carbon. In the beginning of history, it was utilized in the construction of refractory crucibles and the application of graphite began in the late Central European Iron Age. Graphite was mined near Passau (Germany) and utilized to blacken pottery [15]. Later on, the first pencils were manufactured in England in 1550. Graphite was first named as "plumbago," which means "acts (writes) like lead." In 1779, plumbago was analyzed by C.W. Scheele and he demonstrated that plumbago was a form of carbon, not of lead. The term "graphite" originated from the Greek word "graphein" meaning "to write," which was named by A.B. Werner in 1789 [16,17]. At the beginning of the twentieth century, an enormous increase of utilizing graphite as an electrode in electrolytic and electrothermic processes took place [18].

Figure 1.2 (a) sp2-Hybrid orbital, (b) crystal structure, and (c) band structure of graphite [14].

1.2.2 sp2 Hybridization

One s orbital and two p orbitals, i.e. px and py, combine to form sp2 hybridization. These orbitals contribute together to a planar assembly (Figure 1.2a) [14] with a characteristic angle of 120° between the hybrid orbitals forming a s bond. The addition of the pz orbital perpendicular to the sp2 hybrid orbitals forms a p bond. A typical example of an sp2-hybridized crystal structure is graphite, where the carbon atoms are linked together in giant plane networks which are layered parallel to each other (Figure 1.2b). In each layer carbon atoms are organized in a hexagonal lattice. The lattice parameter is 0.142?nm and the distance between planes is 0.335?nm. Each layer of graphite consists of p orbitals, which give rise to weak van der Waals forces between the layers [19]. Figure 1.2c shows the band structure of graphite. The valence and conduction band of graphite consist of bonding (p orbital) and antibonding (p* orbitals), which meet at the K point. Because of this reason, graphite is described as a semimetal [20].

1.2.3 Structure of Graphite

1.2.3.1 Hexagonal Graphite

Hexagonal graphite is the most common form of crystalline graphite with a stacking sequence of -ABABAB-. The carbon atoms in every other layer are superimposed over each other. The crystallographic description of hexagonal graphite is known by the space group D46h-P63/mmc with unit cell constants of a = 45.6?pm and c = 670.8?pm, respectively. Hexagonal graphite is known as the thermodynamically stable form of graphite approximately at 6?GPa and at 2600?K and is found in all synthetic graphite materials.

Figure 1.3a shows a view of the stacking sequence perpendicular to the basal plane of hexagonal graphite [21]. Atoms of the a-type presented with open circles possess neighbor atoms in the adjacent planes directly above and below. Atoms of the ß-type in full circles possess no parallel atoms in these planes.

1.2.3.2 Rhombohedral Graphite

Rhombohedral graphite is another allotrope of graphite. It has a stacking order of -ABCABCABC- and the stacking sequence is perpendicular to the basal plane of rhombohedral graphite, which is viewed in Figure 1.3b [21]. Every third layer of the carbon atoms are superimposed. The crystallographic representation of rhombohedral graphite is acknowledged by the space group D53d-R3m along with its crystal lattice parameters a0 = 0.2256?nm and c0 = 1.006?nm, respectively.

Figure 1.3 Schematic of (a) hexagonal, (b) rhombohedral, and (c) polycrystalline-turbostratic graphite crystal.

Source: Adapted from Yaya et al. 2012, and Dasgupta and Sathiyamoorthy 2003 [21,22].

This form of graphite is thermodynamically unstable. It does not exist in pure form and combines always with hexagonal graphite as an extended stacking fault. It may exist in natural and synthetic graphite up to 40%. During heat treatment above 1300?°C, rhombohedral graphite generally returns to the hexagonal form. Notably, in both rhombohedral and hexagonal graphite structures, no basal plane lies directly over another.

1.2.3.3 Polycrystalline Graphite

Actual polycrystalline graphite is an aggregate of graphite crystallites. For example, pyrolytic graphite, vitreous carbon, carbon black, carbon-fiber-carbon matrix composites, etc. The layer planes of these polycrystalline graphites may or may not be absolutely parallel to each other that depends whether it is graphite or nongraphite.

The aggregates of graphite crystallites have considerably different sizes and properties. Soot comprises exceptionally small crystallites and hence its properties are generally associated to the surface area. But in pyrolytic graphite, aggregates are defect free, reasonably large, and parallel to each other and hence, its structure and properties are closely coupled with the perfect graphitic crystal.

On the other hand, some aggregates of crystallites, such as turbostratic or amorphous graphite, possess random orientation and their bulk properties are fundamentally isotropic (Figure 1.3c) [22].

1.2.3.4 Crystallite Imperfections

All graphites contain defects within their structures [23]. Each crystallite contains many imperfections, which include vacancies, stacking faults, and disclination.

Screw dislocations and edge dislocations are other crystalline imperfections which occur due to growth defects. These imperfections cause a significant impact on the properties of...