**3. Carbon allotropes**

Allotropy is the property of an element that allows it to exist in multiple physical states. Carbon can be divided mainly into two types of allotropes which are as shown in the **Figure 1**:


*Introductory Chapter: Brief Scientific Description to Carbon Allotropes – Technological Perspective DOI: http://dx.doi.org/10.5772/intechopen.107940*

**Figure 1.** *Flow chart of allotropes of carbon.*

Carbon is not the only element that has the ability to mix its atoms to produce allotropes. Silicon, germanium, and tin, all of which are found in the 4th column of the periodic table, share this property. Carbon, on the other hand, is distinctive in terms of its amount and allotrope's variety that it contains, due to its propensity to have variable oxidation states and coordination numbers [2].

#### **3.1 Crystalline carbon allotropes**

Carbon has been thought to have life, in mainly two crystalline allotropic forms for a long period of time: graphite and diamond, which are consist of extensive networks of sp3 and sp2 hybridized carbon atoms, respectively. As a result, we can say that diamond as well as graphite are the two most frequent allotropic forms of carbon. Both kinds have unique physical characteristics. In spite of that, the chemical composition of these two compounds differs significantly in terms of crystal shapes and characteristics [10–14]. Many other types of carbon allotropes may be produced chemically because of valence shell of the carbon atoms [15], which are chemically identical (same building block) but have dramatically different physical properties, such as buckminsterfullerene [16], also known as "Bucky-balls" [2, 17] carbon nanotubes [18], graphene [19, 20], and so on [8]. The discovery of fullerenes by "Harold W. Kroto "in 1985 signaled the start of an era of artificial carbon allotropes that included, in 1991 the production of (CNTs) and in 2004 again the discovery of graphene [20]. Therefore nowadays, carbon is significantly more than charcoal, because it produces the strongest fibers, most excellent lubricants (graphite), the strongest crystal and hardest substance (diamond), optimal gas absorbers (charcoal-gray), and perfect helium gas barriers [2].

#### **3.2 Amorphous carbon allotropes**

Amorphous carbons and carbons with mixed phases exist alongside to crystalline carbon allotropes [17]. Carbon without crystalline structure is referred to as

amorphous form of carbon. Like all other glassy materials, some (specific short-range patterns) may be seen but not any (specific long-range pattern) of the atomic positions are observed. The majority of amorphous carbon is made up of tiny lattices of either graphite or diamond like carbon. This Non-crystalline carbon refers to coal, soot, or carbon black colloquially [17].
