Download this page: doc | docx | pdf
Carbon exists in many different forms. Its allotropes are: diamond, graphite, lonsdaleite, fullerenes (CX, where X = 60, 70, 72, 76, 78, 80, 84, 540, etc., and carbon nanotubes), and amorphous carbon. It should be noted that C60, C70, C72, and so forth consitute a single allotrope of carbon called as fullerenes. The spherical form of fullerenes is called as buckminsterfullerenes. It should also be noted that carbon nanotubes (single wall and multi wall) are also types of fullerenes. So, broadly speaking, carbon has five allotropes. Graphite is one of the allotropes of carbon, which consists of layers of carbon atoms arranged in a hexagonal lattice. Each carbon layer in graphite is bonded to a carbon layer above and a carbon layer below through van der Waals interactions. Each carbon atom in graphite is sp2 hybridized. Graphite exists in two crystallographic forms: hexagonal (alpha) and rhombohedral (beta). The difference in the "stacking" of the layers of hexagonally-linked carbon atoms in graphite gives rise to hexagonal and rhombohedral crystallographic forms.
Flake graphite (often referred to as crystalline flake graphite), amorphous graphite, and vein (or lump) graphite are the three naturally occuring types of graphite. HOPG stands for Highly Ordered (or Oriented) Pyrolitic Graphite, where the stacked planes of carbon atoms are strongly aligned with very low angular spread (called as mosaic angular spread or simply mosaic spread). The lower the mosaic spread, the stronger the orientation, the better the quality of HOPG. It is used as a probe for the calibration of scanning tunneling microscopes. Graphite is a good lubricant in open atmosphere (i.e., in the presence of moisture and air), whereas it is a very poor lubricant in vacuum (i.e., in the absence of moisture and air). The lubricity of graphite depends solely on the environment. In open atmosphere (that has moisture and air), graphite adsorbs moisture and air between the layers that aid them to slide on each other with ease, and hence obtains the self-lubricity property. In vacuum it is not the case, hence it is a poor lubricant in vacuum. Graphene is a single atomic layer of sp2 hybridized hexagonally-bonded carbon atoms. It has very high electron and hole mobilities. Graphene is very strong and its ultimate tensile strength is around 200 times that of steel [ Lee et al (2008), Science 321 (5887) ]. Intercalated graphite (often referred to as graphite intercalation compound) is a compound of graphite formed by intentionally bonding atoms of different elements between the layers of graphite. Chemically, intercalated graphite is represented as XCY, where X is a different element or a molecule and Y is a stoichiometric constant. Some examples of intercalated graphite are calcium graphite (CaC6), potassium graphite (KC8), and ytterbium graphite (YbC6). Expanded graphite (also referred to as Expandable or Intumescent flake graphite) is a form of intercalated graphite is which the crystal planes of carbon atoms are moved apart resulting in the expansion (or swelling) of graphite. As a result of this event, the chemical and physical properties change. The most common of all expanded graphites is graphite bisulfate. It is prepared by soaking natural graphite flakes in chromic acid followed by concentrated sulfuric acid. Expanded graphite(s) is used as fire retardant, electromagnetic radiation shield, conductive filler material, etc. xGnP stands for Exfoliated Graphite Nanoplatelets. It consists of several graphene sheets stacked together to a total thickness between 1 and 15 nanometers (average 5 nanometers), whereas the platelet diameter ranges from sub-micron to over 100 microns. |
|
