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What is Graphite?

1. Physical Properties
2. Chemical Properties
3. Mechanical Properties

Graphite is a common allotrope of carbon and is distinctively the most stable form of carbon (even more so than diamond) under standard conditions. It is unique in that it has properties of both a metal and a non-metal: it is flexible but not elastic, has a high thermal and electrical conductivity, and is highly refractory and chemically inert. Graphite has a low adsorption of X-rays and neutrons making it a particularly useful material in nuclear applications.


1. Physical Properties


Graphite has a melting point similar to that of diamond of around 3600°C, at which point it sublimes rather than melting. Although weaker than structural metals at room temperature, unlike typical metal behaviour it retains its structural strength at high temperatures (in fact it gains strength between 1000°C – 2500°C), it has a low co-efficient of thermal expansion and its high thermal conductivity offers excellence resistance to thermal shock.


2. Chemical Properties


The carbon atoms in graphite are arranged hexagonally in a planar condensed ring system.  The layers are stacked parallel to each other. The molecular structure of graphite is shown below by Figure 40. Each carbon atom uses three of its electrons to form strong covalent bonds to its three close neighbours, leaving a fourth electron to form a weak bond between the ‘layers’ of graphite, known as the π bond. The hexagonal geometry is exactly right for both bond types to have maximum effect, and as a result the basal plane bonds are short and strong.


These delocalised electrons are no longer associated directly with any particular atom or pair of atoms and it is due to this phenomenon that graphite is able to conduct electricity and heat, as well as making graphite ideal for lubrication.


Screen grab from Wikipedia on the Properties on graphite

(a) Elevation view and (b) plan view of the molecular structure of graphite




3. Mechanical Properties


Due to the lattice structure of the graphite layers the mechanical properties are fairly straightforward: shear forces cause the adjacent layers to slip, and it is the ease of shear that causes graphite to leave a trail of black residue (hence pencils, and the Greek origins for its name from writing).


A further consequence of graphite’s anisotropic molecular structure is the major difference between thermal expansion between in-plane and cross-plane directions: up to 400°C the basal plane of graphite shrinks with increasing temperature, although the overall lattice structure volume expands.