What is graphite?

History

Graphite is one of the most interesting elements found on the earth. It is found naturally in its mineral form as well as produced in synthetic processes. The earliest use of graphite dates back to primitive man, who used it to draw on cave walls. It was also used by Egyptians to decorate pottery. During the Middle Ages, graphite was used as a refractory to line molds for the purpose of making smoother cannon balls, which could be fired farther. Almost all of us have had it in our hands at some time or another, mostly in the form of a pencil: Graphite. The term “graphite” is derived from the Greek “graphein” which means “to write”. But the material graphite has many more exciting and unique properties, which we will explore in detail below. Furthermore, we will have a closer look at the manifold areas of application of the various types of graphite.

CHEMISTRY

Graphite and Diamond: it all starts with Carbon… Natural Graphite is one of only two naturally occurring forms of pure carbon, the other being diamonds. Graphite occurs in a two dimensional, planar molecular structure whereas diamonds have a three dimensional crystal structure. They are identical chemically – both are composed of carbon (C), but physically, they are very different. Graphite generally occurs as flakes, which are multiple layers of graphene held together by weak bonds. Natural graphite is the most stable form of carbon under standard conditions. Therefore, it is used in thermochemi-stry as the standard state for defining the heat of formation of carbon compounds. Graphite has a layer structure. In each layer each carbon atom is bound to three others. This results in a two-dimensional network of hexagons. Within each layer there are strong bonds, but between the different layers the bonds are very weak. Thus, the layers can easily be shifted against each other and even separated. This structure is the reason graphite is very soft and is even used as a lubricant. But graphite has other special properties as well.

Graphite Properties

Graphite is a non-metal but has many properties of metals. It is an excellent conductor of heat nd electricity and has the highest natural strength and stiffness of any material. It maintains its strength and tability to temperatures in excess of 3,600°C and is very esistant to chemical attack. At the same time it is one of the ightest of all reinforcing agents and has high natural lubricity.

Types of graphite

Graphite is formed naturally through the metamorphism of carbon rich materials in rock which leads to the formation of either crystalline flake graphite, fine grained amorphous graphite, or crystalline vein or lump graphite.

Amorphous Graphite

Amorphous graphite is the most abundant form of graphite. It is generally a soft, darker black colored graphite, less reflective than other varieties of natural crystalline graphite. Commonly found as “microcrystalline” particles, this variety of graphite has a lower graphitic carbon level when compared to other naturally occurring crystalline graphite varieties. Amorphous graphite is extracted using conventional coal-type mining techniques. Due to its softness, including as an additive to metallic alloys, it can be used as a lubricant or grease in many applications.

Amorphous Graphite

Amorphous Graphite

Flake graphite is the most recognizable natural graphite material. It is commonly used in lead pencils and is desirable for emerging technologies such as lithium-ion batteries. Flake graphite is found in metamorphic rock and can have a carbon range of 85-98%. Depending on the degree of weathering of the ore rock, flake graphite is mined using standard hard or soft rock mining techniques. Unlike other varieties of natural crystalline graphite, flake needs to be mechanically/chemically processed or “upgraded” to reach the necessary carbon levels for industrial applications.

Crystalline Flake Graphite

Crystalline Vein Graphite

Typical veins measure from centimeters to nearly two meters in thickness with the highest purity material being located toward the center of the vein away from contact with the wall rock. Vein graphite is mined using conventional shaft or surface methods typically used to mine vein-type deposits.

In many applications vein graphite may offer superior performance since it has slightly higher thermal and electrical conductivity, which result from its high degree of crystalline perfection. Vein graphite also has the highest degree of cohesive integrity of all natural graphite materials. High cohesive “energy” means that vein graphite is easy to mold and can be formed into solid shapes without the aid of a binder addition.

Crystalline Vein Graphite

Synthetic Graphite

Synthetic graphite, sometimes referred to as artificial graphite, is found in two forms: Primary Synthetic, and Secondary Synthetic. Primary Synthetic is produced through the high temperature treatment of specific coke precursors. Secondary Synthetic is the by-product of graphite electrode and part manufacturing. They have a dark gray to black, flat appearance. Manufactured from calcined cokes, and graphitized at temperatures approaching 3000°C, these materials offer high purity, excellent lubrication, and electrical conductivity. Synthetic graphite is used in many applications including but not limited to friction, foundry, electrical carbons, fuel cell bi-polar plates, coatings, electrolytic processes, corrosion products, conductive fillers, rubber and plastic compounds, and drilling applications.

Synthetic Graphite

Occurrence & production of graphite

Graphite is a non-metal but has many properties of metals. It is an excellent conductor of heat nd electricity and has the highest atural strength and stiffness of any material. It maintains its trength and tability to emperatures in excess of 3,600°C and is very esistant to chemical attack. At the same time it is ne of the ightest of all reinforcing agents and has high atural lubricity.

Manufacturing process of synthetic graphite

The manufacturing processes or synthetic graphite are comparable to those for ceramic materials. The solid raw materials coke and graphite are ground and mixed in mixing units with carbonaceous pitcheses to form a homogeneous mass. This is followed by shaping. Various processes are available for this purpose: isostatic pressing, extrusion, vibration molding or die molding. The pressed “green” bodies are then heated under exclusion of oxygen at about 1000 °C. During this process solid particles are formed. Graphitization – the second thermal processing step – converts the amorphous carbon into three-dimensionally ordered graphite at about 3000 °C. The graphitized molded parts are then mechanically processed into complex components.