In this article, we shall talk in detail about graphite an allotrope of carbon explore its properties, crystal structure, and application. We have also provided a PDF for the same.
What is Graphite
Graphite is a naturally found layered structure of carbon. Specific pressure and temperature conditions lead to its formation. The process goes as beginning with the deposition of carbon-rich substances such as organic sediments in the metamorphism of organic matter in rocks or sedimentary basins.
Over a time period, these rocks or sediments undergo intense pressure and heat treatment deep within the crust of the earth. The temperature is high generally around 750 -1500 0C. Such high temperatures cause a transformation where the rearrangement of carbon atoms into a hexagonal lattice structure. This transformation takes place at a pressure range of 1- 3 Gigapascals
Such conditions cause the carbon atoms to bond together very strongly within each layer establishing a stable sheet of hexagonal structure. The forces holding these layers are weak and cause them to easily slide past each other. This is responsible for the softness and lubricating properties of graphite. The formation process is quite long as geological processes slowly transform the carbon-rich materials
How graphite is obtained
Mining processes such as open pit or underground methods are conducted to obtain graphite from the earth. Mining operation involves the extraction of graphite consisting of ores from the ground. After the ore is obtained it undergoes a beneficiation process to separate the graphite element from the neighboring gangues (rock).
The beneficiation process goes by first handpicking the gangue pieces and screening the product manually. An alternate way for extraction is, the rock is crushed first, and by a flotation process, the graphite is floated. There are some drawbacks to the floatation process during crushing the softness property of graphite tends to leave marks on the gangue materials leading to an impure concentrate
To obtain a commercial product or concentrate, we employ two approaches for the same. The first method is where grinding and floatation are performed repeatedly sometimes even up to 7 times for purification purposes. The second method is acid leaching wherein the gangue is submerged in hydrochloric acid (for carbonate gangue) or hydrofluoric acid (for silicate gangue).
The graphite products and concentrates obtained after the mining process are classified into groups based on their flake sizes. Coarser flake-sized ones are preserved carefully. These are the ones below 8 mesh or 8-20 mesh even 20-50 mesh. Standard blends are prepared from different fractions each with specific carbon content and flake size distribution. Custom blends are prepared based on the individual client requirements
Depending on the end application the graphite processing techniques are varied. These techniques spread from using fine powder in oil coatings for foundry molds to oil drilling and carbon raisers in the steel industry. Powdered petroleum and synthetic graphite powders also serve their purpose as carbon raisers. Its milling procedures have adverse environmental effects as soil contamination due to powder spillages or air pollution caused by fine particulate exposure to workmen.
Talking about the reserves of naturally occurring graphite in the world in 2016 was approx 1.2 million tonnes with China being the largest exporter at 7.8 million which was then followed by India, Brazil, Turkey, and North Korea. Its mining is not done widely in the USA but historic data shows that states like Alabama, Adirondacks, Montana, etc. As per data found in 2010, the USA produces 134K tonnes of synthetic graphite which is valued at 1.1 billion approximately.
Crystal Structure of Graphite
Graphite is a part of the dihexagonal dipyramidal crystal class because it has a hexagonal crystal structure. They usually exist in two groups P63/m mc (flat) and P63 mc (buckled). The unit cell parameters of graphite are a = 2.461 Ã… and c = 6.708 Ã…, with a total of 4 carbon atoms in the unit cell.
Graphite is found in different forms such as granular or compacted masses or tabular six-sided foliated masses. They are found in two colors steel-gray or iron-black. But it also appears blue when observed in transmitted light. Its cleavage is perfect on the {0001} plane and also exhibits twinning.
The physical properties are it is flexible and sectile with a flaky structure. Its luster is metallic or earthy and they exhibit a low Mohs hardness of 1 to 2. The density of graphite is 2.09-2.23g/cm3 whereas the specific gravity of graphite ranges from 1.9- 2.3. The streak of graphite is opaque and black only the thin flakes are exceptions due to being transparent.
Talking about other unique characteristics, I would like to mention its strong anisotropic properties meaning its properties change with different crystallographic directions. Due to its layered structure, it is an excellent conductor of electricity. It leaves marks on the surface and has a greasy feel to it.
In comparison to other allotropes of carbon such as diamond or amorphous carbon, it tends to have its pros and cons while offering high thermal stability, lubricating properties, and excellent conductivity for its application in high-temperature environments and lubrication needs it also has certain cons like less durability compared to diamond, the tendency to leave marks on the surface, etc.
Properties of Graphite
Graphite exhibits several unique properties
- Phase transition
- Anisotropic Properties
- Thermal Stability and Conductivity
- Lubricating Properties
- Pyrolytic Graphite
Under specific temperature and pressure conditions diamond and graphite can transition between each other. At normal temperature and pressure diamond is metastable and graphite is a stable phase of carbon. At temperatures above 4500 kelvin diamond quickly starts converting to graphite. Due to its anisotropic nature, its properties vary as per the directions. Phonons that are responsible for acoustic and thermal properties quickly travel within the tightly bounded graphite planes but are slow when propagating between planes.
It is suitable for applications like refractories and electrodes in high-temperature applications due to its high thermal stability and electrical conductivity properties. It also possesses lubrication properties due to its loose intermolar coupling between sheets where it can be dry lubricating and self-lubricating and helps to reduce friction
The property of good electrical conductivity allows its usefulness in carbon microphones and lamo electrodes. The ability of electrical conduction lies in its plane structure. It can also promote corrosion in certain materials such as galvanic corrosion between two dissimilar metals or pitting corrosion in stainless steel. It can also corrode aluminum metal in the presence of moisture which limits its applications in a few domains.
When crystallographic defects bind the graphite planes together, it loses its lubricating properties and becomes pyrolytic graphite. Pyrolytic graphite is highly anisotropic and diamagnetic.
It’s important to note that natural and crystalline forms are not commonly used as pure structural materials due to their brittleness, shear planes, and inconsistent mechanical properties.
Uses of graphite
- Refractories: It is used in creating materials such as carbon magnesite bricks or alumina-graphite shapes which are used for high-temperature scenarios like foundry facing or steelmaking
- Batteries: Both the forms synthetic and natural serve utilization in batteries as anode materials mainly in lithium-ion batteries and nickel-metal hydride which are used for EV cars and portable electronic equipment.
- The molten state of this material is used for lubrication purposes for extruding hot steel.
- Brake linings or shoes are made from the natural form of these materials making use of asbestos non-existent
- Paints based on these materials are used as mold washes in the foundry to offer easy separation of cast objects.
- It is used to make pencil leads, lubricant for mining machinery, and locks.
- Graphene is used in the production of acoustic drivers in headphones due to their better quality and efficiency
- It can be expanded to create graphite foil, which is used for insulation, firestops, gaskets, heat sinks, and other high-temperature applications.
- Intercalated form creates intercalation compounds with various substances, such as metals and small molecules, which have applications in superconductivity and as an anode material in lithium-ion batteries.
Conclusion
In conclusion, it is a native mineral consisting of layered sheets of carbon atoms arranged in a honeycomb lattice, exhibiting several remarkable properties. Its anisotropic nature allows for high thermal stability and electrical conductivity along the layers, making it suitable for various applications such as electrodes and refractories. Graphite’s self-lubricating and dry lubricating properties have industrial value, although the mechanism behind its lubrication is still under study. However, its tendency to oxidize at high temperatures and promote corrosion in certain environments should be noted.
While it offers advantages like phase transition behavior and conductivity, its use in structural applications is limited due to brittleness. Overall, it showcases a unique combination of properties that contribute to its widespread utility in diverse fields.
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