AMORPHOUS CARBON: TYPES, PROPERTIES AND USES - CHEMISTRY - 2025

The amorphous carbon is any allotropic structures filled with carbon molecular defects and irregularities. The term allotrope refers to a single chemical element, such as the carbon atom, forming different molecular structures; some crystalline, and others, as in this case, amorphous.

Amorphous carbon lacks the long-range crystalline structure that characterizes diamond and graphite. This means that the structural pattern remains slightly constant when viewing regions of the solid that are very close together; and when they are distant, their differences become apparent.

Burning charcoal. Source: Pixabay

The physical and chemical characteristics or properties of amorphous carbon are also different from those of graphite and diamond. For example, there is the famous charcoal, a product of wood combustion (top image). This is not lubricating, and it is not shiny either.

There are several types of amorphous carbon in nature and these varieties can also be obtained synthetically. Among the various forms of amorphous carbon are carbon black, activated carbon, soot, and charcoal.

Amorphous carbon has important uses in the power generation industry, as well as in the textile and health industries.

Types of amorphous carbon

There are several criteria to classify them, such as their origin, composition and structure. The latter depends on the relationship between the carbons with sp ² and sp ³ hybridizations; that is, those that define a plane or a tetrahedron, respectively. Therefore, the inorganic (mineralogical) matrix of these solids can become very complex.

According to its origin

There is amorphous carbon of natural origin, because it is the product of oxidation and forms of decomposition of organic compounds. This type of carbon includes soot, coal, and carbon derived from carbides.

Synthetic amorphous carbon is produced by cathodic arc deposition techniques, and sputtering. Synthetically, diamond-like amorphous carbon coatings or amorphous carbon films are also manufactured.

Structure

Amorphous carbon can also be grouped into three large types depending on the proportion of sp ² or sp ³ bonds present. There is the amorphous carbon, which belongs to the so-called elemental amorphous carbon (aC), the hydrogenated amorphous carbon (aC: H), and the tetrahedral amorphous carbon (ta-C).

Elemental amorphous carbon

Often abbreviated BC or BC, it includes activated carbon and carbon black. The varieties of this group are obtained by incomplete combustion of animal and vegetable substances; that is, they burn with a stoichiometric deficit of oxygen.

They present a higher proportion of sp ² bonds in their structure or molecular organization. They can be imagined as a series of grouped planes, with different orientations in space, product of the tetrahedral carbons that establish heterogeneity in the whole.

From them, nanocomposites have been synthesized with electronic applications and material development.

Hydrogenated amorphous carbon

Abbreviated as BC: H or HAC. These include soot, smoke, extracted coal such as bitumen, and asphalt. Soot is easily distinguishable when there is a fire in a mountain near a city or town, where it is observed in the air currents that carry it in the form of fragile black leaves.

As its name suggests, it contains hydrogen, but covalently linked to carbon atoms, and not of the molecular type (H ₂). That is, there are CH bonds. If one of these bonds is released hydrogen, it will be an orbital with an unpaired electron. If two of these unpaired electrons are very close to each other, they will interact causing the so-called dangling bonds.

With this type of hydrogenated amorphous carbon, films or coatings of lower hardness are obtained than those made with ta-C.

Tetrahedral amorphous carbon

Abbreviated as ta-C, also called diamond-like carbon. It contains a high proportion of sp ³ hybridized bonds.

Amorphous carbon films or coatings with an amorphous tetrahedral structure belong to this classification. They lack hydrogen, have high hardness, and many of their physical properties are similar to those of diamond.

Molecularly, it consists of tetrahedral carbons that do not present a long-range structural pattern; whereas in diamond, the order remains constant in different regions of the crystal. The ta-C can present a certain order or pattern characteristic of a crystal, but only at short range.

Composition

Coal is organized as layers of black rock, containing other elements such as sulfur, hydrogen, nitrogen and oxygen. From here amorphous carbons arise such as coal, peat, anthracite and lignite. Anthracite is the one with the highest carbon composition of all of them.

Properties

True amorphous carbon has localized π bonds with deviations in interatomic spacing and variation in bond angle. It has sp ² and sp ³ hybridized bonds whose relationship varies according to the type of amorphous carbon.

Its physical and chemical properties are related to its molecular organization and its microstructure.

In general, it has properties of high stability and high mechanical hardness, resistance to heat and resistance to wear. Furthermore, it is characterized by its high optical transparency, low friction coefficient, and resistance to various corrosive agents.

Amorphous carbon is sensitive to the effects of irradiation, has high electrochemical stability and electrical conductivity, among other properties.

Applications

Each of the different types of amorphous carbon has its own characteristics or properties, and very particular uses.

Charcoal

Coal is a fossil fuel, and therefore it is an important source of energy, which is also used to generate electricity. The environmental impact of the coal mining industry and its use in power plants are hotly debated today.

Activated carbon

It is useful for selective absorption or filtration of contaminants from drinking water, decolorizing solutions, and can even absorb sulfur gases.

Carbon black

Carbon black is widely used in making pigments, printing inks, and a variety of paints. This carbon generally improves the strength and resistance of rubber items.

As a filler in rims or tires, it increases their resistance to wear, and protects materials from degradation caused by sunlight.

Amorphous Carbon Films

The technological use of amorphous carbon films or coatings in varieties of flat panel displays and microelectronics is growing. The proportion of sp ² and sp ³ bonds means that amorphous carbon films have optical and mechanical properties of variable density and hardness.

Likewise, they are used in anti-reflective coatings, in coatings for radiological protection, among other uses.

References

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