Boron Carbide: Structure, Properties, & Preparation

Boron Carbide is a boron-carbon ceramic that is used to build tank armor, bulletproof vests, etc. It has a very high melting point. When boron oxide is reduced with carbon, it forms Boron Carbide. A high temperature and an electric furnace are required to conduct this process. Boron Carbide is one of the hardest materials known to humans. If boron carbide catches fire, toxic fumes are released. Boron Carbide is black in color. Its chemical formula is B₄C. It is used in a wide range of engineering applications. It has no smell and is insoluble in water. In terms of hardness, boron carbide comes after cubic boron nitride and diamond. It is a covalent substance that has a lot of industrial and automobile applications.

Key Takeaways: Boron Carbide, Boron, Carbon, Boron Nitride, Diamond, Melting point, Carbon Dioxide, Icosahedron, Octahedron, Noble Gases

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Structure of Boron Carbide

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The structure of boron carbide is as shown below:

Structure of Boron Carbide

Boron Carbide Crystal Structure

Boron Carbide has a complex crystal structure. It contains borides that are centered on icosahedrons. A rhombohedral lattice unit is formed by the B₁₂ icosahedra around a C-B-C chain. It is present in the middle of the unit cell. They have three neighboring icosahedra. The role of carbon atoms is to act as a bridge for them. A network plane is formed by the bridging carbon atoms, along with the B₁₂  icosahedra. It runs parallel to the C-plane and forms a layered structure.

The two fundamental structural units of the lattice are- B₁₂ icosahedron and B₆ octahedron. It is very hard to bind the octahedra as it is very small in size. Instead, they bind to B₁₂ icosahedra in the neighboring layers. This, in turn, weakens the c-plane bonding. The ideal boron carbide has a chemical formula B₁₂C₃. The deficiency of carbon in boron carbide can be explained in terms of B₁₂C₃ and B₁₂C units. It happens because of the structural unit-B₁₂.

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Physical Properties of Boron Carbide

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Some of the physical properties of Boron Carbide are:

• Boron Carbide has a density of 2.52 g/cm³.
• It has a melting point of 2445 °C.
• It has a boiling point of 3500 °C.
• Boron Carbide has a hardness ranging between 2900 - 3580Kg / mm²
• It has a fracture toughness of 2.9 - 3.7 MPam- 1/2.
• Young’s modulus value of Boron Carbide is 450 - 470 GPa.
• At 25 °C, boron carbide has an electrical conductivity of 140 S.
• At 25 °C, boron carbide has a thermal conductivity of 30 - 42 W/m.K.
• It has a thermal neutron capture of 600 barns.

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Chemical Properties of Boron Carbide

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The chemical properties of boron carbide are as follows: 

• Boron carbide is tough in nature. It has a toughness ranging from 9.5 to 9.75. It is measured using the Mohs hardness scale. In 2015, Boron carbide was identified as the third hardest material known to humankind. It comes third to diamond and cubic boron nitride.
• It has a large cross-section that is used for neutron absorption.
• Boron carbide has a resistance that ionizes radiation along with many chemicals.
• It is also known as a black diamond.
• Boron Carbide is a semiconductor. It has a hopping-type transport that dominates its electronic properties.
• We determine the energy band gap of Boron carbide using the composition and degree of order.
• The band gap of boron carbide is 2.09 eV.

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Preparation of Boron Carbide

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Henry Moissan first synthesized boron carbide in 1899. The procedure took place in an electric arc furnace. Boron trioxide was reduced by mixing it with carbon or magnesium. In case of carbon, the reaction takes place at a temperature higher than the melting point of B₄C. This reaction releases great amounts of carbon monoxide. If magnesium is used, the byproducts are extracted using the acid treatment. Graphite crucible is used to perform the experiment.

\(\color{red}{2\mathrm{B_2C_3} + 7\mathrm{C} \rightarrow \mathrm{B_4C} + 6\mathrm{CO}}\)

Preparation of Boron Carbide

A 95% pure boron can be obtained from the oxide. Powdered magnesium is combined to achieve this.

\(\color{red}{\mathrm{B}_{2} \mathrm{O}_{3}+3 \mathrm{Mg} \rightarrow 2 \mathrm{~B}+3 \mathrm{MgO}}\)

When a tantalum filament is heated and a mixture of BCl3 or BBr3 is passed through it, we can obtain a high-purity boron. The purity will be > 99.9%.

\(\color{red}{2 \mathrm{BBr}_{3}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{~B}(\mathrm{~s})+6 \mathrm{HBr}(\mathrm{g})}\)

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Reaction of Boron Carbide

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Boron carbide powder is oxidized at low temperatures in the presence of water vapor. It happens at temperatures as low as 250°C. It can also be oxidized in the absence of water vapor. This reaction occurs at 450°C.

Air Oxidation:

\(\color{red}{\mathrm{B}_{4} \mathrm{C}+4 \mathrm{O}_{2} \rightarrow 2 \mathrm{~B}_{2} \mathrm{O}_{3}(\mathrm{I})+\mathrm{CO}_{2}}\)

Water Oxidation:

\(\color{red}{\begin{aligned} &\mathrm{B}_{4} \mathrm{C}+8 \mathrm{H}_{2} \mathrm{O} \rightarrow 2 \mathrm{~B}_{2} \mathrm{O}(\mathrm{I})+\mathrm{CO}_{2}+8 \mathrm{H}_{2} \\ &\mathrm{~B}_{4} \mathrm{C}+6 \mathrm{H}_{2} \mathrm{O} \rightarrow 2 \mathrm{~B}_{2} \mathrm{O}(\mathrm{I})+\mathrm{C}+6 \mathrm{H}_{2} \end{aligned}}\)

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Uses of Boron Carbide

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Since it is one of the hardest materials known to humans, it has a lot of uses. The nanoparticles are used for the purpose of coating and in nuclear reactions.

• It is used to shape hard surfaces.
• Boron Carbide is used in water jet cutters. This equipment is used to cut metals or similar material using a high pressure and velocity jet of water.
• Boron carbide powder is used to coat materials to avoid wear and tear.
• It is also used as a neutron absorber in nuclear reactors.
• It is used to shape and polish work pieces. The work pieces should be made from metals or other hard materials.
• Boron Carbide is used to manufacture padlocks and portable locks.
• Boron Carbide is used in manufacturing grit blasting nozzles.
• It is used in vehicles to line the brakings.

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Preventive Measures

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One should always be very careful when working with boron carbide. It is a toxic material that causes serious damage. The casualties can be managed but only if the actions are taken on time. A few preventive measures to take care of while using Boron Carbide are:

1. If someone ingests it accidently, you should make them drink a lot of water or milk so that it gets diluted. It will induce vomiting as well. If the victim is unconscious, take them to the hospital immediately.
2. If someone inhales boron carbide accidentally, they should be moved to a well ventilated area. They should be given medical attention. Oxygen should be provided to them if they face difficulty in breathing.
3. In case boron carbide comes in direct contact of someone’s skin, make them wash the contaminated area with soap and water.

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Things to Remember

• The chemical formula of Boron Carbide is B₄C.
• It was first synthesized in 1899 by Henri Moissan.
• Boron Carbide has a molar mass of 55.255 g/mol.
• On Mohs scale, it has a hardness of 9 to 10.
• The boiling point of the material is 3500 °C.
• Boron Carbide is odorless and it has a density of 2.52 g/cm³.
• Boron Carbide is toxic in nature. If inhaled or consumed, it can cause serious damage.

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