Boron Carbide

Feb 13, 2022 | SCIENCE, Chemistry

Boron Carbide crystal formation is formed as linear rods consisting of three carbon atoms and twelve boron atoms arranged in a dodecahedral pattern. The material has a chemical formula of “B4C.” Its hardness (Mohs scale 9.3) makes it suitable for use as an abrasive. Various applications such as cutting tools also benefit from its high density. It is even being considered as a rocket fuel.

Boron Carbide

Boron Carbide

Boron Carbide Properties

Boron carbide exhibits the following properties:

High hardness; Low density; High melting point; high elasticity; chemical inertness; high neutron absorption cross-section; excellent thermoelectric properties.

The crystal structure of boron carbide is highly complex. A layer of multiple atoms composes the structure of all borides. In boron carbide, the bandgap energy is about 2.09 eV, so the c-axis runs parallel to the surface. Mid-bandgap states complicate photoluminescence spectra in the crystalline structure.

Boron carbide powder has a density of 2.5 g/cc, 99.4 per cent of its theoretical density. Boron carbide has an average hardness of about three thousand KHN. However, the literature indicates that the optimum hardness ranges from 2,800 to 3,300 KHN.

Boron Carbide should be handled with care. It is highly susceptible to thermal shocks because of its crystalline structure. Even though its chemical structure does not easily change, it does not break in contact with metal. Temperatures as high as 500°F are not a problem. Because of this, the metals in boron carbide can be cut easily by knives. A sharp blade may not perform as well due to the same reason.

Boron Carbide has a melting and boiling point of 2763 degrees Celsius. Consequently, the material is suitable for a wide range of temperatures. Its density is estimated to be 2.52 grams per cubic centimetre.

Production Methods

Boron carbide is produced as large ingots by mixing petroleum coke with boron oxide at high temperatures – 2000 degrees centigrade -. Carbon is reduced with boron anhydride in two steps using the following process:

B₂O₃ + 3CO = 2B +3CO₂
4B + C = B₄C

Above 1400°C, B2O3 reduction with carbon monoxide becomes thermodynamically feasible. The reduction temperature needs to be maintained beyond 2000°C to achieve a faster reduction rate and the formation of B4C during the second stage.

In a resistance or electric arc furnace, boric anhydride (B2O3) is carbothermically reduced at temperatures exceeding 2000°C to produce boron carbide. Arc furnace products contain about 70-75% boron, along with a high percentage of free carbon. In contrast, resistance furnaces have carbide with a boron content close to stoichiometric (78.9%).

The resulting powder is then further heated at 1500 degrees centigrade to produce fine boron carbide particles, which are then further processed for specific applications according to desired properties (e.g. hardness, neutron absorption, and so on).

Carbon /1,2 and boron anhydride can also be reduced by magnetic isothermic to form boron carbide. Due to the reduction process, very fine amorph

In a resistance or electric arc furnace, boric anhydride (B2O3) is carbothermically reduced at temperatures exceeding 2000°C to produce boron carbide. Arc furnace products contain about 70-75% boron, along with a high percentage of free carbon. In contrast, resistance furnaces have carbide with a boron content close to stoichiometric (78.9%).

Carbon /1,2 and boron anhydride can also be reduced by magnetic isothermic to form boron carbide.  Due to the reduction process, the very fine amorphous powder is obtained

The material is well suited to the fabrication of sintered products.

Manufacturing

Boron carbide can be machined as green, biscuit, or fully dense, depending on the machining process. The green or biscuit form allows for relatively easy machining into complex geometries. However, sintering is required to fully densify the material, which causes the Boron Carbide body to shrink by approximately 20%.

Due to this shrinkage, boron Carbide pre-sintering can’t be machined to very tight tolerances. Diamond tools should be used to machine/grind fully sintered material to achieve very tight tolerances.

The desired form is created by rubbing away the material with an exact diamond-coated tool/wheel. The material’s inherent hardness makes this a time-consuming and costly process.

Boron Carbide Applications

Numerous commercial applications make use of boron carbide.

Boron carbide is the material of choice for engineering applications, given its high melting point and thermal stability.

Boron carbide is used in refractory applications. It is also used as an abrasive coating material because it resists abrasion.

In military applications, boron carbide’s hardness and low-density properties are utilised in ballistics and the development of lightweight armour ceramic composites.

Boron carbide is also used in the nuclear sector as it absorbs neutron radiation absorbent. Because it offers good neutron absorption properties in both thermal and epithermal energy ranges, it is widely used in the nuclear industry as a control and shielding material.

Boron carbide is also a suitable coating material for rocket nozzle throats because of its good adhesion properties and resistance to high temperatures and chemicals. When combined with boron carbide in the aerospace industry, it produces an intense amount of heat (12400 kcal/kg).

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