BORON NITRIDE (BN): STRUCTURE, PROPERTIES, PRODUCTION, USES - PHYSICAL - 2025

The boron nitride is an inorganic solid formed by the union of a boron atom (B) with a nitrogen atom (N). Its chemical formula is BN. It is a white solid that is highly resistant to high temperatures and is a good conductor of heat. It is used, for example, to make laboratory crucibles.

Boron nitride (BN) is resistant to many acids, however, it has a certain weakness to attack by hydrofluoric acid and by molten bases. It is a good insulator of electricity.

Structure of boron nitride (BN). Akeramop. Source: Wikimedia Commons.

It is obtained in various crystalline structures, of which the most important are hexagonal and cubic. The hexagonal structure resembles graphite and is slippery, which is why it is used as a lubricant.

The cubic structure is almost as hard as diamond and is used to make cutting tools and to improve the toughness of other materials.

Boron nitride can make microscopic (extremely thin) tubes called nanotubes, which have medical applications, such as transporting within the body and releasing drugs against cancer tumors.

Structure

Boron nitride (BN) is a compound where boron and nitrogen atoms are covalently bonded with a triple bond.

An isolated boron nitride molecule has a boron atom and a nitrogen atom joined by a triple bond. Benjah-bmm27. Source: Wikimedia Commons.

In the solid phase, BN is made up of an equal number of boron and nitrogen atoms in the form of 6-membered rings.

Resonance structures of a BN ring. Author: Teachi. Source: Wikimedia Commons.

BN exists in four crystalline forms: hexagonal (h-BN) similar to graphite, cubic (c-BN) similar to diamond, rhombohedral (r-BN), and wurtzite (w-BN).

The structure of h-BN is similar to that of graphite, that is, it has planes of hexagonal rings which have alternating boron and nitrogen atoms.

Structure in the form of separate planes of hexagonal boron nitride. Benjah-bmm27. Source: Wikimedia Commons.

There is a great distance between the planes of h-BN which suggests that they are joined only by van der Waals forces, which are very weak forces of attraction and the planes can easily slide over each other.

For this reason, h-BN is creamy to the touch.

The structure of cubic BN c-BN is similar to diamond.

Comparison between cubic boron nitride (left) and hexagonal (right). from: Benutzer: Oddball, vector version by chris 論. Source: Wikimedia Commons.

Nomenclature

Boron nitride

Properties

Physical state

Greasy white solid or slippery to the touch.

Molecular weight

24.82 g / mol

Melting point

Sublimates at approximately 3000 ºC.

Density

Hex BN = 2.25 g / cm ³

Cubic BN = 3.47 g / cm ³

Solubility

Slightly soluble in hot alcohol.

Chemical properties

Due to the strong bond between nitrogen and boron (triple bond), boron nitride has a high resistance to chemical attack and is very stable.

It is insoluble in acids such as hydrochloric acid HCl, nitric acid HNO _3, and sulfuric acid H ₂ SO ₄. But it is soluble in molten bases like lithium hydroxide LiOH, potassium hydroxide KOH, and sodium hydroxide NaOH.

It does not react with most metals, glasses or salts. Sometimes it reacts with phosphoric acid H ₃ PO ₄. It can resist oxidation at high temperatures. BN is stable in air but is slowly hydrolyzed by water.

BN is attacked by fluorine gas F ₂ and by hydrofluoric acid HF.

Other physical properties

It has high thermal conductivity, high thermal stability and high electrical resistivity, that is, it is a good insulator of electricity. It has a high surface area.

H-BN (hexagonal BN) is an unctuous solid to the touch, similar to graphite.

On heating h-BN at elevated temperature and pressure it converts to the cubic form c-BN which is extremely hard. According to some sources it is capable of scratching the diamond.

BN-based materials have the ability to absorb inorganic contaminants (such as heavy metal ions) and organic contaminants (such as dyes and drug molecules).

Sorption means that you interact with them and can adsorb or absorb them.

Obtaining

The h-BN powder is prepared by reacting boron trioxide B ₂ O ₃ or boric acid H ₃ BO ₃ with ammonia NH ₃ or with urea NH ₂ (CO) NH ₂ under a nitrogen atmosphere N ₂.

Also BN can be obtained by reacting boron with ammonia at very high temperature.

Another way to prepare it is from diborane B ₂ H ₆ and NH ₃ ammonia using an inert gas and high temperatures (600-1080 ° C):

B ₂ H ₆ + 2 NH ₃ → 2 BN + 6 H ₂

Applications

H-BN (hexagonal boron nitride) has a variety of important applications based on its properties:

-As a solid lubricant

-As additive to cosmetics

-In high temperature electrical insulators

-In crucibles and reaction vessels

-In molds and evaporation vessels

-For hydrogen storage

-In catalysis

-To adsorb pollutants from sewage

Cubic boron nitride (c-BN) for its hardness almost equal to that of diamond is used:

-In cutting tools for machining hard ferrous materials, such as hard alloy steel, cast iron, and tool steels

-To improve the hardness and wear resistance of other hard materials such as certain ceramics for cutting tools.

Some cutting tools may contain boron nitride to exhibit increased hardness. Author: Michael Schwarzenberger. Source: Pixabay.

- Uses of BN thin films

They are very useful in the technology of semiconductor devices, which are components of electronic equipment. They serve for example:

-To make flat diodes; diodes are devices that allow electricity to circulate in one direction only

-In metal-insulator-semiconductor memory diodes, such as Al-BN-SiO ₂ -Si

-In integrated circuits as a voltage limiter

-To increase the hardness of certain materials

-To protect some materials from oxidation

-To increase the chemical stability and electrical insulation of many types of devices

-In thin film capacitors

Some diodes and capacitors may contain boron nitride. Author: Sinisa Maric. Source: Pixabay.

- Uses of BN nanotubes

Nanotubes are structures that at the molecular level are shaped like tubes. They are tubes that are so small that they can be seen only with special microscopes.

Here are some of the characteristics of BN nanotubes:

-They have a high hydrophobicity, that is, they repel water

-They have high resistance to oxidation and heat (they can resist oxidation up to 1000 ° C)

-Exhibit a high hydrogen storage capacity

-Absorb radiation

-They are very good insulators of electricity

-They have a high thermal conductivity

-Its excellent resistance to oxidation at high temperatures means that they can be used to increase the oxidation stability of surfaces.

-Because of their hydrophobicity they can be used to prepare super hydrophobic surfaces, that is, they have no affinity for water and water does not penetrate them.

-BN nanotubes improve the properties of certain materials, for example, it has been used to increase the hardness and resistance to fracture of glass.

Boron nitride nanotubes observed under a microscope. Keun Su Kim et al.. Source: Wikimedia Commons.

In medical applications

BN nanotubes have been tested as carriers for cancer drugs like doxorubicin. Certain compositions with these materials increased the efficiency of chemotherapy with said drug.

In several experiences, BN nanotubes have been shown to have the potential to transport new drugs and release them properly.

The use of BN nanotubes in polymeric biomaterials has been investigated to increase their hardness, degradation speed and durability. These are materials that are used for example in orthopedic implants.

As sensors

BN nanotubes have been used to build novel devices for detecting moisture, carbon dioxide CO _2, and for clinical diagnostics. These sensors have demonstrated a fast response and a short recovery time.

Possible toxicity of BN materials

There is some concern about the possible toxic effects of BN nanotubes. There is no clear consensus about their cytotoxicity, as some studies indicate that they are toxic to cells, while others indicate the opposite.

This is due to its hydrophobicity or insolubility in water, as it makes it difficult to carry out studies on biological materials.

Some researchers have coated the surface of BN nanotubes with other compounds that favor their solubility in water, but this has added greater uncertainty in the experiences.

Although most studies indicate that its level of toxicity is low, it is estimated that more precise investigations should be carried out.

References

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