Preparation of Boron Nitride

Boron Nitride exists in several forms—Hexagonal Boron Nitride (h-BN), Cubic Boron Nitride (c-BN), Wurtzite Boron Nitride (w-BN), and Amorphous Boron Nitride (a-BN). The synthesis method strongly influences which phase is produced.

1. Direct Reaction of Boron and Nitrogen

Method

Heat elemental boron with nitrogen gas (N₂) or ammonia (NH₃) at high temperature.

Reaction

• With nitrogen: 2B + N₂ → 2BN

• With ammonia: B + NH₃ → BN + 3/2 H₂

Conditions

• Temperature: 1000–1500 °C

• Atmosphere: N₂, NH₃, or vacuum

• Used mainly for Hexagonal Boron Nitride (h-BN) synthesis.

Advantages

• Simple and relatively low-cost

• Produces high-purity Boron Nitride (BN)

2. Reaction of Boron Oxide with Ammonia or Nitrogen

This is one of the most common industrial methods.

Reaction

• Convert boric acid → boron oxide: 2 H₃BO₃ → B₂O₃ + 3 H₂O (dehydration)

• React B₂O₃ with ammonia: B₂O₃ + 2 NH₃ → 2 BN + 3 H₂O

Conditions

• Temperature: 900–1200 °C

• Atmospheric or flowing NH₃

Advantages

• Uses inexpensive precursors

• Good for mass production of Hexagonal Boron Nitride (h-BN) powder

3. Carbothermal Reduction–Nitridation

Method

Mix boron oxide (B₂O₃) with carbon and heat in nitrogen.

Reaction

B₂O₃ + 3C + N₂ → 2BN + 3CO

Conditions

• Temperature: 1400–1800 °C

Advantages

• Low-cost and scalable

• Common for industrial Boron Nitride powder

Disadvantages

• Carbon contamination may occur

4. Chemical Vapor Deposition (CVD)

Used to make thin films of high-quality Boron Nitride.

Precursors

• BCl₃ + NH₃

• B₂H₆ + NH₃

• B-trichloroborazine (B₃N₃Cl₃)

General Reaction

BCl₃ + NH₃ → BN + 3HCl

Conditions

• Temperature: 600–1200 °C

• Used to make: Hexagonal Boron Nitride (h-BN) films, coatings, 2D Boron Nitride (“white graphene”)

Advantages

• High-purity, uniform films

• Suitable for electronics and coatings

5. Solvothermal or Polymer-Derived Ceramic (PDC) Method

Use borazine (B₃N₃H₆) or polyborazylene precursors.

Steps

• Polymer formation (cross-linking)

• Pyrolysis at ~1000 °C

• Crystallization into Boron Nitride (BN)

Advantages

• Produces high-purity Hexagonal Boron Nitride (h-BN)

• Good control over morphology (fibers, spheres, nanosheets)

6. High-Pressure High-Temperature (HPHT) Synthesis — For Cubic Boron Nitride (c-BN)

Cubic Boron Nitride (c-BN) requires extreme conditions similar to diamond synthesis.

Method

Heat Hexagonal Boron Nitride (h-BN) under:

• Pressure: 5–7 GPa

• Temperature: 1500–2000 °C

• Catalyst: alkali metals, nitrides, or carbonates

Reaction

• Hexagonal Boron Nitride (h-BN) → Cubic Boron Nitride (c-BN) (phase transformation)

Products

Cubic Boron Nitride (c-BN) (superhard material comparable to diamond)

Summary Table