Question

Preparation of inorganic graphite

Answer 1

Boron Nitride (BN) is known as Inorganic graphite. The most stable crystalline form is the hexagonal one, also called h-BN, α-BN, or g-BN (graphitic BN). ... Its hexagonal form is the most stable and softest among BN polymorphs, and is therefore used as a lubricant and an additive to cosmetic products.

Boron nitride is produced synthetically. Hexagonal boron nitride is obtained by the reacting boron trioxide (B₂O₃) or boric acid (H₃BO₃) with ammonia (NH₃) or urea (CO(NH₂)₂) in a nitrogen atmosphere:

B₂O₃ + 2 NH₃ → 2 BN + 3 H₂O (T = 900 °C) B(OH)₃ + NH₃ → BN + 3 H₂O (T = 900 °C) B₂O₃ + CO(NH₂)₂ → 2 BN + CO₂ + 2 H₂O (T > 1000 °C) B₂O₃ + 3 CaB₆ + 10 N₂ → 20 BN + 3 CaO (T > 1500 °C)

The resulting disordered (amorphous) boron nitride contains 92–95% BN and 5–8% B₂O₃. The remaining B₂O₃ can be evaporated in a second step at temperatures > 1500 °C in order to achieve BN concentration >98%. Such annealing also crystallizes BN, the size of the crystallites increasing with the annealing temperature

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