Boron Carbide: Preparation, Properties, Structure and Uses

Boron Carbide (B₄C) – The Ultimate Ultra-Hard Ceramic

1. Introduction & Formula

• Formula: B₄C (sometimes written B₁₂C₃)
• Common/trade name: Norbide
• Molecular Weight: 55.25 g/mol
• Hardness: 9.5–9.7 on Mohs scale → 3rd hardest material known (after diamond & cubic-BN)
• Density: 2.52 g/cm³ → lightest hard ceramic

Exam favourite line: Boron carbide has a rhombohedral structure consisting of B₁₂ icosahedra linked by linear three-atom chains.

2. Preparation

Method	Reaction	Conditions
Carbothermal reduction (Industrial)	B2O3 + 7C → B4C + 6CO↑	Electric arc furnace, 2200–2500 °C
Magnesio-thermic (Laboratory)	2B2O3 + 6Mg + C → B4C + 6MgO	Then acid leaching to remove MgO
Direct synthesis	4B + C → B4C	Very high temperature, rarely used

Most asked in exams: B₂O₃ + 7C → B₄C + 6CO (high-temperature carbothermal reduction)

3. Crystal Structure (Very Important for JEE Advanced / NET)

• Rhombohedral lattice (hexagonal setting commonly used)
• Basic unit: B₁₂ icosahedra (12 boron atoms at vertices of icosahedron)
• Icosahedra are linked by linear C–B–C chains along the c-axis
• Actual composition varies from B₄C to B_10.5C due to carbon deficiency
• Strong covalent bonding + 3-centre–2-electron bonds in icosahedra → extreme hardness

4. Electron Deficiency & Bonding (CSIR-NET Level)

• Boron has only 3 valence electrons → classic electron-deficient compound.
• Within each B₁₂ icosahedron: 13 skeletal electrons are delocalised using 3-centre–2-electron bonds (typical of boranes and boron-rich solids).
• Inter-icosahedral bonds: normal 2c–2e bonds.
• The linear chain donates electrons to the icosahedra → stabilises the structure.
• Maximum stability at ≈ B₄C composition (C–B–C chain is most favourable).

Most repeated NET/GATE statement: “The extreme hardness of B₄C is due to its complex structure consisting of B₁₂ icosahedra linked by strong covalent bonds and electron-deficient 3-centre–2-electron bonding.”

5. Physical Properties

Property	Value / Nature
Appearance	Shiny black solid
Mohs hardness	9.5–9.7
Density	2.52 g/cm³ (lowest among ultra-hard ceramics)
Melting point	~2450 °C (decomposes above 2700 °C)
Thermal conductivity	Moderate (~30 W/m·K)
Neutron absorption	Very high (¹⁰B isotope: 3840 barns)
Electrical nature	p-type semiconductor

6. Chemical Properties

Boron Carbide is one of the most chemically inert substances known, resisting attack by most acids and bases at room temperature. Its high resistance is due to its strong covalent network structure. However, it can be decomposed by powerful oxidizing agents, molten salts, and halogens at high temperatures.

1. Reaction with Hot Concentrated Oxidizing Acids
   Hot concentrated Nitric Acid or Sulfuric Acid oxidize boron to Boric Acid and Carbon to Carbon Dioxide.
   B₄C + 12HNO₃ → 4H₃BO₃ + 12NO₂ + CO₂
   B₄C + 8H₂SO₄ → 4H₃BO₃ + 8SO₂ + CO₂ + O₂O
2. Reaction with Molten Sodium Hydroxide
   Boron and Carbon are oxidized, forming Sodium Metaborate (NaBO₂) and Sodium Carbonate (Na₂CO₃)
   B₄C + 8NaOH + 3O₂ → 4NaBO₂ + Na₂CO₃ + 4H₂O
3. Reaction with Chlorine Gas at High Temperature
   At high temperatures (600°C 800°C), Boron Carbide reacts readily with Chlorine gas, leading to the formation of volatile halides.
   B₄C + 8Cl₂ → 4BCl₃ + CCl₄

7. Uses of Boron Carbide

Use	Reason
Ballistic armour (bulletproof vests, tank armour)	Lightest + extremely hard
Nuclear reactor control rods & shielding	Excellent neutron absorber
Abrasives (lapping, grinding, polishing of carbides, ceramics, gemstones)	Extreme hardness
Sandblasting nozzles & water-jet cutting nozzles	Superior wear resistance
High-temperature thermocouple tubes	Thermal shock resistance
Nuclear fuel pellet coating (in some designs)	Neutron absorption

8. Comparison with SiC (Frequently Asked in Exams)

Property	B4C	SiC
Hardness (Mohs)	9.5–9.7	9.0–9.5
Density	2.52 g/cm³	3.21 g/cm³
Neutron absorption	Very high	Negligible
Cost	Very expensive	Moderate
Main use	Armour + nuclear	Abrasives + power electronics

9. Key One-Liners for Quick Revision

• Lightest known ultra-hard material → B₄C
• 3rd hardest substance after diamond & cubic boron nitride
• Contains B₁₂ icosahedra + C–B–C chains
• Best neutron absorber among ceramics (used in Chernobyl cleanup also)
• Does NOT conduct electricity well (unlike SiC)
• Reacts with water at red heat: B₄C + 8H₂O → 4H₃BO₃ + CH₄ + 2H₂

Read also: Boron Carbide MCQs Asked in NEET, JEE Main, JEE Advanced, GATE, CSIR-NET, IIT-JAM

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