Carborundum: Preparation, Properties, Structure and Uses

Silicon Carbide (SiC)
Carborundum / Moissanite

1. Introduction

Formula: SiC
Common name: Carborundum
Natural form: Moissanite (very rare mineral)
Molecular weight: 40.10 g/mol
Hardness: 9.5 on Mohs scale (just below diamond)

2. Preparation (Acheson Process) – Industrial Method

Carborundum was first synthesised in 1891 by the American inventor Edward Goodrich Acheson while attempting to create artificial diamonds. He mixed silica (sand) and carbon (coke) and heated them in an electric furnace at high temperatures. Instead of diamonds, he obtained shiny, crystalline particles of silicon carbide.

Silicon carbide is prepared on a commercial scale by heating silica with carbon at a high temperature 2000-2500 in an electric are furnace for ,36–48 hours

SiO₂ (sand) + 3C (coke) → SiC + 2CO↑

Setup: Mixture of pure silica sand + petroleum coke + sawdust + little NaCl placed around a graphite core (acts as electrode). The central zone reaches highest temperature → purest hexagonal α-SiC crystals form.

Exam favourite: Acheson process is asked in JEE Main/Advanced, NEET, GATE, CSIR-NET almost every 2–3 years.

3. Structure

• Giant covalent network (macromolecular) structure like diamond
• Each Si bonded to 4 C and each C bonded to 4 Si → tetrahedral arrangement
• Exists in many polytypes: common ones are α-SiC (hexagonal, wurtzite-like) and β-SiC (cubic, zinc blende)
• Si–C bond length ≈ 1.89 Å, very strong covalent bond

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4. Physical Properties

Property	Value / Nature
Appearance	Colorless in pure state and Black or green iridescent crystals in impure state
Hardness (Mohs)	9.5
Melting point	~2730 °C (decomposes)
Density	3.21 g/cm³
Thermal conductivity	Very high (120–270 W/m·K)
Band gap	2.4–3.3 eV (wide-bandgap semiconductor)
Electrical behaviour	Semiconductor (can be doped n-type or p-type)

5. Chemical Properties

1. Chemical Stability
   Silicon carbide (SiC) resists the attack of most aqueous acids including HF but not H₃PO₄. It is slightly oxidized in air above 1000°C.Silica formed during oxidation forms a protective layer on the surface of SiC and inhibits further oxidation.
2. Reaction with Molten Alkalies
   Silicon carbide (SiC) reacts with molten NaOH to form sodium silicate.
   SiC + 4NaOH + 2O₂ → Na₂SiO₃ + H₂O + CO₂
3. Reaction with Chlorine
   Silicon carbide (SiC) reacts with chlorine to give silicon tetrachloride and carbon tetrachloride.
   SiC + 2Cl₂ —100°C→ SiCl₄ + C
   SiC + 4Cl₂ —1000°C→ SiCl₄ + CCl₄

6. Uses of Silicon Carbide

Use	Reason
Abrasives (grinding wheels, sandpaper)	Extreme hardness
Cutting tools, wire-saw blades for silicon wafers	Hardness + thermal stability
High-temperature ceramics (kiln furniture, brake discs)	High melting point & thermal shock resistance
Power electronics (MOSFETs, Schottky diodes)	Wide bandgap, high breakdown voltage, high thermal conductivity
LEDs (early yellow/blue LEDs used SiC substrate)	Suitable lattice match
Armour plates (military vehicles)	Lightweight + very hard
Gemstone (synthetic moissanite jewellery)	Brilliance & fire close to diamond
Heating elements (silicon carbide rods)	High resistivity + withstands 1600 °C

7. Key Points for Competitive Exams

• Carborundum is SiC, not C (graphite/diamond) or B₄C
• Prepared by Acheson process (electric furnace)
• Structure similar to diamond (tetrahedral covalent network)
• Harder than corundum (Al₂O₃), softer than diamond & cubic-BN
• Used in modern EV inverters because SiC MOSFETs reduce power loss by ~70 % compared to silicon

Important for NEET | JEE Main & Advanced | GATE | CSIR-NET | IIT-JAM

Read also: Silicon Carbide (SiC) – Carborundum MCQs Asked in NEET, IIT-JEE, GATE, CSIR-NET