Structure of Alumina
As stated above, the most common form of alumina or aluminium oxide is known as corundum, which is in a crystalline form. In this, the oxygen ions tend to form a hexagonal close-packed structure. Meanwhile, the aluminium ions fill two-thirds of the octahedral interstices. Each Al3+ centre is octahedral.
Alternatively, alumina also exists in several different metastable phases such as the cubic γ and η phases, the hexagonal χ phase, the orthorhombic κ phase, the monoclinic θ phase and the δ phase. Now, these can either be orthorhombic or tetragonal. Each phase will have a unique crystal structure and properties.
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Properties of Alumina
- Alumina possesses good or high thermal conductivity. At elevated temperatures, it can withstand alkali attacks and strong acid.
- It consists of excellent dielectric properties.
- Metallic Aluminium is highly reactive with atmospheric oxygen.
- It exhibits high stiffness and strength.
- It is available at a purity range from 94%.
- It is insoluble in water.
- Alumina is an amphoteric substance, which means that it can react with both acids and bases.
- Activated alumina is a porous, granular substance.
- Alumina is an excellent ceramic oxide.
- Has high resistance to abrasion and chemical attacks.
- Wear resistance.
Reactions of Alumina
- Reaction with Sodium Hydroxide- Al2O3 + 2NaOH → 2NaAlO2 + H2O
- Reaction with Hydrochloric Acid- Al2O3 + 6HCl → 2AlCl3+ 3H2O
- Reaction with Sulphuric Acid- Al2O3 + H2SO4 → Al2(SO4)3 + H2O
Applications of Aluminium Oxides:
- Alumina is the primary material in the production of aluminium. 90% aluminium oxide is used in the manufacture of aluminium metal mostly by the electrolysis process.
- They are widely used in Electronic Substrates, Thread and Wire Guides, Seal Rings, Ballistic Armour, Thermometry Sensors, Grinding Media, Furnace Liner Tubes, High Voltage Insulators, Laboratory Instrumentary Tubes.
- Various formulation of glass consists of aluminium oxide as an ingredient.
- It is used in purification that is the removal of water from gas streams. They are employed in high-performance applications.
- It is a favoured filler for plastic. It is sometimes present in cosmetics such as nail polish, lipstick and blush.
- They are broadly used as an abrasive. Various sandpaper makes use of aluminium oxide crystals.
- They are employed in the manufacturing of titles where they are attached inside the pulverized fuel lines to protect high wearing areas.
- Alumina is often used extensively in engineered ceramics which is also called advanced or technical ceramics.
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