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Alumina - Aluminium Oxide - Al2O3 - A Refractory Ceramic Oxide

Aluminium Oxide (Al2O3 ) or alumina is one of the most versatile of refractory ceramic oxides and finds use in a wide range of applications.

It is found in nature as corundum in emery, topaz, amethyst, and emerald and as the precious gemstones ruby and sapphire, but it is from the more abundant ores such as bauxite, cryolite and clays that the material is commercially extracted and purified. 

Corundum exists as rhombohedral crystals with hexagonal structure. The unit cell is an acute rhombohedron of side length 5.2Å and plane angle ~55°.  It is the close packing of the aluminium and oxygen atoms within this structure that leads to its good mechanical and thermal properties.

Extraction Routes

The most common process for the extraction and purification of alumina is the ‘Bayer’ process. The first step in the process is the mixing of ground bauxite into a solution of sodium hydroxide. By applying steam and pressure in tanks containing the mixture, the bauxite slowly dissolves. The alumina released reacts with the sodium hydroxide to form sodium aluminate. After the contents of the tank have passed through other vessels where the pressure and temperature are reduced and impurities are removed, the solution of sodium aluminate is placed in a special tank where the alumina is precipitated out. The precipitate is removed from the tank, washed, and heated in a kiln to drive off any water present. The residue is a commercially pure alumina.

Other extraction processes are used including pyrogenic treatment of bauxite with soda, and the extraction of aluminium hydroxide from meta kaolin via either the chloride or sulphate.

The yield of alumina from these processes can approach 90%.

For advanced ceramics uses, the alumina manufactured by these processes requires further purification. This is often achieved by recrystallisation from ammonium alum.

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Key Properties

Detailed below are tables which illustrate typical physical and mechanical properties for different purity grades of Alumina

Table 1. Typical physical properties of Alumina

Property Value
Melting point (°C) 2015±15
Refractive index 1.765
Molecular wt (g.mol-1) 101.96
ΔGf° Free Energy of Formation (kJ.mol-1) -1582.4

Table 2. Typical physical and mechanical properties of 86% to 99.9% Alumina

Property Alumina Grade
86% 94% 97.5% 99.5% 99.9% 99% recry.* Saph**
Density (.gcm-3) 3.5 3.7 3.78 3.89 3.9 3.9 3.985
Dielectric Constant 8.5 9.2 9.5 9 - 10.1 9 - 10.1 7.5 - 10.5
Dielectric Strength
(kVmm-1)
28 30 - 43 10 - 35 10 - 35 17
Volume Resistivity Ohm.cm >1014 >1014 >1014 >1014 >1014 >1014 >1016
Thermal Conductivity
(Wm-1K-1)
15 20 24 26 28-35 28-35 41.9
Thermal Expansion Coefficient
(20-1000 °C x10-6K-1)
7 7.6 8.1 8.3 8 8 5.8
Specific Heat
(JK-1kg-1)
920 900 850 753
Compressive Strength (MPa) 1800 2000 1750 - 2500 2200 - 2600 2200 - 2600 2100
Modulus of Rupture (MPa) 250 330 262 320 - 400 260
Hardness
(Vickers kgf.mm-2)
1500 - 1600 1500 - 1650 1500 - 1650 2500 - 3000

* recrystallised
** Sapphire

Applications

With such a range of composition and properties, alumina ceramics find a wide range of applications. Some of the major application areas can be grouped as shown in table 3.

Table 3. Example applications for a range of Alumina’s

% Al2O3 Grain size Porosity Applications Area
>99.6 Fine closed Electrical, Engineering, Biomedical
>99.8 Fine zero Lamp tubes, Optical
>99.6* (recrystallised) Medium closed High temperature uses
95 – 99.5 Fine closed General electrical, engineering
80 - 95 Fine closed Low duty electrical (spark plugs)
90 - 99.6 Fine/Coarse open Filter media
80 - 90 Fine/Coarse open Abrasive

High Temperature and Aggressive Environments

Its high free energy of formation makes alumina chemically stable and refractory, and hence it finds uses in containment of aggressive and high temperature environments.

Wear and Corrosion Resistance

The high hardness of alumina imparts wear and abrasion resistance and hence it is used in diverse applications such as wear resistant linings for pipes and vessels, pump and faucet seals, thread and wire guides etc.

Biomedical

High purity aluminas are also used as orthopaedic implants particularly in hip replacement surgery.

Metal Cutting Tools

The high “hot” hardness of alumina have led to applications as tool tips for metal cutting (though in this instance alumina matrix composites with even higher properties are more common) and abrasives.

Milling Media

Alumina is used as milling media in a wide range of particle size reduction processes.

Microwave Components

The high dielectric constant coupled with low dielectric loss particularly at high frequencies leads to a number of microwave applications including windows for high power devices and waveguides.

Electrical Insulation

The high volume resistivity and dielectric strength make alumina an excellent electrical insulator which leads to applications in electronics as substrates and connectors, and in lower duty applications such as insulators for automotive spark plugs

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