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Aluminium Oxide: Properties, Production, and Applications

Alumina, commonly known as aluminium oxide (Al2O3), is an inert, odourless, white amorphous material often used in industrial ceramics. Due to its outstanding properties, alumina has contributed to a significant number of life-extending and society-enhancing applications. It is widely used in the medical field and modern warfare [1]. 

Aluminium oxide is a thermally unstable and insoluble compound that occurs naturally in various minerals such as corundum, a crystalline variant of the oxide, and bauxite, which is considered as its principal aluminium ore [1].

Here, you will learn about:

  • What the main properties of aluminium oxide are
  • How aluminium oxide is produced
  • What the main application areas that utilise aluminium oxide are

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Properties of aluminium oxide

Due to its excellent mechanical, chemical and thermal qualities, alumina stands out from many comparable materials by delivering equal or better solutions for low-cost production and manufacturing.

Its performance relies on the following properties [1], [2], [3]: 

Property

Value

Melting point

2,072 °C (3,762 °F; 2,345 K)

Boiling point

2,977 °C (5,391 °F; 3,250 K)

Hardness 

15 – 19 GPa (9 on the Mohs scale)

Electrical resistivity

1012 – 1013 Ωm

Mechanical strength

300 – 630 MPa

Compressive strength

2,000 – 4,000 MPa

Thermal conductivity 

20 – 30 W/mK

Molecular mass

101.96 g/mol

Density

3.95 g/cm3

Appearance

Solid

How is aluminium oxide produced?

Aluminium oxide is often produced by the Bayer process, which means refining bauxite to produce alumina. The following reversible chemical equation describes the grounds of the Bayer process:

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This process starts by drying crushed and washed bauxite, usually containing 30–55% Al2O3 [4]. The bauxite is dissolved in caustic soda to form a slurry, heated to temperatures of about 230–520 °F (110–270 °C). This mixture is then filtered to remove the residue called the “red mud” impurities. 

The filtered alumina solution (aluminium hydroxide) is then transferred or pumped into precipitator tanks where it cools and starts to seed. These seeds stimulate a precipitation process allowing solid aluminium hydroxide crystals to be formed. All the aluminium hydroxide that settles at the bottom of the tank is removed. 

The remaining caustic soda is washed away from aluminium hydroxide, which undergoes various levels of filtering. Finally, it is heated to completely remove excess water. After passing through a cooling stage, the fine white powder is produced [5].

The Bayer process is summarised in Figure 1, showing all the stages and what happens during each stage.

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Figure 1: Bayer process in its modern form (2017), as the removal steps of silica and oxalate are added to the original 1892 Bayer process. Reproduced from Figure 3.1 (p.51) of ref. 1.

Applications of aluminium oxide

Most of the aluminium oxide produced is used to form aluminium metal. Oxygen typically catalyses corrosion in reaction with the metal aluminium. However, when bonded with oxygen to form aluminium oxide, a protective coating forms and prevents further oxidation. This adds strength and makes the material less vulnerable to deterioration [6].

Industries that use aluminium oxide include:

Medical industry

Due to aluminium oxide’s hardness, bio-inertness and chemical properties, it is a preferred material for bearings in hip replacements, as prostheses, bionic implants, prosthetic eye substitutes, tissue reinforcements, dental crowns, abutments, bridges, and other dental implants. It is also used in lab equipment and tools like crucibles, furnaces and other labware [1].

Military and protective equipment

Aluminium oxide’s strength and lightweight qualities contribute to enhancing body armours, like breastplates, as well as vehicle and aircraft armour, which is its biggest market. Aluminium oxide is also used in synthetic-sapphire bulletproof windows and ballistics [1].

Electrical and electronics industry

Its high melting and boiling points, in addition to its excellent thermal resistive properties, make aluminium oxide desirable in the manufacture of high-temperature furnace insulations and electrical insulators. Alumina films are also vital components in the microchip industry. Some of its other uses include spark plug insulators, micro-electric substrates and insulating heatsinks [1]. 

Gem industry

Aluminium oxide is a valuable element in the formation of rubies and sapphires. Its crystalline form, corundum, is the base element for these precious gems. Rubies owe their deep red colour to chromium impurities while sapphires get their variant colours from traces of iron and titanium [6].

Industrial applications

Since alumina is chemically inert, it is utilised as a filler in plastics, bricks, and other heavy clayware, like kilns. Due to its extreme strength and hardness, it is often used as an abrasive for sandpaper. It is also an economical substitute for industrial diamonds [6]. 

Aluminium oxides are used, as well, for the production of piping components such as elbows, tees, straight pipes, hydro cyclones, reducers, nozzles, and valves. Other applications include the production of various machining tools, cutting tools, thermocouple sheaths, wear-resistant pump impellers, and baffle plates [1].

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Sources

[1] A.J. Ruys, Alumina Ceramics: Biomedical and Clinical Applications, UK: Woodhead Publishing, 2018.

[2] CeramTec, The Most Well-known Oxide Ceramic Material, CeramTec, Germany, n.d., Accessed on: Oct. 28, 2019. [Online] Available: https://www.ceramtec.com/ceramic-materials/aluminum-oxide/#

[3] American Elements, Aluminum Oxide, American Elements, USA, n.d., Accessed on: Oct. 28, 2019. [Online] Available: https://www.americanelements.com/aluminum-oxide-1344-28-1

[4] Guichon Valves, Alumina - Manufacturing process of Alumina, Guichon Valves, France, n.d., Accessed on: Oct. 28, 2019. [Online] Available: https://guichon-valves.com/faqs/alumina-manufacturing-process-of-alumina/

[5] “Aluminum,” n.d., Accessed on: Oct. 28, 2019. [Online] Available: http://www.madehow.com/Volume-5/Aluminum.html

[6] “Aluminum Oxide,” n.d., Accessed on: Oct. 28, 2019. [Online] Available: https://aluminumsulfate.net/aluminum-oxide

Fun Facts

  • The only natural materials that are able to scratch aluminium oxide are diamond and moissanite, as aluminium oxide grades 9 on the Mohs scale, following moissanite at 9.5 and diamond at 10 (highest value).
  • Corundum has an unusually high density for a transparent mineral.
  • In the mid-19th century, Swiss watch makers realised that corundum could be the material they need in order to manufacture small abrasion-resistant bearings. By simply drilling a hole into a small corundum piece, they could utilise it as a bearing that runs smoothly and lasts for a very long time. Those bearings were then called "jewel bearings" after corundum became famously used in the jewellery industry.