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

apache-3566191_1920.jpg

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:

Screenshot 2019-10-29 at 09.21.54.png

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.

Alumina Bayer Process.png

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].

sapphire-1433293_1920.jpg

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.

Applications

Pharmaceutical Packaging

Metrology

Electrical & Electronic Testing

Non-Destructive Testing

Mechanical Systems

Industrial Machinery

Wear Parts

Bearings

Plain Bearings

Seals

Electric Motors

Fluid Systems

Nozzles

Manufacturing

Additive Manufacturing

Stereolithography

Materials Processing

Metal Injection Molding

Energy

Burners & Boilers

Gas Fuel Burners

Electrical Engineering

Electrical Insulation

Glass & Ceramic Insulation

Insulation

Insulators

Bushings

Electrical Accessories

Electrical Housing

Electronics

Electronic Components

Sensors

Capacitors

Ceramic & Mica Capacitors

Automotive

Combustion Engines

Emission Control Systems

Packaging

Packaging Machinery

Textile Industry

Textile Machinery

Textile Machinery In General

Clothing Industry

Eyeglasses

Eyeglasses Frame

Sewing Machines

Chemical Industry

Chemical Industry Production

Equipment For Chemical Industry

Other Equipment For The Chemical Industry

Glass & Ceramics

Ceramics

Ceramics In General

Ceramics Raw Materials

Ceramic Products

Advanced Ceramics

Other Standards Related To Ceramics

Paper Industry

Paper Products

Equipment For The Paper Industry

Consumer Goods

Kitchen Equipment

Cooking Ranges

Working Tables

Ovens & Similar Appliances

Dishwashers

Small Kitchen Appliances

Laundry Appliances

Domestic

Commercial & Industrial Heating Appliances

Heating Elements

Bearings General

Rollers

Fittings

Valves

Other Valves

Forming Processes

Machining

Milling Machines

Other Machine Tools

Cutting

Milling Tools

Cutting Tools

Chipless Working Equipment

Punching Die

Punching Tools

Rolling

Extruding & Drawing Equipment

Extrusion Die

Casting Die

Die Casting Plungers

Industrial Furnaces

Furnace Parts

Electronic Components Others

Cooling

Material Handling

Continuous Handling

Continuous Handling Equipment In General

Coating Industry

Paint Coating Equipment

Fasteners

Rings

Construction

Building Elements

Shafts

Ball & Plug Valves

Fluid Power Systems

Pumps & Motors

Forging Presses

Spools

Semiconductor Industry

Telecommunications

Other Telecommunication Components

Laser Equipment

Lasers

Defense

Military Engineering

General Military

Compressors

Abrasives

Analytical Chemistry

Laboratory Ware

Show all