Aluminum oxide (Al₂O₃) is a compound of aluminum metal and oxygen used in the alpha alumina structural form. It is commonly referred to as alumina, a white high-hardness ceramic.  Fully-dense alumina can be translucent.  Aluminum oxide and alumina ceramics have excellent wear characteristics, chemical resistance, compressive strength, high-temperature properties, and dielectric strength.  They are used widely because of their versatility and low cost. The main drawback is its relatively poor thermal-shock resistance due to higher coefficients of thermal expansion and lower thermal conductivity (compared to other pure ceramic materials).

Types of Aluminum Oxide and Alumina Ceramics

Examples include calcium aluminate, sapphire, and alumina-zirconia.

Calcium aluminate (CaAlO₃) is an aluminate ceramic used in refractory cements and shapes, as well as synthetic slag additions for metallurgical operations.  

Sapphire is a high-purity and high-density, single-crystalline form of aluminum oxide that may contain chromia, titania, yttria, or other dopants. Sapphire is usually transparent or translucent. 

Zirconia-toughened alumina (ZTA) and other zirconia-alumina ceramics are often used in wear applications as an intermediate solution between alumina and zirconia. ZTA offers increased fracture toughness over alumina at a lower cost compared to pure or high zirconia ceramics.

Commercial Grades of Alumina

There are many different commercial grades of alumina. Examples include smelter-grade, calcined, low soda, reactive, tabular, fused, and high-purity.

• Smelter alumina is used in the manufacture of aluminum metal, typically in fluid bed or fluid flash calciners.
• Calcined alumina may use mineralizing catalysts and contain sodium oxide as the main impurity. They improve the performance of both shaped and monolithic refractories.
• Low soda alumina is used mainly in electrical and electronic components.
• Reactive alumina has a relatively high purity and small crystal size.  It is used in the production of high-performance refractories that require defined particle packing and consistent placement characteristics.
• Tabular alumina is re-crystallized or sintered alpha alumina. It is used in shaped and unshaped refractories for steel, cement, foundry, glass, petrochemical, and waste incineration applications. Non-refractory applications for tabular alumina include its use in kiln furniture and for metal filtration.
• Fused alumina is made in electric arc furnaces. The fused mass is then crushed and sized to meet application requirements.
• High-purity alumina is used in the manufacture of synthetic gem stones, laser components, and instrument windows.

Applications

Depending on the purity and density, alumina is used for:

• Abrasives
• Analytical labware
• Crucibles
• Refractory cements
• Refractory tubes
• Wear components

Typically, these alumina abrasives are used in sand blasting, polishing, and surface preparation. For example, aluminum oxide grit powder is used to clean engine heads, valves, pistons, and turbine blades. Aluminum oxide abrasive grains are also used in metallizing, plating, and welding operations.

References

Altmatis – Reactive and Calcined Aluminas

Richard Baker Harrison – Fused Alumina

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Insaco, Inc.