Compounds of aluminum and oxygen are referred to as aluminum oxides (Al2O3), whereas compounds with hydroxyl groups are known as hydroxides. Alpha-Al2O3 (corundum)  is the most well-known and significant form of the existing Al2O3 modifications. Besides, there are further aluminum oxides of different structures called transition aluminas [2, 3]. Corundum has a density of 3.98 g/cm³, a high hardness, a melting point of 2053 °C, and a high specific electrical resistance of approximately 1012 ohm·m (at 20 °C) . It is chemically very stable and is almost insoluble in water, acids, and bases. The transition aluminum oxide gamma-Al2O3, in contrast, dissolves in strong acids and in bases. Due to its high surface activity, gamma-aluminum oxide is used as an adsorbent and catalyst material.
In addition to oxides of aluminum, there are different hydroxides, e.g. aluminum hydroxides [Al (OH)3] such as bayerite and gibbsite and the so-called aluminum oxy-hydroxides [AlO(OH)] boehmite and diaspore.
Gibbsite, boehmite, and diaspore are constituents of the technically important aluminum mineral bauxite.
Occurrence and Production
Aluminum oxide is produced industrially from the mineral bauxite. The Bauxite deposits are estimated at approximately 20 billion tons worldwide, the worldwide annual output amounts to about 100 million tons. Australia has the largest output and deposits. The sapphire, well-known as a precious stone, is a quite rare but at the same time the most beautiful modification of aluminum oxide.
Aluminum and aluminum oxide are manufactured by means of the Bayer method: Bauxite is crushed, dried, and dissolved using concentrated sodium hydroxide solution. The impurities iron, silicon, and titanium are separated from the bauxite in the so-called red mud. Aluminum hydroxide is precipitated from the solution and calcinated at 1200-1300°C to form Al2O3.
Due to its high hardness, corundum is used as a bearing jewel in watches and as a grinding and polishing agent for precious stones, metals, and Si wafers. Mixed with binders, especially with other oxides, corundum powder is used for manufacturing crucibles, dishes, sheath tubes, electrical insulators, artificial joints (artificial hip or knee joints), dental ceramics, burner tubes, catalyst carriers, wear protections, hard-facings, furnace linings, and metal forming and machining tools (Fig. 1).
Corundum has a low strength in spite of its hardness and brittleness. To obtain products with improved ductility or strength, partially stabilized zirconium oxide or titanium carbide („black ceramics“) are added to the white corundum. The low electrical conductivity and high dielectric strength of aluminum oxide are exploited in manufacturing ignition plugs and insulators.
Synthetic corundum crystals grown from corundum melts have a high hardness, transparency, and scratch resistance. The precious stones sapphire and ruby consist of corundum with certain additions of iron/titanium or chromium. Synthetically manufactured sapphires and rubies are used in lasers. Sapphires are also used as scratch-resistant watch glasses.
Small additions of foreign substances can make sintered corundum exhibit a broad spectrum of colors (Fig. 2).
Due to their thermal stability, boehmite and other aluminum oxides are used as catalyst carriers and adsorbents in the petroleum and chemical industries. Sintered into porous structures and applied to coarser substrates, nanoscale aluminum oxide can also be used for nanofiltration (Fig. 3).
Aluminum hydroxide Al (OH)3 in powder form is used as a flame retardant and as filler in carpets, rubbers, plastics, and foamed plastics. Moreover, it is used in dentifrices and cosmetics.
Aluminum (hydr) oxides are often used in the dye and plastics industries as thickeners and fillers and as agents that reduce adhesiveness and increase scratch resistance. Besides, they serve to enhance the color saturation of paints and varnishes.
Further applications comprise
- ceramics (to ensure high abrasion and fire resistance),
- additives for paper manufacture (to avoid that the paper adheres to the feed rolls during high-speed processes),
- artificial precious stones such as sapphires or yttrium-aluminum garnets, the latter of which are used, for example, in high-energy lasers, and
- luminescent substances and phosphors with aluminum oxides as substrates.
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- Wefers, K., Misra, C. „Oxides and Hydroxides of Aluminum“, Alcoa Technical Paper No. 19, Alcoa Laboratories, 1987
- Petzold, A.., Ulbricht J. “Aluminiumoxid: Rohstoff, Werkstoff, Werkstoffkomponente, Dt. Verl. Für Grundstoffind., Leipzig, 1991
- Alfrey, A. C., Le Gendre, G. R. & Kachny, W. D.: The dialysis encephalopathy syndro-me: Possible aluminum intoxication, New England J. Med. 294 (1976), 184
- Wikipedia: Aluminium oxide
- Wikipedia: Aluminium hydroxide