![]() See also: List of countries by aluminium oxide productionĪluminium hydroxide minerals are the main component of bauxite, the principal ore of aluminium. The structure of molten alumina is temperature dependent and the fraction of 5- and 6-fold aluminium increases during cooling (and supercooling), at the expense of tetrahedral AlO 4 units, approaching the local structural arrangements found in amorphous alumina. The breakdown of octahedra upon melting is accompanied by a relatively large volume increase (~33%), the density of the liquid close to its melting point is 2.93 g/cm 3. Around 80% of the oxygen atoms are shared among three or more Al-O polyhedra, and the majority of inter-polyhedral connections are corner-sharing, with the remaining 10–20% being edge-sharing. 2/3 of the Al are surrounded by 4 oxygen neighbors), and 1/3 5-coordinated, with very little (<5%) octahedral Al-O present. Molten aluminium oxide near the melting temperature is roughly 2/3 tetrahedral (i.e. ![]() ![]() ![]() The so-called β-Al 2O 3 proved to be NaAl 11O 17. Cubic γ-Al 2O 3 has important technical applications. Each has a unique crystal structure and properties. The primitive cell contains two formula units of aluminium oxide.Īluminium oxide also exists in other metastable phases, including the cubic γ and η phases, the monoclinic θ phase, the hexagonal χ phase, the orthorhombic κ phase and the δ phase that can be tetragonal or orthorhombic. In terms of its crystallography, corundum adopts a trigonal Bravais lattice with a space group of R 3c (number 167 in the International Tables). The oxygen ions form a nearly hexagonal close-packed structure with the aluminium ions filling two-thirds of the octahedral interstices. The most common form of crystalline aluminium oxide is known as corundum, which is the thermodynamically stable form. Amphoteric nature Īluminium oxide is an amphoteric substance, meaning it can react with both acids and bases, such as hydrofluoric acid and sodium hydroxide, acting as an acid with a base and a base with an acid, neutralising the other and producing a salt.Īl 2O 3 + 6 HF → 2 AlF 3 + 3 H 2O Al 2O 3 + 2 NaOH + 3 H 2O → 2 NaAl(OH) 4 ( sodium aluminate) Structure Corundum from Brazil, size about 2×3 cm. Aluminium oxide is on the EPA's Toxics Release Inventory list if it is a fibrous form. The aluminium oxide generated by anodising is typically amorphous, but discharge-assisted oxidation processes such as plasma electrolytic oxidation result in a significant proportion of crystalline aluminium oxide in the coating, enhancing its hardness.Īluminium oxide was taken off the United States Environmental Protection Agency's chemicals lists in 1988. A number of alloys, such as aluminium bronzes, exploit this property by including a proportion of aluminium in the alloy to enhance corrosion resistance. The thickness and properties of this oxide layer can be enhanced using a process called anodising. This layer protects the metal from further oxidation. Metallic aluminium is very reactive with atmospheric oxygen, and a thin passivation layer of aluminium oxide (4 nm thickness) forms on any exposed aluminium surface in a matter of hundreds of picoseconds. Īluminium oxide is responsible for the resistance of metallic aluminium to weathering. In its most commonly occurring crystalline form, called corundum or α-aluminium oxide, its hardness makes it suitable for use as an abrasive and as a component in cutting tools. Properties Aluminium oxide in its powdered formĪl 2O 3 is an electrical insulator but has a relatively high thermal conductivity ( 30 Wm −1K −1) for a ceramic material. An extremely rare δ form occurs as the mineral deltalumite. Sapphires come in different colours given by various other impurities, such as iron and titanium. Rubies are given their characteristic deep red colour and their laser qualities by traces of chromium. Rubies and sapphires are gem-quality forms of corundum, which owe their characteristic colours to trace impurities. Natural occurrence Ĭorundum is the most common naturally occurring crystalline form of aluminium oxide. Al 2O 3 is significant in its use to produce aluminium metal, as an abrasive owing to its hardness, and as a refractory material owing to its high melting point. It occurs naturally in its crystalline polymorphic phase α-Al 2O 3 as the mineral corundum, varieties of which form the precious gemstones ruby and sapphire. ![]() It is commonly called alumina and may also be called aloxide, aloxite, or alundum in various forms and applications. It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula Al 2O 3. ![]()
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