Aluminum Oxide Ceramic

Aluminum Oxide Ceramic

Aluminium oxide is an electrical insulator but has a relatively high thermal conductivity(40 Wm⁻¹K⁻¹) for a ceramic material. In its most commonly occurring crystalline form, calledcorundum or α-aluminium oxide, its hardness makes it suitable for use as an abrasive and as a component in cutting tools.

Aluminium oxide is responsible for resistance of metallic aluminium to weathering. Metallic aluminium is very reactive with atmospheric oxygen, and a thin passivation layer of alumina (4 nm thickness) forms in about 100 picoseconds on any exposed aluminium surface. This layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process called anodising. 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 alumina generated by anodising is typically amorphous, but discharge assisted oxidation processes such as plasma electrolytic oxidation result in a significant proportion of crystalline alumina in the coating, enhancing its hardness.

Aluminium oxide was taken off the United States Environmental Protection Agency's chemicals lists in 1988. Aluminium oxide is on EPA's TRI list if it is a fibrous form.

There are a variety of techniques for firing ceramics and different techniques require different types of ceramic kilns. The earliest type of firing - probably discovered by ancient humans accidentally - is pit firing. This involves putting unfired, or bisque-fired pottery in a large hole in the ground, covering the pottery objects above and beneath with burning materials such as wood or coal and igniting them and leaving them to burn for hours or overnight. Near the end of the burning the pit could be covered with sand or earth to cut off oxygen and make a reducing atmosphere inside the pit. Modern wood-fired kilns are usually made of brick and sometimes contain several chambers for burning and ports for feeding in the fuel. As the wood is burned, its ash - which naturally contains silica, calcia, potash, and other minerals - is deposited on the pottery in the kiln, creating a pleasing wood-ash glaze effect. The individual burning qualities and mineral contents of different woods result in quite different effects; and the firings may require a number of days or weeks.