Aluminum oxide

Aluminum oxide, or alumina, is aluminum and oxygen locked together as Al₂O₃. It is extremely hard, melts only above 2,000 °C, does not conduct electricity and shrugs off most chemicals. As a natural crystal it is corundum; tinted by traces of other metals, it becomes ruby and sapphire.

Its aluminum and oxygen mostly come already joined, in bauxite ore. In the Bayer process the ore is dissolved in hot caustic soda, the impurities settle out, aluminum hydroxide crystallizes from the cooled liquid, and heating it above 1,000 °C drives off water to leave pure white alumina powder. Most alumina is then turned into aluminum metal. It also forms the other way: bare aluminum grows a thin skin of oxide in air, and an electric current can grow it on purpose.

That makes it one of the busiest blocks in a TV:

• pressed and fired into the ceramic bodies of resistors and crystal packages
• grown as one huge perfect crystal into sapphire for LED chips, and part of the YAG phosphor
• an ingredient that strengthens display glass and glass fiber
• the ultra-thin insulating film in an electrolytic capacitor