Beryllium oxide
Beryllium oxide (BeO) is an electrically insulating material with a higher thermal conductivity than most metals and any other non-metal except diamond[1].
For this reason, BeO ceramics are used as substrates in power semiconductors, especially for RF applications. BeO is useful as an intermediary thermal dissipater between silicon and base metals due to its coefficient of thermal expansion. It is also used as a structural ceramic for high-performance microwave devices, vacuum tubes, magnetrons, and gas lasers.
BeO is carcinogenic in powdered form and may cause berylliosis, a chronic lung disease. Once fired into solid form, it is safe to handle if not subjected to machining, crushing or grinding that generates dust.
As a consequence, hermetic packages containing BeO must not be opened, and ceramic packages must not be broken, crushed or otherwise damaged.
Since Beryllium is relatively expensive, its main use is “in high-end products with long life-cycles”[2] where alternatives do not exist. This is one of the factors why BeO is presently not restricted in RoHS.