Aluminium

Aluminium was a metallic element. It could be found on such planets as Earth (K9TV: Dream-Eaters) and Megerra. (BNA: Mean Streets)

The psychic energy given off by the Bodach could be blocked by aluminium, especially in the form of hats. (K9TV: Dream-Eaters)

Erato wove a shell of aluminium around a neutron star that was set to destroy Chloris, allowing the Fourth Doctor to use his TARDIS's tractor beam to pull it off course. (DW: The Creature from the Pit)

Tom Cardwell wore an aluminium hat to protect his mind from alien influences. (BFA: Blood of the Daleks)