Ruthenium dioxide

Ruthenium dioxide is the rare metal ruthenium combined with oxygen, RuO₂. It's a blue-black powder, and it's unusual: most metal oxides are insulators, but this one conducts electricity almost like a metal. It's also very stable, so its properties barely change with heat or age.

Ruthenium is one of the platinum-group metals. It's mined alongside platinum and nickel, mostly in South Africa and Russia, and recovered in small amounts as a by-product of refining them. To make the oxide, ruthenium metal or a ruthenium salt such as ruthenium chloride is heated in air or oxygen, and the product is milled into a very fine powder.

In a thick-film resistor, ruthenium dioxide powder is mixed with powdered glass and an organic binder to make a paste. More ruthenium dioxide means lower resistance; more glass means higher. When the paste is printed and fired, the glass melts and holds the conductive grains in a connected network, giving a film whose resistance hardly changes with temperature. Only a speck is needed in each chip resistor. Ruthenium dioxide is also used to coat electrodes in chemical plants, including those that make chlorine.