16.11 Electrically Conductive Ceramics

Electric arc furnace: schematic and hot
electrode in furnace

Early on, magnetite was discovered to be a conductor as well as having magnetic properties. Other ceramics were found to be conductive, such as zirconia doped with rare earth oxides that when heated to a high temperature by an electrical current was a useful source of white light. Ceramics are also used as furnace heating elements as they can become usefully conductive at high temperatures, and can operate well above the melting temperature of most metals.

Silicon carbide can be used for such heating rods as it is sufficiently conductive at higher temperatures (up to 1700 ºC in an oxidizing atmosphere). It is prepared by pressing the powder into the rod shape and firing at about 2000 ºC. The silicon carbide can resist such temperatures as it forms a protective surface layer of silicon dioxide.

Another example is molybdenum disilicide that is formed into rods or tubes up to a metre long and 20 mm diameter using 20% alumino-silicate glass as a binder, and is usable up to 1800 ºC.

Silicon carbide heating elements
- source Jinyuco

Molybdenum disilicide heating element
- source Careygao

Tin oxide heating elements - source Chida

Lanthanum strontium chromite was developed in the 60’s for the electrodes in a novel type of electrical generator (MHD) running at 2000 ºC. It is now being used in fuel cells. Tin oxide is also used in heating elements, typically for melting glass. The glass constituents are heated conventionally to 1000 ºC when the mixture becomes sufficiently conductive for the tin oxide elements to supply the power to the mix itself to achieve the required temperature of 1300 to 1600ºC. The quality of the glass can be improved as the method retains more volatile components such as lead.

16.12 Superconductors

When some materials are cooled to very low temperatures they can exhibit zero resistivity, meaning that electric current could flow forever through a ring made of such a conductive material. K. Onnes discovered this phenomenon in 1911 using mercury, at minus 269 ºC. This was shortly after helium was first liquefied at minus 272 ºC or 1 degree K (Kelvin).

If a superconductor is placed in a magnetic field, currents are induced in it to oppose the magnetic field, so it is an ideal magnetic screen. However, there are limits to any applied magnetic field, and also limits to the current that can flow through a superconductor, as at a high enough external magnetic field or current density it becomes resistive again.

Using liquid helium for cooling is very expensive, so there is great interest in finding materials that become superconducting at higher temperatures. Superconductivity occurs in various metals and alloys including tin (3.7 degrees K) and niobium/tin alloy (18 degrees K), which is about as high as can be achieved with metal alloys.