Why do zirconia heating elements need a preheater?

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conductive ceramics Figure 1: Schematic diagram of a zirconia oxygen sensor used to monitor automobile exhaust gases. The sensor, approximately the size of a spark plug, is fitted into the exhaust manifold of an automobile engine. The thimble-shaped zirconia sensor, sandwiched between thin layers of porous platinum, is exposed on its interior to outside air and on its exterior to exhaust gas passing through slits in the sensor shield. The two platinum surfaces serve as electrodes, conducting a voltage across the zirconia that varies according to the difference in oxygen content between the exhaust gas and the outside air.

Zirconia (ZrO₂) heating elements require a preheater because they rely on ionic conduction, which only becomes effective at high temperatures.

When zirconia is doped with ions such as calcium or yttrium, oxygen vacancies are created within its structure. At temperatures below 600°C (1,100°F), these ions are not sufficiently mobile to conduct electricity. Once the material is heated to the 600°C threshold, oxygen ions (O²⁻) become mobile and begin to fill these vacancies, enabling the ceramic to function as a conductor. The preheater is therefore necessary to bring the element to this critical temperature, after which it can be used to achieve operating temperatures as high as 2,000°C (3,600°F).1