Why does ionic conductivity increase when a ceramic is heated?

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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.

In ionic conductors, electricity is carried by the movement of charged atoms (ions) through the crystal lattice. At ambient temperatures, these ions are generally locked in place, resulting in low conductivity.

The increase in ionic conductivity upon heating occurs because the thermal energy provided to the material allows ions to overcome the energy barriers required to move between sites. Specifically, the transit of ions occurs via point defects known as vacancies. As the temperature rises, these vacancies become increasingly mobile, facilitating the hopping of ions from one site to another. When this process becomes highly efficient at elevated temperatures, the material exhibits what is known as fast ionic conduction, a property essential for applications such as fuel cells, batteries, and gas sensors.1