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How can impurities make a ceramic electrically conductive?

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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.
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.

While most ceramics are naturally electrical insulators due to their tightly bound electrons, they can be made electrically conductive through the introduction of specific impurities, a process often referred to as doping. This modification occurs through two primary mechanisms:12

  • Electronic Conduction: Impurities with different valences (different numbers of bonding electrons) can be added to the material. These impurities act as donors or acceptors of electrons, creating mobile electrons or electron holes that facilitate current flow. Additionally, transition metals or rare-earth elements can be included to act as centers for polarons—electrons that create small regions of local polarization as they move between atoms.12
  • Ionic Conduction: Certain impurities, such as calcium or yttrium ions, can be introduced to create vacancies in the crystal lattice. At high temperatures, these vacancies become mobile, allowing ions to hop from site to site, which results in ionic conductivity.31

These modifications are essential for creating conductive ceramics used in diverse applications, including oxygen sensors, heating elements, and electroceramics like transparent oxide films.43