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Why do most ceramic materials resist the flow of electric current?

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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.

Most ceramic materials resist the flow of electric current because they lack the "free" electrons necessary for electrical conductivity, which are typically found in metals.12

This nonconductivity is a direct result of the strong chemical bonds that hold ceramic atoms together:

  • Ionic bonding: Bonding electrons are transferred from electropositive metal atoms to electronegative atoms (such as oxygen). Consequently, all electrons are tightly bound to the ions within the crystal structure, leaving none available to move and conduct electricity.12
  • Covalent bonding: Bonding electrons are localized in directional orbitals shared between atoms. Because these electrons are fixed in place, they are not free to facilitate current flow.12

Because of these atomic structures, most traditional ceramics act as effective electrical insulators. While some advanced ceramics can be engineered to conduct electricity—either through the introduction of specific impurities that create mobile electrons or through high-temperature ionic conduction—these are exceptions to the inherent insulating nature of most ceramic materials.12