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Why can impurities at grain boundaries interfere with current flow in ceramic conductors?

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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.

In the field of materials science, the performance of conductive ceramics is heavily dependent on the precise control of their microstructure, particularly at the grain boundaries where adjacent grains meet.1

While some specialized devices, such as varistors and certain capacitors, intentionally utilize grain boundary barriers to function, the segregation of impurities at these boundaries generally has adverse effects on the performance of ceramic conductors and superconductors. These impurities can interfere with current flow by acting as physical or electronic obstacles that impede the movement of charge carriers—whether they are electrons or ions—across the interface between grains.12

Because the quality of these advanced materials is determined by their grain size and boundary chemistry, manufacturers must employ high-purity starting materials and strictly control processing, consolidation, and firing to minimize these detrimental effects.1