How does slow cooling in oxygen form grain-boundary barriers in ceramic capacitors?

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Figure 1: Ferroelectric properties of barium titanate (BaTiO3). (Left) Above 120° C the structure of the BaTiO3 crystal is cubic, and there is no net polarization of charge; (right) below 120° C the structure changes to tetragonal, shifting the relative positions of the ions and causing a concentration of positive and negative charges toward opposite ends of the crystal. — capacitor dielectric and piezoelectric ceramics Figure 1: Ferroelectric properties of barium titanate (BaTiO₃). (Left) Above 120° C the structure of the BaTiO₃ crystal is cubic, and there is no net polarization of charge; (right) below 120° C the structure changes to tetragonal, shifting the relative positions of the ions and causing a concentration of positive and negative charges toward opposite ends of the crystal.

In grain-boundary barrier-layer (BL) capacitors, the formation of resistive barriers is achieved through a two-step process involving the creation of conductive grain cores followed by reoxidation. Initially, the ceramic is subjected to reduction firing or donor doping to establish conductive grains.1

During the subsequent cooling phase, the ceramic is cooled slowly in an atmosphere of air or oxygen. This controlled cooling allows oxygen to diffuse into the grain boundaries, where it reoxidizes the thin layers of material immediately adjacent to those boundaries. This process creates thin, resistive layers that effectively isolate the conductive grains, resulting in very high apparent dielectric constants ranging from 50,000 to 100,000.1

While these capacitors offer high capacitance, they must be used with caution due to their low dielectric breakdown strengths; the extreme thinness of the barriers can lead to intense local fields and potential catastrophic discharge.1