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How do barrier-layer capacitors achieve very high apparent dielectric constants?

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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.

Barrier-layer (BL) capacitors achieve very high apparent dielectric constants—ranging from 50,000 to 100,000—by utilizing specialized internal structures that create thin resistive layers within a conductive ceramic material.1

These capacitors are constructed using one of two primary strategies:

Conductive Formation: The ceramic material is made conductive through donor doping or reduction firing, which creates conductive grain cores or films.1
Oxidation: Thin, resistive barrier layers are then formed by oxidizing the surface or the grain boundaries.1

In surface BL capacitors, this oxidation is achieved by incorporating oxidizing agents like manganese or copper oxide into the electrode paste before firing. In grain-boundary BL capacitors, the material is cooled slowly in an oxygen-rich environment, allowing oxygen to diffuse into and reoxidize the thin layers adjacent to the grain boundaries.1

While these methods produce exceptionally high dielectric constants, they result in very thin barriers that can lead to intense local electric fields, causing the capacitors to have low dielectric breakdown strengths.1