Barrier-layer dielectrics

Barrier-layer dielectrics are bodies exhibiting an anomalously high permittivity as a result of continuous thin layers of low-loss dielectric enclosing regions of high conductivity. This inhomogeneity is brought about in ceramic dielectrics containing titanium dioxide by a reduction process followed by partial reoxidation, the reduced ceramic behaving as an n-type semiconductor. According to the porosity, barrier layers are formed either between the ceramic grains or at the ceramic surface. Dielectrics have been developed with permittivities between 105 and 106, loss tangents below 0.05, and very low temperature coefficients of permittivity between ¿40°C and +150°C. A limitation on the use of such dielectrics is that they are non-ohmic, the resistance falling steeply with increasing voltage and limiting the working potential to a few volts. The dielectrics are shown to be electrically equivalent to two resistance-shunted capacitors connected in series, corresponding to the stoichiometric and reduced portions of the ceramic. An unexpected feature is that the reduced ceramic behaves in itself as a dielectric with anomalously high permittivity. Properties of barrier-layer dielectrics based on a wide range of materials are given, together with preparatory techniques. Data are also presented on two types of commercial barrier-layer capacitors.