Influence of the thickness and area of NiCr/Ag electrodes on the characteristics of BaTiO3- ceramic based positive-temperature-coefficient thermistors

The electrical behaviour of commercial BaTiO3-based positive-temperature-coefficient (PTC) thermistors with NiCr/Ag electrodes was investigated by impedance analysis in the frequency domain. The contact resistance and thus the total resistance of the PTC increases with decreasing thickness as well as with decreasing a rea of the NiCr layer at constant Ag top layer. The increase in the total resistance in both cases is explained by the increase in the contact area of the Ag layer to the ceramic which is represented by a model. The Ag forms a blocking contact to the ceramic in contrast to the NiCr which shows an ohmic behaviour. Using our model, the electrode impedance contribution of very thin NiCr layers can be interpreted in terms of incomplete wetting of the ceramic surface by the NiCr metal during the deposition. Supplementary scanning electron microscopy and high-resolution transmission electron microscopy analyses have been used to study the structure of the electrode interface and the structure of the layers. (C) 1998 Kluwer Academic Publishers.