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-co efficient (PTC) thermistors with NiCr/Ag electrodes was investigated by imp edance analysis in the frequency domain. The contact resistance and thus th e total resistance of the PTC increases with decreasing thickness as well a s with decreasing a rea of the NiCr layer at constant Ag top layer. The inc rease 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 Ni Cr which shows an ohmic behaviour. Using our model, the electrode impedance contribution of very thin NiCr layers can be interpreted in terms of incom plete wetting of the ceramic surface by the NiCr metal during the depositio n. Supplementary scanning electron microscopy and high-resolution transmiss ion electron microscopy analyses have been used to study the structure of t he electrode interface and the structure of the layers. (C) 1998 Kluwer Aca demic Publishers.