Effects of porosity on the electrical characteristics of current-limiting BaTiO3-based positive-temperature-coefficient (PTC) ceramic thermistors coated with electroless nickel-phosphorous electrode

In the present paper, we have intensively investigated the effects of porosity on the electrical characteristics of current-limiting BaTiO3-based positive-temperature-coefficient (PTC) ceramic thermistors coated with electroless nickel-phosphorous electrode by employing scanning electron microscopy (SEM) and ac current inrush test. It was found that the PTCR effect, dielectric property, ac current inrush capability and internal stress of specimens were influenced by porosity. In fact, porosity in BaTiO3 ceramics was related to not only organic and inorganic inclusions in powders but also formation pressure of green compacts. PTCR effect of BaTiO3-based PTCR ceramic thermistors with various porosities could be evaluated by the barrier height of Heywang's double-Schottky barrier model, and dielectric constants could be estimated by the modified Niesel's equation and Maxwell relationship. The results suggested that their PTCR effect and current-withstanding capability of thermistors could be improved by increasing the porosity in ceramic disks. In order to obtain the current-limiting PTCR ceramic disks with high performance, the porosity of disks should be between 6 and 14%, while formation pressure should be chosen 500-1550 kg cm(-2). (C) 2003 Elsevier B.V. All rights reserved.