The ac electrical failure behaviors and mechanisms of current limiting BaTiO3-based positive-temperature-coefficient (PTC) ceramic thermistors coated with electroless nickel–phosphorous electrode

Abstract

The failure behaviors, by the action of ac electric field, of commercial BaTiO 3 -based positive-temperature-coefficient (PTC) current-limiting ceramic thermistors coated with electroless Ni electrodes were investigated. We observed five failure behaviors, namely, (1) comminuted cracking; (2) the rise of resistance value at room temperature; (3) the burnout of electrodes; (4) delamination of ceramic; and (5) the melting of ceramic. Indeed, these phenomena were correlated to the stress (including thermal and internal stresses), electric field distribution, temperature distribution, production and expansion of micro-cracks, and voltage effect. In addition, the ac shock characteristics of BaTiO 3 -based PTCR ceramic were related to the content of phosphorus in the electroless nickel–phosphorus alloy and thickness of the nickel electrodes. According to the experiments, the thickness of the electroless nickel alloy should be <1 μm and the phosphorus content, 4 wt.% in the alloy.