Influences of the electroless nickel electrode on the electrical characteristics of BaTiO3-based PTCR ceramics

This paper investigated the influences of the microstructure of electroless nickel electrode on the a.c. shock characteristics of BaTiO3-based PTCR ceramics and the degradation and failure of PTC thermistors with electroless nickel electrode. In the experiments, we coated metal nickel electrode on the surface of PTCR ceramics samples by employing the different electroless plating baths and analyzed the microstructure of electroless nickel electrodes by SEM. The experimental results indicated that the various baths made the microstructure of nickel electrodes different. The nickel electrodes from acid bath possessed the densified stacked structure, which resulted in being cracked when being shocked by 220 V, 3 A a.c. power; owing to having sparse branch-and-tendril structure, those from alkaline bath had no effects on the ceramics samples being shocked. The work came to the conclusion that the microstructure of electroless nickel electrodes varied the thermal stress in the body of PTCR ceramics. In addition, the a.c. shock characteristics of BaTiO3-based PTCR ceramics 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 less than 1 μm and the phosphorus content, 4 wt.% in the alloy.