Preparation and electrical properties of Ni/(Ba,Sr)TiO3 PTC composite with low resistivity

Ni/ (Ba,Sr)TiO₃ PTC composite of low resistivity was fabricated by a solid state route. A mildly reducing sintering atmosphere was employed to avoid the oxidation of nickel. Metallic nickel is the main chemical state after sintering in the mildly reducing sintering atmosphere. With the increase in nickel amount, the room-temperature resistivity declines and the PTC effect worsens. The quantum mechanical tunneling effect at the Ni–(Ba_,Sr)TiO₃ interface is presumably the prime factor in the deterioration of the PTC effect. PbO–B₂O₃–ZnO–SiO₂ glass was added to modify the interface between nickel and (Ba,Sr)TiO₃ ceramics. The intergranular phase introduced by the glass has an amorphous structure and exists at the interfaces and triple junctions of (Ba,Sr)TiO₃ grains and nickel grains. No obvious diffusion occurs at the interface between crystalline (Ba,Sr)TiO₃ grain and the intergranular phase. Also the added-glass improves the distribution of metal phase. The proper glass addition screens interfacial electron tunneling effect and improves the composite electrical properties. An abundance of the intergranular phase due to excess glass will, however, result in high room-temperature resistivity. The influences of nickel amount and glass amount on the microstructure evolution and electrical properties were analyzed.