Effects of B2O3 addition on electrical characteristics of Bi-based ZnO varistor

ZnO-based varistors are widely used in different circuits and power systems as surge absorbers because of the typical high nonlinear voltage–current (V-I) characteristics. Double Schottky barriers formed between ZnO grains... [ view full abstract ]

ZnO-based varistors are widely used in different circuits and power systems as surge absorbers because of the typical high nonlinear voltage–current (V-I) characteristics. Double Schottky barriers formed between ZnO grains is a main feature of a varistor. By doping several kinds of additives, the V-I characteristics change remarkably. However, the electrical degradation occurs under continued voltage simultaneously. The microstructural and electrical characteristics of Zn-Bi-Mn-Co-Cr-Ni-Y-Sb ceramics doped with B₂O₃ have been investigated in order to increase both the varistor voltage and the resistance of electrical degradation. It was not satisfactory although the resistance of degradation could be improved. It is speculated the electrical degradation is caused by the motion of Zn²⁺ and O^2- ions under voltage application. Thus, in this study, addition of B₂O₃ was examined to block the motion of these mobile species. The V–I characteristics and isothermal Capacitance transient spectroscopy (ICTS) signal were measured. The crystal structure (XRD) and the chemical composition (EDS), and the surface state (SEM, BSE) were evaluated.

It is found from element mapping by EDS that Sb atoms shifted to Bi-Y-O phase along the grain boundaries of ZnO grains by adding the proper amount of B₂O₃(See in Fig.1). Mean grain size did not change much with increasing amount of B₂O₃. For samples with SiO₂, it was found from XRD analysis that the Bi₂O₃ diffraction peaks intensity decreased or disappeared by increasing the amount of B₂O₃. The varistor voltage was stable around a high voltage from 900 to 1100 V/mm(See in Fig.2). It is confirmed by the leakage current-time characteristics measure at 130°C that the resistance to electrical degradation of samples was improved drastically by adding B₂O₃ of 1mol%(See in Fig.3). It was conjectured that excess Sb atoms enhance the motion of mobile species.