Microstructural Electrical Characteristics of ZnO Varistors

A bulk varistor is a complex multijunction device composed of large numbers of grain boundaries and grains connected in a random network. The electrical properties of each individual grain boundary will contribute to the global electrical characteristics of ZnO varistors. The electrical characteristics of grain boundaries in ZnO varistors were systematically analyzed. High nonuniformity exists in barrier voltages and nonlinear coefficients in different grain boundaries. The barrier voltages have normal distributions; only a few grain boundaries are electrically active. The grain boundaries can be simply classified into good, bad, and ohmic according to the electrical characteristics of grain boundaries. A high percent of grain boundaries have bad or ohmic I–V characteristics, and only a small percent of grain boundaries have good I–V characteristics. These few good grain boundaries are responsible for the good varistor effect and control the leakage current of ZnO varistor at low values of applied voltage. Modern scanning probe microscopy based techniques are introduced, which allow to study electric properties of ZnO varistors on the nanometer scale, and combining all methods, a quite comprehensive assessment of the distribution of possible causes of current paths in a varistor ceramic will be presented. At last, a ZnO bicrystal with controlled dopants at the interface is introduced for studying the electrical characteristics and related mechanisms, but the fabrication of Bi-doped bicrystal with excellent nonlinearity is still challenging.