Effects of boron oxide addition on electrical properties of yttrium-doped bismuth-based zinc oxide varistors

The optimal amount of B₂O₃ (0.75 mol%) drastically improved the electrical properties of Y-doped varistors, reducing the leakage current to 0.8 μA/cm² while maintaining exceptionally long-term stability against DC electrical degradation and a varistor voltage around 900 V/mm. Boron ions were found with Bi ions in Y-compounds, which contributed to formation of a Bi-rich glass phase containing B at grain boundaries when the B₂O₃ content was above 0.75 mol%. The change in microstructure affected the donor density and barrier structure, leading to a change in electrical properties. The Bi-rich phase first reduced the Co concentration in the ZnO grains, thereby decreasing the donor density N_D. When large amount of B₂O₃ was added, some Co remained in ZnO grains, causing an increase in N_D. A new analytical method using the differential resistance R_D for the nonlinear voltage–current density relation was proposed to clarify the effects of B₂O₃ addition. This method separates the three differential resistance R_Di values (i = 1, 2, 3) and was used to analyze the corresponding conduction process in forward-biased region I, reverse-biased region III, and interface region II of the energy band. The R_D1 and R_D2 values exhibited junction diode-like behavior in regions I and III. The value of R_D2 that correlated with α_asMAX and the long-term stability of the varistor and R_D3 may be related to region II.