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 mu A/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 Ycompounds, 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 ND. When large amount of B2O3 was added, some Co remained in ZnO grains, causing an increase in ND. A new analytical method using the differential resistance RD for the nonlinear voltage-current density relation was proposed to clarify the effects of B2O3 addition. This method separates the three differential resistance RDi 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 RD1 and RD2 values exhibited junction diode-like behavior in regions I and III. The value of RD2 that correlated with alpha asMAX and the long-term stability of the varistor and RD3 may be related to region II.