The effect of Sm2O3 doping on the microstructure and electrical properties of the ZPCCA-based varistors is comprehensively investigated. The increase of doping content of Sm2O3 results in better densification (from 5.70 to 5.82 g/cm3) and smaller mean grain size (from 7.8 to 4.1 μm). The breakdown electric field increases significantly from 2568 to 6800 V/ cm as the doping content of Sm2O3 increases. The doping of Sm2O3 remarkably improves the nonlinear properties (increasing from 23.9 to 91 in the nonlinear coefficient and decreasing from 35.2 to 0.2 μA/cm2 in the leakage current density). Meanwhile, the doping of Sm2O3 reduces the donor concentration (the range of 2.73 X 1018 to 1.18 X 1018 cm-3) of bulk grain and increases the barrier height (the range of 1.10 to 1.49 eV) at the grain boundary. The density of the interface states decreases in the range of of 5.31 X 1012 to 4.08 X 1012 cm-2 with the increase of doping content of Sm2O3. The dielectric constant decreases from 1594.8 to 507.5 with the increase of doping content of Sm2O3.

This paper focuses on the electrical properties and stability against DC accelerated aging stress of ZnO-V2O5-MnO2- Nb2O5-Bi2O3-Co3O4-Dy2O3 (ZVMNBCD) varistor ceramics sintered at 850 - 925 ℃. With the increase of sintering temperature, the average grain size increases from 4.4 to 11.8 mm, and the density of the sintered pellets decreases from 5.53 to 5.40 g/ cm3 due to the volatility of V2O5, which has a low melting point. The breakdown field abruptly decreases from 8016 to 1,715 V/cm with the increase of the sintering temperature. The maximum non-ohmic coefficient (59) is obtained when the sample is sintered at 875 ℃. The samples sintered at below 900 oC exhibit a relatively low leakage current, less than 60 mA/cm2. The apparent dielectric constant increases due to the increase of the average grain size with the increase of the sintering temperature. The change tendency of dissipation factor at 1 kHz according to the sintering temperature coincides with the tendency of the leakage current. In terms of stability, the samples sintered at 900 ℃ exhibit both high non-ohmic coefficient (45) and excellent stability, 0.8% in ΔEB/EB and -0.7% in Δα/α after application of DC accelerated aging stress (0.85 EB/85 oC/24 h).

This study describes the doping effect of Yb2O3 on microstructure, electrical and dielectric properties of ZnO-V2O5- MnO2-Nb2O5 (ZVMN) ceramic semiconductors sintered at a temperature as low as 900°C. As the doping content of Yb2O3 increases, the ceramic density slightly increases from 5.50 to 5.54 g/cm3; also, the average ZnO grain size is in the range of 5.3-5.6 μm. The switching voltage increases from 4,874 to 5,494 V/cm when the doping content of Yb2O3 is less than 0.1 mol%, whereas further doping decreases this value. The ZVMN ceramic semiconductors doped with 0.1 mol% Yb2O3 reveal an excellent nonohmic coefficient as high as 70. The donor density of ZnO gain increases in the range of 2.46-7.41×1017 cm−3 with increasing doping content of Yb2O3 and the potential barrier height and surface state density at the grain boundaries exhibits a maximum value (1.25 eV) at 0.1 mol%. The dielectric constant (at 1 kHz) decreases from 592.7 to 501.4 until the doping content of Yb2O3 reaches 0.1 mol%, whereas further doping increases it. The value of tanδ increases from 0.209 to 0.268 with the doping content of Yb2O3.

The effect of sintering temperature on the microstructure, electrical and dielectric properties of (V, Mn, Co, Dy, Bi)- codoped zinc oxide ceramics was investigated in this study. An increase in the sintering temperature increased the average grain size from 4.7 to 10.4 μm and decreased the sintered density from 5.47 to 5.37 g/cm3. As the sintering temperature increased, the breakdown field decreased greatly from 6027 to 1659 V/cm. The ceramics sintered at 900 oC were characterized by the highest nonlinear coefficient (36.2) and the lowest low leakage current density (36.4 μA/cm2). When the sintering temperature increased, the donor concentration of the semiconducting grain increased from 2.49 × 1017 to 6.16 × 1017/cm3, and the density of interface state increased from 1.34 × 1012 to 1.99 × 1012/cm2. The dielectric constant increased greatly from 412.3 to 1234.8 with increasing sintering temperature.