**4.2. Effect of Er addition on Co, Nb-doped SnO2 varistors**

The addition of Er2O3(Co, Nb)-doped SnO2 was studied by Qi et al. [70] at different concen‐ trations (0.1, 0.5, 1, and 2 mol%) and different temperatures of sintering (1250 °C, 1300 °C, and 1350 °C for 1 h).The XRD analysis carried out by the authors did not show evidence of the second phase formation into the SnO2-rutile crystalline phase. The SEM micrographs of the varistors prepared are in Figure 18, showing the decreases of grain size associated with the increase of Er2O3 concentration into ceramic matrix. Also, the decreases of grain size occur with lower temperature of sintering. With the addition of 2.0 mol% of Er2O3 modifier agent, the SnO2 grain size was reduced from 12.9 μm to 6.5 μm when the sample sintered at 1350°C for1 h, from 9.7 μm to 3.7 μm when sample was sintered at 1300°C for 1 h, and from 6.8 μm to 2.4 μm when samples were sintered at 1250°C for 1 h.

Figure 19 shows the plots of applied electric field versus current density for different concen‐ trations of Er2O3 sintered at 1350 °C, 1300 °C, and 1250 °C during 1 h. It was observed from Figure 19 that the threshold voltage of the SnO2-based varistors increased significantly from 305 V mm−1 to 1083 V mm−1 with increasing Er2O3 concentrations over the range of 0–2.0 mol % sintered at 1350°C during 1 h and from 1083 V mm−1 to 2270 V mm−1 with decreasing sintered temperatures from 1350°C to 1250°C during1 h. Only the samples sintered at 1300 °C have decrease on nonlinear coefficient with Er2O3 addition. There is no observed significant change on height of the potential barrier for samples sintered ate 1250°C and 1300 °C.

**Figure 18.** Microstructure variation of the SnO2-based varistor system sintered at 1350 °C, 1300 °C, and 1250 °C during 1 h with the composition (all in mol%): 100SnO2 + 0.75Co2O3 + 0.1Nb2O5 + *x*Er2O3: (from top to bottom) *x* = 0.0, *x* = 0.1, *x* = 0.5, *x* = 1.0, *x* = 2.0 [70].

**Figure 19.** E versus J curves for the SnO2-based varistor system sintered at different temperatures during 1 h with the composition (all in mol%) 100SnO2 + 0.75Co2O3 + 0.1Nb2O5 + *x*Er2O3 (*x* ranging from 0.0 to 2.0) [70].
