# Comparative Characterization of Different Cutting Strategies for the Sintered ZnO Electroceramics
## 3 Results and Discussion  
Metal oxide varistors (MOVs) are voltage-dependent resistors (VDRs) having nonlinear non-ohmic current-voltage features that offer safety against high surge current [23]. The non-linear characteristics depend strongly on the microstructure, which is developed intrinsically for a certain chemical composition and applied processing methods [24]. The microstructure of a typical MOV comprises approximately 90% of dark grey ZnO matrix grains a few tens of microns in size, as shown in Figure 6. For a nonlinear non-ohmic response, the varistor forming oxide (VFO), mainly involving Bi2O3, is added to the composition. To augment the threshold voltage and surge current endurance, the number of intergranular (IG) layers in series and parallel, respectively, need to be maximized. Hence, for high surge shielding, finer matrix ZnO grains and high grain boundary (GB) surface area are prerequisites [22; 25; 26; 27; 28; 29]. In addition to VFO, Sb2O3, Al2O3, Cr2O3, SnO2,  
Figure 4: Schematic illustration of waterjet machining setup. Figure 5: Abrasive waterjet cutting operation on ZnO varistors. MnO\({}_{2}\)/Mn\({}_{3}\)O\({}_{4}\), NiO, and Co\({}_{3}\)O\({}_{4}\) are added in minor amounts for gaining non-linearity factor (\(\alpha\)) enhancement, grain size refinement, Schottky barrier height elevation, and inversion boundary (IB) generation [22; 23; 27; 28; 29] for high transient voltage protection [26]. The sintering conditions can have a significant impact on the end microstructure of ZnO MOVs, as shown in Figure 6. The green samples were placed on the refractory ceramic boat during the sintering operation, so the bottom surface was in contact with the refractory, whereas the top surface remained in contact with air. With the ceramic Al\({}_{2}\)O\({}_{3}\) crucible backing layer present during the sintering, the end products are usually flat, and the microstructure is well defined, as shown in Figure 6a,b. These alumina crucibles (>99% Al\({}_{2}\)O\({}_{3}\)) are routinely used for the sintering operations up to 1700 \({}^{\circ}\)C and may contain traces of silica (SiO\({}_{2}\)) and magnesia (MgO). Thus, the bottom part usually retains a well-defined and smooth microstructure, which is unlike the case for the top side, which is in contact with air, as shown in Figure 6c,d. The topography indication (secondary electron imaging--SE) is shown in Figure 6a, and well aligned flat ZnO grains were present with very little porosity. The phase contrast image in Figure 6b with back scattered electron (BSE) detector clearly illustrated the presence of a Bi\({}_{2}\)O\({}_{3}\) intergranular phase (IP). The \(\alpha\)-Bi\({}_{2}\)O\({}_{3}\)-rich IP at the grain boundaries (GBs), triple pockets, and ZnO grain junctions appeared as bright regions in the microstructure, of widths from 0.1 to 1 \(\upmu\)m, reliant on sintering conditions [22; 26]. The Sb\({}_{2}\)O\({}_{3}\) oxide was supplemented to restrict the ZnO grain growth and boost the solubility above the 740 \({}^{\circ}\)C eutectic of Zn in the IG phase. The Sb\({}_{2}\)O\({}_{3}\) oxide supports the phase transformation above 900 \({}^{\circ}\)C to the spinel (Zn\({}_{7}\)Sb\({}_{2}\)O\({}_{12}\)) phase (S) shown in Figure 6b along the IG, and this reaction is activated by pyrochlore (Bi\({}_{3}\)Zn\({}_{3}\)Sb\({}_{3}\)O\({}_{14}\)) phase (P) decomposition above this temperature range during sintering [27; 28]. The spinel (S) phase appears in light greyish tone enclosed within the bright IG phase, whereas the pyrochlore (P) phase can exist as  
Figure 6: SEM images of ZnO varistors after sintering with (**a,b**) having backing ceramic (refractory Al\({}_{2}\)O\({}_{3}\) crucible) to cover the bottom surfaces, whereas (**c,d**) shows sintering of ZnO varistor green compact freely in air, i.e., top side. Here (**a,c**) represent secondary electron (SE) imaging for topographical features, while (**b,d**) represent phase contrast and microstructural composition in backscattered electron (BSE) imaging mode. scattered whitish precipitates at the GBs.