## 3 Results and Discussions
### Effects of Multiple Slurry Coatings on Reticulated Porous Zirconia
Macro voids, which may be caused by non-optimized process conditions, are not shown in Figure 11.  
In Figure 12, at pore densities of 45, 60, and 80 PPI, the dielectric breakdown strength outcomes of the reticulated porous diatomic-kaolin composite sintered at 1200 \({}^{\circ}\)C [26], the reticulated porous alumina sintered at 1600 \({}^{\circ}\)C and the reticulated porous zirconia sintered at 1480 \({}^{\circ}\)C are shown, respectively. To the best of the authors' knowledge, few works have examined the dielectric breakdown strength capabilities of reticulated porous ceramic materials. This topic is difficult to explore because high porosity can decrease the dielectric breakdown strength [27]. The dielectric breakdown strength capabilities of porous ceramics such as porous alumina (porosity \(<\) 10%, 11.8 kV/mm) [28], porous yttria stabilized zirconia (porosity \(<\) 5%, 1.86-2.08 MV/cm) [29], and porous lead zirconate titanate (porosity \(<\) 10%, 6.3 kV/mm) have been investigated under low-porosity conditions. However, for a proper understanding of the effect of high porosity on the dielectric breakdown strength, it is necessary to measure specimens with high porosity. The dielectric breakdown strengths of the reticulated porous zirconia ranged from approximately 10 to 15 kV/cm, regardless of the pore density. Measurements at a pore density of less than 45 PPI were difficult because these specimens were prone to arc generation; thus, the results were not obtained. However, the results in this paper are shown to be excellent when considering the high porosity. Additionally, these results are similar to those in previous studies of reticulated porous alumina and reticulated porous diatomic-kaolin composites, indicating that reticulated porous zirconia is lightweight and has excellent compressive strength while also having adequate dielectric breakdown strength, enabling a wide range of industrial applications, such as dielectric materials. Figure 11: (**a**) A typical three-dimensional microstructure of reticulated porous zirconia at 45 PPI prepared under optimized processing conditions and reconstructed by \(\upmu\)-CT. (**b**) Representative slice extracted to show the corresponding \(\upmu\)-CT reconstructions with a porosity level of 45 PPI. In summary, in a previous study, the authors manufactured reticulated porous zirconia which was sintered at 1600 \({}^{\circ}\)C at 45 PPI, with compressive strength of about 1.26 MP [30]. However, the authors determined that this compressive strength result was insufficient compared to the results of the other types of porous zirconia, such as porous yttria-stabilized zirconia [30,31], fibrous zirconia [32], and ultra-high-porosity zirconia [33], among others. It is necessary to study how to improve the compressive strength of reticulated porous zirconia to expand the application of these materials. This study focused on improving the compressive strength of reticulated porous zirconia by optimizing various process conditions, specifically the particle size, solid loading level, and types of additives. First, the optimal particle size was determined by varying the ball-milling time, and the effect on the compressive strength was observed. When reticulated porous zirconia was ball-milled for 0, 1, 4, and 8 h and sintered at 1480 \({}^{\circ}\)C with 45 PPI, the corresponding compressive strength outcomes increased to approximately 0.14, 0.87, 1.00, and 3.78 MPa. These results depict the effects of a reduction in the particle size as the ball-milling time increased. As the particle size decreases, the specific surface area increases, indicating an increase in the sintering driving force; it has been reported that the particle size is linked to improved compressive strength of the final sintered body [34]. However, if the particle size is reduced to the nano-sized range, this result will be affected by different sintering mechanisms.