## 3 Results and Discussions
### Preparation of Reticulated Porous Zirconia
Therefore, the authors attempted to utilize the ball-milling process to create a finer size and observed the effects of the particle size. The microstructures of the zirconia particles obtained with different ball-milling times of 0, 1, 4, 8, and 24 h are shown in Figure 3. The microstructure of the zirconia particles after ball-milling for 4 h shows an uneven particle distribution, but it was also noted that many particles approximately 0.5 \(\upmu\)m or less in size are distributed throughout the specimen. Furthermore, an examination of the zirconia particles ball-milled for 24 h showed that the particle distribution is uniform and consists of spherical particles. These results indicate that the cohesion of the primary particle size of the raw material was reduced and that more uniform zirconia powder can be prepared using the ball-milling process. Figure 3: _Cont._  
We used distilled water as a solvent for the particle size analysis, and an ultrasonic disperser was used to improve the dispersibility of the zirconia particles in the solution. The particle size distributions of zirconia are shown in Figure 4a and the median particle sizes of zirconia which were measured by laser particle size analysis and SEM image analysis are shown in Figure 4b. As expected from the microstructure observations, the median particle sizes of the zirconia decreased correspondingly in the order of 1.04, 0.62, 0.34, 0.32, and 0.2 \(\upmu\)m with an increase in the ball-milling time. Additionally, expected from the SEM images in Figure 3, the median particle size of zirconia measured by SEM image analysis is not precise. Therefore, it can be suggested that laser particle size analysis is efficient to track a slight change in mean particle size during the ball-milling process. When the ball-milling time was four hours, the greatest decrease in the particle size occurred, and when ball-milling time exceeded 24 h, the particle size approached 0.2 to 0.3 \(\upmu\)m. At this point, even if the ball-milling time was increased continuously, the particle size did not decrease further, Hence, it was judged that the formed particle size using the ball-milling process was limited. In order to investigate the effects of the reduced particle size, we initially measured the viscosity of the zirconia slurry. As shown in the results in Figure 4c, the viscosity of the zirconia slurry increased gradually as the ball-milling time increased. Typically, the viscosity of ceramic slurry is a well-known function of the concentration ratio and the particle size ratio of the total solid loading level. Thus, it is believed that the finer the particle size is, the higher the packing density of the particles in the ceramic slurry becomes, and the more liquid is absorbed into the pores between the particles. It is believed that more liquid moves effectively when flowing, resulting in relatively high viscosity. Figure 3: Scanning electron microscopy (SEM) images of zirconia particles ball-milled for (**a**) 0 h (as-received), (**b**) 1 h (coarse), (**c**) 4 h (intermediate), (**d**) 8 h (fine), and (**e**) 24 h (fine). We confirmed that the viscosity increased as the particle size was decreased. With this in mind, the densities and compressive strength were measured, as shown in Figure 5a. As the particle size of zirconia was decreased, the density and compressive strength of the reticulated porous zirconia clearly increased. However, sharp decreases were noted when ball-milling for more than 24 h. This result is notable in that the highest density and compressive strength did not appear at the highest viscosity. What this result suggests is that the ceramic slurry, when created with a viscosity level past a certain point, deteriorates the density and compressive strength of reticulated porous zirconia. All things considered, the average particle size decreased after ball-milling for 24 h and the viscosity of the zirconia slurry increased above a critical point, indicating that a non-uniform coating could be formed.