## 3 Results and Discussions  
### Preparation of Reticulated Porous Zirconia  
Polyurethane foam with pore densities of 25, 45, 60 and 80 PPI, as shown in Figure 1a, was used as a sacrificial polymer template. It was sintered at 1480 \({}^{\circ}\)C for 1 h to prepare the reticulated porous zirconia, as shown in Figure 1b. When manufacturing reticulated porous ceramics using the replica method, in order to obtain high compressive strength, it is important that the prepared ceramic slurry completely penetrates into the inside of the polymer template. For this reason, the rheological properties of ceramic slurry must be carefully considered when preparing reticulated porous ceramics. There are two considerations when optimizing the rheological properties of ceramic slurry. One is to adjust the properties of the raw materials, such as the particle size, shape, and fluidity of the ceramic powder, while the other is to control the type and content of any additives used, i.e., the binder, plasticizer, dispersant, and thickener, among others [15; 16]. Initially, the authors considered the solid loading amount in the zirconia slurry. If the solid loading level in the ceramic slurry is not sufficient, the result can be a thin coating and poor densification. On the other hand, if the solid loading level of the ceramic slurry is too high, the outcome can be a thick coating and blocked pores. Therefore, the solid loading level in the ceramic slurry must be optimized properly to ensure a uniform coating. Figure 1: Optical images of (**a**) commercial polyurethane foam samples with pore densities of 80, 60, 45, and 25 PPI (left to right, respectively), which were used as sacrificial polymer templates as part of the replica method, and (**b**) reticulated porous zirconia specimens with pore densities of 80, 60, 45, and 25 PPI (left to right, respectively) sintered at 1480 \({}^{\circ}\)C.  
Figure 2a-c show the density, linear shrinkage, and compressive strength outcomes of reticulated porous zirconia specimens with different solid loading levels (75-150 wt.%) in the zirconia slurry. With solid loading of 125 wt.%, the density and compressive strength reached a peak, and when the solid loading level was 150 wt.%, these values decreased. This can be explained as follows: logically, an increase in the solid loading level of the zirconia slurry can result in an increase in the viscosity. Accordingly, the density and compressive strength increase. However, when the solid loading level reaches 150 wt.%, the viscosity is too high and the fluidity of the zirconia slurry drops, resulting in a non-uniform coating and eventually causing the compressive strength to decrease as well. The viscosity outcomes with different solid loading levels (75-150 wt.%) in the zirconia slurry are shown in Figure 2c. As the solid loading level was increased, the viscosity of the zirconia slurry gradually increased, and when the solid loading level was 125 wt.%, the increase in the viscosity was greatest. Although this condition has the highest density and compressive strength, the authors utilized a solid loading level of 100 wt.% in the ensuing experiment. The aim of this paper is to investigate the effects on the compressive strength of reticulated porous zirconia using diverse process conditions. Accordingly, the authors decided to conduct a follow-up experiment under these conditions because the result at 100 wt.% solid loading showed appropriate compressive strength and relatively low viscosity. After fixing the solid loading level in the zirconia slurry to 100 wt.%, the effects of a change in the average particle size on the characteristics of the reticulated porous zirconia  
Figure 2: (**a**) Density and linear shrinkage outcomes, (**b**) compressive strength, and (**c**) viscosity values of reticulated porous zirconia specimens with different solid loading rates of 75–150 wt.%. were investigated. When the number of particles in the ceramic slurry is constant, the viscosity of the ceramic slurry increases as the particle size becomes finer.