# Effect of the Sintering Temperature on the Compressive Strengths of Reticulated Porous Zirconia
## 3 Results
If the viscosity of the zirconia slurry is too high and the fluidity is reduced, the zirconia slurry cannot be completely coated onto the strut walls inside the sacrificial polymer template. Therefore, as shown in Figure 8b, voids arise inside the sacrificial polymer template, reducing the compressive strength. In contrast, if the viscosity of the zirconia slurry is too low and the fluidity is increased, the coating layer of the sacrificial polymer template strut walls becomes non-uniform and very thin. Therefore, the shape of the sacrificial polymer template cannot be maintained, making it difficult to complete the reticulated porous zirconia specimen. In addition, the low viscosity causes the number of cracks and defects to increase as the strut walls are completely uncoated. This reduces the compressive strength of the reticulated porous zirconia. Figure 7: Typical SEM image of the reticulated porous zirconia samples, sintered at (**a**) 1400 \({}^{\circ}\)C, (**b**) 1500 \({}^{\circ}\)C, (**c**) 1600 \({}^{\circ}\)C, and (**d**) 1600 \({}^{\circ}\)C; these reticulated porous zirconia specimens were prepared from fine zirconia particles. When the reticulated porous zirconia was sintered at 1400 \({}^{\circ}\)C, the compressive strength outcomes for each of the sample, from the as-received, coarse, intermediate, and fine zirconia particles, according to the change in the PPI value, are shown in Figure 9a. It has been reported that the larger the average particle size difference is, the more different the sintering behavior itself, and the finer the particles are, the more decreased the compressive strength becomes [26]. In addition, fine particles generally consist somewhat of agglomerated particles, causing non-uniformity and thus reducing the densification when sintered [28]. However, in this study, because the zirconia average particle size difference was not great, the compressive strength increased with the fine average particle size. In addition, as shown in Figure 4b, the compressive strength increases to 60 PPI because the zirconia slurries are uniformly coated onto the strut wall inside of the sacrificial polymer template. Figure 9b shows the density outcome when the reticulated porous zirconia is sintered at 1400 \({}^{\circ}\)C. Overall, this figure shows a trend identical to that of the compressive strength. Figure 8: (**a**) Viscosities of the zirconia slurries for the preparation of reticulated porous zirconia: as-received, coarse, intermediate, and fine zirconia particles; (**b**) fractured reticulated porous zirconia specimen with a pore density of 80 PPI, which possesses internal macro-voids. Figure 9: (**a**) Compressive strength outcomes of the reticulated porous zirconia prepared from the as-received, coarse, intermediate, and fine zirconia particles when sintered at 1400 \({}^{\circ}\)C, and (**b**) the corresponding density levels. When the reticulated porous zirconia was sintered at 1500 \({}^{\circ}\)C, the compressive strength outcomes for each of the cases, from the as-received, coarse, intermediate, and fine zirconia particles, according to the change in the PPI value, are shown in Figure 10a. Overall, as the average particle size decreases, the compressive strength increases. Furthermore, as the degree of the coating of the strut walls with the zirconia slurries increases (as the PPI approaches 60), the compressive strength also increases. It can be seen that the zirconia slurries can be coated up to 60 PPI with the sacrificial polymer template. Figure 10b shows the densities when the reticulated porous zirconia is sintered at 1500 \({}^{\circ}\)C. As the PPI increases and the average particle size decreases, the density increases. When the reticulated porous zirconia was sintered at 1600 \({}^{\circ}\)C, the compressive strength outcomes for each case, from the as-received, coarse, intermediate, and fine zirconia particles, according to the change in the PPI value, are shown in Figure 11a. As the average particle size decreases and the PPI increases, the compressive strength increases.