# Effect of the Sintering Temperature on the Compressive Strengths of Reticulated Porous Zirconia
## 3 Results
The three-dimensional microstructure of a typical reticulated porous zirconia specimen (pore density = 60 PPI), reconstructed by \(\upmu\)-CT, is shown in Figure 4a. A representative cross-section from the micro-CT reconstructions of the specimen is shown in Figure 4b. This figure indicates that the reticulated porous zirconia specimen had well-defined inter-connected pore channels, as intended. The pore channels beneath the surface were not substantially blocked. The figure also demonstrates the effectiveness of how the zirconia slurry penetrated deep inside the sacrificial polymer template, proving that reticulated porous zirconia at 60 PPI can be prepared. The particle size distributions of the zirconia, denoted here as as-received, coarse, intermediate, and fine, is shown in Figure 5. Zirconia particles with an appropriate particle size were selected for use here. If the particle size is too coarse, densification by sintering will be suppressed. Conversely, a nano-sized particle size is well densified, but agglomeration becomes severe such that sintering shrinkage and cracks are generated, reducing the compressive strength. Thus, zirconia with a sub-micron particle size was judged to be appropriate through other experimental results by the present researchers [26]. The particle size is a process variable for obtaining the optimum composition of reticulated porous zirconia specimens. As the particle size decreases, the particle size distribution decreases to 1.435 \(\upmu\)m, 1.236 \(\upmu\)m, 1.020 \(\upmu\)m, and to 0.701 \(\upmu\)m relative to the values above. Figure 6a shows the XRD patterns of the phases of 3 mol% yttria-stabilized zirconia (3 YSZ) with different sintering temperatures. In this study, yttria-stabilized zirconia (YSZ) with the addition of yttria was used. YSZ is well known to prevent the generation of cracks by a phase transformation (tetragonal phase to monoclinic phase) during cooling. For sintered 3 YSZ at 1400, 1500, and 1600 \({}^{\circ}\)C, it was observed that the main phase is tetragonal (t), and no monoclinic (m) phase of ZrO\({}_{2}\) was observed. It was confirmed to have a stable tetragonal phase even at room temperature after cooling. The linear shrinkage outcomes as the sintering temperature is increased are shown in Figure 6b. As the sintering temperature  
Figure 4: (**a**) Three-dimensional microstructure of a typical reticulated porous zirconia specimen (pore density = 60 PPI), reconstructed by \(\upmu\)-CT; and (**b**) representative slice extracted from the \(\upmu\)-CT reconstructions. increases, the shrinkage rate increases. This can be judged as stemming from a decrease in the number of open pores due to densification between the particles as the sintering temperature increases. The density change of the reticulated porous zirconia with an increase the sintering temperature is shown in Figure 6c. Regardless of the change in the sintering temperature, it can be seen that the density increases as the PPI value of the sacrificial polymer template increases. The increase in the density with an increase the sintering temperature and PPI values can be determined to stem from the reduced pore size as the PPI values increase. Figure 6d shows the pore size distributions of the reticulated porous zirconia specimens (pore density = 45 PPI). The integral area is visualized with a quantity of mercury penetrating into the porous interior. At this point, mercury cannot penetrate into the closed pores; it can only penetrate into the open pores. As the sintering temperature increases to 1400 \({}^{\circ}\)C, 1500 \({}^{\circ}\)C, and 1600 \({}^{\circ}\)C, the mercury penetration decreases to 0.142 mL/g, 0.138 mL/g, and 0.097 mL/g, respectively. Because the number of strut walls is fixed, the difference in the pore size distribution varies with the sintering temperature. It is judged that the number of open pores decreased due to densification between the particles as the sintering temperature was increased.