## 3 Results
Therefore, a preliminary experiment was performed with fine alumina. Figure 3: (**a**) Particle size distributions of alumina and zirconia; Typical SEM images of as-received alumina particles are shown: (**b**) coarse, and (**c**) fine. Figure 5a shows a plot of the viscosity vs. the shear rate of the ZTA slurries used to prepare the reticulated porous ZTA. As the content of zirconia increases, the viscosity increases. The viscosities are higher for the fine alumina particles as a matrix than for the coarse alumina particles as a matrix. This occurs because the average particle size decreases, the larger specific surface area increases, and the area where the polyvinyl alcohol (PVA) adheres to the particle increases. The compressive strength values according to the zirconia content are shown in Figure 5b. As the zirconia content increases, the compressive strength increases. When the matrix used is fine alumina, 40 wt. % of added zirconia, the maximum compressive strength is 0.705 MPa. When the matrix is fine alumina, with 40 wt. % of zirconia added, the maximum compressive strength is 3.171 MPa. Figure 5c shows the densities and linear shrinkage rates according to the zirconia content. Overall, the densities show a trend similar to that of the compressive strength. In addition, the shrinkage rate of fine alumina is higher than that of coarse alumina; it can be observed that the strut walls are not fully densified due to their coarse particle size. In addition, as the zirconia content increases, more zirconia particles exist between the alumina particles, meaning that the densification improves, and the shrinkage rate increases. This confirms that the particle size of the raw material affects the densification outcome. Densification also affects the compressive strength as shown in Figure 5b. The X-ray diffraction patterns of the phases of ZTA with different zirconia contents are shown in Figure 5d. The zirconia consisted of a small amount of monoclinic zirconia and tetragonal zirconia. As the content of zirconia increases, the peak intensity of the tetragonal and monoclinic zirconia increases. The alumina peak intensity also decreases. Figure 4: Viscosity at a shear rate of 0.01 s\({}^{-1}\) of the alumina slurries, and the compressive strengths with 58–67 wt. % solid loading. Figure 5: _Cont._  
Figure 6a,b show typical BSE images when using coarse alumina as a matrix with zirconia added at rates of 10 wt. % and 40 wt. %, respectively. The lighter-colored particles are zirconia. The coarse alumina particles do not have enough sintering driving force to be fully densified due to the presence of coarse particles. As the content of zirconia increases, the particle size of alumina decreases, and the particle size of zirconia increases. This occurs because zirconia has some drag force at the alumina matrix grain boundary, thereby inhibiting the grain growth of alumina. In addition, as the amount of zirconia increases, the grain growth of zirconia also increases readily with these combinations. Typical BSE images of reticulated porous ZTA specimens using fine alumina as a matrix with zirconia added at rates of 10 wt. % and 40 wt. % are, respectively, shown in Figure 6c,d. It can be seen that when a smaller particle size of alumina is used as a matrix, the outcome is fewer pores and more densification of the microstructure. It was confirmed that the added zirconia particles were present between the alumina particles in the matrix and thus had the effect of inhibiting mutual particle growth between the alumina and zirconia during the sintering process. The thickener used with coarse alumina as a matrix has relatively low viscosity compared to that used with fine alumina as a matrix. This is done to improve the compressive strength (addition of zirconia: 40 wt. %). The thickener prevents separation between materials by increasing the thickening properties and serves to hold water necessary for hydration, thereby enhancing the mechanical strength of the material.