## 3 Results and Discussions
### Effects of Changes in the Types and Amounts of Binder and Dispersants  
In the previous experimental results, reticulated porous zirconia with the highest compressive strength of 3.78 MPa was prepared after ball-milling for eight hours. Therefore, the subsequent condition of the zirconia slurry used a fixed particle size with eight hours of ball-milling. The second part of this study considered the optimization of additives in the zirconia slurry. Among the various types of additives in use at present, this study concentrated on the type and amount of a binder and a dispersant, which are known to affect the properties of ceramic slurries significantly. Normally, polyvinyl alcohol (PVA), which is often used as an organic binder in ceramic slurries, can cover on the surfaces of these particles to increase the bonding strength between the particles and improve the fluidity. However, because it is removed during heat treatment, when an excessive amount is added, voids may form between the particles, reducing the compressive strength after sintering [17]. Therefore, when PVA is used as an organic binder, optimizing the amount of PVA in the ceramic slurry is important. Figure 6a shows the viscosity of zirconia slurries prepared with 0-1.5 wt.% PVA. As the amount of added PVA in the zirconia slurry increases, the viscosity of the zirconia slurry increases. The density and compressive strength with different amounts from 0 to 1.5 wt.% PVA are shown in Figure 6b. When the added amount of PVA is 1 wt.%, the compressive strength of the reticulated porous zirconia is approximately 3.86 MPa. In contrast, the density of the reticulated porous zirconia remained constant even when the amount of added PVA increased. This result indicates that an increase in the amount of added PVA does not cause an increase in the applied mass, but it was judged that it had the effect of improving the densification by narrowing the distance between the particles. However, when the amount of added PVA was 1 wt.% or more, the compressive strength decreased. This result indicates that high viscosity due to an excessive addition of PVA results in poor compressive strength. Therefore, in this study, the amount of PVA in the zirconia slurry was fixed at 1 wt.%. The pore size distributions of reticulated porous zirconia specimens with changes in the amount of PVA are shown in Figure 6c. Regardless of the increase in the amount of added PVA, the pore size remained uniformly distributed, whereas the log differential intrusions were slightly decreased. These results indicate that the distance between the particles is narrowed, which is believed to have affected the densification. Figure 5: (**a**) Densities and compressive strength outcomes and (**b**) pore size distributions of reticulated porous zirconia specimen ball-milled for 0, 1, 4, 8, and 24 h.  
The authors realized that the particle size can affect the viscosity of the zirconia slurry and that the viscosity should be at a certain level in order to improve the compressive strength. However, as the particle size decreases, agglomeration between the particles and the formation of sediment upon standing appear. These outcomes stem from an increase in the specific surface area. These can degrade the compressive strength by forming an uneven coating during the impregnation step. Therefore, the dispersion properties in the ceramic slurry may be a key factor when attempting to improve the compressive strength. In this study, the zirconia slurry is optimized with two types of dispersants: DARVAN C-N and Dolapix CE 64. DARVAN C-N, an ammonium polymethacrylate solution, is completely soluble in water systems and is commonly applied in the electronics and in specific ceramics fields [18]. It produces slurries with exceptionally low viscosity levels without being prone to foaming. Dolapix CE 64, a carbonic-acid-based polyelectrolyte solution, is also dispersed in water and used in alumina [19], zirconia [20], and cordierite-based-glass ceramics [21].