## 3 Results
### Reticulated Porous ZTA Specim Prepared by Fine Zirconia Particles
#### 3.2.1 The Effect of the Organic Binder on the Reticulated Porous ZTA Specim  
In this study, polyvinyl alcohol (PVA) as an organic binder was added to prepare a uniform zirconia slurry. PVA increases the viscosity of the slurry and prevents the formation of cracks after the coating step [33]. PVA molecules enclose the particles and separate them from other particles, preventing agglomeration between the particles. In addition, PVA particles bond to each other as a matrix to maintain the shape of the sacrificial polymer template [34,35]. The viscosity levels of the ZTA slurries with the amount of PVA are shown in Figure 8a. As the amount of PVA was increased, the viscosity of the ZTA slurries increased. Figure 8b shows the compressive strengths and densities according to the amount of PVA. PVA at 2 wt.% led to the highest compressive strength (approximately 2.43 MPa). PVA assists  
Figure 7: (**a**) Compressive strengths of reticulated porous ZTA specimens with various average particle sizes as controlled by adjusting the ball-milling time and (**b**) density, and (**c**) typical back scattered electron image of reticulated porous ZTA specimens when the average particle size was fine. with contact between the alumina and zirconia particles, making them more viscous and preventing cracks or delamination that may occur during the deposition of the coating layer. However, excessive amounts of PVA cause agglomeration of alumina and zirconia particles. This can lead to an increase in the surface roughness of the coating layer. Therefore, it is important to add an appropriate amount of PVA. Reticulated porous ZTA with a uniform structure can be prepared when using an appropriate amount of PVA. This phenomenon is also seen in the microstructures. Figure 9a shows a typical scanning electron microscopy image of the strut walls of a reticulated porous ZTA specimen without PVA added. It can be seen that the coating layer of the strut walls is very thin, and there are many cracks and defects. These cracks and defects affect the compressive strength. The reticulated porous ZTA specimens prepared without the addition of PVA had a very low compressive strength value of 0.37 MPa. Figure 9b,c show typical backscattered electron images of reticulated porous ZTA specimens when the amount of PVA was 2 wt.% and 10 wt.%, respectively. When the amount of the PVA was 2 wt.%, it was confirmed that fine zirconia particles are uniformly dispersed. Fine zirconia particles are uniformly dispersed, and the grain boundaries between the particles are clearly separated because a uniform PVA adsorption layer was formed on the surfaces of the zirconia and alumina particles to block aggregation effectively between the particles and prevent grain growth. In addition, the uniformity was enhanced, indicating a denser particle arrangement. The enhanced uniformity is initially due to the size of the particles to be filled, which is effectively reduced due to the blocking of agglomeration, thus improving the dispersibility of the slurry. Another cause is the PVA, which is uniformly adsorbed on the particle surface and which increases the fluidity of the slurry such that rearrangements between particles can occur relatively well [36]. However, when the amount of the PVA is 10 wt.%, it can be seen that fine zirconia particles were agglomerated. This affects the compressive strength (approximately 1.81 MPa). The amount of PVA at 10 wt.% had the highest density of approximately 1.0 g/cm\({}^{3}\) because the ZTA viscosity was too high, making it difficult to prepare the reticulated porous ZTA specimens. ZTA slurry when the viscosity was too high becomes thickly attached to the surface of the sacrificial polymer template. In addition, this form of ZTA slurry becomes trapped inside the sacrificial polymer template and cannot be removed. Therefore, when the compressive strength was higher than 8 wt.% PVA, the highest density resulted due to the blocking of the internal pores of the sacrificial polymer template.