# Effect of the Sintering Temperature on the Compressive Strengths of Reticulated Porous Zirconia
## 3 Results
The microstructures of the reticulated porous zirconia when sintered at 1400 \({}^{\circ}\)C, 1500 \({}^{\circ}\)C, and 1600 \({}^{\circ}\)C are shown in Figure 7a-c, respectively. As the sintering temperature was increased, the average particle size increased. In addition, it can be seen that as the sintering temperature increases, the number of open pores decreases due to densification between the particles. When the reticulated porous zirconia is sintered at 1400 \({}^{\circ}\)C, pores exist and necks form between the particles. When the sintering temperature was 1500 \({}^{\circ}\)C, the grain boundaries between the particles became connected. When sintered at 1600 \({}^{\circ}\)C, the grain size grew because there were no pores that interrupted the grain growth. Figure 7d shows the microstructure of the reticulated porous zirconia when reticulated porous zirconia specimens were prepared from fine zirconia particles and sintered at 1600 \({}^{\circ}\)C. As the average particle size decreased, the particles agglomerated. The initial average particle size and particle distribution reportedly play very important roles during the sintering process of ceramics. As the average particle size becomes smaller, the surface energy increases, possibly leading to densification, but aggregation occurs due to electrostatic attraction. Figure 5: Particle size distributions of zirconia, for as-received, coarse, intermediate, and fine. Zirconia slurries were prepared with a combination of a dispersant (Dolapix CE 64), a thickener (MC), and an organic binder (PVA). The dispersant is vital to remove agglomerated zirconia particles and to prepare a uniform zirconia slurry. The organic binder increases the viscosity of the suspension and prevents the formation of cracks after the coating step while also maintaining the shape of the sacrificial polymer template after sintering. The thickener was used to increase the viscosity of the zirconia slurry and to coat the strut walls of the sacrificial polymer template tightly so as to minimize defects that can be caused by the strut walls. Without the addition of an organic binder and thickener, the viscosity of the zirconia slurry is low, making it difficult to coat the strut walls of the sacrificial polymer template tightly. In addition, in order to ensure that the viscosity can sufficiently coat the strut walls of the sacrificial polymer template tightly, zirconia particles must be added indefinitely until the desired viscosity is reached. However, doing this is not efficient, as noted above. Figure 6: (**a**) X-ray diffraction patterns of the reticulated porous zirconia with different sintering temperatures, (**b**) linear shrinkage levels, (**c**) densities, and (**d**) pore size distributions of the reticulated porous zirconia samples when sintered at 1400, 1500, and 1600 \({}^{\circ}\)C.  
Figure 8a shows a plot of the viscosity vs. the shear rate of the zirconia slurries prepared with different average particle sizes. As the average particle size decreases due to ball-milling, the specific surface area increases and the points at which the PVA adheres to the particles increase. Therefore, as the average particle size increases, the viscosity increases. Overall, however, it is apparent that the viscosity tends to decrease as the average particle size decreases. Dolapix CE 64 has a lower molecular weight compared to other dispersants, is completely dissociated in water, and has an electrostatic stabilization effect immediately after it is added [27]. Therefore, it can be judged that the rheological properties of the slurry are optimized. To prepare reticulated porous zirconia specimens with a uniform structure using the replica template method when a sacrificial polymer template is impregnated into the zirconia slurries, proper viscosity and fluidity of the zirconia slurry are important. However, there are several challenges when preparing reticulated porous zirconia specimens under specific conditions.