# Enhancing Compressive Strength of Reticulated Porous Alumina by Optimizing Processing Conditions
## 3 Results  
A commercial polyurethane foam (used as a sacrificial polymer template, 45 PPI), and the as-prepared reticulated porous alumina specimen (45 PPI) created using polyurethane foam and alumina slurry are shown in Figure 1a. The sintering temperature of the specimens was 1600 \({}^{\circ}\)C.  
Generally, the pore density of reticulated porous ceramics that can be practically and easily obtained is less than approximately 45 PPI. Therefore, many researchers have studied reticulated porous ceramics with pore densities of less than 45 PPI. For example, reticulated porous ceramics prepared from various types of ceramic raw materials such as alumina (5 [18] and 20 PPI [19; 20; 21]), alumina-zirconia composites (20 PPI) [22], aluminum nitride (20 PPI) [23], zirconia (45 PPI) [24], silicon nitride (10 PPI) [11], silicon carbide (8 PPI [25] and 10 PPI [26]), corderierte (10 PPI) [27], and mullite (20 PPI) [28] have pore density values equal to or lower than 45 PPI. Figure 1: _Cont._  
In this study, the pore density of the reticulated porous alumina specimens was fixed at 45 PPI. This is because the preparation of a reticulated porous ceramic with a pore density of more than 45 PPI is challenging. As the pore density approaches 80 PPI, it would be difficult to entirely coat the strut walls of the sacrificial template as the viscosity of the ceramic slurry increased. Therefore, there would be a narrow margin to optimize various processing conditions, owing to the decreased upper limit of viscosity. Moreover, the compressive strength of reticulated porous alumina with a pore density of 45 PPI is sufficient for practical applications if it can be enhanced further, owing to its dense strut walls. The main factors governing the properties of a ceramic slurry are the average particle size, solid loading, binder, and dispersant. The required properties of the ceramic slurry depend on the application. Generally, high solid loading and low viscosity are required for applications such as micro-droplet jetting of alumina slurries [29], additive manufacturing of alumina magnesia silicate slurries [30], slip casting of magnesium aluminate spinel slurries [31], additive manufacturing of zirconia toughened alumina slurries [32], and dip coating of cerium oxide slurries [33]. Appropriate solid loading and viscosity are required for the inkjet printing of ceramic ink [34]. In this study, to enhance the compressive strength of reticulated porous alumina, alumina slurries with high solid loading and moderate viscosity were prepared. If the viscosity of the alumina slurry is not high enough, the amount of adsorbed alumina particles in the strut walls of the polyurethane foam would be insufficient, even with a high solid loading. Thus, it cannot completely cover the strut walls of the sacrificial polymer template, which can lead to many defects. Consequently, the overall compressive strength is low. If the solid loading of the alumina slurry is not high enough, the densification of the strut walls will be insufficient, even with a high viscosity. This results in excessive porosity of the strut walls, which leads to low compressive strength. If the alumina slurry is excessively viscous, it is difficult for it to completely penetrate the polyurethane foam. It is also very difficult to remove excess alumina slurry by squeezing the polyurethane foam while maintaining uniform alumina coating layers over the strut walls. In general, the viscosity of the alumina slurry increases significantly with increasing solid loading and decreases with increasing average particle size. Hence, to prepare a high solid loading alumina slurry with moderate viscosity, we attempted to determine the optimal processing conditions; especially, solid loading, average particle size, binder, and dispersant. Although several approaches to obtain an extremely high solid loading have been reported by surface modification [35; 36], it is beyond the scope of this work.