# Enhancing Compressive Strength of Reticulated Porous Alumina by Optimizing Processing Conditions
## 3 Results
Figure 1: (**a**) Optical images of sacrificial polymer templates (polyurethane foam) (left) and reticulated porous alumina specimen (right) with a pore density of 45 PPI, (**b**) three-dimensional microstructure of typical reticulated porous alumina reconstructed by \(\mu\)-CT. (**c**) Representative slice extracted from the \(\mu\)-CT reconstructions. To vary the average particle size of the alumina particles, fine, intermediate, and coarse alumina was used. An optical image of a commercial polyurethane foam used as a sacrificial polymer template is shown in Figure 1a. Figure 1b shows the three-dimensional microstructure of a typical reticulated porous alumina specimen (pore density = 45 PPI) reconstructed by \(\upmu\)-CT. A representative cross-section of the \(\upmu\)-CT reconstruction of the specimen is shown in Figure 1c. The \(\upmu\)-CT images show that the reticulated porous alumina has highly interconnected pore channels. The image does not show any macrovoids in the strut walls inside the specimen that may have induced by unoptimized processing conditions. Typical SEM images of the as-received fine, intermediate, and coarse alumina particles are shown in Figure 2a-c, respectively. The variation in the average alumina particle size affects the morphology of the strut walls of the reticulated porous alumina as shown in Figure 3a-c.  
Figure 3: \(Cont.\)  
Figure 2: SEM images of as-received alumina particles: (**a**) fine (<50 nm), (**b**) intermediate (0.26 \(\upmu\)m), and (**c**) coarse (2–4 \(\upmu\)m). To determine the feasibility of a reticulated porous alumina for industrial applications and to eliminate nonoptimal processing conditions within the numerous experimental batches, we investigated the effect of the average particle size of the raw materials on the compressive strength of a reticulated porous alumina when all the other processing conditions were fixed. The solid loading, binder, and dispersant have not yet been optimized. Among the processing conditions that affect the overall processing costs, the cost of ceramic raw materials (which is generally strongly dependent on the average particle size and purity) overwhelms those of other materials, such as binders and dispersants. For example, in this study, the fine: intermediate: coarse cost ratio was approximately 800: 40: 1. Therefore, even if the overall properties of reticulated porous alumina prepared from fine alumina particles are superior, the industrial applications would be severely limited. Figure 4a shows the compressive strength and density of the reticulated porous alumina specimens prepared from the three average particle sizes, each having a pore density of 45 PPI. The density of the specimens peaked at the intermediate average particle size. Figure 4: _Cont._  
Figure 3: SEM images of fractured strut wall of a reticulated porous alumina specimen prepared from (a) fine, (b) intermediate, and (c) coarse particles. This can be explained as follows. As the average particle size increased, the linear shrinkage of the specimens decreased, as shown in Figure 4b,c depicts the pore size distributions of the reticulated porous alumina specimens (pore density = 45 PPI), prepared using alumina particles of different sizes. For the intermediate alumina, the specimens exhibited a significant decrease in the overall range of pore volume fraction. Because the number of strut walls is invariable and based on the pore density of the polyurethane foam that was used, any difference in the pore size distributions can only be ascribed to the average particle size. The pore size distribution of the reticulated porous alumina prepared from fine alumina particles showed the highest number of pores in the range of 110 \(\mu\)m. This implies that excessive shrinkage during the sintering process, which was induced by nano-sized alumina particles, generated many cracks and defects in the strut walls of the reticulated porous alumina.