# Enhancing Compressive Strength of Reticulated Porous Alumina by Optimizing Processing Conditions
## 3 Results
In contrast, the pore size distribution of the specimen prepared from coarse alumina particles showed the highest number of pores in the range of 0.1 to 1 \(\mu\)m. This indicates that the strut walls of the reticulated porous alumina could not be fully densified by the coarse alumina particles. Because replica processes do not involve conventional cold-isostatic pressing processes, sufficient densification is not achieved when the average particle size is too excessively high. Therefore, the compressive strength of the specimen prepared from intermediate (0.26 \(\mu\)m) alumina particles was higher than those prepared from fine and coarse alumina particles. Although the absolute value of the compressive strength of reticulated porous alumina has not yet been optimized, (1) we can rule out the use of fine and coarse alumina particles, and (2) we can expect wide industrial applications because of the reasonable raw material cost. Figure 5a shows a viscosity vs. shear rate plot of the alumina slurries prepared with different amounts of PVA binder at a solid loading of 63.5 wt. %. At 15 wt. % of PVA addition, the viscosity was approximately ten times higher than at 10 wt. %; therefore, the alumina slurry was excessively viscous and could not penetrate into or be squeezed out of the polyurethane foam completely. Figure 5b shows the viscosity vs. shear rate plot of the alumina slurries prepared with different solid loadings with 10 wt. % PVA. Although, we did not expect a further increase in viscosity at a solid loading of over 63.5 wt. %, it was necessary to verify the possibility of increased densification of the strut walls by increasing the amount of solid loading to 67 wt. %. Figure 4: (**a**) Densities and compressive strengths, (**b**) linear shrinkage, and (**c**) pore size distributions of reticulated porous alumina specimens prepared from fine, intermediate, and coarse alumina particles. Figure 5c shows the compressive strengths and densities of the reticulated porous alumina specimens prepared with different solid loadings (60 wt. %-67 wt. %). At a solid loading of 63.5 wt.%, the compressive strengths of the specimens peaked, which may be due to the viscosity (Figure 5b). Therefore, using this nonoptimized alumina slurry composition, the effect of the more densified strut walls that were less uniformly coated strut walls (67 wt. %) on the compressive strengths of reticulated porous alumina specimens was inferior to that of the more uniformly coated strut walls that were less densified (63.5 wt. %). We observed large voids and defects generated by the uncoated strut walls of the polyurethane foam deep inside the reticulated porous alumina specimen (at a solid loading of 67 wt. %) due to the high viscosity. At this nonoptimized stage, the excessive solid loading could not induce a further increase in compressive strength. In Figure 5d, the pore size distributions (ranging from 0.1 to 100 \(\upmu\)m) of the strut walls of the specimens prepared from different solid loading can be observed. Although there is a negligible difference in the micron to sub-micron range, which represents the degree of densification of the strut walls, there is a noticeable change in the 10-100 \(\upmu\)m range which represents the voids and defects generated by the non-uniformly coated sacrificial polymer template due to the high viscosity. As expected from the values of compressive strengths and densities of the reticulated porous alumina specimens shown in Figure 5c, the pore volume of the reticulated porous alumina specimen prepared at 67 wt. % solid loading was the highest among all the specimens. Figure 5: Viscosity of alumina slurries used for the preparation of reticulated porous alumina: (**a**) 0–15 wt. % of PVA binder with 63.5 wt. % of solid loading, and (**b**) 60–67 wt. % of solid loading with 10 wt. of PVA binder; all other conditions were fixed. (**c**) Density and compressive strength, and (**d**) pore size distributions of reticulated porous alumina specimens prepared from intermediate alumina particles.