# Enhancing Compressive Strength of Reticulated Porous Alumina by Optimizing Processing Conditions
## 3 Results
We are aware that the amount of PVA (10 wt. %) and the solid loading (63.5 wt. %) were not a fully-optimized and could be enhanced further by adding an appropriate type and amount of dispersant. However, considering the number of processing variables, the initial batch (10 wt. % of PVA and 63. 5 wt. % of solid loading) was suitable for use in further experiments. Figure 6a shows a viscosity vs. shear rate plot of the alumina slurries prepared with different amounts of DARVAN C-N dispersant at 10 wt. % of PVA binder addition and 63. 5 wt. % of solid loading. With the addition of 1 wt. % of DARVAN C-N, the viscosity was approximately 100 times lower than with 0 wt. % of DARVAN C-N. Consequently, the alumina slurry would be excessively viscous, and it would not be able to penetrate and be squeezed out of the polyurethane foam completely. However, it should be noted again that the highest compressive strength of the reticulated porous alumina specimen was obtained not when the alumina slurry has the slowest viscosity, but when the alumina slurry has an optimized viscosity. A low viscosity inhibits the adhesion of alumina slurry onto the strut walls of the polyurethane foam and promotes the penetration of the alumina slurry deep inside the polyurethane foam. Figure 6b shows the compressive strengths and densities of the reticulated porous alumina specimens prepared using different amounts of DARVAN C-N dispersant (0-1 wt. %), with all the other processing conditions fixed. When the amount of DARVAN C-N dis  
Figure 6: (**a**) Viscosities, (**b**) densities and compressive strengths, and (**c**) pore size distributions of alumina slurries with 0–1 wt. % of DARVAN C-N dispersant; all other conditions were fixed. (**d**) Typical SEM image of fractured strut wall of reticulated porous alumina specimen prepared with the addition of DARVAN C-N dispersant. persant was increased, the viscosity of the alumina slurry decreased (Figure 6a). The compressive strengths and densities of the reticulated porous alumina specimens did not vary significantly with the addition of DARVAN C-N. This tendency can also be observed in the pore size distributions of the reticulated porous alumina specimens prepared using different amounts of DARVAN C-N dispersant. Pore sizes in the range of 0.1-100 \(\upmu\)m were observed because this corresponds to the pore size distribution of the strut walls. There is also a negligible difference in the 10-100 \(\upmu\)m range, which represents the voids and defects, regardless of the amount of DARVAN C-N dispersant. The lowered viscosity with the aid of DARVAN C-N dispersant enabled the alumina slurry (63.5 wt. % of solid loading) to deeply penetrate the polyurethane foam, which can be confirmed because there was no density decrease, as was the case when 67 wt. % of solid loading was tested (Figure 6c). However, the increase in solid loading did not contribute to the enhancement of the compressive strength of the reticulated porous alumina specimen. This can be explained by the microstructure of the reticulated porous alumina specimens. Typical SEM images of the specimens prepared with DARVAN C-N dispersant are shown in Figure 6d. Many defects and non-uniform agglomerates can be observed in the strut walls, which has a negative effect on the overall compressive strength of the specimen, explaining the negligible effect of the DARVAN C-N dispersant. Therefore, for the investigation of the properties of reticulated porous alumina specimens, it is necessary to carefully analyze the densities, compressive strengths, pore size distributions, \(\upmu\)-CT images, and SEM microstructures of the specimens, because these data are all complementary. To further n investigate further the effect of dispersant, we introduced Dolapix CE 64 dispersant which is also well-known as a good dispersant to alumina particles to compare DARVAN C-N dispersant. Figure 7a shows the viscosity vs. shear rate plot of the alumina slurries prepared with different amounts of Dolapix CE 64 dispersant at 10 wt. % of PVA binder addition and 63. 5 wt.