## 3 Results and Discussions
The identification of the coating quality becomes more difficult as the thickness and PPI of the sacrificial polymer template increase owing to the limitation of optical measurement. Therefore, in this study, non-destructive \(\upmu\)-CT analysis was introduced to investigate the prepared reticulated porous ceramic specimens. Figure 2a shows the three-dimensional microstructure of typical reticulated porous alumina specimen, reconstructed by a \(\upmu\)-CT. A representative slice from the \(\upmu\)-CT reconstructions of the specimen is shown in Figure 2b. It is evident that the specimen had a well-defined inter-connected pore structure as intended. However, Figure 2c shows the three-dimensional microstructure of typical reticulated porous alumina prepared using the un-optimized alumina slurry, reconstructed by a \(\upmu\)-CT. A representative slice from these \(\upmu\)-CT reconstructions is shown in Figure 2d. Although Figure 2c does not show any significant difference at a surface level from Figure 2a, in Figure 2d, the voids and non-uniform distribution of strut walls, inside the reticulated porous alumina specimen can be observed. The pore channels beneath the surface level were severely blocked, which shows the effect of highly viscous alumina slurry that should have penetrated deep-inside the sacrificial polymer template. Figure 3a shows the viscosities as a function of the shear rate of the alumina and diatomic-kaolin slurries with optimized slurry compositions at the highest solid loading. On comparing the viscosity of alumina slurry to that of diatomic-kaolin slurry, it was found to be significantly low, implying that it could more easily penetrate the excessively narrow cells of a sacrificial polymer template whose PPI was greater than 60. This provides a significant advantage of preparing reticulated porous ceramics. Figure 3b shows the sacrificial polymer templates (commercial polyurethane foams) with pore densities of 10, 25, 45, 60, and 80 PPI. These polyurethane foams were used as sacrificial polymer templates during the replica method in this study. Figure 3c shows the as-prepared reticulated porous diatomic-kaolin composite specimens with pore densities of 10, 25, 45, 60, and 80 PPI, prepared using the replica method. In previous reports [21; 22; 35; 36], the sintering temperature of the reticulated porous diatomic-kaolin composite was determined to be 1200 \({}^{\circ}\)C. The authors already recognized that the reticulated porous diatomic-kaolin composite had similar microstructures up to 1200 \({}^{\circ}\)C because above 1200 \({}^{\circ}\)C, a clear coalescence and decrease in the average pore size of diatomic particles began to occur owing to the impurities, such as Na\({}_{2}\)O, K\({}_{2}\)O, Al\({}_{2}\)O\({}_{3}\), CaO and MgO, in the diatomic particles, which easily formed a melt phase in the silica-rich grains [36]. Then, the inherent irregular and porous nature of the diatomic particles diminished. Subsequently, the reason for justification of introducing porous diatomic particles significantly reduced. Definitely, the above-mentioned phenomenon, as described in the literature, was not exactly matched with the reticulated porous diatomic-kaolin composite. However, because diatomic was the main component, the phenomenon corresponding to the sintering temperature was similar to that discussed in the literature. Figure 1: Compressive strengths of the reticulated porous alumina specimens with pore densities of 60 and 80 PPI, which were prepared using the optimized and the un-optimized processing conditions, respectively. Figure 2: Cont. Figure 4a,b show the fractured strut walls of a typical reticulated porous alumina specimen and reticulated porous diatomic-kaolin composite specimen, respectively. It is evident that the strut walls of reticulated porous diatomic-kaolin composite were porous unlike those of reticulated porous alumina. Reasonably, the porous nature of the strut walls of reticulated porous diatomic-kaolin composite degraded the compressive strength to a certain degree.