# Enhancing Compressive Strength of Reticulated Porous Alumina by Optimizing Processing Conditions
## 1 Introduction  
Over the past several decades, researchers have extensively investigated porous ceramics [1] because of their highly stable thermal and chemical properties. However, unstable mechanical properties inhibit their applications compared to competing materials such as porous metals and polymers. Among the various types of porous ceramics, reticulated porous ceramics are of particular interest because of their high permeability and low density. Their advantageous properties originate from the fact that reticulated porous ceramics consist of interconnected pore channels, unlike porous ceramic foams that, similar to soap bubbles or bechives, have fewer interconnected pore channels. Among the various types of highly porous ceramics (porosity \(\geq\) 90%), reticulated porous ceramic prepared by the replica method and porous ceramic foams prepared by the particle-stabilized direct foaming method have closed and open-cell structures, respectively. Reticulated porous ceramics can be prepared by coating a reticulated polyurethane foam (as a sacrificial polymer template) with a slurry consisting of ceramic particles [1]. The ceramic slurry-coated reticulated polyurethane foam is then dried and heat-treated to burn off the reticulated polyurethane foam and sinter the ceramic particles. This results in an open-cell pore structure because the sacrificial polymer template is completely coated with ceramic particles when soaked in the ceramic slurry. Typical examples of the compressive strength of reticulated porous ceramics such as silicon carbide (SiC), zirconia (ZrO\({}_{2}\)), silicon nitride (Si\({}_{3}\)N\({}_{4}\)), mullite (3 Al\({}_{2}\)O\({}_{3}\)-2 SiO\({}_{2}\)), and alumina (Al\({}_{2}\)O\({}_{3}\)) were already addressed in our previous study [2]. As can be understood from the literature data, the low compressive strength of reticulated porous ceramics prevents their use in industrial applications. The compressive strengths of reticulated porous ceramics are strongly dependent on the pore density (PPI) and the processing conditions. Therefore, if the processing conditions of a reticulated porous ceramic are optimized, an enhanced compressive strength can be achieved when using pre-fixed parameters, such as the pore density of the sacrificial polymer template. Consequently, enhancing the compressive strength of reticulated porous ceramics provides an opportunity to achieve synergy between the open-cell structure and high porosity. In this work, we considered a reticulated porous alumina that may have an enhanced compressive strength that is higher than that of all other ceramics. Unlike other ceramics such as plate-like pyrophyllite (Al2 Si4O10(OH)2) [3] and porous, irregularly shaped diatomite (SiO2-nH2O) [4], regular, spherical alumina particles can easily form as rigid strut walls for a reticulated porous ceramic, improving compressive strength; this is a unique advantage of alumina. If the overall permeability of a reticulated porous ceramic is more important than its overall compressive strength, permeable strut walls prepared using porous and irregular particles, such as diatomite, are favorable. However, if the overall compressive strength is more important than the overall permeability, it is necessary to use ceramic materials that have high strength when dense. Hence, the dense strut walls of reticulated porous alumina and the inherent high compressive strength of bulk alumina are essential for increasing the overall compressive strength of the ceramics. The low compressive strength of reticulated porous ceramics can be enhanced by several methods; vacuum infiltration coating [5], dip coating [6], centrifuge coating [7], slurry rheology modification [8], multiple slurry coatings [9; 10], and whisker-reinforcement coating [11] have all been used to improve the mechanical stability of these ceramics.