**5. Conclusions**

Porous ceramics' microstructures and properties are affected by their method of synthesis. Direct foaming can simply, inexpensively and quickly prepare macroporous ceramics. Open or closed porosities of 45%-85% having been demonstrated. The pores produced by this method result from the direct incorporation of air bubbles into a ceramic suspension, elimi‐ nating the need for pyrolysis before sintering. Cellular structures prepared by direct foaming are generally stronger than those prepared by replica synthesis due mainly to the absence of flaws in the cell struts. Given the importance of the chosen synthetic method, this review examines currently available processes for forming porous ceramics. Direct foaming is a simple and versatile process for the low-cost manufacture of porous ceramics for various applications. Its continuous study will result in further improvements of its method and wider applicability

**Figure 24.** Microstructures and thin film (inner cell) of porous ceramics sintered at 30 vol.% Al2O3 with respect to the different mole ratio of SiO2 content.

that at the same concentration of amphiphile, the shortest chain carboxylic acid, i.e., propionic acid, produces relatively large pore size than the longest chain carboxylic acid, i.e., valeric acid. This can be attributed to the fact that greater hydrophobicity is achieved with the aid of long carbon chain present in valeric acid which results in small and uniform pore size. The smaller cell sizes result from the high stability of the foams in the wet state, which impedes bubble coarsening. The dense struts as shown in the inset of Fig. 26a-c plays vital role for improving

**Figure 23.** Average bubble size of suspension with respect to different concentration of amphiphiles.4

Porous ceramics' microstructures and properties are affected by their method of synthesis. Direct foaming can simply, inexpensively and quickly prepare macroporous ceramics. Open or closed porosities of 45%-85% having been demonstrated. The pores produced by this method result from the direct incorporation of air bubbles into a ceramic suspension, elimi‐ nating the need for pyrolysis before sintering. Cellular structures prepared by direct foaming are generally stronger than those prepared by replica synthesis due mainly to the absence of flaws in the cell struts. Given the importance of the chosen synthetic method, this review examines currently available processes for forming porous ceramics. Direct foaming is a simple and versatile process for the low-cost manufacture of porous ceramics for various applications. Its continuous study will result in further improvements of its method and wider applicability

the mechanical strength of the porous ceramics.

**5. Conclusions**

78 Advanced Ceramic Processing

**Figure 25.** Microstructures of (a) AT, (b) ATM1, (c) ATM3, and (d) ATM5 porous ceramics, sintered at 1500°C for 1 hour.32

of its products. Examination of the literature led to the proposal of an equation describing the inverse proportionality of wet foam stability to the surface of the liquid-air interface.

**Figure 26.** Microstructures of porous ceramics using 0.15 mol/L of (a) propionic acid, (b) butyric acid, and (c) valeric acid; the inset in (a), (b), and (c) show single dense struts obtained with direct foaming method.4

#### 1 ( ) *WfS* g¥

*W fS* = Wet foam stability

*γ* = surface tension
