**2.3. Direct foaming**

Direct foaming produces porous materials by the incorporation of air into a suspension or liquid medium. The foam structure is then set by high-temperature sintering to obtain crackfree, high-strength porous ceramics. The suspensions are stabilized *in situ* through the hydrophobization of the suspended particles by short chain amphiphilic molecules. The coated, hydrophobic particles irreversibly adsorb to the air-water interface, thus stabilizing it (Figs. 2(c) and 3) [38]. These wet foams can remain stable for several days and show no bubble coarsening, drainage, or creaming. The short-chain amphiphiles modify *in situ* the wetting behavior of the particles' surfaces, as in a Pickering emulsion. Ultrastable wet foams can be produced by direct foaming using particles instead of surfactants as foams stabilizers [16, 19, 25]. Porous ceramics' properties are also highly influenced by their chemical compositions and microstructures, with porosity, pore morphology, and size distribution being tailored by different compositions, different physical structures of the starting materials, and the use of different amphiphiles [30-36]. This review focuses on this process.

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template, as opposed to the positive morphology obtained from replication. The method of the sacrificial material's extraction from the consolidated composite depends primarily on the type of pore former employed [33]. A wide variety of sacrificial materials can be used as pore formers, including natural and synthetic organics, salts, liquids, metals, and ceramics. This technique is flexible and can employ various chemical compositions. Various oxides have been used to fabricate porous ceramics using starch particles as sacrificial templates [9, 10]. Nonoxide porous ceramics have also been produced using pre-ceramic polymers and various template materials [34, 35]. Since this method produces a ceramic to the negative of the original template, the removal of the sacrificial phase does not lead to flaws in the struts as

directly the pattern of the sacrificial phase and higher mechanical strengths are generally

Fig. 2. Currently available methods of forming porous ceramics.2

**Figure 2.** Currently available methods of forming porous ceramics.2

achievable than by using positive replicas [36, 37].

**Figure 3.** *In situ* hydrophobization of particles and solid foam formation by direct foaming.4
