**2.2. Sacrificial templates**

foam precursor suspensions—bubble size, distribution, contact angle, and surface tension on the resultant porous ceramics' mechanical and physical properties are assessed here. Control of these parameters can allow the tailoring of the microstructures of porous ceramics

The processes for manufacturing porous ceramics can be classified into following four

In this chapter, we reviewed the main processing techniques that can be used for the fabrication of porous ceramics with tailored microstructure. Replica techniques, sacrificial template, and direct foaming techniques is described here and compared in terms of microstructures and mechanical properties that could be achieved. These simple yet versatile approaches give rise to porous ceramics with unique microstructural features that control the properties and

Replica techniques involve the impregnation of a cellular structure with a ceramic suspension or precursor solution to produce a macroporous ceramic exhibiting a similar morphology to the original porous material (Fig. 2(a)). This is followed by the removal of excess slurry, pyrolysis of the polymeric substrate, and sintering to solidify the foam [6, 13]. Therefore, the ceramic foam replicates the original organic polymer structure. Difficulties of slurry impreg‐ nation limit the realization of small cells. The struts contain central holes, which result from the burning out of the polyurethane template. Microcracks and pores also result. Replication generates large amounts of CO2 during firing due to the decomposition of the organic compounds [10, 12]. Suitable biogenic porous structures have been used as templates to form cellular ceramics with particular microstructures that could also be produced by other methods. Those processes for the fabrication of bulk ceramics structures are discussed here.

This technique, reported in the 1960s, is the first method deliberately used for the production of macroporous ceramics [32]. First, polymeric sponges were used as templates to prepare ceramic cellular structures with various pore sizes, porosities and chemical compositions. In the polymer replica approach, a highly porous polymeric sponge is initially soaked in a ceramic suspension until its internal pores are filled. Binders and plasticizers are also added to the initial suspension to provide ceramic coatings sufficiently strong to prevent the struts from

cracking during pyrolysis. This process is explored fully elsewhere [11, 13].

produced by direct foaming.

58 Advanced Ceramic Processing

**i.** Replica techniques

**ii.** Sacrificial template

functions of the ceramic materials.

**iii.** Direct foaming

**2.1. Replica techniques**

**2. Processing routes to porous ceramics**

categories, which is schematically depicted in Fig. 2.

A dispersed sacrificial phase can be homogeneously dispersed throughout a biphasic compo‐ site with a continuous matrix of ceramic particles or ceramic precursors. It is ultimately extracted to generate pores within the microstructure (Fig. 2(b)). This method is analogously opposite to replication and results in a negative replica of the original sacrificial 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 can occur using positive replicas. The microstructures obtained by this technique reflect directly the pattern of the sacrificial phase and higher mechanical strengths are generally achievable than by using positive replicas [36, 37].
