**1. Introduction**

Porous ceramics are widely used in various versatile applications, such as liquid gas filters, catalysis supports, gas distributors, insulators, preforms for metal-impregnated ceramic metal composites, and implantable bone scaffolds [1, 2]. Unlike in metallic or polymeric products, pores have been traditionally avoided in ceramic components because of their inherently brittle nature [3, 4]. However, porous ceramics have become increasingly essential, especially for use in environments involving high temperatures, extensive wear and corrosive media [5,

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6]. Porous ceramics are advantageous in such application areas due to their high melting point, tailored electronic properties, and high corrosion and wear resistance, which combine favorably with the features gained by the introduction of voids into the solid material [7-10]. These features include low thermal conductivity, controlled permeability, high surface area, low density, high specific strength, and low dielectric constant. These properties can be tailored for each specific application by controlling the composition and microstructure of the porous ceramic. Changes in open and closed porosity, pores' size distribution, and pores' morphology can greatly affect a material's properties. These microstructural features are highly influenced by the processing route used to produce the porous material [11-15].

Foaming melts by gas injection creates gas bubbles in the liquid by the admixing of gasreleasing blowing agents into the molten metal, or by causing the precipitation of gas which had been previously dissolved in the liquid [16, 17]. The stabilization of such foams can be achieved by surfactants, which form dense monolayers on foam films. The surfactant films can reduce surface tension, increase surface viscosity, and create electrostatic forces to prevent foam from collapsing. The stabilization and destabilization mechanisms of coated bubbles exposed to surfactants to produce metallic foams are discussed elsewhere [18]. Colombo et al. [19] discussed different novel processing methods for cellular ceramics, including the burning out of fugitive pore formers. Established methods of producing porous ceramics employ the burning out of templates. The impregnating of a polymeric template increases struts through‐ out the material and thus increased the strength of the resulting ceramic foams [20]. The porosity of ceramics produced in this way depends on the template's type, content, and grain size. This limits the maximum useable content of such additives, as too high contents sub‐ stantially weaken the material. Increased porosity can also be achieved by introducing highporosity granules—both natural (e.g., diatomite, tripolite, and swelled perlite) and synthesized (e.g., by the crushing of briquettes prepared by foaming) [21]. Chemical formations of gas bubbles within a ceramic mixture can also increase porosity. These include chemical reactions in the ceramic suspension or the decomposition of gas-forming additives. The kinetics of alumina slip swelling for the production of lightweight corundum materials have been investigated [22]. Another method is the impregnation of a polymer cellular matrix with a ceramic suspension and subsequent squeezing out, drying, and thermal treatment to remove the organic components [23]. The addition or embedding of ceramic fibers into the mixture, followed by molding with binders and the subsequent thermal treatment of the molded products, can also yield porous materials [24].

The introduction of air into a colloidal suspension is widely used during processing of highly porous foam ceramics [25, 26]. Uniform, finely cellular foam can be produced by mixing into the ceramic suspension frothing agents that stabilize the resultant three-phase foam. Such cellular structures are preserved under subsequent drying and firing [27]. Much work has sought to develop processing parameters for such syntheses.

Less defective components, as compared with dry processing, have recently been shown to result from the wet processing of powders. It allows better control of the interactions between the powder and the particles and increases the homogeneity of particles' packing in the wet stage, leading to fewer and smaller defects in the final microstructure. This can be achieved either by consolidating the dispersion medium or by flocculating or coagulating the particles in the liquid medium. Such wet methods have recently been developed to incorporate gaseous phases into ceramic suspensions consisting of ceramic powder, solvent, dispersants, surfac‐ tants, and gelling agents. The process has been called direct foaming by the hydrophobization of particles' surfaces; the incorporation of the gaseous phase can result from mechanical frothing, injection of a gas stream, gas-releasing chemical reactions, or solvent evaporation [28]. Its simplicity, low cost, and versatility has made it popular for the manufacture of porous ceramics. Fig. 1 outlines common methods of preparing porous ceramics and their corre‐ sponding products' degrees of porosity. The fabrication methods of microporous ceramics currently available can be classified as replica techniques, methods that employ sacrificial templates and direct foaming [29]. Ceramics' microstructures and properties depend on their fabrication method. Therefore, consideration of the methods' cost, simplicity, and versatility is important. Stabilization of the introduced species' surfaces is required to overcome coales‐ cence, Ostwald ripening, and phase separation and can be achieved using lower-energy molecules for droplet formation. These provide steric and electrostatic barriers against coalescence [30]. Early twentieth century works by Ramsden and Pickering showed that solid particles adsorbed at liquid-liquid interfaces can stabilize the resulting Pickering emulsions, through the introduced surface active molecules lowering the system's free energy by reducing the liquid-liquid interfacial area [31].

6]. Porous ceramics are advantageous in such application areas due to their high melting point, tailored electronic properties, and high corrosion and wear resistance, which combine favorably with the features gained by the introduction of voids into the solid material [7-10]. These features include low thermal conductivity, controlled permeability, high surface area, low density, high specific strength, and low dielectric constant. These properties can be tailored for each specific application by controlling the composition and microstructure of the porous ceramic. Changes in open and closed porosity, pores' size distribution, and pores' morphology can greatly affect a material's properties. These microstructural features are highly influenced

Foaming melts by gas injection creates gas bubbles in the liquid by the admixing of gasreleasing blowing agents into the molten metal, or by causing the precipitation of gas which had been previously dissolved in the liquid [16, 17]. The stabilization of such foams can be achieved by surfactants, which form dense monolayers on foam films. The surfactant films can reduce surface tension, increase surface viscosity, and create electrostatic forces to prevent foam from collapsing. The stabilization and destabilization mechanisms of coated bubbles exposed to surfactants to produce metallic foams are discussed elsewhere [18]. Colombo et al. [19] discussed different novel processing methods for cellular ceramics, including the burning out of fugitive pore formers. Established methods of producing porous ceramics employ the burning out of templates. The impregnating of a polymeric template increases struts through‐ out the material and thus increased the strength of the resulting ceramic foams [20]. The porosity of ceramics produced in this way depends on the template's type, content, and grain size. This limits the maximum useable content of such additives, as too high contents sub‐ stantially weaken the material. Increased porosity can also be achieved by introducing highporosity granules—both natural (e.g., diatomite, tripolite, and swelled perlite) and synthesized (e.g., by the crushing of briquettes prepared by foaming) [21]. Chemical formations of gas bubbles within a ceramic mixture can also increase porosity. These include chemical reactions in the ceramic suspension or the decomposition of gas-forming additives. The kinetics of alumina slip swelling for the production of lightweight corundum materials have been investigated [22]. Another method is the impregnation of a polymer cellular matrix with a ceramic suspension and subsequent squeezing out, drying, and thermal treatment to remove the organic components [23]. The addition or embedding of ceramic fibers into the mixture, followed by molding with binders and the subsequent thermal treatment of the molded

The introduction of air into a colloidal suspension is widely used during processing of highly porous foam ceramics [25, 26]. Uniform, finely cellular foam can be produced by mixing into the ceramic suspension frothing agents that stabilize the resultant three-phase foam. Such cellular structures are preserved under subsequent drying and firing [27]. Much work has

Less defective components, as compared with dry processing, have recently been shown to result from the wet processing of powders. It allows better control of the interactions between the powder and the particles and increases the homogeneity of particles' packing in the wet stage, leading to fewer and smaller defects in the final microstructure. This can be achieved

by the processing route used to produce the porous material [11-15].

56 Advanced Ceramic Processing

products, can also yield porous materials [24].

sought to develop processing parameters for such syntheses.

**Figure 1.** Typical porosity and average pores sizes achieved via replica, sacrificial templating, and direct foaming routes.2

This chapter explores the stabilization of wet foams by colloidal amphiphilic particles and the development of fabrication techniques of solid macroporous ceramics with tailored micro‐ structures. Each method is discussed and assessed with regard to the versatility and ease of fabrication and its influence on the microstructure and mechanical strength of the resulting macroporous ceramics. Given the importance of ceramics' foam microstructures, the effects of 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 produced by direct foaming.
