**2. Liquid precursor foams**

#### **2.1. Structure of foams**

A bulk foam is a substance formed by trapping gas air bubbles in liquid or solid. A bath sponge and the top of fresh beer are examples of foams. In most cases, the volume of gas is large, with thin films of liquid or solid separating the regions of gas. The equilibrium structure of foam is an elegant and well-defined arrangement of films, plateau borders, and junctions. The bubbles which are pressed together to form the foam are separated by thin films. The films meet along a line or curve, forming a liquid-filled interstitial channel called a plateau border. Where several plateau borders meet to form an interconnected network, they do so at a junction [9].

#### **2.2. Liquid foams**

method, a polymeric sponge with open pores is immersed into ceramic slurry, and after rolling to remove redundant slurry, the coated sponge is dried and pyrolyzed, leaving only the porous ceramic structure. Then, the resultant foam will be sintered for final densification to get required mechanical strength. This method is widely used because it is effective with most kinds of ceramic materials, such as silicon carbide, zirconia, silicon nitride, alumina, silica, mullite, and cordierite. However, large amount of gaseous by-product is released during pyrolysis, and consequently, leaving triangle hollows inside the ceramic struts. Cracking due to difference in thermal expansion coefficient is easy to occur [4]. Hence, there would be defects in ceramic foams fabricated by

**Figure 1.** SEM photographs of ceramic foams consisted of (a) spherical cells and windows and (b) dense triangular struts.

such polymeric foam replication technique, which led to lower mechanical strength [5].

direct foaming method.

32 Recent Advances in Porous Ceramics

**2.1. Structure of foams**

**2. Liquid precursor foams**

Another technique to fabricate ceramic foams is direct foaming method. Ceramic foams are produced by incorporating air into a suspension or liquid media, which is subsequently set in order to keep the structure of air bubbles created. Then, the consolidated foams are afterwards sintered at high temperature to obtain high-strength foams [4]. This method can result in full dense struts without defects by polymeric sponge replication method. Hence, the mechanical strengths of the products are generally higher than those of reticulated porous ceramics. The characteristic of foams by this technique is that most cells are closed or semi-closed, depending on the air bubbles incorporated [6, 7]. **Figure 1(a)** shows the typical morphology of the ceramic foams prepared by the direct foaming method [8], and **Figure 1(b)** is a cross-sectional photograph of the dense struts. This chapter describes the processing of ceramic foams by

A bulk foam is a substance formed by trapping gas air bubbles in liquid or solid. A bath sponge and the top of fresh beer are examples of foams. In most cases, the volume of gas is Liquid foams are thermodynamically unstable systems due to their high gas-liquid interfacial area. Several physical processes take place in wet foams to decrease the system free energy, leading to foam destabilization. The main destabilization mechanisms are drainage (creaming) and coarsening (Ostwald ripening). Drainage is the physical separation between the gaseous and liquid phases of the foam because of the effect of gravity. In draining foams, light gas bubbles move upwards, forming a denser foam layer on the top, while the heavier liquid phase is concentrated on the bottom, as illustrated in **Figure 2** [4]. Coarsening is the gradual change of the foam structure due to gas diffusion through the films. This diffusion is driven by the pressure differences between bubbles. Small bubbles have high pressure, so they lose gas and disappear. Thus, the average bubble size increases with time.

**Figure 2.** Photograph of foam drainage and foam structure [4].

Generally, real liquid foams are only stable if they contain surfactants. Good foams usually contain complex molecules that toughen the walls of the bubbles. Milk fat, for instance, serves this purpose in whipped cream. The way the bubbles stick together or slip past one another determines how the foam behaves.

**2.4. Foams of ceramic slurry**

pore structure.

collapse of fluid films around the bubbles.

In order to manufacture ceramic foams, the liquid foams have to contain abundant ceramic particles in the liquid phase, which are going to be sintered as the main component of the corresponding ceramic foams. **Figure 4** shows the diagram of the related structures for precursor foams and the resulting ceramic foams. The bubbles, which occupy the most volume of liquid foams, turn into cells of the ceramic foams. The films, which comprise liquid and ceramic particles, transform into the cell walls. Generally, the central part of the films is too thin to keep intact during sintering. Hence, there are commonly windows in the cell walls between two neighbor cells. This kind of common constitution is called as semi-open structure. If the films are extremely thin or the solid contents are too low, only plateau border is survived after sintering corresponds to the struts between three or more cells. That generates the strict open

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Generally, the solid content influences the rheology of ceramic suspension. High solid content contributes to high viscosity and shear thin behavior. And the solid content would also associate with the final structure of the ceramic foams. Consequently, it is a practical way to adjust the porosity and structure of ceramic foams by controlling the solid content of the original suspension. Mao et al. [12] manufactured alumina foams with different morphology by changing the solid content using direct foaming and gelcasting method. **Figure 5** shows the rheological flow curves of suspensions with different solid contents. All suspensions reveal pseudoplastic behavior, and the viscosity increases with solid content at the measured shear rate range. In the fabrication of ceramic foams, a slight pseudoplasticity could favor the generation of foams since lower viscosities are obtained under high speed whipping, and the foam stability would be improved because the viscosity recovery under static condition delays the

The relative density of final alumina foams increases with the solid loading, while other processing conditions are constant, as indicated in **Figure 6**. The reason is that high solid content results in high viscosity, corresponding to low foaming capacity and high relative density. It

**Figure 4.** Structural diagram of (a) precursor foams and (b) the sintered ceramic foams.

#### **2.3. Particle-stabilized wet foams**

Solid particles with tailored surface chemistry have lately been shown to efficiently stabilize gas bubbles upon adsorption at the air-water interface. The attachment of particles at gas-liquid interfaces occurs when particles are not completely wet in the liquid phase or, in other words, are partially lyophobic (hydrophobic if the liquid is water). The position of the particles at the interface is ultimately determined by a balance between the gas-liquid, gas-solid, and solid-liquid interfacial tensions, as shown in **Figure 3** [10]. A simple way to describe the particle position at the interface is through the contact angle formed at equilibrium through the liquid phase. Slightly lyophobic particles remain predominantly in the liquid phase and exhibit a contact angle <90°, leading to the formation of air in water mixture, that is, foams [4].

In this method, the amphiphiles added to the suspension let the particles partially hydrophobic by adsorbing with its polar anchoring group on the surface and leaving a short hydrophobic tail in contact with the aqueous phase. Studart et al. [4] summarized some amphiphilic compounds, such as valeric acid, propyl gallate, butyl gallate, and hexyl amine, for surface modification for different particles. After surface modification, air can be easily incorporated by mechanical whipping, injection of gas stream, or initiation of a chemical reaction that releases gaseous by-products directly into the initially fluid suspension.

However, the particle-stabilized wet foams are not strong enough to resist the stress during drying. Hence, they still need to be strengthened before water evaporation, either by coagulating the particles within the foam films or by chemically gelling the foam liquid phase [11].

**Figure 3.** Diagrammatic sketch of the particle-stabilized foams [10].

#### **2.4. Foams of ceramic slurry**

Generally, real liquid foams are only stable if they contain surfactants. Good foams usually contain complex molecules that toughen the walls of the bubbles. Milk fat, for instance, serves this purpose in whipped cream. The way the bubbles stick together or slip past one another

Solid particles with tailored surface chemistry have lately been shown to efficiently stabilize gas bubbles upon adsorption at the air-water interface. The attachment of particles at gas-liquid interfaces occurs when particles are not completely wet in the liquid phase or, in other words, are partially lyophobic (hydrophobic if the liquid is water). The position of the particles at the interface is ultimately determined by a balance between the gas-liquid, gas-solid, and solid-liquid interfacial tensions, as shown in **Figure 3** [10]. A simple way to describe the particle position at the interface is through the contact angle formed at equilibrium through the liquid phase. Slightly lyophobic particles remain predominantly in the liquid phase and exhibit a contact angle <90°, leading to the formation of air in water

In this method, the amphiphiles added to the suspension let the particles partially hydrophobic by adsorbing with its polar anchoring group on the surface and leaving a short hydrophobic tail in contact with the aqueous phase. Studart et al. [4] summarized some amphiphilic compounds, such as valeric acid, propyl gallate, butyl gallate, and hexyl amine, for surface modification for different particles. After surface modification, air can be easily incorporated by mechanical whipping, injection of gas stream, or initiation of a chemical reaction that

However, the particle-stabilized wet foams are not strong enough to resist the stress during drying. Hence, they still need to be strengthened before water evaporation, either by coagulating the particles within the foam films or by chemically gelling the foam liquid phase [11].

releases gaseous by-products directly into the initially fluid suspension.

**Figure 3.** Diagrammatic sketch of the particle-stabilized foams [10].

determines how the foam behaves.

34 Recent Advances in Porous Ceramics

**2.3. Particle-stabilized wet foams**

mixture, that is, foams [4].

In order to manufacture ceramic foams, the liquid foams have to contain abundant ceramic particles in the liquid phase, which are going to be sintered as the main component of the corresponding ceramic foams. **Figure 4** shows the diagram of the related structures for precursor foams and the resulting ceramic foams. The bubbles, which occupy the most volume of liquid foams, turn into cells of the ceramic foams. The films, which comprise liquid and ceramic particles, transform into the cell walls. Generally, the central part of the films is too thin to keep intact during sintering. Hence, there are commonly windows in the cell walls between two neighbor cells. This kind of common constitution is called as semi-open structure. If the films are extremely thin or the solid contents are too low, only plateau border is survived after sintering corresponds to the struts between three or more cells. That generates the strict open pore structure.

Generally, the solid content influences the rheology of ceramic suspension. High solid content contributes to high viscosity and shear thin behavior. And the solid content would also associate with the final structure of the ceramic foams. Consequently, it is a practical way to adjust the porosity and structure of ceramic foams by controlling the solid content of the original suspension. Mao et al. [12] manufactured alumina foams with different morphology by changing the solid content using direct foaming and gelcasting method. **Figure 5** shows the rheological flow curves of suspensions with different solid contents. All suspensions reveal pseudoplastic behavior, and the viscosity increases with solid content at the measured shear rate range. In the fabrication of ceramic foams, a slight pseudoplasticity could favor the generation of foams since lower viscosities are obtained under high speed whipping, and the foam stability would be improved because the viscosity recovery under static condition delays the collapse of fluid films around the bubbles.

The relative density of final alumina foams increases with the solid loading, while other processing conditions are constant, as indicated in **Figure 6**. The reason is that high solid content results in high viscosity, corresponding to low foaming capacity and high relative density. It

**Figure 4.** Structural diagram of (a) precursor foams and (b) the sintered ceramic foams.

can also be seen from **Figure 6** that with the decrease of the relative density, both the mean

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In order to evaluate the influence of the size of ceramic particles, Mao et al. [13] introduced granule particles into the original powders to manufacture the alumina foams. The coarse powder was manufactured by grinding the presintered foams obtained by the fine powder, in order to keep similar sintering ability to fine powder. The flexural strength of the sintered foams with the coarse powder is 25% lower than that by original powder. And the permeability of foams using the coarse powder is about 30% higher than that by original powder. The drop of flexural strength and the rise of permeability are related to high degree of open pores.

Foams and foaming phenomena are common and important in our daily lives. While putting some shaving cream or soap on our faces, and rub gradually, we will create a truly bizarre substance, which are most gas and little bit of liquid. When we whisk air into egg white or cream, bubbles form and linger because the proteins present in these viscous liquids stretch around bubbles and trap them. The foams spout out from the compressed bottle, when we

All these techniques would be applied in the manufacture of ceramic foams. The foaming of ceramic slurries involves dispersing gas in the form of bubbles into ceramic suspension. There are two basic approaches for achieving this: (1) incorporating an external gas by mechanical frothing, or injection of a gas stream and (2) evolution of a gas in situ [14]. In order to stabilize the bubbles developed within the slurry, the surface tension of the gas-liquid interface need to be reduced by, in most cases, adding surfactant or by sometimes partially hydrophobic particles. In some cases, water-soluble polymers are added into the slurry to modify the viscosity,

One of the ways foam is created is through dispersion, where a large amount of gas is mixed with a liquid. Mechanical stirring is the most common technique for gas dispersion. Electric beater or household whisk is convenient choice for foaming of ceramic slurries [15]. The whisking procedure involves incorporating with air-forming bubbles, and at the same time, the bubbles flow up and break because of drainage and coalescence. Hence, generally, surfactant is necessary to reduce the surface tension to stabilize the bubbles. When the speed of bubble generation and burst become equilibrium, the maximum volume of the foam is obtained. **Figure 7** shows the foam volume versus stirring time for alumina suspensions containing two different foaming agents, Triton X114 and Tween 80 [16]. The foam volume increases gradually up to a maximum after approximately 4 min of agitation. During this initial stirring period, gas is entrained into the suspensions and liquid is drawn around each bubble until a thin film is formed. Subsequently, the surfactant molecules of the foaming agent transfer from

cell size and the window size are increased.

**3. Foaming techniques**

style our hair with mousse.

which will affect the foaming results and the stability.

**3.1. Incorporation of an external gas phase**

**Figure 5.** Rheological flow curves of suspensions with different solid contents [12].

**Figure 6.** SEM micrographs of alumina foams with suspension solid content of (a) 76 wt%, (b) 72 wt%, (c) 68 wt%, and (d) 60 wt% [12].

can also be seen from **Figure 6** that with the decrease of the relative density, both the mean cell size and the window size are increased.

In order to evaluate the influence of the size of ceramic particles, Mao et al. [13] introduced granule particles into the original powders to manufacture the alumina foams. The coarse powder was manufactured by grinding the presintered foams obtained by the fine powder, in order to keep similar sintering ability to fine powder. The flexural strength of the sintered foams with the coarse powder is 25% lower than that by original powder. And the permeability of foams using the coarse powder is about 30% higher than that by original powder. The drop of flexural strength and the rise of permeability are related to high degree of open pores.
