**3. Foaming techniques**

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 style our hair with mousse.

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, which will affect the foaming results and the stability.

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

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

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

36 Recent Advances in Porous Ceramics

(d) 60 wt% [12].

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

**4. Foam consolidation**

**4.1. Freezing**

production.

**4.2. Natural polymer denaturation**

ture of the protein molecule.

In direct foaming method, ceramic foams are prepared by introducing large amounts of air bubbles into the slurry. The foam is essentially a metastable system, with some bubbles shrinking and others gathering. It is important to consolidate the foams in certain period, to keep the cellular structure during further heating procedure [21]. In order to prevent the foams from drainage or coarsening, it is necessary to accelerate the consolidation speed and obtain a higher strength. Suitable consolidation method would bring uniform and dense struts, which

Processing of Ceramic Foams

39

http://dx.doi.org/10.5772/intechopen.71006

Freezing method is one of the practical methods to consolidate the foamed slurry [22]. Verma et al. [23] manufactured silica foams with 85 vol% porosity content from ceramic slurries containing ovalbumin as binder along with additives of sucrose and colloidal silica by combination of direct foaming and freeze-casting routes. The foamed slurries were poured into vaseline-coated aluminum molds and cooled using liquid nitrogen for instant freezing of porous structure. The frozen samples were freeze-dried at a low temperature for 24 h. After drying, the dried foam was heated to 1150°C to remove the binder and sinter the pore walls. The advantage of the freezing method is that extra consolidation agent is unnecessary. However, during freezing procedure, the liquid solvent, for example, water, will transfer to solid crystals which entrapped between the agglomerated ceramic particles at the films. These crystals will leave micropores after the evaporation of solvent. Rapid freezing of the solvent leads to formation of fine ice crystals, while long-time freezing procedure would enlarge the size of the crystals. The corresponding large pores may not be removed during sintering and, hence, lead to lower mechanical strength. The freezing time has to be prolonged to cool down the temperature of inside parts for large-sized bulk foams since the foamed slurry is a thermal insulator, which indicates that the frozen crystals will grow during consolidation of large-scale products. Thus, the freezing technique might not be good for high-strength foam

Some natural polymers from animal and plant sources have the properties of liquid-solid transition due to denaturation which has potential applications in the consolidation of foamed slurries. Protein and polysaccharide including starch, agar, and cellulose are often used to manufacture ceramic foams and porous ceramics [23–25]. Proteins are high molecular compounds, which are formally understood as condensation products of amino acids. The amphiphilic character of these molecules causes a decrease of surface tension, therefore good foaming properties. These foaming properties are influenced by the amino acid sequence or rather the number of polar and apolar side chains as well as molecule flexibility [26]. After foaming, the foamed slurries are consolidated by changing conditions, for example, adding acid or heating over 60°C, which would trigger the irreversible changes in the spatial struc-

is a benefit for mechanical properties of the resulting ceramic foams.

**Figure 7.** Foam volume generated with two different foaming agents [16].

interior of the suspension toward the newly created surface, decreasing the surface tension. Increasing the surfactant concentration accelerates this transfer and hence increases the foaming capacity [17]. Sepulveda considers that the maximum foam volume is associated with a minimum thickness of film that can sustain a stable foam. When most surfactant molecules have attached themselves to the gas-liquid interface, the stabilization of new films is no longer possible and the volume increase becomes negligible.

#### **3.2. In situ gas evolution**

The foaming technology of this theme is the presence of a foaming agent that decomposes due to heat or a chemical reaction to generate a gas within a ceramic suspension. Kim [18, 19] used the mixture of the cross-linked polycarbosilane and polysiloxane as the preceramic polymers to manufacture the SiOC foams, which were pressed into disks and CIPed at 340 MPa. The green compacts were placed in a pressure chamber to saturate with gaseous CO2 under a pressure of 5.5 MPa. Then, a thermal dynamic instability was introduced by rapidly dropping the pressure at a rate of 2.9 MPa/s. The foamed preceramic specimens were further cross-linked, then pyrolyzed, and sintered at 1200°C in nitrogen. Takahashi et al. [20] used the blend of methylsilicone resin and polyurethane precursor to prepare the SiOC foams. The foamed blend was prepared in two steps. The first step was the addition of methylsilicone resin dissolved in CH<sup>2</sup> Cl<sup>2</sup> to the mixture of the polyols, the amine catalysts, the surfactant, and the additional dichloromethane. The second step was the addition of polyisocyanate to the solution obtained in the first step. The expansion started during mechanical stirring by the evaporation of the solvent caused by the exothermal reactions occurring in the solution.
