**4. Foam consolidation**

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

#### **4.1. Freezing**

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

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

 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

to the mixture of the polyols, the amine cata-

Cl<sup>2</sup>

lysts, 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

possible and the volume increase becomes negligible.

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

of methylsilicone resin dissolved in CH<sup>2</sup>

**3.2. In situ gas evolution**

38 Recent Advances in Porous Ceramics

occurring in the solution.

CO2

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 production.

#### **4.2. Natural polymer denaturation**

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 structure of the protein molecule.

Garrn et al. [26] used albumin, a major constituent of blood, as a model binder to an aqueous powder suspension to produce ceramic foams. Foaming was done in a planetary mill using PE-milling pots for 15 min. A fine cellular foam structure with approximate diameter of 50–300 μm was formed. Thermal consolidation was done in a conventional household microwave oven with a maximum microwave power of 900 W. After burn out and sintering, final densities in the range from 8 to 20% were achieved. Fish collagen and egg white [27, 28] are other specific examples of protein applied for ceramic foams. They were added into ceramic slurries which would be stirred to become foams. The foamed slurries were then heated at 80°C or higher for consolidation, attributing to the gelation of protein.

**4.3. Gelcasting**

In recent years, the gelcasting method was developed to manufacture ceramic foams by solving the shortcomings of the mentioned natural polymer substances which need heating for denaturation. The water-soluble small molecule compounds are added into the slurries, which will form a gel through radical polymerization. The method was first proposed by Smith [29], which combines the foaming and gelcasting processes, resulting in wet foams with high strength for drying and further handling. Sepulveda and Binner have done a lot of work on the gelcasting of foams, which has been shown to be useful in a variety of ceramic systems such as zirconia, alumina, and hydroxyapatite [30]. The benefit compared with using polymers is the ability to formulate slurries with a lower viscosity, because the size of the organic molecule is smaller, so that higher solid contents can be achieved with good packing densities and excellent green strengths [31]. Such a process yields cellular structures with porosity varying from 40% to >90%, with pores closed or open depending on pore fraction. Mechanical strength of sintered foams is higher than that obtained by other routes, because of the spheri-

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**Figure 9** shows the flow chart for production of ceramic foams by gelcasting of foams. Ceramic slurries with monomers and surfactants are vigorously whisked under inert gas atmosphere to form foams. Afterwards, catalyst and initiator are added to trigger the polymerization reaction, forming a strong three-dimensional gel net. The concentration of these reagents is designed to produce an induction period such that polymerization will be initiated immediately after casting. Within the time allowed by the induction period, the foams could be placed in a desiccator with the pressure reduced using a vacuum pump to produce foams with cell size larger than those obtained directly through foaming [16]. The excellent green body strength is the main advantage of the gelcasting of ceramic foams, which may maintain a porous structure with the porosity up to 90%, compared with other consolidation methods. The investigation to the sintered foams confirms that the solid matrix has very high density, which is much perfect than those by polymeric sponge replication method. Sepulveda produced alumina foams with the bending strength in the range of 2–26 MPa, while their relative

cal pore shape associated with fully dense matrix [15].

**Figure 9.** Flow chart for production of ceramic foams by gelcasting of foams.

density varied in 8–30% [30].

Methylcellulose and polysaccharide, which have similar transformation as protein in case of heating, are also typical agents for consolidation of foamed ceramic slurries. Mao et al. [17] manufactured silica foams based on the generation of foams from composite slurries with cassava starch. These slurries combined with surfactant were vigorously whisked for about 5 min to make foam structure. The as-foamed slurries were then preheated in a microwave oven with a power of 400 W for 60 s, followed by setting in a 70°C oven for 30 min to consolidate the foam structure. After sintering, the resulting silica foams with the relative density of 18–30% were obtained. Because the cassava starch is not soluble in water, the particles will residual pores inside cell walls and struts after debindering and sintering. Hence, the direct foaming and starch consolidation method can produce porous ceramics with hierarchical structures, as shown in **Figure 8**. **Figure 8(a)** shows clearly the spherical cells with the average size of about 50 μm, while **Figure 8(b)** reveals the pores averaging about 10 μm in the cell walls. **Figure 8(c)** further indicates small voids inside ceramic matrix. It can be inferred that the large-sized cells, moderate-sized pores, and small-sized voids were originated from bubbles, elimination of starch particles, and interstices among the silica grains, respectively.

These natural polymers which can be operated in laboratory environment, for example, simply heated at 50–80°C, are excellent agents for ceramic foams. They are environmentally friendly and low cost and, hence, are widely used. However, the consolidation procedure needs heating of the foamed slurries, which would lead to the expansion of air bubbles. The metastable structure would change during the temperature change, which should remain some defects in final ceramic foams.

**Figure 8.** SEM micrographs of sintered ceramics with details of (a) large-sized cells, (b) moderate-sized pores in cell wall, and (c) small-sized voids among silica grains [17].

#### **4.3. Gelcasting**

Garrn et al. [26] used albumin, a major constituent of blood, as a model binder to an aqueous powder suspension to produce ceramic foams. Foaming was done in a planetary mill using PE-milling pots for 15 min. A fine cellular foam structure with approximate diameter of 50–300 μm was formed. Thermal consolidation was done in a conventional household microwave oven with a maximum microwave power of 900 W. After burn out and sintering, final densities in the range from 8 to 20% were achieved. Fish collagen and egg white [27, 28] are other specific examples of protein applied for ceramic foams. They were added into ceramic slurries which would be stirred to become foams. The foamed slurries were then heated at

Methylcellulose and polysaccharide, which have similar transformation as protein in case of heating, are also typical agents for consolidation of foamed ceramic slurries. Mao et al. [17] manufactured silica foams based on the generation of foams from composite slurries with cassava starch. These slurries combined with surfactant were vigorously whisked for about 5 min to make foam structure. The as-foamed slurries were then preheated in a microwave oven with a power of 400 W for 60 s, followed by setting in a 70°C oven for 30 min to consolidate the foam structure. After sintering, the resulting silica foams with the relative density of 18–30% were obtained. Because the cassava starch is not soluble in water, the particles will residual pores inside cell walls and struts after debindering and sintering. Hence, the direct foaming and starch consolidation method can produce porous ceramics with hierarchical structures, as shown in **Figure 8**. **Figure 8(a)** shows clearly the spherical cells with the average size of about 50 μm, while **Figure 8(b)** reveals the pores averaging about 10 μm in the cell walls. **Figure 8(c)** further indicates small voids inside ceramic matrix. It can be inferred that the large-sized cells, moderate-sized pores, and small-sized voids were originated from bubbles, elimination of starch particles, and inter-

These natural polymers which can be operated in laboratory environment, for example, simply heated at 50–80°C, are excellent agents for ceramic foams. They are environmentally friendly and low cost and, hence, are widely used. However, the consolidation procedure needs heating of the foamed slurries, which would lead to the expansion of air bubbles. The metastable structure would change during the temperature change, which should remain

**Figure 8.** SEM micrographs of sintered ceramics with details of (a) large-sized cells, (b) moderate-sized pores in cell wall,

80°C or higher for consolidation, attributing to the gelation of protein.

stices among the silica grains, respectively.

some defects in final ceramic foams.

40 Recent Advances in Porous Ceramics

and (c) small-sized voids among silica grains [17].

In recent years, the gelcasting method was developed to manufacture ceramic foams by solving the shortcomings of the mentioned natural polymer substances which need heating for denaturation. The water-soluble small molecule compounds are added into the slurries, which will form a gel through radical polymerization. The method was first proposed by Smith [29], which combines the foaming and gelcasting processes, resulting in wet foams with high strength for drying and further handling. Sepulveda and Binner have done a lot of work on the gelcasting of foams, which has been shown to be useful in a variety of ceramic systems such as zirconia, alumina, and hydroxyapatite [30]. The benefit compared with using polymers is the ability to formulate slurries with a lower viscosity, because the size of the organic molecule is smaller, so that higher solid contents can be achieved with good packing densities and excellent green strengths [31]. Such a process yields cellular structures with porosity varying from 40% to >90%, with pores closed or open depending on pore fraction. Mechanical strength of sintered foams is higher than that obtained by other routes, because of the spherical pore shape associated with fully dense matrix [15].

**Figure 9** shows the flow chart for production of ceramic foams by gelcasting of foams. Ceramic slurries with monomers and surfactants are vigorously whisked under inert gas atmosphere to form foams. Afterwards, catalyst and initiator are added to trigger the polymerization reaction, forming a strong three-dimensional gel net. The concentration of these reagents is designed to produce an induction period such that polymerization will be initiated immediately after casting. Within the time allowed by the induction period, the foams could be placed in a desiccator with the pressure reduced using a vacuum pump to produce foams with cell size larger than those obtained directly through foaming [16]. The excellent green body strength is the main advantage of the gelcasting of ceramic foams, which may maintain a porous structure with the porosity up to 90%, compared with other consolidation methods. The investigation to the sintered foams confirms that the solid matrix has very high density, which is much perfect than those by polymeric sponge replication method. Sepulveda produced alumina foams with the bending strength in the range of 2–26 MPa, while their relative density varied in 8–30% [30].

**Figure 9.** Flow chart for production of ceramic foams by gelcasting of foams.

However, the usual monomers are acrylamide derivatives, and the polymerization is a free radical reaction which is inhibited by oxygen. For example, just 0.2% oxygen was sufficient to inhibit the reaction completely in foamed suspensions [30]. Thus, the foaming and polymerization procedures have to be carried out in a N<sup>2</sup> -filled chamber to insulate oxygen.

sol, and the mixture was foamed by vigorous agitation. HF was added in order to catalyze the gelation. The foamed gels were cast, aged at 40°C for 72 h, and dried at 40°C for 120 h. Final glass and hybrid foams can be obtained with a high porosity varying from 60 to 95% and

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The consolidated wet foams are a mixture of gas, liquid, and solid, which need to be dried and debindered before sintering to final ceramic foams. Since there are large amount of bubbles dispersed in the bodies, the green strength is much lower than that of normal ceramics. Hence, both drying and debindering procedures should be carried out carefully. However, the bubbles, especially the connected bubbles, would become channels for water, solvent, or pyrolyzate to escape. Generally, the drying and calcination speed should be slowed down to

The foamed green bodies need to be sintered to get sufficient strength for further applications. It is important to modify the sintering schedules to get dense and strong struts and cell walls, to increase the mechanical properties of the ceramic foams. The sintering for ceramic foams is not get equivalent research intension as for foaming and consolidation, since the sintering behavior is dominantly decided by the powders. Especially for the particles inside the struts and the cell walls, the coordination particles are same within normal ceramics. However, for those particle located on the surface of cell walls or in the tip of the strut edges, their coordination particles are less than in dense green bodies. **Figure 10** shows the SEM microstructures of the fracture surface of the struts and the edge of the cell window. We can see clearly that grain size inside the struts is larger than that near the cell wall surface. And the gran size becomes much smaller when the location shifts to the tip of the triangle. The ceramic sintering theory seems not simply suitable to describe the ceramic foams. The difference for grain size

**Figure 10.** Microstructure of alumina foams: (a) fracture surface of the struts and (b) edge of the cell window.

macropore diameters ranging from 10 to 600 μm.

**5. Drying and sintering**

avoid possible crack.

Mao et al. [32] developed a novel gelcasting system based on epoxy resin and polyamine hardener, which could be operated in air atmosphere, because the polymerization between the epoxide group of the epoxy resin and active hydrogen of amine is a nucleophilic addition reaction which is not affected by oxygen in atmosphere. This gelcasting system was then applied to manufacture ceramic foams with some modification [12]. Aqueous suspensions with solids loading of 60–76 wt% were prepared by mixing alumina powder, dispersant, and 5 wt% polyethyleneimine solution. Vigorous stirring about 5 min was applied after adding the surfactant to generate foams. For setting the fluid foams, 10 wt% sorbitol polyglycidyl ether based on the premix solution was added with further stirring about 30 s. The foamed suspensions were immediately poured into plastic molds and sealed at room temperature for gelation.

Yang et al. reported a novel single-component water-soluble copolymer of isobutylene and maleic anhydride, with a commercial name of Isobam, which could be used as both surfactant and gelling agent with the addition much lower than normal gelation systems [33]. Yang et al. developed this system for the consolidation of ceramic foams. A small addition of 0.3 wt% Isobam based on alumina powder is sufficient to consolidate liquid foams and maintain the wet foams for further treatments [34]. Small additive amount is benefit for further heat treatment because the exhaust gaseous by-product can be dramatically reduced. It was confirmed that Isobam could be applied to manufacture variety of ceramic materials, such as mullite and Yb<sup>3</sup> Al<sup>5</sup> O12 [35].

#### **4.4. Sol-gel**

Sol-gel method has been widely used in the preparation of powder, film, and bulk materials. Since the processing of sol-gel is actually a liquid-solid transformation, it can be used to consolidate the liquid foams without any other additive. The advantage of this route is that no contamination is involved, which is suitable for producing high-purity ceramic foams. Silica foams and silica-contained ceramic foams have been manufactured [36–38]. Commercial SiO<sup>2</sup> sol or the hydrolyzate of the precursor tetraethoxysilane was modified by adding acid to the pH value in the range of 5–6. After adding surfactant, the sol is incorporated with air by mechanical stirring or in situ gas evolution. Then, the foamed sol will be gradually consolidated with the sol transfer to gel. The porosity and the pore size distribution may be controlled by changing the viscosity and foaming technology. The silica-based sol-gel system has been used in many ceramic foams, such as silica, boehmite, and zirconia [21]. Pereira et al. [37] manufactured bioactive glass and hybrid scaffolds for bone tissue engineering by sol-gel method. TEOS and calcium chloride were used as the silica and calcium precursors, respectively. The starting sol was prepared by hydrolysis of TEOS in the presence of 1 N hydrochloric acid solution with subsequent addition of calcium chloride. PVA solution, Teepol surfactant, and 5 vol% hydrofluoric acid solution were added to a 40-ml aliquot of the sol, and the mixture was foamed by vigorous agitation. HF was added in order to catalyze the gelation. The foamed gels were cast, aged at 40°C for 72 h, and dried at 40°C for 120 h. Final glass and hybrid foams can be obtained with a high porosity varying from 60 to 95% and macropore diameters ranging from 10 to 600 μm.
