**5. Drying and sintering**

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 polymer-

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

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

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


ization procedures have to be carried out in a N<sup>2</sup>

42 Recent Advances in Porous Ceramics

for gelation.

Yb<sup>3</sup> Al<sup>5</sup>

**4.4. Sol-gel**

O12 [35].

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 avoid possible crack.

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.

is related to the particle coordination, where large coordination number corresponds to large grain size. The possible reason is that more coordination particles indicate abundant mass resource for grain growth.

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## **6. Summary**

Due to its current and potential great application, ceramic foams attracted distinct attentions in past decades with new process routs constantly being developed and reported in the scientific literature and at conferences. As a kind of porous ceramic with special structure, the ceramic foams gradually play irreplaceable roles in many industry fields, such as diesel particulate filters, interpenetrating composites, high-temperature thermal insulators, and biomedical applications. It is very important and valuable to explore novel manufacture routes and continuously improve the performance of ceramic foams. Whereas the polymer replication process is advanced to be in commercial use for decades, now the slurry foaming techniques are developed rapidly, which yields ceramic foams with different morphologies, and hence different properties and potential applications. This provides much greater choice for the end user and far greater potential for the tailoring of structures to meet specific endsue requirement.
