**1. Introduction**

Ceramic foams are tough foams made from ceramics, or ceramics with foam-like structure. It is a kind of porous ceramics with high porosity and sometimes called as cellular ceramics. Because of high amount of pores and surface, ceramic foams are especially suitable for filtering molten metals or hot gases, thermal protection systems, and heat exchangers [1].

The basic structural unit of the ceramic foams is composed of solid struts or walls and the empty cells surrounded by them. If the ceramic phase surrounds entire cells so that each cell is isolated from its adjacent ones, it is called as closed cell structure. If all cells are connected to each other with ceramic phase only in cell edges, it is called as open structure. In fact, ceramic foams often appear in a semi-open structure between the two ideal structures. The basic cell unit is the essential difference between the ceramic foams and general porous ceramics, which is actually a solid with isolated pores. And high porosity is its important characteristic. Gibson and Ashby [2] deem that there is a transition from ceramic foams to general porous ceramics with the relative density at about 0.3.

The earliest and still most common method for creating ceramic foams is the polymeric sponge replication method [3], with the products sometimes called as reticulated porous ceramics. In this

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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

Processing of Ceramic Foams

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

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].

so at a junction [9].

**2.2. Liquid foams**

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

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

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 direct foaming method.
