Ceramic foam is a lightweight structure consisting of a small percentage of very porous ceramic and a high percentage of gas-filled pores. Typical ceramic foams are 75 to 90% porous, although some are even more porous. Such foams have valuable material properties that make them suitable for a variety of engineering and other applications.
Most manufacturing techniques for such foam generally involve impregnation of another foam structure with ceramic slurry. This structure is then fired at high temperature in a kiln. The ceramic hardens while the heat of the kiln destroys the base structure on which it was built.
A lightweight and highly porous ceramic structure is thereby created as the final product of this manufacturing process. The material created may have individually sealed pores. This type of structure is known as closed-cell foam. When the pores of the foam are interconnected throughout the structure, it is called open-cell foam.
The specific characteristics of the particular foam created depend on a variety of factors, but this class of materials is known for certain common properties. Low thermal conductivity makes ceramic foam particularly useful as a thermal insulating material. Its ability to withstand relatively large thermal shocks also makes it suitable for harsh environments such as industrial and aeronautical applications. The high porosity characteristic of these foams means that they are relatively lightweight materials useful in applications where excess weight must be avoided.
Tiles used in the thermal protection systems of the United States’ space shuttles were classic examples of the use of ceramic foam for these properties. Spacecraft traveling between the earth and space, either during launch or re-entry, encounter extreme temperature changes while passing through the atmosphere at high speed. Insulation from these extreme changes was necessary to protect shuttles from this extremely harsh environment. It is very expensive and energy-intensive to move mass into space, however, so weight had to be kept to a minimum. The lightweight nature of ceramic foam combined with its thermal properties made it attractive for use in some of the tiles installed on the shuttle fleet.
In addition to use as an insulating material, ceramic foam is also commonly employed in filtration applications. When used in refining other materials, ceramic foam filters generally provide three modes of filtration. Upon contacting the inlet of the filter, particles larger than the pore size are backed up on the filter’s surface because they are too large to pass through. This backed up material forms a layer, known as the cake layer, on the surface of the filter that in turn captures smaller particles. The finest particles can pass through the cake layer but are then captured within the pores of the filter.
Potential new uses of ceramic foam are also in development. For example, in the medical industry it is being studied for possible use to stimulate bone growth where bone has been removed, such as in cancer patients. Very small segments of ceramic foam are also being considered for use in delivering drugs to the body over time through a controlled release process.