Ceramic foam

Ceramic foams are hardened ceramics with pockets of air or another gas trapped in pores throughout the body of the material. These materials can be fabricated as high as 94 to 96% air by volume with temperature resistances as high as 1700 °C.[1] Because many ceramics are already oxides or other inert compounds, there is little danger of oxidation or reduction of the material.[2]

Previously, pores had been avoided in ceramic components due to their brittle properties.[3] However, in practice ceramic foams have somewhat advantageous mechanical properties compared to bulk ceramics. One example is crack propagation, given by:

${\displaystyle \sigma _{t}=2\sigma \left({\frac {a}{r}}\right)^{\frac {1}{2}}}$

where σ_t is the stress at the tip of the crack, σ is the applied stress, a is the crack size and r is the radius of curvature. For certain stress applications, this means ceramic foams actually outperform bulk ceramics because the porous pockets of air act to blunt the crack tip radius, leading to a disruption of its propagation and a decrease in the likelihood of failure.[4]

Much like metal foams, there are a number of accepted methods for creating ceramic foams. One of the earliest and still most common is the polymeric sponge method.[5] A polymeric sponge is covered with a ceramic in suspension, and after rolling to ensure all pores have been filled, the ceramic-coated sponge is dried and pyrolysed to decompose the polymer, leaving only the porous ceramic structure. The foam must then be sintered for final densification. This method is widely used because it is effective with any ceramic able to be suspended; however, large amounts of gaseous byproducts are released and cracking due to differences in thermal expansion coefficients is common.[3]

While the above are both based on the use of a sacrificial template, there are also direct foaming methods that can be used. These methods involve pumping air into a suspended ceramic before setting and sintering. This is difficult because wet foams are thermodynamically unstable and can end up with very large pores after setting.[3]

A recent method of creating aluminum oxide foams has also been developed.[1] This technique involves heating crystals with the metal and forming compounds until a solution is created. At this point, polymer chains form and grow, causing the entire mixture to separate into a solvent and polymer. As the mixture begins to boil, air bubbles are trapped in solution and locked in to place as the material is heated and polymer is burned off.