Metal foam is a foam — gas bubble-filled substance — made out of metal, frequently aluminum. Metal foam comes in two varieties, closed-cell foam, with sealed pores, and open-cell foam, with open pores. Metal foams have a very high porosity, where 75-95% of the volume of the material is empty, filled by air or hydrogen. Metal foams retain many of the characteristics of the metal used to produce them, such as non-flammability. On the other hand, metal foams are less conductive than solid metal, and are substantially weaker, though rarely weaker than wood. Metal foams are easily recyclable back into the original metal, making them more reusable than polymer foams.
Metal foam is sometimes considered a subset of cellular metallic materials in general, which also includes "metal sponges," though often the term "metal foams" is used interchangeably with all cellular metallic materials. Several categories of cellular metallic materials are distinguished, including cellular metal (metal foam with internal cells, usually closed), porous metal (with closed, smoothly curved voids (pores) rather than jagged or open voids), metallic foams (special cases of porous metals, created by bubbling gas through liquid metal and then letting it solidify), and metal sponges, which is essentially open-cell foam where the entire space of voids is interconnected. These categories are not mutually exclusive, and there are some substances that straddle multiple categories. For instance, a foam with breaks between the cell walls may be on the edge of a metal foam and a metal sponge.
Many metal foams are created by introducing air bubbles into molten metal. Making a foam out of molten metal is not easy, and the material is accordingly expensive. A foaming agent such as powdered titanium hydride, which decomposes into titanium and hydrogen at high temperatures, must be used. Metal foam is a specialty material, used for aerospace, heat exchangers, and other high-performance applications. Because metal foam is stiff and light, it has often been proposed as a futuristic structural material, though it has not yet seriously been used as one. Some commercial metal foams include M-Pore, Porvair, Duocel, Metal Foam Korea, Metafoam and Recemat. The pores in metal foams are usually between 1-8 mm in diameter, but some specialty foams have pores so small they are invisible to the naked eye.
Closed-cell metal foams have only been produced since about 1990. Intriguingly, some metal foams are so diffuse that they float on water. Metal foams are sometimes touted as the ideal material to build floating ocean platforms for solar panels or even cities.