Metal foam is a gas-bubbled substance made of metal, with high porosity and recyclability. It comes in closed or open cell varieties and retains many metal characteristics. Metal foams are less conductive and weaker than solid metal but are used in aerospace and high-performance applications. They are sometimes considered a subset of cellular metal materials and come in several categories, including metallic foams and metal sponges. Closed-cell metal foams have only been produced since about 1990. Some metal foams are so common that they float on water and are touted as the ideal material for building floating ocean platforms.
Foam metal is foam – a gas-bubbled substance – made of metal, often aluminum. Metal foam comes in two varieties, closed cell foam, with sealed pores, and open cell foam, with open pores. Metal foams have very high porosity, where 75-95% of the material volume 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 metal materials in general, which also includes “metal sponges,” although often the term “metal foams” is used interchangeably with all cellular metal materials. Several categories of cellular metallic materials are distinguished, including cellular metal (metal foam with internal, usually closed cells), porous metal (with closed, gently curved voids (pores) rather than jagged or open voids), metallic foams (special cases of metals, created by bubbling gas through liquid metal and then allowing it to solidify), and metal sponges, which is essentially open-cell foam in which the entire space of voids is interconnected. These categories are not mutually exclusive, and there are some substances that straddle multiple categories. For example, a foam with breaks between 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 the molten metal. Making a foam with molten metal is not easy and the material is accordingly expensive. A blowing agent such as titanium hydride powder should be used, which decomposes into titanium and hydrogen at high temperatures. Metal foam is a special material, used for aerospace, heat exchangers and other high performance applications. Because foam metal is stiff and light, it has often been proposed as a futuristic structural material, although it has not yet been used seriously as such. Some commercial metal foams include M-Pore, Porvair, Duocel, Metal Foam Korea, Metafoam and Recemat. The pores in metal foams are usually between 1 and 8 mm in diameter, but some specialty foams have pores so small that they are invisible to the naked eye.
Closed-cell metal foams have only been produced since about 1990. Curiously, some metallic foams are so common that they float on water. Metal foams are sometimes touted as the ideal material for building floating ocean platforms for solar panels or even cities.
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