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Metal foam
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A metal foam is a cellular structure consisting of a solid metal - frequently aluminium - containing a large volume fraction of gas-filled pores. The pores can be sealed (closed-cell foam), or they can form an interconnected network (open-cell foam). The defining characteristic of metal foams is a very high porosity: typically 75-95% of the volume consists of void spaces. The strength of foamed metal possesses a power law relationship to its density; i.e.
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A metal foam is a cellular structure consisting of a solid metal - frequently aluminium - containing a large volume fraction of gas-filled pores. The pores can be sealed (closed-cell foam), or they can form an interconnected network (open-cell foam). The defining characteristic of metal foams is a very high porosity: typically 75-95% of the volume consists of void spaces. The strength of foamed metal possesses a power law relationship to its density; i.e. 20% dense material is more than twice as strong as 10% dense material.
Metallic foams typically retain some physical properties of their base material. Foam made from non-flammable metal will remain non-flammable and the foam is generally recyclable back to its base material. Coefficient of thermal expansion will also remain similar while conductivity will likely be reduced.
Open-cell metal foams
Open celled metal foams are usually replicas using open-celled polyurethane foams as a skeleton and have a wide variety of applications including heat exchangers (compact electronics cooling, cryogen tanks, PCM heat exchangers), energy absorption, flow diffusion and lightweight optics. Due to the high cost of the material it is most typically used in advanced technology aerospace and manufacturing.
Extremely fine-scale open-cell foams, with cells too small to be visible to the naked eye, are used as high-temperature filters in the chemical industry.
Closed-cell metal foams
First patent issued on sponge like metal by Sosnik in 1948 who applied mercury vapor to blow liquid aluminium.
Closed-cell metal foams have been developed since about 1956 by John C. Elliott at Bjorksten Research Laboratories. Although the first prototypes were available in the 50's already serial production is started only in the 90`s by Shinko Wire company in Japan. Metal foams are commonly made by injecting a gas or mixing a foaming agent (frequently TiH2) into molten metal. In order to stabilize the molten metal bubbles, high temperature foaming agent (nano or micrometer sized solid particles) is required. The size of the pores - or 'cell size' - is usually 1 to 8 mm.
Closed-cell metal foams are primarily used as an impact-absorbing material, similarly to the polymer foams in a bicycle helmet but for higher impact loads. Unlike many polymer foams, metal foams remain deformed after impact and can therefore only be used once. They are light (typically 10-25% of the density of the metal they are made of, which is usually aluminium) and stiff, and are frequently proposed as a lightweight structural material. However, they have not yet been widely used for this purpose.
Closed-cell foams retain the fire resistant and recycling capability of other metallic foams but add an ability to float in water.
Naturally formed aluminium foams have been found in the Oklo reactor, apparently formed by the combination of the reactor's extreme heat and the release of stable xenon gas.
Foam metal has also begun to be used as an experimental prosthetic in animals. In this application, a hole is drilled into the bone and the metal foam inserted letting the bone grow into the metal for a permanent connection. The most notable example was performed by Dr. Robert Taylor on a Siberian Husky named Triumph, in which both back legs received foam metal prostheses.
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