Metal foam

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 well over 80% of the volume consists of void spaces.

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Open-cell metal foams

Open celled metal foams have a structure similar to open-celled polyurethane foams 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. Manufacturers include ERG Aerospace (tradename Duocel), Metal Foam Korea, M-Pore, Porvair, Metafoam and Recemat. 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. Manufacturers include Inco Ltd (trade name Incofoam).

Closed-cell metal foams

Closed-cell metal foams have been developed since about 1990, and are commonly made by injecting a gas or foaming agent into molten metal. The size of the pores - or 'cell size' - is usually between 1 mm and 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% 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. They are currently manufactured by Cymat Corporation (Canada) and the Shinko Wire Company (Japan).

Interestingly, naturally formed Aluminum 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.

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See also

• Porous medium
• Foam