Metal foam

In materials science, a metal foam is a material or structure consisting of a solid metal (frequently aluminium) with gas-filled pores comprising a large portion of the volume. The pores can be sealed (closed-cell foam) or interconnected (open-cell foam). The defining characteristic of metal foams is a high porosity: typically only 5–25% of the volume is the base metal. The strength of the material is due to the square–cube law. Metal foams typically retain some physical properties of their base material. Foam made from non-flammable metal remains non-flammable and can generally be recycled as the base material. Its coefficient of thermal expansion is similar while thermal conductivity is likely reduced. Open-celled metal foam, also called metal sponge, can be used in heat exchangers (compact electronics cooling, cryogen tanks, PCM heat exchangers), energy absorption, flow diffusion, scrubbers, flame arrestors, and lightweight optics. The high cost of the material generally limits its use to advanced technology, aerospace, and manufacturing. Fine-scale open-cell foams, with cells smaller than can be seen unaided, are used as high-temperature filters in the chemical industry. Metal foams are used in compact heat exchangers to increase heat transfer at the cost of reduced pressure. However, their use permits substantial reduction in physical size and fabrication costs. Most models of these materials use idealized and periodic structures or averaged macroscopic properties. Metal sponge has very large surface area per unit weight and catalysts are often formed into metal sponge, such as palladium black, platinum sponge, and spongy nickel. Metals such as osmium and palladium hydride are metaphorically called "metal sponges", but this term is in reference to their property of binding to hydrogen, rather than the physical structure. Closed-cell metal foam was first reported in 1926 by Meller in a French patent where foaming of light metals, either by inert gas injection or by blowing agent, was suggested.