Do you know the Metal Foam ?

Metal foam is what you get when you add a foaming agent, powdered titanium hydride, to molten aluminum, then let it cool.The defining characteristic of metal foams is a very high porosity: typically 75–95% of the volume consists of void spaces making these ultralight materials. The strength of foamed metal possesses a power law relationship to its density; i.e., a 20% dense material is more than twice as strong as a 10% dense material.

The result is a very strong substance that is relatively light, with 75–95% empty space. Because of its favorable strength-to-weight ratio, metal foams have been proposed as a construction material for space colonies.  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).

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 thermal conductivity will likely be reduced.

Aluminum Metal Foam

Although there is a very large number of patents describing feasible topological structures, constitutive materials and production methods, metal foams cannot be considered a commodity and relatively few commercial producers are available worldwide of either closed or open cell foams.

Open celled metal foams 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.

“Metal foam” or “metallic foam” has become a very popular term which is nowadays used for almost any kind of metallic material which contains voids. It might be useful to distinguish various expressions:

  1. cellular metal: space is divided into distinct cells. The boundaries of these cells are made of solid metal, the interior are voids. Ideally, the individual cells are all separated from each other by metal but often this restriction is relaxed
  2. porous metal: the metal contains a multitude of pores, i.e. closed, curved gas voids with a smooth surface.
  3. metal(lic) foam: foams are special cases of porous metals. A solid foam originates from a liquid foam in which gas bubbles are finely dispersed in a liquid.
  4. metal sponge: space is filled by pieces of metal that form a continuous network and co-exist with a network of empty space which is also interconnected.

Metallic foams are used in the field of compact heat exchangers to increase heat transfer at the cost of an additional pressure drop. However, their use permits substantial reduction in the physical size of a heat exchanger, and so fabrication costs. To model these materials, most works uses idealized and periodic structures or averaged macroscopic properties.

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. The next two patents on sponge-like metal were issued to Benjamin Sosnik in 1948 and 1951 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 50s, commercial production was started only in the 90s by Shinko Wire company in Japan. 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 an identical non-porous alloy; commonly those of aluminium) and stiff, and are frequently proposed as a lightweight structural material. However, they have not yet been widely used for this purpose.

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