Foams

Manufacturers have pushed the limits of plastic, glass, and other materials by turning them into sturdy foams. Now metals are getting the same treatment.

Materials created by mixing a solid with minute spheres of glass, ceramic, or polymer are finding an increasing range of uses in industrial and high-tech applications. When the spheres are hollow, the materials are foams. They are specified for high-performance aircraft and are used by pattern-makers in factories. Not least among their important characteristics is that materials of this sort are isotropic; that is, they tend to behave the same way on every load-bearing axis.

Researchers exploring metal foams talk about properties that sound almost like the free lunch nature supposedly never serves.

The foams in question are not blown foams created by the injection of gas, nor are they self-expanding, created by chemical evolution. They are syntactic, or assembled.

Blown foams are made by mixing or injecting a gas into a liquid, causing it to froth like soap bubbles in a bathtub. When the bubbles solidify, you have a foam.

Making a self-expanding foam requires the use of at least two chemical constituents: one to decompose into a gas to form the bubbles, and one to form the walls of the cells.

Syntactic foams use "prefabricated," manufactured bubbles that are mechanically combined with a resin to form a composite material. The term "syntactic" is derived from the Greek syntaktikos, meaning "to arrange together." Blown and self-expanding foams develop a fairly random distribution of gas pockets of widely varying sizes and shapes, but the porosity of syntactic foams can be much more closely controlled by careful selection and mixing of the preformed bubbles with the matrix.

While ordinary foams are visibly porous, syntactic foams can have cells so small that the material looks like a homogeneous solid. What's more, syntactic foams behave like homogeneous solids and are easier to use than many other advanced materials.

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