Composite metal foams shield against bullets, radiation and heat

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Hollow spheres in metal foams can be embedded in a couple of ways: encasing pre-made spheres with a powdered version of the same material, or suspending spheres made of a material in one with a lower melting point. – AFSANEH RABIEI / NORTH CAROLINA STATE UNIVERSITY

Don't be fooled by their unsexy name: lightweight composite metal foams are bulletproof, can block radiation and now, new research shows they're great heat insulators.

Researchers from North Carolina State University found steel containing hollow spheres, called steel-steel metal foam, is significantly better at blocking heat than the forged steel used to shield spent nuclear fuel casks.

The work, published in the International Journal of Thermal Sciences, shows how composite metal foam may store and transport nuclear material and explosives, and be used in space exploration.

"This difference in thermal conductivity means that the use of composite metal foams could help avoid circumstances where accidents can lead to explosions," says mechanical engineer and senior author Afsaneh Rabiei.

Like the foam you might make with soap, composite metal foams are packed full of hollow pockets, but unlike soapy bubbles, can be super strong.

Rabiei's lab developed two ways to produce composite metal foams, depending on the materials. The first casts metal with a low melting point, such as aluminium, around hollow spheres made of a material with a high melting point, such as steel. 

The other involves baking metal powder around hollow spheres. Steel-steel metal foam is a product of this technique.

Last year, Rabiei's lab produced a new armour, made of steel-steel metal foam sandwiched between a ceramic plate on the front and a thin sheet of Kevlar or aluminium backing.

When shot by armour-piercing bullets, the metal foam layer absorbed 60-70% of the energy. The bullet penetrated less four millimetres – within the US National Institute of Justice's Standard 0101.06 Ballistic Resistance of Body Armour.

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Lightweight composite metal foams – such as this one pictured here – block X-rays, gamma-rays and neutron radiation, can absorb energy of high impact collisions and insulate heat better than the metal alloys from which they’re made. – AFSANEH RABIEI / NORTH CAROLINA STATE UNIVERSITY

Another paper also published last year, showed "high-Z steel-steel" metal foam – which is made mainly of stainless steel but with a small amount of tungsten – was much better at blocking low-energy gamma rays and neutron radiation as solid steel.

While it wasn't quite as effective as lead at stopping X-ray radiation, it did outperform other materials. It was also as good at blocking high-energy gamma rays as any other.

To see how it fared with heat, Rabiei along with Shuo Chen and Jacob Marx blasted two-centimetre-thick steel-steel metal foam with an 800 ºC flame on one side to see how long it took to heat all the way through. They repeated the experiment with a piece of bulk stainless steel with the same dimensions.

The bulk steel took just four minutes to reach 800 ºC on the non-flame side. But the steel-steel metal foam took eight minutes.

"The presence of air pockets inside [composite metal foams] make it so effective at blocking heat, mainly because heat travels more slowly through air than through metal," Rabiei says.

The composite metal foam expanded at a constant rate as it heated, too. Conventional bulk metals and alloys tend to expand more quickly as temperature increases.

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