Lightweight composite metal foams are effective at blocking harmful radiation, according to a new study carried out by researchers in the US. Indeed, the foams can efficiently block X-rays, gamma rays and neutron radiation, and could also absorb the energy of high-impact collisions. The research may pave the way to metal foams being used in medical imaging, nuclear safety, space exploration and other shielding applications.

With their unusual mechanical and thermal properties, composite metal foams are attracting increasing levels of interest. Afsaneh Rabiei and colleagues at North Carolina State University first began investigating the potential for metal foams to be used in military and transport applications such as for blast protection and as armour, before turning their attention to possible uses in space exploration or nuclear shielding. The team was keen to see if such metal foams could actually block various types of radiation, provide structural support and protect against high-energy impacts.

Foamy tests

Composite metal foams consist of pre-fabricated metallic hollow spheres that are embedded within a metallic matrix. In the latest work, different varieties of the composite foams were made using three main materials – aluminium–steel, steel–steel and tungsten- and vanadium-enriched "high-Z steel–steel" – and in different sphere sizes of 2, 4 and 5.2 mm in diameter.

To test their effectiveness, Rabiei's team measured the attenuation of different types of radiation – specifically, X-rays, gamma rays and neutron radiation. The results of the foam tests were compared with those from bulk samples of an aluminium alloy (A365), lead and steel – three materials that are commonly used in shielding applications. To make the best comparison, the researchers prepared each sample with the same areal density, with the test materials varying in volume but having the same weight.

The foams were seen to perform well, with the most effective shielding material overall being the "high-Z" steel–steel foam. This foam was the best material against neutron radiation and low-energy gamma rays, and was comparable with the alternatives at blocking high-energy gamma rays. For X-ray applications, the high-Z foam was also effective, being bested only by pure lead. The effectiveness of the foam was seen to be largely independent of the size of the sphere used to form the material's cavities.

Environmentally friendly

"This work means there's an opportunity to use composite metal foams to develop safer systems for transporting nuclear waste, more efficient designs for spacecraft and nuclear structures, and new shielding for use in CT scanners," says Rabiei. In addition, she notes, "[The] foams have the advantage of being non-toxic, which means that they are easier to manufacture and recycle."

Russell Goodall, a metallurgist at the University of Sheffield who was not involved in this study, told physicsworld.com that metal foams have "a range of useful properties; unusually, many of these derive from both the material they are made from and the foam structure". Noting that radiation absorption is a property of the foam's material, rather than structure – and therefore that the same degree of protection would be afforded by solid plates of the same material – he adds that it will be important to assess the other advantages foam will provide, such as impact energy absorption or structural support, for potential applications.

"The use of high-tungsten steel as a matrix of these foams is a breakthrough, as it provides excellent radiation shielding via the tungsten, while also offering outstanding strength, ductility and toughness and maintaining low density through the pores," says David Dunand, a metallurgist at Northwestern University in the US, who was also not involved in the current work. "There are no known materials with this combination of properties," he adds.

With its initial study complete, Rabiei's team is looking to modify the composition of its foams to make them more effective than lead at blocking X-rays.

The research is described in the journal Radiation Physics and Chemistry.