Materials

Metal foams could provide lightweight radiation shielding

A sample of the composite metal foam developed by Rabiei's research team
A sample of the composite metal foam developed by Rabiei's research team
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A sample of the composite metal foam developed by Rabiei's research team
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A sample of the composite metal foam developed by Rabiei's research team

Radiation generally comes under the heading of "things you want to stay away from," so it's no surprise that radiation shielding is a high priority in many industries. However, current shielding is bulky and heavy, so a North Carolina State University team is developing a new lightweight shielding based on foam metals that can block X-rays, gamma rays, and neutron radiation, as well as withstanding high-energy impact collisions.

Though they aren't very familiar to the public, foam metals have been around for over a century. In its simplest form, a foam is made by bubbling a gas through molten metal to form a light froth that cools into a lightweight matrix. This produces a foam that is lighter than conventional metals, but has comparable strength.

Foams can also be made by milling or 3D printing, but whatever the method, they are expensive and difficult to manufacture, so their uses are restricted to very specialized applications, such as spacecraft or advanced cooling systems.

The new foam metal being developed by the NC State team led by Afsaneh Rabiei, a professor of mechanical and aerospace engineering, was originally created as a strong, lightweight material for military and transportation applications, but Rabiei became curious about its potential in radiation shielding. The result was a high-Z steel-steel foam, which is a composite made of stainless steel with small amount of tungsten formed into hollow spheres and introduced into the steel matrix to make a foam that was less dense than stainless steel.

According to the team, the foam metal was subjected to multiple tests, which showed that it was effective in blocking X-rays, various forms of higher and lower energy gamma rays, and neutron radiation.

Compared against bulk materials, it demonstrated the same shielding properties for high-energy gamma rays, though its density was lower. In addition, it has better blocking qualities for low-energy gamma rays and neutron radiation. Although it was better than most materials at blocking X-rays, it wasn't quite up to the standard of lead.

"[We] are working to modify the composition of the metal foam to be even more effective than lead at blocking X-rays – and our early results are promising," Rabiei says. "And our foams have the advantage of being non-toxic, which means that they are easier to manufacture and recycle. In addition, the extraordinary mechanical and thermal properties of composite metal foams, and their energy absorption capabilities, make the material a good candidate for various nuclear structural applications."

The results of the study were published in Radiation Physics and Chemistry.

Source: NC State University

8 comments
Bob
I'm curious how lead foam would compare to solid lead?
Nairda
"Compared against bulk materials, it demonstrated the same shielding properties for high-energy gamma rays, though its density was lower." Against what thickness of concrete are we comparing here? 100:1, 10:1, 2:1 ?
Siegfried Gust
Maybe I'm missing something here, but why would the physical shape of the material have any effect on it's blocking of ionizing radiation? Seems that passing through 1mm of a given material would produce the same shielding effects no matter if they were interspersed with gas pockets or not. Are the bubbles filled with some sort of gas that absorbs more radiation? But it shouldn't surprise anyone that the mechanical and thermal properties are better than those of a solid block or sheet. I guessing that this is news because it's a novel method for producing a metal foam. I am curious as to how they produce the hollow spheres in large quantities.
Tyler Totten
Couple of things. First, it may be non-toxic, but after long-term radiation exposure it is radioactive, so it'll still face issues with recycling if used in any high radiation environments. Second, I wonder how it does compared with traditional bulk shielding with long-term neutron exposure. That is a major limiter in plants, particularly with regards to any future for fusion reactors. Neutron radiation really degrades materials.
Bob Flint
Seems that they are using the hollow tungsten spheres as a reflective/deflective technique to reduce the radiation, the lower density contributes to lighter weight, and increased structure also helps slow down impacts of high energy orbiting debris in a space environment. Stainless steel foam invariably stronger than pure lead which has a much higher density, therefore I suspect that the lead foam would reduce radiation, but the trade -off is the high speed particles would make it through.
Kevin Ritchey
I had, as I'm sure many others, this in the back of my head for many years but never understood why it has taken so long to be developed or implemented. Much like how the inclusion of fiberous metal wires increases the durability of concrete structures and how holes punched out of sheet metal increases rigidity, this should aid in areas where solid metals would prove inadequate or restrictive. Certainly worthy of further testing and study. Very glad someone was in a position to make such research possible. Just wish it was me doing it.
DomainRider
I wonder if this kind of material might be useful for long-duration manned space journeys outside Earth's magnetosphere, such as a Mars mission. One of the major problems is high-energy particle radiation, and the search for a low mass shielding solution is ongoing...
ZotricZaoh
Tyler. We might use simplifying assumptions to think about this problem. Consider the shielding effect of any medium against a photon of radiation to depend on only two things. One is the probabiity of a photon hitting an atomic nucleus during its journey through the material. The second is the length of path taken through the material. Considering the photon initially as a classical particle. 1. Consider the path through a homogenoous medium (no spheres) in which there are no interactions other than with nuclei and assuming a single interaction stops the particle dead with no side interactions. In this case the attenuation would depend entirely on the type of atoms and the thicness of the material. Large nuclei would provide, all else being equal, better shielding. 2. Now supposing impurities such as hollow metal sheres are present. One can imagine that at the inteface between the spheres and the bulk medium there could be diffraction effects that might incease the path length by scattering the photon. Now we might have to see the photon as a wave but this is a detail to be put into an actual analysis. If the additional interactions that are due to the increased path length equal or outweigh the reduction due to the voids within the spheres we will see lower density material providing shielding at least as good as a solid shield made of the bulk material. I assume that in 1. and 2. any diffraction effects of in the bulk medium are he same and other simplifying assumptions but I think the above is the wa to think about this kind of problem. In the experiment the materials were kept at constant mass in order, presumably, to cancel out gross density effects.
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