Transparent metal

New processes may lead to metal windows.
U.S. Naval Research Laboratory
October 16, 2015
By U.S. Naval Research Laboratory
The U.S. Naval Research Laboratory uses a hot press to make spinel into conformable optics, like this flat sheet. “Ultimately, we’re going to hand it over to industry,” says Jas Sanghera, who leads the research, “so it has to be a scalable process.” In the lab, they made pieces eight inches in diameter. “Then we licensed the technology to a company who was able then to scale that up to much larger plates, about 30-inches wide.”Imagine a glass window that’s tough like armor, a camera lens that doesn’t get scratched in a sand storm or a smart phone that doesn’t break when dropped. Except it’s not glass, it’s a special ceramic called spinel that the U.S. Naval Research Laboratory (NRL) has been researching over the last 10 years. “Spinel is actually a mineral, it’s magnesium aluminate,” says Jas Sanghera, who leads the research. “The advantage is it’s so much tougher, stronger, harder than glass. It provides better protection in more hostile environments so it can withstand sand and rain erosion.” As a more durable material, a thinner layer of spinel can give better performance than glass. “For weight-sensitive platforms – unmanned autonomous vehicles, head-mounted face shields – it’s a game-changing technology,” he says.NRL invented a new way of making transparent spinel using a hot press, called sintering. It’s a low-temperature process, and the size of the pieces is limited only by the size of the press. “Ultimately, we’re going to hand it over to industry,” Sanghera says, “so it has to be a scalable process.” In the lab, they made pieces eight inches in diameter. “Then we licensed the technology to a company who was able then to scale that up to much larger plates, about 30-inches wide.”The sintering method also allows NRL to make optics in a number of shapes, “conformal with the surface of an airplane or UAV wing,” depending on the shape of the press.In addition to being tougher, stronger and harder, Sanghera says spinel has “unique optical properties. Not only can you see through it, but it allows infrared light to go through it.” That means the military, for imaging systems, “can use spinel as the window because it allows the infrared light to come through.”NRL is also looking at spinel for the windows on lasers operating in maritime and other hostile environments. “I’ve got to worry about wave slap and saltwater and things like that, and gun blasts going off. It’s got to be resistant to all that. And so that’s where spinel comes into its own,” Sanghera says. Says Sanghera, “Everything we do, we’re trying to push the mission. It’s designed to either enable a new application, a new capability, or enhance an existing one.”Spinel can be mined as a gemstone. A famous example is the Black Prince’s Ruby, which is actually spinel with a colour dopant. NRL chemists have also synthesized their own ultra-high purity spinel powder, and other synthetic versions are commercially available. “The precursors are all earth-abundant, so it’s available for reasonably low cost,” Sanghera says. The spinel NRL makes is a polycrystalline material, which means it is made of a lot of crystal particles all pressed together. With glass, “a crack that forms on the surface will go all the way through,” Sanghera explains. Spinel might chip but it won’t crack. “It’s like navigating through the asteroid belt, you create a tortuous path. If I have all these crystals packed together, the crack gets deflected at the hard crystals and you dissipate the crack energy.When scientists first started trying to make glass-like spinel, they were using a crucible instead of a press. “A big problem with growing crystals is that you have to melt the starting powder at very high temperatures: over 2,000 C,” Sanghera says. It’s expensive to heat a material that high, and “the molten material reacts with the crucible, so if you’re trying to make very high-quality crystals, you end up  with a huge amount of defects.” That’s why Sanghera and his colleagues turned to sintering. “You put the powder in a hot press then you press it under vacuum to squash the powder together. If you can do that right, then you can get rid of all the entrapped air and all of a sudden it comes out of there clear-looking.” If the press has flat plates, the spinel will come out flat. “But if I have a ball and socket joint and put the powder in there, I end up with a dome shape,” Sanghera says. “So we can make near-net-shape product that way.”NRL was not the first to try sintering. But previous attempts had yielded “a window where most of it would look cloudy and there would be an odd region here and there – about an inch or so – that was clear, and that would be core-drilled out.”So NRL deconstructed the science. They started with purer chemicals. “Lousy chemicals in, lousy material out,” Sanghera says. Then they discovered a second problem, this time with the sintering aid they were adding to the spinel powder. “It’s about one per cent of a different powder, in this case lithium fluoride,” Sanghera says. This “pixie dust” is meant to melt and “lubricate the powder particles, so there’s less friction, so they can all move together during sintering.” They were putting the powders together in shakers overnight, but “the thing is, on a scale of the powder, it’s never mixed uniformly.” Understanding the problem led to a unique solution for enabling uniform mixing. Now, “there’s only one pathway for densification,” and the spinel will come out clear across the press.To further increase the quality of the optic, “you can grind and polish this just like you would do gems,” Sanghera says. This is the most costly part of the process. “One of the things we’re looking at is, how do we reduce the finishing cost?” The surface of the press is imprinted onto the glass. “If we can improve upon that,” he says, “make that mirror finish, then – and so that’s where we get into a little bit of intellectual property – what’s the best way to do that?”For both the Department of Defense and private industry, “cost is a big driver, and so it’s important for us to make products? that can be affordable.”Unique applications for military and commercial use“There are a lot of applications,” Sanghera says. He mentions watches and consumer electronics, like the smart phone, as examples. The military in particular may want to use spinel as transparent armour for vehicles and face shields. A “bullet-proof” window today, for example, has layers of plastic and glass perhaps five inches thick. “If you replaced that with spinel, you’d reduce the weight by a factor of two or more,” Sanghera says.The military’s also interested in using spinel to better protect visible and infrared cameras on planes and other platforms. Glass doesn’t transmit infrared, so today’s optics are made of “exotic materials that are very soft and fragile,” and have multiple layers to compensate for colour distortions. “So that’s what we’ve been doing now, developing new optical materials,” Sanghera says. Spinel windows could also protect sensors on space satellites, an area Sanghera’s interested in testing.  “You could leave these out there for longer periods of time, go into environments that are harsher than what they’re encountering now, and enable more capabilities,” he says.NRL is also looking at spinel (and other materials) for next-generation lasers. “Lasers can be thought of as a box comprised of optics,” he says. “There’s passive and there’s active components. Passive is just a protective window, active is where we change the colour of light coming out the other end.” For passive laser applications, like exit apertures (windows), the key is high quality. “That window, if it’s got any impurities or junk, it can absorb that laser light,” Sanghera says. “When it absorbs, things heat up,” which can cause the window to break. Sanghera and his colleagues have demonstrated, working with “ultra high purity” spinel powder they’ve synthesized in NRL clean rooms, spinel’s incredible potential. For active laser applications, they’ve demonstrated how sintering can be used with materials other than spinel to make a laser that’s “excellent optical quality.” Instead of spinel, they use, “things like yttria or lutecia and dope them with rare earth ions.” NRL has transitioned both types of laser materials and applications to industry. Editor’s commentAlternative transparent materials in appliactions that require high impact resistance would be a welcome innovation in architectural glazing. Many designers have been looking for solutions to problems with traditional safety glass products. One example that springs to mind is balcony glass. There have been several instances in big Canadian cities of tempered balustrades in high-rise condominiums shattering and showering tempered glass “pebbles” into the street below, much to the concern of passers by and unit owners. The breakages were blamed on the expansion and contraction of nickel sulphide inclusions in the glass which are introduced as a normal byproduct of the primary float glass process. Heat soaking can ensure a lower percentage of inclusions in the glass that survives the process, but adds waste and cost without completely eliminating the problem. New standards for balcony guard construction and the use of laminated glass will probably ameliorate the issue going forward, but at the cost of some design restrictions. Sintered spinel panels would presumably be much stronger, offering increased protection to residents and pedestrians. Another area where safety glass has become not-so-safe is wired glass. Primarily used for fire resistance, wired glass has come under scrutiny lately following instances of people impacting the windows and cutting themselves on the metal wire interlayer. If spinel can take the heat of a laser without shattering, a regular fire should cause no problem. Of course, the cost of producing architectural-size sintered spinel panels is prohibitive right now. Perhaps some form of additive manufacturing process would offer a solution, as some kinds of 3D printers operate in a manner very similar to sintering. Again, technology that is some years off. So were smartphones in 1995. Sometimes it pays to keep an eye on what is coming next.  Reducing costsThe U.S. Naval Research Laboratory uses a hot press to make spinel, a process called sintering. It’s much less expensive than melting, and the size of the pieces is limited only by the size of the press. Lead researcher Jas Sanghera says, “You put the powder in a hot press then you press it under vacuum to squash the powder together. If you can do that right, then you can get rid of all the entrapped air and all of a sudden it comes out of there clear-looking.”  To further increase the quality of the optic, “You can grind and polish this just like you would do gems.”

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