Imagine a glass window that’s tough like armor, a camera lens that doesn’t get scratched in a sand storm, or a smartphone that doesn’t break when dropped. Except it’s not glass; it’s a special ceramic called spinel {spin- ELL} that the Naval Research Laboratory (NRL) has been researching over the last 10 years.

NRL presses spinel powder into transparent domes, sheets, and other shapes. (Photo: Naval Research Lab/Jamie Hartman)

“Spinel is actually a mineral; it’s magnesium aluminate,” said Dr. 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—UAVs (unmanned autonomous vehicles), head-mounted face shields—it’s a game-changing technology.”

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,” said Sanghera, “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 said 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 may be able to use spinel as the “window” for imaging systems.

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,” said Sanghera.

So what, exactly, is spinel? Spinel can be mined as a gemstone; a famous example is the Black Prince’s Ruby, which is actually spinel with a color 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 in reasonably low cost,” said Sanghera. The spinel NRL makes is a polycrystalline material, or a lot of crystal particles all pressed together. Unlike glass, where “a crack that forms on the surface will go all the way through,” spinel might chip but it won’t crack.

When scientists first started trying to make glass-like spinel, they used 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 2000 degrees Celsius,” said Sanghera. It’s expensive to heat a material that high, and additionally, “the molten material reacts with the crucible, and so if you’re trying to make very high quality crystals, you end up [with a] huge amount of defects.”

So Sanghera and his colleagues turned to sintering. “You put the powder in [a hot press], you press it under vacuum, squash this powder together—and 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, put the powder in there, I end up with a dome shape,” said Sanghera, “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. Then they discovered a second problem, with the sintering aid they were adding to the spinel powder. “It’s about one percent of a different powder, in this case lithium fluoride,” said Sanghera. 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,” said Sanghera. 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 said, “make that mirror finished, then — and so that’s where we get into a little bit of IP [intellectual property], is what’s the best way to do that?”

The military in particular may want to use spinel as transparent armor 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 said. The military is 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 color distortions. 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 stated.

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