Durable, Tough & Strong: It’s Spinel!

By Kyra Wiens
U.S. Naval Research Laboratory

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 {spin-ELL} that the U.S. Naval Research Laboratory (NRL) has been researching the last 10 years.

“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.”

The U.S. Naval Research Laboratory invented a new way of making transparent spinel. "We talk to the warfare centers, the systems people—so that what you’re doing really is going to be of value," says lead researcher Dr. Jas Sanghera. In this photo, from left to right: Michael Hunt, Dr. Woohong (Rick) Kim, Dr. Jas Sanghera, Robert Miklos, Dr. Guillermo Villalobos, Dr. Colin Baker, Bryan Sadowski, and Dr. Shyam Bayya. (Photo: Jamie Hartman/U.S. Naval Research Laboratory/Released)

The U.S. Naval Research Laboratory invented a new way of making transparent spinel. “We talk to the warfare centers, the systems people—so that what you’re doing really is going to be of value,” says lead researcher Dr. Jas Sanghera. In this photo, from left to right: Michael Hunt, Dr. Woohong (Rick) Kim, Dr. Jas Sanghera, Robert Miklos, Dr. Guillermo Villalobos, Dr. Colin Baker, Bryan Sadowski, and Dr. Shyam Bayya. (Photo: Jamie Hartman/U.S. Naval Research Laboratory/Released)

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 licensed the technology to a company to scale that up to much larger plates.

The U.S. Naval Research Laboratory made ceramic windows, like this one (with an infrared anti-reflective coating), up to eight inches in diameter, before transitioning the technology to industry. The military 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," says Sanghera. In addition, for imaging systems, engineers "can use spinel as the window because it allows the infrared light to come through." (Photo: Jamie Hartman/U.S. Naval Research Laboratory/Released)

The U.S. Naval Research Laboratory made ceramic windows, like this one (with an infrared anti-reflective coating), up to eight inches in diameter, before transitioning the technology to industry. The military 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,” says Sanghera. In addition, for imaging systems, engineers “can use spinel as the window because it allows the infrared light to come through.” (Photo: Jamie Hartman/U.S. Naval Research Laboratory/Released)

The sintering method 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, harder, spinel has unique optical properties. You can see through it and it allows infrared light to go through it. That’s good news for the military. It means imaging systems can use spinel as the window because it allows that infrared light to pass through.

NRL is also looking at spinel for the windows on lasers operating in maritime and other hostile environments.

What is spinel?
A quick online search defines spinel as “a hard glassy mineral occurring as octahedral crystals of variable color and consisting chiefly of magnesium and aluminum oxides.” It can be mined as a gemstone. A famous example is the Black Prince’s Ruby, which is actually spinel with a color dopant.

So, how is the NRL using it? NRL chemists have synthesized their own ultra-high purity spinel powder, and other synthetic versions are commercially available.

U.S. Naval Research Laboratory (NRL) chemists have synthesized ultra-high purity spinel powder. Other synthetic versions are commercially available: "It's available in reasonably low cost," says Dr. Jas Sanghera, who leads the research. NRL sinters spinel powder to make transparent sheets and other shapes, for unique military and commercial applications, including next generation lasers. (Photo: Jamie Hartman/U.S. Naval Research Laboratory/Released)

U.S. Naval Research Laboratory (NRL) chemists have synthesized ultra-high purity spinel powder. Other synthetic versions are commercially available: “It’s available in reasonably low cost,” says Dr. Jas Sanghera, who leads the research. NRL sinters spinel powder to make transparent sheets and other shapes, for unique military and commercial applications, including next generation lasers. (Photo: Jamie Hartman/U.S. Naval Research Laboratory/Released)

The spinel NRL makes is a polycrystalline material, or a lot of crystal particles all pressed together. Whereas with glass, if a crack forms on the surface, it can go all of the way through. Spinel might chip but it won’t crack.

Transferable & Scalable Process
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 2000 degrees Celsius,” says 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.”

That’s why 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,” says 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.”

Understanding the problems with how scientists had been trying to make spinel, starting in the 1960s, led the U.S. Naval Research Laboratory to a unique solution for uniformly distributing the sintering aid. Added to the spinel powder, before it goes in the hot press, is "about one percent of a different powder, in this case lithium fluoride," says optical scientist Dr. Jas Sanghera. With a novel method for mixing the "pixie dust" and spinel powder, "there's only one pathway for densification," and the finished product comes out of the press completely clear. (Photo: Jamie Hartman/U.S. Naval Research Laboratory/Released)

Understanding the problems with how scientists had been trying to make spinel, starting in the 1960s, led the U.S. Naval Research Laboratory to a unique solution for uniformly distributing the sintering aid. Added to the spinel powder, before it goes in the hot press, is “about one percent of a different powder, in this case lithium fluoride,” says optical scientist Dr. Jas Sanghera. With a novel method for mixing the “pixie dust” and spinel powder, “there’s only one pathway for densification,” and the finished product comes out of the press completely clear. (Photo: Jamie Hartman/U.S. Naval Research Laboratory/Released)

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. This “pixie dust” is meant to melt and lubricate the powder particles, so there’s less friction. The scientists were putting the powders together in shakers overnight, but, the powders 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.

Unique Applications for Use
Spinel can be used in watches and consumer electronics, like smartphones. For the military, in particular, spinel can be used as transparent armor for vehicles and face shields. Spinel may also be used to better protect infrared cameras on planes.

NRL is also looking at spinel (and other materials) for next generation (NEXTGEN) lasers. Sanghera acknowledges, “In DoD, we are the premier place for development of fiber lasers. It’s something we are heavily involved with, all the different types of fibers and configurations and materials required to enable these eye-safer and NEXTGEN lasers.”

Story and information provided by the U.S. Naval Research Laboratory
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