[Image above] A sample of transparent polycrystalline cubic silicon nitride. Credit: Norimasa Nishiyama; DESY/Tokyo Tech

It must have been clear to the Deutsches Elektronen-Synchrotron researchers who pulled their sample out of the furnace that what they had created was big news.

The originally opaque sample of silicon nitride—a super hard and industrially useful ceramic used for ball bearings, cutting tools, and high-performance engine components—emerged optically clear, flirting its potential as an incredibly strong window material for extreme conditions.

Transparent ceramics tout durability and high mechanical, thermal, and chemical stability, making them useful in a variety of applications where other materials often fail, including transparent armor windows and night vision devices.

And while researchers have gotten glimpses of the possibilities of transparent cubic silicon nitride in the past, it’s remained difficult to synthesize samples of the material. “The cubic phase of silicon nitride was first synthesized by a research group at Technical University of Darmstadt in 1999, but knowledge of this material is very limited,” explains lead author Norimasa Nishiyama in a DESY press release.

Nishiyama and a team of scientists have done it now, however—by processing samples of silicon nitride under high pressure and heat, the researchers converted the originally hexagonal crystal structure into optically transparent cubic silicon nitride (c-Si3N4).

Bright-field TEM image of cubic silicon nitride, with average grain size of ~150 nm. Credit: Norimasa Nishiyama;, DESY/Tokyo Tech

But it didn’t come easily—the DESY researchers used pressures of 15.6 GPa, or ~156,000 times atmospheric pressure, and a temperature of 1,800ºC to catalyze the phase transformation of opaque hexagonal to clear cubic silicon nitride.

The emergent material happens to be one the hardest nanoceramics out there, only topped by diamond.

“The transformation is similar to carbon that also has a hexagonal crystal structure at ambient conditions and transforms into a transparent cubic phase called diamond at high pressures,” Nishiyama says in the release. “However, the transparency of silicon nitride strongly depends on the grain boundaries. The opaqueness arises from gaps and pores between the grains.”

While the transformation of a material from opaque to clear may seem like materials magic, it’s simple science. The area between the crystals in a polycrystalline material, called grain boundaries, can disrupt visible light waves, making an object appear opaque.

However, the high pressure and heat treatment by DESY researchers caused structural changes in the silicon nitride, rearranging its crystals.

Those adjustments minimized grain boundaries, making them smaller than the wavelength of light—allowing visible light waves to pass through with little disruption, and making the material appear clear.

“Also, in the high-pressure phase oxygen impurities are distributed throughout the material and do not accumulate at the grain boundaries like in the low-pressure phase. That’s crucial for the transparency,” Nishiyama adds in the release.

Atomic-resolution STEM image of a triple junction of grains in cubic silicon nitride. Thickness of the grain boundaries is <1 nm. Credit: Norimasa Nishiyama; DESY/Tokyo Tech

And because silicon nitride is more thermally stable than other hard, transparent materials, it could be an ideal material for armor windows, for example to protect optical sensors and detectors in extreme environments.

“Cubic silicon nitride is the third hardest ceramic known, after diamond and cubic boron nitride,” Nishiyama explains in the release. “But boron compounds are not transparent, and diamond is only stable up to approximately 750ºC in air. Cubic silicon nitride is transparent and stable up to 1,400ºC.”

Although not everyone may agree—researchers created the first inklings of transparent cubic boron nitride a few years ago—cubic silicon nitride is surely something special.

In any case, there’s only one small problem—because of the intense pressure and temperature required to fabricate transparent silicon nitride, the researchers can make only small samples of the material. The sample synthesized so far is just 2 mm, and the researchers speculate that only sizes of around 10 mm are feasible with current equipment.

The open-access paper, published in Scientific Reports, is “Transparent polycrystalline cubic silicon nitride” (DOI: 10.1038/srep44755).