Making Super-Hard Windows from Nanoceramics

A sample of transparent polycrystalline cubic silicon nitride, synthesised at DESY. (Image courtesy of Norimasa Nishiyama/DESY/Tokyo Tech.)

Researchers have synthesized the first transparent sample of a popular industrial ceramic. The result is a super-hard window made of cubic silicon nitride that can potentially be used under extreme conditions like in engines, as the Japanese-German team writes in the journal Scientific Reports.

Cubic silicon nitride (c-Si3N4) forms under high pressure and is the second hardest transparent nanoceramic after diamond but can withstand substantially higher temperatures.

"Silicon nitride is a very popular ceramic in industry," explained lead author Dr. Norimasa Nishiyama from Deutsches Elektronen-Synchrotron (DESY) who now is an associate professor at Tokyo Institute of Technology. "It is mainly used for ball bearings, cutting tools and engine parts in automotive and aircraft industry."

The ceramic is extremely stable because the silicon nitrogen bond is very strong. At ambient pressures, silicon nitride has a hexagonal crystal structure, though the sintered ceramic of this phase is opaque.

A bright-field transmission electron microscope image of cubic silicon nitride. The average grain size is about 150 nm. (Image courtesy of Norimasa Nishiyama/DESY/Tokyo Tech.)

At pressures above 130 thousand times atmospheric pressure, silicon nitride transforms into a crystal structure with cubic symmetry that experts call spinel-type, in reference to the structure of a popular gemstone.

Artificial spinel (MgAl2O4) is widely used as transparent ceramic in industry.

"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," said Nishiyama. His team used a large volume press (LVP) at DESY to expose hexagonal silicon nitride to high pressures and temperatures.

At approximately 156 thousand times the atmospheric pressure (15.6 gigapascals) and a temperature of 1800° C, they were able to form a transparent piece of cubic silicon nitride with a diameter of about two millimeters. "It is the first transparent sample of this material," emphasized Nishiyama.

Analysis of the crystal structure at DESY's X-ray light source PETRA III showed that the silicon nitride had completely transformed into the cubic phase. "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," explained Nishiyama. "However, the transparency of silicon nitride strongly depends on the grain boundaries. The opaqueness arises from gaps and pores between the grains."

Investigations with a scanning transmission electron microscope at the University of Tokyo showed that the high-pressure sample has only very thin grain boundaries.

An atomic-resolution scanning transmission electron microscope image of a triple junction of grains in cubic silicon nitride. The thickness of the grain boundaries is less than one nanometer. (Image courtesy of Norimasa Nishiyama/DESY/Tokyo Tech.)

"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," said Nishiyama.

According to Nishiyama, cubic silicon nitride is the hardest and toughest transparent spinel ceramic ever made. The scientists foresee diverse industrial applications for their super-hard windows. "Cubic silicon nitride is the third hardest ceramic known, after diamond and cubic boron nitride," said Nishiyama. "But boron compounds are not transparent, and diamond is only stable up to approximately 750° Celsius in air. Cubic silicon nitride is transparent and stable up to 1400° Celsius."

However, because of the large pressure needed to synthesize transparent cubic silicon nitride, the possible window size is limited for practical reasons. "The raw material is cheap, but to produce macroscopic transparent samples we need approximately twice the pressure as for artificial diamonds," said Nishiyama. "It is relatively easy to make windows with diameters of one to five millimeters. But it will be hard to reach anything over one centimeter."

For more materials news, find out why silicon nanosheets offer an alternative to graphene.

Source: DESY