The ceramic is extremely stable, because the silicon nitrogen bond is very strong. At ambient pressures, silicon nitride has a hexagonal crystal structure and sintered ceramic of this phase is opaque.
At pressures above 130,000 times atmospheric pressure, silicon nitride transforms into a crystal structure with cubic symmetry that experts call spinel-type. Artificial spinel (MgAl2O4) is widely used as transparent ceramic in industry.
The team, led by Dr Norimasa Nishiyama from DESY who now is an associate professor at Tokyo Institute of Technology, used a large volume press (LVP) to expose hexagonal silicon nitride to high pressures and temperatures. At approximately 156,000 times atmospheric pressure and a temperature of 1800°C a transparent piece of cubic silicon nitride formed with a diameter of about 2mm.
Analysis of the crystal structure 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 Dr 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. “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 Dr Nishiyama.
“Cubic silicon nitride is the third hardest ceramic known, after diamond and cubic boron nitride,” explained Dr Nishiyama. “But boron compounds are not transparent, and diamond is only stable up to approximately 750°C in air. Cubic silicon nitride is transparent and stable up to 1400°C.”
The scientists foresee diverse industrial applications for their super-hard windows. However, because of the large pressure needed to synthesise 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 Dr Nishiyama. “It is relatively easy to make windows with diameters of 1 to 5mm. But it will be hard to reach anything over 1cm.”