Transparent Ceramics Make Super-hard Windows
Silicon nitride is mainly used for ball bearings, cutting tools and engine parts in the automotive and aviation industry. The ceramic is extremely stable and has a hexagonal crystal structure under normal conditions. At pressures above 13 gigapascals, the structure of silicon nitride assumes a cubic symmetry. Researchers at DESY subjected hexagonal silicon nitride to 15.6 gigapascals at 1,800 degrees Celsius, forming a transparent piece of cubic silicon nitride with a diameter of about two millimetres in the process, the first transparent sample of this material. Tokyo Institute of Technology, Ehime University, the University of Bayreuth, Hirosaki University and the Japanese National Institute for Materials Science were also involved in this research.
The transparency of silicon nitride strongly depends on the grain boundaries. Its opaqueness arises from gaps and pores between the grains. Investigations have shown that the high-pressure sample of the material has only very thin grain boundaries. It is crucial for the transparency that oxygen impurities are distributed throughout the material in the high-pressure phase and do not accumulate at the grain boundaries as under normal conditions.
The scientists foresee diverse industrial applications for their super-hard windows since cubic silicon nitride is the third hardest ceramic known, after diamond and cubic boron nitride. However, boron compounds are not transparent, and diamond is only stable up to approximately 750 degrees Celsius in air. Cubic silicon nitride is transparent and stable up to 1,400 degrees Celsius, but the window size is limited for practical reasons to under one centimetre due to the high pressure needed to synthesise it.