Requirements for durable, ultra-hard windows might be fulfilled by a cubic silicon nitride ceramic fabricated by researchers from Japan and Germany. The polycrystalline material forms under high pressure and is the second hardest transparent nanoceramic after diamond but can withstand substantially higher temperatures.
Silicon nitride is a stable ceramic widely used for components in the automotive and aviation sectors. At ambient pressures, it has a hexagonal crystal structure and the sintered ceramic of this phase is opaque.
Pressures above 130 thousand times atmospheric pressure transform silicon nitride into a crystal structure with cubic symmetry known as spinel-type in reference to the structure of a popular gemstone. The researchers used a large volume press (LVP) to expose hexagonal silicon nitride to high pressures and temperatures, and a 2mm-diameter piece of transparent cubic silicon nitride formed at about 156 thousand times atmospheric pressure (15.6 gigapascals) and 1800 degrees C.
Scanning transmission electron microscopy showed that the high-pressure sample has only very thin grain boundaries. High-pressure phase oxygen impurities are distributed throughout the material, a property essential for transparency, say the researchers.
Diamond and cubic boron nitride are the hardest ceramics known, but boron compounds are not transparent, and diamond is only stable up to approximately 750 degrees C in air. The new ly produced cubic silicon nitride is transparent and stable up to 1400 degrees C.
While the scientists foresee diverse industrial applications for their super-hard windows, possible window size is limited due to the large pressure needed to synthesize transparent cubic silicon nitride. Producing macroscopic transparent samples requires about twice the pressure as for artificial diamonds, so fabricating anything over 1 cm in size will be extremely difficult.
Deutsches Elektronen-Synchrotron (DESY),Tokyo Institute of Technology, Ehime University, the University of Bayreuth, Japanese National Institute for Materials Science, and Hirosaki University were involved in this research.