Clear silicon nitride could be a cheaper alternative to diamond for ultra-tough windows

One of the hardest materials known – silicon nitride – has been turned transparent for the first time. The see through ceramic could be used for ultra-tough windows able to withstand extreme conditions.

Windows that let users peer into engines and industrial reactors, or protect optical sensors from high pressures or heat are usually made of diamond – an expensive material that becomes unstable at temperatures above 750°C.

Norimasa Nishiyama, from the German Electron Synchrotron DESY, and an international group of researchers from Germany and Japan have now created a disc of transparent silicon nitride 2mm in diameter. The ultra-hard but cheap material can withstand temperatures up to 1400°C.

Silicon nitride is usually opaque, but at 1800°C and 15.6 gigapascals its crystal structure changes from a hexagonal to a cubic arrangement. The cubic structure is transparent as it has fewer crystal defects on which impurities like oxygen, which can make the material opaque, accumulate.

The size of any transparent silicon nitride window might be limited, however. The extreme pressure it takes to transform the ceramic into its transparent form requires specialised instruments, the largest of which could only create windows up to 1cm in diameter . Diamond windows 2.5cm in diameter are already on the market.