Carbon nitrides are forever

BY LEDETTA ASFA-WOSSEN | 17 JANUARY 2024

An international group of scientists has discovered a near-unbreakable material that could be used in a range of applications, from coatings to solar panels and photodetectors.

Researchers at the University of Edinburgh, UK, University of Bayreuth, Germany, and University of Linköping, Sweden, found that when carbon and nitrogen precursors are subjected to extreme heat and pressure, the resulting materials – known as carbon nitrides – are tougher than cubic boron nitride, and almost as tough as diamond (Advanced Materials, doi: 10.1002/adma.202308030).

The team exposed the carbon nitrogen precursors to pressures of between 70 and 135 gigapascals – around one million times atmospheric pressure – while heating them to temperatures of more than 1,500°C.

To identify the atomic arrangement of the compounds under these conditions, the samples were illuminated by an intense X-ray beam at three particle accelerators – the European Synchrotron Research Facility in France, the Deutsches Elektronen-Synchrotron in Germany and the Advanced Photon Source, based in the US.

Their investigations found that three carbon nitride compounds have the necessary building blocks for super-hardness, while further tests revealed even more properties.

‘Even though the materials rival diamond in terms of their predicted hardness values, a similarly important point is their multi-functionality,’ explains Florian Trybel, Assistant Professor at University of Linköping. This feature could be useful in applications where diamond doesn’t offer the desired combination of properties.

‘These carbon nitrides have demonstrated multifunctional properties that diamond cannot have,’ elaborates Dominique Laniel, UKRI Future Leaders Fellow at University of Edinburgh, UK.

‘Besides their very high hardness, these materials exhibit piezoelectricity, tuneable photoluminescence, a direct and wide electrical band gap and high energy density. These properties could have potential for smart machining tools, optoelectronics such as lasers and LEDs, high power devices and environmentally-friendly mining explosives, respectively.

‘They are also expected to have a number of other properties, including superconductivity, high temperature stability and be chemically resistant to iron, though these remain to be confirmed experimentally,’ Laniel says.

The predicted hardness of these materials makes them very appealing for wear-and-temperature resistant coatings, too.

‘Boron carbide is often used for body and tank armour. However, it has a hardness of only slightly above 30 gigapascals. The carbon nitrides, by comparison, have a hardness of between 78 and 87 gigapascals,’ Laniel notes.

The team believes potential coating applications could be wide ranging, from cutting tools to aeronautical, automotives and spacecrafts.

At present, the team is still some way from achieving the sample sizes needed for industrial applications, only producing a few micrograms at a time – and even these small quantities are more expensive than they’d like.

‘The very first of these materials was discovered nearly two years ago, in the middle of the night at a particle accelerator. Since then, we have made significant efforts to reproduce these results and expand on them, both from an experimental perspective but also from a theoretical one,’ says Laniel.

‘Given the small sample sizes, experiments can only go so far in measuring their properties and, as such, theoretical calculations have been essential to better understand the full potential of these solids. As it stands, we are considering different pathways to increase the production scale – the biggest hurdle for commercialisation. This is not a trivial task and could take many years,’ adds Laniel.

Scientists have attempted to uncover the potential of carbon nitrides since the 1980s but after several attempts to synthesise them, no credible results have been reported until now. Both Trybel and Laniel note that the synergy between different institutes, backgrounds and scientific fields has been instrumental to their success.

Upscaling production in a way that is both energy- and cost-efficient is an area the team is keen to explore, potentially with industrial partners. The team is also looking to gain a deeper fundamental understanding of this material class and its properties. Further research and funding will be required.

‘Currently, the pressures and temperatures necessary to synthesise these materials are too high to produce large quantities efficiently. Theoretical simulations will play an important role in predicting new synthesis pathways at less extreme pressure and temperature conditions. As soon as we are able to produce large amounts, we can further investigate different properties and move towards industrially relevant scales,’ adds Trybel.