A combined team of researchers from Nanyang Technological University (NTU Singapore) and Rice University in the US have unearthed the secret that lies behind one of the world’s toughest materials. Published in the top scientific journal Nature this June, the paper uncovers the strength of hexagonal boron nitride (h-BN). After one thousand hours of lab experiments and taking advantage of computer simulations, the scientists traced the vastly different fracture toughness of graphene and h-BN to their chemical compositions. h-BN can resist ten times the amount of force that graphene can, one of the toughest materials known to man. Its unique properties could, the team claim, lead to improvements in the designing of novel flexible materials for the electronics industry.
First used in cosmetics in the 1940s before being abandoned because of its high price, h-BN made a comeback in the latter half of the 1990s after technological advances made its manufacture more cost-effective. A two-dimensional (2D) material, h-BN has a thickness of just one atom.
These days almost all leading producers of cosmetic products use. h-BN has the capability to absorb excess facial sebum and disperse pigment in an even manner. Additionally, as it insulates against electricity and can sustain temperatures of up to 1000 °C, it is used as a protective layer in 2D electronics.
On inspection, the scientists reported the h-BN that had been exposed to stress with breakages in the material branched “like forks in a road”. This is opposed to travelling straight through the material (see image above), signalling that fractures in h-BN are far less likely to grow when more stress is placed on it.
“Our experiments show that h-BN is the toughest nanomaterial measured to date. What makes this work so exciting is that it unveils an intrinsic toughening mechanism in this material — which should be brittle as it is only one atom thick. This is unexpected as there is often a trade-off between the strength and brittleness of nanomaterials,” Professor Gao Huajian, a Distinguished University Professor in NTU’s School of Mechanical and Aerospace Engineering, who led the study, said on the importance of the team’s research.
This is just another of Gao Huajian’s achievements in the field of applied mechanics. Not long ago he was awarded the prestigious 2021 Timoshenko Medal by the American Society of Mechanical Engineers (ASME). This was in recognition of his groundbreaking work in nanomechanics of engineering and biological systems, a new research field at the interface of solid mechanics, materials science and biophysics.
Professor Jun Lou, from Rice University’s Department of Materials Science and NanoEngineering, who also led the study, said:
“In the real world, no material is free from defects, which is why understanding fracture toughness — or resistance to crack growth — is so important in engineering. It describes how much punishment a real-world material can withstand before failing.”
Similar to a honeycomb, h-BN and graphene are both arranged in interconnecting hexagons, though the hexagons in graphene are composed of only carbon atoms. The hexagon structure in h-BN, meanwhile, comprises three nitrogen and three boron atoms. The contrast in the composition is why a moving crack in h-BN branches off its trajectory path. This forking is significant, as it reveals more energy is required for a crack to be driven further into it. Conversely, graphene breaks more easily, as fractures move straight through the material “like a zipper”.
h-BN’s astounding toughness could give rise to it being used for tear-resistant flexible electronics, such as is found in wearable medical devices and foldable smartphones, say the researchers, while also added to strengthen electronics made from two-dimensional (2D) materials, which tend to be brittle. Its heat resistance and chemical stability, too, would allow h-BN’s to be used as a supporting base and an insulating layer between electronic components. This would make it stand out from other traditional materials used so far employed in electronics.
“Our findings also point to a new route to produce tough materials by adding structural asymmetry into their designs. This would reduce the likelihood of materials fracturing under extreme stress, which may cause the devices to fail and lead to catastrophic effects,” said Gao Huajian’s on the efficacy of the material for future applications.
While Lou Jun added:
“The niche area for 2D material-based electronics like h-BN is in flexible electronic devices. In addition to applications like electronic textiles, 2D electronic devices are thin enough for more exotic applications like electronic tattoos and implants that could be attached directly to the brain.”