Hexagonalboron nitride has a structural similarity to graphene. It is composed of a planar network of atoms interconnected in hexagons. The only difference between graphene and H-BN is that all atoms in graphene are carbon. In H-BN, every hexagon contains three boron and three nitrogen atoms.
Graphene has a stronger carbon-carbon bond than H-BN. The strengths and elastic modulus for the two materials are very similar. H-BN is slightly less than graphene, which has a strength of 130GPa and young’s modulus around 1.0TPa. HBN has a strength and modulus respectively of 100GPa (and 0.8 TPA).
Graphene is not only strong in mechanical properties but it also has low crack resistance which makes graphene brittle.
British engineer Griffiths published in 1921 a theoretical study on fracture mechanics. This included a description of the failures of brittle materials as well as the relationship between the size cracks and the force necessary to make them grow. Engineers and scientists have used this theory for hundreds of decades to predict and determine the toughness of materials.
A study conducted by Jun Lou at Rice University in 2014 showed that graphene has a high degree of fracture toughness. It is consistent to Griffith’s theory about fracture mechanics. Graphene cracks will propagate when the stress applied is greater than the force keeping it together.
Due to its structural similarity with graphene H-bn could also be vulnerable. But this is not true.
H-BN was found to be 10 times more ductile that graphene, according to scientists.
Professor Jun Lou, Nanyang Technological University Singapore and Prof. Hua Jian gao, of Rice University found that HBN was 10 times stronger than graphene for cracking resistance. This discovery is in direct contradiction to Griffith’s fracture theory. Such anomalies have never before been seen in two-dimensional materials. The Nature article entitled “Intrinsic Toughening in Hexagonal Boron Nitride” published the related research results.
Mechanism of H-BN’s Extraordinary Strength
To discover why, the team applied stress on the HBN sample using scanning electron microscopes, transmission electron microscopes, and other tools. The mystery was solved after over 1,000 hours of experiments, theoretical analysis and further research.
H-Bn graphene and graphene are structurally identical, but the boron atoms and nitrogen atoms differ. HBN also has an asymmetric arrangement in hexagonal lattice. This is in contrast to graphene’s carbon hexagon. Graphene’s cracks tend to penetrate the symmetrical hexagonal structure, opening the bond like an open zipper. H-BN has a hexagonal structure that is slightly asymmetric, due to the stress contrast of boron with nitrogen. Because of this, cracks can bifurcate and form branches.
The crack that splits means it’s turning. To make the crack harder to propagate, this steering crack needs additional energy. H-Bn is more elastic than graphene.
H-BN’s excellent heat resistance and chemical stability have made it an important material for two-dimensional electronic devices and other 2-bit devices. hBN’s toughness makes them an ideal choice for flexible electronic. This is also important for the development and use of flexible 2D materials in two-dimensional electronics.
Future uses for h-BN include electronic textiles that are flexible and electronic skin, as well implantable electronics that can connect directly to the brain.
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