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.
H-BN is theoretically more powerful than graphene because of its strong carbon-carbon bonds. The strengths and elastic modulus are identical, with h-BN slightly lower than graphene. Graphene is stronger than H-BN at 130GPa and has a young’s modulus around 1.0TPa. The strength and modulus for H-BN are 100GPa (and 0.8 TPA respectively).
Graphene, despite its exceptional mechanical properties is very brittle.
British engineer Griffiths published in 1921 a theory on fracture mechanics. This study described the failures of brittle materials as well as the relationship between crack sizes 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 considered to 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 more crack resistant than graphene. This discovery is in direct contradiction to Griffith’s fracture theory. Such anomalies have never before been observed 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
The team applied stress on the HBN sample using scanning electron microscopes, transmission electron microscopes, and other tools to discover the cause. 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 excellent 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, and implantable electronics that connect directly to the brain.
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