Lou Jun and Gao Huajian, Academician of the Sixth Chinese Academy of Sciences, "Nature": Discovered that the super toughness of hexagonal boron nitride is that of graphene

Hexagonal boron nitride (h-BN) is a two-dimensional layered wide band gap insulating material with good heat resistance, chemical stability and dielectric properties, and is widely used in electronic devices
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? From a structural point of view, hexagonal boron nitride and graphene are very similar, and both consist of atoms arranged in a plane lattice of interconnected hexagons
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The only difference is that in graphene, all atoms are carbon, while in h-BN, each hexagon contains three nitrogen atoms and three boron atoms
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? Carbon-carbon bond is one of the strongest bonds in nature, so in theory graphene is much stronger than h-BN
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Although the strength and elastic modulus of these two materials are similar, h-BN is slightly lower: the strength of graphene is about 130 GPa, and the Young's modulus is about 1.
0 TPa; while the strength and modulus of h-BN are respectively 100 GPa and 0.
8 TPa
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? However, in the real world, no material is without defects, and so is graphene
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Although it has excellent mechanical properties, graphene has low crack resistance
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In other words, graphene is brittle! In 1921, British engineer Griffith published a groundbreaking theoretical study of fracture mechanics, describing the failure of brittle materials and the relationship between the size of the crack in the material and the force required to propagate the crack
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For hundreds of years, scientists and engineers have been using this theory to predict and define the toughness of materials
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? In 2014, a research by Professor Lou Jun's team from Rice University in the United States showed that the fracture toughness of graphene is consistent with Griffith's theory of fracture mechanics: when the stress applied to graphene is greater than the force to fix it together, The crack will grow; and the difference in energy is released in the crack propagation
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? Given the similar structure of h-BN and graphene, it is thought that it is also very brittle
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However, it is not
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? Violation of a century of fracture mechanics theory! Scientists found that h-BN is 10 times more tough than graphene? Recently, Professor Lou Jun from Rice University in the United States and Academician Gao Huajian from Nanyang Technological University in Singapore (Academician of the American Academy of Sciences, Academician of the American Academy of Engineering, Academician of the American Academy of Arts and Sciences, Academician of the German Academy of Sciences, A joint team of foreign academicians of the Chinese Academy of Sciences and academicians of the European Academy of Sciences published an article in "Nature" that they found that the brittle h-BN in common sense actually exhibits super crack resistance (ie toughness), which is 10 times that of graphene.
! This discovery runs counter to Griffith's fracture theory, and this anomaly has never been observed in two-dimensional materials before
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Relevant research results are titled "Intrinsic toughening and stable crack propagation in hexagonal boron nitride" and published in the top international journal "Nature"! ? The mechanism behind the super toughness of h-BN In order to find out the reason, the research team applied stress to the h-BN sample and used scanning electron microscope and transmission electron microscope to observe how cracks occurred as much as possible
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After more than 1,000 hours of experimentation and subsequent theoretical analysis, they discovered the mystery
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? Figure 1.
SEM observation of the fracture and stable crack growth of the pre-cracked monolayer h-BN
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? Although graphene and h-BN may be structurally similar, boron and nitrogen atoms are different, so the hexagonal lattice in h-BN inherently has an asymmetric arrangement, which is different from the carbon hexagon in graphene
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In simple terms, in graphene, cracks tend to pass straight through a symmetrical hexagonal structure from top to bottom, opening the bond like a zipper
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Due to the stress contrast between boron and nitrogen, the hexagonal structure of h-BN is slightly asymmetric.
The inherent asymmetry of this lattice will cause cracks to bifurcate and form branches
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? Figure 2.
Crack initiation in h-BN and graphene
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? And if the crack is bifurcated, it means it is turning
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The existence of this turning crack requires additional energy to further promote the propagation of the crack, which makes it more difficult for the crack to propagate and effectively enhances the toughness of the material
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"This is the reason why h-BN exhibits superior elasticity to graphene
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? Figure 3.
Crack growth and effective energy release rate in h-BN samples
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? Due to its excellent heat resistance, chemical stability and dielectric properties , H-BN has become an extremely important material for two-dimensional electronics and other two-position devices, not only as a supporting substrate, but also as an insulating layer between electronic components
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Today, h-BN's surprising toughness makes it It has become an ideal choice for flexible electronic products and is of great significance to the development of flexible 2D materials for applications such as two-dimensional electronics
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? Academician Gao Huajian of Nanyang Technological University in Singapore said: "This work is surprising and exciting.
, Because it reveals the internal toughening mechanism of a material that is said to be very brittle
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"Obviously, even Griffith could not foresee such a big difference in the fracture behavior of two brittle materials with similar atomic structures
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In the future, in addition to flexible electronic textile applications, h-BN with super toughness can also be used as a flexible electronic skin and implantable electronic devices that can be directly connected to the brain
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