As we all know, two-dimensional materials have excellent mechanical properties. In addition to Young's modulus and mechanical strength, fracture toughness is also a very important mechanical index, reflecting the material's ability to resist crack growth instability.
Since the material has inherent defects in practical applications, the fracture toughness in many cases can better reflect the mechanical properties of the material. However, for materials with very high Young's modulus and mechanical strength, the fracture toughness is not necessarily as high. Graphene is proved to be the material with the highest Young's modulus and strength found so far, but its fracture toughness is only about 16 J/m2, which is lower than some traditional materials. The brittle mechanical properties of two-dimensional materials lead to One of the important reasons for its low fracture toughness.
Numerous studies have shown that the introduction of nano-scale defects in two-dimensional materials can enhance the plastic properties of the material and thus increase its fracture toughness. However, this method requires extremely high experimental skills and expensive equipment to accurately control the size of the introduced defects. And shape, and can only be achieved in a very small area, it is difficult to scale into two-dimensional materials.
In view of this, Yu Sun, Chandra Veer Singh and Tobin Filleter of the University of Toronto developed a new strategy of chemical means to change the mechanical properties. By introducing chemical functional groups into two-dimensional materials and increasing the thickness of graphene, it has greatly improved the The plastic properties and fracture toughness of two-dimensional materials are expected to be widely used in two-dimensional material applications.
Using transmission electron microscopy, the researchers first etched approximately 10% of the width of the multilayer graphene oxide by an electron gun, and then performed in-situ tensile testing of the cracked graphene oxide in a transmission electron microscope through a MEMS device. It completely breaks.
In the experiment, multilayer graphene oxide was found to have a mechanism to inhibit cracks. When the crack began to expand, it did not instantaneously spread to the edge of the film like graphene, but stopped in the middle. After the stress was further increased, the crack was extended to the film. At the edge, this crack-inhibiting mechanism was first discovered in two-dimensional materials.
In addition, since the stress-strain curve of multi-layer graphene oxide is non-linear, the Griffith Fission theorem cannot be used to calculate its fracture toughness. The team proved in the paper that the application of Griffith's crack theorem to nonlinear two-dimensional materials is not Rationality, and pointed out that the fracture toughness of non-linear materials should be calculated by the J-integration theory proposed by Hutchinson and Rice. The team calculated the multi-layer graphene oxide by using the finite element method and the J-integral theory. The fracture toughness is more than 3 times that of single-layer pure carbon graphene.
Through the method of molecular dynamics simulation, the Toronto team pointed out that the potential cause of crack inhibition in multilayer graphene oxide is: For a single layer of graphene oxide, once the crack begins to expand, it will naturally chase The carbon atoms of the oxidized functional group, when they encounter SP2 bonds that require more energy to be destroyed, require more strain energy to stop the extension; while graphene does not have oxidizing functional groups, it needs to be higher under the same conditions. The stress causes the cracks to begin to expand, but does not exhibit crack suppression. In addition, the potential reason for the higher fracture toughness of multi-layer graphene oxide than single-pure pure carbon graphene is: The pure carbon graphene has the same spreading cracks in each layer. For multi-layer graphene oxide, due to the disordered distribution of oxidized functional groups, the crack propagation path of each layer is different, resulting in the need for more strain energy.
In summary, this study provides a theoretical and experimental basis for chemically regulating the mechanical properties of two-dimensional materials, and guides new directions for the preparation of two-dimensional materials with enhanced fracture toughness! Changhong Cao, Sankha Mukherjee, Yu Sun, Chandra Veer Singh, Tobin Filleter et al., Nonlinear fracture toughness measurement and crack propagation resistance of functionalized graphene multilayers. Sci. Adv. 2018; 4: eaao7202.