Text|Qing Shirt Xun
Editor|青贵洵
preface
As a single-layer two-dimensional material, graphene is composed of carbon atoms through covalent bonds, and has a unique multi-scale structure and excellent mechanical properties. Its mechanical properties are determined by its special structure and interatomic interactions.
At the nanoscale, the structural characteristics of graphene have a significant impact on its mechanical properties. Lattice defects, boundary effects, local strains, and deformations all have important influences on the mechanical behavior of graphene.
In addition, the multilayer structure and interlayer interaction of graphene at the micron scale also have a significant impact on its mechanical properties. Factors such as interlayer slip, peeling and stacking are important determinants of the mechanical properties of graphene at the micron level.
As a material with many advantages, what factors are affected by graphene-based?
Graphene
Over the past two decades, graphene has sparked great interest in two-dimensional materials. As the first 2D material to be discovered, graphene is the thinnest, strongest, and most flexible material known to exist.
Moreover, graphene has a large specific surface area and excellent thermal and electrical conductivity, which is considered an ideal material for various advanced applications, such as flexible electronics, sensors, and biomedicine.
In order to take advantage of the extraordinary mechanical properties of graphene in actual engineering structures, graphene is often assembled into macroscopic structures. However, in existing work, the mechanical properties are reduced by about two orders of magnitude in macroscopic graphene components such as monolayer graphene to graphene-based paper and fibers, and this experiment shows a multi-scale mechanical degradation mechanism from nanoscale to millimeter.
Therefore, bringing the excellent mechanical properties of single-layer graphene into macroscopic components of different length scales is extremely important for the practical application of graphene-based materials, and many theoretical, simulation and experimental work has been carried out to reveal the multi-scale structure-property relationship of graphene-based materials to further improve their mechanical properties.
Single-layer graphene is the smallest building block of graphene-based materials. In general, graphene can be manufactured by two methods, chemical vapor deposition and wet chemical synthesis.
However, in the process of manufacturing these graphene groups, defects will inevitably be introduced, which cannot be ignored when determining the mechanical properties of graphene. On the other hand, the hexasymmetry of graphene indicates the isotropic mechanical properties of the base surface, but under finite deformation, the mechanical properties of graphene are anisotropic.
Therefore, some work has focused on revealing the structure-property relationship of single-layer graphene. For example, a continuum model is developed to predict the elastic and failure behavior of a single layer of graphene mediated by defects. In addition, the mechanical behavior of defective graphene based on molecular dynamics simulation has received more attention.
Although, in general, defects are one of the main sources of mechanical degradation of monolayer graphene. However, for some special cases, defects increase failure strain, fracture toughness, and out-of-plane deformation. In experiments, defects were found to increase Young's modulus to almost twofold with a vacancy content of ~0.2%. Until now, accurately controlling the type and distribution of defects in a single layer of graphene, as well as predicting the mechanical properties of graphene with arbitrary defects, remains a huge challenge.
Graphene layered nanostructures — the nanoscale building blocks of graphene-based materials — can be formed by assembling single layers of graphene in a layer-by-layer manner. According to the continuity of graphene sheets in GLNs, GLNs can be divided into two types, namely nacre structure and layered structure.
In general, the in-plane mechanical behavior of nacre structures and the out-of-plane mechanical behavior of laminar flow structures are of great significance for nacre structures subjected to in-plane tensile loads.
In addition, the in-plane stress in the graphene sheet can be transferred to the adjacent graphene sheet through interlayer shearing. The related continuum model shows that the in-plane tensile behavior of this structure mainly depends on the balance of in-plane and interlayer load transfer.
In particular, the interlayer interaction of primordial graphene will be too weak to transmit sufficient in-plane tensile load, which greatly limits the improvement of mechanical properties. Therefore, many strategies were proposed to enhance layer-to-layer load transfer in the course of the experiment, including chemical crosslinking and nanostructure modification.
On the other hand, the out-of-plane behavior of laminar flow structures, including bending deformation, failure, and compression buckling, is also understood. Due to weak interlayer interactions, laminar flow structures often exhibit some unusual mechanical behaviors, such as intrinsic buckling without the structure being elongated.
Although interlaminar mechanical properties are known to play an important role in the overall mechanical behavior of nacre and layered structures, there are few opportunities to explore the influence of interlamellar interactions on the bending behavior of layered structures. In addition, the cause of the bending behavior of nacre-like structures remains unknown.
Although the further assembly of GLNs can form graphene base materials, its mechanical properties not only depend on the performance of GLNs, but also depend on the multi-scale assembly structure, such as folds, GLN orientation, etc. There are also three macroscopic graphene components, such as graphene paper, graphene fiber, and graphene aerogel.
In addition, in the structure of GLNs, GP and GF are obtained when GLN is assembled in a compact manner, where GA has a cellular structure, but their cell wall is composed of GLN. Different methods have been developed to manufacture different graphene macro components. For example, vacuum filtration yields a compacted paper-like structure, while a compacted fibrous structure comes from the wet process, where the porous structure is produced by solution assembly and then the lyophilization process.
In addition, some porous films or fibers exhibit a paper or fibrous form because they have a porous structure inside, and they are produced by a manufacturing method that combines compacted structure and porous structure. Of course, in general, structural compaction and uniformity are two important parameters that affect the load-bearing performance of GLM in the test.
This is why GFs have better mechanical properties than GPs, especially in terms of Young's modulus and tensile strength. Although the reasons for the degradation of mechanical properties of monolayer graphene to macroscopic graphene components, and the relationship between multi-scale mechanisms and the structure-properties of GLM have not been clearly understood. But experiments to study various graphene, polymer nanocomposites have been successful.
In addition, due to its excellent mechanical properties, graphene is considered to be a promising reinforcing filler, requiring only a small amount of graphene to significantly improve the Young's modulus and strength of composites.
However, how to uniformly disperse graphene in composites with large graphene content has become a big problem, because the weak vdW interface between graphene and polymer matrix also provides a source of crack initiation, which hinders the further mechanical improvement of graphene and polymer composites.
However, several strategies have been proposed to optimize the interface structure and properties to improve the overall mechanical properties, Young's modulus, strength and toughness of graphene nanocomposites. In addition, other structures, such as inverse nacre or double continuous lamellar microstructures, have been prepared, both of which can improve the strength and toughness of graphene nanocomposites.
Of course, although some reviews have focused on graphene monolayers, GP, GFs, gas and graphene, as well as polymer nanocomposites, the mechanism of mechanical degradation of GLMs at multiple scales has been rarely discussed. To know that the nanoscale to the macroscale requires a comprehensive understanding of the structural mechanical properties of graphene-based materials, which can provide some multi-scale optimization strategies for improving their mechanical properties.
In addition, attention should be paid to the layered structure of GLM and the associated mechanical behavior, as well as theoretical models of monolayer graphene, nacre and laminar flow GLN. Finally, it should be noted that the theoretical model of nanoscale layered structures is also applicable to macroscopic layered structures.
For example, the mechanical behavior of intraplane tensile and interlaminar shear behavior of single-layer graphene, and the bending behavior of GLN. Discussing large-scale graphene flakes in solution is also known as the conformation of graphene macromolecules in solution, which may affect the microstructure of graphene components. This is because of its graphene polymer nanocomposites with different microstructures.
A theoretical model of single-layer graphene tension behavior
However, classical mechanical models may not be suitable for describing the mechanical properties of atom-thick two-dimensional materials, so it is necessary to develop new theoretical models to accurately predict the structural property relationship of two-dimensional materials, which is very important for designing the mechanical properties of two-dimensional materials.
Of course, the in-plane Young's modulus of graphene in the deformation state shows the isotropy of the six-fold symmetric atomic structure, but under the finite deformation, the overall stress-strain relationship, strength and toughness are related to the direction.
For example, the ultimate stress along the armchair direction and the zigzag direction varies to 2GPa in terms of Cauchy stress simulated by density functional theory (DFT), while the MD simulation using different force fields is 2~11GPa, and several superelastic-based continuum models have been proposed by this method, which elucidate the nonlinear behavior of primordial graphene with finite deformation under uniaxial loading, biaxial loading or nanoindentation, where the elastic constant is determined by the least squares fit calculated ab initio.
In addition, under uniaxial tension, cracks always nucleate and propagate in different directions in the zigzag direction. Based on this observation, they proposed a simplified formula to characterize the chiral dependence of strength and toughness, and by considering nonlinear elastic deformation, they found that the formula was in good agreement with the MD results.
However, according to the second law of thermodynamics, single-layer graphene inevitably introduces defects during the manufacturing process. After reviewing the effects of several defects, the underlying mechanisms including point defects, vacancies, dislocations, S-W defects, line defects, grain boundaries, GBs, pattern defects, chemical functionalization and surface defects, holes and cracks, and fracture behavior were finally discovered.
However, in the course of the research, few theories were found to predict the overall mechanical properties of defective graphene, which is quite a challenge. Finally, in limited theoretical works, the energy and motion of GB between two crystallites are predicted using a dislocation model, which is suitable for small-angle GBs. Based on the discrete model, an elastic field caused by discrete dipoles is derived, which is suitable for small-angle and large-angle GBs.
Then, taking into account the interaction between discrete dipoles and following the rule that the bond with the maximum residual stress starts at the hexagon-heptagon ring share, the relationship of strength to tilt angle is theoretically given at the GB's GBs at a tension perpendicular to the GB, a result that is very consistent with the MD simulation.
Taking into account the loading direction, using continuum theory and the MD simulation system, the failure strength of GBs under tension in all possible directions between 0° and 90° was discovered. In addition, a statistical model was developed to predict the strength and toughness of the triple junction of polycrystalline graphene containing GBs with different sample sizes, grain sizes and strain rates.
They then obtained scaling laws for polycrystalline graphene strength and toughness, and predicted the conclusion that toughness is independent of grain size above 25.6 nm, and its breaking strength always depends on grain size and sample size. Finally, it was found that dislocation shielding had a very high influence on crack propagation.
A theoretical model of the tensile behavior of nacre structures
In addition, it was found that for nacre-like GLN, the intraplane tension is transmitted to adjacent platelets through interlaminar shear stress. Therefore, it is necessary to establish a theoretical model to describe tensile-shear load transfer to further predict the mechanical properties of such structures.
First, a tensile shear chain TSC model needs to be proposed, which can be used to describe the mechanical behavior of biological materials such as nacre and bone, in which lined up mineral tablets can be joined layer by layer by soft and tough proteins. The TSC model shows that the tensile load in the mineral table is mainly transferred by the large shear band of the protein matrix.
As a result, the mineral sheet carries most of the tensile load, and the protein matrix also provides it with large shear deformation. Therefore, the rigidity of minerals and the toughness of proteins can be maintained in minerals at the same time. Finally, a quasi-self-similar layering model is developed based on the TSC model, which gives the optimal level of layered nacre material, and it is found that it is very consistent with natural observations.
summary
In the experimental process, the multi-scale structure, mechanical behavior, mechanical degradation mechanism and corresponding preparation methods of graphene base materials were discovered, and it was also possible to achieve from single-layer graphene to macroscopic graphene assembly. Finally, a multi-scale mechanical optimization strategy based on mechanical degradation mechanism is proposed.
Through retrospective analysis, it can be seen that significant efforts have been made to produce the extraordinary performance of monolayer graphene to macroscopic graphene components. However, considering the huge gap in mechanical properties between single-layer graphene and macroscopic graphene components, there is still more room for improvement in the mechanical properties of graphene-related materials.
In addition, at the nanoscale, some work has been carried out to understand the effect of several specific defects on the mechanical properties of monolayer graphene. However, defect types and distributions in actual materials can be more complex because the influence of curvilinear grain boundaries on mechanical properties is rarely addressed.
Therefore, it is also necessary to mediate the theoretical numerical method through machine learning to understand the defect performance relationship of two-dimensional materials, and only after doing these can we further use defect engineering to regulate mechanical properties on this basis. For example, with the increase of grain boundary density, the Young's modulus and strength of graphene sheets first decrease and then increase, so the grain size of graphene in the experiment can be adjusted by controlling the growth conditions, that is, temperature, of polycrystalline graphene, so as to obtain the expected mechanical properties.
On the other hand, based on experimental observations, independent single-layer graphene has a large number of ripples at a limited temperature, and since the material is usually used at a limited temperature, relevant theories should be developed to consider the influence of ripples on the mechanical behavior of single-layer graphene. Finally, in terms of graphene layered nanostructures, the in-plane mechanical behavior of nacre structures and the bending behavior of layered structures are well carried out in two-dimensional models.
Further mechanical models should be proposed to describe the three-dimensional deformation of GLN and the bending behavior of nacre-like structures. In addition, more experimental and theoretical work should be carried out to understand the mechanical behavior of the layered structure under finite deformation, which, if successful, can play an important role in practical applications such as flexible electronics and biosensors.