Mechanical Properties Of Graphene Sheets Research Articles

Triangular graphene nanosheets, structures with extraordinary bending behavior.

Sometimes, manipulating the geometry or creating a defect can help the engineering of the nanostructured properties, such as increasing the frequency sensitivity of graphene nanosheets as a mass sensor by changing the geometry of the structure in a triangular shape and also by creating a vacancy or making nanodiodes in a triangular shape, which restricts the flow of heat in one direction and expands it in another direction. Then, the mechanical properties of the triangular geometry of graphene nanosheets have an excellent value for future devices. The results of the molecular dynamics study can illustrate the mechanical properties of graphene sheets well, which is particularly important. In this study, using molecular dynamics (MD) simulation, the bending stiffness of triangular graphene sheets was investigated to present a broader view of engineering applications of graphene sheets. Analyzing stress-displacement results of triangular graphene sheets in both zigzag and armchair directions under bending load reveals the effect of the directions of sheets on their bending stiffness. Also, comparing the bending stiffness of pristine and defective sheets showed that bending stiffness was decreased when an atom was removed from the middle of a sheet and made a single vacancy defected sheet. Obtained results suggest that triangular graphene nanosheets with higher bending stiffness, compared to rectangular sheets with the same number of atoms, are better for applications requiring higher bending properties, such as nanoelectromechanical devices. The reported results of this work provide an overall viewpoint concerning the geometry and defect effects on the graphene nanosheets' mechanical properties. The bending stiffness of triangular sheets was 20 and 10 times that of intrinsic and defective rectangular sheets, respectively.

Evaluation of the Stress-Strain of Single-Layer Graphene Sheet Including a Crack

Graphene is extremely thin, which exhibits extraordinary mechanical, physical and electrical properties, such that there are applications for this material in batteries and flexible displays. However, during the fabrication of commercial graphene devices, defects such as impurities and vacancies inevitably occur and the resulting cracks or notches can deteriorate the performance of graphene-based structures. Thus, it is necessary to understand the mechanical properties of graphene sheets including cracks so as to design healthy graphene-based structures. The present study investigated the effects of cracks (at various positions, distances and angles) on the strength of graphene. Four initial positions were considered in conjunction with uniaxial tensile loading, along with nine different distances and three initial angles. It was found that a crack located at the top center of the sheet represents the most vulnerable stress point, while a crack located further than one quarter of the sheet from the loading point had the greatest effect on the stress. In addition, the maximum stress was obtained in conjunction with a crack inclined at 45° relative to the loading direction. These results are expected to assist in the future evaluation for the safety usage of graphene sheets.

Application of modified couple-stress theory to stability and free vibration analysis of single and multi-layered graphene sheets

With the development of nanotechnology, research activities on carbon nanostructures have increased rapidly. In recent years, due to the extraordinary mechanical properties of graphene sheets, there has been a growing interest in investigating the mechanical response of these carbon nanostructures. In this article, the modified couple-stress theory (MCST) is first employed to study the free vibration and mechanical buckling of single-layered graphene sheets (SLGSs). To this end, SLGS is modeled as a nanoplate and the two-variable refined plate theory (TVRPT) is adopted to extend the finite strip method (FSM) formulation. The natural free vibration frequency and mechanical buckling loads of the sheet are then obtained by solving the proper eigenvalue problems. Mechanical buckling and free vibration of multi-layered graphene sheets (MLGSs) are also investigated considering the effects of van der Waals (vdW) bonds between the layers. Modified couple-stress theory is applied to consider the small-scale effects of the graphene sheets. The results obtained by the proposed method are validated against those available in the literature. Finally, a comprehensive parametric study is performed to investigate the effects of different parameters such as loading schemes, nanoplate dimensions and boundary conditions.

A New Approach of Mathematical Analysis of Structure of Graphene as a Potential Material for Composites.

The new analysis of a simplified plane model of single-layered graphene is presented in this work as a potential material for reinforcement in ultralight and durable composites. However, owing to the clear literature discrepancies regarding the mechanical properties of graphene, it is extremely difficult to conduct any numerical analysis to design parts of machines and devices made of composites. Therefore, it is necessary to first systemize the analytical and finite element method (FEM) calculations, which will synergize mathematical models, used in the analysis of mechanical properties of graphene sheets, with the very nature of the chemical bond. For this reason, the considered model is a hexagonal mesh simulating the bonds between carbon atoms in graphene. The determination of mechanical properties of graphene was solved using the superposition method and finite element method. The calculation of the graphene tension was performed for two main directions of the graphene arrangement: armchair and zigzag. The computed results were verified and referred to articles and papers in the accessible literature. It was stated that in unloaded flake of graphene, the equilibrium of forces exists; however, owing to changes of inter-atom distance, the inner forces occur, which are responsible for the appearance of strains.

Research on Strain Rate Effect of the Mechanical Properties of Graphene Sheet Containing Randomly Distributed Defects

Strain rate effects on the mechanical properties of graphene sheets (GSs) contain randomly distributed defects investigated by molecular dynamics simulations, and the results have been discussed. The strain rate has a significant effect on the tensile strength, while fracture strain and failure mechanism of GSs and elastic modulus are insensitive to the strain rate. At room temperature of 300 K, GSs without defects show that brittle failure is at a low strain rate and linear hardening is at a high strain rate. The strain rate is higher, the GSs are harder. However, GSs with a large number of randomly distributed defects only show brittle failure under high temperature. The tensile strength and fracture strain increase with the strain rate increasing. The tensile strength of GSs without defects has a linear relation to the logarithm of the strain rate, but the GSs with defects do not exhibit this phenomenon because the defects are sensitive to the strain rate. The present study provides a theoretical optimization method for the preparation and performance of GSs.

Investigation on fracture of pre-cracked single-layer graphene sheets

Peridynamics (PD) is employed to model the fracture of pre-cracked graphene sheets under mode-I loading condition to show its application at nanoscale. Then the related mechanisms at atomic scale are revealed by using molecular dynamics (MD) simulation, with which the mechanical properties and fracture mechanisms of full atomistic pre-cracked single-layer graphene sheets (SLGSs) with different types of crack tip micro-structures are investigated. The results such as the fracture forms obtained from the PD and MD simulations show good consistency. The MD results show that different crack tip structures have distinct effect on the mechanical properties of graphene sheets. The pre-cracked SLGSs with ω-type crack tip show higher fracture strength and strain than the ones with u-type crack tip. For the pre-cracked SLGSs with u-type crack tip, the fracture strength and strain of the armchair sheet are higher than the zigzag one. However, there are almost no differences for the armchair and zigzag sheets with ω-type crack tip. Due to the difference between armchair and zigzag structures, carbon polygon ring can form in armchair sheets and its formation is related to the crack tip structures and the crack width. In addition, the crack propagation can be characterized by the local stress state, which for crack initiation is different from steady crack propagation.

Effect of topological defects on mechanical properties of graphene sheets: a molecular dynamics study

The mechanical properties of graphene may be strongly influenced by topological defects such as vacancies, Stone-Thrower-Wales (STW) defects. In this research article, the effects of Stone Thrower Wales (STW) defect on the mechanical properties such as intrinsic strength and fracture toughness of graphene sheet have been investigated by using molecular dynamics based simulations. The intrinsic properties of graphene sheet are strongly affected by the orientation of STW defects. For STW2 defected graphene sheet, the strength of the armchair direction is much higher than that of zigzag direction. While the scenario for STW1 defected graphene is opposite, the strength of the zigzag direction is much higher than that of armchair direction. Furthermore, we investigated that STW defects can be used for tailoring the fracture toughness of graphene sheet. Significant improvement in the fracture toughness of graphene in armchair direction was predicted, while STW defects were positioned in front of the crack-tip. Interaction of stress fields generated from crack tip as well as from STW defects helps in tailoring the fracture behavior of graphene.

Anomalous strength characteristics of Stone-Thrower-Wales defects in graphene sheets - a molecular dynamics study.

Graphene, viz., the one-atom-thick sheet of carbon, exhibits outstanding mechanical properties, but defects, which are inevitable at the time of synthesis, may strongly affect these properties. In this study, the effects of two types of Stone-Thrower-Wales (namely, STW-1 and STW-2) defects on the mechanical properties of graphene sheets at different temperatures and strain rates were investigated on the basis of molecular dynamics simulations. The authors also investigated the effect of the strain rate and defect concentration on the failure morphology of STW-1 and STW-2 defected graphene sheets. It was observed that, irrespective of the strain rate, the fracture strengths of STW-1 and STW-2 defected graphene sheets are identical in the zigzag and armchair directions, respectively, at low temperatures. It was also observed that the fracture strengths of graphene sheets with STW-1 defects in the armchair direction and STW-2 defects in the zigzag direction decrease drastically at higher temperatures and also at lower strain rates. On the other hand, it was noticed that the fracture strengths of graphene sheets with STW-1 defects in the zigzag direction and STW-2 defects in the armchair direction decrease gradually with an increase in the temperature and a decrease in the strain rate. It was also predicted that the failure morphology of graphene sheets with STW-1 defects in the zigzag direction and STW-2 defects in the armchair direction depends on the defect concentration and the strain rate.

Enhanced photocatalytic degradation of Rhodamine B by reduced graphene oxides modified Bi12TiO20 under visible light

In this work, Bi12TiO20/reduced graphene oxide (Bi12TiO20/rGO) photocatalysts were successfully synthesized via a simple microwave hydrothermal process. The as-prepared products were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible absorption spectroscopy. The photocatalytic activity was investigated using Rhodamine B under visible-light irradiation. The Bi12TiO20/rGO composites presented much higher photocatalytic performance than pure Bi12TiO20. The B-5rGO sample exhibits the highest photocatalytic activity. The high performance could be attributed to the excellent electrical and mechanical properties of graphene sheets. The present study could provide new direction into the research of the Bi12TiO20/rGO composite materials and promote their application.

Finite element simulation of mechanical properties of graphene sheets

Graphene is the material for the twenty first century applications. In this paper, the elastic properties of monolayer and double layer Graphene sheets, typically less than 10nm in size are investigated through linear finite element simulations. The effect of aspect ratio, sizes and chirality of the Graphene sheet on the Young’s modulus, Shear modulus and Poisson’s ratio are studied. By using structural mechanics approach combining atomistic and equivalent continuum techniques, the Young’s modulus, shear modulus and the Poisson ratio were found and they slightly increase with the aspect ratio but decrease with the size of the Graphene sheet. These simulated properties compliment the mechanical properties of Graphene found in literature.

Wet mechanochemical approach assistance to the green synthesis of graphene sheet at room temperature and in situ anchored with MnO2 in a green method

In this study, we report a green and eco-friendly method for direct synthesis of graphene sheet from natural graphite under simple conditions and in situ anchored with MnO2 via an environmental route. A wet mechanochemical approach is used to exfoliate the graphite to graphene sheet by using citric acid as a non-toxic and friendly intercalator agent instead of strongly corrosive acids such as sulfuric and nitric acids at the shortest time and environmental condition. We carried out various microscopic and spectroscopic measurements such as UV–visible spectroscopy, Fourier transform Infrared (FT-IR), Thermogravimetric Analysis (TGA), X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM) Atomic Force Microscopy (AFM) and Handle Raman Microscopy to confirm the exfoliation of natural graphite to graphene under mild condition. Moreover, we took nanoindentation test for investigation of mechanical properties of graphene sheets. The hardness and elastic modulus obtained from this analysis are about 4.1 and 37.2 GPa, receptivity. At the following work, one pot preparation of graphene-MnO2 nanocomposite can be performed on the large scale and be utilized in a wide variety of applications. This synthesis, which is economically suitable, is performed without any reduction agent and much purification. Facile synthesis of graphene and graphene-MnO2 nanocomposite via a green and simple way makes it scalable to apply in many fields.

Nanocomposites of Metal Oxide@Graphene@Ni Foam for Energy Storage Applications

Nanocomposites of metal oxide@graphene@Ni foam have been developed for energy storage applications such as supercapacitors. Electrochemical performance of the graphene-based metal compounds nanocomposites has been improved due to the synergistic effect of excellent conductivities and mechanical properties of graphene sheets and the high pseudocapacitance of metal oxides and metal sulfides. Most metal oxides and sulfides are low costs and doping of graphene with metal oxide or sulfide nanoparticles enhances the energy density remarkably, resulting in an excellent performance as an electrode material for supercapacitors. Recently, metal foam has been getting tremendous attention as it provide a large surface area and large pores that enables electrolytes and charges penetrates freely. However, to meet the demands of industry for higher capacitance, energy density and power density, a new approach is required. Various metal oxides and sulfides will be compared. The benefits of using graphene will also be discussed. In the synthesis of these materials, a cleaner technology has been employed. Nanocomposites that show high surface areas, high conductivities, and remarkable electrochemical properties will be introduced and, especially, materials for high performance supercapacitors will be presented for future energy storage application.

How to Enhance the Performance of Graphene/Metal Oxide Nanocomposite Supercapacitors

Graphene/metal oxide nanocomposites have been developed for energy storage applications such as supercapacitors, catalysts, sensors, and batteries. Electrochemical performance of the nanocomposites has been improved so much due to the synergistic effect of excellent conductivities and mechanical properties of graphene sheets and the high pseudocapacitance of metal oxides. For the applications of these nanocomposites to the real supercapacitors, a huge enhancement in capacitance, cycling stability, energy density, and power density should be achieved, resulting in an excellent performance as an electrode material for supercapacitors.In this study, we have tested several methods to enhance the supercapacitor performance, such as doping of graphene with other elements and using mixed metal oxides and conducting polymers. We have achieved high surface area, high conductivity, and remarkable electrochemical properties (specific capacitance of over 4,000 F/g). Cleaner synthetic methods have also been involved in the synthesis process.

Doping of graphene using ion beam irradiation and the atomic mechanism

In this study, graphene doping using N ion beam irradiation is experimentally realized and the doped results are identified by X-ray photoelectron spectroscopy and Raman spectrum method. In order to explain the doping mechanism, classical molecular dynamics simulation and first principle calculation are applied to observe dynamics variation of the structure and the final steady state. Then the two-step doping mechanism is proposed to explain the experimental phenomena. Meanwhile, the optimum parameters (irradiated energy and ion dose) for getting an ideal doping piece are revealed. At last, tensile test simulations are proceeded to illuminate the mechanical properties of graphene sheets with different doping results.

Thermal Conductivity and Pressure-Dependent Raman Studies of Vertical Graphene Nanosheets

Thermal and mechanical properties of graphene sheet are of significant importance in the areas of thermal and stress management, respectively. Here, we report the thermal conductivity and high-pressure behaviors of unsupported vertical graphene nanosheets (VGNs) grown by electron cyclotron resonance-plasma enhanced chemical vapor deposition method. Structural morphology of the as-grown VGNs on SiO2/Si substrate suggests a homogeneous, uniformly interconnected network of graphene sheets standing vertically on a basal nanographitic layer. On examination of edges of exfoliated sheets using transmission electron microscopy, seven layers of graphene is estimated. The frequency of the G-band (E2g-in plane mode) is found to vary linearly with temperature. The first-order temperature coefficient for G-band is found to be 1.47(1) × 10–2 cm–1 K–1. Using the G-band temperature coefficient and its position dependence on excitation laser power, the thermal conductivity of the VGNs at room temperature is estimated to b...

Anisotropic compressive response of Stone–Thrower–Wales defects in graphene: A molecular dynamics study

The mechanical properties of graphene sheet can be tailored with the help of topological defects. In this research article, the effects of Stone–Thrower–Wales (STW) defects on the mechanical properties of graphene sheet was investigated with the help of molecular dynamics based simulations. Authors has made an attempt to analyse the stress field developed in and around the vicinity of defect due to bond reorientation and further systematic evaluation has been carried out to study the effect of these stress fields against the applied axial compressive load. The results obtained with the pristine graphene were made to compare with the available open literature and the results were reported to be in good agreement with theoretical and experimental data. It was predicted that graphene with STW defect cannot able to bear compressive strength in zigzag direction, whereas on the other hand it was predicted that graphene sheet containing STW defect can bear higher compressive load in armchair direction, which shows an anisotropic response of STW defects in graphene. From the obtained results it can be observed that orientation of STW defects and the loading direction plays an important role to alter the strength of graphene under axial compression.

Finite temperature effect on mechanical properties of graphene sheets with various grain boundaries**Project supported by the Nation Natural Science Foundation of China (Grant Nos. 11347219 and 11404147), the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20140519), the Training Project of Young Backbone Teacher of Jiangsu University, the Advanced Talents of Jiangsu University, China (Grant No. 11JDG118), the Practice

The mechanical properties of graphene sheets with various grain boundaries are studied by molecular dynamics method at finite temperatures. The finite temperature reduces the ultimate strengths of the graphenes with different types of grain boundaries. More interestingly, at high temperatures, the ultimate strengths of the graphene with the zigzag-orientation grain boundaries at low tilt angles exhibit different behaviors from those at lower temperatures, which is determined by inner initial stress in grain boundaries. The results indicate that the finite temperature, especially the high one, has a significant effect on the ultimate strength of graphene with grain boundaries, which gives a more in-depth understanding of their mechanical properties and could be useful for potential graphene applications.

Effects of SI, N and B doping on the mechanical properties of graphene sheets

Molecular dynamics (MD) simulations were performed to stretch the rectangular graphene sheets doped with silicon, nitrogen or boron atoms. Young's modulus, ultimate stress (strain) and energy absorption were measured for the graphene sheets with the doping concentration (DC) ranging from 0 to 5%. The emphasis was placed on the distinct effects of each individual dopant on the fundamental mechanical properties of graphene. The results indicated that incorporating the dopants into graphene led to an almost linear decrease in Young's modulus. Monotonic reductions in ultimate strength, ultimate strain and energy absorption were also observed. Such doping effects were found to be most significant for silicon, less pronounced for boron, and small or negligible for nitrogen. The outputs provide an important guidance for the development and optimization of novel nanoscale devices, and facilitate the development of graphene-based M/NEMS.

Mechanical properties of fully hydrogenated graphene sheets

Graphane is a two-dimensional structure consisting of a flat monolayer graphene sheet fully covered with hydrogen atoms attached to its carbon atoms in an alternating pattern. The unique properties of graphane make it suitable for different applications. In this paper, the mechanical properties of the most stable conformer of graphane, the so-called chair-like, are extensively investigated using density functional theory (DFT) scheme within the framework of the generalized gradient approximation (GGA) and the well-known Perdew–Burke–Ernzerhof (PBE) exchange correlation. It is shown that the hydrogenation has significant influences on the mechanical properties of graphene sheet. In particular, it is found that the elastic, bulk and shear moduli and Poisson’s ratio of the chair–graphane are significantly smaller than those of graphene.

Cover Picture: Mechanical Properties and Failure Mechanisms of Graphene under a Central Load (ChemPhysChem 13/2014)

The cover image shows the two failure mechanisms that occur when graphene is under a central load; in brittle bond breaking, cracks occur along the loading domain edge, whereas in plastic relaxation, cracks form inside the thermal control domain. In their paper on p. 2749, H. Chen et al. reveal that the mechanical properties of graphene sheets are sensitive to both temperature and the symmetry of the loading domain.