Mechanical Properties Of Polycrystalline Graphene Research Articles

Research on fracture mechanism and mechanical properties of polycrystalline graphene by nanoindentation: A molecular dynamics study

Randomness of grain boundaries makes it difficult to reach a broad consensus about mechanical properties of polycrystalline graphene (PG). In the present paper, based on principle of Voronoi diagram, the models of PG with different grain sizes were established, and the fracture mechanism and mechanical properties were investigated by molecular dynamics (MD). The results showed that the crack initiation point of PG always located at the multiple junction of grain boundaries, and the crack propagation and fracture mode of PG was mainly dependent on not only the relative size but also the relative location of the indenter and grain boundaries. Additionally, the effects of grain size, indentation speed, temperature and indenter diameter on the mechanical properties were studied, which showed some interesting and different phenomena from the tensile case, e.g., the grain size seems no regular effect on mechanical properties. Furthermore, the ultimate indentation force, indentation depth and fracture showed an increase trend with the increase of indenter diameter and indentation speed, while they decreased with the increase of temperature. But when it came to the elastic modulus, it showed a decreasing trend with the increase of indenter diameter and indentation speed, while it first increased and then decreased with the increase of temperature.

A deep learning model for predicting mechanical properties of polycrystalline graphene

Molecular dynamics simulations have been widely used to understand the properties of nanomaterials. The main issue associated with molecular dynamic modeling is the high computational costs. Machine learning (ML) models can be used as an alternative method for predicting the behavior of materials. After training, machine learning models can provide instantaneous results and avoid the computational costs of molecular dynamic simulations. We develop a deep convolutional neural network model to predict the mechanical properties of polycrystalline graphene. The data required for training our machine learning model is generated using molecular dynamics simulations by modeling the behavior of polycrystalline graphene under uniaxial tensile loading. More than 2000 data points are generated for graphene sheets of different grain sizes and grain orientations. The goal is to train the network such that it can predict the Young’s modulus and fracture stress of graphene sheets by analyzing an image of the polycrystalline sheet.

Atomic-scale mechanical behaviors of polycrystalline graphene under biaxial loadings and high temperature

Graphene is widely utilized due to its excellent properties. Nevertheless, the failure mechanism previously acquired by uniaxial tension needs to be optimized urgently as its service conditions become more and more demanding. Here, the mechanical behaviors of polycrystalline graphene under tensile-shear biaxial strains and high temperature were investigated by molecular dynamics simulations. We proved that the shear strain dominated the failure of polycrystalline graphene when tensile and shear loading were applied simultaneously. As the temperature rises, the structural destruction of polycrystalline graphene changes from stress-dominated to temperature-dominated. Moreover, the ultimate shear stress increases with the increase of grain size, while the ultimate strain is the opposite. The polycrystalline graphene with a large grain size has better high temperature resistance. These results extend our understanding of the mechanical properties of polycrystalline graphene and guide the design of devices composed of 2D materials under extreme conditions.

Atomic-Layer-Deposition-Made Very Thin Layer of Al2O3, Improves the Young’s Modulus of Graphene

Nanostructures with graphene make them highly promising for nanoelectronics, memristor devices, nanosensors and electrodes for energy storage. In some devices the mechanical properties of graphene are important. Therefore, nanoindentation has been used to measure the mechanical properties of polycrystalline graphene in a nanostructure containing metal oxide and graphene. In this study the graphene was transferred, prior to the deposition of the metal oxide overlayers, to the Si/SiO2 substrate were SiO2 thickness was 300 nm. The atomic layer deposition (ALD) process for making a very thin film of Al2O3 (thickness comparable with graphene) was applied to improve the elasticity of graphene. For the alumina film the Al(CH3)3 and H2O were used as the precursors. According to the micro-Raman analysis, after the Al2O3 deposition process, the G-and 2D-bands of graphene slightly broadened but the overall quality did not change (D-band was mostly absent). The chosen process did not decrease the graphene quality and the improvement in elastic modulus is significant. In case the load was 10 mN, the Young’s modulus of Si/SiO2/Graphene nanostructure was 96 GPa and after 5 ALD cycles of Al2O3 on graphene (Si/SiO2/Graphene/Al2O3) it increased up to 125 GPa. Our work highlights the correlation between nanoindentation and defects appearance in graphene.

Fracture toughness of various percentage of doping of boron atoms on the mechanical properties of polycrystalline graphene: A molecular dynamics study

This article investigates the effect of different initial crack sizes on the mechanical response of single-layer boron doped polycrystalline graphene nanosheets by Molecular Dynamics (MD) simulations. We study 1%, 3%, 6% and 10% of boron doped polycrystalline graphene nanosheets with grain sizes of 10 and 15 nm for eight different initial crack lengths of 0.02L, 0.04L, 0.08L, 0.12L, 0.16L, 0.2L, 0.24L, and 0.32L, where L is the initial length of the nanosheet. We found that 1. brittle fracture for boron doped polycrystalline graphene as the failure occurs without any sign of plastic deformation and low energy absorption and 2. the ultimate tensile strength is independent of the initial crack size. For identical grain sizes and doping of boron atoms, the ultimate tensile stress and strain decrease as the crack lengths increases. In contrast, a clear trend was not observed in the ultimate tensile strength for the same crack length and doping of boron atoms as the grain size increases. The highest difference of 13.4% in the ultimate tensile strength was observed between grain sizes of 10 and 15 nm for 3% of boron doped nanosheets with an initial crack size of 20 Å.

Effects of Grain Size Distribution on the Mechanical Properties of Polycrystalline Graphene

One of the characteristics of polycrystalline graphene that determines its material properties is grain size. Mechanical properties such as Young’s modulus, yield strain and tensile strength depend on the grain size and show a reverse Hall-Petch effect at small grain size limit for some properties under certain conditions. While there is agreement on the grain size effect for Young’s modulus and yield strain, certain MD simulations have led to disagreement for tensile strength. Song et al. showed a decreasing behavior for tensile strength, that is, a pseudo Hall-Petch effect for the small grain size domain up to 5 nm. On the other hand, Sha et al. showed an increasing behavior, a reverse Hall-Petch effect, for grain size domain up to 10 nm. Mortazavi et al. also showed results similar to those of Sha et al. We suspect that the main difference of these two inconsistent results is due to the different modeling. The modeling of polycrystalline graphene with regular size and (hexagonal) shape shows the pseudo Hall-Petch effect, while the modeling with random size and shape shows the reverse Hall-Petch effect. Therefore, this study is conducted to confirm that different modeling is the main reason for the different behavior of tensile strength of the polycrystalline structures. We conducted MD simulations with models derived from the Voronoi tessellation for two types of grain size distributions. One type is grains of relatively similar sizes; the other is grains of random sizes. We found that the pseudo Hall-Petch effect and the reverse Hall-Petch effect of tensile strength were consistently shown for the two different models. We suspect that this result comes from the different crack paths, which are related to the grain patterns in the models. Key words: Polycrystalline graphene, Molecular dynamics, Mechanical properties, Hall-Petch effect

Investigation into the effect of doping of boron and nitrogen atoms in the mechanical properties of single-layer polycrystalline graphene

In the present work, classical Molecular Dynamic simulations have been performed to peruse the effect of different grain sizes, temperatures and strain rates for different percentage of boron and nitrogen doping on the mechanical properties of polycrystalline graphene. Therefore, we studied 1%, 3%, 6% and 10% of boron or nitrogen doping for grain sizes of 1, 5, 10, 15, and 20nm of graphene nanosheets at room temperature. The effect of different temperatures (100, 300, 600 and 900K) on the mechanical response is investigated for five grain sizes (1, 5, 10, 15 and 20nm) and 6% of boron or nitrogen doped polycrystalline graphene. Moreover, we studied different engineering strain rates on the mechanical response of six percentage of boron or nitrogen doped polycrystalline graphene for all simulation samples at 300K. Our findings revealed that the mechanical response of polycrystalline graphene decrease as nitrogen or boron atoms are substituted into the nanosheet. By increasing of nitrogen atoms into polycrystalline graphene, a destructive affect occurs on the ultimate failure strain and tensile strength. Furthermore, the tensile strength tends to increase as the grain sizes increase from 1 to 20nm. On the contrary, the failure strain has a tendency to decrease. As the temperatures increase in boron or nitrogen doped polycrystalline graphene, the tensile strength decreases as well as the failure strain. An increasing trend has been observed for mechanical properties of boron or nitrogen doped polycrystalline graphene as the strain rate increase. However, the effect of nitrogen on the tensile strength of nanosheet is more significant compared to boron doped nanosheet.

Grain Size Effect on Mechanical Properties of Polycrystalline Graphene

Characteristics of nanocrystalline materials are known substantially dependent on the microstructure such as grain size, crystal orientation, and grain boundary. Thus it is desired to have systematic characterization methods on the various nanomaterials with complex geometries, especially in low dimensional nature. One of the interested nanomaterials would be a pure two-dimensional material, graphene, with superior mechanical, thermal, and electrical properties. In this study, mechanical properties of “polycrystalline” graphene were numerically investigated by molecular dynamics simulations. Subdomains with various sizes would be generated in the polycrystalline graphene during the fabrication such as chemical vapor deposition process. The atomic models of polycrystalline graphene were generated using Voronoi tessellation method. Stress strain curves for tensile deformation were obtained for various grain sizes (5~40 nm) and their mechanical properties were determined. It was found that, as the grain size increases, Young`s modulus increases showing the reverse Hall-Petch effect. However, the fracture strain decreases in the same region, while the ultimate tensile strength (UTS) rather shows slight increasing behavior. We found that the polycrystalline graphene shows the reverse Hall-Petch effect over the simulated domain of grain size (< 40 nm).

Molecular dynamics study of the tensile mechanical properties of polycrystalline graphene

Grain boundaries (GBs) are known to have an important influence on material properties, so understanding how GBs in graphene change its physical properties is important both scientifically and technologically. In this paper, we perform a series of molecular dynamics simulations to investigate the energies, mechanical properties and fracture process of 29 graphene GBs (symmetric and nonsymmetric) under tensile strains. With different arrangements of the pentagonal and heptagonal rings, the misorientation angle () ranges from 3.5 to 27.8. The GBs defects in graphene can produce a pre-strain that will lead to an increase of the energy of GBs. We study the atomic energy distribution around GBs and define a new parameter: single defect energy (Esingle) to calculate the average energy per GBs defect. It is found that Esingle shows a clear linear relation between and defect density (), because pre-strain filed can be cancelled out locally with the increase of defect density. And this pre-stain can reduce the strength of the C-C bond contained in GBs defects. Hence, with very few exceptions, mechanical failure always starts from the defective region. Furthermore, the energy of GBs can be used to reflect the strength of GBs indirectly. The simulated results show that the tensile strength of GBs is linearly related to the highest atomic energy (Emax), and it also depends on Esingle monotonically. Owing to the pre-strain, load distribution along GBs is uneven. Because some bonds are stretched while others are compressed, that is, the real number of bearing carbon bonds is less than the nominal number. Therefore, at the beginning of tension, the Young's modulus of polycrystalline graphene is significantly lower than that of the monocrystal one. But with the increase of strain, it becomes comparable to that of the monocrystal graphene at sufficiently large strain. The results of fracture process indicate that formation and propagation of crack are both dependent on strength GBs. For low GB strength, the fracture mechanism is transgranular fracture in the form of direct fracture of C-C bonds. When stress reaches a critical value, the weakest C-C bonds in GBs will breakdown and form a fracture site. Because of the uneven bearing condition, the C-C bonds in front of the crack possess considerable residual strength and could prevent crack from propagating. As a result, many other fracture sites in the GBs defect can be produced with the increase of strain, and finally, these sites emerge gradually along GBs and form a sawtooth crack. In contrast, the fracture process of high strength GBs is always accompanied with the variations of Stone-Wales (S-W) transformation and direct fracture of C-C bonds. Once the fracture site forms, the crack will propagate rapidly predominantly along armchair or zigzag direction and finally could cross GBs, this process can be called intergranular fracture. Our present work provides fundamental guidance for understanding how defects affect the mechanical behaviour, which is important for further research and application of graphene devices.

Computational study on the effects of annealing on the mechanical properties of polycrystalline graphene

The feasibility of mass production of polycrystalline graphene (polygraphene) has motivated intensive efforts to boost its promising applications in electronics, energy storage, composites and biomedicine. As the grain boundaries in graphene-related 2D materials can strongly affect their material properties and the performance of graphene-based devices, it is highly desirable to better understand the effects of grain boundaries and make them as stable as possible. Here we employ molecular dynamics simulation to explore the annealing process of polygraphene and compare mechanical properties of annealed and unannealed polygraphene to pristine graphene under the same conditions, with a focus on the stability and energy of the grain boundaries. It is shown that the annealing process has a strong effect on the strength and stiffness of polygraphene due to the rearrangement and stabilization of the grain boundaries. However, our results show that irrespective of grain size, the annealing process makes polygraphene both tougher and stiffer, with a higher Young’s modulus, strength, and ultimate strain under a tensile test compared to the same samples prior to the annealing process.

Effects of grain size, temperature and strain rate on the mechanical properties of polycrystalline graphene – A molecular dynamics study

We investigate the mechanical properties of polycrystalline graphene deforming under uniaxial tension by using molecular dynamics simulations, focusing on the effects of grain size, temperature and strain rate. It is found that polycrystalline graphene with smaller grain size shows a larger drop of Young’s modulus and fracture strength. For the polycrystalline graphene with grain size of 7.5nm, the Young’s modulus and fracture strength is about 80% and 40% that of single-crystalline graphene, respectively. Our simulation results also reveal that the Young’s modulus and fracture strength of polycrystalline graphene are more sensitive to the changes of temperature and strain rate than that of single-crystalline graphene. When temperature increases from 100 to 1200K, the fracture strength of polycrystalline graphene reduces by around 45%. At room temperature (300K), the fracture strength of polycrystalline graphene increases by 10% as the strain rate increases from 5×10−5 to 5×10−3ps−1. Furthermore, the strain rate has a stronger influence on the fracture strength of polycrystalline graphene at a higher temperature than at a lower temperature; while the temperature has a stronger influence at a lower strain rate than at a higher strain rate. Our study thus provides a comprehensive understanding of the mechanical properties of polycrystalline graphene.

On the failure load and mechanism of polycrystalline graphene by nanoindentation.

Nanoindentation has been recently used to measure the mechanical properties of polycrystalline graphene. However, the measured failure loads are found to be scattered widely and vary from lab to lab. We perform molecular dynamics simulations of nanoindentation on polycrystalline graphene at different sites including grain center, grain boundary (GB), GB triple junction, and holes. Depending on the relative position between the indenter tip and defects, significant scattering in failure load is observed. This scattering is found to arise from a combination of the non-uniform stress state, varied and weakened strengths of different defects, and the relative location between the indenter tip and the defects in polycrystalline graphene. Consequently, the failure behavior of polycrystalline graphene by nanoindentation is critically dependent on the indentation site, and is thus distinct from uniaxial tensile loading. Our work highlights the importance of the interaction between the indentation tip and defects, and the need to explicitly consider the defect characteristics at and near the indentation site in polycrystalline graphene during nanoindentation.

Inverse pseudo Hall-Petch relation in polycrystalline graphene.

Understanding the grain size-dependent failure behavior of polycrystalline graphene is important for its applications both structurally and functionally. Here we perform molecular dynamics simulations to study the failure behavior of polycrystalline graphene by varying both grain size and distribution. We show that polycrystalline graphene fails in a brittle mode and grain boundary junctions serve as the crack nucleation sites. We also show that its breaking strength and average grain size follow an inverse pseudo Hall-Petch relation, in agreement with experimental measurements. Further, we find that this inverse pseudo Hall-Petch relation can be naturally rationalized by the weakest-link model, which describes the failure behavior of brittle materials. Our present work reveals insights into controlling the mechanical properties of polycrystalline graphene and provides guidelines for the applications of polycrystalline graphene in flexible electronics and nano-electronic-mechanical devices.

Hydrogenated Grain Boundaries Control the Strength and Ductility of Polycrystalline Graphene

In the growth of polycrystalline graphene via chemical vapor deposition, grain boundaries (GBs) were shown to be energetically favorable sites for hydrogenation. Thus, it is of both scientific interest and technological significance to understand how hydrogenation on GBs affects the mechanical properties of polycrystalline graphene. Here we perform molecular dynamics simulations to investigate the mechanical properties of hydrogenated polycrystalline graphene. Our simulations reveal that the fracture strength of hydrogenated polycrystalline graphene is significantly reduced by the combined weakening effect of bond prestraining in highly defective GBs and sp3 hybridization of hydrogenated GB atoms. In addition, this reduction in fracture strength due to GB hydrogenation is observed in polycrystalline graphene samples of different grain sizes ranging from 2.5 to 10 nm. Our findings show that the loss of mechanical strength due to GB hydrogenation must be taken into account in the application of polycrystall...

Charge transport in polycrystalline graphene: challenges and opportunities.

Graphene has attracted significant interest both for exploring fundamental science and for a wide range of technological applications. Chemical vapor deposition (CVD) is currently the only working approach to grow graphene at wafer scale, which is required for industrial applications. Unfortunately, CVD graphene is intrinsically polycrystalline, with pristine graphene grains stitched together by disordered grain boundaries, which can be either a blessing or a curse. On the one hand, grain boundaries are expected to degrade the electrical and mechanical properties of polycrystalline graphene, rendering the material undesirable for many applications. On the other hand, they exhibit an increased chemical reactivity, suggesting their potential application to sensing or as templates for synthesis of one-dimensional materials. Therefore, it is important to gain a deeper understanding of the structure and properties of graphene grain boundaries. Here, we review experimental progress on identification and electrical and chemical characterization of graphene grain boundaries. We use numerical simulations and transport measurements to demonstrate that electrical properties and chemical modification of graphene grain boundaries are strongly correlated. This not only provides guidelines for the improvement of graphene devices, but also opens a new research area of engineering graphene grain boundaries for highly sensitive electro-biochemical devices.

Atomistic modeling of mechanical properties of polycrystalline graphene

We performed molecular dynamics (MD) simulations to investigate the mechanical properties of polycrystalline graphene. By constructing molecular models of ultra-fine-grained graphene structures, we studied the effect of different grain sizes of 1–10 nm on the mechanical response of graphene. We found that the elastic modulus and tensile strength of polycrystalline graphene decrease with decreasing grain size. The calculated mechanical proprieties for pristine and polycrystalline graphene sheets are found to be in agreement with experimental results in the literature. Our MD results suggest that the ultra-fine-grained graphene structures can show ultrahigh tensile strength and elastic modulus values that are very close to those of pristine graphene sheets.

20101 引張荷重下における多結晶グラフェンの力学特性解析

20101 引張荷重下における多結晶グラフェンの力学特性解析

1528 多結晶グラフェンの力学的特性解析

1528 多結晶グラフェンの力学的特性解析

Mechanical properties of polycrystalline graphene based on a realistic atomistic model

Graphene can at present be grown at large quantities only by the chemical vapor deposition method, which produces polycrystalline samples. Here, we describe a method for constructing realistic polycrystalline graphene samples for atomistic simulations, and apply it for studying their mechanical properties. We show that cracks initiate at points where grain boundaries meet and then propagate through grains predominantly in zigzag or armchair directions, in agreement with recent experimental work. Contrary to earlier theoretical predictions, we observe normally distributed intrinsic strength (~ 50% of that of the mono-crystalline graphene) and failure strain which do not depend on the misorientation angles between the grains. Extrapolating for grain sizes above 15 nm results in a failure strain of ~ 0.09 and a Young's modulus of ~ 600 GPa. The decreased strength can be adequately explained with a conventional continuum model when the grain boundary meeting points are identified as Griffith cracks.

Intrinsic Strength and Failure Behaviors of Graphene Grain Boundaries

As one-dimension line defects, grain boundaries (GBs) can affect many intrinsic properties of graphene. In this paper, the mechanical properties of 20 representative graphene grain boundaries were studied using density functional theory and molecular dynamics. With different arrangements of the pentagonal and heptagonal rings, the grain boundary may remain flat or become inflected up to 72°. For the flat GBs, the intrinsic tensile strength decreases linearly with the formation energy with a maximum value of 93 GPa, close to that of a perfect graphene. The intrinsic tensile strength of the inflected GBs is found to generally decrease with increasing inflection angle. Stone-Wales transformation is identified as the major failure mechanism of graphene GBs at high temperatures, whereas the initial fracture site can be either on the boundary line or inside the domain. These theoretical results constitute a useful picture of the grain boundary effect on the mechanical properties of polycrystalline graphene.