Exploring and understanding the structure‐mechanical property relations in hierarchical graphene assemblies is crucial for optimizing their mechanical properties and developing new functionalities, ...as the tensile strength is two orders of magnitude degradation from pristine graphene to graphene assemblies. Yet, quantifying the strength degradation across multiscale is a challenge due to the complex hierarchical structures. Thus, key structures and dominated factors at different lengthscales that affect the mechanical properties of graphene assemblies should be extracted for the reasonable unveiling of this problem. In this study, the multiscale mechanical degradation of graphene assemblies through practical microstructure‐guided multiscale modeling is characterized. Combining with experimental observations, three representative models are developed to study the mechanical behaviors of graphene assemblies at different lengthscales. Then, the dominated factors affecting the strength at these lengthscales are identified, that is the defects in monolayer graphene, tension‐shear load transfer for stacked graphene, and uniformity of graphene assemblies. Based on the simulation results, the structure‐strength relation of graphene assemblies is given, and practical strategies are proposed followed by experimental realization, to significantly improve the mechanical properties of graphene‐based nanocomposites.
In this study, the multiscale mechanical degradation of graphene assemblies is characterized through practical microstructure‐guided multiscale modeling. Three representative models are developed to study the mechanical behaviors of graphene assemblies at different lengthscales. The structure‐strength relation of graphene assemblies is given, and practical strategies are proposed followed by experimental realization, to significantly improve the mechanical properties of graphene‐based nanocomposites.
Monolayer molybdenum disulfide (MoS2), a new two-dimensional material beyond graphene, has attracted tremendous attention recently. Its applications in nanoelectronic and thermoelectric devices ...usually require manipulating the thermal transport properties. Using nonequilibrium molecular dynamics simulations, we investigated the effects of lattice defects and mechanical strain on the thermal conductivity of MoS2. We found that the thermal conductivity of monolayer MoS2 can be effectively tuned by introducing even a small amount of lattice defects. For example, a 0.5% concentration of mono-Mo vacancies is able to reduce the thermal conductivity by about 60%. Remarkably, the thermal conductivity of the defected sample can further be tuned by mechanical strain. For example, a 12% tensile strain is able to reduce the thermal conductivity by another 60%. We also found that the tensile strain exerts almost the same impact on the thermal conductivity of both pristine and defective MoS2, which signifies that there is no apparent coupling between defects and strain in affecting the thermal conductivity. Our analyses of the vibrational density of state and spectral energy density show that the underlying mechanisms for these drastic changes are (1) the reduction of the phonon relaxation time arising from phonon–defect scattering and (2) the reduction of the group velocity and heat capacity caused by tensile strain. Our findings here provide important insights and guidelines for the use of monolayer MoS2 in thermal management and thermoelectric devices.
Owing to the superior thermal properties of graphene, graphene-reinforced polymer nanocomposites hold great potential as the thermal interface materials (TIMs) dissipating heat for electronic ...packages. However, this application is greatly hindered by the high thermal resistance at the interface between graphene and polymer. In this paper, some important aspects of the improvement of the thermal transport across the interface between graphene and epoxy in graphene–epoxy nanocomposites, including the effectiveness of covalent and noncovalent functionalization, isotope doping, and acetylenic linkage in graphene are systematically investigated using molecular dynamics (MD) simulations. The simulation results show that the covalent and noncovalent functionalization techniques could considerably reduce the graphene–epoxy interfacial thermal resistance in the nanocomposites. Among different covalent functional groups, butyl is more effective than carboxyl and hydroxyl in reducing the interfacial thermal resistance. Different noncovalent functional molecules, including 1-pyrenebutyl, 1-pyrenebutyric acid, and 1-pyrenebutylamine, yield a similar amount of reductions. Moreover, it is found that the graphene–epoxy interfacial thermal resistance is insensitive to the carbon isotope doping in graphene, while it can be reduced moderately by replacing the sp 2 bonds in graphene with acetylenic linkages.
Entropy is a universal concept across the physics of mixtures. While the role of entropy in other multicomponent materials has been appreciated, its effects in polymers and plastics have not. In this ...work, it is demonstrated that the seemingly small mixing entropy contributes to the miscibility and performance of polymer alloys. Experimental and modeling studies on over 30 polymer pairs reveal a strong correlation between entropy, morphology, and mechanical properties, while elucidating the mechanism behind: in polymer blends with weak interactions, entropy leads to homogeneously dispersed nanosized domains stabilized by highly entangled chains. This unique microstructure promotes uniform plastic deformation at the interface, thus improving the toughness of conventional brittle polymers by 1–2 orders of magnitude without sacrificing other properties, analogous to high‐entropy metallic alloys. The proposed strategy also applies to ternary polymer systems and copolymers, offering a new pathway toward the development of sustainable polymers.
Common brittle plastics are toughened by 10–100 folds by simple mixing. They could withstand tensile deformations up to 500% and high loads up to 60 MPa without fracture, due to entropy‐driven dense entanglements. This finding provides new design strategy for high‐performance sustainable polymer materials.
We investigate the thermal conductivity of hydrogenated graphene using non-equilibrium molecular dynamics simulations. It is found that the thermal conductivity greatly depends on the hydrogen ...distribution and coverage. For random hydrogenation, the thermal conductivity decreases rapidly with increasing coverage up to about 30%. Beyond this limit, however, the thermal conductivity is almost insensitive to the coverage. For patterned hydrogenation with stripes parallel to the heat flux, the thermal conductivity decreases gradually with increasing coverage from 0% to 100%. In contrast, when the stripe direction is perpendicular to the heat flux, a small (5%) coverage causes a sharp (60%) drop of thermal conductivity. The deterioration of thermal conductivity is due to the sp
2-to-sp
3 bonding transition upon hydrogenation, which softens the G-band phonon modes. Percolation theory can be used to explain the variation of thermal conductivity at different hydrogenation distributions and coverages. The applicability of the rule of mixtures in predicting the thermal conductivity is also discussed. The work suggests that hydrogenation is a possible route to tune graphene thermal conductivity and manage heat dissipation in graphene-based nanoelectronic devices.
•Interfacial thermal transport of multilayer GBN heterostructures are analyzed by NEMD.•The tunable ITC of the heterostructure is achieved by modifying interface morphologies.•Interface engineering ...for composition diffusion, defect density and distribution patterns, hydrogenation, and isotope.•ITC can be improved significantly by hydrogenation and composition diffusion.
The combination of dissimilar materials to build heterostructures has opened up new possibilities for the use of graphene in a variety of applications. Among these, the stacking of graphene and h-BN into graphene/h-BN (GBN) van der Waals (vdW) heterostructures is particularly promising for use as thermal interface materials and underfill materials in new electronic devices because of their remarkable thermal properties and electrical insulation endowed by h-BN if it encapsulates graphene. The heat dissipation performance of these GBN heterostructures depends heavily on the interfacial thermal conductance (ITC) of graphene and h-BN. Herein we explore the heat transfer across multilayer GBN vdW heterostructures by nonequilibrium molecular dynamics simulations and put emphasis on the interface morphology. The tunable ITC of the heterostructure can be achieved by modifying the interfacial layers through composition diffusion, vacancy density and pattern, hydrogenation, and isotope. The simulation results demonstrate that hydrogenation is the most efficient approach to tailor the ITC with a large variation, followed by the composition diffusion. Composition diffusion can induce a stronger interaction between graphene and h-BN, resulting in a 70% improvement in the ITC of the GBN vdW heterostructure. This study offers valuable perspectives to the physical mechanisms behind the interface morphology-dependent ITC, and can facilitate the design of novel GBN vdW heterostructures for efficient heat dissipation.
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The recently fabricated monolayer MoS2–WSe2 lateral heterostructures are promising for many interesting applications, such as p–n diodes, photodetectors, transistors, sensors, light-emitting diodes ...and thermoelectric and flexible nanodevices. In this work, we study the mechanical and thermal properties of MoS2–WSe2 lateral heterostructures by using molecular dynamics (MD) simulations based on the recently parameterized Stillinger-Weber (SW) potential. It is found that the fracture strength and fracture strain of MoS2–WSe2 lateral heterostructures are dictated by the mechanical properties of MoS2, and are very sensitive to temperature. However, when a crack is introduced in the MoS2–WSe2 heterostructures, failure may occur either in MoS2 or WSe2, depending on the crack length and location. Interestingly, the fracture strengths obtained from our MD simulations are in agreement with those obtained from the Griffith theory. Our MD simulations further reveal that, in addition to the low thermal conductivities of MoS2 and WSe2, the MoS2–WSe2 heterojunctions exhibit a very low interfacial thermal conductance, which is about one order of magnitude lower than that of graphene–hBN heterojunctions.
Three-dimensional hexagonal boron nitride aerogels (hBNAGs) are novel porous materials with many promising applications such as energy storage, thermal insulation and sensing. However, the ...structure-property relationships of hBNAGs in complicated thermo-mechanical coupled environments are still not clear. In this study, we employed a binary phase-field crystal (PFC) model to construct the atomic structures of hBNAGs, upon which the mechanical and thermal behaviors of hBNAGs were systematically investigated using large-scale atomistic simulations. It is found that the hBNAG geometry and topological defects strongly affect the mechanical and thermal properties. For example, the Young's modulus and tensile strength follow the scaling laws of mass density with a power factor of about 1.4 and 1.2, respectively, indicating that the stretching and bending combine toward tensile deformation. In addition, cracks nucleate around the octagon defects, indicating that the tensile strength is also influenced by the topological defects. Under compression, complicated crumpled deformations and ridges in the entire region are observed and the compression strength follows the scaling law of mass density with a power factor above 2.0, which means that a large portion of the hBNAGs do not contribute to the compression load bearing. We find that hBNAGs have a very low thermal conductivity of about two orders of magnitude lower than that of a hBN sheet. Also, the thermal conductivity of hBNAGs increases with increasing mass density, which also follows a scaling law. The power of the scaling law is about 0.5, indicating that the thermal conductivity has a strong nonlinear dependence on the mass density. Our work provides a deep understanding of the structure-property relationships of hBNAGs, which is useful for the engineering applications of hBNAGs.
The mechanical and thermal behaviors of three-dimensional hexagonal boron nitride aerogels (hBNAGs) are studied using molecular dynamics simulations based on a binary phase-field crystal (PFC) model.
As a new two-dimensional (2D) material, phosphorene has drawn growing attention owing to its novel electronic properties, such as layer-dependent direct bandgaps and high carrier mobility. Herein we ...investigate the in-plane and cross-plane thermal conductivities of single- and multi-layer phosphorene, focusing on geometrical (sample size, orientation and layer number) and strain (compression and tension) effects. A strong anisotropy is found in the in-plane thermal conductivity with its value along the zigzag direction being much higher than that along the armchair direction. Interestingly, the in-plane thermal conductivity of multi-layer phosphorene is insensitive to the layer number, which is in strong contrast to that of graphene where the interlayer interactions strongly influence the thermal transport. Surprisingly, tensile strain leads to an anomalous increase in the in-plane thermal conductivity of phosphorene, in particular in the armchair direction. Both the in-plane and cross-plane thermal conductivities can be modulated by external strain; however, the strain modulation along the cross-plane direction is more effective and thus more tunable than that along the in-plane direction. Our findings here are of great importance for the thermal management in phosphorene-based nanoelectronic devices and for thermoelectric applications of phosphorene.
We investigated the in-plane and cross-plane thermal conductivities of single and multi-layer phosphorene using non-equilibrium molecular dynamics simulations.
Bismuth telluride (Bi2Te3), a thermoelectric material, has gained tremendous attention for its high thermoelectric figure of merit (ZT) at room temperature. Recent experimental studies have ...demonstrated that incorporating graphene into Bi2Te3 can further enhance the ZT by reducing the thermal conductivity and increasing the power factor of the Bi2Te3/graphene composites. The interfacial thermal resistance (ITR) between Bi2Te3 and graphene plays a crucial role in determining the thermal conductivities of these composites. In the present study, we investigate the ITR between Bi2Te3 and graphene using nonequilibrium molecular dynamics simulations. We systematically explore the effects of graphene layer number, defects in graphene, and mechanical strain on the ITR. Our results reveal that the ITR increases with an increase in the number of graphene layers, reaching a stabilized value at five layers. Furthermore, the presence of vacancy defects in graphene reduces the ITR, with a higher defect density leading to greater reduction in the ITR. Remarkably, the mechanical strain has a profound impact on the ITR. At a tensile strain of 2.5%, the ITR more than quadruples, while a compressive strain of 4% reduces the ITR by about 60% compared to the strain-free condition. These findings offer valuable insights for manipulating the ITR in Bi2Te3/graphene composites with enhanced thermoelectric performance.
