Yuli Chen

Mechanics of nanoscale wrinkling of graphene on a non-developable surface

As a two-dimensional crystal, the graphene sheet is often used with substrate materials because the freestanding graphene tends to corrugate and can hardly display its extraordinary properties in devices. However, the substrate is rarely perfectly-flat, but has microscopic roughness. Whether or not the graphene sheet can conform fully to a substrate with nano- and micro-roughness is essential for the performance of the graphene-based devices. In this paper, a theoretical model is developed to predict the morphology of a monolayer graphene sheet attaching to the substrate with microscopic non-developable roughness by an energy-based analysis. The final graphene morphology is revealed to result from the competition between two energy terms: the adhesion energy between graphene and substrate, and the strain energy stored in the graphene due to the deformations. Thus, by accounting for these two parts of energy, the critical condition to predict the morphology conversion from full conformation to wrinkling is established, which agrees well with the results of molecular dynamics simulations. This study has significant meanings for design and fabrication of high quality nanostructured coatings on substrates with complex surfaces, and can offer a guide for designing new functional graphene electronical devices such as nano-sensors and nano-switches as well.

A micromechanics-based degradation model for composite progressive damage analysis:

A new material degradation model only with fundamental material properties required is proposed for composite progressive damage analysis based on micromechanics. For different failure modes, the effects of fiber and/or matrix damage on the composite material properties are explored, from which the material degradation factors for these failure modes are deduced. The material degradation model is then implemented for progressive damage analyses, using user subroutines in the commercial code ABAQUS®, accompanying with a modified Hashin type failure criterion and finite element models for six commonly used double-lap composite bolted joints with various layups, geometry dimensions, and fasteners. The numerical predictions of failure loads, failure patterns, and load–displacement curves are compared with results obtained from static tests and further ultrasonic C-scan detection. Good agreements between numerical failure predictions and experimental outcomes indicate the effectiveness and suitability of the proposed model for progressive damage analyses of composite bolted joints.

Robustly Engineering Thermal Conductivity of Bilayer Graphene by Interlayer Bonding.

Graphene and its bilayer structure are the two-dimensional crystalline form of carbon, whose extraordinary electron mobility and other unique features hold great promise for nanoscale electronics and photonics. Their realistic applications in emerging nanoelectronics usually call for thermal transport manipulation in a controllable and precise manner. In this paper we systematically studied the effect of interlayer covalent bonding, in particular different interlay bonding arrangement, on the thermal conductivity of bilayer graphene using equilibrium molecular dynamics simulations. It is revealed that, the thermal conductivity of randomly bonded bilayer graphene decreases monotonically with the increase of interlayer bonding density, however, for the regularly bonded bilayer graphene structure the thermal conductivity possesses unexpectedly non-monotonic dependence on the interlayer bonding density. The results suggest that the thermal conductivity of bilayer graphene depends not only on the interlayer bonding density, but also on the detailed topological configuration of the interlayer bonding. The underlying mechanism for this abnormal phenomenon is identified by means of phonon spectral energy density, participation ratio and mode weight factor analysis. The large tunability of thermal conductivity of bilayer graphene through rational interlayer bonding arrangement paves the way to achieve other desired properties for potential nanoelectronics applications involving graphene layers.

Strong Surface Orientation Dependent Thermal Transport in Si Nanowires

Thermoelectrics, which convert waste heat to electricity, offer an attractive pathway for addressing an important niche in the globally growing landscape of energy demand. Research to date has focused on reducing the thermal conductivity relative to the bulk. Si nanowires (NWs) have received exceptional attention due to their low-dimensionality, abundance of availability, and high carrier mobility. From thermal transport point of view, the thermal conductivity of Si NWs strongly depends on the detailed surface structure, such as roughness and surface orientation. Here, direct molecular dynamics simulations and theoretical models are used to investigate the thermal transport in Si NWs with diverse surface orientations. Our results show that the thermal conductivity of Si NWs with different surface orientation can differ by as large as 2.7~4.2 times, which suggests a new route to boost the thermoelectric performance. Using the full spectrum theory, we find that the surface orientation, which alters the distribution of atoms on the surface and determines the degree of phonon coupling between the core and the surface, is the dominant mechanism. Furthermore, using spectral thermal conductivity, the remarkable difference in the thermal conductivity for different surface orientation is found to only stem from the phonons in the medium frequency range, with minor contribution from low and high frequency phonons.

The morphology of graphene on a non-developable concave substrate

The performances of graphene sheet in micro- and nano-electronics and devices are significantly affected by its morphology, which depends on the surface features of the supporting substrate. The substrates with non-developable concave surface are widely used with graphene sheet in applications but rarely studied. Therefore, a theoretical model is established based on the energy analysis to explain the adhesion mechanisms and predict the morphology of the graphene sheet on a non-developable concave surface. Four different morphologies of the graphene sheet are revealed, and the critical conditions are established to predict which morphology the graphene/substrate system belongs to. For the monolayer graphene sheets much larger than the concave of substrate, the final equilibrium morphology is dominated by the half cone angle of the concave. The graphene sheet conforms completely to the SiO2 substrate if the half cone angle is less than 27.5° and spans over the concave if the angel is larger than 27.5°. For...

Size effect on interlayer shear between graphene sheets

Interlayer shear between graphene sheets plays an important role in graphene-based materials and devices, but the effect of in-plane deformation of graphene, which may depend on the graphene size, has not been fully understood. In this paper, the size effect on interlayer shear behavior between two graphene sheets is studied based on a non-linear shear-lag model with energy barrier analysis, in which both the lattice registry effect and the elastic deformation of graphene are taken into account, and molecular dynamics (MD) simulations are carried out to verify the model. Both theoretical prediction and MD simulations show that the maximum interlayer shear force of short graphene sheets increases with the graphene length and width. However, if the sheet length is beyond 20u2009nm, the maximum shear force cannot be further increased by increasing the graphene length due to the non-uniform relative displacement between graphene layers, which is caused by the in-plane deformation of graphene. The upper bound of t...