Yilun Liu

Observation of microscale superlubricity in graphite.

Through experimental study, we reveal superlubricity as the mechanism of self-retracting motion of micrometer sized graphite flakes on graphite platforms by correlating respectively the lock-up or self-retraction states with the commensurate or incommensurate contacts. We show that the scale-dependent loss of self-retractability is caused by generation of contact interfacial defects. A HOPG structure is also proposed to understand our experimental observations, particularly in term of the polycrystal structure. The realisation of the superlubricity in micrometer scale in our experiments will have impact in the design and fabrication of micro/nanoelectromechanical systems based on graphitic materials.

Mechanical properties of graphene papers

Abstract Graphene-based paper materials attract particular interests recently owing to their outstanding properties, the key of which is their layer-by-layer hierarchical structures similar to many biological materials such as bone, teeth and nacre, combining intralayer strong sp2 bonds and interlayer crosslinks for efficient load transfer. Here we firstly study the mechanical properties of various interlayer and intralayer crosslinks through first-principles calculations, and then perform continuum model analysis for the overall mechanical properties of graphene-based paper materials. We find that there is a characteristic length scale l0, defined as D h 0 / 4 G , where D is the stiffness of the graphene sheet, h0 and G are height of interlayer crosslink and shear modulus respectively. When the size of the graphene sheets exceeds 3l0, the tension–shear (TS) chain model, which is widely used for nanocomposites, fails to predict the overall mechanical properties of the graphene-based papers. Instead we proposed here a deformable tension–shear (DTS) model by considering elastic deformation of graphene sheets, also the interlayer and intralayer crosslinks. The DTS is then applied to predict the mechanical properties of graphene papers under tensile loading. According to the results we thus obtain, optimal design strategies are proposed for graphene papers with ultrahigh stiffness, strength and toughness.

Water transport inside carbon nanotubes mediated by phonon-induced oscillating friction

The emergence of the field of nanofluidics in the last decade has led to the development of important applications including water desalination, ultrafiltration and osmotic energy conversion. Most applications make use of carbon nanotubes, boron nitride nanotubes, graphene and graphene oxide. In particular, understanding water transport in carbon nanotubes is key for designing ultrafiltration devices and energy-efficient water filters. However, although theoretical studies based on molecular dynamics simulations have revealed many mechanistic features of water transport at the molecular level, further advances in this direction are limited by the fact that the lowest flow velocities accessible by simulations are orders of magnitude higher than those measured experimentally. Here, we extend molecular dynamics studies of water transport through carbon nanotubes to flow velocities comparable with experimental ones using massive crowd-sourced computing power. We observe previously undetected oscillations in the friction force between water and carbon nanotubes and show that these oscillations result from the coupling between confined water molecules and the longitudinal phonon modes of the nanotube. This coupling can enhance the diffusion of confined water by more than 300%. Our results may serve as a theoretical framework for the design of new devices for more efficient water filtration and osmotic energy conversion devices.

Mechanics of carbon nanotube networks: microstructural evolution and optimal design

Carbon nanotube networks feature outstanding mechanical performance, and also hierarchical structures and network topologies. In this paper we investigate their structure–property relationship through mesoscale molecular dynamics simulations. We find that their microstructures undergo remarkable evolution under mechanical loads. The correlation between applied strain, microstructural evolution and failure mechanism, especially the bundling process and evolution of bridging carbon nanotubes, is discussed based on the simulation results. Based on the insights of the underlying mechanisms, further engineering approaches on the carbon nanotube networks towards enhanced mechanical properties are proposed and validated, e.g., by including intertube cross-links that resist shear, maintain the network topology and improve strain affinity.

Mechanical properties of nanoporous graphene membrane

Nanoporous graphene holds great promise in the application of filtration such as seawater desalination, gas separation, and ionic channels. In this paper, we study the mechanical properties of nanoporous graphene with different size, shape, and density of nanopore. The strength decreases as the size and porosity of the nanopore increases. However, the rough edges of the nanopore has significant influence to the strength where the blunt tip perpendicular to the loading direction has higher strength. The effective tensile modulus is only determined by porosity of the nanopore as ΔE ∼ -p0.64, while the strength is determined by the size, shape, and porosity of the nanopore, for the same type of nanopore the strength scales with the porosity as Δσs ∼ −p. In contrast, the effective fracture strain increases as porosity increases for small and moderate porosities. The work is a first study of the relation between mechanical properties and porosity of nanoporous graphene and is helpful to the design of high perf...

Mechanics of coordinative crosslinks in graphene nanocomposites: a first-principles study

Graphene-based nanocomposites have recently received rising interest owing to their outstanding performance. Crosslinkers, such as a divalent ion based coordinative complex, have been proved to significantly improve load transfer between adjacent graphene sheets, which subsequently defines the overall mechanical properties of the composites. In this paper, the structures and mechanical properties of both interlayer and intralayer coordinative bonds are quantified through density functional theory based first-principles calculations. The structural deformation and failure mechanism are investigated under interlayer sliding and separation loads. Extensive discussion is made by comparing coordinative bonds with other types of crosslinks such as covalent bonds, van der Waals interactions and hydrogen bonds. Moreover, the impacts from the crosslink mechanisms to overall properties of nanocomposites are projected towards the applications in high-performance multifunctional materials.

Interfacial shear strengths between carbon nanotubes

Interfacial shear strengths or static frictions between carbon nanotubes (CNT) in contact at different cross angles are studied by using atomic mechanics. It is shown that the axial interfacial shear strengths between parallel CNTs in commensurate are two orders of magnitude greater than those in incommensurate. This strong chiral dependence is not surprising and is similar to that of the friction between two graphite basal planes. In contrast, we find that the interfacial shear strengths of crossly contacted CNT pairs are much less dependent upon chirality. The estimated values of interfacial shear strengths, ranging from 0.05 to 0.35 GPa, agree very well with experimentally measured results available in the literature. These results may thus be used as a basis for explaining the observed tension strengths of CNT bundles and films that are mainly bonded by van der Waals interactions and the mechanical behaviors of composite materials with highly concentrated CNTs.

A graphite nanoeraser

We present here a method for cleaning intermediate-size (up to 50 nm) contamination from highly oriented pyrolytic graphite and graphene. Electron-beam-induced deposition of carbonaceous material on graphene and graphite surfaces inside a scanning electron microscope, which is difficult to remove by conventional techniques, can be removed by direct mechanical wiping using a graphite nanoeraser, thus drastically reducing the amount of contamination. We discuss potential applications of this cleaning procedure.

The high-speed sliding friction of graphene and novel routes to persistent superlubricity

Recent experiments on microscopic graphite mesas demonstrate reproducible high-speed microscale superlubricity, even under ambient conditions. Here, we explore the same phenomenon on the nanoscale, by studying a graphene flake sliding on a graphite substrate, using molecular dynamics. We show that superlubricity is punctuated by high-friction transients as the flake rotates through successive crystallographic alignments with the substrate. Further, we introduce two novel routes to suppress frictional scattering and achieve persistent superlubricity. We use graphitic nanoribbons to eliminate frictional scattering by constraining the flake rotation, an approach we call frictional waveguides. We can also effectively suppress frictional scattering by biaxial stretching of the graphitic substrate. These new routes to persistent superlubricity at the nanoscale may guide the design of ultra-low dissipation nanomechanical devices.

The Role of Computer Simulation in Nanoporous Metals—A Review

Nanoporous metals (NPMs) have proven to be all-round candidates in versatile and diverse applications. In this decade, interest has grown in the fabrication, characterization and applications of these intriguing materials. Most existing reviews focus on the experimental and theoretical works rather than the numerical simulation. Actually, with numerous experiments and theory analysis, studies based on computer simulation, which may model complex microstructure in more realistic ways, play a key role in understanding and predicting the behaviors of NPMs. In this review, we present a comprehensive overview of the computer simulations of NPMs, which are prepared through chemical dealloying. Firstly, we summarize the various simulation approaches to preparation, processing, and the basic physical and chemical properties of NPMs. In this part, the emphasis is attached to works involving dealloying, coarsening and mechanical properties. Then, we conclude with the latest progress as well as the future challenges in simulation studies. We believe that highlighting the importance of simulations will help to better understand the properties of novel materials and help with new scientific research on these materials.