Guoxin Xie

Mechanical properties of nanoparticles: basics and applications

The special mechanical properties of nanoparticles allow for novel applications in many fields, e.g., surface engineering, tribology and nanomanufacturing/nanofabrication. In this review, the basic physics of the relevant interfacial forces to nanoparticles and the main measuring techniques are briefly introduced first. Then, the theories and important results of the mechanical properties between nanoparticles or the nanoparticles acting on a surface, e.g., hardness, elastic modulus, adhesion and friction, as well as movement laws are surveyed. Afterwards, several of the main applications of nanoparticles as a result of their special mechanical properties, including lubricant additives, nanoparticles in nanomanufacturing and nanoparticle reinforced composite coating, are introduced. A brief summary and the future outlook are also given in the final part. (Some figures may appear in colour only in the online journal)

Direct Electrochemical Synthesis of Reduced Graphene Oxide (rGO)/Copper Composite Films and Their Electrical/Electroactive Properties

Electrical contact materials with excellent performances are crucial for the development and safe use of electrical contacts in different applications. In our work, reduced graphene oxide (rGO)/copper (Cu) composite films, as potential electrical contact materials, have been synthesized on copper foil with one-step electrochemical reduction deposition method. Cyclic voltammetry (CV) was used to define the deposition conditions, and confocal Raman microscopy (CRM), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to characterize the chemical compositions, molecular and micro- and nano-structures of the composite films. Atomic force microscopy/scanning Kelvin probe force microscopy (AFM/SKPFM), conductive AFM (C-AFM) as well as impedance analysis were employed to evaluate the electroactive/electrical properties of the prepared composite films, respectively. The CRM and XPS results suggest that the rGO/Cu composite films can be synthesized through one-step electrochemical codeposition using suitable precursor solutions. Within a short deposition period, the growth of discrete nanograins in the composite film predominates, whereas pine-tree-leaf nanostructures are formed in the composite film when the deposition period is long, due to the chelating role of GO or rGO to regulate the growth rate of metallic copper nanograins. The electrical resistivity of the composite films is lower than the polished Cu foil and the electrodeposited Cu film, probably due to the higher conductivity (enhanced transfer of charge carriers) of the rGO incorporated in the composite films. The Volta potential variation in the rGO/Cu composite film is quite different from that in the electrodeposited Cu film. The electroactivity of the rGO/Cu composite films is higher than the electrodeposited Cu film, but lower than polished Cu foil, and the underlying mechanisms have been discussed.

Nanoconfined ionic liquids under electric fields

The effect of external electric fields (EEFs) on ionic liquid films confined within a nanogap has been investigated by measuring the film thickness with the thin film interferometry and calculating the effective viscosity. Experimental results indicated that the film thickness of ionic liquids could be increased obviously by the application of EEFs with strengths weaker than the electric interactions between cationic head groups and anions. The effect of EEFs on the confined ionic liquid film with a shorter alkyl side chain is more noticeable. It is thought that the charged anions and headgroups of the cations are structured near electrified walls to form ordered layers and short alkyl side chains at the interfaces are aligned along the EEF direction due to induced dipoles.

Abrasive rolling effects on material removal and surface finish in chemical mechanical polishing analyzed by molecular dynamics simulation

In an abrasive chemical mechanical polishing (CMP) process, materials were considered to be removed by abrasive sliding and rolling. Abrasive sliding has been investigated by many molecular dynamics (MD) studies; while abrasive rolling was usually considered to be negligible and therefore was rarely investigated. In this paper, an MD simulation was used to study the effects of abrasive rolling on material removal and surface finish in the CMP process. As the silica particle rolled across the silicon substrate, some atoms of the substrate were dragged out from their original positions and adhered to the silica particle, leaving some atomic vacancies on the substrate surface. Meanwhile, a high quality surface could be obtained. During the abrasive rolling process, the influencing factors of material removal, e.g., external down force and driving force, were also discussed. Finally, MD simulations were carried out to examine the effects of abrasive sliding on material removal under the same external down force as abrasive rolling. The results showed that the ability of abrasive rolling to remove material on the atomic scale was not notably inferior to that of abrasive sliding. Therefore, it can be proposed that both abrasive sliding and rolling play important roles in material removal in the abrasive CMP of the silicon substrate.

Elastic properties of polystyrene nanospheres evaluated with atomic force microscopy: size effect and error analysis.

The mechanical properties of polystyrene (PS) nanospheres of ca. 50-1000 nm in diameter were evaluated by using an atomic force microscope (AFM). The compressive elastic moduli of individual nanospheres were obtained by analyzing the AFM force-displacement curves on the basis of the Hertz and JKR contact theories. The results showed that the elastic moduli of PS nanospheres of different sizes were in the range of 2-8 GPa. The elastic modulus of PS nanospheres increased with the decrease of the sphere diameter, especially when the diameter was less than 200 nm. The measurement errors due to tip wear and the deformation at the bottom of the sphere were analyzed. Mechanisms for the size dependence on the elastic modulus of PS nanospheres were also discussed.

Effect of surface charge on water film nanoconfined between hydrophilic solid surfaces

The effect of surface charge on the water film confined between hydrophilic solid surfaces has been investigated using thin film interferometry. Experimental results indicate that the higher surface charge density induces the thicker water film for model surfaces, silica, and alumina. It is thought that the strong surface field induced by the surface charge establishes a more ordered hydrogen-bonding network that promotes the forming of thicker water lubrication film between hydrophilic solid surfaces.

Effect of external electric field on liquid film confined within nanogap

A strong and reproducible effect of an external electric field (EEF) on liquid films confined within a nanogap between a highly polished steel ball and a smooth glass disk is described. Induced by the EEF, microbubbles were observed at the edge of the contact region. This phenomenon is more obvious in polar liquid films than that in nonpolar ones. A stronger EEF causes a decrease in the film thickness in the contact region initially, and then the variation becomes much smaller. When the ball is applied with a positive EEF, the emergence of microbubbles is stable over time, while it becomes much less as time progresses. Different mechanisms of these experimental phenomena have been discussed.

Tribological characteristics of aloe mucilage

Abstract Increased concerns about environmental damage caused by many lubricants, has created a growing worldwide trend of promoting new environmentally friendly lubricants. The tribological characteristics of aloe mucilage as a kind of original biolubricant have been investigated in the present work. The experimental results indicate that the variation of the film thickness of aloe mucilage is not the same as that in traditional elastohydrodynamic lubrication, but conforms to the lubrication regime of thin film lubrication under the present experimental conditions. The coefficient of friction (COF) of the aloe mucilage among different tribological pairs is significantly decreased by the increase in velocity, while there is little variation when the normal load is increased. The COF of aloe mucilage between WC and DLC surfaces is very small with a value of 0˙04, and the wear resistance of the aloe mucilage between WC/DLC is better than that between WC/Si and WC/steel.

Water film confined in a nanoscale gap: Surface polarity and hydration effects

The water film nanoconfined between solid surfaces was investigated using thin film interferometry. Experimental results indicate that surface polarity and hydration effects can strongly influence on the property of water film. It is thought that water molecules strongly bind to surface containing hydrophilic groups and a preferential slip plane is created against the adjacent layer between the hydration sheath and bulk water. The slip plane dramatically weakens the entrainment effect in the inlet zone of lubrication by breaking the hydrogen-bonding network.

Electrospreading of dielectric liquid menisci on the small scale

The effects of an external electric field (EEF) on dielectric liquid menisci formed in a small gap between a smooth plate and a high precision steel ball have been investigated. It was found that thin spreading films were pulled out and moved away from the menisci of low permittivity dielectric liquids after exposure to the EEF. During the initial period of spreading, a strong “electric wind” due to the gas discharge in the vicinity of the three-phase contact line (TCL) of the liquid meniscus was observed. However, such electrospreading phenomena were absent for the menisci of conductive liquids which were commonly used in classical electrowetting studies. It is deduced that the spreading of liquid menisci under EEFs is driven by the thermocapillary force near the TCL.