Dry tribology and nanomechanics of gaseous flame soot in comparison with carbon black and diesel soot

Ethylene gas is burnt and the carbon soot particles are thermophoretically collected using a home-built equipment where the fuel air injection and intervention into the 7.5-cm long flame are controlled using three small pneumatic cylinders and computer-driven controllers. The physical and mechanical properties and tribological performance of the collected soot are compared with those of carbon black and diesel soot. The crystalline structures of the nanometric particles generated in the flame, as revealed by high-resolution transmission electron studies, are shown to vary from the flame root to the exhaust. As the particle journeys upwards the flame, through a purely amorphous coagulated phase at the burner nozzle, it leads to a well-defined crystalline phase shell in the mid-flame zone and to a disordered phase consisting of randomly distributed short-range crystalline order at the exhaust. In the mid-flame region, a large shell of radial-columnar order surrounds a dense amorphous core. The hardness and wear resistance as well as friction coefficient of the soot extracted from this zone are low. The mechanical properties characteristics of this zone may be attributed to microcrystalline slip. Moving towards the exhaust, the slip is inhibited and there is an increase in hardness and friction compared to those in the mid-flame zone. This study of the comparison of flame soot to carbon black and diesel soot is further extended to suggest a rationale based on additional physico-chemical study using micro-Raman spectroscopy.

Friction-formed liquid droplets

The formation of nanoscale liquid droplets by friction of a solid is observed in real-time. This is achieved using a newly developed in situ transmission electron microscope (TEM) triboprobe capable of applying multiple reciprocating wear cycles to a nanoscale surface. Dynamical imaging of the nanoscale cyclic rubbing of a focused-ion-beam (FIB) processed Al alloy by diamond shows that the generation of nanoscale wear particles is followed by a phase separation to form liquid Ga nanodroplets and liquid bridges. The transformation of a two-body system to a four-body solid-liquid system within the reciprocating wear track significantly alters the local dynamical friction and wear processes. Moving liquid bridges are observed in situ to play a key role at the sliding nanocontact, interacting strongly with the highly mobile nanoparticle debris. In situ imaging demonstrates that both static and moving liquid droplets exhibit asymmetric menisci due to nanoscale surface roughness. Nanodroplet kinetics are furthermore dependent on local frictional temperature, with solid-like surface nanofilaments forming on cooling. TEM nanotribology opens up new avenues for the real-time quantification of cyclic friction, wear and dynamic solid-liquid nanomechanics, which will have widespread applications in many areas of nanoscience and nanotechnology.

Nonlocal continuum mechanics based ultrasonic flexural wave dispersion characteristics of a monolayer graphene embedded in polymer matrix

Wave propagation in graphene sheet embedded in elastic medium (polymer matrix) has been a topic of great interest in nanomechanics of graphene sheets, where the equivalent continuum models are widely used. In this manuscript, we examined this issue by incorporating the nonlocal theory into the classical plate model. The influence of the nonlocal scale effects has been investigated in detail. The results are qualitatively different from those obtained based on the local/classical plate theory and thus, are important for the development of monolayer graphene-based nanodevices. In the present work, the graphene sheet is modeled as an isotropic plate of one-atom thick. The chemical bonds are assumed to be formed between the graphene sheet and the elastic medium. The polymer matrix is described by a Pasternak foundation model, which accounts for both normal pressure and the transverse shear deformation of the surrounding elastic medium. When the shear effects are neglected, the model reduces to Winkler foundation model. The normal pressure or Winkler elastic foundation parameter is approximated as a series of closely spaced, mutually independent, vertical linear elastic springs where the foundation modulus is assumed equivalent to stiffness of the springs. For this model, the nonlocal governing differential equations of motion are derived from the minimization of the total potential energy of the entire system. An ultrasonic type of flexural wave propagation model is also derived and the results of the wave dispersion analysis are shown for both local and nonlocal elasticity calculations. From this analysis we show that the elastic matrix highly affects the flexural wave mode and it rapidly increases the frequency band gap of flexural mode. The flexural wavenumbers obtained from nonlocal elasticity calculations are higher than the local elasticity calculations. The corresponding wave group speeds are smaller in nonlocal calculation as compared to local elasticity calculation. The effect of y-directional wavenumber (eta(q)) on the spectrum and dispersion relations of the graphene embedded in polymer matrix is also observed. We also show that the cut-off frequencies of flexural wave mode depends not only on the y-direction wavenumber but also on nonlocal scaling parameter (e(0)a). The effect of eta(q) and e(0)a on the cut-off frequency variation is also captured for the cases of with and without elastic matrix effect. For a given nanostructure, nonlocal small scale coefficient can be obtained by matching the results from molecular dynamics (MD) simulations and the nonlocal elasticity calculations. At that value of the nonlocal scale coefficient, the waves will propagate in the nanostructure at that cut-off frequency. In the present paper, different values of e(0)a are used. One can get the exact e(0)a for a given graphene sheet by matching the MD simulation results of graphene with the results presented in this article. (c) 2012 Elsevier Ltd. All rights reserved.