Atomic Scale Sliding and Rolling of Carbon Nanotubes

A carbon nanotube is an ideal object for understanding the atomic scale aspects of interface interaction and friction. Using molecular statics and dynamics methods different types of motion of nanotubes on a graphite surface are investigated. We found that each nanotube has unique equilibrium orientations with sharp potential energy minima. This leads to atomic scale locking of the nanotube. The effective contact area and the total interaction energy scale with the square root of the radius. Sliding and rolling of nanotubes have different characters. The potential energy barriers for sliding nanotubes are higher than that for perfect rolling. When the nanotube is pushed, we observe a combination of atomic scale spinning and sliding motion. The result is rolling with the friction force comparable to sliding.Comment: 4 pages (two column) 6 figures - one ep

Atomic scale study of friction and energy dissipation

Cataloged from PDF version of article.This paper presents an analysis of the interaction energy and various forces between two surfaces, and the microscopic study of friction. Atomic-scale simulations of dry sliding friction and boundary lubrication are based on the classical molecular dynamics (CMD) calculations using realistic empirical potentials. The dry sliding of a single metal asperity on an incommensurate substrate surface exhibits a quasi-periodic variation of the lateral force with two different stick-slip stage involving two structural transformation followed by a wear. The contact area of the asperity increases discontinuously with increasing normal force. Xe atoms placed between two atomically flat Ni surfaces screen the Ni-Ni interaction, decrease the corrugation of the potential energy as well as the friction force at submonolayer coverage. We present a phononic model of energy dissipation from an asperity to the substrates. (C) 2003 Elsevier Science B.V. All rights reserved

More Than Meets the Eye in Bacterial Cellulose: Biosynthesis, Bioprocessing, and Applications in Advanced Fiber Composites

Bacterial cellulose (BC) nanofibers are one of the stiffest organic materials produced by nature. It consists of pure cellulose without the impurities that are commonly found in plant-based cellulose. This review discusses the metabolic pathways of cellulose-producing bacteria and the genetic pathways of Acetobacter xylinum. The fermentative production of BC and the bioprocess parameters for the cultivation of bacteria are also discussed. The influence of the composition of the culture medium, pH, temperature, and oxygen content on the morphology and yield of BC are reviewed. In addition, the progress made to date on the genetic modification of bacteria to increase the yield of BC and the large-scale production of BC using various bioreactors, namely static and agitated cultures, stirred tank, airlift, aerosol, rotary, and membrane reactors, is reviewed. The challenges in commercial scale production of BC are thoroughly discussed and the efficiency of various bioreactors is compared. In terms of the application of BC, particular emphasis is placed on the utilization of BC in advanced fiber composites to manufacture the next generation truly green, sustainable and renewable hierarchical composites

Transport through the intertube link between two parallel carbon nanotubes

Quantum transport through the junction between two metallic carbon nanotubes connected by intertube links has been studied within the TB method and Landauer formula. It is found that the conductance oscillates with both of the coupling strength and length. The corresponding local density of states (LDOS) is clearly shown and can be used to explain the reason why there are such kinds of oscillations of the conductances, which should be noted in the design of nanotube-based devices.Comment: 6 pages, 4 figure

Controlled lateral and perpendicular motion of atoms on metal surfaces

We present the theoretical study of the controlled lateral and perpendicular motion of Xe on the Pt(111) surface. The lateral translation of Xe is manipulated by a tungsten tip of a scanning tunneling microscope. Using molecular statics and dynamics the energetics and different modes of atom translation are revealed. In the controlled and reversible transfer of Xe between two flat Pt(111) surfaces, effective charge on Xe, and the dipole moment of the Xe-Pt bond, are calculated as functions of the Xe-surface separation. The contributions of various mechanisms to the transfer rate of Xe are investigated by using the calculated quantum states of Xe under the applied bias voltage. These are tunneling and ballistic transfer, dipole excitation and excitation due to resonant tunneling of electrons, and electron wind force. We found that a single power law for the transfer rate does not exist in the whole range of applied pulse voltage. At high pulse voltage the transfer rate is dominated by the inelastic electron tunneling. At low pulse voltage the rate due to thermally assisted tunneling and ballistic transfer becomes important

Theoretical study of boundary lubrication

We analyzed the dynamics of xenon atoms as lubricant between two Ni(110) slabs in relative motion. Atomic simulations are carried out by using classical molecular dynamics with realistic empirical potentials, where nickel as well as xenon atoms are relaxed. The resistance of the xenon layer against the loading force is examined and critical forces are determined to destroy the lubricant layer at different coverages. The relative motion of slabs in the lateral direction is investigated under constant normal force as a function of coverage ranging from zero to the monolayer xenon. Important lubrication properties of xenon atoms are analyzed by calculating the variation of potential energy, lateral force, and local hydrodynamic pressure. It is predicted that the corrugation of the potential energy associated with the sliding has a minimum value at submonolayer coverage. A phononic energy dissipation mechanism together with the theoretical analysis is proposed. ©1999 The American Physical Society

Quantum heat transfer through an atomic wire

We studied the phononic heat transfer through an atomic dielectric wire with both infinite and finite lengths by using a model Hamiltonian approach. At low temperature under ballistic transport, the thermal conductance contributed by each phonon branch of a uniform and harmonic chain cannot exceed the well-known value which depends linearly on temperature but is material independent. We predict that this ballistic thermal conductance will exhibit stepwise behavior as a function of temperature. By performing numerical calculations on a more realistic system, where a small atomic chain is placed between two reservoirs, we also found resonance modes, which should also lead to the stepwise behavior in the thermal conductance.Comment: 14 pages, 2 separate figure

Interplay between stick-slip motion and structural phase transitions in dry sliding friction

Simulations of dry sliding friction between a metal asperity and an incommensurate metal surface reveal unusual atomic processes. The lateral force exhibits a quasiperiodic variation with the displacement of an asperity; each period consists of two different stick-slip processes involving structural transitions. While one layer of asperity changes and matches the substrate lattice in the first slip, two asperity layers merge into a new one through a structural transition during the second slip. This leads to wear. The lateral force decreases abruptly during these slip stages, but it increases between two consecutive slips and resists the relative motion. The analysis of the order suggests that each structural transition is associated with a first-order phase transition. Nonadiabatic atomic rearrangements during these phase transitions involve a new kind of mechanism of energy dissipation in the dry sliding friction

Electron field emission properties of closed carbon nanotubes

Recent experiments have shown that carbon nanotubes exhibit excellent electron field emisson properties with high current densities at low electric fields. Here we present theoretical investigations that incorporate geometrical effects and the electronic structure of nanotubes. The electric field is dramatically enhanced near the cap of a nanotube with a large variation of local field distribution. It is found that deviation from linear Fowler-Nordheim behavior occurs due to the variation of the local field in the electron tunneling region. The maximum current per tube is of the order of [Formula presented]. Local and microscopic aspects of field emission from nanotubes are also presented

Atomic-scale study of dry sliding friction

We present a theoretical study of dry sliding friction, which has a close bearing on the experiments done by using the atomic and friction force microscope. By performing atomic-scale calculations for the friction between a single atom and monoatomic infinite chain, we examined the effect of various material parameters on the stick-slip motion. We found that the perpendicular elastic deformation of the substrate that is induced by the sliding object is crucial for the energy damping in friction. In this case, the average friction force strongly depends on the perpendicular force constant of the substrate and the friction constant varies with the normal force. In particular, soft materials that continue to be elastic for a wide range of perpendicular compression may exhibit a second state. As a result, the hysteresis curve in the stick-slip motion becomes anisotropic