417 research outputs found
Vacancy-Vacancy Interaction in Copper
The binding energy of two vacancies in a static lattice as a function of their separation and the positions of their displaced neighboring atoms has been calculated using a Morse potential function model for copper. It was found that two vacancies attract one another at separation less than about 7 A. At separations greater than 7 A the vacancies do not interact appreciably. The most stable separation was found to be the first nearest- neighbor separation or the divacancy configuration, for which the binding energy was found to be 0.64 ev. Based on these calculations, it is shown that third-stage annealing in irradiated copper may be accounted for by divacancy migration. The role of the divacancy in copper self-diffusion is also explained
The relation between solar cell flight performance data and materials and manufacturing data Final report
Flight performance data for solar cell power systems in satellites and correlation with manufacturing methods and material
Energy of Cohesion, Compressibility, and the Potential Energy Functions of the Graphite System
The lattice summations of the potential energy of importance in the graphite system have been computed by direct summation assuming a Lennard-Jones 6-12 potential between carbon atoms. From these summations, potential energy curves were constructed for interactions between a carbon atom and a graphite monolayer, between a carbon atom and a graphite surface, between a graphite monolayer and a semi-infinite graphite crystal and between two graphite semi-infinite crystals. Using these curves, the equilibrium distance between two isolated physically interacting carbon atoms was found to be 2.70 a, where a is the carbon-carbon distance in a graphite sheet. The distance between a surface plane and the rest of the crystal was found to be 1.7% greater than the interlayer spacing. Theoretical values of the energy of cohesion and the compressibility were calculated from the potential curve for the interaction between two semi-infinite crystals. They were (delta)E(sub c) = -330 ergs/sq cm and beta =3.18x10(exp -12)sq cm/dyne, respectively. These compared favorably with the experimental values of (delta)E(sub c) = -260 ergs/sq cm and beta = 2.97 X 10(exp -2) sq cm/dyne
Effect of Static Strains on Diffusion
A theory is developed that gives the diffusion coefficient in strained systems as an exponential function of the strain. This theory starts with the statistical theory of the atomic jump frequency as developed by Vineyard. The parameter determining the effect of strain on diffusion is related to the changes in the inter-atomic forces with strain. Comparison of the theory with published experimental results for the effect of pressure on diffusion shows that the experiments agree with the form of the theoretical equation in all cases within experimental error
Vacancy Relaxation in Cubic Crystals
The configuration of the atoms surrounding a vacancy in four face-centered cubic and three body-centered cubic metals has been computed, using a pairwise, central-force model in which the energy of interaction between two atoms was taken to have the form of a Morse function. Only radial relaxations were considered. The first and second nearest-neighbor relaxations for the face-centered systems were found to be: Pb (1.42,0.43), Ni (2.14,0.39), Cu(2.24,0.40) and Ca (2.73,0.41, expressed in percentages of normal distances. For the body-centered systems the relaxations out to the fourth nearest neighbors to the vacancy were: Fe (6.07,2.12, 0.25, ), Ba (7.85, 2.70, 0.70, 0.33) and Na (10.80, 3.14, 3.43, 0.20). The positive signs indicate relaxation toward the vacancy and the negative signs indicate relaxation away from the vacancy. The energies of relaxation (eV) are: Pb (0.162), Ni (0.626), Cu (0.560), Ca (0.400), Fe (1.410), Ba (0.950) and Na (0.172)
Infinite compressibility states in the Hierarchical Reference Theory of fluids. II. Numerical evidence
Continuing our investigation into the Hierarchical Reference Theory of fluids
for thermodynamic states of infinite isothermal compressibility kappa[T] we now
turn to the available numerical evidence to elucidate the character of the
partial differential equation: Of the three scenarios identified previously,
only the assumption of the equations turning stiff when building up the
divergence of kappa[T] allows for a satisfactory interpretation of the data. In
addition to the asymptotic regime where the arguments of part I
(cond-mat/0308467) directly apply, a similar mechanism is identified that gives
rise to transient stiffness at intermediate cutoff for low enough temperature.
Heuristic arguments point to a connection between the form of the Fourier
transform of the perturbational part of the interaction potential and the
cutoff where finite difference approximations of the differential equation
cease to be applicable, and they highlight the rather special standing of the
hard-core Yukawa potential as regards the severity of the computational
difficulties.Comment: J. Stat. Phys., in press. Minor changes to match published versio
van der Waals interaction in nanotube bundles : consequences on vibrational modes
We have developed a pair-potential approach for the evaluation of van der
Waals interaction between carbon nanotubes in bundles.
Starting from a continuum model, we show that the intertube modes range from
to . Using a non-orthogonal tight-binding approximation
for describing the covalent intra-tube bonding in addition, we confirme a
slight chiral dependance of the breathing mode frequency and we found that this
breathing mode frequency increase by 10 % if the nanotube lie inside a
bundle as compared to the isolated tube.Comment: 5 pages, 2 figure
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
Phase behavior and material properties of hollow nanoparticles
Effective pair potentials for hollow nanoparticles like the ones made from
carbon (fullerenes) or metal dichalcogenides (inorganic fullerenes) consist of
a hard core repulsion and a deep, but short-ranged, van der Waals attraction.
We investigate them for single- and multi-walled nanoparticles and show that in
both cases, in the limit of large radii the interaction range scales inversely
with the radius, , while the well depth scales linearly with . We predict
the values of the radius and the wall thickness at which the gas-liquid
coexistence disappears from the phase diagram. We also discuss unusual material
properties of the solid, which include a large heat of sublimation and a small
surface energy.Comment: Revtex, 13 pages with 8 Postscript files included, submitted to Phys.
Rev.
Carbon nanotubes adhesion and nanomechanical behavior from peeling force spectroscopy
Applications based on Single Walled Carbon Nanotube (SWNT) are good example
of the great need to continuously develop metrology methods in the field of
nanotechnology. Contact and interface properties are key parameters that
determine the efficiency of SWNT functionalized nanomaterials and nanodevices.
In this work we have taken advantage of a good control of the SWNT growth
processes at an atomic force microscope (AFM) tip apex and the use of a low
noise (1E-13 m/rtHz) AFM to investigate the mechanical behavior of a SWNT
touching a surface. By simultaneously recording static and dynamic properties
of SWNT, we show that the contact corresponds to a peeling geometry, and
extract quantities such as adhesion energy per unit length, curvature and
bending rigidity of the nanotube. A complete picture of the local shape of the
SWNT and its mechanical behavior is provided
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