12,262 research outputs found
Mechanical strength of atomic chains, surface skins, and nanograins
This report deals with the correlation between the mechanical strength and
thermal stability of systems extending from monatomic chains to surface skins
and solids over the whole range of sizes with emphasis on the significance of
atomic coordination imperfection. Derived solutions show that a competition
between the bond order loss and the associated bond strength gain of the lower
coordinated atoms dictate the thermo-mechanics of the low dimensional systems.
Bond order loss lowers the atomic cohesive energy that determines the
temperature of melting (Tm), or the activation energy for atomic dislocation,
whereas bond strength gain enhances the energy density, or mechanical strength,
in the surface skin. Therefore, the surface is harder at T << Tm whereas the
surface becomes softer when the T approaches the surface Tm that is lower than
the bulk due to bond order loss. Hence, the strained nanostructures are usually
stiffer at low T whereas the harder skins melt easier. Quantitative information
has been obtained about the bonding identities in metallic monatomic chains and
carbon nanotubes. Solutions also enable us to reproduce the inverse Hall-Petch
relationship with clarification of factors dominating the transition from
hardening to softening in the nanometer regime.Comment: Review 42 pages, 12 figures 183 reference
Elastic Coulomb-levitation: why is ice so slippery?
The elastic, less dense, polarized, and thermally stable supersolid skin
lubricates ice. Molecular undercoordination shortens the H-O bond and lengthens
the O:H nonbond through O-O repulsion, which is associated with low-frequency
and high-magnitude of O:H vibration and a dual O-O polarization. The softer O:H
springs attached with stronger molecular dipoles provide forces levitating
objects sliding on ice, like Maglev or Hovercraft
The strongest size in the inverse Hall-Petch relationship
Incorporating the bond-order-length-strength correlation mechanism [Sun CQ,
Prog Solid State Chem 35, 1 -159 (2007)] and Borns criterion for melting [J.
Chem. Phys. 7, 591(1939)] into the conventional Hall-Petch relationship has
turned out an analytical expression for the size and temperature dependence of
the mechanical strength of nanograins, known as the inverse Hall-Petch
relationship (IHPR), that has long been a topic under debate regarding the
possible mechanisms. Reproduction of the measured IHPR of Ni, NiP and TiO2
nanocrystals revealed that: (i) the size induced energy densification and
cohesive energy loss of nanograins originates the IHPR that could be activated
in the contact mode of plastic deformation detection; (ii) the competition
between the inhibition of atomic dislocations, via the surface energy density
gain and the strain work hardening, and the activation for dislocations through
cohesive energy loss determine the entire IHPR profile of a specimen; (iii) the
presence of a soft quasisolid phase is responsible for the size-induced
softening and the superplasticity as well of nanostructures; (iv) the bond
nature involved and the T/Tm ratio between the temperature of operating and the
temperature of melting dictate the measured strongest sizes of a given
specimen
Pressure-stiffened Raman Phonons in Group III Nitrides
It has long been puzzling regarding the atomistic origin of the
pressure-induced Raman phonon stiffening that generally follows a polynomial
expression with coefficients needing physical indication. Here we show that an
extension of the bond-order-length-strength (BOLS) correlation mechanism to the
pressure domain has led to an analytical solution to connect the
pressure-induced Raman phonon stiffening directly to the bonding identities of
the specimen and the response of the bonding identities to the applied
stimulus. It is found that the pressure-induced blue-shift of Raman phonons
arises from the bond compression and energy storage exerted by the compressive
stress. Agreement between predictions and experimental measurement leads to the
detailed form for the polynomial coefficients, which offer an atomic
understanding of the physical mechanism of the external pressure induced energy
gain, thermally induced bond expansion as well as means of determining the mode
atomic cohesive energy in a specimen
Hidden force floating ice
Because of the segmental specific-heat disparity of the hydrogen bond (O:H-O)
and the Coulomb repulsion between oxygen ions, cooling elongates the O:H-O bond
at freezing by stretching its containing angle and shortening the H-O bond with
an association of larger O:H elongation, which makes ice less dense than water,
allowing it to float
Perspective: Supersolidity of the Confined and the Hydrating Water
This work reviews the progress in STM/S, XPS, NEXFAS, SFG, DPS, ultrafast UPS
and FTIR observations and quantum theory calculations on the
bond/electron/phonon correlation in the supersolid phase derived by molecular
undercoordination (confinement) and aqueous charge injection
Tetra-bonding of C, N and O at solid surface
In order to gain advanced understanding of the kinetics and dynamics of C, N,
and O reacting with a solid surface, it is necessary to consider the reaction
from the perspectives of bond formation, bond dissociation, bond relaxation,
bond vibration, and the associated charge redistribution and polarization and
the energetic response of the involved atoms and valence electrons. The
sp-orbital hybridization is found necessary for these concerned reactions
associated with strongly anisotropic bonding and valence identities and the
localized energy states of bonding pairs, nonbonding lone pairs, and the lone
pair induced antibonding dipoles, as well as the hydrogen bond like and C-H
bond like states, which could unify the observations using atomistic
microscopy, crystallography, electronic spectroscopy, vibronic spectroscopy,
and thermal desorption spectroscopy and provide guidelines for materials
design.Comment: Book Chapter (invited
The kinetics and modes of gold nanowire breaking
Molecular dynamics calculations revealed that the temperature of operation
and the applied tensile force (f) determine not only the kinetics but also the
mode and duration of Au nanowire breaking. In the tensile force range of 0.018
and 0.1 nN/atom, structure transformation of the wire occurs prior to breaking
at random positions. The gold wire breaks abruptly when the f is stronger than
0.1nN/atom but no rupture occurs at all when the f is weaker than 0.018
nN/atom. At higher temperatures and under stronger tensile forces, the wire
breaks even faster
Scaling relation for the bond length, mass density, and packing order of water ice
The packing order of molecules and the distance between adjacent oxygen atoms
(dOO) in water and ice are most basic yet puzzling. Here we present a scaling
solution for this purpose based only on the mass density (gcm-3), (Equation)
where dL is the length ({\AA}) of the O:H van der Waals bond and dH the H-O
polar-covalent bond projecting on the O---O line. Validated by the measured
proton symmetrization of compressed ice, dOO of water and ice, and dOO
expansion at water surface, this solution confirms that the fluctuated,
tetrahedrally-coordinated structure is unique for water ice
An approach to local band average for the temperature dependence of lattice thermal expansion
It has long been puzzling regarding the mechanism behind the nonlinearity of
lattice thermal expansion at low temperatures despite modeling considerations
from various perspectives in classical or quantum approximations. An analytical
solution in terms of local bond average is presented herewith showing that the
thermal expansion coefficient follows closely the specific heat of Debye
approximation without the involvement of mode Gruneisen constant or the bulk
modulus. Matching predictions to experimental observations using the Debye
temperature and the atomic cohesive energy as input evidences that the current
approach may represent the true situation of temperature induced lattice
expansion though the exact form of phonon density of states need to be
considered for further refinement
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