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

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

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 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

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

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

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

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

Thermally driven elastic weakening of crystals

An analytical solution has been developed clarifying that the thermally driven elastic softening of crystals can be directly related to the length and strength of the representative bonds of the crystal and to the response of the bonding identities to the change of temperature. Reproduction of the experimental results Ag, Au, MgO, Mg2SO4, Al2O3, and KCl derived mean atomic cohesive energy of the specimen may evidence the validity of the proposed approach without involving parameters using in classical thermodynamics