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Introduction
 The mechanical behavior of monatomic chains, nanotubes, nanowires, solid and liquid
surface skins, nanocavities, and grains in nanometer and micrometer regimes, as a whole,
and their temperature dependence are indeed fascinating
 Questions and emerging problems due to the change of solid size and operation
temperature form a great challenge to classical continuum medium and quantum
approaches
 The impact of bond order imperfection dominates nanostructures
 A new approach combining theory and experiment from the perspective of bond formation,
dissociation, relaxation and vibration is highly desirable to compensate the existing
approaches
Principles
 If one bond breaks, the nearby ones become shorter and stronger, associated with local
strain and quantum trap surrounding the defect
 Localized densification of charge and energy in the relaxed region contributes to the
Hamiltonian and the bond order loss to the atomic cohesive energy, both of which dominate
the detectable quantities of a substance
 Atoms in the surface skins originate the unusual behavior of nanostructures yet atoms in the
core interior remain as they are in the bulk
 The entire specimen can be viewed as one bond averaged over all the bonds involved
 A detectable quantity can be connected to the averaged bond and its geometrical and
energetic response to the externally-applied stimulus such as coordination environment,
temperature, pressure, etc.
Liquid and solid surfaces
 It is essential to introduce the concepts for the surface properties: (i) energy density in the
surface skin and (ii) residual cohesive energy per discrete surface atom
 The excessive surface energy, stress and tension originate from the broken-bond-induced
bond contraction and the associated bond strength gain; the lowered atomic cohesive
energy arises from bond order loss
 A strained, solid-like, and well-ordered liquid skin serves as an elastic covering sheet for a
liquid drop or a gas bubble
 A solid surface melts first and a liquid surface solidifies easier than the liquid core; a solid
skin is generally harder than the core interior
 Measurement of the temperature dependence of surface tension can reveal the atomic
cohesive energy at the surface; the temperature dependence of elastic modulus gives the
mean atomic cohesive energy of the specimen
 Charge repopulation and polarization dominates the adsorbate-induced stress change
 The nonbonding states of nitrogen and oxygen play an important role in determining
mechanical behavior
Monatomic chains: bond length, strength and maximal strain
 With involvement of neither atomic gliding dislocation nor bond unfolding, monatomic chain
forms an ideal prototype of bond stretching investigation
 A bond in a monatomic chain contracts by 30% at equilibrium and the bond energy
increases by 43% compared with the bond in the bulk counterpart
 A monatomic chain melts at 0.418 Tm; metallic chains could form at 0.4Tm with caution in
operation
 The breaking limit of the bond varies exponentially with the inverse separation (Tmi – T)
between the chain melting point and the temperature of operation
Nanotubes and nanowires
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It is found true that the open edge of a CNT melts at 1593 K and the stiffness is 0.3685
(GPa·nm); one can actually only measure the stiffness rather than the wall thickness or the
young’s modulus separately.
 The C-C bond in a CNT is 0.116 nm in length and 1.42 thick with 68% rise in bond energy;
the Young’s modulus is 2.595 TPa; the C-C bond in the open edge is even shorter and
stronger.
 The Young’s modulus of a hollow tube increases with the inverse of wall thickness, rather
than its radius; a hollow tube is more elastic than a rod of the same radius because of the
relatively high portion of surface atoms
 The process of bond unfolding dominates nanostructures’ superplasticity as the maximal
strain in the vicinity of Tm is 150% for metallic atomic chains.
 The breaking mode of nanowires should change with operating temperature
Nanograins: I. Elasticity and extensibility
 Size induced elastic enhancement originates, intrinsically, from the broken-bond-induced
energy density gain in the skin
 Elasticity of a specimen may rise or drop with size reduction, depending on the separation
between melting point and the temperature of operation
 Thermally induced elastic softening arises from bond expansion and bond energy
weakening
 The thermal expansion coefficient, Debye temperature, Young’s modulus, and specific heat
of a specimen are strongly correlated and they are all size and temperature dependent
Nanograins: II. Plastic deformation and yield strength
 The competition between the energy density gain and the cohesive energy loss in surface
skin originates the IHPR yet the competition between the activation and the inhibition of
dislocation motion activates the IHPR
 The strongest size in the IHPR is determined by: (i) the nature of the bonds involved; (ii) the
temperature of operation, and (iii) the artifacts in the measurement
 A quasi-molten phase presents in which the superplasticity takes place
Atomic vacancy, nanocavity, and metallic foams
 Atomic vacancies, point defects, and nanometer sized pores cause the unusual properties
of the specimens – they are of light weight, high strength yet thermally less stable
 A broken bond not only serves as a center initiating mechanical failure but also provides a
site pinning dislocations
 Bonds between the under-coordinated atoms in the negatively curved surfaces perform the
same as those in a flat or a positively curved surface
Compounds and nanocomposites
 The reinforcement of a compacted interface in multilayers originates from the excessive
energy due to bond contraction and bond nature alteration
 A dissociated interface is identical to a free surface with a barrier and a trap in the skin
 Mechanical reinforcement of a nanocomposite may come from the filler, interface, or the
nearly-free surface surrounding the filler, depending on the intermixing conditions and the
strength of the filler
Concluding remarks
 The LBA could be an effective way of compensating the continuum and the quantum
approaches
 Developed approach has led to quantitative information about the cohesive energy and the
bonding identities in atomic chains or a CNT
 The broken bonds and the associated strain and quantum trapping provides should provide
profound impact to surface and nanosolid sciences
 The thermo-mechanics regarding atomic defects, impurities, adsorbed surfaces, liquid
surfaces, junction interfaces, and systems under other stimuli such as pressure, electronic
and magnetic fields remain challenge