arXiv subjects

Chang Q. Sun

Publications and source records attributed to Chang Q. Sun.

28 records · Page 2Linked to original sources

arXiv2008

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.

cond-mat.mtrl-sci↗

arXiv2008

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.

cond-mat.mtrl-sci↗

arXiv2008

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.

cond-mat.mtrl-sci↗

arXiv2008

Nanocavity hardening: impact of broken bonds at the negatively curved surfaces

It is expected that atomic vacancies or nanometric cavities reduce the number of chemical bonds of nearby atoms and hence the strength of a voided solid. However, the hardness of a porous specimen does not always follow this simple picture of coordination counting. An introduction of a certain amount of atomic vacancies or nanocavities could, instead, enhance the mechanical strength of the porous specimen. Understanding the mechanism behind the intriguing observations remains yet a high challenge. Here we show with analytical expressions that the shortened and strengthened bonds between the under-coordinated atoms and the associated local strain and energy trapping [Sun, Prog Solid State Chem 35, 1-159 (2007)] in the negatively curved surface skins dominate the observed nanocavity hardening. Agreement between predictions and the experimentally observed size-dependence of mechanical strength of some nanoporous materials evidences for the proposed mechanism.

cond-mat.mes-hall↗

arXiv2008

Stimulus dependence of the collective vibration of atoms in an icosahedral cluster

Molecular dynamics calculations of the vibrational behavior of atoms in a Lennard-Jones 147-atom cluster revealed that the relaxation and the stability of the collective vibration of atoms in the cluster depend on the extent of the mechanical disturbance. A relatively larger-scale perturbation will cause a faster decay of the vibration magnitude, the potential and the kinetic energy compared to the vibration stimulated by a small-scale stimulus.

cond-mat.mes-hall↗

arXiv2008

Shell-resolved melting kinetics of an icosahedral cluster

Molecular dynamics calculations of the fluctuation of bond vibration revealed the shell-resolved kinetics of surface melting of closed-shelled cluster containing 147atoms with Lennard-Jones type interaction. It is found that the surface melting is imitated by the migrating of the vertex atoms and the melting process can be divided into three major stages, i.e., vertex migrating, surface melting, and general melting. Although the melting process of the LJ147 cluster could be divided into discrete stages of shell-by-shell surface melting, in general, there is still a continuous process of melting from the surface to the core interior.

cond-mat.mes-hall↗

arXiv2008

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.

cond-mat.mes-hall↗

arXiv2008

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.

cond-mat.mtrl-sci↗

arXiv2005

Nature evolution of the shortened bond in atomic clusters and at junction interfaces

Thermally stimulated process such as evaporation, phase transition, or solid-liquid transition of a solid consumes each a certain portion of the solid cohesive energy that is the sum of bond energy over all the coordinates of all the involved atoms. Generally, the critical temperatures for such processes drop with solid size, unless hetero capping or interfacial interaction becomes dominant, because of the increased portion of the lower-coordinated surface atoms [Sun et al., J. Phys. Chem. B108, 1080 (2004)]. It is intriguing, however, that the melting point (Tm) of a solid containing III-A or IV-A atoms oscillates with size (the Tm drops first and then rises as the solid size is reduced) and that the Tm of chemically capped nanosolid often increases with the inverse size. Here we show that bond nature evolution is essential for the selective nanosolids and at the junction interfaces, which is responsible for the superheating of the smallest nanosolids, chemically capped clusters, and junction interfaces that exhibit insulating nature with high mechanical strength.

cond-mat.mtrl-sci↗

arXiv2005

On the unusual behavior of nitride compounds

This report presents consistent insight into the mechanism behind the unusual behavior of nitride compounds from the perspective of tetrahedron bond formation and its consequence on valence density of states. An extension of the recent bond-band-barrier (BBB) correlation mechanism for oxidation [Sun CQ, Prog Mater Sci 2003;48:521-685] to the electronic process of nitridation has led to the essentiality of sp-orbital hybridization for a nitrogen atom upon interacting with atoms in solid phase of arbitrary less-electronegative element. In the process of nitridation, a nitrogen atom forms a quasi-tetrahedron with surrounding host atoms through bonding and nonbonding interaction associated with production of electronic holes and antibonding dipoles, which add corresponding density of states to the valence band of the host. It is suggested that the valance alteration of the system takes the responsibility for the blue shift in photoluminescence, lowered work function for cold cathode field emission, corrosion and wear resistant, high elasticity, and magnetic modulation as well.

cond-mat.mtrl-sci↗