Surface Thermodynamic Quantities Research Articles

Role of Surface Effects in the Vibrational Density of States and the Vibrational Entropy in Spin Crossover Nanomaterials: A Molecular Dynamics Investigation

Size reduction effects on the lattice dynamics of spin crossover (SCO) thin films have been investigated through molecular dynamics (MD) simulations of the density of vibrational states. The proposed simple model structure and reduced force field allows us to obtain good orders of magnitude of the sound velocity in both spin states and takes into account the contribution of free surfaces in the vibrational properties of very thin films (below a thickness of 12 nm). The slab method issue from the field of surface physico-chemistry has been employed to extract surface thermodynamic quantities. In combination with the related slab-adapted method, the slab approach provides a powerful numerical tool to separate surface contributions from finite-size effects. Due to the relatively low stiffness of SCO materials, the lattice dynamics seems to be governed by surface instead of confinement effects. The size evolution of thermodynamic quantities is successfully reproduced, especially the increase of the vibrational entropy with the size reduction, in good agreement with experimental observations.

The size dependence of surface thermodynamic quantities of nano-Cu2(OH)2CO3: Theory and experimental research by thermal decomposition

The peculiar chemical properties of nanoparticles are quite different from their bulk counterparts, the differences are related to their surface thermodynamic quantities, mainly depending on their particle sizes. In this paper, the relations between solid-gas surface thermodynamic quantities and particle size at constant temperature and constant volume were derived. And the theory of decomposition thermodynamics at constant temperature and constant volume was proposed and the relations of equilibrium constants and thermodynamic properties of decomposition reactions with particle size were derived respectively. Applying the theory, the method of obtaining the surface thermodynamic quantities by thermal decomposition was proposed. Experimentally, the thermal decomposition of nano-Cu2(OH)2CO3 with different particle sizes was taken as a research system, the standard equilibrium constants, the thermodynamic properties and the heat capacity difference for decomposition of nano-Cu2(OH)2CO3 with different sizes were obtained by calculating the partial pressure and the fugacity coefficient of each gas component by RK equation at different temperatures. Further, the solid-gas surface thermodynamic properties, surface tension and surface heat capacity were obtained. And the size dependence of surface thermodynamic quantities and decomposition thermodynamic quantities of nano-Cu2(OH)2CO3 are consistent with the theoretical relations above.

Simple model for interface stresses with application to misfit dislocation generation in epitaxial thin films

A simple model for the interfacial free energy of a semicoherent interface is used to develop expressions for interface stresses, which are surface thermodynamic quantities associated with solid–solid interfaces. An analysis of the thermodynamics of thin film epitaxy is presented that incorporates the effects of free surface and interface stresses, and an expression for the critical thickness for thin film epitaxy is obtained. Based on this analysis, the concept of effective pressures exerted by the thin film free surface and film–substrate interface is introduced. If it is assumed that misfit dislocations are generated at the film–substrate interface as a result of glide of threading dislocations, the thermodynamics and kinetics of stress relaxation can be discussed in terms of a balance of Peach–Koehler forces acting on the threading dislocations owing to the surface and interface pressures as well as to the coherency stress. An example is given that shows that, if the film has a relatively large surface pressure that opposes lattice matching, the dependence of the coherency strain on film thickness can be very different from that obtained from conventional analyses which ignore the effect of the free surface; specifically, the largest equilibrium coherency strain of the same sign as the misfit can be much smaller than the total misfit, and an “anomalous” coherency strain of sign opposite that of the misfit can be thermodynamically favorable at small film thicknesses. The analysis used to obtain the critical thickness for thin film epitaxy is extended to give an expression for the critical thickness for misfit dislocation generation at the interface between a substrate and a superlattice thin film. It is shown that this critical thickness depends on a superlattice pressure associated with the interlayer interface stress in addition to the free surface and film–substrate interface pressures.

Electrolytes at charged interfaces: Integral equation theory for 2–2 and 1–1 model electrolytes

The structure and thermodynamics of both 2–2 and 1–1 model electrolytes at a charged interface have been determined. The solvent is modeled as a structureless dielectric continuum. The structure is calculated from the ‘‘singlet’’ version of the Ornstein–Zernike integral equation, using as input the structure of the bulk electrolyte from a recent integral equation theory. The approximation in the theory is the wall–ion bridge function, which is investigated for two levels of approximation. Surface thermodynamic quantities calculated from this structural information are compared with the classical Gouy–Chapman–Stern approximation for the interfacial region, computer simulations, and selected experimental data. Higher order structure predicted by the integral equations indicates that caution should be used when interpreting results of the classical approximation.

Review: Surface tension and its relations with adsorption, vapourization and surface reactivity of liquid metals

The interactions of a gaseous phase with a liquid metallic surface are analysed in terms of surface tension, adsorption and chemical reactivity. It is shown that thermodynamic equilibrium considerations are not sufficient to interpret surface tension measurements especially at high temperatures, and that diffusion and interactions in the vapour phase must also be considered. Surface tension isotherms are discussed and models allowing calculations of maximum adsorption and surface tension values are presented. These are of particular interest when dealing with systems forming very stable oxides. The influence of surface thermodynamic quantities on chemical reactivity is also shown by means of classical thermodynamic reasoning. One of the main results presented here is that reactions favoured in the bulk phase may not take place at the surface, where the presence of the excess free energy term may favour another reaction, thermodynamically impossible in the bulk phase.

Surface thermodynamic properties of a semi-infinite Ising ferromagnet in the presence of a surface transverse field.

By use of a combination of the two-spin cluster approximation with the discretized path-integral representation, the surface phase diagrams and surface thermodynamic quantities, such as free energy, specific heat, and susceptibility, of a semi-infinite Ising ferromagnet in the presence of a surface transverse field are investigated.

Statistical ensembles for surface problems

The calculation of the thermodynamic properties of the crystal-vapour interface using canonical and grand canonical ensembles is discussed. It is shown that the latter should be used in problems concerning crystal-vapour equilibria and that there is a continuous transition between the results for the two ensembles as the number of vapour atoms is increased from zero, for the canonical ensemble, to infinity for the grand canonical ensemble. We discuss how the Gibbs dividing surface may be positioned in order to ensure that the excess number of atoms is zero at equilibrium and that the derivatives of surface thermodynamic quantities with respect to the chemical potentials are well defined.

Surface Thermodynamic Functions for Noble-Gas Crystals

Surface thermodynamic quantities have been calculated as functions of temperature between 0°K and the melting temperature for the (111), (100), and (110) surfaces of the noble-gas solids Ne, Ar, Kr, and Xe. The method of calculation differs from the methods used in previous treatments of surface thermodynamic functions in that the atomic vibrations are taken into account, and the vibrational frequencies are properly calculated rather than obtained from a Debye model or an Einstein model. The quantities calculated are the static surface energy, vibrational surface energy, surface entropy, vibrational surface free energy, and surface specific heat. In addition, a surface frequency-distribution function f8(ω) has been calculated; f8(ω) is positive at low frequencies, because of the presence of surface modes of vibration, and negative at higher frequencies. This behavior of f8(ω) produces a narrow peak in the graph of the surface specific heat as a function of temperature.