John R. Smith

Scaling relations in the equation of state, thermal expansion, and melting of metals

A simple and yet quite accurate prediction of volume as a function of pressure for metals and alloys is presented. Thermal expansion coefficients and melting temperatures are predicted by simple, analytic expressions and results compare favorably with experiment for a broad range of metals. All of these predictions are made possible by the discovery of universality in binding energy relations for metals.

Self-consistent electronic structure calculation for nitrogen chemisorbed on copper (100)

Abstract A fully self-consistent calculation of the electronic structure of a N covered Cu slab is reported. The density of states shows changes relative to a clean Cu slab that reproduce the changes observed in photoemission spectra when N is chemisorbed on Cu(100). Thus initial state information is correctly mirrored in the photoemission spectra in this case. The changes are due to the presence of the N 2p-bands, and to significant alteration of the surface Cu 3d-bands by the chemisorption of N. The localization in space of the chemisorption bond and the importance of self-consistency are demonstrated.

Enhanced Raman scattering on metal surfaces

Abstract We present a theory which describes enhanced Raman scattering from molecules adsorbed on metals. The enhancement is due to the screening fields induced by the optically polarized adsorbate in the presence of the substrate. These fields interact coherently with the applied optical field and the vibrational motion of the adsorbed molecule. The enhancement is shown to be coverage dependent and also rather sensitive to the optical dielectric function of the substrate. Predicted enhancements compare well with experiment.

METAL/CERAMIC ADHESION : A FIRST PRINCIPLES STUDY OF MGO/AL AND MGO/AG

The energetics of adhesion and the electronic structure are determined for MgO/Ag(100) and MgO/Al(100) interfaces via local density-functional calculations. At the interface, both Ag and Al atoms energetically favour the site directly above the O atom, which is consistent with recent high-resolution transmission electron microscopy experiments on MgO/Ag(100). For that site, electron density distributions in the metal surface regions are reminiscent of a charge array imaging only the surface Mg and O ions. This implies a substantial ionic component to the adhesive bond. The screening charge distributions appear smoother in the A1 case than in the Ag case, suggesting a direct role of the Ag d electrons in the screening. Despite the ionic contribution to the bonding, the adhesive energy versus interfacial separation curves obey the same, universal form originally discovered for bimetallic adhesion. This could be explained by a significant metallic and/or covalent contribution to the metal/ceramic bonds. Furt...

Universal binding energy relations in metallic adhesion

Abstract Rose, Ferrante, and Smith (ref. 7) have discovered scaling relations which map the adhesive binding energy of Ferrante and Smith (ref. 6) onto a single universal binding energy curve. The energies in ref. 6 are calculated for all combinations of Al(111), Zn(0001), Mg(0001), and Na(110) in contact. The scaling involves normalizing the energy to the maximum binding energy and normalizing distances by a suitable combination of Thomas-Fermi screening lengths. A simple mathematical expression is found to accurately represent the universal curve, E* (a*) = – (1 + βa*) exp (−βa*) where E* is the normalized binding energy, a* is the normalized separation, and β is the fitting parameter. Rose et al. (ref. 7) have also found that the calculated cohesive energies of K, Ba, Cu, Mo, and Sm scale by similar simple relations suggesting the universal relation may be more general than for the simple free electron metals for which it was derived. In this paper we outline these results and discuss them in relation to topics of interest in adhesion, friction, and wear.

Self-Consistent Screening of Charges Embedded in a Metal Surface

This paper first develops a general theory of the screening change induced in an inhomogeneous electron gas by a small perturbing charge. This theory is applied to the screening of a point charge embedded in a metal surface. The first stage of this program which is reported here, uses the jellium model for the unperturbed metal surface and treats both the unperturbed and perturbed systems in the Thomas–Fermi approximation. Self-consistency, which is maintained in the present paper, leads to results which differ significantly from earlier results obtained with infinite barriers, which are by their nature not self-consistent.

Impurity effects on adhesion at an interface between NiAl and Mo

Abstract Fully self-consistent (LDA) density functional calculations have been performed to obtain the ideal adhesion energy and peak interfacial stress for rigid fracture of an [001] interface between NiAl and Mo with and without interfacial impurities. These calculations shed light on interfaces in NiAl–Mo eutectic composites in which [001] interfaces between NiAl (CsCl structure) and Mo (b.c.c.) are prominent. In all cases impurities reduce the adhesion energy, from 3.3xa0J/m2 for the clean interface to 2.4, 1.5, and 1.4xa0J/m2 for C, O, and S impurities, respectively. Interestingly, however, C increases (40.0xa0GPa), while O (26.0xa0GPa) and S (18.0xa0GPa) decrease the peak interfacial stress from that for the clean interface (37.0xa0GPa). The Harris functional was also tested, and it was found that the ordering of adhesion energies and peak interfacial stresses were the same as for the fully self-consistent results, but errors in the magnitudes were significant. Misfit dislocations play an important role and are estimated to reduce the adhesion energy of the clean interface by an amount of the order of 10%.

Universal behavior in ideal slip

The slip energies and stresses are computed for defect-free crystals of Ni, Cu, Ag, and Al using the many-atom approach. A simple analytical expression for the slip energies is obtained, leading to a universal form for slip, with the energy scaled by the surface energy and displacement scaled by the lattice constant. Maximum stresses are found to be somewhat larger than but comparable with experimentally determined maximum whisker strengths.

Impurity effects on adhesive energies

Some simple arguments are made about the effect of impurities on adhesive interactions at solid junctions. A universal adhesive force relation is derived for brittle adhesion. The adhesive binding energy, ΔE, is an important parameter in brittle adhesion forces. ΔE has also been shown by others to be important when there is plastic flow. We found that impurity effects on ΔE are determined by the segregation energies to the junction and to the free surfaces. At low temperatures, if it is energetically more favorable for impurities to segregate to the surfaces than to the junction, then the impurities will decrease ΔE. The converse is also true. For example, for self junctions which are in registry, ΔE is decreased if surface segregation is exothermic and increased if it is endothermic. These segregation energy relationships are consistent with the results of a number of experiments on the effects of impurities on adhesion forces and grain boundary embrittlement.

Electronic structure and magnetism of a Pd monolayer on Fe(100)

Abstract Spin-polarized self-consistent localized-orbital (SCLO) calculations have been performed for a five-plane slab simulating Pd/Fe(100). Pd monolayers were placed in registry with both faces of a three-plane Fe(100) slab. We chose a Pdue5f8Fe bond length equal to the sum of metallic radii, 2.62 A, and an Feue5f8Fe bond length equal to the bulk value, 2.49 A. The computed energy bands for the Pd/Fe 3 /Pd slab resemble those calculated for a five-plane Fe(100) slab, except for a positive work-function shift of 0.5 eV. The Pd monolayer has a magnetic moment of 0.37 μ B /atom. The magnetic moment of an adjacent iron atom is 2.74 μ B , slightly smaller than the value 2.89 μ B at the surface of Fe 5 , but still significantly larger than the central-plane value 2.37 μ B . The d bands of the two metals are strongly hybridized, but very little charge transfer takes place across the interface. Compared with the isolated Pd(100) monolayer, or the clean Fe(100) slab, the surface density of states of Pd/Fe(100) is rather weak near the Fermi level, suggesting a reduced chemical reactivity for this surface.