FULL SPECTRAL CALCULATION OF NONRETARDED HAMAKER CONSTANTS FOR CERAMIC SYSTEMS FROM INTERBAND TRANSITION STRENGTHS

The van der Waals (vdW) interaction is one of the key terms in the for ce balances dictating wetting behavior and intergranular film thicknes ses. The characteristics of thin intergranular or surficial glass film s are of increasing importance due to their role in determining the pr operties of polycrystalline ceramics. The Hamaker constant scales the London dispersion force part of the vdW interaction for a particular c onfiguration of grains and films and is a direct function of the inter band optical properties of the interatomic bonds of the materials. For ceramics, much previous work focused on simplified models, such as th e Tabor-Winterton approximation (TWA), to determine Hamaker constants based on refractive indices. Herein we develop full spectral calculati ons of the Hamaker constants for various ceramic systems using experim entally determined interband transition strengths ((J) over cap(omega( cv))) to directly derive the London dispersion spectra (epsilon(2)(xi) ) from which spectral difference functions lead to direct determinatio n of the Hamaker constants, The results affirm the expectation that tr ansitions involving valence electrons provide the predominant contribu tion to the dispersion forces for the compounds examined. Calculations have been done for the planar case of a gap between two semi-infinite bodies containing either vacuum or an intervening glassy layer. The r esults indicate that the TWA is useful for oxides with relatively low refractive indices, i.e., n similar to 1.4-1.8. However, when any of t he materials have larger indices, this approximation becomes inexact, and no obvious, simple correction to the TWA gives uniformly good resu lts, as the behavior differs for simple covalent materials and for oxi des with partially filled d-shells but having similar refractive indic es. An important consequence is that Hamaker constants are smaller for such high index materials, especially oxides, with intervening glassy films than might be expected from approximations. Calculations have a lso been done for two other geometries, i.e., for an intervening film with a layer of a third material at both interfaces and for glass coat ed free surfaces. The former of these provides first insights regardin g the behavior with nonuniform films which often differs markedly from that expected for homogeneous films of the same average composition.