6. NDE for Advanced Materials
Inelastic electron tunneling spectroscopy, or lETS, provides an extremely sensitive method for monitoring the chemical and physical state of a molecular substance adsorbed onto an oxide surface. Inelastic tunneling data directly reflect the molecular vibrational frequencies of the first monolayer of adsorbed molecules and changes in the vibrational spectrum can be correlated with changes in the chemical state of the molecule/oxide interface. We have carried out lETS experiments on the components of the commercial adhesive, Hercules 3501. This epoxy system consists ·of two molecular components; diamino diphenyl sulfone (DPS) and tetraglycidycl 4,4' diamino diphenyl methane (DPM). lETS spectra of the individual components and of the epoxy mixture adsorbed on aluminum oxide have been obtained and the vibrational modes and frequencies assigned by comparison with computer calculations and existing infrared optical spectra. Some evidence for an aging effect has been observed for the adsorbed DPS. This effect appears as a dramatic change in the low frequency vibrational modes and may be associated with the formation of hydrogen bonds or the polymerization of the DPS. Further studies of this effect are in progress. The effects of water permeation may be studied using D2O as a tracer. The vibrational modes of D20 are easily distinguished from those of water which may be present as a contaminant. If the exchange reaciton D2O + HCR → DHO + DCR occurs, it would be easily detected in the lETS spectrum. Initial experiments performed by simply immersing the tunnel junction into liquid D2O for several hours were unsuccessful because severe corrosion of the tunnel junction resulted. Experiments employing aluminum/aluminum oxide/adhesive/gold thin film junction for the study of H2O permeation are in progress. Further studies are planned to monitor the effects of heat treatment on the adhesive components and mixture.
White, H. W.; Godwin, L. M.; and Wolfram, T., "Microscopic Features of Adhesive Bonds for Non-Destructive Measurements" (1978). Proceedings of the ARPA/AFML Review of Progress in Quantitative NDE, September 1976–June 1977. 37.