Diffusion in evaporated films of silver–aluminium

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Abstract
Measurements of optical reflectivity have been used to investigate diffusion and intermetallic compound formation in thin-film diffusion couples of silver–aluminium. The couples were prepared by vacuum deposition of the two metals in succession on glass substrates. The specimens were annealed at temperatures up to 240°C and reflectivity measurements were made at both the free surface and the substrate side. An intermediate zone of Ag2Al was formed by diffusion and a marked change in reflectivity occurred as this compound approached the surface. Electron diffraction was used to confirm the presence of this compound. The results indicated an extremely rapid initial diffusion occurring over a limited distance without formation of any intermetallic compound, and it appeared necessary to assume a high initial concentration of vacancies to explain this. It is suggested that vacancies in the diffusion zone condense at precipitate particles as soon as they start to form and that this is followed by normal diffusion. This explains observed differences in behaviour between Ag/Al and Au/Al. The rate of diffusion in the Ag2Al compound has been calculated, taking account of concentration limits, and gives D 0 = 9 cm2/sec and an activation energy of 26·4 kcal/mole.

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