The influence of impurities on the kinetics and morphology of reaction layers in diffusion couples

The growth and morphology of reaction layers in diffusion couples is probably more frequently influenced by impurities than is generally realized. We found drastic effects of impurities on the reaction between Cu and Si and on the reaction between Cu20 and Ni or Co. In the first case the always present thin Si02-film on Si plays an important role: it hampers the reaction between Cu and Si, giving rise to an incubation period. This reaction barrier can be removed completely by only a few ppm of phosphorus in the copper starting material, which enables the formation of Cu3Si at much lower temperatures than in the absence of phosphorus. In the latter example the presence of chloride ions prevents NiO or CoO, which is formed during the displacement reaction, from building up a continuous, closed and rate-limiting layer. Instead, precipitates of these oxides in a matrix of copper are formed . This results in a much higher reaction rate since then the much faster diffusion of oxygen through copper becomes rate-limiting.