Review Room-temperature reactions in thin metal couples

Abstract

Results of a 40 year-long investigation of room-temperature formation of compounds in 144 thin-film couples obtained by combining 23 metals (Ag, Al, Au, Bi, Cd, Co, Cr, Cu, Ga, Ge, In, Mg, Mn, Ni, Pb, Pd, Pt, Sb, Sm, Sn, Te, Ti, Zn) are presented. The data from published papers of the present and other authors have been complemented by unpublished data of the present authors. The systematized results and their analysis point to the following. In 39 couples, a total of 65 compounds are formed. The reactions of compound formation last from ∼1 min to ∼10 y, depending on the specimen type and procedure of film deposition. In the bulk–film specimens, the number of compounds formed is smaller and the reaction is slower than in the film–film specimens prepared by thermal evaporation. In the specimens with the sputtered top layer, a greater number of compounds is formed and the compound formation reaction proceeds more rapidly than in the couples prepared by thermal evaporation of both metals. The compounds formed can be transformed into others containing higher percentage of one of the constituents, until the specimen contains an excess of that constituent, provided that such compounds exist in the respective phase diagram. At the beginning of the reaction, compounds are formed in a broad concentration range, while at the reaction end the range is narrowed down, becoming close to that in the corresponding phase diagram. The optimum conditions for compound formation exist in the couples consisting of a high-melting metal and a low-melting one, provided that the potential compound is not high-melting. If the potential compound is high-melting, or if both metals in the couple have melting points in the same temperature range, no compound formation takes place at room temperature. In the couples consisting of a given high-melting metal and one of the low-melting metals, a linear relationship exists between the interdiffusion coefficient and the melting point of the low-melting metal. If, for a couple consisting of a high-melting metal and a low-melting one, there is a solid solubility range of the low-melting metal, during the course of long ageing, the compound formed is decomposed and the low-melting metal is amorphized. During the long ageing, the ambient atmosphere acts on the metal films (alone or in a couple) leading to formation of oxides, hydroxides or carbonates. The results obtained complement the low-temperature range results in the respective phase diagrams.