Proceedings of the 12^th International Students Day of Metallurgy. Ostrava: VŜB,  – 2005. - p. 65-69.

THE STRUCTURE OF REACTION DIFFUSION ZONE IN HIGH-TEMPERATURE COPPER OXIDATION

K.A. Lebedev, V.V. Prisedsky, V.M. Vinogradov

(Donetsk National Technical University, Ukraine))

          The unique role of cuprates among high-temperature superconductors and other advanced materials intensively studied during the last decades supports interest to many aspects of copper physical and chemical behavior and, in particular, to the properties of copper oxides and their formation.

Despite vast literature on the mechanism and kinetics of copper oxidation, the details of deeper oxidation to single-phase CuO as an equilibrium product are much less lucid.  In fact, there are no reports on kinetics studied in the course of complete copper oxidation. At high temperatures, when the process of copper oxidation, according to Wagner theory, ought to obey the parabolic law, some researchers have noticed differences between experimental and theoretical data. 

The aim of this work is the study of oxidation kinetics and also composition, structure and development of reaction diffusion zone in the course of copper oxidation in air in temperature interval 600-900 ^oC.

          Using thermogravimetry, microscopy and X-ray diffraction, the process of high-temperature oxidation of copper wires and plates has been studied. An abrupt decrease in reaction rate after complete consumption of metal phase but long before reaching the equilibrium has been found. This phenomenon is connected to an irregular character of the development of the reaction diffusion zone. In contrast to the usually applied layer model, initially formed oxide layers separates into numerous aggregates of Cu₂O crystals chaotically scattered throughout the zone between CuO grains. Such fragmentation of the diffusion zone is induced by macro- and microcracks formed in copper scale under influence of mechanical stresses at metal-oxide phase boundary due to the difference in molar volume between copper and its oxides. The pattern of cracks provides channels of fast diffusion and maintains the reaction rate at high level but only until the source of cracks formation remains in action.      

The structure of the diffusion zone undergoes a series of basic rearrangements in the process of deep copper oxidation. They take place against the background of increasing packing density of recrystallising tenorite CuO grains in oxide layer. At the very first moment of oxidation numerous CuO whiskers appear on the surface of copper specimen. Then these black needle-like crystals thicken into a porous layer of lusterless black phase similar to soot in consistency. This layer is growing rapidly. In further oxidation, increasing density of needle-like crystals leads to their recrystallization into grains of almost equiaxial habitus. In some time on outer surface of the specimen a continuous dense layer of recrystallized grains is formed over lusterless black phase of fine-grained tenorite phase CuO. After that, well-edged crystals grains of bright-red phase Cu₂O appear at the phase boundary with metal and then quickly grow. The next stage of oxidation brings even more profound rearrangements in the diffusion zone. The layer of copper scale now breaks into many Cu₂O crystals aggregates separated by thinner layers of black CuO crystals. Such fragmentation of the reaction diffusion zone comprises all the volume of copper scale with the exception of thin outer layer of already recrystallized CuO grains and inner layer consisting of only red Cu₂O crystals.

The moment of disappearance metal phase corresponds to immediate slow down in oxidation speed. It can be caused by deceleration of process of formation of cracks and other ways of the fast diffusion. This fragmented two-phase mixture of oxides is very stable to further oxidation.

These results show that widely accepted layer model of reaction diffusion zone is far from reality in deep copper oxidation.