INTERNAL REDUCTION OF (MG, CU)O

(Mg, Cu)O single crystals are internally reduced at temperatures ranging from 1273 to 1673 K in the presence of either a C/CO buffer or a CO/CO2 gas flow. As a result, a reduction scale containing discrete precipitates develops. At low reaction temperature, this scale is divided in an outer part where discrete copper precipitates are present in the MgO matrix, and an inner part where both metallic copper and cuprite Cu2O precipitates coexist in the MgO matrix. At high temperatures, the inner scale is very narrow. Transmission electron microscopy (TEM) investigations of the reduction scale reveal special orientation relationships between the MgO lattice (m) and that of the precipitates (p): T: (001)m//(001)p and [100]m//[100]p D: (001)m//(011BAR)p and [001]m//[110]p DA: (001)m//(011BAR)p and [011]m//[100]p M: (001)m//(112BAR)p and [010]m//[111BAR]p or [100]m//[111BAR]p. The interfaces between precipitate and matrix have a well defined structure. An extended dislocation network interconnects the precipitates. A mechanism is proposed for the internal reduction of (Mg, Cu)O, where reduction would proceed in two separate steps. First, cuprite (Cu2O) precipitates form with one of the three special orientation relationships identified by TEM. Second, while the reaction front penetrates further into the matrix, Cu2O particles are reduced to metallic copper. The orientation relationship between matrix and precipitates is maintained during the loss of oxygen and, in a later step, evolves into a new and energetically more favourable one. A model based on this mechanism is developed and a comparison of the computed and measured reduction kinetics is presented.