Partial reduction of NiAl₂O₄ - mechanism and the influence of doping

Metal-ceramic microstructures were formed in situ by partial reduction of the spinel compound NiAl₂O₄. At 1300°C the reduction reaction passed through an intermediate stage involving the formation of Ni and metastable 'defect spinel' phases, before the appearance of the equilibrium phases, Ni + α-Al₂O₃. At 1100°C, depending on the reduction time, the products of the reduction reaction were Ni and the metastable 'defect spinel' for less than 24 hours; Ni + θ-Al₂O₃ for 24-48 hours; and Ni + α-Al₂O₃ for 144-168 hours. The effects of dopants, such as Cr₂O₃, MgO, TiO₂ and Y₂O₃, on the reduction mechanism and kinetics were examined and shown, in the case of TiO₂, to have a controlling influence. A reduction reaction mechanism is proposed.