Kinetics and Mechanism of the Reduction of Nickel Oxide in Aligned Nickel Oxide-Zirconia and Nickel Oxide-Yttria Eutectic Structures

Unidirectional solidification experiments were carried out at the eutectic composition of the systems NiO-ZrO₂ and NiO-Y₂O₃, yielding materials made of regularly aligned lamellar structures. Eutectic samples were submitted to a chemical reduction treatment at 1075°C under CO/CO₂ mix tures corresponding to Po ₂ values for which only NiO should be reduced to the metal. The rapid formation of nickel be tween the zirconia or yttria platelets and the morphology of the nickel layer resulting from reduction establish that the reduction kinetics were controlled predominantly by the dif fusion of CO to the Ni/NiO interface through the porous Ni product. Experimental data of the variation of reduction depth of NiO as a function of time are qualitatively inter preted using a simple model of interdiffusion of CO in N₂/ CO/CO₂ mixtures. This model also explains the observation of an influence of the dilution of CO on the reduction depth.