Defect Reactions and the Controlling Mechanism in the Sintering of Magnesium Aluminate Spinel

Abstract

Pressureless sintering studies have been conducted for excess Al 2 O 3 , stoichiometric, and excess MgO compositions of MgAl 2 O 4 at 1500‐1625°C. Initial powders of various compositions are prepared by solid‐state reaction of MgO and Al 2 O 3 . A Brouwer defect equilibrium diagram is constructed that assumes intrinsic defects of the Schottky type. The densification rate derived from sintering kinetics is compared with the compositions investigated when the concentration is converted to the activity of the two oxide components in MgAl 2 O 4 . The grain‐size exponent of p similar/congruent 3 suggests that densification takes place by a lattice‐diffusion mechanism in the solid state. Determined activation enthalpies of 489‐505 kJmol ‐1 are close to those obtained from oxygen self‐diffusion derived in previous sintering studies. It is, therefore, proposed that oxygen lattice diffusion through vacancies is the rate‐controlling mechanism for the sintering of nonstoichiometric MgAl 2 O 4 compositions. The discrepancy between densification‐rate ratios in experimental results and oxygen vacancy concentration in the Brouwer diagram is accounted for by the defect associates formed in the nonstoichiometric compositions.