Effect of Physical Nature of Powders and Firing Atmosphere on ZnAI ² O ⁴ Formation

Abstract

The rate of ZnA1 2 O 4 formation for binary powder mixtures of ZnO and α‐Al 2 O 3 (dense coarse particles and weak agglomerates of fine powder) fired in air or O 2 atmospheres was measured and the microstructures of those systems were observed by scanning electron microscopy. With dispersed dense particles of α‐Al 2 O 3 , the Al 2 O 3 surfaces were covered with ZnO and the spinel grew into the particles maintaining essentially a constant reaction interface area. Calculations based on geometric measurements and use of Jander's equation gave a similar high activation energy, 354 kJ/mol, which corresponds to the activation energy of volume diffusion of Zn 2+ in ZnAl 2 O 4 . An oxygen atmosphere had no effect. With a matrix of fine α‐Al 2 O 3 powder and dispersed granules of ZnO, a higher reaction rate occurred because of an increase in reaction interface area due to penetration of the powder compact matrix by ZnO vapor, which was enhanced by an O 2 atmosphere. The reaction layer grew into the alumina matrix adjoining the ZnO granules with a parabolic rate law. Apparent activation energies below ∼200 kJ/mol were calculated.