Ferrite Formation Mechanism

Abstract

The most significant information, for the interpretation of the ferrite formation mechanism is provided by methods based on the study of diffusion couples in a semi-infinite medium between two oxides. It is by such experiments that many investigators studied the solid state reaction between Fe₂O₃ and MgO (1,2,3,4,5). However studies of diffusion couples are really convenient only if the examinations lead to quantitative results:

• 1. To start with, good contact must be achieved between the oxides to avoid any local gas phase transport at the interface. In our diffusion experiments, the specimens were kept in contact through the application of pressure by means of alumina rods. Moreover, marking of the initial interface was achieved by platinum sputtering, forming a fine layer about 500 A thick. This layer is transformed into globules about 11.ím in diameter during heat treatment. This marker technique does not disturb contact and we were able to confirm that it does not interfere with diffusion.

• 2. Local variations in volume, or material transport, linked to the formation of the intermediate compound, must be brought into evidence at all points of the diffusion couples.

• 3. Concentration variations throughout the diffusion couple must be known for a correct evaluation of the flux of diffusing elements, and to interpret measurements of spinel quantities formed on both sides of the initial interface.

• 4. It is necessary to determine the diffusion coefficients of the cations in the same conditions of temperature, partial oxygen pressure and composition as those in which the spinel is formed.