Kinetics of the Internal Oxidation of (Mg, Fe)O Solid Solutions

Single crystals of Mg_1‐xFe_xO,x= 0.01, have been oxidized in air at temperatures between 1146 and 1389 K to determine the kinetics of the internal oxidation reaction. A two‐phase region composed of MgFe₂O₄ precipitates in a (Mg, Fe)O matrix grows inward from the surface as the reaction progresses. The thickness of this layer as a function of time has been determined by optical microscopy; the growth kinetics are parabolic. In addition, an external oxide layer of (Mg, Fe)O grows outward from the original surface. The thickness of this external oxide layer as a function of time was determined by following the displacement of inert markers with Rutherford backscattering spectrometry; the kinetics of the growth of this external layer are also parabolic in time. The parabolic reaction rate constant determined for the growth of the external layer accurately predicts that for the growth of the internally oxidized region.