Effect of electric fields on solid-state reactions between oxides

Interdiffusion in sintered polycrystalline pellets of MgO and Al₂O₃ at temperatures in the range 1300 to 1450° C was shown by scanning electron microscope energy dispersive X-ray probe (EDAX) and X-ray diffraction techniques to be enhanced by the application of direct current electric fields, particularly when the alumina pellet is of negative polarity, under which conditions the diffusion activation energy of Mg in Al₂O₃ is significantly increased. The diffusion process involved, principally, the transport of Mg by grain-boundary diffusion and possibly vapour-phase transport into the interstices of the comparatively porous alumina pellets; only minimal counter-migration of aluminium was observed in these experiments. The Mg-content of the discrete spinel grains within the reaction interface region is increased by electrolysis above the values calculated from solid solubility data, particularly where the alumina pellet is of positive polarity. The excess Mg is most apparent at the side of the spinel grains nearest the reaction interface, and precipitates as a surface MgO layer on the grains. The effective electrical mobility of the migrating species was calculated from the down-field shift of the diffusion profile and compared with values calculated from the electrical conductivity of the system.