Effect of Magnesia on Mass Transport at the Surface of Alumina
The modification of surface mass transport rates due to the presence of a divalent dopant in alumina has been studied. The composition of the prismatic (1010) surface of sapphire, doped with MgO by two different techniques, has been monitored with Auger electron spectroscopy. Mass transport at these doped surfaces has been measured by observing the rate of decay of a periodic surface profile. Short wavelength patterns were used so that the transport process would be dominated by solid state diffusion. The analysis assumes an isotropic surface free energy, the implications of which are discussed. It was found that during annealing in air, both surface and bulk diffusion mechanisms contribute to ionic mobility. The effect of Mg ions seems to depend on its thermodynamic activity. When the surface coverage is low, up to about 20%, the effect of the dopant is not significant. When the surface concentration of Mg increases to that corresponding to spinel (about 33%), the mass transport rate increases significantly. From these observations, it is implied that a second phase may be required to markedly enhance the surface mass transport rate of alumina in air. The results obtained in this study have been critically analyzed and are discussed in light of other data for mass transport in alumina.
Mukhopadhyay, S. M.,
& Blakely, J. M.
(1988). Effect of Magnesia on Mass Transport at the Surface of Alumina. MRS Proceedings, 138.