Ionic Double Layers at the Surface of Mg-doped Aluminum Oxide: Its Effect on Segregation Properties
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A model involving ionic double layers at the surface has been constructed for magnesia-doped sapphire based on earlier models which were developed for cubic halides. The model takes into account the presence of electrostatic potentials, isolated point defects, defect complexes, and special surface sites which can act as sources and sinks for ions. Equations have been set up for the various defect concentrations, and Poisson's equation has been solved numerically to give depth profiles for defects and corresponding electric fields. The calculations suggest that Mg2+ ions can segregate both to the free surface and to the space charge region. The effective (or Langmuir) enthalpy of segregation depends not only on the actual binding energies of the dopant ion, but also on other parameters such as the density of special surface sites. Over the temperature range studied, the variation of the calculated surface magnesium concentration with temperature is found to be approximately Arrhenius in nature, as was observed in segregation experiments.
Mukhopadhyay, S. M.,
& Blakely, J. M.
(1991). Ionic Double Layers at the Surface of Mg-doped Aluminum Oxide: Its Effect on Segregation Properties. Journal of American Ceramic Society, 74 (1), 25-30.