Doping of alumina ceramics by manganese – Thermodynamical and experimental approach

The preparation of a transparent ultra-fine-grained doped ceramics requires a homogeneous dopant distribution in a matrix. In the present work, two thermodynamical phenomena allowing the preparation of such ceramics (the dissolution of the dopant and the formation of undesirable secondary phases) were experimentally and theoretically studied. A general thermodynamic-kinetic model was developed for dopant dissolution, which was verified for the experimental conditions used in this work. The model and experiment showed that Mn3O4 dopant with overall concentration of 1 at.% and particle size of 50 nm is dissolved and homogenized in a fine-grained alumina matrix within less than one hour at a temperature of 1220 °C. For the purposes of the study of the formation of undesired secondary phase, the phase diagram of the Al2O3-Mn3O4 system was calculated using the CALPHAD approach. Detailed STEM observations combined with EDX and EELS chemical analyses showed that the data used for the calculation of the phase diagram need some modifications because they overestimate the solubility of Mn in the alumina and underestimate the solubility of Mn in the MnAl2O4 spinel