SIZE-DEPENDENT SOLUTE SEGREGATION AND TOTAL SOLUBILITY IN ULTRAFINE POLYCRYSTALS - CA IN TIO2

Grain boundary segregation at ultrafine grain sizes has been studied. Using a STEM microanalysis technique to quantify the grain boundary coverage of calcium (0.34 mol%) in TiO2 ranging in grain size from 50 to 750 nm, it is found that below grain sizes of 150-350 nm, segregation deviates from conventional isotherms, exhibiting a clear size dependence. In this size regime the interfacial area to volume ratio is as important as temperature and composition in determining grain boundary coverage. In the present system, grain boundaries become saturated with calcium when the coverage reaches approximately one half of an equivalent monolayer. The experimental results can be modeled by a statistical thermodynamical treatment of segregation which takes into account the large density of grain boundary sites in this size range. We also find direct evidence of enhanced total solubility at very fine grain sizes due to grain boundary segregation.