Molecular dynamics (MD) computer simulations are used to evaluate the effect of composition of the IGF on the location and energetics of La ions in silicon oxy-nitride intergranular films (IGFs) between basal and prism oriented silicon nitride crystals. Results of the simulations show La adsorption at the four sites on the prism oriented nitride surface that are the same as those observed with HAADF-STEM. In addition, the simulations also show the effect of composition on this behavior, in which La segregation to at least the two closest surface sites on the prism surface is observed at almost all compositions, whereas La segregation to the basal interface is significantly affected by composition. With increasing La content, La site-filling increases on both prism and basal interfaces. Because of Lanthanum's role as a network modifier in a pure silica glass, IGFs with no N and low La content show complete segregation of La to the crystal surfaces. Increasing N content in the IGF reduces this La segregation to the interfaces, allowing for more La in the interior IGF. This is consistent with the change in binding energy of the La ions in the interior IGF in comparison to that at the interfaces, reducing the driving force for segregation. Because the additive rare earth 'poisons' the surface to which it adsorbs from further growth, the change in segregation is important in the development of microstructure in the ceramic. Importantly, increasing N content in the IGF causes preferential La depletion from the basal surface, with no such significant depletion of La on the prism surface, providing a mechanism for the anisotropic grain growth observed in this system.
ASJC Scopus subject areas
- Materials Chemistry