Molecular Dynamics Simulations of the Locations of La Ions in La–Si–O–N Intergranular Films in Silicon Nitride

Abstract

Molecular dynamics computer simulations were used to determine the locations and energies of La ions in silicate intergranular films (IGFs) between basal and prism‐oriented silicon nitride crystals as a function of composition. La segregation to the interfaces occurs at each surface with no nitrogen present in the IGF, but segregates preferentially to the prism surface with nitrogen in the IGF. La adsorption onto the prism surface occurs in the simulations at the two sites observed previously in studies using high‐angle annular dark field scanning transmission electron microscopy. The binding energies of La at the two sites differ with no nitrogen in the IGF, and both are more strongly bonded than for La in the interior of the IGF. However, with nitrogen in the IGF, the binding energies are less different between the two prism surface sites and the interior of the IGF, indicating a decreased driving force for segregation. The simulations provide evidence justifying the preferential segregation of La ions to the prism surface and also indicate that a variation in the local composition would cause some IGFs to show segregation of all La to the interface, leaving little La within the glassy IGF, while other IGFs may show more La remaining in the glassy IGF.

Phase transformation and interface segregation behavior in Si3N4 ceramics sintered with La2O3–Lu2O3 mixed additive

“…The contradiction might arise from their assumptions that the coordination numbers of RE–N and Si–O are 4 and 6, respectively, which usually are 6 or more and explicitly 4, respectively, in the silicon‐based oxynitride glass. The molecular dynamics computer simulation predicts that Lu shows no adsorption onto the nitrogen‐terminated prism surface, but La does. With O replacing N, the simulations match the experimental coordinates of the adsorbed Lu ions.…”

“…With O replacing N, the simulations match the experimental coordinates of the adsorbed Lu ions. Based on the assumption that the prism surface be terminated with nitrogen, it was again proposed that La readily “poisons” the prism surface, inhibiting the grain growth to this direction . However, it would be expected that the prism surface should be terminated at least partially with oxygen, which was confirmed by the electron energy‐loss near‐edge structures of the interface between Si 3 N 4 grain and IGF .…”

“…The theory predicts that, of the RE, La should have the strongest and Lu the weakest preferential segregation to the grain surfaces. Segregation of RE ions to the β‐Si 3 N 4 grain prism surface would impede the attachment of Si‐based silicon nitride growth units, so‐called “poisoning effect”. The role of oxygen in the anion sites in the β‐Si 3 N 4 prism surface on Lu and La adsorption from the intergranular film was investigated via molecular dynamics computer simulation . No Lu adsorption onto the β‐Si 3 N 4 prism surface was observed at N terminated prism surface, and the simulations match the experimental coordinates of the adsorbed Lu ions at the surface when N atoms were replaced by O.…”

“…As a result, the segregation of RE ions at the interface was clearly visualized at the atomic scale by high‐angle annular dark‐field (HAADF) scanning transmission electron microscopy (STEM) in the last decade . Moreover, several numbers of theoretical calculations were also reported at the atomic scale, based on experimental results . The first research group reported that no specific segregation position was observed for largest RE ion, La, whereas other RE ions such as Sm, Er, Yb, and Lu showed clear segregation .…”

“…Once it has been understood that RE ions segregate at the interface, next question would be how it should be related to the grain growth behaviors, and several research groups proposed plausible mechanisms that evidently conflict each other as follows:…”