Atomistic Segregation Behaviors of Ti in Al₂O₃ Grain Boundaries

Grain boundaries (GBs) have a great influence on the properties of polycrystalline Al₂O₃. Doping foreign elements has been known as one of the most popular strategies to alter the GB behaviors and hence material performances in Al₂O₃. Among various dopants for polycrystalline Al₂O₃, titanium (Ti) can strongly affect the resultant properties, such as conductivity [1], GB diffusivity [2] and creep resistance [3]. Although the structure-property relationships in the Al₂O₃ GBs have been widely studied [4], mechanisms of dopant segregation in the GBs are still under investigation. For instance, it is unclear how the GB structures affect the segregation of a certain element and how the dopant segregation affects GB electronic band structures. With the aid of the state-of-the-art scanning transmission electron microscopy (STEM), it becomes possible to uncover atomic segregation sites in doped GBs. In this study, we investigated the atomistic segregation behaviors of Ti dopants in two Al₂O₃model GBs (Σ7{4-510} and Σ7{2-310}) and the effect of Ti dopants on the GB bandgaps, using STEM, energy dispersive X-ray spectroscopy (EDS) and valence electron energy-loss spectroscopy (VEELS). Bicrystal-based model experiments were performed for the analysis of atomic structures, which enable us to find out the decisive factors governing the GB segregation. Our results indicate that the preferential segregation of Ti at specific atom sites in GBs was mainly driven by the ionic size mismatch between Ti³⁺ and Al³⁺. Furthermore, VEELS measurements showed that Ti doping has introduced the extra impurity state in Al₂O₃GB bandgaps.<br/> <br/>References<br/>[1] H. Unno, et al. J. Electron Microsc. 59 (2010) S107.<br/>[2] H. Yoshida, et al. Mater. Trans. 50 (2009) 1032.<br/>[3] H. Yoshida, et al. Acta Mater. 50 (2002) 2955.<br/>[4] J. P. Buban, et al. Science 311 (2006) 212.