Analytical investigation of random grain boundaries of Zr-doped sintered α-Al₂O₃ by transmission electron microscopy and scanning transmission electron microscopy

The effects of the Zr doping in fine-grained alumina (α-Al₂O₃) on fine structures are studied. The combination of high-resolution transmission electron microscopy (HRTEM) and field emission scanning transmission electron microscopy has been a powerful tool for material investigations and it was employed to investigate grain-boundary structures, chemical composition and chemical bonding of 1000 ppm Zr-doped polycrystalline α-Al₂O₃. HRTEM revealed that the atomic structure of grain-boundary regions was not strongly distorted from that of the surrounding bulk, α-Al₂O₃. It was found that there was no amorphous phase at any grain boundaries. Energy-dispersive X-ray spectroscopy (EDS) and electron—energy-loss spectroscopy in a dedicated scanning transmission electron microscope used for analytical investigations provided spatial difference information about elemental-composition since the electron probe size in the scanning transmission electron microscope is as small as 3 Å. It was shown by the EDS that Zr segregated in the grain-boundary regions with the Zr-to-Alatomic ratio of 1.6 ± 0.2%. Normalization and subtraction of the matrix spectra from the interface spectra yield grain-boundary-sensitive near-edge structure of energy-loss near-edge structure (ELNES). Similar features are found for both the OK edge and the Al L_2.3 edge ELNES at the boundary. The origin of these features is ascribed to the heavily misshapen defects at the random boundary.