1. Sub-nanometer Scale Atomic Bonding Characterization via HAADF STEM
Yu Shi
Materials Science and Engineering

2. Old Question about Structural Ceramics
Substance Material Product Application
Intrinsic brittleness of β-Si3N4
Rare earth oxides  elongated grains  reinforced toughness
Why?
Zhu et al. Sci. Technol. Adv. Mater. 9, (2008)

3. Electron Microscopy Ernest Ruska Nobel Prize in Physics 1986
"for his fundamental work in electron optics, and for the design of the first electron microscope"
Source: Wikipedia.org, FEI.com, nobelprize.org

4. New-generation TEM STEM Scanning transmission electron microscopy
HAADF High-angle annular dark field
EELS Electron-energy-loss Spectroscopy
Z-contrast imaging Chemical Analysis
Cs-corrected Atomic resolving power ~1Å
Simultaneous EELS Electronic structure and bonding
Scan coil
Electron beam
Specimen
Z-contrast STEM
EELS
Browning et al. Nature 366, 143 (1993)

5. Preferential segregation in silicon nitride
Amorphous intergranular film (IGF)
La (Z = 57) segregates in IGF
Maximum intensity at IGF/grain interface
Partially occupied of Si terminating positions
Shibata et al. Nature 428, 730 (2004)

6. Preferential segregation in silicon nitride
Position of La identified (vaguely…)
Longer La-N bonding length than Si-N
Theoretical calculation shows eV higher bonding energy of La-N
Shibata et al. Nature 428, 730 (2004)

7. Further study: range of rare earth atoms
La: no periodic atomic sites associated
Sm: A and B sites occupied
Er: atom pairs (1.48 ± 0.04 Å)
Yb: atom pairs (1.46 ± 0.05 Å)
Lu: atom pairs (1.43 ± 0.07 Å)
Ziegler et al. Science 306, 1768 (2004)

8. Further study: bonding characteristics
Sm N45: A = B  same bonding
Si L23: A ≠ B  A: O-Si-N B: Si-N
Ziegler et al. Science 306, 1768 (2004)

9. Conclusion
Preferential segregation of rare earth atoms results in the suppression of grain growth in diametrical direction and the enhancement of growth in c-axis.
Depending on the atom size, electronic configuration, and the presence of oxygen at the interface, rare-earth atom bonds to different locations.
Reference: Shibata et al. Nature 428, 730 (2004) Ziegler et al. Science 306, 1768 (2004)

10. Thank you!
Q & A
Yu Shi