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Sub-nanometer Scale Atomic Bonding Characterization via HAADF STEM

Sub-nanometer Scale Atomic Bonding Characterization via HAADF STEM. Yu Shi Materials Science and Engineering. Old Question about Structural Ceramics. Intrinsic brittleness of β-Si 3 N 4 R are earth oxides  elongated grains  reinforced toughness.

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Sub-nanometer Scale Atomic Bonding Characterization via HAADF STEM

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

  2. Old Question about Structural Ceramics • Intrinsic brittleness of β-Si3N4 • Rare earth oxides  elongated grains  reinforced toughness Substance Material Product Application Why? Zhu et al. Sci. Technol. Adv. Mater. 9, 033001 (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 • STEMScanning transmission electron microscopy • HAADFHigh-angle annular dark field • EELSElectron-energy-loss Spectroscopy • Z-contrast imagingChemical Analysis • Cs-correctedAtomic resolving power ~1Å • Simultaneous EELSElectronic 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 • Maximum intensity at IGF/grain interface • Partially occupied of Si terminating positions • Amorphous intergranular film (IGF) • La (Z = 57) segregates in IGF Shibata et al. Nature 428, 730 (2004)

  6. Preferential segregation in silicon nitride • Theoretical calculation shows 0.5-5.3 eV higher bonding energy of La-N • Position of La identified (vaguely…) • Longer La-N bonding length than Si-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

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