Post on 01-Jan-2016
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SUB-NANOMETER SCALE ATOMIC BONDING CHARACTERIZATION VIA HAADF STEM Yu Shi
Materials Science and Engineering
OLD QUESTION ABOUT STRUCTURAL CERAMICS
Intrinsic brittleness of β-Si3N4
Rare earth oxides elongated grains reinforced toughness
Substance Material Product Application
Zhu et al. Sci. Technol. Adv. Mater. 9, 033001 (2008)
Why?
ELECTRON MICROSCOPY
Source: Wikipedia.org, FEI.com, nobelprize.org
Ernest Ruska Nobel Prize in Physics 1986
"for his fundamental work in electron optics, and for the design of the first electron microscope"
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)
PREFERENTIAL SEGREGATION IN SILICON NITRIDE
Amorphous intergranular film (IGF) La (Z = 57) segregates in IGF
Shibata et al. Nature 428, 730 (2004)
Maximum intensity at IGF/grain interface Partially occupied of Si terminating positions
PREFERENTIAL SEGREGATION IN SILICON NITRIDE
Position of La identified (vaguely…) Longer La-N bonding length than Si-N
Shibata et al. Nature 428, 730 (2004)
Theoretical calculation shows 0.5-5.3 eV higher bonding energy of La-N
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)
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)
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)
THANK YOU!
Q & A
Yu Shi