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