Structure of defects and microstructure evolution
Kazuhiro Yasuda
Department of Applied Quantum Physics and Nuclear Engineering,
Kyushu University, JAPAN
MINOS 2nd Int. Workshop Irradiation of Nuclear Materials:
Flux and Dose Effects
November 4-6, 2015, CEA – INSTN Cadarache, France
Structure of Defects and Microstructure Evolution in Oxide Ceramics
– Role of Electronic Excitation and Selective Displacement Damage –
Excellent radiation resistance• Resistance to amorphization and volumetric swelling
• Successful achievements as LWR fuels: UO2, (U,Pu)O2
• Potential applications to inert fuel matrix, transmutation target: stabilized ZrO2
• A surrogate of UO2:CeO2
Oxide ceramics: fluorite- and spinel-type
Radiation effects and radiation resistance•Production rate of point defects
⇒ difference between cations and anions
•Recombination rate of point defects
⇒ structural vacancy: cation site (spinel), anion site (YSZ)
•Stability of extended defects: size of stable nuclei (ex. dislocation loop)
•Sensitivity to electronic excitation
⇒ defect kinetics (rather low electronic stopping)
⇒ ion tracks (high electronic stopping)
Topics
Selective displacement damage of oxygen sub-lattice
Structure of ion tracks
Stability of dislocation loops under electronic excitation
CeO2Ce ion
(cation)
O ion
(anion)Ref.
mass [amu] 140 16
44~58 <30 [1]
移動の活性化エネルギー
2.1~5.4 0.5~0.6 [1],[2]
移動の活性化エネルギー
6.1 1.1 [2]
Ce ion
O ion
[1] K. Yasunaga, et al, NIMB 266 (2008) 2877.
[2] A. Guglielmetti et al, NIMB (2008) 5120.
Displacemend energy
V - migration enegy
I - migration energy
[eV]
[eV]
[eV]
⇒Role of oxygen point defects is important for defect
kinetics in fluorite-type oxides.
High production rate and mobility of oxygen defects
(Fluorite-type)
0.00 s 174.00 s 388.30 s 388.35 s288.35 s
408.00 s 445.00 s 497.00 s500 nm
a b c d e
f g h ji
521.00 s 689.00 s
K Yasuda et al, JNM 319 (2003) 74.
Anomalous defects in YSZ: selective displacement of O-ions
300 keV O ions ⇒ 200 keV electrons
0.00 s 174.00 s 388.30 s 388.35 s288.35 s
408.00 s 445.00 s 497.00 s500 nm
a b c d e
f g h ji
521.00 s 689.00 s
K Yasuda et al, JNM 319 (2003) 74.
Anomalous defects in YSZ: selective displacement of O-ions
300 keV O ions ⇒ 200 keV electrons
• Strong stress and strain field around the defect
• Entirely different growth process
• Multiplication of dislocations during the growth.
Similar oxygen-type defects (loops) in CeO2
100 nm
200 keV 500 keV 750 keV 1000 keV 1250 keV
(at 300 K. F ~ 3×1026 e/m2)
With decreasing e-energy, loop size is increased and density is decreased.
B // <111>, on (111) planes K Yasunaga et al, NIMB (2008)
A model for charged disl. loop consist of O-ions
accumulation of oxygen ions,
preferentially at dislocations or
invisible defect clusters
oxygen ions are considered to
lose electrons during diffusion
process
the defect clusters are considered
to trap free electrons and grow as
a charged dislocation loop
Y
ik
E
ikik driven by an electric field
driven by an elastic strain
n
1200
1000
800
600
400
200
Rc/a
0.4 0.5 0.6 0.7 0.8 0.9 1.0
Zr O e
A. Ryazanov et al, JNM 323 (2003) 372.
Charge of O-n ions
HAADF STEM image of Dislocation loops in CeO2
[111]
[111][011]
200 keV electron irradiation at 300 K
S. Takaki et al. Mater. Res. Soc. Symp. Proc.
1514 (2013) 93.
HAADF STEM image
Lattice planes are strongly distorted around the dislocation loop.
No additional Ce-plane is inserted at the dislocation loop, indicating that this is not the perfect dislocation loop.
0.31 nm
Ce ion
O ion
2 nm
HAADF STEM image
2 nm
P1
P5
P10
P15
P20
P25
P30
P1
P5
P10
P15
P20
P25
P30
0.31 nm
0.49 nm
⇒The loop is suggested to be on a (111) plane consist of
oxygen ions.
Topics
Selective displacement damage of oxygen sub-lattice
Structure of ion tracks
Stability of dislocation loops under electronic excitation
Fission fragments (E~70-100 MeV)
• high-density electronic excitation
(Se ~20 keV/nm)
• ion tracks
S.J. Zinkle et al., NIMB 141 (1998)
swift heavy ion
ion track
Ion tracks in fluorite and spinel-type oxides
• radiation resistant: no amorphization by individual ions
• threshold Se for continuous track formation
~10 keV/nm: MgAl2O4, ~15 keV/nm: CeO2
• At high fluence ⇒dislocation structure
MgAl2O4: amorphization (1020 m-2)
(Zinkle 2000)
CeO2: sub-grain formation
(Sonoda 2010, Garrido 2009)
13
3 x1011 (cm-2)
(a) Under-focus (b) Over-focus
Ion tracks appear as Fresnel contrast ⇒ decrease in atomic density.
Fluorite structure is retained.
10 nm
Bright-field kinematical TEM images
CeO2 irradiated with 200 MeV Xe : Se=27 keV/nm
K. Yasuda NIMB 314 (2013) 185.
MgAl2O4: Bright-field TEM images
Over focus Under focus
Core damage regions (2-3 nm insize) show Fresnel contrast.
5 x1015 (m-2)kinematical (off-Bragg) diffraction condition
K. Yasuda et al. Int. J. Mater. Res. (2011) 140.
2 nm
HAADF STEM Image of an Ion Track
CeO
0.27 nm
0.27 nm
3 x1012 (cm-2)
S. Takaki et al. NIMB 326 (2014) 140.
CeO2with 200 MeV Xe
Signal intensity profile around the ion track
Y10 Y20 Y30 Y40 Y50Y0
3
4
Inte
nsity (
a.u
.) (b)
X10 X20 X30 X40 X50X0
3
4
(c)
Inte
nsity (
a.u
.)
Ce-signal intensity decreases at the center of ion track (~2-3 nm).
The size, where the Ce signal intensity is decreased, is comparable to
the size of Fresnel contrast in BF image .
(a)
2 nm
Y30
Y40
Y20
Y10
X30 X40X20X10
(a)
CeO
[001]
[200]
[020]
(a)ABF STEM Image of an Ion Track
2 nm
CeO
[001]
[200]
[020]
0.27 nm
0.27 nm
3 x1012 (cm-2)
S. Takaki et al. NIMB 326 (2014) 140.
(c) core region
(b) peripheral region
CeO
Ce
[001]
[200]
[020]
(a)
2 nm
Accumulation of core damage region
BF-TEM Fresnel contrast
(a) Under-focus
1014
1015
1016
1014
1015
1016
1017
1018
1019
CeO2 irradiated with 200 MeV Xe
CeO2 irradiated with 210 MeV Xe
MgAl2O4 irradiated with 200 MeV Xe
Fluence (ions/m2)
Are
al density o
f io
n tra
cks (
m-2
)
spinel
ceria
Yasuda NIMB 314 (2013) 185.
The density is saturated at high fluence, although damage area does not
covers the whole region. ⇒ balance between the production and recovery
Accumulation of core damage region
BF-TEM Fresnel contrast
1014
1015
1016
1014
1015
1016
1017
1018
1019
CeO2 irradiated with 200 MeV Xe
CeO2 irradiated with 210 MeV Xe
MgAl2O4 irradiated with 200 MeV Xe
Fluence (ions/m2)
Are
al density o
f io
n tra
cks (
m-2
)
influence region
r
pre-existing core
damage region
incident ion
spinel
ceria
Yasuda NIMB 314 (2013) 185.
Interstitials are generated during the recovery process in the influence
region to create core damage regions with high concentration of vacancy.
spinel: r=7.0 nm
ceria: r=8.4 nm
Structure of ion tracks in CeO2
200 MeV Xe ions induces the decrease in the coordination
number of Ce ion (EXAFS), and also the generation of
ferromagnetism.
⇒ suggesting oxygen vacancy
formation in ion tracks.
H. Ohno et al., NIMB 266 (2008) 3013.
A. Iwase et al., NIMB 267(2009) 969.
MD simulation with highly energetic thermal
spike (36 keV/nm) induced vacancy clusters
at the core of ion tracks and interstitial
clusters at surrounding region.
C.A. Yablinsky et al., JMR (2015) .
36ps after thermal spike introduction 50 nm
Cross sectional view at ~1 mm depth
Dislocation network in CeO2 at 1x1014 cm-2
22
200 nm
Formation of subgrains at near surface region
K. Yasuda NIMB 314 (2013) 185.
210 MeV Xe ions: 1x1016 ions/cm2 at 573 K⇒ overlap with ~104 times
Topics
Selective displacement damage of oxygen sub-lattice
Structure of ion tracks
Stability of dislocation loops under electronic excitation
1017
1018
1019
1020
1021
1022
1023
10 100 1000 104
Loo
p D
ensity (
m-3
)
Electron-hole pairs/dpa Ratio
ZrFe
Fe
Mg Al
Al
Mg
C C He He H
650°C
10-6
to 10-4
dpa/sFe Al
C
He
H
H
Al2O
3
MgAl2O
4
Zr
Mg
Fe
MgO
He
H
Mg
S.J. Zinkle, MRS Symp. Proc. 439 (1997)
Loop formation is suppressed by electronic excitation.
Spinel is most sensitive to electroic exciation.
500 nm
500 nm
300 keV O+ ions
300 keV O+ ions
+ 200 keV electrons
K. Yasuda, Philos Mag.78 (1998)
Displacement damage and electronic excitation
Elimination of dislocation loops under electronic excitation
Electron flux:7.0×1022 e/m2s
Electronic stopping of 200 keV :~1 eV/nm
300 K MgAl2O4
K. Yasuda NIMB 266 (2008) 2834.
Areal density of dislocation loops vs electron fluence
0
1
2
3
4
5
0 2 4 6 8 10 12
300 K
350 K
400 K
450 K
Dis
location
Loo
ps D
ensity [×
10
15 m
-2]
Electron Fluence [×1025
e-/m
2]
0CL = C L exp(-lf t)
CL : loop density,
f : electron flux,
t : irradiation time ,
l : cross section
⇒ Evaluation of elimination cross section: l
Cross section(l) for elimination of loops
200 300 400 500 600 700 800
spinel
alumina
0
2
4
6
8
10
l[ x
10
-26 m
2]
T [K]
Same temp.dependence⇒same mechanism:loops dissociate into isolated interstitials
Loops in spinel is more unstable than alumina
MgAl2O4
a-Al2O3
K. Yasuda NIMB 266 (2008) 2834.
K. Yasuda NIMB 191 (2002) 559.
0
1
2
3
4
5
6
0 1 2 3 4 5 6
Loop1Loop2Loop3Loop4Loop5
Dis
loca
tio
n L
oo
p S
ize
[n
m]
Electron Fluence [×1025
e-/m
2]
0
2
4
6
8
10
12
0 1 2 3
Loop1Loop2Loop3Loop4Loop5
Dis
loca
tio
n L
oo
p S
ize [
nm
]
Electron Fluence [×1025
e-/m
2]
300K 450K
Size variation vs. electron fluence
Schematic showing for the elimination of loops
30
Summary
Selective displacement damage in fluorite-type oxides:
- Charged dislocation loops with oxygen ions are formed on (111)
planes
Ion tracks in fluorite- and spinel-type oxides:
- the core region is underdensed (with vacancies).
- influence region ~15 nm in diameter (invisible in TEM)
- interstitial generation to develop dislocation structure
Instability of dislocation loops under electronic excitation:
- dissociate loops into isolated interstitials
- stability of loops: MgAl2O44<Al2O3
31
Thank you for your attenstion