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