Transcript Penn State visit 18092006

Equipe mixte CEA-CNRS-ECP
Irradiation induced structural transformations in
normal spinels, potential materials for nuclear waste
management
G. Baldinozzi,
D. Simeone, D. Gosset, L. Lunéville, S. Surblé
Matériaux fonctionnels pour l’énergie
SPMS, CNRS - École Centrale Paris & DEN/DMN/SRMA, CEA Saclay
OUTLINE:
Coll. Synchrotron Soleil & ESRF
ISCSA TEM
JANNUS Orsay & Saclay
GANIL Caen
 Scientific Context
 Spinel structure
 Experimental & Results
 Model & Conclusion
G. Baldinozzi – June 23rd 2008, SUPELEC
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Equipe mixte CEA-CNRS-ECP
Ceramics are complex materials
 Understand their behavior at equilibrium (length scales)
 Characterize the equilibrium properties
ZrO2, HfO2, UO2+x
 Nanostructured ceramics: mechanical properties TiC, ZrC, SiC
 … and out of thermodynamic equilibrium (time frames)
 Material in working conditions (massive production of defects, ...)
 Modeling of the elementary mechanisms
Irradiation
Defect engineering to control the material properties at
different length scales and for different time frames
G. Baldinozzi – June 23rd 2008, SUPELEC
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Scientific aim
 Why do we study the behavior of spinel structures under
irradiation ?
 To understand and to model the properties of ceramics kept far from
their thermodynamic equilibrium
 To study the elementary mechanisms and to forecast the ceramics
behavior under irradiation
 Test models, impact of chemical bonds
 Driven alloys like in metals? Bragg William models ?
 Industrial context
 Radiation tolerant materials
 Disposal and transmutation of high level waste
G. Baldinozzi – June 23rd 2008, SUPELEC
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AB2O4 spinels
A cation is tetrahedrally
coordinated (divalent)
B cation is octahedrally
coordinated (trivalent)
 Filled octahedra form criss-cross rows, with alternating layers
of parallel rows offset as shown on the right side of the
picture. The square holes enclosed by the rows of octahedra
are filled with tetrahedra
G. Baldinozzi – June 23rd 2008, SUPELEC
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Spinel Chemistry: chemical bond & site selectivity
Charge density in slabs
ELF = 0.6 in glyphs
MgAl2O4
ZnAl2O4
•Chemical bonds and site selectivity
– Hybridization
– Crystal Field
– Then charge and size considerations kick in …
MgCr2O4
G. Baldinozzi – June 23rd 2008, SUPELEC
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Order & disorder in AB2O4 spinels
 “Normal” spinel structure: (A)[B2]O4
MgAl2O4, ZnAl2O4, FeCr2O4, FeAl2O4, MgCr2O4, …

Violates Pauling’s rules as larger cation (Mg) in tetrahedral site A. Controlled by crystal field
stabilization energies rather than simple packing geometry. Results in lower free energy
configuration.
 “Inverse” spinel structure: (B)[AB]O4
(Fe3+)[Fe2+Fe3+]O4, (Fe2+)[Fe2+Ti4+]O4

More standard, but still violates Pauling’s rules. Half of octahedral sites filled by larger A
cation.
 Annealing: nonconvergent cation disordering
(Navrotsky & al, J Inorg Nucl Chem 1961 – O’Neill & al, J Phys Chem Minerals 1994)
 a spectrum of disordered spinels that ranges from normal to
inverse
G. Baldinozzi – June 23rd 2008, SUPELEC
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GIXRD
•
•
•
Above the critical angle, the instrumental
broadening is reduced
SRIM calculations of ion implantation
and damage profile allow to optimize the
grazing angle
Microstructural information (lineshapes):
–
–
MgCr2O4
•
Structural information :
–

Radiation damage of a PWR simulated
by ion irradiation (JANNUS)
 The irradiated layer is rather thin
(0.2 µm – GIXRD)
strain fields
size of diffracting domain
–
–
Phase identification, amorphous
fraction
LRO - crystalline phases (atomic
positions, site occupancies)
SRO - amorphous phases (bond
distances, coordination number)
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Equipe mixte CEA-CNRS-ECP
Spinel irradiation at room temperature
Simulation of neutron irradiation by low energy ions (cascades)
and of fission products by swift heavy ions
PSD Detector
Equatorial Sollers Slits
XRD
Monochromator
or parabolic mirror
Beam
Sample
holder
± 1µm,
± 0,02°
50µm*5mm
X-ray diffraction:
Asymmetric reflection setup
Au @ 4 MeV
(fixed, grazing impinging beam)
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II) Structural evolution of spinels under irradiation
Behavior of spinel under irradiation: Modification of Diffraction patterns
High E.
ions
@RT
MgAl2O4
MgCr2O4
ZnAl2O4
Vanishing of odd
Bragg reflexions
Vanishing of odd
Bragg reflexions
Non vanishing of odd
Bragg reflexions
D. Simeone, J. Nucl. Mat. 2002
G. Baldinozzi, Nucl. Inst
Meth B. 2007
D. Simeone, J. Nucl. Mat. 2002
K Yasuda, MINB 2006, JNM
2007…
Low E.
ions
@ 140 K
Vanishing of odd
Bragg reflexions
Low E.
ions
@ RT
Non vanishing of odd
Bragg reflexions
Vanishing of odd
Bragg reflexions
Non vanishing of odd
Bragg reflexions
D. Gosset, J. Eur. Ceram. 2005
D. Gosset, J. Eur Ceram Soc,
2005
G. Baldinozzi, Nucl. Inst Meth
B. 2005
L.M. Wang, MRS 1995, R.
Devanathan, Phil Mag Let
1995
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XRD before & after irradiation at room T
MgAl2O4
MgCr2O4
• Structural information:
–
–
–
ZnAl2O4
–
The (ooo) peaks depend on the cation
distributions
In ZnAl2O4 no symmetry change
In the other two compounds the (ooo) peaks
have nearly or totally vanished
The structural refinements provide the
localization of the charge density in real space
G. Baldinozzi – June 23rd 2008, SUPELEC
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Equipe mixte CEA-CNRS-ECP
Electron density from X-ray diffraction
MgAl2O4
MgCr2O4
•
Fourier syntheses derived from the
observed diffracted intensities
indexed in the Fd3m space group
ZnAl2O4
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Equipe mixte CEA-CNRS-ECP
Comparison between the three spinel structures
 In ZnAl2O4 no symmetry change is detected and an increase of the
inversion parameter occurs as a function of the ion fluence

Irradiation induces a cations exchanges between the tetrahedral (8a)
and octahedral (16d) sites : isostructural phase transition similar to
the phase transition observed in spinel out of irradiation
 The charge density distribution is different in magnesium
compounds


A and B occupy 8a, 8b, 16c and 16d and 48f in MgAl2O4
A and B occupy mostly 16c and 16d MgCr2O4
 The charge density distributions in Mg spinels can be described in
the Fm-3m space group (a’=a/2): cations occupy the 4a and 8c
Wyckoff sites in Fm-3m
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Local structure: TEM in MgCr2O4
222
FFT
400
512 pixels = 23 nm
222
400
•
Odd Bragg reflections always exist but
they are broadened
– The structure is Fd-3m at the local
scale (20 nm)
•
The average structure is Fm3m over 500
nm (domain)
Diffraction from a 500 nm region
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Local structure : Raman scattering
 Active Raman Irr. Reps. For
ideal normal spinels:
MgCr2O4
Low energy ions
@RT
i 2T2g 2A1g Eg

MgAl2O4
Swift ions@RT
The number of frequencies shows that
tetrahedral sites are still occupied !
Raman shift : the inversion parameter is
about 18 %.
G. Baldinozzi – June 23rd 2008, SUPELEC
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Summary of structural results
 ZnAl2O4
 Under irradiation, a cations exchange occurs without any space
group modification
 Mg based spinels
 XRD diffraction : odd Bragg reflexions vanish
 Apparent space group Fm-3m
 Cations only on octahedral sites (4a Wyckoff positions): no tetrahedra,
disagrees with Raman scattering
 Cations on 4a and 8c sites: tetrahedral sites agree with Raman but
interatomic distances are too short
 TEM observations
 At the mesoscopic scale (20 nm in MgCr2O4) , the space group
is still Fd-3m
G. Baldinozzi – June 23rd 2008, SUPELEC
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Equipe mixte CEA-CNRS-ECP
Structural model
Radiation damage in these three compounds acts at two
different scales
 At the atomic scale:
 The local structure consists of octahedra and tetrahedra
 The space group is unchanged (Fd-3m) for all spinels
 Cation interchange occurs as in the thermal picture
 At the mesoscopic scale (few nanometers)
 Damage induced by a ion impact is spatially localized
 Coherent nanoregions are produced in spinels sharing the anion sublattice
 Spatial interference ‘averages’ their contributions over a large number of
regions of the crystal and it leads to an apparent symmetry change (Fm3m,
a’=a/2) in Mg based spinels
How to confirm this model ?
G. Baldinozzi – June 23rd 2008, SUPELEC
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Equipe mixte CEA-CNRS-ECP
Thermal annealing after irradiation
1200 K
(111)
(333/511)
600 K
(400)
 Thermal annealing of the extended
defects increases the size of the
coherent diffracting domains restoring
the “normal” structure
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Summary
 What can we learn from irradiation of spinel compounds?
 Irradiation acts at two different length scales
 Locally in a similar way as temperature does
 Since the spinel structure is the only stable one vs. temperature increase, only
cation inversion is observed
 Atoms cannot be freely mixed as in metal alloys because of atomic charges :
fewer new phases are expected in ceramics under irradiation …
 Radiation damage modifies the material at the mesoscopic scale
 Anion sublattice must provide charge balance
 The characteristic domain length scale possibly depends on the elements in
the material and on the energy deposition mode
 The correlation length seems to be
 Very large in ZnAl2O4: no scattering coherence
Fd-3m
 Very small for Mg based spinels: strong scattering coherence
Fm-3m
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