Radiation-Enhanced Diffusion of La in Ceria

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Radiation-Enhanced Diffusion of La in Ceria

• Summary
• NERI-C collaboration to study actinide surrogate
  and fission gas behavior in thin film UO2.
• Started with CeO2development of UO2 fabrication
  facilities required time.
• Use of thin film samples with controlled
  microstructure and impurity content.
• Behaviors of interest diffusion, segregation,
  bubble formation influence of grain boundaries.
• Techniques ExperimentalSIMS, XAS, XPS, RBS,
  TEM. ComputationalkMC, DFT, MD.
• Outline
• Introduction to thermal diffusion and
  radiation-enhanced diffusion (RED).
• CeO2 systemcation vs. anion sublattice, film
  characterization
• Experimental resultsSIMS profiles, analysis to
  determine diffusivities.
• Discussion of resultsdiffusivity vs.
  temperature, three temperature regimes, influence
  of vacancies on oxygen anion sublattice.
• Preliminary results of UO2 Nd film growth.

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Acknowledgements

• University of Illinois
• J. Stubbins, R. Averback. P. Bellon, J. Eckstein
• H. Pappas, M. Strehle, H. Ju, M. El-Bakhshwan, X.
  Han, D. Heuser.
• T. Spilla, D. Jeffers, S. Burdin
• Funding
• DOE NEUP/NERI-C program
• UIUC MRL and DOE

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DiffusionMicroscopic point of view w/point
defects
Diffusion processes at microscopic scale coupled
to point defects in crystalline solid
D(T)Do exp(-Ea/kT)
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DiffusionActivation Energy of point defects
D(T)Do exp(-Ea/kT)
Activation Energy, Ea Vacancy Interstitial Ea
Ef Em 1 eV 2 eV Ef energy of
formation 0.2 eV 2 eV Em energy of
migration 1 eV 0.1 eV Interstitial defects
more costly to make, but easier to move. As a
consequence, VSD dominate mechanism for
self-diffusion.
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Radiation Damage ProcessFreely-migrating
defects, FMDs
Few point defects (FMDs) survive displacement
cascade quenching
FMDsvacancies and Interstitials in equal
numbers
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Radiation-Enhanced Diffusion (RED)Combination
of Elevated Point Defect Populations and Elevated
Temperature
Thermal VSD
Fate of FMDs
Sink-Limited Kinetics
Recombination- Limited Kinetics
Ballistic Mixing
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CeO2 and UO2 have same structureCeria often used
as surrogate for Urania.
Fluorite Structureanions red, cations white
Epitaxial relationship Fluorite
structureR-plane Sapphire
CeO2 Tm2673 K a5.4114 A UO2 Tm3138 K a5.466 A
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Sample Architecture w/La Impurity Layer

• Two ways to consider La?CeO2
• Tracer or marker layer for cation diffusion
• 3 dopant in CeO2
• La is 3 actinide surrogate
• (Am, for example) and high-yield
• (A139) fission product.

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Experimental Facilities at Illinois

• Microanalytical AES, SIMS, RBS, XRD/XRR, TEM.
• Implantation/Bombardment Van de Graaff (0.5-2.3
  MeV H, He, Xe, Kr, Ne 100 nA).
• 1.8 MeV Kr ions 100 nA variable fluence
  variable temperature.

Physical Electronics PHI Trift III SIMS Instrument
High Voltage Engineering Van de Graaff Accelerator
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Ion BombardmentTRIM results
1.8 MeV Kr implantation into CeO2 on sapphire
1.8 MeV Kr Energy to RecoilsFD (need later)
CeO2
FD 115 eV/Å/ion
sapphire
FD
Kr
CeO2
Variable temperature, constant fluence
bombardment F 1x1016 ions/cm2
? 0.02 FIMA 2 burnup
NERI-C PROJECT NO. 08-041
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Secondary Ion Mass Spectroscopy (SIMS)
O or Cs sputter beam rastered over 400 x 400 mm2
area
Au analytical beam Beam rastered over 50 x 50 mm2
area
Residual positive charge on sample surface after
O sputter beam raster







Sample surface
Positive-charged species liberated by analytical
beam accelerated across voltage biasmass
separated by time-of-flight
CeO2
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XRD Analysis of MBE CeO2 film
Specular Scan
Rocking Curve
In-plane f Scan
CeO2 is single crystalno grain boundaries.
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SIMS ResultsRT
1.8 MeV Kr bombardment Variable fluence
constant T
Ballistic mixing parameter x Dt /FFD Relates
to energy deposition to RMS distance
La depth profiles
1-D Diffusion Geometry s 2 Dt 2Dt (sirr)2
(sref) 2
As grown s26Å

• 4 Å5/eV in CeO2
• 120 Å5/eV in Au
• 1-5 Å5/eV in MgO

CeO2
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SIMS ResultsElevated T


1.8 MeV Kr bombardment Variable T constant
fluence
Kinetic Rate Theory
Time rate of change Production Loss to sinks
- Loss via
recombination
La depth profiles
RT irradiated s36Å
KFrenkel pair production rate K0.02 1/s (heavy
ion) K10-10 1/s (fast neutron) Kv,idefect
removal rates at sinks v,ipoint defect fractions
induced by bombardment vothermal equil.
vacancy fraction niinterstitial jump frequency
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Steady-State Solutions to Kinetic Rate Theory
Total vacancy fraction
Total interstitial fraction
Diffusivities due to Frenkel defects
Total diffusivity
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Three Temperature Regimes
Recombination limited vi0
Low T lt800K
Sink limited v ? dislocation i ? dislocation
Intermediate T
D ? f(T)
High T gt1100K
VSD
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Diffusivity versus Temperature
D(T)Do exp(-Ea/kT)
VSD
VSD
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Discussion

• Cation vs. Anion diffusion.
• 3 dopant-anion vacancy cluster.
• No influence from grain boundaries.

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Magnetron Sputtering System at Illinois
Targets depleted U Ce Nd Mo Power Supply 3
DC 1 RF Gas Supply O2 1x10-9 to 1x10-3 T
Ar 1 to 100 sccm Max. Ts850 C
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UO2 Single Crystal Film Growth on YSZ
Strain free UO2
RBSUO2
Smooth surface
Single crystal domain
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SIMS on UO2 Nd
Nd isotopes
s31 Å
U-235 region
U isotopes