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Radiation-Enhanced Diffusion of La in Ceria

Radiation-Enhanced Diffusion of La in Ceria. Summary NERI-C collaboration to study actinide surrogate and fission gas behavior in thin film UO 2 . Started with CeO 2 —development of UO 2 fabrication facilities required time.

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Radiation-Enhanced Diffusion of La in Ceria

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  1. 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 CeO2—development 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: Experimental—SIMS, XAS, XPS, RBS, TEM. Computational—kMC, DFT, MD. Outline Introduction to thermal diffusion and radiation-enhanced diffusion (RED). CeO2 system—cation vs. anion sublattice, film characterization Experimental results—SIMS profiles, analysis to determine diffusivities. Discussion of results—diffusivity vs. temperature, three temperature regimes, influence of vacancies on oxygen anion sublattice. Preliminary results of UO2 + Nd film growth. NERI-C PROJECT NO. 08-041

  2. 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 NERI-C PROJECT NO. 08-041

  3. Classical picture—transition state theory yields jump frequency over saddle point saddle point Interstitial self-diffusion-- dumbbell arrange- Ment. Diffusion—Microscopic point of view w/point defects Vacancy self-diffusion VSD Diffusion processes at microscopic scale coupled to point defects in crystalline solid D(T)=Do exp(-Ea/kT) NERI-C PROJECT NO. 08-041

  4. Diffusion—Activation 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. NERI-C PROJECT NO. 08-041

  5. Radiation Damage ProcessFreely-migrating defects, FMDs Frenkel pair population inside displacement cascade Few point defects (FMDs) survive displacement cascade quenching Fast neutron 1st struck atom (PKA) FMDs—vacancies and Interstitials in ~ equal numbers • Three phases • Formation • Recombination • Thermal spike Displacement Cascade—high Density of Frenkel Pairs (vac. + int.) NERI-C PROJECT NO. 08-041

  6. Radiation-Enhanced Diffusion (RED)—Combination of Elevated Point Defect Populations and Elevated Temperature Thermal v/i population>> Frenkel pairs Thermal VSD T>1100K Fate of FMDs sink sink Sink-Limited Kinetics T~850-1050K i v v+i recombination T<800K Recombination- Limited Kinetics v i Ballistic Mixing T~295K Temperature

  7. CeO2 and UO2 have same structure—Ceria often used as surrogate for Urania. Fluorite Structure—anions red, cations white Epitaxial relationship— Fluorite structure:R-plane Sapphire CeO2 Tm=2673 K a=5.4114 A UO2 Tm=3138 K a=5.466 A NERI-C PROJECT NO. 08-041

  8. 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 • (A=139) fission product. CeO2 370 Å ~3 Å 1ML LaCeO2 CeO2 Sapphire NERI-C PROJECT NO. 08-041

  9. 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 NERI-C PROJECT NO. 08-041

  10. Ion Bombardment—TRIM results 1.8 MeV Kr+ implantation into CeO2 on sapphire 1.8 MeV Kr+ Energy to Recoils—FD (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

  11. Secondary Ion Mass Spectroscopy (SIMS) O or Cs sputter beam rastered over 400 x 400 mm2 area Residual positive charge on sample surface after O sputter beam raster Au analytical beam Beam rastered over 50 x 50 mm2 area + + + + + + + Sample surface Positive-charged species liberated by analytical beam & accelerated across voltage bias—mass separated by time-of-flight CeO2 NERI-C PROJECT NO. 08-041

  12. XRD Analysis of MBE CeO2 film Specular Scan Rocking Curve In-plane f Scan CeO2 is single crystal—no grain boundaries. NERI-C PROJECT NO. 08-041

  13. SIMS Results—RT 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: s~26Å CeO2 • = 4 Å5/eV in CeO2 • = 120 Å5/eV in Au • ~ 1-5 Å5/eV in MgO NERI-C PROJECT NO. 08-041

  14. SIMS Results—Elevated 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: s~36Å K—Frenkel pair production rate K~0.02 1/s (heavy ion) K~10-10 1/s (fast neutron) Kv,i—defect removal rates at sinks v,i—point defect fractions induced by bombardment vo—thermal equil. vacancy fraction ni—interstitial jump frequency NERI-C PROJECT NO. 08-041

  15. Steady-State Solutions to Kinetic Rate Theory Total vacancy fraction Total interstitial fraction Diffusivities due to Frenkel defects Total diffusivity NERI-C PROJECT NO. 08-041

  16. Three Temperature Regimes Recombination limited: v+i=0 Low T <800K Sink limited: v  dislocation i  dislocation Intermediate T D’ ≠ f(T) High T >1100K VSD NERI-C PROJECT NO. 08-041

  17. Diffusivity versus Temperature D(T)=Do exp(-Ea/kT) VSD VSD RED NERI-C PROJECT NO. 08-041

  18. Discussion • Cation vs. Anion diffusion. • +3 dopant-anion vacancy cluster. • No influence from grain boundaries. NERI-C PROJECT NO. 08-041

  19. Ar2 Air Ar1 O2 Sputter Deposition Facility Schematic Foreline pump MFC1 MFC2 FV1 VV1 FV2 SV4 SV6 TP1 PG TCG Mass Spec. GV1 APC RV CG1 CM2 TP2 VV2 IG1 CM1 GV2 Sample Trans. Arm CM3 Primary Chamber Load- lock Thickness Monitor VLV CG2 IG2 SV5 TP—turbo pump GV—gate valve FV—foreline valve VV—vent valve SV—solenoid valve RV—relief valve VLV—variable leak valve CG—convectron gauge IG—ion gauge TCG—thermocouple gauge PG—Pirani gauge CM—capacitance manometer MFC—mass flow controller S—sputter gun S1 S2 S3 SV2 SV1 SV3

  20. 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. Ts=850 C NERI-C PROJECT NO. 08-041

  21. UO2 Single Crystal Film Growth on YSZ Strain free UO2 RBS—UO2 Smooth surface Single crystal domain NERI-C PROJECT NO. 08-041

  22. SIMS on UO2 + Nd Nd isotopes s~31 Å U-235 region U isotopes NERI-C PROJECT NO. 08-041

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