Surface Analysis Surface interface controls many aspects of chemistry Catalysts Corrosion Thin films Surfaces Methods

Surface Boundary between solid and other phase Gas, vacuum, liquid Surface differs from solid bulk Decarburised surface layer on the seal rim. Preferential grain boundary oxidation evident

Spectroscopic Surface Methods Incident beam and secondary beam Photons, electrons Incident particle not same as secondary

X-ray photoelectron spectroscopy Examination of surface with x-rays and measurement of electrons Evaluation of elements on surface by x-rays and Auger A+g->A+*+e- Electron energy (Ek) is measured Eb=g-Ek-w w is work function

XPS The momentum p of the outgoing electron is determined from the kinetic energy

XPS spectra Chemical shifts can be observed Variation with oxidation state Substituant groups

Auger Electron Spectroscopy A+e1-->A+*+e1-’+eA- Relaxation can occur in two ways A+* => A++ + eA- eA- = Auger electron A+* => A+ + gf Auger emission types KLL LMM MNN Removal, transition to removed state, ejection of electron favored by low atomic number elements hnf => fluorescence photon

Auger Electron Spectroscopy

Similar to other procedures for fuel preparation Ceramic Synthesis Dried at 90°C Calcination, Reduction, Sintering Final Product Precipitate-acetone mix from Zr, Th, U salts with NH4OH ≈5 g total salt Similar to other procedures for fuel preparation

Ceramic Synthesis Parameters Use of H2 and reduction step examined Calcination Performed in air at 750 oC for one hour Reduction One hour at 600 oC under Ar/4% H2 Powders placed in 5 mm die and cold pressed at 55 MPa for 1-2 minutes Low pressure, higher surface area Not to standard fuel surface areas Sintering Performed in air or Ar/4% H2(g) at 1500 oC for 4 hours

Ceramic Characterization EDX (Energy Dispersive X-ray) (e-) Emission of characteristic X-rays XRD (X-ray diffraction) (g) EELS (Electron Energy Loss Spectroscopy) (e-) Loss of energy by monoenergetic e- Can be used to determine oxidation state 3d3/2 -> 5f5/2 (M4) and 3d5/2 -> 5f7/2 (M5) ratio Based on lanthanides XANES/EXAFS (g) Oxidation state and near neighbor chemistry

Element bright in EDX mapping EDX Results 0.5m TEM Picture Th Zr U Mg Element bright in EDX mapping Two phases found Low mutual solubility of Zr and Th Zr rich and Th rich phase Little solubility of Th in Zr U and Mg distributed throughout the ceramic

XRD Results: Standards U3O8 ThO2 UO2

Influence of synthesis conditions Zr6Th3UO20 Calcined in air/No reduction Calcined in air/Reduction No effect on the inclusion of reduction step: U as U(IV)

XRD Results U is reduced to the tetravalent state in Zr-Th-U ceramic Th and Zr stabilize tetravalent U Calcine in air, no reduction step, sinter under Ar/4% H2 Zr-U ceramic requires reduction step Calcination performed under reducing conditions more U incorporated into the ZrO2 lattice structure Unit Cell Measured for Th3UO8 5.57+0.01 Å

Th-U solid solution cell parameters Black points from Hubert et al (2001)

EELS Spectra ZrTh3UO10

EELS analysis Evaluation of U oxidation state Multiple analysis of samples Evaluation of M4/M5 ratio for U UO2: 0.41±0.03 U3O8: 0.48±0.04 Th3UO8: 0.40±0.03 ZrTh3UO10: 0.40±0.03 Tetravalent U for above samples U oxidation with higher Zr is noted in some samples Air ingress into furnace

X-ray Absorption Spectroscopy Utilizes x-rays from synchrotron source to probe local structure High intensity, broad spectral range Spectra can be separated into regions containing different information Global technique yields average structure of sample

XAS setup

XAS spectrum

XANES Spectroscopy X-Ray Absorption Near Edge Structure Region between absorption edge and start of EXAFS oscillations, up to 40 eV above edge Absolute position of edge contains information on oxidation state Also contains information on vacant orbitals, electronic configuration, and site symmetry

EXAFS Spectroscopy X-ray Absorption Fine Structure Above absorption edge, photoelectrons created by absorption of x-ray Backscattering photoelectrons effect x-ray absorption Oscillations in absorption above edge Oscillations used to determine atomic number Distance coordination number of nearest neighbors

XAS Procedure Scanned U, Th, and Zr separately Th L 3 edge to k = 13 U L 3 edge to k = 14 Zr K edge to k = 14 U L 2 edge also scanned Th EXAFS interference in U spectra due to proximity of edges

Th and U Edges

Uranium XANES Tetravalent U for Zr=0 or 1 U oxidation evident with higher Zr Agrees with EELS

Th3UO8 EXAFS

EXAFS Analysis EXAFS equation Phase(k) and Amp(k) calculated from theory Fit data to determine: N coordination number R bond length s Debye-Waller term

EXAFS Zr and U interchangeability limited Mg affects U solubility Increase in Mg decrease in U solubility ThO2 structure U and Th completely interchangeable in lattice Th-Th(U): 3.941 + 0.010Å Th-O: 2.402 + 0.005 Å

Th-Th(U) and Th-O distance

Characterization Results Two phases Th rich and Zr rich Tetravalent U in ZrTh3UO10 and Th3UO8 Identified by EELS and XANES Unit Cell Parameter and Th interatomic distances agree with other work Solubility Experiments pH 4, 7, and 10 under Ar, pH 4, 5.25, 6.5 under Ar/10% CO2 Collect samples up to 5 months