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
Final Product Ceramic Synthesis Dried at 90°C • Calcination, Reduction, Sintering 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
0.5m 0.5m U Zr TEM Picture Th Mg 0.5m 0.5m 0.5m EDX Results 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
