13-1 Surface Analysis Surface interface controls many aspects of chemistry §Catalysts §Corrosion §Thin films Surfaces Methods.

13-1

Surface Analysis

• Surface interface controls many aspects of chemistry Catalysts Corrosion Thin films

• Surfaces• Methods

13-2

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

13-3

Spectroscopic Surface Methods

• Incident beam and secondary beam Photons, electrons

Incident particle not same as secondary

13-4

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+->A+*+e-

Electron energy (Ek) is measured

* Eb=Ek-w

w is work function

13-5

XPS

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

13-6

XPS spectra

• Chemical shifts can be observed Variation with

oxidation state Substituant groups

13-7

Auger Electron Spectroscopy

• A+e1-->A+*+e1

-’+eA-

Relaxation can occur in two ways A+* => A++ + eA-

* eA- = Auger electron• A+* => A+ + f

Auger emission types KLL LMM MNN

Removal, transition to removed state, ejection of electron

• favored by low atomic number elements

hf => fluorescence photon

13-8

Auger Electron Spectroscopy

13-9

Ceramic Synthesis

Precipitate-acetone mix from Zr, Th, U salts with NH4OH

≈5 g total salt

Dried at 90°C

Final Product•Calcination, Reduction, Sintering

Similar to other procedures for fuel preparation

13-10

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

13-11

Ceramic Characterization

• EDX (Energy Dispersive X-ray) (e-) Emission of characteristic X-rays

• XRD (X-ray diffraction) ()

• 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 () Oxidation state and near neighbor chemistry

13-12

0.5m 0.5m

TEM Picture Th Zr U Mg

0.5m0.5m 0.5m

• 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

EDX ResultsElement bright in EDX mapping

13-13

XRD Results: Standards

ThO2

UO2

U3O8

13-14 No effect on the inclusion of reduction step: U as U(IV)

Influence of synthesis conditions

Zr6Th3UO20

Calcined in air/No reduction

Calcined in air/Reduction

13-15

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

13-16

Th-U solid solution cell parameters

0,0 0,2 0,4 0,6 0,8 1,0

5,46

5,48

5,50

5,52

5,54

5,56

5,58

5,60

5,62

a (Å) = 5.598(4) - 0.124(6) x (U)

Uni

t ce

ll p

aram

eter

(Å

)

Substitution ratio (x)

Black points from Hubert et al (2001)

13-17

EELS Spectra

0

500

1000

1500

2000

3400 3500 3600 3700 3800 3900

Th rich phaseZr rich phase

Co

un

ts

Energy Loss (eV)

Th M4 Edge

Th M5 Edge

U M5 Edge

U M4 Edge

ZrTh3UO10

13-18

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

13-19

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

13-20

XAS setup

13-21

XAS spectrum

13-22

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

13-23

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

13-24

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

13-25

Th and U Edges

13-2617160 17190 17220 172501.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

No

rma

lize

d A

bso

rba

nce

eV

UO2 Std

Th3UO

8

ZrUO2

ZrTh3UO

xx

ZrTh3UO

xx+ Mg

Zr3Th

3UO

xx

Zr3Th

3UO

xx+ Mg

Zr6Th

3UO

xx

Zr6Th

3UO

xx+ Mg

Uranyl Std

U3O

8 std

Uranium XANES

• Tetravalent U for Zr=0 or 1

• U oxidation evident with higher Zr Agrees with

EELS

13-27

Th3UO8 EXAFS

13-28

EXAFS Analysis

• EXAFS equation

• Phase(k) and Amp(k) calculated from theory• Fit data to determine:

N coordination number

R bond length

Debye-Waller term

13-29

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 latticeTh-Th(U): 3.941 + 0.010ÅTh-O: 2.402 + 0.005 Å

13-30

Th-Th(U) and Th-O distance

13-31

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