1. Impedance Spectroscopy: Can it be used to probe Radiation Damage
Impedance Spectroscopy: Can it be used to probe Radiation Damage? Richard Veazey Supervisors: Prof Derek Sinclair Dr Julian Dean Dr Amy Gandy Prof Simon Pimblott (Uni of Manchester) Department of Materials Science & Engineering, The University of Sheffield Joint ICTP/IAEA Workshop on Radiation Effects in Nuclear Waste Forms and their Consequences for Storage and Disposal Trieste, Italy, Thursday 15th September 2016

2. What is Pyrochlore? General formula - A2B2O7 A = 2+/3+, B = 5+/4+
2x2x2 superstructure of Fluorite (AO2)
All atoms reside on specific positions
2 Cation sites
3 Anion sites
Anion vacancy - 8a
Oxygen positional parameter x48f defines the relaxation from ideality
Lattice parameter ~ 10Å, ap=2af
A-cation
B-cation
Oxygen
Oxygen vacancy

3. Fluorite – Pyrochlore differences
AO2
Fluorite
Disordered
A2B2O7Vo
Pyrochlore
Ordered

4. Why Radiation damage in Pyrochlore?
Novel Fuel types
Candidate matrices for the immobilisation of Nuclear Waste
Ca0.89Gd0.22Hf0.23U0.44Pu0.22Ti2O7 A2
B2O7
Model Systems
Develop an understanding of how ceramics behave in radiation environments.
How does the composition and structure effect the response to radiation damage?
Pyrochlore structure – variety of compositions, little change in structure.
Image Credit: NRC

5. Standard techniques to characterise damage
GAXRD
Raman
Peaks/ Spots from the crystaline phase decrease in intensity.
Diffuse scattering.
Structural properties
TEM
Impedance Spectroscopy?!?!?
Electrical properties
Electrical properties more sensitive to defects – Easier to see changes in a material as a result of damage
Time dependence of recovery
Expensive and hard
Tedious sample prep
Li, Y., 274, , Park, S., 93, 1-11, Lian, J., 51, , 2003.

6. What is Impedance Spectroscopy?
Simple technique for measuring the electrical properties of materials.
Powerful technique as it is possible to separate contributions from electrically distinct regions, typically the bulk and grain boundaries.
Electrodes (Au or Pt) fired onto sample. An alternating voltage is passed through the sample.
Measure current which can be separated into in-phase and out-of-phase components – Complex impedance.
Measurement is repeated over a range of frequencies (Hz – MHz).
Formalisms
Z* - Impedance
M* - Electric modulus
Y* - Admittance
ε* - Permittivity
Kidner, N.,91, 6, 1733, 2008

7. Impedance Data Equivalent circuit – best represents the data
Individual components
Z” ∝ R – highlights large resistances
M” ∝ 1/C – highlights small capacitances
Sinclair, D.C., 66, , 1989
Li, M.., 13, 31-35, 2014

8. Experimental Solid state synthesis Characterisation Irradiation
X-ray Diffraction
Raman spectroscopy
Transmission electron microscopy
Scanning electron microscopy
Impedance spectroscopy
Irradiation
Ion implantation

9. Y2B2O7 (B= Ti and Zr) Synthesis
Stoichiometric amounts of Y2O3, ZrO2 and TiO2
Composition
Method
YTO-1
Uniaxial press, heated at 1500 °C in air for 8 hrs
YTO-2
Uniaxial press, CIP, heated at 1675 °C in air for 8 hrs
YTO-3
PVA, Unaxial press, CIP, heated at 1675 °C in air for 8 hrs
YZO-1
YZO-2
Uniaxial press, CIP, heated at 1750 °C in air for 8 hrs
YZO-3
SPS at 1500 °C and 50 MPa for 20 mins
CIP – Cold Isostatic Press
PVA – Polyvinyl alcohol ground into powder prior to pressing
SPS – Spark Plasma Sintering
Mill in IPA and milling media
Dry and sieve the powder
Zr only
Heat in Furnace – 1500°C for 8 hours
Pyrochlore
Fluorite
Composition
ρGeom (g cm-3) | % ρTh
ρArch (g cm-3) | % ρTh
YTO-1
4.20 | 84.7
4.46 | 89.8
YTO-2
4.70 | 94.7
4.72 | 95.0
YTO-3
4.52 | 91.2
4.73 | 95.2
YZO-1
4.43 | 79.9
5.23 | 94.3
YZO-2
4.67 | 84.3
5.24 | 94.5
YZO-3
4.85 | 87.5
5.37 | 96.9

10. Y2B2O7 (B= Ti and Zr) SEM 1500°C 1675°C 1675°C
Porosity low – density measurements reasonable
1675°C
1750°C
1500°C SPS
Porosity high – density measurements unreasonable

11. Importance of dense ceramics
Damage typically 1 μm deep – large pores can have large effects.
Characterisation techniques are surface sensitive – homogeneous damaged region required.
Porosity has significant effects on Impedance data.
Constriction Resistance
Dense ideal arc
Porous elongated arc
Credit: Lena Sjögren
Current takes longer path length to get around the pores

12. Modelling Impedance Spectroscopy Macroscopic contacts Local micro-contacts

13. Model Setup Model drawn using CAD Ra Ca Rb Cb Material a – Amorphous
σa = 1 μS/m εr,a = 1000
Material b - Bulk crystalline
σb = 100 μS/m εr,b = 1000
Current flow homogeneous

14. Macroscopic Contacts
Model solved using ElCer. Dean, J., J. Am. Ceram. Soc., 97, 3,
σa = 1 μS/m
εr,a = εr,b = 1000
Amorphous response
Amorphous response
Bulk response Rt Ca Ct
σb = 100 μS/m
Bulk response

15. Macroscopic Contacts
Model solved using ElCer. Dean, J., J. Am. Ceram. Soc., 97, 3,
ωRC=1
σa = 1 μS/m
σb = 100 μS/m

16. Macroscopic Contacts
Amorphous
Bulk
Bulk
Generated using SRIM
Amorphous
Unless amorphous region is very resistive, signal will be very small.
Need a method to achieve a greater signal from amorphous region with as little from the bulk as possible.
Microcontacts…

17. Local Microcontacts
Bulk GB
By picking certain electrodes, can fine tune the experiment to detect the response from certain components.
Bulk
Fleig,J.App.Phys.,87,5,2000

18. Local Microcontacts ?? Variables Erad, Esep R1 C1 Bulk crystalline
σb = 100 μS/m εr,b = 1000 ??
Current flow heterogeneous

19. Local Microcontacts
90%

20. Local Microcontacts Macroscopic contacts
Current penetrates further into the material with increasing Esep.
Current flow through material in this setup not understood.
How does electrode geometry impact the impedance response?
Once understood, increase the complexity of the model by introducing amorphous region.
Can control how deep into the material we probe!
(Am-2)

21. Conclusions Yet to acquire correct density for YZO samples.
SPS heat treatments planned.
Density important because of surface sensitive characterisation techniques and impedance.
Microcontact impedance planned.
Impedance with macroscopic contacts unlikely to probe radiation damage. Local microcontacts identified to achieve a greater signal from damaged region.
Initial results of Impedance with mirocontacts are promising.
Current density max at surface and drops rapidly with depth.
At closest separation, 90% of current density passes through a region 2.1 μm from surface.
Impedance Spectroscopy: Can it be used to probe Radiation Damage?
Preliminary results from the modelling suggest it can!
But, further modelling is vital to be able to interpret the data and understand the response that is obtained from experimental work.

22. Thanks for Listening!
Any Questions?