Early Damage Mechanisms in Nuclear Grade Graphite under Irradiation

1. Early Damage Mechanisms in Nuclear Grade Graphite under Irradiation Jacob Eapen•, Ram Krishna •, T. D. Burchell† and K. L. Murty• •Department of Nuclear Engineering North Carolina State University, Raleigh, NC 27695 †Carbon Materials Technology Group Oak Ridge National Laboratory, Oak Ridge, TN 37831

2. Objective and Summary • Disordering mechanisms in graphite have conflicting view points. • We use Raman spectroscopy, XPS and TEM to investigate the early damage mechanisms in NBG-18 under neutron/ion irradiation. • Our results show evidence for topological defects under irradiation, even at high doses. • Amorphization by direct collapse of vacancies is deemed unlikely, – instead it is likely mediated through multiplication of dislocations.

3. Disordering Mechanisms: Traditional View • Large number of interstitial and vacancies are generated. • Point defects agglomerate – collapse into dislocation loops. • Additional planes are formed between graphitic layers leading to expansion perpendicular to basal plane (c-axis). • In nuclear graphite collapse of micro-cracks initially masks the expansion along c-axis. Expansion: c-axis Contraction: a-axis

4. Disordering Mechanisms: Experimental Evidence • Bending and warping of basal planes are observed. • Strong evidence for formation of new planes/sheets from interstitials is lacking. Heggie, S.–Martinez, Davidson, Haffenden J. Nuclear Materials, 413 p. 150 (2011) • Current work: • Neutron irradiated NBG-18 • 0.002 dpa • 325 K

5. Disordering Mechanisms: Experimental Evidence Koike & Pedraza, J. Mater. Res. 1994;9(7):1899-1907. Tanabe et al, App. Phys. Lett. 61 p. 1638 (1992) • e- irradiation, no interstitial loops, large expansion along c-axis. • Bending and fragmentation of basal planes into nanocrystallites. • Partial-to-full amorphozation, at low temperatures and high dpa. ANS Meeting - Atlanta

6. Amorphization Mechanisms Disordered Region Model: • The defect (D) peak in Raman spectra is correlated to vacancy or vacancy clusters. Also mentions ‘in-plane’ defects. • On saturation, vacancies transform into disordered and amorphous regions (beyond a critical irradiation dose). Niwase, Physical Review B. 52, p. 15785 (1995)

7. Amorphization: Two Mechanisms Dislocation Accumulation Model: • Frenkelpairs generated by radiation give rise to divacancies. • They morph into stable dislocation dipoles that multiply with increasing irradiation dose. Niwase, Phil. Mag. Lett, 82, p. 401 (2002) Niwase, Int. J. Spectroscopy, ID: 197609 (2012)

8. Disordering Mechanisms: Recent Progress Karthik, Kane, Butt, Windes and Ubic J. Nuclear Materials, 412 p. 321 (2011) • Vacancy loops dissociate into prismatic dislocations. Incomplete planes, formed by climb mechanism, lead to expansion along c-axis

9. Disordering Mechanisms: Recent Progress Ruck and Tuck Mechanism Heggie, S.–Martinez, Davidson, Haffenden J. Nuclear Materials, 413 p. 150 (2011) • Basal edge dislocation (a) sweeping right to left, (b) climbs a plane, and (c) extends the ruck and tuck defect. (d) DFT simulation result.

10. Current Work • Neutron Irradiation • Low dpa: PULSTAR Reactor, NC State University. • Fast neutron flux: 2×1012 n/(cm2-s) • dpa = 0.002 – 0.01 dpa • Temperature: 325 K • High dpa: Oak Ridge Research Reactor (ORR). • dpa: 6.6/10.1 dpa • Ion Irradiation • University of Wisconsin through NSUF • dpa:1–25 • Temperature: 300 K – 900 K

11. High Temperature Materials Testing Capsule at NC State

12. Results: Raman Spectra Eapen, Krishna, Burchell and Murty, Materials Research Letters, In Press (2013)

13. Raman Spectra of Ion Bombarded Mono-layer Graphene G peak: arises from atomic vibrations . G’ peak: overtone of G peak. D peak: disorder peak arising from breathing modes of closed rings. D peak can emerge only from sp2 bonds. D’ peak: minor defect peak. LD: measure of the amount of disorder; the distance between defected regions. Cançadoet al. Nano Lett. 11(8), p. 3190 (2011) ANS Meeting - Atlanta

14. sp2 and sp3 Bonds sp2 sp3

15. Interpretation of Peaks in Raman Spectra • Solid-state Picture and the Molecular Picture • G Peak: • Stems from a single resonance process. • Corresponds to E2g phonon scattering with zero momentum at Γ point. • Proportional to the sp2 carbon sites. • Represents bond stretching of sp2 sites. • It occurs at the same Raman shift for defected and pristine crystalline structures. Ferrari and Robertson, Phys. Rev. B. 61, p. 14 095 (2000)

16. Interpretation of Peaks in Raman Spectra • Solid-state Picture and the Molecular Picture • D Peak: • Stems from a higher order, double resonance process. • Excitation of momentum q≠0 process: allowed for defected samples but not for crystalline structure. • Stems from the A1gbreathing mode. • No rings – No D peak. Ferrari and Robertson, Phys. Rev. B. 61, p. 14 095 (2000), Phil. Trans. R. Soc. Lond. A 362, p. 277 (2004).

17. Topological Defects • Maintain sp2 connectivity • Stone-Wales Defect: • Created by rotation of bonds. • No bonds are broken. • More Complex Defect Types

18. Topological Defects • More Complex Defect Types • Created by rotation of bonds, without broken bonds. • maintain sp2 bond structure, connectivity. • Interacts with point defects.

19. Topological Defects

20. Changes to Bond Structure with Irradiation Ferrari and Robertson, Phil. Trans. R. Soc. Lond. A 362, p. 277 (2004).

21. Changes to Bond Structure with Irradiation

22. Changes to Bond Structure with Irradiation sp2 sp3 Telling, Ewels, El-Barbary and Heggie, Nature, 2, p. 333 (2003)

23. Raman Spectra at 25 dpa with Ion Irradiation

24. Transmission Electron Microscopy at High Resolution (HRTEM) Virgin NBG-18 Irradiated NBG-18 (0.002 dpa) ANS Meeting - Atlanta

25. Evidence of Dislocation Loops & Partial Dislocations in NBG-18 (b) (a) (c) (d) Basal Dislocations splitting into partials ANS Meeting - Atlanta

26. Summary • We use Raman spectroscopy, XPS and TEM to investigate the early damage mechanisms in NBG-18 under neutron/ion irradiation. • Our results show evidence for topological defects under irradiation, even at high doses. • Amorphization by direct collapse of vacancies is deemed unlikely, – instead it is likely mediated through multiplication of dislocations.

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