Early Damage Mechanisms in Nuclear Grade Graphite under Irradiation

Jacob Eapen•, Ram Krishna •, T. D. Burchell† and K. L. Murty•

•Department of Nuclear EngineeringNorth Carolina State University, Raleigh, NC 27695

†Carbon Materials Technology GroupOak Ridge National Laboratory, Oak Ridge, TN 37831

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Objective and SummaryDisordering 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.

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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

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Disordering Mechanisms: Experimental Evidence

Bending and warping of basal planes are observed.

Strong evidence for formation of new planes/sheets from interstitials is lacking.

Current work: Neutron irradiated NBG-18 0.002 dpa 325 K

Heggie, S.–Martinez, Davidson, HaffendenJ. Nuclear Materials, 413 p. 150 (2011)

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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.

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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)

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Amorphization: Two MechanismsDislocation Accumulation Model:

Frenkel pairs 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)

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Disordering Mechanisms: Recent Progress

Karthik, Kane, Butt, Windes and UbicJ. Nuclear Materials, 412 p. 321 (2011)

Vacancy loops dissociate into prismatic dislocations. Incomplete planes, formed by climb mechanism, lead to expansion along c-axis

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Disordering Mechanisms: Recent Progress

Heggie, S.–Martinez, Davidson, HaffendenJ. 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.

Ruck and Tuck Mechanism

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Current WorkNeutron 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

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High Temperature Materials Testing Capsule at NC State

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Results: Raman Spectra

1200 1300 1400 1500 1600 1700

0

1000

2000

3000

4000

5000

6000

7000 Virgin Irradiated

D' Peaks

G Peaks

D PeaksIn

tens

ity (a

rb. u

nits

)

Raman Shift (cm-1)

Eapen, Krishna, Burchell and Murty, Materials Research Letters, In Press (2013)

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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.

Cancado et al. Nano Lett. 11(8), p. 3190 (2011)

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sp2 and sp3 Bonds

sp2

sp3

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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)

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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 A1g breathing 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).

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Topological Defects

Maintain sp2 connectivity

Stone-Wales Defect:

Created by rotation of bonds. No bonds are broken.

More Complex Defect Types

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Topological DefectsMore Complex Defect Types

Created by rotation of bonds, without broken bonds. maintain sp2 bond structure, connectivity. Interacts with point defects.

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Topological Defects

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Changes to Bond Structure with Irradiation

Ferrari and Robertson, Phil. Trans. R. Soc. Lond. A 362, p. 277 (2004).

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Changes to Bond Structure with Irradiation

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Changes to Bond Structure with Irradiation

Telling, Ewels, El-Barbary and Heggie, Nature, 2, p. 333 (2003)

sp2

sp3

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Raman Spectra at 25 dpa with Ion Irradiation

1200 1300 1400 1500 1600 1700

0

500

1000

1500

2000

2500

3000

3500

G Peak

Inte

sity

(arb

. uni

ts)

Raman Shift (cm-1)

D Peak

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Transmission Electron Microscopy at High Resolution (HRTEM)

Virgin NBG-18 Irradiated NBG-18 (0.002 dpa)

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Evidence of Dislocation Loops & Partial Dislocations in NBG-18

(b)(a)

(d)(c)

[1210] [1100] [0110]3 3 3

a a a

Basal Dislocations splitting into partials

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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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Thank you!