Annealing Studies of Irradiated HOPG using X-ray Measurements

Nidia C. GallegoRobbie a. MeisnerTim D. Burchell

Oak Ridge National Laboratory

INGSM-13

Seattle, WA – September 15, 2013

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Irradiation damage in graphite induces dimensional changes

• Polygranular graphite exhibits a polycrystalline structure, usually with significant texture resulting from the method of forming during manufacture.

• Structural and dimensional changes in polygranular graphite are a function of the crystallites dimensional change and the graphite’s texture.

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Tirr: 710°C.

Irradiation-induced anisotropic dimensional changes in graphite crystal

Interstitial defects will cause crystallite growth perpendicular to the layer planes (c-axis direction), whereas coalescence of vacancies will cause a shrinkage parallel to the layer planes (a-axis direction)

Image from J-P Bonal, et al., MRS Bulletin, Vol. 34 (2009)

After B.T. Kelly and Brocklehurst, Carbon 9, 783 (1971)

Highly oriented pyro-graphite

Dim

ensi

onal

Cha

nge

(%)

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Objective

• Single crystal (HOPG) annealing studies. In support of the IAEA CRP on mechanisms in irradiation creep of graphite, a new annealing study of the d-spacings in HOPG is being conducted at both ORNL and INL. ORNL will perform annealing studies on one-half of the HOPG specimens irradiated in the AGC-1 capsule and INL will analyze the other half.

• HOPG were included in experiment AGC-1 in piggy-back locations to provide data for the dimensional change rates of graphite crystals. These data in turn will be used in models for the dimensional change rates of polycrystalline graphites.

This work is Task 3.1.11 VHTR TDO Graphite R&D at ORNL

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About the HOPG Samples• GE Advanced Ceramics (now Momentive)• Strongsville, OH 44149• Pyrolitic graphite Monochromator• Grade ZYB• 5mm x 5mm x 2mm

http://www.momentive.com/Products/Main.aspx?id=22817

• HOPG samples were irradiated in the AGC-1 capsule, ATR reactor

• Target conditions:– Tirr = 600 °C

– Dose: 1.5 – 7 dpa

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HOPG Samples and their Irradiation Conditions

Sample Tirr [°C] Dose [dpa]

HOPG-unirradiated - -

CPB1 619 3.41

CPB21 678 5.06

CPB51 744 6.49

CPB71 751 6.82

CPB101 726 6.62

CPB121 690 6.01

CPB141 622 4.88

CPB161 511 3.09

Irradiation conditions fromDocument ID ECAR-1943 (Table 1)

ΔT = 240°C

ΔT ~ 110°C

ΔT ~ 57°C

ΔT ~ 73°C

ORNL received from INL 8 irradiated HOPG samples

There is a significant gradient of the irradiation temperature !!

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Results from Dimensional AnalysisPIE was carried out at both INL and ORNL

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Initial Experimental Plan

• Perform x-ray scans on irradiated HOPG samples– Monitor crystal lattice parameters <a> and <c>

(from d002, and d110) and crystallite size parameters, Lc, and La

• Conduct step-wise annealing of (all) irradiated HOPG and repeat x-ray scans after each annealing

• Proposed annealing temperatures– 700°C (assumed as the average irradiation temperature)– 1000, 1200, 1400 and 1600°C

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Diffracted x-ray beam

Experimental Setup

qq

Incident x-ray beam

(002)

(004)

(006) (008)

(100)

(110)

(200)

XRD was performed using PANAlytical machine using Cu-kα radiation, λ, of 0.154056 nm, 2Ɵ range of 10 to 140°

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Approach• Bragg Law was applied to calculate the interplanar spacing from the diffraction peaks at 2θ =

26.603° (002) and 2θ = 77.697° (110) :

nλ = 2d sinθ (1)

• Apparent crystallite sizes (or coherence length), in the <a> and <c> directions (La and Lc, respectively), were calculated applying the Scherer equation:

L = Kλ / β Cosθ (2)

• where K is a shape factor equal to 1 for highly graphitic materials and equal to 1.84 for poorly ordered carbons, β is the intrinsic breadth of the diffraction peak (FWHM value of the relevant diffraction peak), and θ is the diffraction angle of the relevant peak.

• Measured peak widths were corrected to allow for machine line broadening effects.

• Diffraction patterns were analyzed using “Jade™” software to derive the crystallographic parameters.

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Results to date

Sample Tirr [°C] Dose [dpa] Surface scan Edge scan

HOPG-unirradiated - -

CPB1 619 3.41 CPB21 678 5.06 CPB51 744 6.49 -

CPB71 751 6.82 -

CPB101 726 6.62 CPB121 690 6.01 -

CPB141 622 4.88 -

CPB161 511 3.09 -

Low dose

Medium dose

High dose

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Analysis of the (002) peakSample d(002) Lc

HOPG unirrad 3.3508 702

CPB1 3.3651 294

CPB21 3.3658 260

CPB51 3.3673 235

CPB71 3.3614 202

CPB101 3.3650 231

CPB121 3.3649 231

CPB141 3.3676 238

CPB161 3.3699 236

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Analysis of the (002) peakInterplanar spacing and thickness

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What happens with the Lc??

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Analysis of the (110) peak

Sample d(110) La

HOPG unirrad 1.2323 544

CPB1 1.2318 337

CPB21 1.2300 307

CPB101 1.2302 321

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Analysis of the (110) peak

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What happens with the La??

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Summary• ORNL has initiated annealing studies of HOPG samples

• Initial XRD scans after irradiation have been completed: – flat-on for all 8 irradiated HOPG– Edge-on for only 3 of the irradiated HOPG

• Preliminary analysis show (as expected):– Expansion on the c-axis direction– Shrinkage on the a-axis direction

• However, there is a huge difference in the magnitude of the changes of crystal dimension (<c> and <a>) and of ‘bulk’ dimensions of the HOPG

• This difference may be attributed to porosity generated during irradiation (reflected in a significant decrease in Lc and La), however additional research is needed to verify this.

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Future Work

• Complete analysis of current XRD data• Carry out step-wise annealing of selected

irradiated HOPG and repeat x-ray scans after each annealing

• Proposed annealing temperatures– 700°C (assumed as the average irradiation

temperature)– 1000, 1200, 1400 and 1600°C 1500 °C