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Annealing Studies of Irradiated HOPG using X-ray Measurements

Annealing Studies of Irradiated HOPG using X-ray Measurements. Nidia C. Gallego Robbie a. Meisner Tim D. Burchell Oak Ridge National Laboratory. INGSM-13 Seattle, WA – September 15 , 2013. Irradiation damage in graphite induces dimensional changes.

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Annealing Studies of Irradiated HOPG using X-ray Measurements

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  1. Annealing Studies of Irradiated HOPG using X-ray Measurements Nidia C. Gallego Robbie a. Meisner Tim D. Burchell Oak Ridge National Laboratory INGSM-13 Seattle, WA – September 15, 2013

  2. 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 polygranulargraphite are a function of the crystallites dimensional change and the graphite’s texture.

  3. Irradiation-induced anisotropic dimensional changes in graphite crystal 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 Dimensional Change (%) Tirr: 710°C. 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)

  4. Objective This work is Task 3.1.11 VHTR TDO Graphite R&D at ORNL • 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.

  5. About the HOPG Samples • GE Advanced Ceramics (now Momentive) • Strongsville, OH 44149 • Pyrolitic graphite Monochromator • Grade ZYB • 5mm x 5mm x 2mm • HOPG samples were irradiated in the AGC-1 capsule, ATR reactor • Target conditions: • Tirr = 600 °C • Dose: 1.5 – 7 dpa http://www.momentive.com/Products/Main.aspx?id=22817

  6. HOPG Samples and their Irradiation Conditions ORNL received from INL 8 irradiated HOPG samples ΔT ~ 73°C ΔT ~ 57°C ΔT = 240°C ΔT ~ 110°C Irradiation conditions from Document ID ECAR-1943 (Table 1) There is a significant gradient of the irradiation temperature !!

  7. Results from Dimensional Analysis PIE was carried out at both INL and ORNL

  8. 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

  9. Experimental Setup (002) Incident x-ray beam Diffracted x-ray beam (004) q (006) (008) q (110) (100) (200) XRD was performed using PANAlyticalmachine using Cu-kα radiation, λ, of 0.154056 nm, 2Ɵ range of 10 to 140°

  10. 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.

  11. Results to date Low dose Medium dose High dose

  12. Analysis of the (002) peak

  13. Analysis of the (002) peakInterplanar spacing and thickness

  14. What happens with the Lc??

  15. Analysis of the (110) peak

  16. Analysis of the (110) peak

  17. What happens with the La??

  18. 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.

  19. 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°C1500 °C

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