Multiple length-scale microstructural characterisation of four grades of fine-grained graphite

Eric Jiang1, Kavin Ammigan2, George Lolov2, Patrick Hurh2, DongLiu1

1SchoolofPhysics,UniversityofBristol,UnitedKingdom

2Target Systems Department, Fermi National Accelerator Laboratory, United States

mj14259@bristol.ac.uk (Outlook)mj14259@my.bristol.ac.uk (Gmail)

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Introduction Materials and experimental methods

• Four different grades of fine-grained graphite• X-ray tomography

• FIB-SEM tomography

• Raman spectroscopy Results

• Microstructure/porosity

• Crystal properties Conclusions Ongoing work and planning for next

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Outline

Introduction

Many different grades classified (ASTM7219) into:Coarse-grained (filler < 4 mm);Medium grained (< 2 mm); Fine-grained (< 100 µm), etc

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The schematic of DUNE [1].

[1] Acciarri R, et al. Long-Baseline Neutrino Facility (LBNF) and Deep Underground Neutrino Experiment (DUNE) Conceptual Design Report Volume 1: The LBNF and DUNE Projects. arxivorg. 2016 1;

[2] Holt M. Issues of scale in nuclear graphite components (Doctoral dissertation, University of Hull).

An Advanced gas-cooled reactor (AGR) Gilsocarbongraphite core [2].

Task/objective: characterise graphite’s microstructure

Artificial nuclear graphite has multiple length-scale porosity and mechanical properties

Gilsonite cokeCalcination (~1300°)

Milled and sized

Filler and flourCoal tar

pitch

Green article

Baked article

Graphite

Baking at ~800°C

Re-impregnated with pitchBaking

Graphitisation (~3000°C)

Cooled

Moulded

Hot mixed

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[3] Marrow TJ, Liu D, Barhli SM, etal. Carbon. 2016 Jan 1;96:285-302.[4] Liu D, Flewitt P, etal. Journal of Nuclear Materials. 2017 Sep 1;493:246-54.[5] Liu D, Cherns D. Philosophical Magazine. 2018 May 13;98(14):1272-83.[6] Krishna R, Wade J, Marsden BJ. etal. Carbon. 2017 Nov 1;124:314-33.

Nano-scale [5]

Macro-scale [3]

Micro-scale [4]

atomic-scale [6]

Taking near-isotropic Gilsocarbon graphite as an example:Bulk dimensional change

• Shrink then swell; as a function of temperature

• The higher the temperature, the quicker the ‘turn around’

• The target graphite temp <430 °C so the shrinkage will be slower

• Above 900 °C is a different story

• Fine grain graphite is different from Gilsocarbon, but similar behaviour expected.

Bulk Young’s modulus change

• Increase plateau decrease

• High temperature quicker reaching plateau

Gilsocarbon graphite Young’s modulus change

Gilsocarbon graphite dimensional change

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Marsden BJ, Hall GN. 4.11 Graphite in Gas-Cooled Reactors. Reference Module in Materials Science and Materials Engineering. 2020 Aug 3;14:357-421.

430°C600°C

430°C600°C

EDND: equivalent DIDO nickel dose

NT02 target US centre >1 dpa and up to 370 °C.BLIP experiment is only ~0.1 dpa (0.7×1020 n/cm2 EDND) and up to 200 °C.

1430°C

1430°C

Materials:

• Samples of them have been irradiated in BLIP, BNL, 180MeV, 24kW and peak dpa of 0.095~0.12, irradiation temp of up to 200°C.

• We dealt with un-irradiated samples only in this work.

• Porosity at µm scale and sub-µm scale, crystal properties by spectroscopy.

• Gain a relatively more comprehensive picture of these candidates.

• 4 different grades of fine-grained nuclear graphite:CL 2020, IG 430, SGL R7650 and POCO ZXF-5Q

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Experimental methods: X-ray (computed) tomography (XCT)

Zeiss Xradia Versa 520 Tomography

Simplified schematic of how X-ray tomography worksLandis EN, Keane DT. X-ray microtomography. Materials characterization. 2010 Dec 1;61(12):1305-16.

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Results: X-ray tomography: pixel size 0.2 µm, 2000 slices.

One single 2D slice of X-ray tomographic image, showing distinct porosity in these four

X-ray scanned volume is reconstrued to show spatial evolution of porosity

Dimension: ~ 1200×1200×1900 voxels3

~physical volume of 0.02 mm3 each.

Detailed analysis of porosity. This process can be computationally intensive

Dimension: ~ 500×500×500 voxel3~physical volume of 0.001 mm3 each.

Distinct pore structures

Local variation of porosity

Largely inter-connected

Data are listed in our paper

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Experimental methods: focused ion beam-scanning electron microscope (FIB-SEM tomography)

Simplified schematic of how FIB tomography worksLiu Y, King HE, Van Huis MA, Drury MR, Plümper O. Nano-tomography of porous geological materials using focused ion beam-scanning electron microscopy. Minerals. 2016 Dec;6(4):104.

FEI Helios NanoLab 600i DualBeam

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One single 2D slice of FIB tomography image, showing distinct porosity in these four, at much higher pixel resolution than µXCT, pixel size in these SEM images is only ~250 nm/pixelphysical volume milled ~ 4800 µm3 each.

Again, 3D reconstruction and detailed porosity statistics

Dimension: ~ 800×300×150 voxels3

~physical volume of 2800 µm3 each.

Equivalent diameter: Example of POCO ZXF-5Q graphite porosity analysis, which is the candidate in NuMI’s NT02 target.

Again: Distinct pore structures

Local variation of porosity

POCO inter-connected

Data are also listed in our paper

Results: FIB-SEM tomography: in plane pixel size: 0.02 µm, lateral pixel size: 0.13 µm; 150 slices each.

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Porosity is never 100% accurately known.

Compromise must be made between length scales and resolution.

Nano-porosity is vital, but nano-scale pores and cracks were not characterised in this work.

Porosity shows good agreement.

Key messages from porosity characterisation:

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• Raman is surface-based laser technique.

• G-band is graphite and graphitic materials’ common band.

• D-band is an indicator of crystal damage level, A1g symmetry of the in-plane breathing mode at K point in Brillouin zone and therefore, in aromatic rings.

• D’-band is also crystal defect induced.

• The 2D (G’) band at 2700~2750 cm-1 is very sensitive to defects in the stacking direction (Lc direction) due to the stacking order defects.

• Defect-induced bands are also sensitive to wavelength used.

Experimental methods: Raman spectroscopy

D

G 2D

D’

Raman shift (cm-1)

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Results: Crystal property (mainly La) characterisation by Raman:

d (002) spacing, (Å) [9]

Graphitisation degree (%)

Graphitisation Temp (°C)

Average crystalline size (La), (nm)

IG 430 3.3681 83.6 > 2800 291.4 ± 200

CL 2020 N/A N/A N/A 157.2 ± 78.3

SGL R7650 3.3794 70.5 N/A 120.8 ± 70.7

POCO ZXF-5Q 3.3833 65.9 ~ 2500 84.5 ± 9

[9] Simos N, etal. Physical Review Accelerators and Beams. 2017 Jul 24;20(7):071002.[10] Cançado LG, etal. Applied Physics Letters. 2006 Apr 17;88(16):163106.[11] Feret FR. Analyst. 1998;123(4):595-600.

Degree of graphitisation: [11]

3.354 Å marks the ‘ideal’ (002) plane spacing

Estimated characteristic in-plane crystalline size La

[10]

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Conclusions:

Measured porosity is consistent with manufacturers’ datasheet.

Very distinct microstructure across multiple length scales.

The developed methodology can be used for irradiated graphite as well and experiment is ongoing.

Pore population with modulus, crystallite size with thermal conductivity, have a one-to-one correlation, other properties are not directly correlated. The underlying mechanisms are not clear.

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Conference and paper:

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Oral presentation (online)

Ongoing work:o NuMI/MINOS NT02 target POCO ZXF-5Q characterisation (PIE).

• 340 kW, 120GeV, beam 1σ = 1.1mm, 9.8 µs beam pulse length and peak fluence of 8.6×1021 p/cm2, Temp range: 60-370°C. • Fractured piece stored at the Materials Research Facility (MRF) of UKAEA.• Essentially carrying out experimental measurements across proton beam profile and therefore, irradiation gradient.

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o Raman mapping of NT02 irradiated POCO 532 nm (2.33eV) green laser at ~10mWo Mapping across proton beam profile, from beam centre all the way to 6000 µm away (more than 5σ).o Hundreds of spectra in 1 map, and there are 48 maps in total---data analysis is still ongoing.

Ongoing work:

Representative Raman spectra taken from maps in Linescan01 direction.

Beam 1σ = 1100 µm

1000 1450 1900 2350 2800 3250 Ramanshift, cm−1

Intensity, Arbitrary. Unit

D

G 2D

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Origin, failed

20µm topmiddle(c)

4µmmiddle toporigin(e) (f) (g)

origin

o FIB tomography of NT02 irradiated POCO

(d)(b)

Ongoing work:

Origin: 100 µm from beam centre

middle: 2200 µm from beam centre

top: 5600 µm from beam centre

o TEM thin foil of NT02 irradiated POCO

FIB tomography animation

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o FIB tomography of NT02 irradiated POCO• Planning to use machine learning-based analysis technique - more reliable• To see any correlation of porosity with thermo-mechanical properties change

o TEM experiment for nano porosity is to be proposed

o Raman spectral interpretation of these results

o Neutron diffraction for NT02 whole fins lattice strain mapping (ENGIN-X, in June time).

Future work:

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• Thank you very much!

• Acknowledgement:We would like to express our deep appreciation and gratitude to a lot of people from the University of

Bristol, Materials Research Facility of UKAEA, Science and Technology Facility Council (STFC) and RaDIATE collaboration, for their helpful and generous assistance in our work. The names are too many and will not be listed here for all.

Contact: mj14259@bristol.ac.uk

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