Multiple length-scale microstructural characterisation of four grades of fine-grained graphite Eric Jiang 1 , Kavin Ammigan 2 , George Lolov 2 , Patrick Hurh 2 , Dong Liu 1 1 Schoolof Physics,Universityof Bristol, United Kingdom 2 Target Systems Department, Fermi National Accelerator Laboratory, United States [email protected](Outlook) [email protected] (Gmail) 1
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Multiple length-scale microstructural characterisation of four grades of fine-grained graphite
Eric Jiang1, Kavin Ammigan2, George Lolov2, Patrick Hurh2, DongLiu1
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.
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:
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.