Micro-mechanics on Nuclear Graphite
Dr. Dong Liu
EPSRC Postdoctoral Research Fellow
1851 Exhibition Brunel Research Fellow
Junior Research Fellow, Mansfield College
Department of Materials, University of Oxford, U.K.
Research Affiliate
Lawrence Berkeley National Laboratory, U.S.A.
Outline
Background
o The material
o Microstructure over multiple length-scales
o Irradiation damage in nuclear graphite
Micro-mechanical testing over multiple length-scales
o Ex situ and in situ nano-indentation
o In situ micro-cantilever testing
Key messages
Nuclear Graphiteo Graphite has been widely used as a moderator, reflector and fuel matrix in various
types of nuclear reactors, such as gas-cooled reactor (e.g. AGR, MAGNOX), Russian
RBMK reactors, high temperature gas cooled reactor (Dragon, Peach Bottom, AVR,
THTR-300, Fort St. Vain, HTTR, HTR-10 ) etc.
o Gilsocarbon graphite is used as moderators and structural components in operating
Advanced Gas-cooled Reactors (AGRs) in the UK; Life-limiting as it is not replaceable.
Nuclear Graphite
Filler particle
Micro-scale Mrozowski cracks
100 µm
500 µm 500 µm
Filler
Binder
Threshold image
• Macro-scale • Micro- and Nano- scale
Background: Microstructure
Room temperatureElevated temperature (1000 - 1200°C)
Raman spectroscopy
Crystal bonding
Neutron diffraction
Lattice strain
X-ray tomography
Micro-scale deformation
Macro-scale deformation
Background: Multiple length-scale
• Micro-crack closure from expansion in the c-direction and• Dimensional change from irradiation induced creep
[Equivalent DIDO Nickel Dose]
Marsden et al, International Materials Reviews, 2016
Fast Neutron Irradiation Effect on Graphite Properties: Dimensional change
Nightingale et al
• Dimensional changes are correlatedwith irradiation and temperature
• Expansion of c-direction as a functionof neutron flux at different temperature(1Mwd/At = thermal energy output forone tonne of nuclear fuel produced bya flux of 3.5x1020 n.m-2 in the reactor,this corresponds to about 3.1x1023
displacement/m2)
Fast Neutron Irradiation Effect on Graphite Properties: Dimensional change
Kelly & Rappeneau et al.
Fast Neutron Irradiation Effect on Graphite Properties: Thermal conductivity
• Comparison between theoretical andempirical values in the fractionalchange of the thermal resistance as afunction of neutron dose. K0 and K arethe thermal conductivity values beforeand after irradiation, respectively.
𝐾0
𝐾-1• Fractional change:
S. Ishiyama et al. Journal of Nuclear Materials 230 (1996) 1-7
Fast Neutron Irradiation Effect on Graphite Properties: modulus and strength
UKAEA data on near isotropic graphites irradiated in the Dounreay Fast Reactor, R. Price.
𝑆
𝑆0= (
𝐸
𝐸0)𝑘
Fast Neutron Irradiation Effect on Graphite Properties: modulus and strength
Micro-mechanical testing over multiple length-scales
200 µm
Setup 1: Nano-indentationNano Indenter G200Ex situ test
Setup 4: Micro-cantilever bendingTriangular section, 100-200 µm in lengthIn situ test
1 mm200 µm
Indenter
Graphite cantilevers
Setup 3:Micro-cantilever bendingRectangular section, 10-20 µm in length In situ test
Micro-cantilever
Loading probe
5 µm
10 µm
Indenter Graphite surface
Setup 2: Nano-indentationNano Indenter inside a SEMIn situ test
Liu et al. Journal of Nuclear Materials, 2017
Nano-indentation (Ex situ)
• Load control• Displacement changes dramatically • Large scatter in the modulus measurements (similar as in hardness)• Which of these data can we trust?
Liu et al. Journal of Nuclear Materials, 2017
Nano-indentation (In situ)
10 µm 10 µm 10 µm
Indenter Sample surface
Sample surface
Sample surface
Indenter Indenter
Load
(m
N)
0 2 4 6 8Displacement (µm)
0
2
4
6
8
10
0 1 2 3Displacement (µm)
0
2
4
6
8
10
Load
(m
N)
0 0.2 0.4 0.6 0.8
0
2
4
6
8
1
0
Load
(m
N)
Displacement (µm)
Liu et al. Journal of Nuclear Materials, 2017
• Dualbeam workstation (FEI Helios NanoLab 600i Workstation)
• Force measurement system (FMS) (Kleindiek Nanotechnik)
• Workstation stage monitored and debris collected
• Calibrated against a spring standard and on glassy carbon
• Step I • Step II • Step III
In situ micro-cantilever bending
1.0 1.5 2.0 2.5 3.0 3.50
10
20
30
40
50 E
E (
GP
a)
Section size (m)
E = 29 4.5 GPa1.75x1.75x11.5µm
1.25x1.25x8.80µm
2.0x2.0x17.0µm
3.25x3.25x21µm
0 1 2 3 40
50
100
150
E= 40.5 GPa
Cantilever 3
Linea fit
Loa
d (
N)
Displacement (m)
Slope=32.990.21Lo
ad (
µN
)
Fracture
Displacement (µm)
Calibration: Glassy carbon
• Specimens prior to failure • Linear load-displacement relation • Small variation in E with sample size
• Repeatable modulus and flexural strength as measured at macro-scale
• Similar brittle fracture modes observed at micro-scale as in macro-size samples
Liu et al. Carbon, 2017
• Load-displacement curve for a cantilever with less surface defects showing the linear and non-linear stages prior to fracture;
• Cantilevers at this length-scale with varied surface defects that lead to scatter in the measured modulus and strength. Liu et al. Journal of Nuclear Materials, 2017
In situ micro-mechanical testing: small cantilevers
Micro-cantilever bendingTriangular section, 100-200 µm in lengthIn situ test
1 mm200 µm
IndenterGraphite
cantilevers
Micro-cantilever bendingRectangular section, 10-20 µm in length In situ test
Micro-cantilever
Loading probe
5 µm
Liu et al. Journal of Nuclear Materials, 2017
In situ micro-mechanical testing: small & large cantilevers
‘Small’ cantilever ‘Large’ cantilever
20 µmCantilever specimen
Loading probe
Loading arm length
20 µm
Fracture path
Cantilever root
10 µm 90°
Triangular cross-section
30 µm
Side surface of the triangle cantilever
xc
yc
b
h
0 20 40 600.0
0.5
1.0
1.5
2.0
2.5
3.0
0 20 40 600.0
0.5
1.0
1.5
2.0
2.5
3.0
0 20 40 600.0
0.5
1.0
1.5
2.0
2.5
3.0
0 20 40 600.0
0.5
1.0
1.5
2.0
2.5
3.0
0 20 40 600.0
0.5
1.0
1.5
2.0
2.5
3.0
Lo
ad
(m
N)
Displacement (m)
cycle 1
Lo
ad
(m
N)
Displacement (m)
cycle 2
Lo
ad
(m
N)
Displacement (m)
cycle 5
Lo
ad
(m
N)
Displacement (m)
cycle 4
Lo
ad
(m
N)
Displacement (m)
cycle 3
Linear
Non-linear
Post-peak progressive
failure
Liu et al. Journal of Nuclear Materials, 2017
In situ micro-mechanical testing: large cantilevers
Liu et al. Journal of Nuclear Materials, 2017Liu et al. Nature Communications, 2017
Indentation modulus
• The irradiated PGA graphite (6018-12/3/3)
Weightloss
Diameter(mm)
Length(mm)
Mass(g)
Neutron doseDIDO equiv. (n·cm-2)
Temp.(K)
15% 12.1 7.1 1.01 33.2 × 1020 560
Radiolytically-oxidised (CO2 environment) PGA graphite samples from a Magnox reactor supplied by Magnox Ltd.
10 µm40 µm
Filler particle
Matrix
Liu et al. Carbon, 2017
In situ micro-mechanical testing: irradiated PGA
• Irradiated filler particle• E = 40 to 86 GPa• σf = 600 to 1300 MPa
• Irradiated matrix• E ≤ 10 GPa• σf ≤ 500 MPa
• Unirradiated PGA graphite
• E = 10 to 20 GPa• σf = 200 to 500 MPa
Irradiated filler particle
Irradiated matrixLiu et al. Carbon, 2017
HOPG
Highly Oriented Pyrolytic Graphite
http://nanoprobes.aist-nt.com/apps/HOPGinfo.htm
An angular spread of the c-axes of the crystallites is of the order of 1 degree
Samples Temp (celcius) dpa
HOPG1 633 4.19
HOPG2 760 6.71
Irradiated HOPG
• Micro-mechanical testingo In situ testing in a Dualbeam chambero Un-irradiated specimen as reference
• Micro-Raman analysis
o 60 nm penetration deptho 1.5 µm laser spoto 488 nm wavelength
Microstructural Characterisation
5 µm
FEG-SEM image of a FIB cross-section
10 µm
Trench created by FIB in the middle of HOPG sample:
o Focused ion beam cross-sectioning o The material is free of large pores
SEM sample holder
Orientation of the basal plane to the loading direction
Orientation of the basal plane to the loading direction
0 1 2 30
5
10
15
20
25
30
35
Load (
N)
Displacement (m)
Cantilever 1
Linear fit
565057005750580058505900595060006050610061506200625063006350640064506500
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
C
A
raw U/V
Load-displacement curve
250 nm
Fractured surface
o Modulus and flexural strength can be measured.
Fractured at root
Twinning
Elastic
Plasticity?
• Modulus/strength increased by a factor of about 2 after irradiation at 760°C for 6.71 dpa!
𝑆
𝑆0= (
𝐸
𝐸0)𝑘• R. Price
k = 0.5 to 1For HOPG, k=1
UKAEA data on near isotropic graphitesirradiated in the Dounreay Fast Reactor
J. Hinks et al IOP Journal of Physics Conference Series, 2012 Wen et al, Journal of Nuclear Materials, 2008
• Densification
• Transformation from crystallite material to other
0
200
400
600
0 1000 2000 3000
Co
un
ts
Wavenumber (cm-1)
Crossed polarisation Parallel polarisation
Unirradiated HOPGRaman spectrum
G
0 1000 2000 3000
Inte
nsity (
a.u
.)
Wavenumber (cm-1)
760C_6.7dpa
633C_4.19dpa
Irradiated HOPG
GD
Ferrari AC, Robertson J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys Rev B, 2000
0
0.2
0.4
0.6
0.8
1
1.2
1574 1576 1578 1580 1582 1584
ID/I
G
G peak position
633C_4.19dpa
760_6.7dpa
unirradiated
• Transformation from crystallite material to other
HOPG2: 633°C for 4.19 dpa
Each image is about 60 µm wide
Key Messages
• Micro-scale testing can potentially describe the mechanical properties in graphite over
a rang of length-scales
• The filler particles and binder matrix react differently to neutron irradiation
• Elastic modulus and flexural strength in HOPG doubled at 760C 6.71 dpa
• These approaches could well be applied to target graphite materials
Acknowledgements
D.L. acknowledges the EPSRC fellowship grant: EP/N004493/1
D.L. acknowledges the Royal Commission for the Exhibition of 1851 Research Fellowship
Collaborators:
Idaho National Laboratory, USA
Dr. Joshua Kane, Dr. William Windes
Centre for Device Thermography and Reliability, Bristol, UK
Prof. Martin Kuball, Dr. James Pomeroy
National Physical Laboratory, UK
Dr. Ken Mingard, Dr. Mark Gee