An In Situ transmission electron microscopy study on the synergistic effects of Au-ion irradiation and high temperature on nuclear graphite microstructure

The combined effects of high-temperature and heavy-ion irradiation on Mrozowski cracks (MC) and nuclear graphite crystallographic dimensions have been studied using in situ heating and in situ ion-irradiation in the transmission electron microscope (TEM). Electron transparent lamella of nuclear graphite, IG-110, was irradiated using a 2.8 MeV Au beam at an ion flux of 3.991 ×1010 ion cm−2 s−1 for 70 min at 800 °C. Upon high-temperature irradiation, Mrozowski crack closure was studied quantitatively. The analysis showed linear, positive expansion of nuclear graphite which is significantly different from the dimensional changes previously reported for low-dose neutron irradiation of nuclear graphite in which the material undergoes negative to positive expansion via a turnaround radiation dose. The trend of the thermal expansion coefficient (CTE) of pristine IG-110 in this study is consistent with previous reports in the 100 °C–800 °C temperature region in which the dimensional change ranges from negative to positive values.


Introduction
Graphite has been used in nuclear reactors since the 1940s due to its excellent moderator and reflector characteristics [1][2][3].It is an ideal material for use as a neutron moderator, fuel and coolant channels, and rod control channels in high-temperature gas-cooled reactors (HTGRs) because of its high neutron scattering crosssection, low neutron absorption cross-section, and temperature resistance.However, when operating HTGRs at temperatures around 800 °C, graphite can undergo dimensional and property changes due to extreme stimuli such as neutron irradiation, high temperature, and load, which can cause stress, strain, and microstructural defects in the material.These changes can lead to material failure, such as blockages in coolant channels and component failure, that ultimately can hinder reactor performance [4][5][6].
Pristine nuclear graphite is nearly pyrolytic and comprises the binder phase and coke filler particles.Filler particles contain micrometer size domains of crystalline particles stacked along the c-axis and lenticular cracks in between the basal planes.Irradiation-induced changes can alter graphite's microstructure [4,7,8].Given these subtle changes in microstructure and defects can make significant changes in material properties, it is important to study the microstructural aspects and physical properties of nuclear graphite as a function of its operating conditions to get an understanding of the material's structural integrity to ensure safe operation and enhanced performances in HTGRs [4,5,9].This study concentrates on both irradiation and temperature-induced dimensional and microstructural changes in nuclear graphite, IG-110.
Ion irradiation can provide fundamental insight into radiation damage similar to neutron irradiation conditions without exposing materials to or creating radioactive elements.It is also an advantageous method to achieve high fluence values in a relatively short time.Ion irradiation is a great alternative when neutron irradiation and neutron-irradiated materials are restricted or inaccessible [10].This method employs a wide spectrum of ions and irradiation conditions ranging from light-ion irradiation (proton, He) to heavy-ion irradiation (Ni, Cr, Au, U).Although there are differences between neutrons and ions in terms of size and charge, ion irradiation remains the best alternative to simulate the irradiation-induced damage in materials due to its versatility [10][11][12][13].In situ transmission electron microscopy (TEM) is a unique technique that allows direct observation of structural changes that materials undergo in real-time in response to external stimuli such as heat or radiation.In contrast, ex situ studies provide information only before and after the actual reaction, thus insufficient to bridge the gap.In situ TEM is crucial in bridging the gap by providing real-time data as the changes occur in the material, thus providing an understanding of the fundamental mechanisms that the material undergoes in the presence of external stimuli [14].The current study utilized two in situ TEM techniques: in situ heating and in situ ion irradiation in the TEM to study the microstructural changes in IG-110 as they occur in the microscope.
Under irradiation nuclear graphite structure expands in the c-direction and contracts in the direction parallel to the basal planes, which is often called the 'swelling' of nuclear graphite [5,8,15].In earlier reports, this change in dimension has been attributed to displacement of carbon atoms upon neutron irradiation [16][17][18].Recent neutron and irradiation studies have attributed this phenomenon to the formation of edge dislocation via vacancy loops and interstitial fullerene-like structures which contributes to expansion in the material resulting in Mrozowski crack (MC) closure [19][20][21][22][23].This work repots a study on Mrozowski cracks upon irradiated with 2.8 MeV Au-ion at high temperature.By utilizing complementary in situ TEM techniques together, i.e., heavy-ion irradiation and heating, the combined effect of irradiation and temperature on expansion in IG-110 and Mrozowski crack closure is studied in detail.

Experimental
Fine-grained IG-110 graphite used in this work was manufactured by Toyo Tanso Co., Kagawa, Japan.An electron-transparent lamella was prepared via in situ lift-out onto a Mo half-moon grid from Ted Pella, Inc., using a Scios 2 Dual Beam Scanning Electron Microscope-Focused Ion Beam (SEM-FIB) (Thermo Fisher Scientific).The in situ heating with Au-ion irradiation was conducted inside the In situ Ion Irradiation TEM (I 3 TEM), a modified JEOL 2100 TEM integrated with a 6 MV tandem accelerator at the Sandia National Laboratories 11 , as can be seen in figure 1. Electron transparent lamella of pristine IG 110 was heated using the Gatan double tilt heating holder (equipped with a water-cooling system in the JEOL I 3 TEM at 200 kV.In this setup, the sample was heated to 800 °C in 100 °C increments at nominally 0°tilt.At every heating step, the sample was held at that temperature for 5 min to minimize drift., then TEM micrographs and diffraction patterns were acquired before heating to the next temperature point.Upon reaching 800 °C, the sample was tilted + 30°(alpha) toward the ion beam and was exposed to the 2.8 MeV Au 4+ ion beam for 1 h and 10 min (final fluence of 1.6762 × 10 14 ion cm −2 ) at a flux of 3.991 × 10 10 ion cm −2 s −1 with direct acquisition of TEM bright-field micrographs and select area electron diffraction patterns throughout the experiment.
Irradiation damage in a 100 nm-thick specimen was calculated using the Stopping and Range of Ions in Matter (SRIM-2013) with Ion Distribution and Quick Calculation of Damage mode.In this calculation, the displacement energy and the surface binding energy for sputtering were set to 28 eV and 7.4 eV, respectively.The angle of incidence in the SRIM calculation is set to 60°as the sample is tilted + 30°toward the ion beam which is perpendicular to the electron beam.The irradiation damage in displacement per atom (dpa) as a function of sample thickness is shown in figure 2.

Results and discussion
The scanning electronic microscopy (SEM) characterization of pristine IG-110 bulk sample surface is shown in figure 3(a).The nuclear graphite sample is consistent with filler particles (figure 3 Figure 4 shows stages of microstructural changes, i.e.Mrozowski crack closure upon irradiation with the 2.8 MeV Au 4+ ion beam at 800 °C constant temperature.The left-most micrograph was acquired prior to Au beam irradiation, which shows typical Mrozowski cracks in IG-110 as appeared prior to ion irradiation on the lamella.The Mrozowski cracks of interests are numbered 1-4 in all three micrographs.The TEM micrograph in the middle was acquired mid-irradiation, specifically, at a fluence of 7.423 × 10 13 ion cm −2 (0.22 dpa).The right-most micrograph was taken towards the end of the irradiation experiment (at a fluence of 1.486 × 10 14 ion cm −2 (0.43 dpa)).In this image sequence, it is apparent that the Mrozowski cracks (MC) (labeled as 1 to 4) in the right-most micrograph are visibly obscure when compared with that of the left-most micrograph.
A qualitative analysis of the dimension change of the Mrozowski cracks labelled in figure 4 is presented in figure 5.The left TEM micrograph is acquired prior to Au beam irradiation, thus, it represents the typical form of Mrozowski cracks present in IG-110.The right TEM micrograph is acquired at the end of the irradiation experiment (at a fluence of 1.6762 × 10 14 ion cm −2 (0.49 dpa)).The dimensions (widths and lengths) of each crack (previously labeled as 1-4) are measured using the Gatan Digital Micrograph ® software by drawing a distinct intensity profile across the width (w) and length(l) of each crack (figure S1), and the measurements that were taken from the resulting histograms are reported in each micrograph.At the end of the irradiation experiment, the width and length of MC-1 have decreased to 1.4 μm and 103.1 μm, from 3.9 μm and 129.1 μm, respectively.MC-2 which was originally 3.9 μm (w) and 239.6 μm (l) has healed upon irradiation leaving three  [22,24].As TEM characterization provides a 2D projection of the specimen, the study of depth's effect in crack closure would warrant a detailed analysis.
The selected area electron diffraction (SAED) patterns were recorded throughout the heating profile (25 °C-800 °C), and the irradiation experiment at 800 °C.To analyze the effect of 2.8 MeV Au-ion irradiation on expansion of IG-110, the reciprocal space spot distances were measured in SAED patterns acquired at various stages in the irradiation experiment using Gatan Digital Micrograph ® software's intensity profile histograms (figure S2).Their real space spacings and dimensional expansions were computed and tabulated in table S1.The Au radiation-dependent expansion plot constructed from table S1 is shown in figure 6, which indicates a linear  increase of expansion in the material with 2.8 MeV Au 4+ ion irradiation at a flux of 3.991 × 10 10 ion cm −2 s −1 throughout the irradiation experiment up to a fluence of 1.6762 × 10 14 ion cm −2 .The linear trendline is drawn for the easy visualization of the trend.As the graph shows, the IG 110 lamella continuously expands with Au-ion beam fluence at a constant temperature of 800 °C.Upon low doses of neutron irradiation (0.02 dpa), Mrozowski crack closure accommodate for c-axis expansion and the shrinkage in a-axis of nuclear graphite produce net shrinkage in material.At a 'turnaround' dose of neutron irradiation, in the event where all Mrozowski cracks are closed and cannot further account for the c-axis expansion the material will result in a net expansion [22,25].However, the present study shows results that are in agreement with high-temperature high-dose neutron irradiated IG 110 (0.25 dpa) reported by Lv et al [25], comparably during ion-irradiation at 800 °C (0.22-0.49dpa), IG-110 showed net positive expansion throughout the course of irradiation without any material shrinkage.
The SAED patterns acquired at every 100 °C step during the heating ramp (25 °C-800 °C) prior to the irradiation experiment were quantitatively analyzed to compute the thermal expansion coefficient of IG-110 lamella.As illustrated in figure S2, the reciprocal distances of SAED spots were measured and the real space dimensions were computed accordingly (table S2).The resulting temperature dependence expansion graph is given in figure 7.In comparison with previously reported CTE values for graphite (figure 7), the trend in thermal  expansion graph experimental CTE data for present study is consistent with previously reported data for thermal expansion of graphite in-plane direction [26][27][28].The slight deviation in the numerical values of basal plane expansion is likely a result of confined polycrystalline microstructure and defect formation (such as ripplocations) during the extreme high-stress manufacturing conditions of nuclear graphite.As shown in figure 7, the thermal expansion coefficient for 1G-110 varies from negative to positive values due to in-plane dimensions of IG-110 decreases before increasing in this temperature range, which is in consistent with previous reports on graphite.This negative CTE in graphite basal planes can be attributed to the large increase of dimensions in the c direction due to Poisson contraction in basal planes [27].

Conclusions
The SEM micrographs confirm that IG-110 microstructure consists of filler particles, binder phase, and lenticular Mrozowski cracks in filler particles.Upon Au-ion irradiation at 800 °C, the closure of Mrozowski cracks was visible and at least one crack completely healed without any traces at the fluence value of 1.6762 × 10 14 ion cm −2 .The quantitative analysis performed using TEM micrographs acquired at various points of the  high-temperature ion-irradiation experiment further confirms Mrozowski crack closure is induced by a synergistic effect of Au-ion radiation and high temperature.The Mrozowski crack closures were not observed during the initial temperature ramp from 25 °C-800 °C without any ion-beam irradiation.The SAED patterns collected throughout both initial heating ramp and high temperature ion-irradiation experiments were used to quantitatively analyze the dimensional changes in the material by converting reciprocal space measurements to real space.The percent dimension change as a function of Au-ion beam dose follows a linear trend line in contrast to negative to positive dimensional change previously reported for low-dose neutron irradiated samples [22].This behavior in IG-110 could be accounted to plastic crystal strain and generation of nano sized cracks in the sample preventing the material to converting back to its original pristine state dimensions [29,30].The thermal expansion of pristine IG-110 studied during the initial heating ramp follows a similar trend with previously reported data for graphite in-plane direction.The slight deviation could be attributed to size confinement and quantum effects aspects of the nano-sized IG-110 lamella.This study provides insight on understanding how the microstructure and dimensions of nuclear graphite changes under its operating environment thereby aiding ensure safe operations and extended lifetime of next-generation HTGRs.

Figure 1 .
Figure 1.(a) JEOL 2100 TEM with key beamline and stage capabilities in the I 3 TEM facility of Sandia National Laboratories (inset schematic shows directions of the electron beam and ion beams aligned to the same 3 mm spot in the I 3 TEM.Reproduced from [11].CC BY 4.0.

Figure 2 .
Figure 2. Depth distributions of damage in displacement per atom (dpa) as calculated by SRIM-2013 code.The area marked by orange represents the sample thickness used in this study.

Figure 3 .
Figure 3. SEM micrographs showing the microstructure of IG-110 nuclear graphite including filler particles, binder phase and pores.(b and (c) are the magnified SEM micrographs of the areas marked by dashed-rectangular regions showing a filler particle with lenticular porosity and binder phase, respectively.

Figure 4 .
Figure 4. TEM micrographs showing the IG-110 nuclear graphite lamella microstructure, including Mrozowski cracks at three stages of Au-ion irradiation (a): before irradiation, (b): mid-irradiation, (c): towards the end of irradiation).The Mrozowski cracks of interest are numbered 1-4 in each micrograph for easy navigation.Scale bar indicates 100 μm.

Figure 5 .
Figure 5. Dimensions (lengths and widths) of each Mrozowski crack (1-4 as marked in figure 3) before irradiation (t = 0 s) and at the end of irradiation (t = 4200 s) as measured from TEM micrographs.All measurements are in μm.

Figure 6 .
Figure 6.Expansion of IG-110 as a function of 2.8 MeV Au 4+ -ion Irradiation at a Constant Temperature of 800 °C.The linear trendline is added for easy visualization of the trend.

Figure 7 .
Figure 7. Experimental CTE of IG-110 as a function of temperature (25 °C-800 °C) in comparison with the reported CTE values.It should be noted that the data point corresponding to 200 °C is considered an outlier and is therefore excluded from the analysis.Reprinted from [25], Copyright (2005), with permission from Elsevier.