2.1. Materials

Two types of graphite were used in this study. The IG-110 graphite, from Toyo Tanso Inc., is produced from petroleum coke with an average grain size of ~20 μm. The NBG-18 graphite, from SGL Group, is manufactured from pitch coke with a maximum grain size of 1600 μm. According to the standard ASTM D7219, NBG-18 and IG-110 are regarded as medium-grained and superfine-grained graphite, respectively. Typical properties of the two graphite grades are listed in Table 1.

Firstly, samples of the two graphite grades were ground using a 1500 grit emery paper to remove the entire original rough surface. Thereafter, the samples were polished successively on a cloth using 3000 nm and 50 nm diamond suspensions. Notably, mechanical polishing produces surface defects, thereby resulting in an increase in the D peak intensity in the Raman spectrum. However, a flat surface is obtainable through polishing and is suitable for analyzing the evolutions of filler particles, pores, and other microstructures during irradiation. The polished samples were cut into small slices (length × width × thickness: 7 × 1.5 × 1 mm³) for irradiation.

2.2. Ion irradiation

Two samples, IG-110 and NBG-18, were irradiated by (7 + 4 + 2) million electron volt (MeV) xenon ions. Irradiation was performed at the 320 kV high-voltage Experimental Platform in the Institute of Modern Physics, Chinese Academy of Science. Twenty-six electrons were stripped off the xenon atoms (Xe²⁶⁺) to attain the energy of 7 MeV, while twenty electrons were stripped off (Xe²⁰⁺) to reach the energy of 2 and 4 MeV. Xenon ions are adopted in this study because they are chemically inert and can create displaced carbon atoms at high speeds. Multi-energy xenon ions were chosen to irradiate the samples in this study to produce a uniformly damaged layer in samples. The xenon ion beam was scanned in two directions. An annular Faraday cup was used as an aperture in front of the target for uniform irradiation by the xenon ions. The sample was fixed on a metallic plate facing the xenon ion beam. The metallic plate was installed in the middle of a circular heater and the temperature of the sample was maintained to 600 °C during irradiation. The temperature of the sample was measured and monitored by a thermocouple. Another Faraday cup was placed behind the target to measure the ion beam passing through the aperture. When the target is mounted and ready for irradiation, the ion beam is blocked by the target and cannot be measured by the Faraday cup. Therefore, the current of the ion beam was monitored during irradiation by the annular Faraday cup in front of the target. The beam currents of the xenon ions were recorded to be in the range of 1.6–2.6 μA. A high vacuum (pressure < 5 × 10^-4 Pa) in the irradiation chamber was kept during heating and irradiation.

The xenon ion irradiation process was simulated using "The Stopping and Range of Ions in Matter" (SRIM) program. A displacement threshold energy (E_d) value is required for a proper SRIM simulation. Upon evaluating the neutron irradiation-induced displacements in carbon materials, the value of E_d was reported to be 31 eV [20]. A theoretical study reported that the E_d had a value of 25 eV at 300 K [21]. At a temperature of 900 K, the value of E_d increases to 30 eV [21], which is approximately the value used to calculate the neutron irradiation damage. Therefore, a value of 31 eV was applied for E_d in this study. Notably, changing the value of E_d only alters the calculated number of displacement atoms and does not influence their profile.

Subsequently, SRIM simulations with two modes— "Ion Distribution and Quick Calculation of Damage" and "Detailed Calculation with full Damage Cascades" —were performed to calculate the number of displacement atoms. The number of displacements in nickel and iron calculated using these two modes were compared with that derived from the well-known model developed by Norgett, Robinson, and Torrens (NRT model) [22]. It was found that the SRIM simulation with the "Quick Calculation mode" produced displaced atoms in nickel and iron comparable to the NRT model, while the "full Damage Cascade mode" produced much more displaced atoms [23]. Therefore, the SRIM calculation with the "Quick Calculation mode" has been recommended. However, in the case of xenon ion irradiation into graphite, the number of displaced atoms calculated with the SRIM program with the two modes are comparable and are 1/3 less than those derived from the NRT model. In this study, inert xenon ion irradiation at high temperatures was applied to graphite to simulate the neutron irradiation damage, which had been calculated using the NRT model [20]. Therefore, the number of displaced atoms derived from the NRT model were used and were determined to be 19180, 14916, and 10026 displacements/ion for 7, 4, and 2 MeV xenon ions, respectively. Next, the profile of displacements per atom (dpa) was derived from the ion fluence (ions/cm²). The dose was accumulated to 4, 15, and 25 dpa, where 1 dpa equals to (8.6 + 3.44 + 2.58) × 10¹⁴ ions/cm².

2.3. Characterization techniques

Thin foils of un-irradiated graphite were prepared for TEM observation. Samples with diameters of ~3 mm were cut from the un-irradiated graphite materials and were then ground to disks with thicknesses of ~50 μm. A dimple grinder was used to further grind the center of the disks. Thereafter, a penetration hole was formed in the disks by low-energy (3–5 keV) argon ion milling using a Gatan Model 691 system. A 200 kV FEI Tecnai G2 F20 TEM was used to characterize the QI particles in the two graphite grades. In addition, the NBG-18 sample was heated to 600 °C during TEM observation for in situ electron irradiation. The electron beam (200 kV), with a diameter of ~100 nm and current of ~15 nA, was focused on one QI particle.. The image of the QI particle was captured every 2 min during irradiation. The electron fluences were accumulated to 1.4 × 10²³, 2.8 × 10²³, 4.2 × 10²³, and 5.6 × 10²³ electrons/cm².

One specimen for TEM characterization was also prepared for the NBG-18 graphite sample irradiated to 25 dpa to investigate the structures of the irradiated QI particles. To observe the cross section of the irradiated sample, two slices were cut from the xenon-ion-irradiated NBG-18 sample. The irradiated surfaces of the two slices were glued facially. The edges of the specimen were milled until they could be inserted into a 3-mm-diameter copper tube. Then, the tube was cut into small pieces. The remaining processing steps with the small pieces are the same as those mentioned above.

The surface of the polished samples was observed via scanning electron microscopy (SEM) before and after ion irradiation to 15 and 25 dpa to study the surface morphology changes and QI particle evolution during irradiation.

The Raman scattering spectra were collected from the NBG-18 graphite before irradiation and after irradiation at each dose. A Bruker SENTERRA confocal Raman microscope was used to measure the Raman spectra at room temperature. The excitation beam has a wavelength of 532 nm.

A simple fit with two symmetric Lorentzians is often used for the Raman spectra of crystalline graphite structures. Nevertheless, two-peak fitting is unsuitable for the Raman spectra of highly disordered carbon materials. Typically, a fit with four symmetric peaks (D peak, G peak, and two other peaks) involving Gaussian lines has been used for the Raman spectra of highly disordered carbon materials. However, four Gaussian lines can usually provide good fitting with more than one possibility for the Raman spectra of highly disordered graphite. A unique and appropriate four-peak fit cannot be distinguished from all the possibilities, which may cause an inconsistency in interpreting the Raman spectra of irradiated graphite within various literature.

Rather than using a four-peak fitting, the Raman spectra were fitted with a symmetric Lorentzian for the D peak and an asymmetric Breit-Wigner-Fano (BWF) line for the G peak in this study [24]. Because of the peak broadening induced by irradiation, the small D’ peak that emerged in the Raman spectrum of the as-polished graphite was unrecognizable after xenon ion irradiation. Therefore the fitting of the D’ peak was not conducted.

3.1. The QI particles in graphite before irradiation

Figure 1 shows a typical QI particle in IG-110 graphite. There are many small packets of graphite sheets in QI particles. Empty voids were found in-between the packets. The c axis of these packets is directed radially towards the center of the QI particles [19]. Usually, QI particles are surrounded by folded graphite structures, which are shown outside the dashed circle in Fig. 1.

Fig. 1.Full-screen View

TEM micrograph of a typical QI particle (indicated by a dashed circle).

TEM micrographs of QI particles in NBG-18 and IG-110 graphite are shown in Figs. 2(a) and 2(b), respectively. It is evident that QI particles tend to agglomerate in nuclear graphite. The concentration of QI particles in IG-110 is much less than that in NBG-18 graphite, which is consistent with the conclusion of the previous study [18].

Fig. 2.Full-screen View

TEM micrographs of microstructures with QI particles in (a) NBG-18 and (b) IG-110.

3.2. Identifying the QI particles on irradiated graphite surface

The SEM micrographs of the irradiated surfaces of NBG-18 and IG-110 are shown in Figs. 3(a) and 3(b), respectively. The polished surface became rough after irradiation. Wrinkle-like textures (marked by W) are found on the surfaces of both graphite samples. These wrinkle-like textures were first identified on needle-like fillers in the IG-110 graphite after argon ion irradiation [15]. Therefore, they are believed to be the graphite structures with the long-range order. The "wrinkles" show the directions of curved graphite sheets. The size of the wrinkle-like textures represents the size of these graphite structures.

Fig. 3.Full-screen View

SEM micrographs of polished surfaces of (a) NBG-18 and (b) IG-110 after irradiation to 15 dpa. The "wrinkle-like" structures shown on the irradiated surface are marked by "W."

The main difference between the surface morphology of the two types of graphite is the concentration of small round hillocks (indicated by arrows in Fig. 3). These hillocks have very small sizes and tend to agglomerate, especially in NBG-18. Therefore, they are considered to be the QI particles. To prove this ideology, two QI particles were identified on the as-polished surface of the NBG-18 sample (Fig. 4(a)). Typical QI particle features, loose cores surrounded by folded graphite sheets (indicated by two dashed circles), are evident in Fig. 4(a). After irradiation, two hillocks emerged at the same place. Apparently, the hillocks, especially the left-hand side one, shrink radially and grow in height during irradiation. Faint round wrinkles are found on the two hillocks.

Fig. 4.Full-screen View

SEM micrographs showing two QI particles evolve into hillocks. (a) Two QI particles indicated by dashed circles on the as-polished surface, (b) after irradiation at 15 dpa, and (c) after irradiation at 25 dpa.

To show the distribution of QI particles in NBG-18 graphite, Fig. 5 shows an SEM micrograph of the irradiated NBG-18 surface. The entire irradiated surface is occupied by agglomerations of hillocks, along with the wrinkle-like structures. Large cracks, shown in Fig. 5, which are formed during the calcination of the filler coke, are typical for the fillers of NBG-18 graphite. This kind of calcination cracks are also present in the fillers of nuclear graphite Gilsocarbon and PCEA [19, 25]. Fig. 5 reveals an area in a typical filler of NBG-18 graphite. It was interesting to see a large number of QI particles in the filler since they were always reported to be formed in the binder phase [8,19]. Based on the morphology of the irradiated NBG-18 surface, it can be deduced that the filler in NBG-18 graphite is composed of many graphite structures (represented by the wrinkle-like structures with sizes from several microns to tens of microns), which are separated by the agglomerations of QI particles (represented by the hillocks).

Fig. 5.Full-screen View

SEM micrograph of irradiated NBG-18 surface showing calcination cracks and a large number of hillocks.

3.3. The Raman spectra of irradiated graphite

The Raman spectra were measured at the regions of QI particle agglomeration on the polished NBG-18 surface. Figure 6(a) shows the Raman spectra before and after xenon ion irradiation. The irradiation doses are also given in Fig. 6(a). Three peaks (The G mode appearing at 1582 cm^-1, the D mode located at ~1350 cm^-1, and the small peak D’ at 1620 cm^-1) emerged in the spectrum of un-irradiated graphite. During xenon ion irradiation, the graphite structures were damaged gradually by nuclear collisions, resulting in peak broadening and overlapping.

Fig. 6.Full-screen View

(Color online) (a) The Raman spectra and fitting curves of the agglomeration of QI particles before and after xenon ion irradiation to three doses. (b) The full width at half maximum (FWHM) of the G peak at various doses. (c) I_D/I_G at various doses.

Figures 6(b) and 6(c) show the full width at half maximum (FWHM) of the G peak and the ratio of the intensities of D and G peaks (I_D/I_G), respectively. The FWHM of the G mode of the as-polished NBG-18 sample is ~30 cm^-1. During xenon ion irradiation at 600 °C, the FWHM increased monotonically to a value of ~150 cm^-1 (25 dpa). It was reported that the irradiation-induced disordering of the graphite lattice might cause a strong coupling between the discrete G mode and a continuum of phonon states, resulting in the broadening of the G peak [26].

The D mode was explained as a "breathing mode" of sp² carbon atom rings and was forbidden in a perfect graphite lattice [10]. In-plane defects have to be introduced into graphite to excite the D peak. The region of agglomeration of QI particles contains many small graphitic structures with abundant defects in boundaries. Therefore the I_D/I_G ratio was high (0.84), even without irradiation. At the beginning of irradiation, although the number of in-plane defects increased, a sufficient amount of carbon atom rings was preserved, resulting in an increase in the D peak intensity with limited overlapping of peaks [27]. It has been reported that the I_D/I_G ratio increases fast and reaches a maximum of ~1.4 at quite low doses (<0.02 dpa) for graphite irradiated at room temperature [10,16,27]. Along with the increase in the I_D/I_G ratio, the graphite structure evolved into nano-crystalline graphite. With further irradiation, the lattice disordering increased, while the number of ordered rings decreased, resulting in a reduction in the D peak intensity and broadening of Raman scattering peaks. Fig. 6(a) shows that the Raman peaks at 4 dpa are already highly broadened; thus, the maximum of the I_D/I_G ratio should appear at lower doses than 4 dpa. It is worth mentioning that an inversely proportional relationship between the I_D/I_G ratio and the in-plane crystallite size was proposed and has been used to characterize the crystallinity of graphitic structures [28]. However, this relationship can only be used to calculate the crystallite size in the increasing stage of I_D/I_G ratio [24]. At higher irradiation doses, the I_D/I_G ratio decreases, and this relationship is apparently invalid presently.

Both the G peak width and the I_D/I_G ratio tended to be stable at doses higher than 15 dpa, indicating that the graphite structure at high irradiation doses, although damaged, was very stable. It was reported that the I_D/I_G ratio decreased toward 0.2 in an amorphous carbon material [24,29]. Fig. 6(c) shows that the values of I_D/I_G ratio at high doses are approximately 0.9, indicating that graphite is not amorphous after xenon ion irradiation to 25 dpa at 600 °C.

3.4. Irradiation effects in QIs in nuclear graphite

A QI particle was in situ irradiated by the electron beam in TEM at 600 °C. Figs. 7(a)–(c) show the QI particle before and after irradiation for 4 and 8 min (with electron fluences of 2.8 × 10²³ and 5.6 × 10²³ electrons/cm²), respectively. During irradiation, the small packets of graphite sheets swelled in the c axis and shrank in the basal plane. The c-axis swelling was readily absorbed by the voids, and the shrinkage in basal plane resulted in the densification of the QI particle. The black arrows show the evidence of shrinkage of the QI particle. Densification of QI particles has also been reported in neutron irradiated NBG-18 [8].

Fig. 7.Full-screen View

Evolution of a QI particle in NBG-18 during in situ electron beam irradiation at 600 °C. Electron dose: (a) 0, (b) 2.8 × 10²³, and (c) 5.6 × 10²³ electrons/cm².

Figure 8(a) shows the TEM micrograph of NBG-18 irradiated to 25 dpa. This TEM sample was processed not only by the xenon ion irradiation but also by subsequent low energy argon ion milling during the TEM sample preparation. Caution must be exercised to distinguish the damage induced by xenon ion irradiation from that induced by low energy argon ion milling. Fig. 8(a) shows a highly damaged surface layer with a thickness of ~3.1 μm which is consistent with the xenon ion range (3 μm) obtained from the SRIM simulation. Therefore, the damaged surface layer is induced by xenon ion irradiation. The boundary (indicated by a white dashed line) between the xenon-ion-irradiated layer and the un-irradiated substrate was evident in the TEM micrograph. The structures in the substrate were not irradiated by xenon ions but subjected to low energy argon ion milling. Typical QI particles and graphite structures were found in the un-irradiated substrate, indicating that the damage induced by low energy argon ion milling could be ignored compared to the damage induced by high dose xenon ion irradiation.

Fig. 8.Full-screen View

(a) TEM micrograph of the NBG-18 sample irradiated to 25 dpa. The white dashed line indicates the end of the ion range. The areas indicated by the two black dashed squares are magnified in (b) and (c).

Many densified QI particles with a spherical appearance are shown in the irradiated layer. Two areas indicated by the dashed squares are magnified in Figs. 8(b) and 8(c). Irradiated and un-irradiated QI particles are identified in Fig. 8(b). The loose core of the QI particles and the surrounding folded graphite sheets shrank and evolved into the densified spheres, which were found to be weakly bonded with the adjacent structures.

There are plenty of lenticular cracks with widths of several nanometers or tens of nanometers shown in Figs. 8(a)–(c). Lenticular cracks are typical structures in nuclear graphite and are known as Mrozowski cracks. During cooling down from the graphitization temperature, cracks opened between graphite basal planes by anisotropic thermal shrinkage. Both heating and irradiation could cause a remarkable expansion of the graphite structure in the c axis, which could be readily absorbed by these cracks. Owing to these lenticular cracks, graphite has a good resistance to thermal shock and irradiation-induced swelling and disintegration. After irradiation to 25 dpa, graphite structures were expected to swell in the c axis and close the lenticular cracks. However, Fig. 8 reveals that the graphite structures between the QI particles still show plenty of cracks lying between the graphite sheets because of the shrinkage of the adjacent QI particles.