2.1. Specimen preparation and irradiation

The ultra-fine grain graphite HPG-510 and fine grain graphite IG-110, which are both isostatic molded graphite grades, were manufactured by Toyo Tanso Co (Kagawa, Japan). Their physical and mechanical properties are listed in Table 1.

Specimens in a size of 10 mm×10 mm×2 mm were polished manually with 1200 grit sandpaper to remove surface scratches, and then rotary polished with polishing compound (0.05µm-Al₂O₃) to achieve a smoother surface. They were ultrasonically cleaned in acetone and deionized water successively to remove surface contaminations, and vacuum-heated at 120°C to remove moisture.

The samples were irradiated at room temperature to 1×10¹⁴, 5×10¹⁴, 1×10¹⁵ and 5×10¹⁵ ions/cm² by 7 MeV-Xe²⁶⁺ ion beams on a terminal of the 320 kV High-voltage Experimental Platform equipped with an electron cyclotron resonance ion source in the Institute of Modern Physics, Chinese Academy of Science[24]. The irradiation processes were simulated by SRIM 2008 program using the “Detailed Calculation with full Damage Cascades” mode [23]. The averaged irradiation damages were D = 0.10, 0.52, 2.61 and 5.23 dpa (displacement per atom) for corresponding irradiation doses, respectively, calculated by D =N_displacement/N_atom=Φn_dis/ρ_atom, where Φ is the ion fluence, n_dis is the number of displacements per ion, and ρ_atom is the atomic density. The depth profiles of displacement damage are shown in Fig. 1. Notably, some of the displaced atoms are so close to vacant lattice sites that they recombine immediately, and at high radiation fluxes, annealing of damage due to the temperature rise in the irradiation process, becomes prominent[15, 25].

Fig. 1Full-screen View

(Color online) Depth profile of displacement induced by 7 MeV-Xe²⁶⁺ ion irradiation in graphite calculated by the SRIM-2008 code.

2.2. Characterization

The SEM (LEO1530 VP, Germany) was used to study the surface morphological changes of graphite. The comparison of graphite in the microstructure, that is, Mrozowski cracks, was studied using a TEM (Tecnai G 2 F20, America) operated at 200 kV. The TEM specimens (as-received) were cut and sanded into Φ3 mm× ~50 µm disks, and spattered by 4-keV Ar ions in an incident angle of ~4° from an ion-beam thinner (Gatan PIPSion mill, America). A Raman spectrometer (XploRA INV, France) was used to record the Raman spectroscopy of the virgin and irradiated specimens with excitation laser wavelength of 532 nm.

After the microstructure characterization, the changes in hardness and modulus were measured using a nano-indentation with Berkovich indenter (G200, USA). The geometry of Berkovich indenter was three sided pyramids with a sharp tip end. The Berkovich indenter was modified to have the same projected area as the Vickers indenter at given indentation depth.

3.1. Microstructure changes

3.1.1. Changes in surface morphology

The surface morphology evolutions of the graphite samples irradiated with 7 MeV Xe²⁶⁺ ion beams are shown in Fig. 2. Fig. 2(a) and 2(f) show typical polished surface of IG-110 and HPG-510 before irradiation, with irregular pores. After irradiation, fragmented-shaped surface can be seen obviously, indicating that the microstructure of graphite can be damaged by ion beam bombardment, and the degree of damage increases with the irradiation dose. Comparing Figs. 2(a–e) with Figs. 2(f–j), the IG-110 samples show more fragmentation than that of HPG-510, i.e. the IG-110 is more susceptible to surface damage by ion bombardment.

Fig. 2Full-screen View

SEM images of the IG-110 (a–e) and HPG-510 (f–j) before and after irradiation.

3.1.2. TEM microstructure analysis

Artificial graphite is imperfect with Mrozowski cracks, which is formed by anisotropic shrinkage effects due to greater shrinkage perpendicular to the layers than parallel. Mrozowski cracks affect graphite properties of coefficient of thermal expansion [7], thermal conductivity [26], radiation resistance[27] and so on. Fig. 3 shows the TEM images of as-received HPG-510 and IG-110. Although the average length of cracks is smaller in HPG-510 than that of IG-110, HPG-510 is of larger crack density than IG-110. Microcracks play an important role in accommodating the bulk volume changes of polycrystalline graphite by irradiation. Graphite with more microcracks means higher capacity to accommodate the irradiation-induced volume changes, hence the higher radiation resistance of HPG-510 than IG-110.

Fig. 3Full-screen View

TEM images of as-received graphite: (a) IG-110, (b) HPG-510.

3.1.3. Raman spectroscopic analysis

Raman spectroscopy is an appropriate tool for microstructure characterization of irradiated graphite due to its sensitivity to various defects [28, 29]. Figs. 4(a) and 4(f) show Raman spectra of as-received HPG-510 and IG-110, respectively. They are very similar. The G resonance peak at 1582 cm⁻¹ is Raman active for the sp² carbon networks[30]. The peak intensity depends on the size of graphite microcrystal in the sample[31]. The D peak at 1352 cm⁻¹ indicates the existence of disorder and/or presence of defects such as vacancy defects and edges[32,33]. The intensive D resonance peaks of IG-110 and HPG-510 indicate that they have plenty of defects before irradiation. The D- and G-peaks overlapped at 0.52 dpa, and the outline of the Raman spectra does not change above 0.52 dpa. This may be related to the irradiation-induced amorphization of graphite [34, 35].

Fig. 4Full-screen View

(Color online) Raman spectra of IG-110 (a–e) and HPG-510 (f–j) before and after irradiation.

In Fig. 4, to show the ion beam effects on microstructure of IG-110 and HPG-510, the Raman spectra were deconvoluted into four Lorentzian peaks marked as ν₁, D, ν₃ and G bands at 1150, 1352, 1500 and 1582 cm⁻¹, respectively[36]. Fig. 5 shows the peak positions as a function of the displacement per atom. Both the D and G peak positions of IG-110 shift more than those of HPG-510. The peak position shifts of the D and G bands reveal an occurrence of strain in graphite duo to ion irradiation[37]. The results indicate the strain induced by ion irradiation in IG-110 is higher than that in HPG-510, i.e. IG-110 is more sensitive than HPG-510 to 7 MeV Xe²⁶⁺ ion irradiation.

Fig. 5Full-screen View

(Color online) Comparison of peak position changes of the irradiated IG-110 and HPG-510.

The D peak intensity (I_D) and ν₃ peak intensity (I_ν3) normalized by the G peak intensity (I_G) are shown in Fig. 6. I_D/I_G is widely used for characterizing the defect quantity in graphitic materials[33, 38]. Irradiation-induced changes in the Raman spectrum are usually explained in terms of a reduction in the phonon correlation length due to defects [39, 40]. In Fig. 6(a), I_D/I_G increases with the displacement per atom, with the IG-110 being in a greater changing rate than HPG-510.

Fig. 6Full-screen View

(Color online) Comparison of I_D/I_G and Iν₃/I_G of the irradiated IG-110 and HPG-510_.

The I_D/I_G is inversely proportional to the in-plane “crystallite size” (La), that is, either the cluster diameter or the in-plane correlation length[41]. According to the empirical Tuinstra–Koenig equation (valid for graphitic materials at La>2 nm [39]), the La of IG-110 decreases from 22.17 to 10.65 nm, while the La of HPG-510 decreases from 21.37 to 12.62 nm, i.e. after Xe ion irradiation IG-110 decreases more in crystal size than HPG-510.

The intensity of ν₃ peak increases with the dpa (Fig. 4) and the integrated intensity ratio to G peak (I_ν3/I_G) increases with the dpa, too (Fig. 6b). This indicates rapid accumulation of the interstitial defects [32, 42]. The interstitial defects in graphite pin the dislocations and increase the elastic modulus. Fig. 6(b) also shows that IG-110 has more interstitial defects than HPG-510.

Another parameter to monitor the carbon bonding is the FWHM of the D and G peak. The FWHM of D band is generally considered as a result of the cluster distribution with different dimensions and orders in amorphous phases[43]. The FWHM of G peak is proportional to the disorder of the carbon phase, which always increases with the disorder[44]. Fig. 7 shows the FMHW of D- and G-peaks of the irradiated IG-110 and HPG-510 graphite. Both the FWHMs of the G- and D-peak increase before 0.52 dpa, and from 0.52 dpa to 5.23 dpa they have little changes considering the measurement error. The FWHM increases indicate the increased disorders due to the ion irradiation. The FWHMs of the G- and D-peaks in IG-110 increase faster than that in HPG-510, i.e. IG-110 is of higher irradiation sensitive than HPG-510.

Fig. 7Full-screen View

(Color online) FMHW of the D- and G-peak in Raman spectra of the irradiated IG-110 and HPG-510 graphite.

3.2. Hardness and Young’s modulus

Mechanical properties of graphite are important for its application in nuclear reactors. Hardness and Young’s modulus of irradiated graphite are frequently characterized by nanoindentor as an indicator for changes in mechanical properties [32]. Neutron irradiation experiments in Refs. [45, 46] indicated that the Young’s modulus of graphitic increased with fast neutron fluence (at low irradiation fluence) because of the dislocation pinning and a closure of the fine pores. With an increase in fast neutron fluence, dislocation pinning quickly reduced and then structural changes took precedence. These changes led to an increase in the Young’s modulus first and then decrease rapidly as the structure disintegrates towards the critical fluence.

Hardness and Young’s modulus of the irradiated IG-110 and HPG-510 graphite are compared in Fig. 8, in the depths from 2.2 µm to 2.6 µm. The data are averaged from 36 tests and the error bars are the standard deviation. The hardness of IG-110 at 2300 nm increased from 0.06 GPa to 1.09 GPa after the irradiation (Fig.8a), while that of HPG-510 increased from 0.41 GPa to 1.49 GPa after the irradiation (Fig.8c). The Young’s modulus of IG-110 changed from 4.23 GPa to 12.25 GPa after irradiation (Fig.8b), while that of HPG-510 changed from 10.53 GPa to 13.83 GPa (Fig. 8d). Both the hardness and Young’s modulus increase after irradiation, indicating an obvious irradiation-hardening effect, which is consistent with the neutron irradiation experiments[46]. The increases in hardness and Young’s modulus are presumably caused by pinning of the basal plane dislocations as observed in single crystal graphite.

Fig. 8Full-screen View

(Color online) Hardness and Young’s modulus of IG-110 and HPG-510 graphite irradiated by 7MeV-Xe²⁶⁺, as a function of depth.

Changes in the Hardness and Young’s modulus are shown in Fig. 9, as a function of displacement per atom. In Figs.9(a) and 9(b), the Hardness and Young’s modulus increase with the dpa, but the hardness of IG-110 decreased after being irradiated to 2.61 dpa, due to probably by the saturation of defects in graphite. In Fig. 9(c), the hardness of IG-110 increases rapidly with the irradiation dose, being 17 times higher at 2.61 dpa than that of the as-received samples. And it is larger than that of HPG-510 (less than 3 times). In Fig. 9(d), the modulus of IG-110 is 2 times higher than that of HPG-510 (less than 0.32 times). And it is pretty obvious that the hardness and modulus of HPG-510 are larger than of IG-110 at 5.23 dpa.

Fig. 9Full-screen View

(Color online) Changes in hardness and Young’s modulus of the irradiated IG-110 and HPG-510 graphite.

As we discussed above, many evidences indicate that HPG-510 is more irradiation resistant than IG-110. Although, no theory could predict the irradiation behaviors of graphite yet, it is agreed that the microcrack is essential to the irradiation behaviors of graphite, for it accommodates the interstitial atom induced expansion along the c-axis of graphite crystal. HPG-510 has more microcrack than IG-110 (Fig. 2), and this may explain their irradiation behavior difference.