Irradiation Effects of Fiber and Matrix Induced by He+ Ion for High

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Irradiation Effects of Fiber and Matrix Induced by He+ Ion for High Performance C/C Composites Shanglei Feng, Yingguo Yang, Huihao Xia, Li Li, Qiantao Lei, Derek K. L. Tsang, and Xingtai Zhou ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00362 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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ACS Applied Nano Materials

Irradiation Effects of Fiber and Matrix Induced by He+ Ion for High Performance C/C Composites Shanglei Fenga, b,#,*, Yingguo Yang a,b,c,#, Huihao Xiaa, Li Li a,c, Qiantao Lei a, Derek K. L. Tsang a, Xingtai Zhoua,* a Shanghai

Institute of Applied Physics, Chinese Academy of Sciences, 2019 Jialuo Road, Shanghai 201800, China b University

c Shanghai

of Chinese Academy of Sciences, Beijing 100049, China

Synchrotron Radiation Facility, Shanghai Advanced Research Institute, Chinese Academy of Sciences, 239 Zhangheng Road, Shanghai 201204, China

Abstract To optimize the performance of the carbon fiber reinforced carbon matrix (C/C) composites by controlling the microstructure for more reliable and safety application in Thorium Molten Salt Reactor (hereafter TMSR), we have investigated the irradiation effects of fiber, matrix and their interfaces in C/C composite induced by He+ ions and further reveal their corresponding micro-mechanism. Compared with fibers, an obviously fragmented surface morphology in matrix appears and then gradually becomes widespread around the surface of C/C composite with increasing dose of irradiation. This found is attributed to the breakage of crystallites observed by synchrotron-based grazing incidence X-ray diffraction (GIXRD) and the increase of defect state density revealed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, respectively. Three different microstructure evolutions in fiber, matrix and fiber-matrix interface induced by irradiation damage have been further revealed in detail by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). It is found that the layered structure gradually loses its initial ordering and the nanostructural degradation in carbon matrix is much more serious than that of the fiber, resulting in breaks and bends in the lattice with increasing dose. Observed by nanoindentation experiment, the enhancement of the hardness and modulus of the matrix is more significant than that in fiber, which can be attributed to the more obviously pinning of basal plane dislocations in the matrix due to lattice defects induced by He+ irradiation. These discoveries are properly contribute to improve the performance of the C/C composites by regulatory microstructure composition, such as fiber and matrix. Keywords: C/C composites, irradiation effect, fiber-matrix interface, microstructure, 1

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synchrotron-based GIXRD, Thorium Molten Salt Reactor 1. Introduction Carbon fiber reinforced carbon matrix (C/C) composite has been considered as one of the promising candidates in the Next Generation Nuclear Plant(NGNP)1-2 to replace the metallic alloy to ensure the higher outlet temperature and more freedom in the reactor scram procedure, since its superior material properties of low density, low coefficient of thermal expansion, low neutron absorption cross-section, high specific strength, high thermal conductivity, etc3-5. Research on the application of C/C composite in High Temperature Gas-cooled Reactor and Very High Temperature Reactor was first performed in the 1990s6. One typical potential application of C/C composite is control rod component6-8. It is noteworthy that C/C composites will suffer strength degradation and change in material properties as a result of carbon atom displacements and crystal lattice damage caused by high energy fission neutrons collision. Therefore, to reveal and understand the irradiation effect and its mechanism is critical to the extensive application of C/C composites. However, effects of irradiation on C/C composite materials have received less attention than nuclear graphite, though similar trends have been observed9-11. Recently, the irradiation effect of C/C composites has been reviewed and highlighted limited amount of work on the subject in the literatures12-16. Snead and Gray et al. found that the carbon fiber after neutron irradiation shows a shrink along its length while the fiber diameter initially shrank and then swelled14, 17. T.D. Burchell et al. have reported that neutron irradiation can induce the changes in dimension and thermal conductivity of C/C composite in terms of their architecture (two and three-dimensional (3D)) and fiber type5, 14. However, micro-structural evolution of C/C composite induced by irradiation has been rarely reported. The effects of ion bombardment on the material properties are similar to those of neutron irradiated reactor materials, therefore, ion irradiation has been widely used to study the irradiation damage in nuclear graphite due to its obvious advantage for higher damage rate, ease of operation and hazards associated with active material5, 18-19. Herein, C/C composites have been irradiated by 2 MeV He+ ions at room 2


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temperature to reveal the microstructures evolution of C/C composite induced by irradiation with different doses. With increasing dose of irradiation, the surface morphology change in C/C composite was observed by scanning electron microscopy (SEM), the crystal structure degradation during irradiated damage area was monitored by synchrotron-based grazing incidence X-ray diffraction (GIXRD), the defect state density in the fiber and matrix of C/C composite was tracked by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Three different micro-structural morphologies in fiber, matrix and fiber-matrix interface have been further revealed in detail by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). Furthermore, the hardness and modulus of the fiber and matrix in C/C composite induced by He+ ion irradiation were measured by the nano-indentation experiment. 2. Experiments 2.1. Sample preparation The C/C composite samples used in this study were fabricated by a pitch impregnation process as shown in the scheme of Fig. 1. High modulus polyacrylonitrile (PAN) carbon fibres (average diameter 6.0 μm) are used as reinforcement for the C/C composites. The structure of fiber preform is alternately laminated with the fiber web and the non-woven fabric in the XY direction, and the needle is formed in the Z direction (shown in Fig.1). The carbon fiber preforms were annealed for 1 hour at ~1800℃ under Ar atmosphere and then fast impregnated with molten pitch at 0.1 MPa and subsequently baked at 1000 °C to convert the pitch into a carbonaceous material. Further cycles of impregnation are used to densify the composite. Finally, the composite is graphitized by heating to 2500℃. Properties of final C/C composite are showed in Table S1 (seen in supplementary material). Bulk density of C/C composite is around 1.95 g·cm-3.

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Fig. 1-

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A scheme for the fabrication of C/C composite

The dimensions of each sample are 1.0 cm1.0 cm0.3 cm (sample size). These samples are divided into three groups (A, B and C). Group A is pristine samples for the purpose of comparison. The samples in Group B and Group C are irradiated at room temperature by a constant-energy 2 MeV He ions beam. The final fluences for Group B and C are 11015 and 11016 ions cm-2, respectively. Prior to irradiation, each specimen was finely polished (using 0.05 µm Al2O3 polish solution and a cloth polishing), and then ultrasonically cleaned in ethanol followed by acetone and finally by distilled water to remove surface contaminants. Afterwards, each specimen was baked in a vacuum furnace for about 24 h at 100oC. The damage profiles, irradiation doses, and atomic displacement values were calculated with SRIM-2010 program

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using a displacement energy (Ed) of 28 eV. 2.2. Characterizations SEM images were obtained using a field-emission scanning electron microscope (LEO 1530 VP) in the secondary electron image mode and backscattered electron image mode. Synchrotron GIXRD measurements were performed at BL14B1 beamline of Shanghai Synchrotron Radiation Facility, the wavelength of the X-ray is 1.54 Å. One dimensional GIXRD spectrum was collected by a NaI point detector with a step of 0.02 degrees and a counting time of 0.5 second per step. The grazing incidence angle of Xray was fixed at 0.4° with respect to the surface plane of the sample. XPS spectroscopy 4


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was measured with a Kratos spectrometer (Axis UltraDLD) using mono-chromatic Al Ka (1486.6 eV).The incident light spot size of ~ 13 mm2 is considerably larger than the diameter of fiber in C/C composite specimen. Raman spectroscopy was performed at room temperature using a Bruker SENTERRA Dispersive Raman microscope and spectrometer at 532 nm equipped with a CCD camera. The laser beam was focused to about 1.4 μm in diameter at a power of less than 5 mW at the most disordered zone (the calculated projected damage range Rd) on the cross-sectional specimen. TEM specimen preparation steps used were exactly the same as the used for the graphite 21. Specimens (both unirradiated and irradiated) were prepared by slicing into 3 mm disks using an ultrasonic cutting machine. In order to achieve the microstructure in the irradiation damage peak area, the irradiated specimens were finely ground and polished to remove the thickness of 6.0 microns in the side with the irradiation. The thicknesses of these samples were measured with a micrometer caliper. The irradiated disks were then mechanically thinned down to 20–30 μm in thickness through grind the back of the irradiated side. Finally, a precision ion polishing system (PIPS 691; Gatan Inc., USA) was utilized to obtain the final electron transparency. In order to minimize the ion-beam damage during the ion beam polishing process, the specimens were milled for about 2030 min at 5o followed by a low angle milling step at 2o for 10 min. TEM and HRTEM images were obtained using a transmission electron microscope (FEI Tecnai G2 F20 S-TWIN) with a high spatial resolution 0.19 nm. Finally, the changes of hardness and modulus in C/C composite induced by He+ irradiation were measured using a nano-indenter (G200, America).

3. Results and discussion

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Fig. 2 - FE-SEM micrographs of (a) pristine C/C composite, (b) the interface among pyrolytic carbon at higher magnification, and irradiated C/C composite at the fluences of (c) 11015 ions·cm-2 ,(d) 11016 ions·cm-2, respectively; (e) GI-XRD patterns of C/C composite, pristine (black line) and ion-irradiated samples at fluences of 11015 ions·cm-2 (red line) and 11016 ions·cm-2(blue line), (f) (002) diffraction peak FWHM and peak area for these samples are plotted as a function of fluence. Curve-fitted C 1s XPS spectra of (g) pristine C/C composite, irradiated C/C composite at the fluences of 11015 ions·cm-2 (h) and 11016 ions ·cm-2 (i). 6


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Figs. 2a-d show the morphology changes for the virgin and as-irradiated C/C composite specimens with different doses. Fig. 2a illustrates the carbon fibers within a matrix of graphitized pitch. The cross section of fiber is nearly circular with an average diameter ~6.0 μm. According to SRIM's calculations, the whole damage depth is estimated to be ~6.3 μm (see Supplementary Fig. S1), which mean that the He+ ion beam can effectively penetrate through fiber cross section. Before irradiation, the junction between adjacent matrixes is clear, and an obvious order graphite planes can be seen in Fig. 2b. After irradiation at the fluence of 11016 ions ·cm-2, a so-called clastic morphology appears on the carbon matrix (Figs. 2c and d) and such a widespread fragmented shape surface was observed in Fig. 2d, which is attributed to the breakage of crystallites initiating at the grain boundaries of C/C composite induced by He+ implantation 22. In order to reveal the breakage of crystallites on the surface of C/C composite, the GI-XRD patterns of pristine and ion-irradiated samples at the fluences of 11015 and 11016 ions·cm-2 are presented in Fig. 2e. A set of strong regular peaks is observed at 2 @ ~26.16, ~42.54, ~44.74, and ~54.97 corresponding to (002), (100), (101) and (004) crystal planes of graphite, respectively, indicating that graphite phase of C/C composite exhibit mainly hexagonal (graphite-2H ， space group: P63/mmc, JCPDS #:41-1487)22. It is notable that the irradiated C/C composites alike with the other graphite materials also keep the same graphite phase

23.

The enlarged view of (002)

peak (inset) shows that the He+ irradiation results in an obviously peak position shift to the lower diffraction angle, indicating that the interlayer spacing of graphene layers becomes larger (showed in Fig. 2f) from 0.3398 to 0.3406 nm. As shown in Fig. 2f, the FWHM of (002) diffraction peak for the C/C composite increased from 0.264 to 0.406 nm-1 with increase of irradiation fluence, which demonstrates that the degree of crystalline order has decreased24. Figs. 2g-i show high resolution C1s spectra of the un-irradiated and irradiated C/C composite, respectively, which are fitted by Doniach-Sunjic functions. It can be seen that the binding energy (BE) of C1s peak covers a wide region from BE = 282 to 293 eV, indicating a complex neighbor chemical states in carbon atoms with a series of 7



binding energy25. Fig. 2g shows that four prominent peaks located at around 284.5, 285.5, 287.1 and 289.5 eV were fitted for the C 1s spectrum of the virgin sample, which presents a quite complicated just as in other polycrystalline carbonaceous materials, indicating an inhomogeneous surface of the sample26-27. The dominant peak located at ~ 284.5 eV is assigned to the typical graphitic sp2 C=C bond28-31. The peak at around 285.5 eV is accounted to sp3 C-C and C-H bond, which correspond to the “defect” peak32-33. Another two peaks located at peak 287.1 and 289.5 eV are generally related to oxygen functionalities, which most likely arise from C-OH, C-O-H, or C-OOH29, 3235.

Figs. 2h and i show the C 1 s spectra of the C/C composite irradiated at the fluences

of 11015 and 11016 ions·cm-2, which present much less C–O and COOH peaks compared with that of pristine C/C composite. After ion irradiation at the fluence of 11016 ions·cm-2, the total amount of sp2 C=C

decreases from ~48.46% to ~19.02%,

while the ‘‘defect’’ peak increases obviously from ~38.16% to ~76.28%, which implies that ion irradiation results in the change of localized electronic states on the surface of C/C composite.

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Fig. 3 - Low magnification TEM bright field micrograph of C/C composite (a), and three SAED patterns (b-c) recorded from the area labelled 1(b), 2(c) and 3(d) shown in (a). Raman spectra recorded from carbon matrix (e-g) and carbon fiber (h-j) in C/C composite before irradiation(e and h), and after irradiation at the fluences of 11015 ions·cm-2 (f and i) and 11016 ions·cm-2(g and j).

Nanoscale analyses on the microstructural evolution of C/C composite induced by ion irradiation were conducted using TEM. As shown in Fig. 3a, it is a representative of TEM bright field micrograph including the carbon fiber, carbon matrix and the fiber– matrix interface. Figs.3b-d show the selected area diffraction (SAED) images of the 9



carbon fiber (labelled 1 in Fig. 3a), fiber–matrix interface (labelled 2 in Fig. 3a) and the carbon matrix (labelled 3 in Fig. 3a) areas. All of these SAED patterns showed a set of observable reflection rings of (002), (100) and (004) respectively, which demonstrates that the high crystalline graphite in these samples. To analyze the texture degrees of domains in C/C composite, the orientation angles (OA) are acquired from the SAED images, which denote the arc length and can be discussed by the FWHM of the azimuthal distribution of (002) diffraction reflection 22. According to the relationship between OA and texture 36, PAN-based fiber in C/C composite possessed of a highly textured structure with an OA ~33o. The fiber–matrix interface shows relatively much more organized microstructure with many tiny, yet quite sharp spots showed in SAED image (Fig. 3c), which is attribute to the degree of graphitization of pitch-based matrix is routinely much more than that of PAN-based fiber. There are some microcracks in the interface regions (pointed by the arrow in Fig. 3a) parallel to the basal planes and highly aligned to the fiber axis, which are attributing to delamination of basal planes induced by the thermal contraction in the graphitization process. Notably, agglomeration of small particles with a roughly spherical appearance was observed in the matrix, which has been previously observed in the binder of pitchbonded graphite and has been referred to as rosettes by Jones et al.37. And they proposed that these rosette particles were part due to the quinoline insoluble (QI) the fraction of the coal tar pitch binder. During C/C composite graphitization process, these particles are formed by graphitize of aromatic molecules, and thereby resulting in the generation of these rosette shaped particles composed of a spiral of graphite strands. As shown in Fig. 3d, the faintly visible diffraction spots in SAED ring pattern illustrated that the QI particles were mostly surrounded by well-graphitized crystallites, with the graphite layers in the surrounding crystallites sometimes adopting the same orientation as the graphite layers of the outermost packets in a QI particle. And the ring pattern demonstrated that the continuous variation of the orientation of the basal planes in the QI particles. The non-uniform intensity distribution in the ring pattern also revealed the presence of texture owing to the imperfectly spherical QI particles. Figs. 3e-j show the typical Raman spectra of recorded from the matrix and fiber in 10


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the unirradiated and irradiated C/C composite. The pseudo-Voigt function is used to fit these Raman spectrum which are deconvoluted with the D band at ~1355 cm-1, D' band at ~1620 cm-1, D1 band at ~1500 cm-1, D2 band at ~1150 cm-1, and G band at ~1580 cm-1. The origin of each band has been discussed in literatures

38-43.

The spectra

recorded from the matrix and fiber in unirradiated C/C composite exhibit a dominant G band, and weaker D, D', D1 and D2 bands, indicating a small amount of defects existence. The degree of disorder in carbon material can be illustrated by the integrated intensity ratio ID/IG. It is notable that the ratio ID/IG of fiber is slightly larger than that of the matrix in unirradiated C/C composite, implying a slightly more disorder in the fiber than that in the matrix before the irradiation. The intensity ratio ID/IG also revealed the negative correlate evolution of the inplane ‘‘crystallite size’’ (La) denoted by either the diameter of clusters or the in-plane coherent length of grains44. According to the empirical Tuinstra–Koenig equation44, the crystallite size La of matrix is estimated to be decreased from 10.0 nm to 2.9 nm with irradiation at fluence of 11016 ions·cm-2 whereas La of fiber is decreased from 5.9 nm to 2.4 nm, which is well consistent with a length decrease of the intact basal planes obtained from the following TEM result. Based on above findings, it can be concluded that the structural degradation of the matrix is much more serious than in the fiber after ion irradiation. According to the multicomponent spectral analysis, the FWHM (the full width half maximum) of the G band in the fiber or matrix increases with increasing irradiation dose and whereas the area of the G band decreases, implying that a large amount of in-plane defects generated in the graphite crystal induced ion irradiation. Figs. 3g and j show the significant intensity of D1 and D2 bands of the samples with irradiation at fluence of 11016 ions·cm-2, respectively, and their corresponding ID1/IG and ID2/IG become 4-5 times higher than that of the un-irradiated samples. These increase values of ID1/IG and ID2/IG imply that the interstitial defects which pin the dislocations are rapidly accumulated induced by irradiation, influencing the elastic modulus of graphite materials45, which would be further confirmed by the following nano-indentation experiment in the present study. Through comparative analysis, it is found that the density of defect states induced by irradiation in the matrix is much larger than that of the fiber, implying that the micro-structure of the matrix is more sensitive to ion irradiation than that in fiber.

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Fig. 4 - The HRTEM micrographs recorded from the QI particle regions in the C/C composite without (a) and with irradiation at the fluences of 11015 ions·cm-2 (b) and 11016 ions·cm-2 (c). High magnification HRTEM medium micrograph (d) of fiber in virgin C/C composite shows ribbon-like microstructure in the longitudinal direction, (e) and (f) high magnification HRTEM medium micrographs of fiber with irradiation at fluences of 11015 ions·cm-2 and 12


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11016 ions·cm-2.

Figs. 4a-c show the HRTEM micrographs recorded from the QI particle regions of C/C composite with and without irradiation. The HRTEM micrograph of QI particle in virgin C/C composite as seen in Fig. 4a shows the finer crystal structure of graphitic strands. After the irradiation (Figs. 4b and c), the graphite crystals with a layered structure gradually loses its initial ordering due to the lattice broken and the turbostratic structure appeared at the fluence of 11016 ions·cm-2. Notably, a series of dark spots, which are highlighted by a red circle with a size of 1~2 nm, generated after the irradiation at fluence of 11015 ions·cm-2, indicating the presence of defect clusters induced by ion irradiation45. It is well consistent with the previous literature reported by Telling et al that many similar defects in a multi-scale from Å to nm and even μm have been examined46. As is well-known, the interstitials are more mobile than the vacant sites which are found to be at higher concentrations because of their lower rate of absorption and aggregation at boundaries46, which indicates that defect clusters shown in Fig. 4b are mainly attribute to interstitial clusters. Figs. 4d-f show HRTEM micrographs of the lattice fringe recorded from the longitudinal section of fiber regions of C/C composite with and without irradiation. Fig. 4d shows the medium magnification lattice fringe HRTEM micrograph of fiber. The fiber-axial orientation is marked by arrow. As seen in high magnification image (inset in Fig. 4d), the PAN fibers consist of nanometer crystallites (~5 nm) which are stacked by a series of graphite layers. The dis-organized carbon is filled among the betterorganized crystalline regions. In the longitudinal direction, it is typical that these crystallites are attached to each other and thereby formed the dominant ribbon-like microstructure, which is well consistent with the observation of Guigon et al. 47. With increase of ion irradiation fluence (Figs. 4e and f), the layer-by-layer structure of the ribbon-like crystalline regions gradually loses its initial ordered degree, which is attributed to the breaks and bends of the graphite lattice (as marked by dotted rectangles).

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Fig.5–High resolution TEM images of fiber–matrix interface in virgin C/C composite (a), and irradiated C/C composite at the fluences of 11015 ions·cm-2 (b) and 11016 ions·cm-2 (c). Three noise filtered images (d-f) extracted respectively from the sections of (a-c).

Fig. 5 presents the lattice fringe image recorded along a normal direction of (001) crystalline orientation and reveals the sequential microstructure change of the (002) 14


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basal planes in fiber–matrix interface of C/C composites before and after irradiation, respectively. Fig. 5a shows the HRTEM image in the area of fiber–matrix interface in the C/C composite before irradiation, which illustrates the layer-by-layer carbon atom forming over 10 nm length intact basal planes. From these ordered arrangement of carbon atoms along the out-of-plane and in-plane directions, the crystallite size of graphite domains (Fig. 5a) can be deduced to be about 20 nm, and the size of the intact region without basal dislocations to be over 10 nm (highlighted in a red dotted rectangle in Fig. 5a), respectively. After irradiation (shown in Figs. 5b and c), the basal planes of fiber–matrix interface in C/C composite presented bending (marked by red dotted circles) and were broken into a series of small fragments (marked by red solid ellipses), leading to the average length of intact planes decreased from ~20 nm to ~2-5 nm. However, it is obvious that each part of these disordered zones still kept an original lamellar nanostructure, which is confirmed by the globally parallel fringes. After irradiation to a fluence of 11016 ions·cm-2, most of intact regions significantly reduced to the typical structure of nano-crystalline graphite (with size of ~2-5 nm), which is marked by red dotted rectangle in Fig. 5c. Obviously, the bending and breaking phenomenon of basal planes after irradiation at the fluence of 11016 ions·cm-2 illustrated an observable damage of fiber–matrix interface in C/C composite induced by He+ ion irradiation. Three noise filtered images shown in the Figs. 5d-f are extracted respectively from the magnified sections of Figs. 5a-c. The fiber–matrix interface in the virgin specimen has a small number of the broken and/or bent lattice fringes, as highlighted in red dotted ellipses in Fig. 5d, differing from that of lattice fringe in virgin highly oriented pyrolytic graphite which has orderly perfect lattice fringes. After ion irradiation, many isolated graphene layers present (marked by red dotted circles in Fig. 5e). It also shows in Fig. 5e that the generation of extended interlayer defects referring to the adjacent basal planes performed some new dislocations which are marked by dotted rectangles. As shown in Fig. 5f, the basal plane bending and basal dislocations become remarkably and spread all over the structure with increasing irradiation fluence up to 11016 ions·cm-2, which are similar to the earlier reports of graphite irradiated by in situ 15



electron beam48. After analyzed three different microstructural morphologies of fiber, matrix and fiber-matrix interface, the interphase interface was best in this material prior to ion irradiation. The layered structure of either the matrixes, fibers or fiber-matrix interface is all gradually lose its original ordering due to the breaks and bends in graphite lattice with increasing irradiation fluence.

Fig. 6- (a-b) Hardness- and (c-d) Elastic modulus-Indentation depth relationship for the unirradiated, 2 MeV He+ irradiated C/C composite at the fluences of 11015 ions·cm-2 and 11016 ions·cm-2, respectively.

To further reveal the mechanical behaviors and their comparison with other radiation effect, Fig. 6 showed the influences of the hardness (Fig.6 a-b) and elastic modulus (Fig.6 c-d) of fiber and matrix in virgin C/C composite and irradiated C/C composite with the fluences of 11015 ions·cm-2 and 11016 ions·cm-2. The data points are the average values obtained from 10 tests of each type sample and their error bars were the standard deviation calculated from these10 tests 45.In the damage area from 16


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surface to ~500 nm displacement, the hardness and Young’s modulus of C/C composites after irradiation show an obvious enhancement with increasing of the irradiation fluence, whereas in the much deeper region the growth rate of both physical quantities begin to decrease and gradually keep unchanged with increase of displacement. Moreover, all of the hardness and Young’s modulus of C/C composites after irradiation increased significantly compared with the original value, indicating an obvious irradiation-hardening phenomenon, which is well consistent with the results obtained from the proton or neutron irradiation reports45, 49-50. Similar to the founds reported by Kelly, the hardness increase in C/C should be ascribed to pinning effect caused by irradiation-induced point defects and defect clusters, meanwhile the elastic modulus enhancement attributed to point defects via pin dislocations caused by irradiation 51. Notably, the maximum increase amplitude of the average hardness (400 nm~900 nm) in the matrix is ~3.5 times (from 0.63 to ~2.22 Gpa) after irradiation in Fig. 6b and that of elastic modulus in matrix is ~2.4 times (from ~12.20 to ~29.25 Gpa) in Fig. 6d, which showed more significant increase than the changes in fiber as shown in Figs. 6a and 6c, indicating that the matrix is more sensitive to irradiation effect. The reason for this result would be attribute to the easier produce of point defects and defect clusters, rapid accumulation of the interstitial defects and the increase in basal plane dislocations in matrix after the irradiation, which have been observed by the Raman, XPS and TEM images in present study. In addition, the error bars in fiber and matrix with irradiation at fluence of 11016 ions·cm-2 are larger, which cloud be attributed to the change of surface roughness and hardness, and more defects produced by the higher fluence irradiation52-54. This phenomenon agrees well with the published data45, 55-57.

4. Conclusions In summary, the irradiation effect of 2-MeV He+ ions on PAN–pitch-based C/C composite was characterized by the SEM, GI-XRD, XPS, TEM and HRTEM analyses. After exposure to He+ ion irradiation, a fragmented surface morphology appears and then gradually becomes widespread around the surface of C/C composite, which is attributed to the breakage of crystallites. Due to the ion irradiation, the total amount of 17



aromatic C–C (sp2) decreases, while the ‘‘defect’’ peak increases. With increased irradiation dose, the interlayer spacing between graphene layers becomes larger and the corresponding crystalline order degree decreases obviously. The crystalline size LC002 of C/C composite is estimated to have decreased from 27 to 17 nm due to irradiation damage fluence of 11016 ions·cm-2. Raman studies revealed that a corresponding rapid increase in the interstitial and vacancy defects in matrix and fiber, and the crystallite size La of matrix and fiber are respectively decreased from 10.0 nm to 2.9 nm and from 5.9 nm to 2.4 nm induced by irradiation at the fluence of 11016 ions·cm-2, which demonstrated that the nano-structural degradation in the matrix is much more serious than that in the fiber after He+ ion irradiation. Three different microstructural morphologies in fiber, matrix and fiber-matrix interface were analyzed in detail. The interphase interface was best in this material prior to ion irradiation. The layered structure in either the matrixes, fibers or fiber-matrix interface all gradually lose its initial ordering, resulting in breaks and bends in the lattice with increasing fluence. The enhancement in the hardness and modulus of C/C composites induced by ion irradiation can be attributed to the pinning of basal plane dislocations caused by the lattice defects. These discoveries are properly contribute to improve the performance of the C/C composites by controlling the microstructure for more reliable and safe application in TMSR, spacecraft, etc. Associate content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxx. Damage profile induced by 2MeV He+ ions according to an estimation from SRIM-2010; key parameters for C/C composite Additional information Corresponding Author *Correspondence and requests for materials should be addressed to Shanglei Feng (email: [email protected]) or Xingtai Zhou ([email protected]). Notes #These

authors (Shanglei Feng and Yingguo Yang) contribute equally to this paper. The

authors declare no competing financial interest. 18


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Irradiation Effects of Fiber and Matrix Induced by He+ Ion for High

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