High resolution transmission electron microscopy and electron energy loss ... Oriented Pyrolytic Graphite (HOPG) by using HRTEM and Electron Energy Loss.
STRUCTURAL CHANGE OF GRAPHITE DURING ELECTRON IRRADIATION;, J. KOIKE1 and D. F. PEDRAZA2 1 2
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ABSTRACT Highly oriented pyrolytic graphite was irradiated at room temperature with 300-keV electrons. High resolution transmission electron microscopy and electron energy loss spectroscopy were employed to study the structure of electron-irradiated graphite. All the results obtained here consistently indicated the absence of long-range order periodicity in the basal plane and the loose retention of the c-axis periodicity. The structure was modeled based on a mixture of sixfold and non-sixfold atom rings. The formation of non-sixfold atom rings was related to the observed buckling and discontinuity of the original graphite basal plane. INTRODUCTION Graphite materials have been used as moderators and structural components in fission reactors as well as plasma facing materials in experimental fusion reactors. In a nuclear reactor environment, graphite is subject to irradiation damage and undergoes degradation in its physical properties. Numerous works have been done to understand structural and property changes during irradiation [1,2]. Based on Raman spectroscopy results for ion and neutron-irradiated graphite, it has been suggested that amorphization takes place at 425 and 475 K [3]. However, this suggestion was countered by Kelly [4] for the case of neutron irradiation in fission reactors on the basis of the highly anisotropic dimensional changes experienced by irradiated graphite. Consistent High Resolution Transmission Electron Microscopy (HRTEM) results have been reported for graphite irradiated with 14 MeV neutrons [5], hydrogen [6], and helium ions [7]. More recently, renewed efforts have been directed towards the understanding of fundamental problems of structural damage by using graphite fibers irradiated with high energy electrons in order to take advantage of the relative simplicity of the crystalline structure and the damage production process [8, 9]. However, reported results are in controversy concerning the possibility of amorphization. In the present work, we studied the structure of electron-irradiated Highly Oriented Pyrolytic Graphite (HOPG) by using HRTEM and Electron Energy Loss Spectroscopy (EELS). Based on the obtained results, a structural model is proposed. EXPERIMENTAL PROCEDURE Two types of thin foils were prepared from HOPG. One foil has its foil normal parallel to the graphite c-axis and the other perpendicular to the c-axis. The parallel foil was prepared by cleaving HOPG along a basal plane with an adhesive tape followed by dissolving the tape in toluene and scooping the foil onto a Cu-mesh grid. The perpendicular foil was prepared by slicing a thin plate across the basal plane of HOPG. The thin plane mounted on a slotted Cu grid was then mechanically ground and ion milled to perforation. Electron irradiation was performed with 300 keV electrons in a Philips CM 30T transmission electron microscope at room temperature. The electron beam was focused to about 0.3 \xm in diameter during irradiation with j I _ -111( an electron flux of 6x10 23 e.nv2 that corresponds to a displacement rate of 5.5xl0"4 dpa/s for a displacement threshold energy of 30 eV [10] and a displacement cross section of 8.7xlO"28 nr 2 [11]. A maximum beam heating was estimated to be ~ 10 °C by using a model proposed by Fischer [12]. The irradiation-induced effects on the structure were investigated by taking high resolution lattice fringe images and selected-area diffraction patterns in situ during irradiation. Electron energy loss spectra were also taken with a Philips EM 400T/FEG STEM equipped with a Gatan parallel detection EEL spectrometer. infttrft DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED
RESULTS Figure 1 shows selected-area diffraction patterns of specimens irradiated to a dose of 1 dpa; (a) a parallel specimen with the electron beam // [0001] and (b) a perpendicular specimen with the electron beam // [1210]. As the figures show, the diffraction pattern of the perpendicular specimen exhibits the structural anisotropy indicated by the strong diffuse intensity around the original 0002 crystalline spots. In order to examine the effects of the irradiation direction on the resultant structure, the parallel specimen was tilted by 46° from the orientation in Fig. 1 (a). Although the result is not shown here, the structural anisotropy was observed as evidenced by a similar intensity distribution to Fig, l(b). These results indicate that the long-range periodicity in the basal plane is destroyed by electron irradiation, but the periodicity along the c-axis is maintained with a small deviation in the spacing and orientation of the basal planes. The periodicity along the c-axis can be visualized in Fig. 2 where high resolution lattice fringe images of the perpendicular specimen before and after irradiation are shown. Before irradiation, periodic (0002) lattice fringes are observed, indicating the periodicity along the caxis. After irradiation to 1 dpa, the lattice fringes are broken to small segments of 0.5 to 5 nm in length. Buckling and lattice-spacing variation of the basal planes can also be seen in the figure. High resolution imaging of the structure in the basal plane was attempted by using a JEOL 4000EX HRTEM, but the structural image of atomic columnswas not resolved very well after only a short irradiation time, possibly due to the overlapping of the misoriented segments. It should also be mentioned that in lower magnification images, the strain contrast associated with defect clusters, if there is any, was not observed in either type of specimen. Figure 3 shows EEL spectra near the carbon K-edge of unirradiated and irradiated areas in the (a) parallel and (b) perpendicular specimens. The spectra from the unirradiatcd area show sharp a* and K* peaks. After irradiation, these peaks broaden, due to structural disordering in the basal plane and variation of the basal plane spacing. Because of the anisotropic nature of the a* and K* bonds in the graphite structure, the intensity ratio of K*/C* depends on the relative orientation of the specimens with respect to the electron beam. Before irradiation, K*/G* = 0.26 for the parallel specimen and K*/G* = 0.6 for the perpendicular specimen. After irradiation, the intensity ratios, calculated from the intensity at the same energy loss values as in the unirradiated specimen, are found to be only slightly changed; K*/O* = 0.28 and 0.51 for the parallel and perpendicular specimens, respectively. This indicates that the a bonds still lie on the buckled basal planes and the n bonds are localized perpendicular to the basal planes. The formation of sp 3 bonding is unlikely because no decrease of the n* peak intensity was observed at the same time in both specimens. Slight variation of the intensity ratios after irradiation is probably caused by the misorientation of crystallographic directions on a localized scale as shown in the high resolution lattice fringe image, increasing the structural isotropy on a larg scale. The density change during electron irradiation was calculated from plasmon peak shift in the energy loss spectra by assuming that the number of valence electron per carbon atom remains constant during structural change. The plasmon peak position was determined by fitting an experimentaly observed curve by a Lorenzian function. According to a free electron approximation [13], an observed decrease of the plasmon peak position by 1.2 eV after irradiation corresponds to a density decrease of 8.9%. This value is smaller than the density change of > 11.5% expected in transformation from crystalline graphite to amorphous carbon, indicating an intermediate degree of structural disorder in the electron-irradiated graphite. DISCUSSION We have shown that electron irradiation to a dose of 1 dpa (l.lxlO 2 7 em*2) destroys the long-range structural periodicity in the basal plane. In contrast, the long-range periodicity along the c-axis was found to be loosely maintained. These results are in good agreement with recent results reported in electron irradiation of graphite fibers [9]. Electron irradiation generally produces interstitials and vacancies. The formation energies of an isolated interstitial and vacancy in graphite were calculated to be 7.1 and 7.3, respectively [14]. Although these values appear rather large, the formation of di-interstitials and di-vacancies
have been calculated to release a large portion (5.7 and 5.1 eV [14]) of the energies associated with isolated defects. Maeta et al [2] has proposed the presence of di-interstitials at 80 to 120 K in neutron-irradiated graphite, based on electrical resistivity measurement. These di-interstitials were thought to be mobile at room temperature and form clusters. However, we did not observe the formation of defect clusters in any stages of electron irradiation. The clusters of more than a few nanometers are generally visible in TEM images as strain contrast. Absence of such strain contrast suggests that large defect clusters are not formed in electron-irradiated graphite under the present experimental conditions. Since the atomic density of the electron-irradiated graphite was found to be very close to that of amorphous carbon, the highly distorted structure observed in the present work can be better understood by considering structural models of amorphous carbon. Various types of amorphous carbon have been synthesized by vapor deposition techniques [15] We will discuss the structure of predominantly sp2-bonded amorphous carbon with little or no sp3 bonds since the formation of sp3 bonds was not noticeable in electron-irradiated graphite. According to a first principle molecular dynamics calculation [16], amorphous carbon having 85% of sp 2 bonds and 15% of sp3 bonds is characterized by buckled atom sheets that cross each other orthogonally and consist of fivefold, and sevenfold atom rings in addition to sixfold atom rings. Townsend et al [17] theoretically studied the structure of sp2-bonded amorphous carbon in terms of randomly curved graphite-like atom sheets. A stable configuration of carbon atoms on such a smooth curved surface was found to be a mixture of sixfold and non-sixfold rings. Their results suggest that when a graphite basal plane is buckled by irradiation-induced lattice defects, carbon atoms in the basal plane rearrange and form non-sixfold rings to relax the strain around the lattice defects. Based on these theoretical calculations, we propose a structural model of electronirradiated graphite in Fig. 4. Absence of long-range order in the basal plane, indicated experimentally by the continuous diffuse ring intensity, can be explained by the presence of nonsixfold atom rings. The non-sixfold atom rings would accompany the deviation of a bond angle as well as rotation of neighboring sixfold atom rings with respect to the original atom ring arrangement in HOPG. As we can see in the figure, a trigonal nature of the a bonds is not altered, consistent with the nearly constant n*/a* intensity ratio in the EEL spectra. Elastic strain associated with buckling of the basal plane can be relaxed by the formation of non-sixfold atom rings. Strain normal to the basal plane would not be significant due to a weak Wan der Waals force between the adjacent basal planes. Discontinuity of the basal planes, as observed in the lattice fringe image, may occur in some parts of the buckled sheets to relax a substantial elastic strain accumulated on a localized scale. The density decrease, observed as a plasmon peak shift, can be explained by the combination of buckling and discontinuity of the basal planes. In a separate experiment that will be published elsewhere, HOPG was irradiated under the same condition to a dose of more than 5 dpa. Although the degree of buckling and discontinuity increased to some extent, the c-axis periodicity was found to persist. This result indicates that the proposed structure is a steady state structure and is unlikely to transform to three dimensionally random amorphous carbon structure. SUMMARY Highly oriented pyrolytic graphite was irradiated with 300 keV electrons at room temperature to a dose of 1 dpa. The damage structure was studied by means of electron diffraction, high resolution lattice fringe imaging, and electron energy loss spectroscopy. The long-range structural order in the basal plane was destroyed by irradiation. Small segments of the basal plane structure were found to align loosely along the c-axis and maintain the c-axis periodicity. The structure of electron-irradiated graphite was proposed to be a mixture of sixfold and non-sixfold atom rings lying on the buckled basal plane. ACKNOWLEDGEMENT This research was sponsored by the Office of Fusion Energy, U. S. Department of Energy, under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc. The
use of the SHARE electron microscopy facility at ORNL are gratefully acknowledged. The authors are most grateful to Dr. E. A. Kenik for his valuable assistance in the experiments. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
B. T. Kelly, Physics of Graphite (Applied Science Publishers, Essex, England 1981) H. Maeta, T. Iwata, and S. Okuda, J. Phys. Soc. Japan, 39, 1558 (1975) K. Niwase, K. Nakamura, T. Shikama, and T. Tanabe, J. Nucl. Mater. 170, 106 (1990) B. T. Kelly, J. Nucl. Mater.172, 237 (1990) T. Tanabe, S. Muto, Y. Gotoh, and K. Niwase, J. Nucl. Mater, 175, 258 (1990) Y. Gotoh, H. Shimizu, and M. Murakami, J. Nucl. Mater. 162-164, 865 (1989) K. Niwase, K. Nakamura, I. Tanaka, Y. Miyamoto, and T. Tanabe, J. Nucl. Mater. 179181, 214(1991) A. Matsunaga, C. Kinoshita, K. Nakai, and Y. Tomokiyo, J. Nucl. Mater. 179-181, 457 (1991) T. Tanabe, S. Muto, and K. Niwase, Appl. Phys. Lett. 61, 1638 (1992) J. Koike and D. F. Pedraza, Proc. of Int. Conf. on Beam Processing of Advanced Materials, to be published. O. S. Oen, Cross Section for Atomic Displacements in Solids by Fast Electrons, ORNL4897 (National Technical Information Service, U. S. Dept. of Commerce, Springfield, VA 1973), p. 29 S. B. Fisher, Rad. Effects 5, 239 (1970) R. F. Edgerton, Electron Energy Loss Spectroscopy in the Electron Microscope (Plenum Press, New York 1986), p. 152 C. H. Xu, C. L. Fu, and D. F. Pedraza, in preparation D. R. MacKenzie, D. A. Muller, E. Kravtchinskaia, D. Segal, D. J. H. Cockayne, D. Amaratunga, and R. Silva, Thin Solid Films, 206, 198 (1991) G. Galli, R. M. Martin, R. Car, and M. Parrinello, Phys. Rev. Lett. 62, 555 (1989) S. J. Townsend, T. J. Lenosky, D. A. Muller, C. S. Nichols, and V. Elser, Phys. Rev. Lett. 69, 921 (1992)
DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
Fig. 1 Selected area diffraction patterns of graphite after irradiation to a dose of a 1 dpa. T h e electron beam is (a) parallel and (b) perpendicular to the c-axis.
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Fig. 2 High resolution lattice fringe images of graphite (a) before and (b) after irradiation.
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Fig. 3 Effects of irradiation on electron energy loss spectra of the carbon K edge. The electron beam is (a) parallel and (b) perpendicular to the c-axis.
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Fig. 4 A schematic structural model of electron-irradiated graphite consisting of a mixture of sixfold and non-sixfold atom rings lying on the buckled basal planes.
