CONF-921183—2 ' ' DE92 040863
STRUCTURAL CHANGES INDUCED BY ELECTRON IRRADIATION IN GRAPHITE* J. Koike, Oregon State University, Corvallis, OR 97331-6001
and
D.F. Pedraza, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6069 SUMMARY
Highly oriented pyrolytic graphite was irradiated at room temperature with 300 kV electrons. Transmission electron microscopy and electron energy loss spectroscopy were employed to study the structural changes produced by irradiation. The occurrence of a continuous ring intensity in the selected area diffraction (SAD) pattern obtained on a specimen irradiated with the electron beam parallel to the c-crystallographic axis indicated that microstructural changes had occurred. However, from the SAD pattern obtained for the specimens tilted relative to the irradiation direction, it was found that up to a fluence of 1.1x102? e/m2 graphite remained crystalline. An SAD pattern of a specimen irradiated with the electron beam perpendicular to the c-axis confirmed the persistence of crystalline order. High resolution electron microscopy showed that ordering along the c-axis direction remained. A density reduction of 8.9% due to irradiation was determined from the plasmon frequency shift. A qualitative model is proposed to explain these observations. A new determination of the threshold displacement energy, Ed, of carbon atoms in graphite was done by examining the appearance of a continuous ring in the SAD pattern at various electron energies. A value of 30 eV was obtained whether the incident electron beam was parallel or perpendicular to the c-axis, demonstrating that Ed is independent of the displacement direction.
INTRODUCTION
Graphite materials have been utilized as moderators and structural components in fission reactors since the dawning of the nuclear industry. Recently, they have also been used as plasma facing materials in experimental tokamaks. Owing to their importance in both fusion and fission reactor technologies, their structural evolution and physical property changes under irradiation have been the subject of numerous studies. In many investigations, post-irradiation annealing was conducted for the purpose of analyzing point defect behavior [1].
Graphite has a layered crystalline structure consisting of hexagonal planes stacked in an ABAB... sequence, as illustrated in Figure 1. Van der
"The submitted manuscript has been authored by a contractor of the U.S. Government under contract No DE-AC05-84OR2I400. Accordingly, the U S Government retains a nonexclusive royalty-free license to publish or reproduce
Waals attractive forces keep together the graphite layers, whose atoms are
covalently (sp2) bonded. The physical properties of graphite are very
dependent upon crystallographic direction because of the high degree of anisotropy in its structure [2]. Nuclear-grade graphites have been developed to minimize the overall structural anisotropy. Due to their complex polycrystalline microstructure, however, many investigations have been made using highly oriented pyrolytic graphite (HOPG) as a model of their crystallite components. The structural perfection of HOPG is akin to that of natural single crystals.
There is ample experimental evidence that interstitials in irradiated graphite aggregate into clusters. This clusters may eventually grow and form dislocation loops, depending on temperature and dose. Loop formation is the cause of irradiation growth in the c-axis direction [2]. The vacancy behavior, on the other hand, is not well understood. However, it is highly possible that this behavior is related to the basal plane contraction observed after neutron irradiation [2]. In recent years, a renewed effort has been directed towards investigating further point defect behavior and structural changes produced by neutron, ion and electron irradiation [3,4]. Based on Raman spectroscopy results for ion and neutron irradiated graphite, it was suggested that neutron irradiation at 425 to 475 K could promote amorphization [5]. This suggestion was countered by Kelly [6] on the basis of neutron-irradiation induced dimensional changes, which exhibit a strong spatial anisotropy, hardly accountable for in an amorphous structure. In this work, we investigate structural changes during electron irradiation to gain further understanding of the behavior of graphite materials in a radiation environment.
For electrons, the mean free path between collisions with atoms in a solid is large [7] and the energy transferred to the atom is a small fraction of the electron energy. Hence, electrons having energies between the threshold energy and 2 MeV produce only few displacements per collision, typically 1 to 2 Frenkel pairs [8]. For fast neutrons (En>0.1 MeV), on the
other hand, the mean number of displacements per collision varies between
-300 for En=0.1 MeV and 500 for En= 10 MeV [2]. However, no thermal
spikes are produced and the displacement damage consists of a series of well-separated groups containing on the order of ten Frenkel pairs. At regularly utilized irradiation fluxes, considerable Frenkel pair recombination takes place even at room temperature because the interstitial mobility in graphite is substantial [9]. This relatively fast recombination rate ensures that the point defect spatial distribution becomes randomized promptly. Thus, electron irradiation offers the possibility of in-situ studies of irradiation damage albeit at a much faster dose rate than is produced in a reactor.
tool for studying displacement threshold energies in solids through the observation of radiation damage. Typically, the first manifestation of damage appears as black and white spots, sometimes too small to be identified as anything else but defect clusters. Ohr et. al conducted the first TEM study aimed at determining the threshold displacement energy, Ed, for graphite [10]. They examined, in-situ, graphite specimens irradiated at 300 and 600° C and derived a value Ed = 24 eV from the minimum electron accelerating voltage, Ee = 120 kV, required to produce observable damage.
They could also identify the spots, 5 to 10 nm in diametei, as basal interstitial loops in specimens irradiated at electron energies higher than Ee.
At room temperature, damage evolution appears to be quite different. Matsunaga et al. [11] have recently reported the occurrence of amor-phization induced by electron irradiation at temperatures up to ~ 200°Cv
using electron fluxes between 4.78 and 5.25 x 102 3 e/m2s. They studied the
fluence required to amorphize graphite as a function of electron energy and, fitting the Mc-Kinley-Feshbach formula to their data for energies higher than 180 keV, estimated a value of Ed = 12 eV. The authors point out that their experimental data below 170 keV indicate a higher fluence to induce amorphization than extrapolated from the curve fitting, suggesting that this value of Ed is somewhat low. Previous determinations of Ed, including that of Ohr et al. [9] and others based on electrical resistivity measurements [12-14] and on an etch-decoration technique to reveal the presence of vacancy clusters [15, 16], are certainly much higher than 12 eV. They range between 24 and 60 eV for an electron beam incident in a direction parallel to the c-axis. A dependence of Ed on the direction of incidence of the electron beam relative to the c-axis was also suggested in the experiments of Montet [15] who determined values Edn = 31 eV of Edp = 60 eV for electrons hitting the specimen normal and parallel to the basal plane, respectively. Also Iwata and Nihira [13] found a dependence of Ed on the incident direction of the electrons, but lesser (Edn = 28 eV and Edp = 42 eV). In view of the discrepancies found in the literature, we decided to re-determine the value of Ed as well as its orientation dependence.
EXPERIMENTAL PROCEDURE
folding Cu grid. The a-foil was prepared by cutting graphite across the basal plane with a low speed diamond saw. The small slab was mounted on a slotted Cu grid with the basal planes edge-on, then mechanically ground and dimpled to form a small hole in the center. Lastly, in order to obtain an electron transparent area, ion milling was conducted using 5 keV Ar ions at a glancing angle of 15 degrees. A damaged layer less than a few nanometers thick might have formed during ion milling. However, the damaged layer thickness is much less than the specimen total thickness, and thus, had a negligible effect on the TEM images or the SAD patterns.
For structural studies, the specimens were irradiated with 300 keV electrons at room temperature in a Philips CM-30T TEM, with the beam normal to the foil face. The electron flux was 6xlO2 3 e/m2.s which
corresponds to a dose rate of 5.22xlO"4 dpa/s for a displacement threshold
of 30 eV, newly determined in the present work, and a displacement cross section of 8.7 x 10"2& m2 [17]. The total fluence reached was 1.1 x
1 02 7 e/m2 which corresponds to a dose of almost 1 dpa. For the
displacement threshold energy studies, the accelerating voltage was varied from 200 kV down to 130 kV at 10 kV steps, using fluxes between 1.5 and 2.1 x 102 3 e/m2.s, depending on the accelerating voltage, and a fluence of
1.0 to 1.2 x 1027 e/m2, and in some cases 6.0 x 102 7 e/m2.
The irradiation-induced structural changes were investigated in situ by examining SAD patterns obtained in the TEM. High resolution lattice fringe images were also taken for the a-foil. The irradiated specimens were next transferred to an analytical TEM (Philips EM-400T/FEG) equipped with a parallel detection electron energy loss spectrometer (EELS) to further investigate the effect of irradiation on the structural stability comparing the carbon K-edge peak shape in irradiated and unirradiated areas. Specimen thickness in observation areas was determined by EELS from the intensity ratio of zero loss over inelastically scattered electrons. These ratios yielded a specimen thickness of 63 nm in an irradiated area of a c-foil and of 147 nm in an irradiated area of an a-c-foil.
electron beam. Thus, a strong a* peak is observed for electron incidence along the c-axis whereas a strong JI* peak results for electron incidence parallel to the basal plane. Since amorphous carbon does not exhibit the orientation dependence of the EELS spectrum due to its isotropic structure [18], analysis of the orientation dependence of the EELS spectrum provides information on structural disordering.
It is also known that the plasmon peak position, Ep, in the EELS
spectrum is proportional to the square root of the density of valence electrons as described by [20]
where h is Planck's constant, e the electron charge, n the number of valence electrons per unit volume, eo the dielectric constant, and me the mass of the
free electron. The plasmon peak calculated using this formula has been reported to agree well with a wide range of materials including diamond, Si, Ge, InSb, GaAs, and NaCl [21]. It can be seen that any shift of the plasmon peak position would be proportional to the change in atomic density provided that the number of valence electrons per atom remains unaltered by the radiation displacement damage or irradiation-induced structural changes. Since the number of valence electrons remains the same in all the allotropic structures of carbon, atomic density changes caused by irradiation were determined from the plasmon peak position of the irradiated specimen relative to the peak position measured before irradiation.
RESULTS 1. Structure Evoiution
of crystalline graphite as indicated by single arrows. Weak intensities of arc shape can also be seen, indicated by double arrows, extending along the reciprocal [0001] direction; these arcs are not a part of diffuse rings. These results indicate that the long range six-fold order in the basal plane is destroyed by electron irradiation. However, the long range structural order along the c-axis is still maintained with local variation of the basal plane spacing and orientation relative to the unirradiated specimen, as revealed by the broad intensities around the 0002 and 0004 spots.
The persistence of crystalline order along the c-axis upon irradiation to 1 dpa was confirmed by high resolution lattice fringe images of an a-foil. Figure 4 shows the structural change produced by electron irradiation. Before irradiation (Figure 4a), straight (0001) lattice fringes of the basal plane indicative of crystalline perfection are observed. After irradiation (Figure 4b) the lattice fringes of the basal plane are broken up into small segments of 0.5 to 5 nm in length. These segmented fringes exhibit a small tilt angle ( ~ 15 degrees) relative to the original (0001) basal planes, thus remaining closely aligned normal to the [0001] direction. These results demonstrate that crystalline order along the c-axis remains, in agreement with the SAD data. High resolution imaging of the basal plane structure was also attempted using a JEOL 4000EX, but the atomic image was not well resolved, possibly due to the overlapping of nanometer size crystals or misorientation of the nanocrystals with respect to the electron beam.
Figure 5 shows the EELS spectra near the carbon K-edge before and after irradiation with the electron beam parallel (a) and perpendicular (b) to the c-axis. Before irradiation, the relative intensities of the a * and JC* peaks depend on the crystalline orientation with respect to the incident electron beam direction. The %*/a* peak intensity ratio is 0.26 and 0.6 for the electron beam parallel and perpendicular to the c-axis, respectively. After irradiation, the it* peak broadens while the a * peak decreases considerably. The %*/o* peak intensity ratios were calculated using the intensity values determined at the same energy positions for the two peaks prior to irradiation. The intensity ratios are still found to depend upon crystailographic orientation, having values of 0.28 and 0.51 for the electron beam parallel and perpendicular to the c-axis, respectively. The deviations from the unirradiated values can also be attributed to the puckering of the basal planes. The anisotropy of the n*/a* ratio indicates independently the presence of crystalline order after irradiation, in consistency with the results obtained from the SAD patterns and the high resolution lattice images.
saturated peaks centered around the zero energy loss are due to elastically scattered electrons, and the broad peaks at their right are due to energy loss by plasmon excitation. Fitting of the plasmon peaks was done using a Lorentzian function. The plasmon peak position before irradiation is 26.2 eV, in fair agreement with previously reported values [22]. After irradiation, the peak position shifts to 25 eV, yielding an atomic density reduction of 8.9 %. This density change is less than the minimum density difference (11.5%) between graphite and amorphous carbon.
2. Displacement Threshold Energy
Figure 7 shows SAD patterns from areas irradiated with the electron beam parallel to the c-axis. After irradiation v/ith 140 kV electrons to a fluence of 1.2 x 1027 e/m2, the SAD pattern exhibits sharp crystalline spots
indicating no change of crystalline structure. The SAD pattern remain unchanged after additional irradiation to a total fluence of 6.0 x 102 7 e/m2 ,
demonstrating the stability of the crystalline structure under 140 kV electron irradiation. By contrast, after irradiation with 150 kV electrons to a fluence of 1.2 x 1027 e/m2, a diffuse ring pattern is observed, indicating
that atomic displacements have occurred and that the crystalline structure has become disordered.
Figure 8 shows SAD patterns after irradiation with the electron beam normal to the c-axis. Similar to the previous case, crystalline spots are observed following electron irradiation to a fluence of 1.0 x 1027 e/m2, at
an accelerating voltage of 140 kV. Some diffuse intensities observed in the pattern are obtained before irradiation as well. They are due to local bending of graphite basal planes, as revealed by TEM bright field images, and not to irradiation damage. The overall intensity distribution remains the same after further irradiation to a total fluence of 6.0 x 102 7 e/m2.
Again, electron irradiation at 150 kV to a fluence of 1.0 x 102 7 e/m2
produces structural disorder, as seen in the SAD pattern. As in the preceding studies (Figures 2a and 3) the SAD patterns of the disordered structures depend on the irradiation direction, due to the loss of long range order in the basal plane and the loose crystalline periodicity along the c-axis direction.
Ed = 2 me (E + 2meC2) E /
where M is the carbon atom mass, me is the electron mass, c is the velocity
of light and E is the bombarding electron energy. It should be noted that the displacement threshold energies are the same for both electron beam incidence directions and, thus, independent of the crystallographic direction.
DISCUSSION AND CONCLUSIONS
We have found that room temperature electron irradiation to a fairly high fluence, 5.4 x 102^ e/m2 does not produce a complete loss of the
crystalline structure. The SAD patterns from irradiated specimens show that irradiation produces a significant degree of disorder in the crystalline structure. However, even at the very high fluences used in this study, a certain degree of crystalline order remains. The basal planes become disordered, but the weak van der Waals forces still keep a loose stalking of 'puckered' basal planes. HREM images corroborate this conclusion showing that the layer plane disordering is due, at least partly, to a fragmentation of the structure into small units having an average rotation relative to each other of, at most, 15 degrees. Similar structural changes were detected by HREM at the edge of graphite whiskers implanted with 1 keV H+ ions to fluences from lxlO1 8 to lxlO2 5 H+/m2, accompanied by a dramatic increase
of the c-lattice spacing in the implanted region, attributed to the implanted hydrogen occupying interstitial (inter-plane) sites [23]. In the present work, also EELS spectra demonstrate that irradiated graphite retains a certain degree of crystalline order and, consistent with it, that the density decrease is less than expected for amorphous carbon. If all the density change is attributed to an increase in the c-lattice parameter it would be similar to the increase found upon neutron irradiation to a fluence of ~ 3 x l 02 4 n/m2 at 343 K [6]. However, let us note that under neutron
irradiation the a-lattice spacing is found to decrease ( by ~2% under the above conditions).
randomly distributed throughout the lattice. For a random distribution of vacancies, it can be shown that the probability of finding clusters of even vacancy pairs is negligible for vacancy concentrations as high as 2 at.% [25].
A high concentration of immobile vacancies in the basal plane is a source of considerable perturbation. For instance, it has been related to the decrease of the a-lattice parameter observed upon neutron irradiation [26]. In the case of electron irradiation, radiation damage appears to evolve towards a state other than the planar layer with vacancies plus a relaxed configuration of the atoms neighboring the vacancy. Nonetheless, it can be suggested that the presence of vacancies contributes in a significant manner to the fragmentation of the basal planes. Considering that the linear dimension of the basal plane fragments varies from 0.5 to 5 nm and assuming that each fragment is associated with a vacancy, a vac; ncy concentration per atom of ~ 5 x 10"-* for an average fragment size of 1 T.m would account for the fragments (the area per atom in the basal plane is 0.026 nm2). However, the rotation of the fragments relative to the original
basal plane is perhaps too large to be caused by vacancies alone, as the above estimated vacancy concentration might account for a tilt angle of at most 1 to 2 degrees [27]. Hence, it nwy be suggested that the other product of radiation damage, i.e., the interstitials, must also play an active role in the overall irradiation effect.
Although single interstitials have a high mobility, interstitial clusters might not only have their mobility limited by the basal plane deformation but also contribute importantly to that deformation. The puckering of the basal planes involves a considerable relaxation demonstrated by the 15 degrees rotation of the segmented regions relative to the original basal plane. It can then be argued that the substantial relaxation around a vacancy may generate a barrier for Frenkel pair recombination. The presence of such a thermal barrier was suggested by the occurrence of an inverse annealing peak in the electrical resistance at -100 K [28]. No interstitial sinks other than vacancies appear to be present during room temperature electron irradiation of HOPG since interstitial loops are not seen to develop. Hence, vacancies and interstitiais should be in equal numbers, the interstitial population including single ones and those in small clusters.
10
- j - ^ = K - a Ci cv (1)
where K is the production rate of Frenkel pairs per atom, a is the recombination rate per atom, and cj is the concentration of free (non-clustered) interstitials. In equation (1) it is assumed that clusters have a very low mobility compared to free interstitials and do not contribute significantly to vacancy annihilation. The rate coefficient a can be estimated
as 12DjR/Ar using a simple random walk approximation, R being the
recombination barrier and Ar the recombination area. Taking Di = 1 x 10"4
exp(-Qi/kt) m2/s, with Qi = 0.1 eV [3], R = exp(- Qr/kT), with Qr = 0.31 eV
[21], and Ar = 6.25 nra2 ( this value of Ar corresponding to the average
linear dimension for basal plane fragments in the HREM image), yields a = 2.5 x 107 s-l.
Our observations demonstrate that total disordering, i.e., amorphization, is not achieved under electron irradiation at room temperature after a high fluence. It can therefore be concluded that the rate of change of the point defect concentration becomes very slow after a
dcv
certain dose, virtually attaining a steady state. For this condition, i.e., ~TT=0, equation 1 leads to
«•£; <«
Using K=5.22 x 10"4 dpa/s and the value estimated above for cv yields an
interstitial concentration cj = 4 x 10-9, much lower than cv. The value
derived for ci thus supports the concept that most interstitials should be in small clusters.
11 displace an atom permanently from its site (i.e., without the possibility of spontaneous recombination) is only a function of the energy required to break the atomic bonds, which is intrinsically independent of the irradiation direction.
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ACKNOWLEDGMENTS
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 ihe SHARE electron microscopy facilities at ORNL are gratefully acknowledged. The authors are most grateful to Dr. E.A. Kenik for his valuable assistance in the experiments and his careful review of the manuscript. The detailed review and helpful comments of Drs. T.D. Burchell and E.H. Lee are deeply appreciated.
DISCLAIMER
13 FIGURE CAPTIONS
Figure 1. Ideal crystalline structure of graphite showing the hexagonal unit cell, the crystal axes and the coordinates of the four different lattice sites.
Figure 2. Selected area diffraction pattern of a specimen irradiated along a c-foil [0001] direction to a dose of 1 dpa (1.1 x 1027 e/m2). (a) Foil normal
parallel to the electron beam; (b) foil normal tilted 40 degrees with respect to the electron beam.
Figure 3. Selected area diffraction pattern of a specimen irradiated along an a-foil [12~10] direction to a dose of 1 dpa (1.1 x 102 7 e/m2). Foil normal
parallel to the electron beam.
Figure 4. Lattice fringe images of a-foil: (a) showing straight fringes before irradiation; (b) showing fragmentation of basal planes after irradiation to 1 dpa.
Figure 5. Electron energy-loss spectra before (lower curves) and after (upper curves) irradiation to 1 dpa. (a) electron irradiation parallel to he crystaliographic axis; (b) electron irradiation perpendicular to c-crystallographic axis.
Figure 6. Electron energy-loss spectra showing zero-loss and plasmon peaks: (a) before irradiation; (b) after irradiation to 1 dpa.
Figure 7. Selected area diffraction patterns of a c-foil after irradiation to
a fiuence of 1.2 x 1 02 7m -2 at an accelerating voltage of 140 kV (a) and
150 kV (b).
Figure 8. Selected area diffraction patterns of an a-foil after irradiation
to a dose of 1.2 x 102 7 m-2 at an accelerating voltage of 140 kV (a) and
ha 5
PhotodJode Counts
1000
;<a 7