The neutron-induced swelling of SiC has been well studied for low and intermediate temperatures (^293- 1273 K). Originally, this material was investigated in
support of nuclear fuel coating1-9 and more recently, for various nuclear applications such as structural SiC composites.10 Before proceeding, it is important to distinguish neutron-induced effects on high-purity materials, such as single crystal and most forms of chemical vapor deposited (CVD) SiC, from those on lower purity forms such as sintered with additives, reaction-bonded, or polymer-derived SiC. It is well understood that the presence of significant second phases and/or poorly crystallized phases in these mate­rials leads to unstable behavior under neutron irradia­tion,11-14 as compared to stoichiometric materials, which exhibit remarkable radiation tolerance. Discus­sion and data for this section refer only to high purity, stoichiometric, near-theoretical density SiC, unless otherwise specified. Rohm and Haas (currently Dow Chemicals) CVD SiC is an example of such material.

The irradiation-induced microstructural evolu­tion of CVD SiC is roughly understood and has been reviewed recently by Katoh et a/.15 An updated version of the microstructural evolution map is shown in Figure 1. However, the contribution of the defects themselves to the swelling in SiC is less understood. Below several hundred Kelvin, the observable

Figure 2 Microstructure for CVD neutron irradiated at 573 and 1073 K.

microstructure of neutron-irradiated SiC is described as containing ‘black spots, which are most likely tiny clusters of self-interstitial atoms in various indeterminate configurations. For irradiation tem­peratures less than about 423 K, accumulation of strain due to the irradiation-produced defects can exceed a critical level above which the crystal becomes amorphous. This has been shown in the case of both self-ion irradiation and fast neutron irra- diation.20-22 As shown by Katoh et al.,23 the swelling at 323 K under self-ion irradiation increases logarithmi­cally with dose until amorphization occurs. The swelling of neutron — and ion-amorphized SiC has been reported to be 10.8% for 343 K irradiation.22 However, there is evidence that the density of amor­phous SiC will depend on the conditions of irradiation (dose, temperature, etc.)24

For temperatures above the critical amorphiza­tion temperature (423 K), the swelling increases logarithmically with the dose until it approaches saturation, with a steady decrease in the saturation swelling level with increasing irradiation tempera­ture. The dose exponents of swelling during the logarithmical period are in many cases close to two — thirds, as predicted by a kinetic model assuming planar geometry for interstitial clusters.25 This tem­perature regime is generally referred to as the point — defect swelling regime and can be roughly set between 423 and 1273 K. As an example of how these ‘black spot’ defects mature in the point-defect swelling regime, Figure 2 shows neutron-irradiated microstructures at 573 and 1073 K for doses
consistent with a saturation in density. While these microstructural features are generically classified as ‘black spots,’ the defects formed at 1073 K are clearly coarser compared to those formed under 573 K irradiation.

The approach to saturation swelling is shown for High Flux Isotope Reactor (HFIR) neutron irra­diated Rohm and Haas CVD SiC in Figure 3. In this figure, the swelling is depicted in both logarith­mic (Figure 3(a)) and linear (Figure 3(b)) plots. In addition to the approach to saturation, this figure highlights two other characteristics of neutron — induced swelling of SiC. First, the swelling of SiC is highly temperature dependent. For the data given in Figure 3, the 1 dpa and saturation values of swelling at 473 K are approximately five times that for 1073 K irradiation. This reduced swelling with increasing irradiation temperature is primarily attrib­uted to enhanced recombination of cascade — produced Frenkel defects due to lower interstitial clustering density at higher temperatures. The sec­ond characteristic swelling behavior to note is that the swelling saturates at a relatively low dose. For damage levels of a few dpa (typically months in a fission power core), the swelling in the point-defect recombination range has found its saturation value.

At higher temperatures such as 1173-1673 K,4,18,26 Frank faulted loops of the interstitial type become the dominant defects observed by transmission elec­tron microscopy (TEM). It has also been reported that Frank faulted loops appear for lower tempera­ture neutron irradiation at extremely high doses.27

Tirr=200°C ■ Tirr=300°C ♦ Tirr=400°C Tjrr=500°C a Tirr=600 °C a Tirr=800°C

Figure 3 Swelling of SiC in the intermediate temperature point defect swelling regime. Reproduced from Snead, L. L.; Nozawa, T.; Katoh, Y.; Byun, T-S.; Kondo, S.; Petti, D. A. J. Nucl. Mater. 2007, 371, 329-377.

Under silicon ion irradiation at 1673 K, the develop­ment of Frank loops into dislocation networks through unfaulting reactions at high doses is reported.26 The volume associated with dislocation loops in irradiated SiC has been estimated to be on the order of 0.1%28 At temperatures where vacancies are sufficiently mobile, vacancy clusters can be formed. Three­dimensional (3D) cavities (or voids) are the only vacancy clusters known to commonly develop to large sizes in irradiated SiC. The lowest temperature at which void formation was previously reported under neutron irradiation is 1523 K.4 Senor reported the lack of void production after neutron irradiation to 0.9 dpa at 1373 K, although voids were observed after subsequent annealing at 1773 K for 1 h.18 Under silicon ion irradiation, voids start to form at 1273 K at very low density and become major contributors to swelling at irradiation conditions of 1673 K at >10 dpa.29 Positron annihilation and electron para­magnetic resonance studies have shown that the silicon vacancy in cubic SiC becomes mobile at 1073-1173 K.30,31 Therefore, it would not be sur­prising for void swelling to take place at as low as 1273 K at high doses, particularly for low damage rate irradiations.

As previously mentioned, data on swelling of SiC in the high-temperature ‘void swelling’ regime has been somewhat limited. Recently, however, work has been carried out in the 1173—1773 K range for Rohm and Haas CVD SiC irradiated in HFIR. Of particular significance to that experiment is the confidence in irradiation temperature owing to the melt-wire passive thermometry. 2 Recent TEM imaging by Kondo28 clearly shows the evolution of complex defects. As an example, Figure 4 indicates sparse void formation on stacking faults for material irradiated at 1403 K. Significant growth of voids commences at 1723 K. The well-faceted voids appeared to be tetrahedrally bounded by planes, which likely provide the lowest surface energy in cubic SiC. In many cases, voids appeared to be aligned on stacking faults at all temperatures. However, intra­granular voids unattached to stacking faults were also commonly observed at 1723 K. The evolution of dis­location microstructures at 1403—1723 K is shown in Figure 5. In this temperature range, dislocation loops are identified to be Frank faulted loops of inter­stitial type. Evolution of the dislocation loops into dislocation networks was confirmed for irradiation at 1723 K.

Figure 6 plots both historical data, recently pub­lished, and unpublished data on the swelling behavior of SiC over a wider range of temperature.16,33 This plot is limited to literature data on high-purity CVD SiC. A divergence from point-defect ‘saturated’ swelling to unsaturated swelling is observed in the 1273-1473 K range, although additional data in this temperature range as a function of fluence would be required to precisely define such behavior. Above 1373 K, there exists a clear unsaturated swelling behavior for CVD SiC. The three divergent curves

drawn in Figure 6 represent data taken at nomi­nally 1.75, 5.0, and 8.5 x 1025nm~2 (E> 0.1 MeV) (assumed 1.75, 5.0, and 8.5 dpa). In the 1373-1473 K temperature range, volumetric swelling is apparently at a minimum, although it increases from ^0.2% to ^0.4% to ^0.7% for ~1.75, 5.0, and 8.5 dpa, respectively. Clearly, the swelling in this temperature range has not saturated by 10 dpa. Above this minimum in swelling, the data indicates a continual swelling increase to the highest irradiation temperature of ^1773-1873 K. At 1773 K, measured swelling

was ~0.4, 1.0, and 2.0% for ~1.75, 5.0, and 8.5 dpa, respectively. It was also noted in the study by Snead eta/.33 that at 1773 K, surface reaction between SiC and the graphite holder had taken place. However, a loss of silicon from the surface cannot be ruled out.

Figure 6 includes historical data for swelling above 1273 K.3,4,18,22,34,35 Specifically, Senor eta/.18 report swelling for the same type of CVD SiC irra­diated in this study when irradiated in a water­moderated fission reactor (the ATR) as well. Their maximum dose, irradiation temperature, and swelling data were ^1 dpa, 1373 ± 30 K, and 0.36 ± 0.02%, respectively. The irradiation temperature quoted in Senor et a/.’s work was a best estimate, although the authors also provide an absolute bound of 1073-1473 K for their experiment. The maximum swelling in their work (0.36 ± 0.02% at ^1dpa) is somewhat higher than the ^0.25% swelling at 2 dpa, ^1373 K, of the trend data in Figure 6. This is seen from the rightmost figure of Figure 6. Also seen in the figure is the high-temperature swelling of Price.3,4,34 The Price data, which are in the dose range of about 4-8 dpa, are in fair agreement with the
measured swelling of the Snead data16,33 of Figure 6. The highest swelling material (^ 1523 K, ^6 and 10 dpa) shows the largest discrepancy, although if the temperature error bands quoted by the various authors are taken into account, the data are conceivable more in alignment. It is also noted that the Price material may have had some excess silicon leading to higher swelling as compared to stoichiometric material.

As mentioned earlier, the microstructural evolu­tion of irradiated SiC is roughly understood, at least for temperatures up to 1373 K. The swelling near the critical amorphization temperature (^423 K) is classically described as the differential strain between the single interstitial, or tiny interstitial clusters, immobile vacancies, and antisite defects. As the tem­perature increases above the critical amorphization temperature, the number of defects surviving the postcascade thermally activated recombination is reduced and the mobility of both silicon and carbon interstitials becomes significant. For temperatures exceeding ^1273 K, microstructural studies have noted the presence of both Frank loops and tiny voids, indicating limited mobility of vacancies.

The apparent increase in swelling with dose in the 1373-1873 K range seen in Figure 6 and the observed production of voids are interesting considering that the maximum irradiation temperature (^1773 K) in Figure 6 is ^0.65 of the melting (dissociation) temperature (Tm) for SiC. Here, we have assumed Olesinski and Abbaschian’s36 value of 2818 K where stoichiometric SiC transforms into C + liquid phase. This value of 0.65 Tm is high when viewed in compar­ison to fcc metal systems where void swelling typi­cally begins at ~-0.35 Tm, goes through a maximum value, and decreases to nil swelling by ^0.55 Tm. (It is noted that the melting and dissociation temperatures of SiC are somewhat variable in the literature. How­ever, even considering this variability, the previous statement is accurate). If, as the swelling data seems to indicate, the voids in SiC are continuing to grow in SiC irradiated to 1773 K, the energies for diffusion of either the Si or C vacancy or both must be quite high, as are the binding energies for clustered vacancies. This has been shown through theoretical work in the literature.37-40 However, it is to be noted that the defect energetics obtained from this body of work, and in particular those of the Si and C vacancies within SiC, vary widely. Perhaps, the work of Bockstedte eta/.,39 which follows an ab initio approach, is the most accurate, yielding a ground state migra­tion energy of 3.5 and 3.4 eV for Si and C vacancies, respectively. It was also noted by Bockstedte et a/.41 that the assumed charge state of the vacancy affects the calculated migration energy. Specifically, the car­bon vacancy in the +1 and +2 charge state increases from 3.5 to 4.1 and 5.2 eV, respectively, and that of silicon in the -1 and -2 charge state decreases from 3.4 to 3.2 and 2.4 eV, respectively. Several papers discuss the vacancy and vacancy cluster mobil­ity measured experimentally. The silicon monova­cancy has been shown to be mobile below 1273 K. Using electron spin resonance, Itoh et a/.30 found the irradiation-produced T1 center in 3C-SiC disappear­ing above 1023 K. The T1 center was later confirmed to be an Si vacancy.31 Using electron spin resonance, the carbon vacancy in 6H-SiC is shown to anneal above 1673 K.42 Using isochronal annealing and positron lifetime analysis, Lam et a/.40 have shown a carbon — silicon vacancy complex to dissociate above 1773 K for the same 6H single crystal materials studied here.

From a practical nuclear application point of view, the swelling data for CVD SiC can be broken down into the amorphization regime (<423 K), the saturable point-defect swelling regime (423-1073 K) range, and the unsaturated void swelling regime, which occurs for irradiation temperature >1273 K. From the data of Figure 6, it is still unclear where the actual transition into the unsaturated swelling begins. Furthermore, while there is an increase in swelling in the 1273-1773K range, as the dose is increased from ~1.75, 5.0, and 8.5 x 1025nm~2 (E > 0.1 MeV), swelling is close to linear with neutron doses, and it is unclear how swelling will increase as a function of dose above 10 dpa. For example, swelling by voids estimated from the TEM examina­tion accounts for only relatively small fractions of the total swelling even in the void swelling regime. Analo­gous to the typical swelling behavior in metals, void growth may cause steady-state swelling after a certain transition dose regime. However, dose dependence of the swelling due to the nonvoid contribution remains to be understood. Extrapolation of swelling outside of the dose range of Figure 6 is therefore problematic.