RH. Jones et al. I Journal of Nuclear Materials 307-311(2002)1057-1072 oxidation. Therefore, SiCrSic composites have the po Table 2 tential for excellent oxidation resistance in He +O, en- Defect formation energies in 3C-SiC [4] Defect type Formation energy (el 4. Challenges of engineering the properties of Sic / Sic Given the positive attributes of Sic /SiC composites, they would be the obvious first choice for structural applications in fusion energy systems if there were no issues in their use. However there are some unresolved 71 issues associated with their use as outlined in table l Considerable progress has been made in understanding these issues and in some cases improvements have beer C-Si(100) made. Since the properties of these composites are 9.32 engineerable, there is the potential, to some extent,to (110 engineer around these issues. The purpose of this paper is to summarize some of the understanding and im provements made in these materials suggest that the c interstitials should migrate from sublattice to sublattice. The most stable configuration for Si interstitials is in a tetrahedrally-coordinated in- 5. Fundamentals of radiation damage in SiC terstitial site surrounded by c atoms on the C sublattice Ongoing theoretical and computational 5.1. Defect formation energi migration of these stable defect configurations will yield the necessary parameters to model radiation damage Density functional theory dFT), based on the processes at higher temperatures and over longer time pseudopotential plane-wave method within the frame- scales in SiC using rate-theory approaches or kinetic york of the local density approximation (LDA), has Monte Carlo methods been used to study the formation and properties of na tive defects in 3C-SiC (cubic SiC), as described in detail 5.2. Damage production and accumulation elsewhere [3-5]. The formation energies for vacancie antisite defects and interstitials in 3C-SiC are summa Molecular dynamics(MD) simulations of displace rized in Table 2. Two types of vacancies form, namely C nent cascades and cascade overlap events have been and si vacancies. In addition, two types of antisite de performed using a modified version of the code mOl fects are formed by atoms located on the wrong sub- DY [6]. with either constant volume or constant pressure lattice. For interstitial defects, there are ten possible and periodic boundary conditions. Details of the md configurations, four tetrahedral and six dumbbell(split) simulations and interatomic potentials employed are configurations. It is found that the most stable config urations for C interstitials are C-C and C-Si split in The short cascade lifetime in SiC is illustrated in Fig. terstitials along the(100)and(110) directions, which 1, where the numbers of interstitials and antisite defects produced in a 10 keV Si cascade are shown as a function Table I of time. The number of interstitials and vacancies(not Critical issues associated with the use of SiC SiC composites in shown) reaches a peak at about 0. 1 ps and then de- nuclear environments creases due to defect recombination [7, 9, 10]. The defect Priman Secondary concentrations attain steady state values after about 0.4 Thermal conductivity Chemical compatibility(He) ps. The cascade lifetime has been found to be slightly Radiation stability Carbon interfaces longer(about 0.7 ps)for a 50 keV Si PKa in SiC [7, 91 Fibers-polymer derived Thermal fatigue and shock These lifetimes are about an order of magnitude smaller Lack of a database than the values reported for metals using similar PKA + Matrices-CVI and polymer Long-term thermal stability energies [6, 11]. impregnated Design codes The results from MD simulations [10 for the ne Transmutations displacements and antisite defects produced by a 10 keV Hermetic behavior Si primary knock-on atom(PKA) are shown in Fig. 2 as Joining technology a function of PKA energy. The number of net dis- Chemical compatibility placements is defined as the sum of the total number of interstitials (or vacancies) and antisite defects. Theoxidation. Therefore, SiCf/SiC composites have the po￾tential for excellent oxidation resistance in He þ O2 en￾vironments. 4. Challenges of engineering the properties of SiCf/SiC Given the positive attributes of SiCf/SiC composites, they would be the obvious first choice for structural applications in fusion energy systems if there were no issues in their use. However, there are some unresolved issues associated with their use as outlined in Table 1. Considerable progress has been made in understanding these issues and in some cases improvements have been made. Since the properties of these composites are engineerable, there is the potential, to some extent, to engineer around these issues. The purpose of this paper is to summarize some of the understanding and im￾provements made in these materials. 5. Fundamentals of radiation damage in SiC 5.1. Defect formation energies Density functional theory (DFT), based on the pseudopotential plane-wave method within the frame￾work of the local density approximation (LDA), has been used to study the formation and properties of na￾tive defects in 3C–SiC (cubic SiC), as described in detail elsewhere [3–5]. The formation energies for vacancies, antisite defects and interstitials in 3C–SiC are summa￾rized in Table 2. Two types of vacancies form, namely C and Si vacancies. In addition, two types of antisite de￾fects are formed by atoms located on the wrong sub￾lattice. For interstitial defects, there are ten possible configurations, four tetrahedral and six dumbbell (split) configurations. It is found that the most stable config￾urations for C interstitials are C–C and C–Si split in￾terstitials along the h100i and h110i directions, which suggest that the C interstitials should migrate from sublattice to sublattice. The most stable configuration for Si interstitials is in a tetrahedrally-coordinated in￾terstitial site surrounded by C atoms on the C sublattice. Ongoing theoretical and computational studies of the migration of these stable defect configurations will yield the necessary parameters to model radiation damage processes at higher temperatures and over longer time scales in SiC using rate-theory approaches or kinetic Monte Carlo methods. 5.2. Damage production and accumulation Molecular dynamics (MD) simulations of displace￾ment cascades and cascade overlap events have been performed using a modified version of the code MOL￾DY [6], with either constant volume or constant pressure and periodic boundary conditions. Details of the MD simulations and interatomic potentials employed are described elsewhere [7–10]. The short cascade lifetime in SiC is illustrated in Fig. 1, where the numbers of interstitials and antisite defects produced in a 10 keV Si cascade are shown as a function of time. The number of interstitials and vacancies (not shown) reaches a peak at about 0.1 ps and then de￾creases due to defect recombination [7,9,10]. The defect concentrations attain steady state values after about 0.4 ps. The cascade lifetime has been found to be slightly longer (about 0.7 ps) for a 50 keV Si PKA in SiC [7,9]. These lifetimes are about an order of magnitude smaller than the values reported for metals using similar PKA energies [6,11]. The results from MD simulations [10] for the net displacements and antisite defects produced by a 10 keV Si primary knock-on atom (PKA) are shown in Fig. 2 as a function of PKA energy. The number of net dis￾placements is defined as the sum of the total number of interstitials (or vacancies) and antisite defects. The Table 1 Critical issues associated with the use of SiCf /SiC composites in nuclear environments Primany issues Secondary issues Thermal conductivity Chemical compatibility (He) Radiation stability þ Carbon interfaces þ Fibers-polymer derived Thermal fatigue and shock Interphases-C, porous Lack of a database þMatrices-CVI and polymer impregnated Long-term thermal stability Design codes Transmutations Hermetic behavior Joining technology Chemical compatibility (Pb–Li) Table 2 Defect formation energies in 3C–SiC [4] Defect type Formation energy (eV) Vc 5.48 VSi 6.64 CSi 1.32 SiC 7.20 CTC 6.41 CTS 5.84 SiTC 6.17 SiTS 8.71 Cþ–Sih100i 3.59 Cþ–Ch100i 3.16 C–Siþh100i 10.05 Siþ–Sih100i 9.32 Cþ–Ch110i 3.32 Cþ–Sih110i 3.28 1060 R.H. Jones et al. / Journal of Nuclear Materials 307–311 (2002) 1057–1072