Distribution of black spot defects and small clusters in 1 MeV krypton irradiated 3C-SiC has been investigated using advanced scanning transmission electron microscopy (STEM) and TEM. We find that two thirds of clusters smaller than 1 nm identified in STEM are invisible in TEM images. For clusters that are larger than 1 nm, STEM and TEM results match very well. A cluster dynamics model has been developed for SiC to reveal processes that contribute to evolution of defect clusters and validated against the (S)TEM results. Simulations showed that a model based on established properties of point defects (PDs) generation, reaction, clustering, and cluster dissociation, is unable to predict black spot defects distribution consistent with STEM observations. This failure suggests that additional phenomena not included in a simple pointdefect picture may contribute to radiation-induced evolution of defect clusters in SiC and using our model we have determined the effects of a number of these additional phenomena on cluster evolution. Using these additional phenomena it is possible to fit parameters within physically justifiable ranges that yield agreement between cluster distributions predicted by simulations and those measured experimentally.2 Keywords: silicon carbide, black spot defects, cluster dynamics, STEM, TEM, size distribution 22]. This assumption is also adequate for small clusters, which are neither spherical nor planar [23]. The absorption rate is then defined as [12,14]

Experimental and modeling efforts were combined to investigate the role of black spot defects (BSD) in swelling of carbon-and krypton-irradiated 4H-SiC. Samples were exposed to conditions favoring BSD formation: irradiation at temperatures 600− 950 • C and damage levels of 0.4− 0.8 dpa. The maximum XRD swelling values, corrected for the effect of the rigid substrate, of 0.58% for C and 0.75% for Kr-irradiation were measured at the lowest irradiation temperature of 600 • C and decreased with increasing temperature. The swelling values estimated from TEM are on the same order of magnitude, but usually 40 − 70% lower than those measured by XRD. The contribution of BSDs to the overall swelling is 62% and the remainder of the swelling is caused by isolated point defects. The obtained results contribute to understanding of what defect types account for swelling and how their concentration evolves with the irradiation temperature and damage level.

Silicon carbide is of great interest as a structural material in nuclear systems because of its high strength, corrosion resistance and high thermal conductivity. An important material property related to failure is irradiation induced volume increase, or swelling. Swelling saturates in silicon carbide at relatively low irradiation temperatures (~< 1000 °C) and doses (~< 10 dpa or displacements per atom), when the major defects appear in bright field transmission electron microscopy (TEM) images as nanometer scale black spots and are referred to as black spot defects (BSD) [1]. The detailed internal structure of BSD is unknown. We are working towards understanding the structures and evolution of BSD by combining high resolution scanning transmission electron microscopy (STEM) and defect structure modeling [2]. We have irradiated single-crystal 4H-SiC and polycrystalline 3C-SiC with 3.15 MeV carbon ions to a dose of 5.14 × 10 16 C/cm 2 at 600 °C, 800 °C, and 950 °C. The corresponding average damage level is 0.5 dpa. TEM observation was performed at 300 kV in a FEI Tecnai TF30 and STEM observation was performed at 200 kV in a probe Cs-corrected FEI Titan.

Silicon carbide is of great interest as a nuclear fuel cladding material. At relatively low irradiation temperatures (< 1000 C) and doses (< 10 dpa or displacements per atom), the major irradiation induced defects are black spot defects (BSD), which appear as nanometer scale black spots in bright field transmission electron microscopy (TEM) images [1,2]. BSDs are associated with radiation-induced swelling [1]. The detailed internal structure of BSD is unknown. We are working towards understanding the structures and evolution of BSD by combining high resolution scanning transmission electron microscopy (STEM) and defect structure modeling [3]. We have irradiated single-crystal 4H-SiC and polycrystalline 3C-SiC with two ion species. One is 3.15 MeV carbon ions to a dose of 5.14 × 10 16 at/cm 2 at 870 K, 1070 K, and 1250 K. The other is 1 MeV krypton ions to a dose of 3 × 10 14 at/cm 2 and 6 × 10 14 at/cm 2 at 870 K, 1070 K. The corresponding damage levels are 0.4 dpa for carbon irradiation at 1 μm implantation depth, and 0.4 dpa / 0.8 dpa for krypton irradiation at 0.3 μm depth. TEM observation was performed at 300 kV in a FEI Tecnai TF30 and STEM observation was performed at 200 kV in a probe Cs-corrected FEI Titan.