Abstract
Silicon Carbide (SiC) is a promising cladding material for accident-tolerant fuel in light water reactors due to its excellent resistance to chemical attacks at high temperatures, which can prevent severe accident-induced environmental disasters. Although it has been known for decades that radiation-induced swelling at low temperatures is driven by the formation of black spot defects with sizes smaller than 2 nm in irradiated SiC, the structure of these defect clusters and the mechanism of lattice expansion have not been clarified and remain as one of the most important scientific issues in nuclear materials research. Here we report the atomic configuration of defect clusters using Cs-corrected transmission electron microscopy and molecular dynamics to determine the mechanism of these defects to radiation swelling. This study also provides compelling evidence that irradiation-induced point defect clusters are vacancy-rich clusters and lattice expansion results from the homogenous distribution of unrecovered interstitials in the material.
Highlights
BSDs, types of point defect cluster composed by vacancies and interstitials in irradiated Silicon Carbide (SiC), have been characterized mainly using TEM by many researchers
We discovered the regions including these black spot defects under TEM images for SiC samples irradiated by 5.1 MeV Si-ions under 400 °C and 20 dPa
Focusing the electron beam of the TEM, we artificially constructed nine holes to locate and identify a region including a black dot structure (Fig. 1a) in order to exactly observe the same region of atom columns when shifting the microscope into the annular bright-field (ABF)-STEM (Fig. 1b), HAADF-STEM (Fig. 1c), and HR-TEM (Fig. 1d) modes
Summary
BSDs, types of point defect cluster composed by vacancies and interstitials in irradiated SiC, have been characterized mainly using TEM by many researchers. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) is an imaging technique which can detect individual atoms at an atomic resolution[10]. The annular-shaped high-angle detector behind the sample collects the signal dominated by Rutherford and thermal diffuse scattering. When applied in a restricted zone-axis orientation, the HAADF scattering signal from a single column of atoms is strongly dependent on the atomic number (roughly Z1.7, it is referred to as Z-contrast images) and the thickness of the sample[11,12]. The annular bright-field (ABF) imaging technique collecting lower-angle signals is able to directly detect the position of light atoms[13,14] (e.g., oxygen, lithium, and carbon, which cannot be significantly imaged by HAADF images). Nanoclusters in irradiated 3C-SiC using a Cs-corrected STEM (JEOL, JEM-ARM200F) at an accelerating voltage of 200 kV with an ultra-high spatial resolution of approximately 0.12 nm
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