Ag diffusion in SiC high-energy grain boundaries: kinetic Monte Carlo study with first-principle calculations

摘要

The diffusion of silver (Ag) impurities in high energy grain boundaries(HEGBs) of cubic (3C) silicon carbide (SiC) is studied using an ab initio basedkinetic Monte Carlo (kMC) model. This study assesses the hypothesis that theHEGB diffusion is responsible for Ag release in Tristructural-Isotropic fuelparticles, and provides a specific example to increase understanding ofimpurity diffusion in highly disordered grain boundaries. The HEGB environmentwas modeled by an amorphous SiC. The structure and stability of Ag defects werecalculated using density functional theory code. The defect energeticssuggested that the fastest diffusion takes place via an interstitial mechanismin a-SiC. The formation energy of Ag interstitials and the kinetic resolvedactivation energies between them were well approximated with Gaussiandistributions that were then sampled in the kMC. The diffusion of Ag wassimulated with the effective medium model using kMC. At 1200-1600C, Ag in aHEGB is predicted to exhibit an Arrhenius type diffusion and with a diffusionprefactor and effective activation energy of (2.73+-1.09)*10-10 m2s-1 and2.79+-0.18 eV, respectively. The comparison between HEGB results to othertheoretical studies suggested not only that GB diffusion is predominant overbulk diffusion, but also that the HEGB is one of fastest grain boundary pathsfor Ag diffusion in SiC. The Ag diffusion coefficient in the HEGB shows a goodagreement with ion-implantation measurements, but is 2-3 orders of magnitudelower than the diffusion coefficients extracted from integral releasemeasurements. The discrepancy between GB diffusion and integral releasemeasurements suggests that other contributions are responsible for the fastrelease of Ag in some experiments and we propose that these contributions mayarise from radiation enhanced diffusion.