Stability and kinetics of helium interstitials in boron carbide from first principles

International audienceWhen boron carbide is used in nuclear reactors as a neutron absorber, helium concentrations on the order of a few atomic percent can be attained. It is thus of primary importance to know the distribution and kinetics of helium atoms in boron carbide. In spite of a variety is of experimental works devoted to the characterization of the microstructure and He bubbles in boron carbide irradiated in reactor, there is a serious lack of knowledge concerning the basic mechanisms governing helium kinetics. This study is devoted to the stability and mobility of helium interstitial atoms in carbon rich boron carbide. The lowest energy He insertion sites were screened through density functional theory and the most probable migration paths and energy barriers were investigated using the nudged elastic bands (NEB) approach. The results suggest that in a wide range of temperatures He interstitials undergo 2D diffusion confined between two 111 planes. The onset of 3D diffusion is expected, according to our calculations, with an activation energy close to 2 eV.Our result is in qualitative agreement with the observation of flat bubbles with 111 orientation, although a quantitative comparison with He diffusion data is hindered by discrepancies and microstructure issues in available experimental results

Molecular Dynamics Modeling of the Evolution of SiC Thermal Conductivity with Accumulation of Irradiation Damage

International audienceSiC thermal conductivity is known to decrease under irradiation. To numerically model this effect, we studied the variation of the thermal conductivity of cubic SiC with the defect accumulation induced by displacement cascades. We use empirical potential of the Tersoff type in the framework of non-equilibrium molecular dynamics. The conductivity is found to decrease with dose in very good quantitative agreement with low temperature irradiation experiments. The results are analyzed in view of the amorphization states that are created by the cascade accumulation simulations. The decrease of the conductivity observed at lower doses is related to the creation of point defects. The calculated values are close to the smallest measured values after high temperature irradiation. A subsequent decrease takes place upon further cascade accumulation. It is characteristic of the amorphization of the material and is experimentally observed for low temperature irradiation only

First principle study of neutral and charged self-defects in amorphous SiO2

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Computational RBS/C method based on Molecular Dynamic simulations for investigating irradiation induced defect: UO2 as test-case material

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Opposite trends in the microstructural evolution of irradiated ceramics driven by asymmetric defect migration energies

International audienceX-ray diffraction experiments complemented with numerical simulations reveal that two ceramic oxide materials, c-ZrO2 and MgO, under similar ion irradiation conditions, exhibit an unexpected, opposite response with varying the irradiation temperature. In fact, the final damage state is reached faster in c-ZrO2 with increasing temperature than in MgO. Rate equation cluster dynamics simulations show that defect clustering is favored in c-ZrO2 while defect annealing is enhanced in MgO. This contrasting behavior can be rationalized in terms of the asymmetry in the interstitial vs. vacancy defect migration energies. We demonstrate that these trends allow qualitatively reproducing the evolution with temperature of the experimental disorder

Nanoscale spectroscopy with polarized X rays by NEXAFS TXM

International audienceNear-edge X-ray absorption spectroscopy (NEXAFS)1 is an essential analytical tool in material science. Combining NEXAFS with scanning transmission X-ray microscopy (STXM) adds spatial resolution and the possibility to study individual nanostructures2, 3. Here, we describe a full-field transmission X-ray microscope (TXM) that generates high-resolution, large-area NEXAFS data with a collection rate two orders of magnitude faster than is possible with STXM. The TXM optical design combines a spectral resolution of E/ΔE = 1 × 104 with a spatial resolution of 25 nm in a field of view of 15-20 µm and a data acquisition time of ~1 s. As an example, we present image stacks and polarization-dependent NEXAFS spectra from individual anisotropic sodium and protonated titanate nanoribbons. Our NEXAFS-TXM technique has the advantage that one image stack visualizes a large number of nanostructures and therefore already contains statistical information. This new high-resolution NEXAFS-TXM technique opens the way to advanced nanoscale science studies