Atomistic simulations of threshold displacement energies in SiO 2
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Atomistic simulations of threshold displacement energies in SiO2 F.Mota1), M.-J.Caturla2,3), J.M.Perlado1), E.Dominguez1) , A. Kubota3 1 Instituto de Fusión Nuclear, Universidad Politécnica, Madrid, Spain 2 Universidad de Alicante, Dep. Física Aplicada, Alicante, Spain 3 Lawrence Livermore National Laboratory, Livermore CA, USA. ABSTRACT Silica is one of the candidate materials for final focusing mirrors in inertial fusion reactors. This material will be exposed to high neutron irradiation fluxes during operation. Radiation damage results in point defects that can lead to obscuration of this material; that is, degradation of the optical properties of silica. In this paper we present molecular dynamic simulations of defect production in silica glass. Results on the threshold displacement energies due to oxygen Primary Knock-on Atoms (PKA) are reported concluding that a range of energies (20 – 40 eV) exists in which the defects have a probability to be created. In addition, we determine a range of distances for a PKA to become a stable defect out of its original position. Our present analysis is focused on the formation of Oxygen Deficient Centers (ODC). INTRODUCTION The development of fusion reactors requires an understanding of materials performance under the extreme conditions that will take place in such an environment. Fused silica is one of the materials considered as the final optics in inertial confinement fusion reactors. These lenses will be exposed to high energy neutrons (14 MeV) and ions. The radiation could affect the optical properties of the material, creating point defects that can act as color centers and therefore produce obscuration of these optics [1]. Indeed, experiments have shown that neutron irradiation induces the production of oxygen deficient centers (ODC) [2, 3] and their conversion to E' centers after gamma irradiation [3]. However, significant reduction of the absorption coefficient of these irradiated samples is also observed experimentally through high temperature annealing ( > 350C) pointing to a process of diffusion and recovery of the damage of the sample at these temperatures [3, 4]. Unlike metals where the production and annihilation of defects using molecular dynamics simulations has been widely studied [5], there are very few calculations of radiation effects in oxides using these techniques. Several issues could be responsible for this, from the reliability of the interatomic potentials used to the difficultly of identifying defects in compound and amorphous materials such as fused silica. However, the success of such models to interpret the processes occurring during irradiation of metals and semiconductors lead us to follow similar studies in this material. We present calculations of the minimum energy to produce a stable defect of the type 3-fold coordinated silicon, which is related to the oxygen deficient centers that can be observed experimentally. The study is focused on the defects produced by an initial energetic oxygen atom and a fixed temperature, 300K.
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