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Molecular Dynamics Simulations of Radiation Damage in Amorphous Silica: Effects of Hyperthermal Atomic Oxygen Vanessa Oklejas Space Materials Laboratory The Aerospace Corporation Motivation Neutral atomic oxygen is the dominant species in low earth orbit (LEO). Spacecraft materials undergo significant erosion due to atomic oxygen in LEO; the mitigation of radiation damage of spacecraft materials is a major concern. Use MD simulations to understand how radiation modifies the molecular and mechanical properties of materials. © 2013 The Aerospace Corporation This work was supported under The Aerospace Corporation's Independent Research and Development Program. Results Conclusions & Future Work Significant displacement damage was observed for simulations of AO projectile atoms with 100 eV energies, which was accompanied by changes in ring structure. Limited damage was also observed for AO projectiles with energies below those typically used to model displacement damage (i.e., the threshold displacement damage, E d ). Simulate and identify the formation of stable silica defects due to radiation- induced damage. Examine radiation-induced changes in the material at the atomic level Radiation-induced chemical changes High energy AO projectiles introduce chemical changes in the silica model that, over time, will develop into stable defects. Defects are associated with performance degradation of amorphous silica (e.g., darkening, development of charge traps, decrease in mechanical strength). Averaged Displacement Damage An increase in the larger ring sizes and a loss of most stable ring sizes was observed. Methods A reactive force field (ReaxFF) was used to represent inter-atomic interactions. 1 Preparation of amorphous silica substrate: A simulation box of 2000 SiO 2 moieties was heated to 4000 K and cooled at a rate of ~ K/sec. ( = 0.25 fs) The first annealing cycle was performed using the NVT ensemble followed by a final annealing cycle in the NPT ensemble. A free surface was generated at the xy- plane. Radiation Simulations MD simulation parameters: NVE ensemble; = fs. Oxygen atom placed 5 Å above silica surface. Total simulation time was 32 ps. Image details: AO shown as red sphere. Displaced atoms shown as pink (O) or cyan (Si) spheres. Original positions of atoms are rendered transparent. Silicate network shown as transparent sticks. Silica tetrahedrons are connected to one another by bridging O atoms 12 node ring Silicate ring structure describes connectivity in amorphous silicate materials. 2 Larger rings are associated with nanoscale void formation. Molecular dynamics simulations were used to investigate the damage created in amorphous silica by hyperthermal atomic oxygen (AO) as a function of incident kinetic energy. 100 eV 4 eV 30 eV Small amount of displacement damage observed for AO with energies below E d. AO Penetration Depth surface 100 eV Penetration depth is commensurate with displacement damage. 4 eV 30 eV Radiation-induced changes in silicate ring structure 100 eV 4 eV 30 eV References 1.Fogarty, J.C.; Aktulga, H.M.; Grama, A.Y.; van Duin, A.C.T.; Pandit, S.A., J. Chem. Phys., 132, (2010). 2.Le Roux, S.; Jund, P., Comp. Mater. Sci., 49, (2010). Amorphous Silica Model α-quartz amorphous silica
Water cluster- silica collision Water cluster, 104 H 2 O Simulation time, 17ps NVE simulations O 1560 Si atoms Molecular mass g/mole.
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