Ramaswami Devanathan

Atomic scale simulation of defect production in irradiated 3C-SiC

Molecular dynamics simulations using a modified Tersoff potential have been used to study the primary damage state and statistics of defect production in displacement cascades in 3C-SiC. Recoils with energies from 0.25 to 50 keV have been simulated at 300 K. The results indicate that: (1) the displacement threshold energy surface is highly anisotropic; (2) the dominant surviving defects are C interstitials and vacancies; (3) the defect production efficiency decreases with increasing recoil energy; (4) defect clusters are much smaller and more sparse compared to those reported in metals; and (5) a small fraction of the surviving defects are antisite defects.

Displacement Energy Surface in 3C and 6H SiC

Abstract The phase stability of 3C–SiC upon heating and the threshold displacement energy ( E d ) surfaces for C and Si primary knock-on atoms (PKAs) in 3C–SiC and 6H–SiC have been investigated using molecular dynamics simulations. A recently optimized Tersoff potential is used in conjunction with an ab initio repulsive potential to represent the interactions between atoms. The simulations provide important insights into phase separation of SiC upon heating, and indicate a strong anisotropy in the E d surface for both Si and C PKAs. The two polytypes show many similarities in the nature of the E d surface. The average displacement energy is separately determined by simulating 30 different 500 eV cascades in 3C–SiC. The minimum displacement energies of 21 eV for C and 35 eV for Si are in excellent agreement with interpretation of experimental observations and the simulations of 500 eV cascades.

Displacement threshold energies in β-SiC

We have calculated the displacement threshold energies (E d ) for C and Si primary knock-on atoms (PKA) in β-SiC using molecular dynamic simulations. The interactions between atoms were modeled using a modified form of the Tersoff potential in combination with a realistic repulsive potential obtained from density-functional theory calculations. The simulation cell was cubic, contained 8000 atoms and had periodic boundaries. The temperature of the simulation was about 150 K. Our results indicate strong anisotropy in the E d values for both Si and C PKA. The displacement threshold for Si varies from about 36 eV along [001] to 113 eV along [111], while E d for C varies from 28 eV along [111] to 71 eV along [111]. These results are in good agreement with experimental observations.

Defect Production, Multiple Ion-Solid Interactions and Amorphization in SiC

Abstract Recent progress in the atomic-scale simulations of fundamental damage production processes in SiC is reviewed, which includes the displacement threshold energy surface, the primary damage state and statistics of defect production, multiple ion–solid collision events and structural evolution in SiC. The threshold energy surface, E d , appears to be highly anisotropic, and the results of molecular dynamics (MD) simulations, in conjunction with experimental studies, suggest that E d values of 20 eV for C and 35 eV for Si should be used in Kinchin–Pease calculations. The Si displacement cascades with energies up to 50 keV show that the surviving defects are dominated by C interstitials and vacancies, consistent with experimental observations. The defect production efficiency decreases with increasing recoil energy, but the number and size of clusters or complex domains formed at the end of cascades are very small, independent of cascade energy. A large number of 10 keV displacement cascades were randomly generated in a model crystal to simulate multiple ion–solid interaction and damage accumulation. The coalescence of clusters represents an important mechanism leading to the complete amorphization of SiC, and the relative disorder and swelling behavior show an excellent agreement with experimental observations. HRTEM images simulated from the MD cell reveal the microstructural evolution of multiple ion–solid collision events, and provide atomic-level interpretations of experimentally observed features in SiC.

Heavy-ion irradiation effects on structures and acid dissolution of pyrochlores

Abstract The temperature dependence of the critical dose for amorphization, using 0.6 MeV Bi+ ions, for A2Ti2O7 pyrochlores, in which A=Y, Sm, Gd and Lu, exhibits no significant effect of A-site ion mass or size. The room temperature dose for amorphization was found to be ∼0.18 dpa in each case. After irradiation with 2 MeV Au2+ ions glancing-incidence X-ray diffraction (XRD) revealed that each pyrochlore underwent an irradiation-induced structural transformation to fluorite in conjunction with amorphization. The effect of amorphization on the dissolution rates of fully dense pyrochlores, at 90°C and pH 2 (nitric acid) varied from a factor of 10 to 15 increase for Gd2Ti2O7 to none for Y2Ti2O7. Significant differences were observed in the A-site dissolution rates from the crystalline pyrochlores, indicating differences in the manner in which the A-site cations are incorporated into the pyrochlore structure. These indications were supported by Raman spectroscopy.

Atomistic simulation of water percolation and proton hopping in Nafion fuel cell membrane.

We have performed a detailed analysis of water clustering and percolation in hydrated Nafion configurations generated by classical molecular dynamics simulations. Our results show that at low hydration levels H(2)O molecules are isolated and a continuous hydrogen-bonded network forms as the hydration level is increased. Our quantitative analysis has established a hydration level (λ) between 5 and 6 H(2)O/SO(3)(-) as the percolation threshold of Nafion. We have also examined the effect of such a network on proton transport by studying the structural diffusion of protons using the quantum hopping molecular dynamics method. The mean residence time of the proton on a water molecule decreases by 2 orders of magnitude when the λ value is increased from 5 to 15. The proton diffusion coefficient in Nafion at a λ value of 15 is about 1.1 × 10(-5) cm(2)/s in agreement with experiment. The results provide quantitative atomic-level evidence of water network percolation in Nafion and its effect on proton conductivity.

Computer simulation of a 10 keV Si displacement cascade in SiC

Abstract The threshold energy for atomic displacement and the evolution of high-energy displacement cascades in β -SiC have been examined using molecular dynamics simulations. A modified form of the Tersoff potential was used in combination with a repulsive potential obtained from density functional theory to describe the interactions between the atoms. The evolution of lattice damage in a 10 keV Si cascade was studied for about 10 ps. The system size varied from 8000 atoms for the displacement energy calculations to 192,000 atoms for the cascade simulations. The results indicate that the minimum displacement energy is about 36 eV for Si and 28 eV for C. The cascade lifetime was found to be of the order of 0.1 ps, and the surviving C vacancies and interstitials outnumbered the corresponding Si defects by a factor of about 3. These results are discussed in light of previous theoretical and experimental studies of radiation damage in SiC.

Nanoscale Phase Transitions under Extreme Conditions within an Ion Track

The dynamics of track development due to the passage of relativistic heavy ions through solids is a long-standing issue relevant to nuclear materials, age dating of minerals, space exploration, and nanoscale fabrication of novel devices. We have integrated experimental and simulation approaches to investigate nanoscale phase transitions under the extreme conditions created within single tracks of relativistic ions in Gd2O3(TiO2)x and Gd2Zr2–x TixO7. Track size and internal structure depend on energy density deposition, irradiation temperature, and material composition. Based on the inelastic thermal spike model, molecular dynamics simulations follow the time evolution of individual tracks and reveal the phase transition pathways to the concentric track structures observed experimentally. Individual ion tracks have nanoscale core-shell structures that provide a unique record of the phase transition pathways under extreme conditions.

Effects of Cation Disorder on Oxygen Vacancy Migration in Gd2Ti2O7

Atomistic simulations were used to calculate defect formation and migration energies for oxygen vacancies in the pyrochlore Gd2Ti2O7, with particular attention to the role of cation antisite disorder. Oxygen occupies two crystallographically distinct sites (48f and 8a) in the ordered material, but the 8b sites become partially occupied with disorder. Because cation and anion disorder are coupled, oxygen vacancy formation and migration energetics are sensitive to the configuration of the cation disorder. The VO8a vacancy and VO8a + O8bi Frenkel defects are energetically favored in the ordered material, but VO8a is favored at higher disorder. The VO8a + O8bi Frenkel is favored for some disorder configurations. Eight possible oxygen vacancy migration paths converge toward a unique migration energy as cation disorder increases, reflecting a reversion towards the fluorite structure. Oxygen vacancy migration is determined by O48f → O48f transitions along the shortest edges of the TiO6 octahedra. The transition V48a → V48f is also possible for low disorder, and can activate the V48f → V48f migration network by depositing vacancies there. The reverse transition may occur at very high disorder to retard ionic conduction, and is consistent with Frenkel defect stabilities. Local regions of ordered and disordered material both appear necessary to explain the observed trends in ionic conductivity.

Structure and dynamics of N,N-diethyl-N-methylammonium triflate ionic liquid, neat and with water, from molecular dynamics simulations.

We investigated by means of molecular dynamics simulations the properties (structure, thermodynamics, ion transport, and dynamics) of the protic ionic liquid N,N-diethyl-N-methylammonium triflate (dema:Tfl) and of selected aqueous mixtures of dema:Tfl. This ionic liquid, a good candidate for a water-free proton exchange membrane, is shown to exhibit high ion mobility and conductivity. The radial distribution functions reveal a significant long-range structural correlation. The ammonium cations [dema](+) are found to diffuse slightly faster than the triflate anions [Tfl](-), and both types of ions exhibit enhanced mobility at higher temperatures, leading to higher ionic conductivity. Analysis of the dynamics of ion pairing clearly points to the existence of long-lived contact ion pairs. We also examined the effects of water through characterization of properties of dema:Tfl-water mixtures. Water molecules replace counterions in the coordination shell of both ions, thus weakening their association. As water concentration increases, water molecules start to connect with each other and then form a large network that percolates through the system. Water influences ion dynamics in the mixtures. As the concentration of water increases, both translational and rotational motions of [dema](+) and [Tfl](-) are significantly enhanced. As a result, higher vehicular ionic conductivity is observed with increased hydration level.