Driving mechanism of neutron-irradiation-induced amorphization in silicon carbide

In this study, irradiation-induced amorphization in silicon carbide (SiC) by 1 MeV neutrons was investigated using molecular dynamics (MD) simulations. The crystalline-to-amorphous (c-a) transition occurred at 0.27 dpa with a structure relaxation of the whole lattice. Fast neutrons have produced many displacement spikes with unsaturated coordinated atoms at the center. Our results have shown that the two-coordinated Si atoms play a key role in defect accumulation and amorphization. There are two types of such defects: displaced-atom-induced (D-type) defect and vacancy-induced (V-type) defect. The D-type defect tends to form clusters and promotes the formation of C Frenkel pairs after 0.13 dpa. The V-type defect enhances the driving force of c-a transition and finally triggers amorphization at high concentration based on thermodynamics.     

Microstructure evolution of zircaloy-4 during Ne ion irradiation and annealing: An in situ TEM investigation

The microstructural evolution of zircaloy-4 was studied, including the amorphization and recrystallization of Zr（Fe, Cr）2 precipitates, and the density of dislocations under in situ Ne ion irradiation and post annealing. The results show that irradiation at a relatively high temperature and dose induces the formation of nanocrystals in pre-amorphized Zr（Fe, Cr）2 precipitates. The recrystallized nanocrystals also have the structure of hcp-Zr（Fe, Cr）2. The formation of the nanocrystals is thought to be the consequence of competition between atomistic disordering and the recrystallization of precipitates under ion irradiation. The free energy of the nanocrystal is lower than that of the amorphous state, which is another reason for the recrystallization of the precipitates. With increased annealing temperature, the density of the nanocrystals is increased. The dislocation density sharply decreases with the increase in the annealing temperature, and its size increases.     

Evidence for direct impact damage in metamict titanite CaTiSiO?

We have measured the dose dependence of the degree of amorphization of titanite, CaTiSiO(5). Titanite is an often metamict mineral which has been considered as a matrix for the encapsulation of radiogenic waste, such as Pu. The amorphous fraction p of geologically irradiated samples (ages between 0.3 and 1 Ga) follows p = 1 - exp(-B(a)D) where D is the total dose and the characteristic amorphization mass is B(a) = 2.7(3) × 10(-19) g. Amorphization follows the direct impact mechanism where each α-decay leads to a recoil of the radiogenic atoms (mostly Th and U), which then, in turn, displaces some 5000 atoms of the titanite matrix. The amorphization behaviour is almost identical with that of zircon, ZrSiO(4), which has a similar molecular mass. While the recrystallization mechanism and elastic behaviour of the two minerals are very different, we do not find significant differences for the amorphization mechanism. Our samples have undergone little reheating over their geological history, since heating over 800 K would lead to rapid recrystallization for which we have found no evidence.     

Plastic deformation under high-rate loading: The multiscale approach

A two-level approach has been proposed for describing the plastic deformation under high-rate loading of metals. The characteristics of the motion of dislocations under shear stresses have been investigated at the atomistic level by using the molecular dynamics simulation. The macroscopic motion of a material has been described at the continuum level with the use of the model of continuum mechanics with dislocations, which uses information obtained at the atomistic level on the dislocation dynamics. The proposed approach has been used to study the evolution of the dislocation subsystem under shock-wave loading of an aluminum target. The behavior of the dynamic yield stress with an increase in the temperature has been analyzed. The results of the calculations are in good agreement with experimental data.     

Sub-gap absorption study of defects in ion-implanted and annealed Si layers

Sub-gap absorption measurements are presented as a tool to characterize the amorphization and recrystallization processes in ion-implanted and annealed Si layers. The gap state density associated with the disorder introduced in the target crystalline lattice has been shown to saturate once the amorphization dose is exceeded. The doping effect due to implantation of impurity species is also reported. The absorption spectra have also been shown to be very sensitive to defects associated with precipitation of the implanted atoms.     

Structural properties of defected ZnO nanoribbons under uniaxial strain: Molecular dynamics simulations

Structural properties of various type and position defected zinc oxide nanoribbons with armchair and zigzag edges have been investigated via classical molecular dynamics simulations. An atomistic potential energy function has been used to represent the interactions among the atoms. A uniaxial strain has been applied to the generated ZnO nanostructures at two different temperatures of 1 K and 300 K. It has been found that ZnO nanoribbons under strain application exhibit a structural change depending on the temperature; the position and type of the defect; and the edge geometries of the nanoribbons.     

Modelling transient heat conduction in solids at multiple length and time scales: A coupled non-equilibrium molecular dynamics/continuum approach

A method for controlling the thermal boundary conditions of non-equilibrium molecular dynamics simulations is presented. The method is simple to implement into a conventional molecular dynamics code and independent of the atomistic model employed. It works by regulating the temperature in a thermostatted boundary region by feedback control to achieve the desired temperature at the edge of an inner region where the true atomistic dynamics are retained. This is necessary to avoid intrinsic boundary effects in non-equilibrium molecular dynamics simulations. Three thermostats are investigated: the global deterministic Nosé–Hoover thermostat and two local stochastic thermostats, Langevin and stadium damping. The latter thermostat is introduced to avoid the adverse reflection of phonons that occurs at an abrupt interface. The method is then extended to allow atomistic/continuum models to be thermally coupled concurrently for the analysis of large steady state and transient heat conduction problems. The effectiveness of the algorithm is demonstrated for the example of heat flow down a three-dimensional atomistic rod of uniform cross-section subjected to a variety of boundary conditions.     

Surface elasticity and size effect on the vibrational behavior of silicon nanoresonators

Predominance of nano-scale effects observed in material behavior at small scales requires implementation of new simulation methods which are not merely based on classical continuum mechanic. On the other hand, although the atomistic modeling methods are capable of modeling nano-scale effects, due to the computational cost, they are not suitable for dynamic analysis of nano-structures. In this research, we aim to develop a continuum-based model for nano-beam vibrations which is capable of predicting the results of molecular dynamics (MD) simulations with considerably lower computational effort. In this classical-based modeling, the surface and core regions are taken to have different mechanical properties, where core atoms are assumed to have macroscale properties whereas surface layer is showing a different elastic modulus from the core components. By estimating physical parameters of proposed classical model using molecular dynamics results and the genetic algorithm, calibrated classical Euler–Bernoulli and Timoshenko beam models are developed. The results demonstrates that a Timoshenko beam model incorporating surface effects and having calibrated parameters, is able to provide almost the same results as molecular dynamics method which can be used to predict the vibrational behavior of nano-beams at different scales from nano to macro.     

Supercoiling and denaturation of DNA loops

We study the statistical mechanics of small DNA loops emphasizing the competition between elasticity, supercoiling, and denaturation. Motivated by recent experiments and atomistic molecular dynamics simulation, we propose a new coarse-grained phenomenological model of DNA. We extend the classical elastic rod models to include the possibility of denaturation and nonlinear twist elasticity. Using this coarse-grained model, we obtain a phase diagram in terms of fractional overtwist and loop size that can be used to rationalize a number of experimental results which have also been confirmed by atomistic simulations.     

Lattice dynamics studies of grain boundary structures: I. Method and application to stability

The method of lattice dynamics has been applied to an atomistic model of a grain boundary with emphasis on its utility in examining structural instability. A 36.9° [001] coincidence symmetrical tilt boundary was used with a Morse pairwise potential. Several structures derived by a static relaxation procedure were studied. Stable structures are characterized by a vibrational spectrum consisting of real frequencies only, while one of the statically relaxed structures yielded a pair of imaginary frequencies and therefore was unstable. A comparison of the lattice dynamics method with other methods for testing stability of a defect configuration is given.     

SiC晶界薄膜的变电荷分子动力学模拟

基于迭代变电荷方法,用分子动力学模拟了SiC中的晶界薄膜．从原子尺度上模拟了不同的晶界薄膜的结构．观察到了晶粒与晶界薄膜间的电荷转移并且晶界薄膜的厚度与电荷转移有关．该结果提供了晶界存在空间电荷的直接证据,并证明静电作用与晶界薄膜的平衡厚度密切相关． 关键词： 分子动力学 变电荷 晶界薄膜     

Thermal processes in multilayer second-generation HTSC under swift heavy-ion irradiation

On the basis of the thermal spike model, the estimations with regard to tapes of the second-generation Ag/YBaCuO/MgO/Hastelloy HTSC under irradiation with Ar, Kr, and Xe ions of an energy of about 1.2 MeV/amu have been carried out. The results have been compared with the available experimental data. In addition, the possibility of processes such as melting, recrystallization, amorphization, and other phase transitions in multilayer structures under ion irradiation has been studied.     

Molecular dynamic simulation of disorder induced amorphization in pyrochlore

The defect accumulation mechanism of amorphization has been studied for the La2Zr2O7 pyrochlore by means of classical molecular dynamic simulations. Present calculations show that the accumulation of cation Frenkel pairs is the main driving parameter for the amorphization process, while the oxygen atoms simply rearrange around cations. Under Frenkel pair accumulation, the structure follows the pyrochlore-fluorite-amorphous sequence. Present results consequently provide atomic-level interpretation to previous experimental irradiation observations of the two-step phase transition.     

Inelastic neutron scattering and lattice dynamics of minerals

We review current research on minerals using inelastic neutron scattering and lattice dynamics calculations. Inelastic neutron scattering studies in combination with first principles and atomistic calculations provide a detailed understanding of the phonon dispersion relations, density of states and their manifestations in various thermodynamic properties. The role of theoretical lattice dynamics calculations in the planning, interpretation and analysis of neutron experiments are discussed. These studies provide important insights in understanding various anomalous behaviour including pressure-induced amorphization, phonon and elastic instabilities, prediction of novel high pressure phase transitions, high pressure-temperature melting, etc.      

EPitaxial regrowth and defect formation during pulsed nanosecond annealing of amorphous layers. Part II: Model of liquid phase epitaxy and secondary defect formation

The atomistic model of the liquid phase epitaxial regrowth has been suggested. The model suggestes that the multiatomic (n > 3) nuclei are formed near the regrowth front and they as a whole are attached to the growing crystal. The present model is able to account for the orientation dependency of the crystal growth and to explain the pecularities of the defect formation during the pulsed nanosecond annealing.     

Search for a precursor crystal-to-crystal phase transition to amorphization in α-GeO2 and α-AlPO4 under pressure

Recent X-ray diffraction studies on α-quartz (SiO₂) by Kingmaet al [1], have shown the occurrence of a reversible, crystalline-to-crystalline, phase transition just prior to amorphization at ≈ 21 GPa. This precursor transition has also been confirmed by our recent molecular dynamics simulation study [2]. In order to investigate the possibility of a similar behaviour in other isostructural compounds, which also undergo pressure induced amorphization, α-GeO₂ and α-AlPO₄ (berlinite form) were studied using energy dispersive X-ray diffraction. In either of these materials, no such phase transition is detected prior to amorphization. The onset of amorphization and its reversal is found to be time dependent in GeO₂.     

Peculiarities of radiation-induced defect formation in nanocrystals embedded in a solid matrix

The modification of basic radiation-physics effects (elastic defect formation and amorphization) in nanoclusters placed in a solid matrix compared to the same processes in single crystals is considered theoretically. We introduce a diffusion model of defect formation and study the influence of the nanocluster-matrix interface on the displacement energy. Based on the percolation treatment, we show a significant change in the critical nanocluster amorphization dose compared to the case of a crystal. The influence of various types of irradiation on the amorphization processes is considered. Judgments about the variety of radiation-physics effects in nanoobjects are expressed.     

In-situ transmission electron microscopy observations and molecular dynamics simulations of dislocation-defect interactions in ion-irradiated copper

An in-situ transmission electron microscopy straining technique has been used to investigate the dynamics of dislocation-defect interactions in ion-irradiated copper and the subsequent formation of defect-free channels. Defect removal frequently required interaction with multiple dislocations, although screw dislocations were more efficient at annihilating defects than edge dislocations were. The defect pinning strength was determined from the dislocation curvature prior to breakaway and exhibited values ranging from 15 to 175 MPa. Pre-existing dislocations percolated through the defect field but did not show long-range motion, indicating that they are not responsible for creating the defect-free channels and have a limited contribution to the total plasticity. Defect-free channels were associated with the movement of many dislocations, which originated from grain boundaries or regions of high stress concentration such as at a crack tip. These experimental results are compared with atomistic simulations of the interaction of partial dislocations with defects in copper and a dispersed-barrier-hardening crystal plasticity model to correlate the observations to bulk mechanical properties.     

Internally consistent verification of mean-field models for aggregation using large-scale molecular dynamics

The underlying atomistic mechanisms that govern vacancy aggregation in crystalline silicon are probed using a parametrically consistent, two-scale approach. The essential ingredient in this framework is a direct, quantitative comparison between the predictions of atomistic and continuum simulations for the transient size distribution of vacancy clusters. The former is carried out with parallel molecular dynamics simulation of a silicon system containing 215?000 atoms and 1000 vacancies. The continuum model is based on a sequence of coupled Master equations and is parametrized based on the same empirical potential used to perform the atomistic aggregation simulation. An excellent representation of the cluster size distribution can be obtained with consistent parameters only if the relevant physical mechanisms are captured correctly. The inclusion of vacancy cluster diffusion and a model to capture the dynamic nature of cluster morphology at high temperature are necessary to reproduce the results of the large-scale atomistic simulation. Finally, the continuum model is used to investigate cluster evolution for longer times, which are relevant for process simulation of defect-optimized silicon substrates for microelectronic device fabrication.     

Self-trapped interstitial-type defects in iron

Small interstitial-type defects in iron with complex structures and very low mobilities are revealed by molecular dynamics simulations. The stability of these defect clusters formed by nonparallel {110} dumbbells is confirmed by density functional theory calculations, and it is shown to increase with increasing temperature due to large vibrational formation entropies. This new family of defects provides an explanation for the low mobility of clusters needed to account for experimental observations of microstructure evolution under irradiation at variance with the fast migration obtained from previous atomistic simulations for conventional self-interstitial clusters.