Investigation of radiation defects in Fe-Cr alloy

Abstract

The kinetics of radiation defect accumulation and subsequent recovery during/after electron irradiation below 273 K, at 323 K and 373 K were investigated for the Fe-15.7 at.% Cr using positron annihilation measurements at room temperature. Formation of vacancy clusters was observed at all of the irradiation temperatures. The formation of clusters and kinetics of their accumulation point to mobility of vacancies at least at room temperature. The cluster rearrangement and variations in the cluster configuration take place during annealing.     

低能Pt原子团簇沉积过程的分子动力学模拟

利用分子动力学模拟系统研究了低能Pt38,Pt141和Pt266原子团簇与Pt(001)表面的相互作用过程,详细分析了初始原子平均动能为0.1,1.0和10eV的原子团簇的沉积演化过程及其对基体表面形貌的影响.研究表明,初始原子平均动能是描述低能原子团簇的重要参量.当团簇的平均原子动能较低时,团簇对基体表层原子点阵损伤较小,基本属于沉积团簇;随着入射团簇的原子平均动能的增加,团簇对表层原子点阵结构的破坏能力增强,当团簇的原子平均动能增加到10eV时,团簇已经显现出注入特征.低能原子团簇对基体表面形貌的影响 关键词： 分子动力学模拟 低能原子团簇 载能沉积     

Percolation approach for atomic and molecular cluster formation

We apply a percolation approach for the theoretical analysis of mass spectra of molecular microclusters obtained by adiabatic expansion technique. The evolution of the shape of the experimental size distributions as function of stagnation pressure and stagnation temperature are theoretically reproduced by varying the percolation parameter. Remaining discrepancies between theory and experiment are discussed. In addition, the even-odd alternation as well as the “magic” shell structure within metallic, secondary ion mass spectra are investigated by introducing statistical weights for the cluster formation probabilities. Shell correction energies of atomic clusters as function of cluster size are deduced from the experimental data.     

Dislocation decoration and raft formation in irradiated materials

Experimental observations of dislocation decoration with self-interstitial atom (SIA) clusters and of SIA cluster rafts are analysed to establish the mechanisms controlling these phenomena in bcc metals. The elastic interaction between SIA clusters, and between clusters and dislocations is included in kinetic Monte Carlo (KMC) simulations of damage evolution in irradiated bcc metals. The results indicate that SIA clusters, which normally migrate by 1D glide, rotate due to their elastic interactions, and that this rotation is necessary to explain experimentally-observed dislocation decoration and raft formation in neutron-irradiated pure iron. The critical dose for raft formation in iron is shown to depend on the intrinsic glide/rotation characteristics of SIA clusters. The model is compared with experimental observations for the evolution of defect cluster densities (sessile SIA clusters and nano-voids), dislocation decoration characteristics and the conditions for raft formation.     

Dynamic Scaling of Ramified Clusters Formed on Liquid Surfaces

A comprehensive simulation model -- deposition, diffusion, rotation, reaction and aggregation model is presented to simulate the formation processes of ramified clusters on liquid surfaces, where clusters can disuse and rotate easily. The mobility （including diffusion and rotation） of clusters is related to its mass, which is given by Dm = Dos^-γD and θm = θos^-γθ, respectively. The influence of the reaction probability on the kinetics and structure formation is included in the simulation model. We concentrate on revealing dynamic scaling during ramified cluster formation. For this purpose, the time evolution of the cluster density and the weight-average cluster size as well as the cluster-size distribution scaling function at different time are determined for various conditions. The dependence of the cluster density on the deposition flux and time-dependence of fractal dimension are also investigated. The obtained results are helpful in understanding the formation of clusters or thin film growth on liquid surfaces.     

The role of small clusters in the transformation of two-ring clusters into fullerenes

Possible reactions by which small carbon molecules C₂ and C₃ can join a cluster at an intermediate assembly stage from a two-ring cluster to a fullerene are considered. For analysis of the separate stages of this type of reaction, methods of quantum chemistry are used. The generalized reaction probabilities obtained are then included into the general scheme of assembly kinetics alongside monomolecular reactions. It has been shown that at a small cluster concentration of about 10¹⁴ cm⁻³ the assembly time drops severalfold. Also given are generalized results of the effect on the assembly time of the multiplicity of paths of fullerene assembly from two-ring clusters.     

碱金属团簇Na2在重离子作用下的离化

在时间相关的局域密度近似理论框架下研究了重离子Ar8+ 和碱金属团簇Na2 随时间演化的相互作用 .给出了电子系统的总能量 ,产生Na2 的各种电荷态的几率和Na2 价电子的平均逃逸数等物理量随时间的演化 ,进而得到了Na2 的离化几率 .在快重离子与团簇作用中 ,可以得到低激发态的团簇离子. Based on the Time Dependent Local Density Approximation(TDLDA), the processes of excitation of sodium clusters by highly charged Ar 8+ions are simulated. The time-dependent probabilities of escaped electrons are obtained, furthermore, the ionization probabilities of the Na 2clusters are deduced.In fast ion-cluster collisions, Na 2cluster ions can be produced with minimum or even no excitation of the cluster itself     

The DLCA-RLCA transition arising in 2D-aggregation: simulations and mean field theory

A sticking probability model based on the average cluster lifetime is employed for deducing a kernel capable to describe the kinetics of computer simulated irreversible aggregation processes in two dimensions. The deduced kernel describes not only the time evolution of the cluster size distribution for diffusion limited aggregation (DLCA) and reaction limited aggregation (RLCA) but also for the entire transition region between both regimes. The model predicts a crossover to diffusion limited cluster aggregation for all sticking probabilities at long aggregation times. The time needed for reaching the DLCA limit increases for decreasing sticking probability. Received 16 April 2001 and Received in final form 24 May 2001     

Sedimentation rate of erythrocyte from physics prospective

The European Physical Journal E - A two-dimensional multiphase cellular automaton (CA) model is proposed for the prediction of growth kinetics and microstructural evolution during peritectic...     

Kinetics of the total cluster perimeter in thin film nucleation on solid surfaces

The time evolution of the total perimeter of clusters growing on a surface has been described on the basis of the JMAK (Johnson-Mehl-Avrami-Kolmogoroff) statistical theory. A general formula, which can be easily extended to any space dimension, is obtained. When particular nucleation functions and the cluster growth law are considered the kinetics of the perimeter can be explicitly calculated and moreover, it can be expressed as a function of the covered surface. Experimental data on the efficiency of a Cu/CuO_x model catalyst, towards imide formation, have been satisfactorily described by the model. Moreover, the growth of Ag on a GaAs(110) surface studied via photoelectron spectroscopy has been qualitatively explained by the proposed model. In the model case of cylindrical clusters the knowledge of the evolution of the total perimeter allows the entire area of the film to be evaluated.     

The kinetics of formation and the parameters of radiation defect clusters in silicon

An approximate analytic solution of the system of equations describing the kinetics of formation and the parameters of radiation defect clusters in crystals is obtained. A model corresponding to the real parameters of Si is assumed as a basis for calculation and it is shown that two types of secondary radiation defect clusters may be realized corresponding to the congealing and spreading of the initial vacancy clusters. The cluster type depends on the incident energy of the particle which creates the effects and on the physical parameters of the crystal under irradiation. For a congealing cluster most of the initial vacancies react within the original volume of the damage cascade and for a spreading cluster the final concentration of divacancies and A-centres within the original cascade volume is much less than the initial vacancy concentration, i.e. most of the vacancies form divacancies and A-centres dispersed throughout the crystal volume.

The definition of the concept “threshold energy of cluster formation” as the minimum primarily displaced atom energy for the creation of a “congealing” vacancy cluster is proposed.

It is shown that A-centres form a belt surrounding a central divacancy cluster.     

Molecular dynamics simulations of the freezing of gold nanoparticles

A set of molten gold clusters, each with 1157 gold atoms, was studied by molecular dynamics simulations as the clusters underwent freezing at three different temperatures. Most of the clusters attained an icosahedral structure upon freezing, a structure found to be stable to mild annealing. Other structures observed were imperfect truncated decahedral, truncated octahedral and hexagonal close packed structures. The role of kinetics in the process of cluster solidification is discussed. Received 6 November 2000     

Role of cluster transient mobility in pulsed laser deposition-type growth kinetics

The ballistic motion of kinetically hyperthermal clusters on corrugated potential energy surfaces is studied by molecular dynamics simulations. Under pulsed laser deposition conditions, such clusters are generated due to the supersaturation flux and nonthermal energies of the incident species, a fraction of which is transferred to the clusters. This energy transfer, together with the flattening of the potential energy surface due to the cluster inner bonds, causes a lengthening of the ballistic paths as the cluster size increases. These results help explain the enhancement of growth kinetics by pulsed laser deposition.     

Mechanism of void nucleation and growth in bcc Fe: atomistic simulations at experimental time scales

Evolution of small-vacancy clusters in bcc Fe is simulated using a multiscale approach coupling an atomistic activation-relaxation method for sampling transition-state pathways with environment-dependent reaction coordinate calculations and a kinetic Monte Carlo simulation to reach time scales on the order of ~10? s. Under vacancy-supersaturated condition, di- and trivacancy clusters form and grow by coalescence (Ostwald ripening). For cluster size greater than four we find a transition temperature of 150 °C for accelerated cluster growth, as observed in positron annihilation spectroscopy experiments. Implications for the mechanism of stage-IV radiation-damage-recovery kinetics are discussed.     

Simulation of the first growth phase of single-walled carbon nanotubes using a model based on a cellular automaton

This paper presents a theoretical study on the first growth phase of single-walled carbon nanotubes. The model adopted is based on a cellular automaton containing as the elementary entities the metal particles and the carbon atoms. It uses an interaction based on phenomenological potentials to define the update function of the automaton. The purpose of this study is to establish the saturation condition of the metal particle with the carbon atoms when the formation of clusters of carbon begins. The results show that the first phase of formation of the carbon clusters can start with the particle partially saturated with carbon atoms.     

Speech utterance clustering based on the maximization of within-cluster homogeneity of speaker voice characteristics

This paper investigates the problem of how to partition unknown speech utterances into a set of clusters, such that each cluster consists of utterances from only one speaker, and the number of clusters reflects the unknown speaker population size. The proposed method begins by specifying a certain number of clusters, corresponding to one of the possible speaker population sizes, and then maximizes the level of overall within-cluster homogeneity of the speakers' voice characteristics. The within-cluster homogeneity is characterized by the likelihood probability that a cluster model, trained using all the utterances within a cluster, matches each of the within-cluster utterances. To attain the maximal sum of likelihood probabilities for all utterances, the proposed method applies a genetic algorithm to determine the cluster in which each utterance should be located. For greater computational efficiency, also proposed is a clustering criterion that approximates the likelihood probability with a divergence-based model similarity between a cluster and each of the within-cluster utterances. The clustering method then examines various legitimate numbers of clusters by adapting the Bayesian information criterion to determine the most likely speaker population size. The experimental results show the superiority of the proposed method over conventional methods based on hierarchical clustering.     

Evolution of three-shell onion-like and core-shell structures in （AgCo）201 bimetallic clusters

<正>This paper studies the structural evolution of（AgCo）₂₀₁ clusters with different Co concentrations under various temperature conditions by using molecular dynamics with the embedded atom method.The most stable position for Co atoms in the cluster is the subsurface layer at low temperature(lower than 200 K for the Ag₂₀₀Co₁ cluster).The position changes to the core layer with the increase of temperature,but there is an energy barrier in the middle layer. This makes the Ag-Co cluster form an Ag-Co-Ag three-shell onion-like configuration.When the temperature is high enough[higher than 800 K for（AgCo）₂₀₁ clusters with 50%Co],Co atoms can obtain enough energy to overcome the energy barrier and the cluster forms an Ag-Co core-shell configuration.Amorphization for the onion-like and core-shell clusters is induced by the large lattice misfit at Ag-Co interfaces.The structural evolution in the Ag-Co cluster is related to the release of excess energy.     

Dynamical phase transitions in one-dimensional stochastic cellular automata

We study the influence of dynamic noise and disorder on the evolution of a chaotic cellular automaton model. Three distinct phases are identified corresponding to ordered, random and damage spreading evolution. The time evolution of the associated order parameters is investigated and the critical exponents are calculated close to the phase transition.