Number Of Self-interstitial Atoms Research Articles

What promotes derected self assembly (DSA)?

A low-energy electron beam (EB) can create self-interstitial atoms (SIA) in a solid and can cause directed self-assembly (DSA), e.g. {311}SIA platelets in c-Si. The crystalline structure of this planar defect is known from experiment to be made up of SIAs that form well aligned 〈110〉 atomic rows on each (311) plane. To simulate the experiment we distributed Frenkel pairs (FP) randomly in bulk c-Si. Then making use of a molecular dynamic (MD) simulation, we have reproduced the experimental result, where SIAs are trapped at metastable sites in bulk. With increasing pre-doped FP concentration, the number of SIAs that participate in DSA tends to be increased but soon slightly supressed. On the other hand, when the FP concentration is less than 3%, a cooperative motion of target atoms was characterized from the long-range-order (LRO) parameter. Here we investigated the correlation between DSA and that cooperative motion, by adding a case of intrinsic c-Si. We confirmed that the cooperative motion slightly promote DSA by assisting migration of SIAs toward metastable sites as long as the FP concentration is less than 3%, however, it is essentially independent of DSA.

Radiation damage in nano-crystalline tungsten: A molecular dynamics simulation

This study presents MD simulations for the radiation damage of single- and nano-(poly)crystalline tungsten. The nano-(poly)crystalline microstructure was constructed using a new construction method combined with the phase field model. At the maximum damage state, the recombination rate of self-interstitial atoms (SIAs) and vacancies is faster in the single-crystal that in the poly-crystal structure. In the steady state, the number of SIAs is twice as large in the nano-crystal, as caused by the attractive interaction between the grain boundary and the SIAs.

Atomistic simulation of helium-defect interaction in alpha-iron

Molecular dynamics (MD) methods are utilized to study the formation of vacancy clusters created by displacement cascades in α-Fe containing different concentrations of substitutional He atoms. Primary knock-on atom energies, Ep, from 500eV to 20keV are considered at a temperature of 100K, and the results are compared with those performed in pure α-Fe. There are distinct differences in the number and size of vacancy clusters within displacement cascades with and without substitutional helium atoms. It is found that many large vacancy clusters can be formed within cascade cores in α-Fe with helium atoms, in contrast to a few small vacancy clusters observed in pure α-Fe. The number and size of helium/vacancy clusters generally increase with increasing helium concentration and PKA energy. One of the striking results is that the number of self-interstitial atoms (SIAs) and the size of interstitial clusters are much smaller than those in pure α-Fe.

A Coalescence Mechanism of Impurity Atoms Implanted into a Crystalline Target at Low Temperatures

We have studied the coalescence mechanism of impurity atoms implanted into a crystalline target at low temperatures, paying attention to the dimer formation mechanism. A classical molecular-dynamics (MD) calculations was used in the case of 1 keV B ion implantation at 100 K into a crystalline silicon (c-Si) target that was supersaturated with pre-embedded B atoms at a concentration of 3 at. %. The initial phase of dimer formation was investigated in terms of the temperature distribution and the degradation of the long-range order (LRO) parameters defined in the pixel mapping (PM) method. One result was the locally high temperature and big temperature gradient that revealed the acceleration of collisional diffusion of light impurity atoms in the host medium. The other finding was the decrease of the LRO parameters. This fact associated with the number of self-interstitial atoms (SIAs) implies a deformed potential-field in a crystal under ion irradiation. These results indicate that dynamically enhanced diffusion or collisional-diffusion is the origin of dimer formation.

Mechanism of one-dimensional glide of self-interstitial atom clusters in α-iron

Abstract Recent molecular dynamics (MD) computer simulations of pure copper and iron have shown that clusters consisting of up to a few tens of self-interstitial atoms (SIAs) are highly mobile along close-packed crystallographic directions. This effect has important consequences for microstructure evolution in irradiated metals and so it is desirable to investigate the mechanisms of cluster motion. In the present paper, results of MD modelling of the thermally-activated motion of clusters of three, nine and 17 SIAs in α-iron in the temperature range from 90 to 1400 K are analysed. The correlation between the motion of the centre of mass of a cluster and the individual jumps of its constituent SIAs is revealed. It is found that the SIAs in a cluster jump almost independently and their jump frequency depends on the number of SIAs in the cluster. This leads to a simple relationship between the jump frequency of a cluster and the number of SIAs in it. The reason for the deviation of the cluster jump frequency from a simple Arrhenius relationship is discussed. It is shown that such clusters only exhibit an effectively random walk, that is a correlation factor of one, when the jump length defining diffusion is taken to be 3b to 4b, where b is the magnitude of the vector ½(111).

On The Mechanism of Interstitial Cluster Migration in α -FE

AbstractRecent molecular dynamics (MD) computer simulations have shown that clusters consisting of up to a few tens of self-interstitial atoms (SIAs) are highly mobile along closed-packed crystallographic directions in pure copper and iron. This effect has important consequences for microstructure evolution in irradiated metals and so it is desirable to investigate the mechanisms of the cluster motion. In the present paper the results of MD modelling of the thermally-activated motion of clusters of 3, 9 and 17 SIAs in α-Fe in the temperature range from 90 to 1400 K are analyzed. The extensive MD data has enabled the migration of clusters, as well as that of individual SIAs in the clusters, to be treated with high statistical accuracy. The correlation between the motion of the centre of gravity of a cluster and the jumps of individual SIAs in the cluster is revealed. It is found that the SIAs in a cluster jump almost independently and their jump frequency depends on the number of SIAs in the cluster. This leads to a simple relationship between the jump frequency of a cluster and the number of SIAs in it. The cluster jump frequency exhibits a deviation from the Arrhenius relationship. The reason for this is discussed.

Silicon Self-Interstitial Clusters

AbstractA tight-binding molecular dynamics investigation on the structure and energetics of self-interstitial clusters in silicon is presented. We discuss how a small number of self-interstitial atoms give rise to the formation of tedrahedally-shaped clusters, while a larger number of defects exhibit a self-organization mechanism driving the system to form rod-like defects.

Investigation of the behaviour of helium and radiation defects after room temperature he-implantation of nickel and copper

Abstract Helium was implanted in a homogeneous distribution into thin foils of nickel and copper at room temperature. The development of the irradiation defects and the helium precipitates was investigated by the combined measurements of the change in length and the change in lattice parameter (differential dilatometry). The results show: (a) over the range of He-concentration between 1000 to 10,000 appm of He, the number of vacancies observed in the metals is larger than the number of self-interstitial atoms. Therefore, the majority of the interstitial atoms is not present in small separate dislocation loops but rather in a dislocation network. (b) The number of vacancies cv is larger than the number of the implanted He atoms. This result indicates that highly overpressurized He-bubbles are not the dominant helium-vacancy agglomerates and consequently the growth of the small precipitates is not controlled by a loop punching mechanism or by SIA-emission.