Diffusion Of Substitutional Impurities Research Articles

Helium impurity transport on grain boundaries: Enhanced or inhibited?

We present atomistic simulations that show that transport of helium is inhibited on grain boundaries in tungsten. This finding is contrary to self-diffusion, or diffusion of substitutional impurities in metals, for which transport is generally enhanced along grain boundaries, but is similar to the behavior observed for hydrogen in past studies on low-angle grain boundaries, for which transport also occurs via interstitial diffusion. In the case of helium transport in tungsten, diffusion is biased toward grain boundaries, but once a helium atom or group of atoms is on a grain boundary,diffusion is impeded rather than enhanced. The reduced rate of diffusion on grain boundaries produces a higher concentration of helium in the grain boundary regions. The effect arises from the relative insolubility of helium inmost materials combined with the size mismatch between helium and tungsten, which results in an interstitial diffusion mechanism rather than diffusion that relies on the presence of self-vacancies. In light of this, it is important to note that grain boundaries will not facilitate transport of helium in tungsten and other metals, but in fact that helium is immobilized on grain boundaries.

Structural transformations, distribution of nonequilibrium vacancies, and anomalous diffusion in plastically deformed metals and alloys upon mechanical alloying

Processes of structural transformations leading to the generation of fluxes of nonequilibrium point defects and anomalously high rates of diffusion of substitutional impurities in nanostructured metallic systems upon mechanical alloying have been investigated. The basic contribution to the anomalous acceleration of diffusional mass transfer was shown to come from the nonequilibrium vacancies generated by processes of the deformation-induced migration of intercrystallite boundaries. The influence of interstitial atoms on the diffusion and processes of recrystallization was found to be negligible. The specific features of the formation of strongly supersaturated substitutional solid solutions upon severe plastic deformation of metals and alloys have been examined.

Diffusion simulation of Cr–Fe bcc systems at atomic level using a random walk algorithm

AbstractThis paper proposes a model to simulate the diffusion of impurities in bcc atomic lattices. It works with three‐dimensional volume, divided in small cubic elements (voxels), containing more than one atomic cell each.Once the domain is discretized, impurities jump from one voxel to another according to certain probability that takes into account the composition and geometry of the target voxel.In the present work, a model was applied to a prismatic volume and in order to deduce the relationship between the atomic jumping frequency and the temperature two different cases were studied. One consists of a Fe matrix with Cr impurities, and the other is based on a Cr matrix with Fe impurities. Results obtained from these simulations were compared with profiles obtained by Dictra software.Results for the atomic jumping frequencies were fitted to an Arrhenius type equation, as shown in following expressions:magnified imageFrom these equations it is possible to obtain an activation energy for the atomic jumping phenomenon of ∼306 kJ/mol and ∼411 kJ/mol for the Fe‐matrix and Cr‐matrix systems, respectively. These energies match the empirical measured values for the diffusion of Cr and Fe impurities, 250 kJ/mol and 407 kJ/mol, respectively.Results obtained in this work assure that the proposed model is suitable for simulating the three‐dimensional diffusion of substitutional impurities in Cr and Fe bcc systems. It could be easily expanded to other bcc matrix systems. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ab initio study of magnetic effects on diffusion inα-Fe

A deviation from the Arrhenius law inα-Fe self-diffusion and also in the diffusion of substitutional impurities is foundexperimentally. Below the Curie temperature the diffusion coefficients have lower valuesthan those extrapolated from the paramagnetic region and the Arrhenius plot shows anupward curvature.In this work we attempt to understand this behaviour from first-principles calculations.Formation and migration energies for self-diffusion and also for the diffusion of somesubstitutional impurities are calculated. Spin-polarized and non-spin-polarizedcalculations are assumed to approximately represent ferromagnetic and paramagneticα-Fe respectively.The calculations were performed using the WIEN97 code, with a supercell of 36 atomswhich allows us to include both vacancies and impurities and therefore to study themigration along the direction. The increment in the diffusion barrier due to the total magnetic alignment at0 K, with respect to the paramagnetic case, is almost constant for non-magnetic impurities,as it is in the experiments, whereas for magnetic impurities it depends on the diffusingatom.

Monte Carlo study of vacancy-mediated impurity diffusion in silicon

We present a kinetic lattice Monte Carlo study of the behavior of a dopant flux driven by a vacancy gradient, associated with the diffusion of substitutional impurities via a vacancy mechanism in silicon. Recent ab initio results are used for a quantitative description of the impurity-vacancy interaction. We find a dopant flux smaller than, but in the same direction as, that predicted by the pair diffusion model, with the deviation greatest at high temperatures.

As-vacancy interaction and ring mechanism of diffusion in Si

Vacancy-mediated diffusion of substitutional impurities in Si drastically depends on the character of the impurity-vacancy interaction. The attractive interaction which extends to at least the third nearest neighbors allows a vacancy to move along a ring of Si sites around the impurity. This process enables the impurity-vacancy pair to move and underlies the ``ring mechanism'' of diffusion. To evaluate the importance of this mechanism for As impurity in Si we performed density functional calculations of the As-vacancy interaction. We also calculated the vacancy migration barriers on the circular path around the impurity. The binding energy of the As-$V$ pair was found to be 1.17 eV. This energy decreases down to 0.36 eV when the vacancy moves onto the third-nearest-neighbor site, which is the most remote site on the ring path. We found that the migration barriers for the vacancy hops around the impurity are an order of magnitude smaller than in a bulk Si. The effective barrier along the ring path is therefore mostly determined by the difference of the binding energies on the first- and the third-nearest-neighbor sites and was found to be about 0.81 eV, in a fairly good agreement with experiment. Using ab initio binding energies and migration barriers we carried out the Monte Carlo simulation of As diffusion in silicon. We found that in a temperature range between 700 K and 1300 K the vacancy-assisted As diffusion is totally dominated by the ring mechanism.

Diffusion of substitutional impurities in silicon at short oxidation times: An insight into point defect kinetics: D A Antoniadis and I Moskowitz, J Appl Phys, 53 (10), 1982, 6782-2796

Diffusion of substitutional impurities in silicon at short oxidation times: An insight into point defect kinetics: D A Antoniadis and I Moskowitz, J Appl Phys, 53 (10), 1982, 6782-2796

Diffusion of substitutional impurities in silicon at short oxidation times: An insight into point defect kinetics

Oxidation-enhanced diffusion of phosphorus, arsenic, and boron and oxidation-reduced diffusion of antimony in silicon have been studied as a function of oxidation time. Data for the early phase of oxidation in dry oxygen from 5 to 60 min have been obtained. Oxidation-enhanced diffusivities show a steady decrease with decreasing oxidation rate for phosphorus, arsenic, and boron, with enhancements at long oxidation times in agreement with previously reported results. Antimony shows a reduction in diffusivity during oxidation. A model allowing calculation of diffusivity enhancement or reduction for all elements and oxidation times has been developed. The present data support the theory of a dual vacancy-interstitialcy diffusion mechanism for all the elements studied. The fraction of interstitialcy diffusion fI has been calculated, yielding fI=0.38 for phosphorus at 1000 °C, fI=0.30 for boron at 1000 °C, fI=0.35 for arsenic at 1090 °C, and fI=0.015 for antimony at 1100 °C. It has also been shown that the oxidation-induced supersaturation of self-interstitials is accompanied by an undersaturation of vacancies during oxidation. This undersaturation can be explained by a rate-limited bimolecular annihilation mechanism. This theory yields, for the first time, values for the vacancy-interstitial recombination-limited intrinsic vacancy lifetime in silicon under near-equilibrium conditions at high temperature; it also indicates the presence of an energy barrier to this recombination of the order of 1.4 eV.

Nuclear Spin Relaxation by Quadrupolar Interactions. The Diffusion of Substitutional Impurities in Ionic Crystals

AbstractSpectral densities are calculated for nuclear spin relaxation via quadrupolar interactions caused by the vacancy diffusion of impurities in NaCl‐type ionic crystals. The treatment, which takes correlation effects into account, is based on the encounter model. NMR cw‐studies on the diffusion of Na+ ions in AgBr are analysed in the framework of this model. The impurity diffusion coefficients derived are found to be in good agreement with tracer diffusion data.