Deuterium accumulation in tungsten under low-energy high-flux plasma exposure

The accumulation of deuterium implanted in tungsten is simulated within the framework of kinetic diffusion theory. The influence of the tungsten microstructure (dislocation density and impurity concentration) on the process of deuterium capture and accumulation is considered. It is established that, under the chosen irradiation conditions, deuterium accumulation in the near-surface region is determined by capture at defects formed during implantation. The deuterium concentration gradient, together with the material microstructure, determines its accumulation in tungsten. Variation in the dislocation density and impurity concentration does not affect the simulation results, which is, first, related to the fact that the model used does not contain alternative mechanisms for the formation and growth of vacancy clusters under the subthreshold irradiation mode. The simulation results are compared with experimental data, and ways of improving the model are discussed in order to explain the deuterium-saturation effect for high fluences (more than 10²³ m^?2).     

Theoretical investigation of excited states of C(3)

In this work, we present ab initio calculations for the potential energy surfaces of C(3) in different electronic configurations, including the singlet ground state [X (1)Sigma(g) (+),((1)A(1))], the triplet ground state [a (3)Pi(u),((3)B(1), (3)A(1))], and some higher excited states. The geometries studied include triangular shapes with two identical bond lengths, but different bond angles between them. For the singlet and triplet ground states in the linear geometry, the total energies resulting from the mixed density functional--Hartree-Fock and quadratic configuration interaction methods reproduce the experimental values, i.e., the triplet occurs 2.1 eV above the singlet. In the geometry of an equilateral triangle, we find a low-lying triplet state with an energy of only 0.8 eV above the energy of the singlet in the linear configuration, so that the triangular geometry yields the lowest excited state of C(3). For the higher excited states up to about 8 eV above the ground state, we apply time-dependent density functional theory. Even though the systematic error produced by this approach is of the order of 0.4 eV, the results give different prospective to insight into the potential energy landscape for higher excitation energies.     

Glide of dislocations in <1 1 1>{3 2 1} slip system: an atomistic study

Atomistic calculations are performed to investigate plastic slip in the <1?1?1>{3?2?1} system in body-centred cubic iron. Several modern interatomic potentials, developed over the last decade, are applied to compute the stacking fault γ-line energy in the {3?2?1} plane and the results are compared with the ab initio prediction. The applied potentials have shown strong deviations, but several potentials acquired good qualitative agreement with the ab initio data. Depending on the applied potential, the lowest value of the Peierls stress for the edge dislocation (ED) is 50 MPa (Ackland and Bacon from 1997) and the highest is 550 MPa (Dudarev and Derlet from 2005), while for the screw dislocation it is much higher, in the range 1–2 GPa. At finite temperature, however, the flow stress of the ED is found to decrease exponentially reaching a negligible value at about 200 K, irrespective of the applied potential. On the basis of the data obtained using Ackland–Mendelev potential from 2004, we conclude that the slip resistance of the <1?1?1>{3?2?1} system is in between the resistance of the <1?1?1>{1?1?0} and <1?1?1>{1?1?2} slip systems.     

Effect of isotropic stress on dislocation bias factor in bcc iron: an atomistic study

The effect of externally applied stress on the dislocation bias factor (BF) in bcc iron has been studied using a combination of atomistic static calculations and finite element integration. Three kinds of dislocations were considered, namely, a₀/2〈1 1 1〉{1 1 0} screw, a₀/2〈1 1 1〉{1 1 0} edge and a₀〈1 0 0〉{0 0 1} edge dislocations. The computations reveal that the isotropic crystal expansion leads to an increasing or constant dislocation bias, depending on the Burgers vector and type of dislocation. On the other hand, compressive stress reduces the dislocation bias for all the dislocations studied. Variation of the dislocation BF depending on dislocation type and Burgers vector is discussed by analysing the modification of the interaction energy landscape and the capture efficiency values for the vacancy and self-interstitial atom.     

Modeling the Angular Momentum Control of Small Spacecraft with a Magnetic Attitude Control System

Doklady Physics - The problem of modeling the angular momentum control modes of small spacecraft using electromagnetic systems interacting with the Earth’s magnetic field is considered. The...     

Modelling the diffusion of self-interstitial atom clusters in Fe–Cr alloys

The results of molecular dynamics simulations of the diffusion of self-interstitial atom clusters in Fe–Cr alloys of different Cr content are presented. It is shown that, with increasing Cr concentration, the cluster diffusivity first decreases and then increases, in accordance with the predictions of a model developed recently and based on molecular static calculations. The minimum diffusivity is found at about 10 at% Cr for small clusters and it shifts towards lower concentration with increasing cluster size. The migration energy of SIA clusters is found to lie in between the binding energy of a Cr atom with a crowdion and half of it. This indicates that the mechanism of cluster migration is via the movement of individual crowdions from one Cr atom to another. The values obtained statically are much higher and are argued to be more reliable due to better sampling of different configurations in a bigger simulation box.     

Determination of the structural features of <Emphasis Type="Italic">O</Emphasis>-alkyl-<Emphasis Type="Italic">S</Emphasis>-2-(<Emphasis Type="Italic">N,N</Emphasis>-dialkylamino) ethyl alkylthiophosphonates from their electron ionization mass spectra

It is believed that information about the molecular structure of highly toxic O-alkyl-S-2(N,N-dialkylamino) ethyl alkylthiophosphonates (V-gases) obtained from their EI mass spectra is too insufficient. In particular, the determination of molecular weights and structures of radicals at phosphorus and oxygen atoms causes great difficulties. In this paper, solutions of these problems are proposed.