On the binding energies and configurations of vacancy and copper–vacancy clusters in bcc Fe–Cu:a computational study

The properties of trapping centres in – as grown – Tl₄GaIn₃S₈ layered single crystals were investigated in the temperature range of 10–300 K using thermoluminescence (TL) measurements. TL curve was analysed to characterize the defects responsible for the observed peaks. Thermal activation energies of the trapping centres were determined using various methods: curve fitting, initial rise and peak shape methods. The results indicated that the peak observed in the low-temperature region composed of many overlapped peaks corresponding to distributed trapping centres in the crystal structure. The apparent thermal energies of the distributed traps were observed to be shifted from ~12 to ~125 meV by increasing the illumination temperature from 10 to 36 K. The analysis revealed that the first-order kinetics (slow retrapping) obeys for deeper level located at 292 meV.     

First principle study of nitrogen vacancy in aluminium nitride

In the framework of density functional theory, using the plane-wave pseudopotential method, the nitrogen vacancy （VN） in both wurtzite and zinc-blende AlN is studied by the supercell approach. The atom configuration, density of states, and formation energies of various charge states are calculated. Two defect states are introduced by the defect, which are a doubly occupied single state above the valance band maximum （VBM） and a singly occupied triple state below the conduction band minimum （CBM） for wurtzite AlN and above the CBM for zinc-blende AlN. So VN acts as a deep donor in wurtzite AlN and a shallow donor in zinc-blende AlN. A thermodynamic transition level E（3＋/＋） with very low formation energy appears at 0.7 and 0,6eV above the VBM in wurtzite and zinc-blende structure respectively, which may have a wide shift to the low energy side if atoms surrounding the defect are not fully relaxed. Several other transition levels appear in the upper part of the bandgap. The number of these levels decreases with the structure relaxation. However, these levels are unimportant to AlN properties because of their high formation energy.     

First principles study of behavior of helium at Fe(110)–graphene interface

Recently,metal-graphene nanocomposite system has aroused much interest due to its radiation tolerance behavior.However,the related atomic mechanism for the metal-graphene interface is still unknown.Further,stainless steels with Fe as main matrix are widely used in nuclear systems.Therefore,in this study,the atomic behaviors of point defects and helium(He) atoms at the Fe(110)-graphene interface are investigated systematically by first principles calculations.The results indicate that graphene interacts strongly with the Fe(110) substrate.In comparison with those of the original graphene and bulk Fe,the formation energy values of C vacancies and Fe point defects decrease significantly for Fe(110)-graphene.However,as He atoms have a high migration barrier and large binding energy at the interface,they are trapped at the interface once they enter into it.These theoretical results suggest that the Fe(110)-graphene interface acts as a strong sink that traps defects,suggesting the potential usage of steel-graphene with multiply interface structures for tolerating the radiation damage.     

First-principles calculations of solute–vacancy interactions in aluminum

The interactions of solute atoms with vacancies play a key role in diffusion and precipitation of alloying elements,ultimately influencing the mechanical properties of aluminum alloys. In this study, first-principles calculations are systematically performed to quantify the solute–vacancy interactions for the 3 d–4 p series and the 4 d–5 p series. The solute–vacancy interaction gradually transforms from repulsion to attraction from left to right. The solute–vacancy binding energy is sensitive to the supercell size for elements at the beginning. These behaviors of the solute–vacancy binding energy can be understood in terms of the combination and competition between the elastic and electronic interactions. Overall, the electronic binding energy follows a similar trend to the total binding energy and plays a major role in the solute–vacancy interactions.     

Positron annihilation study of the vacancy clusters in ODS Fe–14Cr alloys

Abstract

Oxide dispersion strengthened Fe14Cr and Fe14CrWTi alloys produced by mechanical alloying and hot isostatic pressing were subjected to isochronal annealing up to 1400 °C, and the evolution and thermal stability of the vacancy-type defects were investigated by positron annihilation spectroscopy (PAS). The results were compared to those from a non-oxide dispersion strengthened Fe14Cr alloy produced by following the same powder metallurgy route. The long lifetime component of the PAS revealed the existence of tridimensional vacancy clusters, or nanovoids, in all these alloys. Two recovery stages are found in the oxide dispersion strengthened alloys irrespective of the starting conditions of the samples. The first one starting at T > 750 °C is attributed to thermal shrinkage of large vacancy clusters, or voids. A strong increase in the intensity of the long lifetime after annealing at temperatures in the 800–1050 °C range indicates the development of new vacancy clusters. These defects appear to be unstable above 1050 °C, but some of them remain at temperatures as high as 1400 °C, at least for 90 min.     

First–principles study of potassium adsorption and diffusion on graphene

Potassium–ion batteries (KIBs) are a new–type of energy storage devices that have attracted increasing attention due to their low cost and the abundant resource of K in the Earth’s crust. Monolayer and multilayer graphene are promising electrode materials for KIBs. Herein, the adsorption and diffusion of potassium atoms on the surface of graphene were studied using the first–principles calculations including the van der Waals interaction. It was determined that K atoms can stably adsorb on the surface of graphene. The climbing image nudged elastic band method was employed to calculate the diffusion barriers of a single K atom and two K atoms on the surface of graphene. The results demonstrated that the diffusion barrier of a single K atom on graphene was low. The interaction between K atoms was considered and it facilitates the K atom diffusion to the second and third nearest–neighbour site of the K adatom, but prevents the K atom diffusion to the far nearest–neighbour site of the K adatom. Moreover, the difference in charge density demonstrates that there was a significant charge transfer from two K adatoms to its nearest–neighbour carbon atoms.     

First principles study on the electronic structure and magnetism of Fe1-xCoxSi alloys

Electronic and magnetic properties of Fe1-xCoxSi alloys were investigated by using a full-potential linear augmented-plane-wave method based on density functional theory. Electronic structure calculation demonstrates that half-metallic property appears in the Fe-rich region of 0 〈 x ≤ 0.25, while the alloys turn out to be a magnetic metal for x 〉 0.25. The concentration dependence of the magnetic moment of the alloys can be understood by the fixed Fermi level at minority band in Fe-rich region, as well as at the majority band in Co-rich region. In Fe-rich alloys, the electronic structure and the magnetic properties at Fe site depend mainly on the spin-polarization of nearest neighbouring Co atoms, while in Co-rich alloys, these features at Co site arise mainly from the neighbours of Fe atoms.     

First principles study of half-metallic ferromagnetism in zincblende Cd1−xVxSe

The structural, electronic and magnetic properties of zincblende (ZB) Cd_1?xV_xSe for different values of x were investigated using the full-potential linearized augmented plane wave plus local orbital (FPLAPW+lo) method based on spin-polarized density functional theory (DFT). It was confirmed that for all values of x, the ferromagnetic (FM) state is more stable than antiferromagnetic (AFM) state. The results show that Cd_0.25V_0.75Se and Cd_0.5V_0.5Se compounds exhibit a half-metallic (HM) characteristic while Cd_0.75V_0.25Se has nearly a HM nature. The obtained HM band gaps with EV-GGA have been considerably improved with respect to PBE-GGA scheme. The analysis of density of states (DOSs) curves confirms the hybridization between V d and Se p states in all compounds. The total magnetic moments of Cd_0.25V_0.75Se and Cd_0.5V_0.5Se compounds are 3μ_B (integer values), confirming their HM characteristic. Finally, the robustness of half-metallicity with changing the lattice parameters of Cd_1?xV_xSe alloys was discussed.     

Deformation-induced atomic redistribution in bcc Fe–Mn alloy

Deformation-induced iron and manganese atomic separation in a bcc solid solution is detected via Mössbauer spectroscopy in Fe_93.2Mn_6.8 alloy after high pressure torsion deformation in Bridgman anvils. The rate of short-range separation grows along with the temperature of deformation.     

Correlation factor for diffusion in cubic crystals with solute–vacancy interactions of arbitrary range

Using a double Laplace and Fourier transform of the transport equation for the vacancy, we obtain the exact values of the return probabilities of the vacancy in the close vicinity of the tracer atom in the presence of a solute-vacancy interaction of arbitrary range. The study of model cases shows that taking into account the interaction up to the third neighbour shell is mandatory to obtain the solute diffusivity in BCC and FCC structures with a good precision. A thorough ab initio evaluation of all the migration barriers is rarely available in the literature; it is shown that the approximations often used to overcome this lack of data must be chosen with care in order to avoid puzzling conclusions. An examination of dilute systems studied in the recent literature is presented.     

Gasification and removal of carbon materials and redeposited boron–carbon layers exposed in an oxygen–ozone mixture

The results from experiments on measuring the rate of gasification for carbon and boron–carbon films and carbon fiber composite (CFC) exposed in oxygen–ozone mixtures are presented. The rate of gasification is 0.4–0.6 μm h^–1 (at temperatures of 220–250°C, a pressure of 0.3 atm, and an ozone concentration of 0.6 at %) for carbon films; plane CFC samples; gaps 1 and 2 mm wide with walls of stainless steel; and gaps 1 mm wide with walls of CFC. It is 15 μm h^–1 for plane CFC at a temperature of 250°C, a pressure of 1 atm, and an ozone concentration of 10 at %. The rate of gasification for boron–carbon films is from 3 to 30 nm h^–1 for B/C ratios of 2.1 to 0.8 (at 250°C, 1 atm, and ozone concentration of 10 at %).     

First principles study on the ferroelectricity of the perovskite ABO3 ferroelectrics

In order to understand well the different ferroelectric behaviour of quantum paraelectrics and ferroelectrics and the origin of the ferroelectricity of the solid solution KTa0.5Nb0.5O3(KTN),we calculated the electronic structure of CaTiO3,BaTiO3 and KTN by first principles calculation.From total energy analysis,it is shown that,with increasing cell volume,the crystals (CaTiO3,SrTiO3) will have a ferroelectric instability.For BaTiO3,the ferroelectricity will disappear as the cell volume is decreased.From the density of states analysis,it is shown that the hybridization between B d and O p is very important for the ferroelectric stability of ABO3 perovskite ferroelectrics.This is consistent with the analysis of band structure.     

A first-principles study of the effect of oxygen vacancy on rutile Ti1−xCdxO2

A first-principles study has been performed to understand the effect of oxygen vacancy on the electronic properties of cadmium doped rutile TiO₂. We observe that Cd incorporation on rutile TiO₂ induces Cd p-states on the top of the valence band which is consistent with an earlier result of Zhang et al. (2008) [5]. Furthermore, by creating an oxygen vacancy, some new states are induced, which originate from the Ti 3d electrons at the middle of the band gap and spread up to the conduction band. Therefore, the band gap of the material reduces significantly, making it suitable to act as a better photocatalyst.     

Global study of electron–quark unparticle interactions

We perform a global fit on parity-conserving electron–quark interactions via spin-1 unparticle exchange. Besides the peculiar features of unparticle exchange due to non-integral values for the scaling dimension \(d_{\mathcal {U}}\) and a non-trivial phase factor \(\exp (-id_{\mathcal {U}}\pi)\) associated with a time-like unparticle propagator, the energy dependence of the unparticle contributions in the scattering amplitudes are also taken into account. The high energy data sets taken into consideration in our analysis are from (1) deep inelastic scattering at high Q ² from ZEUS and H1, (2) Drell–Yan production at Run II of CDF and DØ, and (3) e ⁺ e ^?→ hadrons at LEPII. The hadronic data at LEPII by itself indicated a 3–4 sigma preference of new physics over the Standard Model. However, when all data sets are combined, no preference for unparticle effects can be given. We thus deduce an improved 95% confidence level limit on the unparticle energy scale \(\varLambda_{\mathcal {U}}\).     

First principles study of electronic structure and carrier mobility in β-armchair antimony nanotubes

In recent work, we have investigated the structure and stability of β-armchair antimony nanotubes (SbNT) using density functional theory (DFT). We studied electronic properties like electronic band structure, density of states (DOS) and mechanical properties such as stiffness constant, Poisson's ratio, and mechanical strength for these nanotubes. We found that these nanotubes are energetically stable and semiconducting in nature with band-gap varying between 1.32 eV to 1.47 eV. We have also calculated effective mass and carrier mobility for these nanotubes. Furthermore, stiffness constant and mechanical strength of these nanotubes increases with increase in diameter. While, $(4,4)$ nanotube shows anomalously higher strength than other nanotubes. The results of effective mass and carrier mobility for these nanotubes shows that electrons have higher effective mass and therefore lesser mobility than holes for most of the nanotubes. Our calculations show that β-armchair antimony nanotubes (SbNT) could be use in nano-electronics.