Defect Binding Energies Research Articles

Hydrogen Plasma Inhibits Ion Beam Restructuring of GaP.

Focused ion beam (FIB) techniques are employed widely for nanofabrication and processing of materials and devices. However, ion irradiation often gives rise to severe damage due to atomic displacements that cause defect formation, migration, and clustering within the ion-solid interaction volume. The resulting restructuring degrades the functionality of materials and limits the utility of FIB ablation and nanofabrication techniques. Here we show that such restructuring can be inhibited by performing FIB irradiation in a hydrogen plasma environment via chemical pathways that modify defect binding energies and transport kinetics, as well as material ablation rates. The method is minimally invasive and has the potential to greatly expand the utility of FIB nanofabrication techniques in processing functional materials and devices.

A continuum scale chemo-mechanical model for multi-trap hydrogen transport in deformed polycrystalline metals

This study presents a coupled multi-trap hydrogen diffusion and crystal plasticity model within a thermodynamically consistent framework. The developed modeling scheme incorporates the hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced strain-induced plasticity (HESIV) based on thermodynamic considerations. The theoretical formulations are implemented in the finite element method, which considers the concentration- and chemical potential-based balance equations. The validity of the coupled model is established through preliminary single element simulations under isothermal conditions, showcasing the predictability of evolution of hydrogen concentration and occupancies in the deformed microstructure of polycrystalline metal. Then, the developed computational approach is applied to two materials exhibiting hydrogen-induced hardening and softening behaviors in accordance with the HELP mechanism and vacancy-hydrogen complex (VaH) formation under isothermal condition. The model successfully reproduces the change of mechanical behavior induced by hydrogen in both materials. The effects of hydrogen diffusivity and the binding energy of defects on hydrogen transport are also investigated. Furthermore, the sensitivity of hydrogen diffusion to different trapping mechanisms, local equilibrium trapping and kinetics trapping, is examined. The proposed modeling framework can be applied to new material design strategies for developing metals resistant to hydrogen embrittlement.

Atomic study of the trapped and migration patterns of point defects around screw dislocation in tungsten

The interaction of 1/2 〈1 1 1〉 screw dislocation and point defects, including vacancy and self-interstitial atom (SIA), was investigated by molecular dynamics (MD) simulations. Molecular statics (MS) are used to analyze the factors affecting the binding energy of point defects around the dislocation. The results show that the point defects around the dislocation core have the highest binding energy. In addition, we use elasticity theory to find that the point defects binding is interrelated to strain distribution of dislocation and the residual stress of point defects. Furthermore, nudged elastic band (NEB) calculations have been applied to find that vacancy and SIA tend to migrate along the dislocation line around the core region. The results can clearly show the interaction behavior of point defects and screw dislocation, and provide a reasonable explanation for the diffusion mechanism of mono-vacancy and a single SIA around the dislocation line.

Atomistically-informed modeling of point defect clustering and evolution in irradiated ThO2

A cluster dynamics (CD) model has been developed to investigate the nucleation and growth of point defect clusters, i.e., interstitial prismatic loops and nanoscale and sub-nanoscale voids, in ThO2 during irradiation by energetic particles. The model considers cluster off-stoichiometry due to the asymmetry of point defect generation on the O and Th sublattices under irradiation, as well as the point defect diffusivities and the defect binding energies to clusters. The energies were established using detailed molecular dynamics simulations considering the statistical variability of cluster configuration. A high-order adaptive time-integration has been used to solve the model. The predicted loop density and their average size is in good agreement with reported experimental observations for proton irradiated ThO2 at 600°C. The model did not predict void evolution due to the sluggish kinetics of cation vacancies, explaining the absence of voids in proton irradiated ThO2 (and other oxides) at relatively low temperatures.

A web application to calculate the mass defect and nuclear binding energy per nucleon

The discovery of radioactivity proved that the energy stored in the nucleus could be released as radioactive particles’ kinetic energy. Fission and fusion reactions generate enormous amounts of energy that could be used to generate electricity in nuclear power plants. The energy released or absorbed in these nuclear reactions comes from the different binding energies associated with each nucleus. Nuclear binding energy is the amount of energy required to keep a nucleus’s protons and neutrons together, and nuclear power is the amount of energy released during nuclear fission or fusion. Each type of nucleus has its own binding energy, which is the amount of energy lost when its nuclei came together and are the same as the amount of energy needed to split them. The paper describes a web application for computing nuclear binding energy per nucleon. The calculation of nuclear binding energy in terms of MeV/ nucleon entails determining the mass defect and converting the mass defect into energy per nucleon.

Visible light induced photocatalytic performance of Mn-SnO2@ZnO nanocomposite for high efficient cationic dye degradation

In this work, we have synthesized Mn-doped SnO2@ZnO nanocomposite for photo degradation of Methylene blue and Rhodamine B dyes upon visible light irradiation. The crystal structure, functional group, optical absorption, defect related emission, morphology, purity and binding energy state of synthesized samples were identified by using various analytical tools. The crystal structure revealed the rutile tetragonal, hexagonal wurtzite for SnO2 and ZnO samples and the average crystal sizes were found in the range of 23.3 nm to 16.7 nm for the synthesized samples. The optical absorption peaks were shifted to higher wavelength side and optical band gap values were found between 3.52 eV and 2.77 eV which confirm the formation of hetero-junction of SnO2@ZnO composites. The field emission scanning electron spectroscopy (FESEM) revealed the spherical grain morphology for pure and composite samples. The energy dispersive spectra (EDS) and element mapping confirms the purity of the synthesized samples. The X-ray photoelectron spectroscopy (XPS) revealed that the composition and energy state of Mn4+, Sn4+ and Zn2+ for composite samples. The photocatalytic degradation results clearly indicate that the Mn-doped SnO2@ZnO nanocomposite has higher degradation efficiency of 98% and 92% for the Methylene blue and Rhodamine B dyes, respectively and is higher than the other synthesized samples. The present study reveals a low cost and highly efficient photo-catalyst which works up on visible light irradiation for the purification of waste water from industries.

Molecular dynamics study of the point defects in bcc uranium

Thermodynamic properties of bcc uranium with point defects are studied using ab initio molecular dynamics (MD) simulations at 1100 K. The simulations were performed with canonical ensembles of ${\mathrm{U}}_{127}{\mathrm{M}}_{1}$, ${\mathrm{U}}_{128}{\mathrm{M}}_{1}$, and ${\mathrm{U}}_{126}{\mathrm{M}}_{1}{\ensuremath{\square}}_{1}$ for M = $\ensuremath{\square}$, He, Ne, Ar, Kr, Xe, Sr, Zr, I, Cs, and Pu disposed on a bcc lattice lying within a $4\ifmmode\times\else\texttimes\fi{}4\ifmmode\times\else\texttimes\fi{}4$ cubic supercell. This work provides formation energies of substitutional, self, and solute interstitial atom defects as well as binding energies of M-$\ensuremath{\square}$ pair defects. This work demonstrates that our computational scheme based on MD simulations gives reliable formation and binding energies of atomic defects in bcc uranium compared to conventional density functional theory calculations. The equilibrium volume, bulk modulus, and thermal expansion coefficient of pure bcc uranium obtained from our MD simulations compare very well with corresponding experimental results. The vacancy formation energy is predicted to be 0.88 eV. The experimental vacancy formation energy remains uncertain. Experimental study of the formation and binding energies of other point defects as well as the bulk modulus and thermal expansion coefficients of uranium with these defects is also not found in the literature. This work shows that point defects tend to decrease the bulk modulus and increase the thermal expansion coefficient of bcc uranium. The solute formation energies of noble gas atoms show a bearing on their size. A large solute (Xe) has a high formation energy, and vice versa. This size effect is not quite evident for the chemically reactive solutes, namely, Sr, Zr, I, Cs, and Pu. Our MD simulations further show that vacancies are the favorable point defects in bcc uranium rather than both vacancies and self interstitials as predicted by earlier calculations. The formation energies of self interstitial atoms are found to be lower than those of solute interstitial atoms, each calculated in six different basic interstitial dumbbell configurations. That is, bcc U accommodates self interstitials more easily than decay or fission gas interstitials (He, Kr, and Xe). Further, He atoms are found to have comparable formation energies in the substitutional and interstitial locations. The fission product atoms Kr and Xe prefer to occupy vacant substitutional lattice sites rather than interstitial sites. Binding energies of divacancy and solute-vacancy pairs (0.31 vs $\ensuremath{-}0.69$ eV for the Xe-$\ensuremath{\square}$ pair, for instance) from our MD simulation show that nucleation and growth of fission gas bubbles are supported by a thermodynamic driving force, whereas vacancies tend to stay apart. This is in agreement with literature reporting that bcc uranium softens and swells mainly by agglomeration of noble gas bubbles.

Structural and electronic properties of SnO2 doped with non-metal elements.

Crystal structure and electronic properties of SnO2 doped with non-metal elements (F, S, C, B, and N) were studied using first-principles calculations. The theoretical results show that doping of non-metal elements cannot change the structure of SnO2 but result in a slight expansion of the lattice volume. The most obvious finding from the analysis is that F-doped SnO2 has the lowest defect binding energy. The doping with B and S introduced additional defect energy levels within the forbidden bandgap, which improved the crystal conductivity. The Fermi level shifts up due to the doping with B, F, and S, while the Fermi level of SnO2 doped with C or N has crossed the impurity level. The Fermi level of F-doped SnO2 is inside the conduction band, and the doped crystal possesses metallicity. The optical properties of SnO2 crystals doped with non-metal elements were analyzed and calculated. The SnO2 crystal doped with F had the highest reflectivity in the infrared region, and the reflectance of the crystals doped with N, C, S, and B decreased sequentially. Based on this theoretical calculations, F-doped SnO2 is found to be the best photoelectric material for preparing low-emissivity coatings.

Defect properties in Yb3+-doped CaF2 from first-principles calculations: a route to defect engineering for up- and down-conversion photoluminescence

The formation energy, transition level and binding energy of defects in Yb3+-doped CaF2 were calculated with DFT/HSE06.

Influence of codoping on the conductivity of Sc-doped zirconia by first-principles calculations and experiments

Doped zirconia is a promising electrolyte candidate for Solid Oxide Fuel Cells (SOFCs). The effects of dopants and their interactions with oxygen vacancies on the conduction properties have been systematically investigated by density functional theory (DFT) calculations and experiments. The influences of dopants' valences and radii on the defect formation energies and defect binding energies are discussed. Our results show that, for Scandia stabilized zirconia (ScSZ) co-doped with In3+, Yb3+, or Bi3+, the defect formation energy differences and binding energy differences of different sites are low, which indicates that for these co-doping cases the oxygen vacancies distribute more randomly, and are more beneficial for improving the oxygen vacancy diffusing efficiency. The inner influence mechanisms of the co-dopants are discussed from the aspects of elastic and electrostatic effects. Oxygen diffusion barriers in ScSZ co-doped with various elements are also calculated by the saddle point method. The calculated results indicate that the diffusion barrier orders for different edges are ESc-Zr < EZr-Zr < ESc-M for RM > RZr and ESc-Zr ≈ ESc-M < EZr-Zr for RM < RZr. Furthermore, relative experiments are carried out to verify the theoretical results.

Role of point defects in self-diffusion along low-angle twist boundaries in fcc metals: A molecular dynamics study

The interaction of point defects with low-angle (100), (110) and (111) twist boundaries in fcc metals Ni, Cu, Al and role of the point defects in self-diffusion along considered boundaries were studied by the molecular dynamics method. The binding energies of point defects with low-angle twist boundaries were calculated. It was found that the point defects are mainly fixed in the nodes of grain-boundary screw dislocations network. It was shown that the introduced vacancies play an important role in diffusion along twist grain boundaries. The contribution of introduced interstitial atoms to grain-boundary diffusion under thermodynamic equilibrium condition is less by several orders of magnitude in comparison with the contribution of vacancies.

Enhancing the electrical conductivity and thermoelectric figure of merit of the p-type delafossite CuAlO2 by Ag2O addition

(CuAlO2)1-x(Ag2O)x specimens with 0 ≤ x ≤ 0.06 were prepared through the sintering of mixtures of CuO, Al2O3 and Ag2O powders at 1373 K. Hall effect, Seebeck coefficient and electrical conductivity measurements were subsequently employed to assess the electrical transport properties. The electrical conductivity of the as-sintered samples was found to increase with Ag2O addition as a result of increases in the carrier density. Over the temperature range of 323–623 K, the transport properties can be attributed to thermally activated transitions from the acceptor state to the valence band. In contrast, the variable range hopping theory is applicable over the temperature range of 623–873 K. Ag2O addition evidently reduces the defect binding energy in the electronic structure of the CuAlO2. The addition of this compound also obstructs the formation of both a spinel phase and CuO, such that the oxygen off-stoichiometry value and the carrier density are increased with increasing Ag2O levels. The presence of Ag metal has the main effect on thermal conductivity below 400 K, while above 400 K increases in the phonon concentration affect the conductivity. The highest value obtained for the figure of merit was 0.0044 at 573 K, from a sample containing 0.2 at.% Ag2O.

Retention and thermal desorption of helium in pure tungsten

Tungsten (W) is cited as a candidate first-wall material in fusion reactors owing to its outstanding thermal properties and erosion resistance. An important issue is that the energetic isotopes of hydrogen, tritium (T) and deuterium (D) and helium (He) particles damage the surface of W in fusion reactors. He particles cause more notable damage than D or T because the binding energy of defects and He is larger than that of defects and D or T. In this study, well-annealed W specimens were implanted with 5 keV He ions at room temperature and irradiation dosages of 1.0 × 1020 and 2.5 × 1021 ions/m2. Then, thermal desorption spectroscopy analysis was performed by heating the samples to 1973 K at a ramping rate of 0.5 K/s. Thermal desorption of He in the sample irradiated with a low dosage occurred at 1400 and 1960 K, whereas that in the sample irradiated with a high dosage occurred at 740, 1050 and 1500 K. According to the microstructures observed using transmission electron microscopy, both peaks in the former case were attributed to He de-trapping from irradiation to nduced helium-vacancy clusters of different sizes, whereas the peaks in the latter case were attributed to He de-trapping from surface defects, irradiation-induced dislocation loops and tiny helium-vacancy clusters, respectively.

Atomistic simulation of the trapping capability of He-vacancy defects at Ni grain boundary

He atoms tend to cluster and precipitate into bubbles that prefer to grow in the grain boundaries, resulting in high temperature He embrittlement with significantly degraded material properties. This is a major bottleneck in employing Ni-based alloys for applications such as molten salt reactors (MSRs). This paper focuses on understanding how the local grain boundary structure interacts with He atoms and how the local atomistic environment in the grain boundary influences the binding energy of He defects. Using molecular dynamics simulations, we have investigated the trapping capability of the Ni grain boundary to He defects (HeN) and to He-vacancy defects (HeNVM). The two defects in the Ni grain boundary exhibit geometries with high symmetry. The binding energy of an interstitial He atom to HeNVM defects is found to be generally larger in pure Ni than that in the grain boundary. We compared the binding energy of HeN defects to the Ni vacancy and to the Ni grain boundary, finding that the Ni vacancy possesses a higher trapping strength to HeN. We also found that the binding strength of HeN to the grain boundary is stronger than that of HeNVM to the grain boundary. The He-vacancy ratio in HeNVM defects does not significantly affect the binding energy in the grain boundary plane. The current work will provide insight in understanding the experimentally observed He bubble formation in Ni-based alloys and bridge atomic scale events and damage with macroscopic failure.

Effects of He, D interaction on thermal desorption of He and D2 and microstructural evolution in pure Fe

He and H atoms are produced in (n, α) and (n, p) nuclear reactions. In fusion reactors, energetic T and D, being isotopes of H, and He particles damage the surface materials. To investigate the He-D interaction, Fe, which is a model metal of choice in ferritic stainless steel that is used in fusion reactors, was irradiated separately by He or D2 ions and by combinations of He + D2 or D2 + He ions with the energy of 5 keV. The dose for single-species irradiation and each step of double-species irradiation was 1.0 × 1020 ions/m2. Thermal desorption analysis indicates that, in the case of single ion species irradiation, thermal desorption of D occurs at temperatures below 700 K, while the main thermal desorption of He occurs at 750 K and above 1200 K. The binding energy of He and defects is higher than that of D and defects. In the case of irradiation with combinations of ions species, however, the obtained thermal desorption spectra are the same, although the peak intensities are different, suggesting that the He-D interaction is weak. The sorption of D is more predominant for irradiations with He + D2. On the microstructure level, the irradiated samples exhibited larger voids following combined irradiations compared with those for irradiation with a single ion species after annealing to 1323 K. During the He + D2 irradiation, D atoms are effectively trapped owing to the defects induced by pre-irradiation with He.

Interatomic potential for Fe–Cr–Ni–N system based on the second nearest-neighbor modified embedded-atom method

The interatomic potential for Fe–Cr–Ni–N system based on the second nearest-neighbour modified embedded-atom method has been developed in this work. The potential is based on those for the corresponding lower order systems. The potential parameters for the binary systems, Cr–N, Ni–N, Ni–Fe and Ni–Cr, were determined by fitting the lattice constants, elastic properties, heat of solution and defect binding energies. The potential parameters for the ternary systems were calculated based on the corresponding binary systems. Then, all of them were applied to the quaternary system Fe–Cr–Ni–N to confirm their validity by a simulation of the lattice constants of AISI 316 austenitic stainless steel with a range of nitrogen content. The results were in good agreement with the previous observations and calculations.

Identification of dominant damage accumulation processes at grain boundaries during irradiation in nanocrystalline α-Fe: A statistical study

Radiation defect accumulation in metals with high interface to volume ratios, such as nanocrystalline metals, is strongly dependent on the ability of these interfaces to act as unbiased sinks for defects. Multi-scale simulations of damage accumulation in such materials necessarily depend on parameters describing defect behaviors inside grain boundaries, such as migration and binding energies within grain boundaries as well as binding energies of defects to grain boundaries. In general, these behaviors are sensitive to the grain boundary structure, making atomic-scale quantification of such parameters challenging due to the large number of variables in the input space of such a problem. The goal of the present study is to identify which of these parameters most strongly influence defect accumulation and grain boundary sink efficiency in α-Fe during Frenkel pair implantation at room temperature and a dose rate of 10−7 dpa⋅ s−1. Defect accumulation inside grains and in grain boundaries is simulated using spatially resolved stochastic cluster dynamics (SRSCD). Using this methodology, sensitivity studies investigating vacancy accumulation in grain boundaries, sink efficiency η of grain boundaries, and vacancy cluster profiles inside grains are performed by varying model input parameters such as defect migration and binding energies inside grain boundaries. Principal component analysis of these model input parameters is then carried out to identify which defect behaviors are strongly correlated with changes in damage accumulation metrics when varying many parameters at once. In both of these analyses, single vacancy and self-interstitial diffusion inside grain boundaries, small vacancy cluster diffusion inside grain boundaries, and the binding energy of vacancies and self-interstitials to grain boundaries are shown to have the greatest impact on defect accumulation.

The potential existence of mixed defect incorporation modes for rare-earth doped cubic BaTiO3

The existence of two novel rare-earth incorporation mechanisms in cubic barium titanate is demonstrated using classical simulations. The dependence of solution, binding and modified solution energies with respect to the ionic radii of dopants is determined. Strong defect binding energies are obtained, similar to Er-doping, which has been reported previously. Dopants with small ionic size can potentially be incorporated via either the Ba-vac or Ti-vac mode, whereas large dopants prefer the Ti-vac mixed mode. Although the mixed modes reveal somewhat higher energies with respect to the traditional self-compensation mechanism, they are likely to be active under certain experimental conditions. The abstract figure shows two mixed incorporation mechanisms in doped BaTiO3 labelled as (a) Ti-vac and (b) Ba-vac. Green, grey, blue, and red spheres correspond to Ba2+, Ti4+, rare-earth (RE3+), and O2- ions, respectively.

First-principles calculation of defect free energies: General aspects illustrated in the case of bcc Fe

The first-principles calculation of contributions of phonon and electron excitations to free formation, binding, and migration energies of defects is illustrated in the case of bcc-Fe. First of all, the ground state properties of the vacancy, the foreign atoms Cu, Y, Ti, Cr, Mn, Ni, V, Mo, Si, Al, Co, O, and the O-vacancy pair are determined under constant volume (CV) as well as zero pressure (ZP) conditions. Second, the phonon contribution to defect free energies is calculated within the harmonic approximation using the equilibrium atomic positions determined in the ground state under CV and ZP conditions. Additionally, quasi-harmonic corrections are applied to the ZP-based data. A simple transformation similar to the quasi-harmonic approach is found between the CV- and ZP-based frequencies. Therefore, it is not necessary to calculate these quantities and the corresponding defect free energies separately. In contrast to ground state energetics the CV- and ZP-based defect free energies do not become equal with increasing supercell size. Third, it was found that the contribution of electron excitations to the defect free energy can lead to an additional deviation of the total free energy from the ground state value or can compensate the deviation caused by the phonon contribution. Finally, self-diffusion via the vacancy mechanism is investigated. The ratio of the respective CV- and ZP-based results for the vacancy diffusivity is nearly equal to the reciprocal of that for the equilibrium concentration. This behaviour leads to almost identical CV- and ZP-based values for the self-diffusion coefficient. Obviously, this agreement is accidental. The consideration of the temperature dependence of the magnetization yields self-diffusion data in very good agreement with experiments.

Simultaneous visualization of graphene grain boundaries and wrinkles with structural information by gold deposition.

Although line defects such as grain boundaries (GBs) and wrinkles are unavoidable in graphene, difficulties in identification preclude studying their impact on electronic and mechanical properties. As previous methods focus on a single type of line defect, simultaneous measurements of both GBs and wrinkles with detailed structural information have not been reported. Here, we introduce effective visualization of both line defects by controlled gold deposition. Upon depositing gold on graphene, single lines and double lines of gold nanoparticles (NPs) are formed along GBs and wrinkles, respectively. Moreover, it is possible to analyze whether a GB is stitched or overlapped, whether a wrinkle is standing or folded, and the width of the standing collapsed wrinkle. Theoretical calculations show that the characteristic morphology of gold NPs is due to distinct binding energies of line defects, which are correlated to disrupting diffusion of NPs. Our approach could be further exploited to investigate the defect structures of other two-dimensional materials.