Vacancy Binding Energy Research Articles

Elucidation of amphoteric nature of Pr2O3 using XPS and conductivity measurement in lead-free NKBT host lattice

The study of the piezoelectric response in praseodymium-doped (Na0.41K0.09Bi0.5)TiO3 (NKBT) revealed an unusual behaviour at doping level near 0.5 and 1.0 at%. Although there is a marginal increment in the piezoelectric charge coefficients for samples with praseodymium doping of 0.5–1.0 at% compared to undoped NKBT, importantly, the response drastically diminishes for doping concentrations exceeding 1.0 at%. To comprehend the observed anomaly, X-ray photoelectron spectroscopy (XPS) and temperature dependent conductivity measurement was carried out. These investigations provided critical insights into the role of oxygen vacancy and the behaviour of praseodymium ions in the NKBT matrix. The XPS analysis revealed the presence of core level peak corresponding to O-1s with asymmetric broadening at higher energy side implying the presence of oxygen bound to the perovskite lattice (binding energy: 529.2±0.3 eV) along with oxygen vacancy (binding energy: 531.5±0.4 eV). The XPS study further revealed that the concentration of the oxygen vacancy is lowest around 0.5 at% of praseodymium doping and beyond this the oxygen vacancy increases. The high temperature conductivity measurement supported these findings by showing similar trends in the electrical conductivity. Both the XPS and conductivity measurements further corroborate the conjecture that praseodymium behaves as amphoteric ion. The praseodymium occupies the A-site of perovskite (Na0.41K0.09Bi0.5)TiO3 for lower praseodymium concentration (<1.0 at%), whereas for higher doping concentration (>1.0 at%) it occupies the B-site exhibiting acceptor like behavior that deteriorates the piezoelectric response.

The coordination environment dominated clustering behavior of vacancy and rhenium in molybdenum

Refractory molybdenum-rhenium (Mo-Re) alloys are considered as the best core structural materials in the space nuclear reactor. Irradiation will induce obvious microstructure damage and formation of Re-rich precipitates, but the underlying mechanism remains unclear due to the complicated interaction between Re and defects. In this work, by using first-principles and molecular static calculations, we obtain the thermodynamically stable configurations and binding energies of vacancy and vacancy-Re clusters in bulk Mo. A general expression of the vacancy binding energy for a stable vacancy cluster is proposed, which is determined by the numbers of the formed 1st and 2nd nearest neighboring vacancy-vacancy pairs. A scenario for the clustering of vacancy and Re is also revealed. With the presence of solute Re in Mo, the vacancy clustering initiates when the vacancy number reaches a certain value. Prior to the vacancy aggregation, the newly-added vacancy combines with the present Re and other vacancies, due to the strong attraction of Re-vacancy. It is noteworthy that the formed vacancy-Re and vacancy-Re-vacancy pairs contribute to the vacancy binding energies in Mo-Re system. Especially for the formed V2-Re1 complex, it could not only indicate the initiation of vacancy clustering, but also be the nucleus for further Re segregation towards the vacancy cluster. Our results provide fundamental insight to the irradiation-induced formation of Re-rich clusters in Mo-Re system, from the aspect of the atomic interaction between Re and vacancy.

First-principles investigation of lanthanides diffusion in HCP zirconium via vacancy-mediated transport

The diffusion of lanthanide fission products plays an important role in the growth of the fuel-cladding chemical interaction (FCCI) region in metallic fuels. The use of a Zr interdiffusion barrier may mitigate the transport of lanthanides from the fuel to the cladding, but the efficacy of such a liner is not yet known. In this paper, the stability and vacancy-mediated diffusion of La, Ce, Pr, and Nd in hexagonal close-packed (HCP) Zr is investigated via density functional theory (DFT) calculations and self-consistent mean field (SCMF) analysis. DFT is used to calculate the formation, binding, and migration energies of vacancies and vacancy-solute pairs. The DFT energetics are used in the KineCluE code to calculate the Onsager transport coefficients. La is found to be the fastest diffusing species in HCP Zr and experiences an almost isotropic diffusion behavior. The other three species (Ce, Pr, and Nd) demonstrate anisotropic diffusion where the diffusion in the basal planes is significantly faster than that along the c-axis. The calculated lanthanide diffusivities in HCP Zr are fitted to an Arrhenius relation and the activation energies and prefactors are reported for the first time. Furthermore, the vacancy drag and the segregation tendencies were analyzed using the calculated off-diagonal transport coefficients. According to our vacancy-mediated diffusion model, lanthanides are expected to be enriched at vacancy sinks at low temperatures, while at high temperatures, lanthanides are depleted at sinks and will preferably diffuse into the bulk. The enrichment/depletion transition temperature depends on the diffusion direction (basal or axial) and hence will be controlled by the grain texture and orientation.

Regulating the microstructure and mechanical properties in cast Al–3Li–2Cu-0.15Zr alloy via Sn addition and heat treatment

The cast Al–3Li–2Cu-0.15Zr alloy is a promising material due to its high strength-to-weight ratio and stiffness. In this work, the Sn addition and heat treatment were employed to regulate its microstructure and mechanical properties. The results indicate that Sn exists in the matrix as Sn solute atoms and Sn-rich particles including Li5Sn2, Li13Sn5, Li7Sn2, and Li22Sn5. The number of these particles increases as the Sn content rises. The addition of Sn significantly enhances the grain refinement, reducing the average grain size to 50.2 μm when 0.3 wt% Sn is incorporated. During the aging process, the δ′ and δ'/β′ precipitates gradually grow and coarsen, while the T1 and θ′ precipitates nucleate and grow. Some δ′ precipitates attach to the board of θ′ to form sandwich-like phases, such as δ'/θ'/δ'. The addition of Sn promotes the nucleation of θ′ and T1 precipitates. This is achieved by leveraging the Sn-rich particles as nucleation sites and utilizing Sn solute atoms to decrease the interface energy between these precipitates and α-Al. Moreover, the addition of Sn with a high vacancy binding energy and the reduction of aging temperature can effectively decrease the diffusion rate of Li atoms and vacancies, thereby inhibiting the broadening of the δ′-PFZ. After a suitable heat treatment, the Sn-modified Al–Li alloy exhibits a microstructure characterized by fine and equiaxed grain, fine and uniform δ′ precipitates, and a narrow δ′-PFZ, contributing to excellent mechanical properties of the alloy. Significantly, the underlying mechanisms behind the aforementioned observations were discussed analytically.

Atomic diffusion in bcc Fe–Mn alloys: Theoretical analysis and experimental measurements across the Curie temperature

The influence of a magnetic transition on Fe and Mn diffusion in bcc Fe–Mn alloys as a function of Mn concentration is studied combining experimental measurements and DFT-informed modeling. The radiotracer technique in combination with precise mechanical sectioning or ion-beam sputtering is used to determine the diffusion rates of 59Fe and 54Mn in a series of Fe–Mn alloys with up to 1.9 at.%Mn. The solvent and solute enhancement factors are determined. While the diffusion rates of Fe atoms are found to be almost independent on the Mn content in the Fe–Mn alloys in both para- as well ferro-magnetic states, Mn diffusion is strongly enhanced by an increase of Mn concentration in these alloys, especially in the paramagnetic state. The experimental findings are supported by theoretical results, which are analyzed in terms of an equilibrium vacancy concentration, atomic jump frequencies, Mn–vacancy binding energy, short-range ordering tendency and kinetic correlation factors.

Investigation of the Y Effect on the Microstructure Response and Radiation Hardening of PM V-4Cr-4Ti Alloys after Irradiation with D Ions

In the current work, an analysis of the effects of Y on the radiation hardening and microstructure response of a V-4Cr-4Ti alloy has been conducted after 30 keV D ion irradiation at room temperature using transmission electron microscopy (TEM) and nanoindentation. The results show that the formation of large Y2O3 and small Y2V2O7 nanoparticles was confirmed, indicating that the addition of Y reduces the amount of dissolved oxygen. The addition of Y has been shown to affect the radiation-induced dislocation loops, radiation hardening, and Ti-rich segregation of the V-4Cr-4Ti alloy. With the addition of Y, the mean size of the radiation-induced dislocation loop decreased, which may result from the strong sink strength of the nanoparticle/matrix interface, interactions between Y atoms and SIA clusters, and the strong binding energy of vacancy–oxygen pairs. Some particles with core–shell structures were observed after ion irradiation, where Ti-rich segregations at the nanoparticle/matrix interface were confirmed. These results indicate that Y might promote abnormal segregation. Possible causes for this include the lower interface energy at the particle/matrix interface and the interaction between oxygen and solute atoms.

First-principles study of hydrogen-vacancy interactions in CoCrFeMnNi high-entropy alloy

Vacancies can easily capture H atoms in metals, forming hydrogen-vacancy complexes/clusters. In this work, the hydrogen-vacancy interactions in CoCrFeMnNi high-entropy alloy (HEA) were studied with first-principles calculations based on the density functional theory (DFT). The special quasi-random structure (SQS) method was used to construct a chemically disordered HEA, and the effects of solute H atoms on the formation energy of monovacancy, the formation energy and binding energy of multi-vacancy cluster were calculated. It is found that an H atom prefers to occupy an octahedral interstitial site neighboring a vacancy and attracts the charge from the surrounding first-nearest neighbor atoms (e.g. Co, Ni, Fe or Mn atom, excluding Cr atom), weakening the stability of the atoms around the vacancy and reducing the vacancy formation energy in CoCrFeMnNi HEA. After introducing H atoms, the formation energies of both vacancy and vacancy cluster decrease in CoCrFeMnNi HEA, but they are still higher than those in pure Fe and Ni. In addition, the reduction of the binding energy of vacancies in CoCrFeMnNi HEA is much lower than that in pure Fe and Ni, and the binding energy of vacancies even increases in some cases. The results of the first-principles calculations indicate that the solute hydrogen atoms, although promoting vacancies, unfavorably combining vacancies into clusters to form micro-voids. This provides a good explanation for the good resistance to hydrogen embrittlement of CoCrFeMnNi HEA observed in experiments.

Investigation of vacancy formation energy and binding energy in fcc crystals by pseudopotential technique

The parameters of Heine and Abarenkov model potential have been computed for 21 face centered cubic (fcc) closed pack crystals and a comparison is made with their magnitudes to the Ashcroft model potential. The magnitudes of monovacancy and divacancy formation energy are calculated for different fcc crystals and compared with the literature. Huge values of divacancy formation energy and divacancy binding energy in some higher valence fcc metals are attributed to the probability of instability of these defects there. Lastly, the changes in monovacancy formation energy and divacancy formation energy in presence of an impurity in three simple metals Cu, Ag, Au are calculated.

First-principles screening of Pt doped Ti2CNL (N = O, S and Se, L = F, Cl, Br and I) as high-performance catalysts for ORR/OER

Developing high-activity and low-cost oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) bifunctional catalysts is strategic to metal-air batteries. Herein, based on the idea that functionalized atoms can adjust the electronic properties of MXenes-based catalysts, we have screened some catalysts with high catalytic activity for ORR/OER using first-principles calculations. Specifically, we firstly explored the stability of Pt doped Ti2CNL ((N = F, Cl, Br and I, L = O, S and Se)) and Ti2CN(L)2 though calculating the vacancy formation energy and binding energy. Moreover, compared the overpotential of Pt doped Ti2CN(L)2 and Ti2CNL for ORR and OER, the Janus-MXenes configurations can tune the d-band center, work-function, density of state (DOS) and charge distribution of the catalysts, and then improve the catalytic performance of catalysts. Pt-VS-Ti2CSBr, Pt-VI-Ti2COI and Pt-VS-Ti2CSBr as ORR, OER and ORR/OER bifunctional catalysts were revealed due to their ultra-low overpotential of 0.35, 0.34 and 0.79 V, respectively. Additionally, compared to H ions, the catalyst preferentially adsorbs O2, thus ensuring the ORR advantage to proceed rather than hydrogen evolution reaction (HER). Even so, Pt-VO-Ti2COI, Pt-VS-Ti2CSI and Pt-VSe-Ti2CSeI have high activity for HER. Our work not only screened out catalysts with encouraging catalytic performance but also guided the design of high-performance catalysts.

Effects of Cd addition in welding wires on microstructure and mechanical property of wire and arc additively manufactured Al−Cu alloy

Wall structures were made by cold metal transfer-based wire and arc additive manufacturing using two kinds of ER2319 welding wires with and without Cd elements. T6 heat treatment was used to improve mechanical properties of these wall structures. Due to the higher vacancy binding energy of Cd, Cd-vacancy clusters are formed in the aging process and provide a large number of nucleation locations for θ′ phases. The higher diffusion coefficient of the Cd-vacancy cluster and the lower interfacial energy of θ′ phase lead to the formation of dense θ′ phases in the heat-treated α(Al). According to the strengthening model, after adding Cd in ER2319 welding wires, the yield strength increases by 43 MPa in the building direction of the heat-treated wall structures.

Xenon mobility in γ-uranium and uranium–molybdenum alloys

Diffusion in bcc uranium and U–Mo alloys is of great interest because fission gas and other fission products impact the performance of nuclear fuels. We investigate the mobility of xenon and molybdenum in bcc uranium (γ-U) and metallic U–Mo alloys by calculating the migration energies of xenon and molybdenum for various local compositions using density functional theory. We also calculate the solute–vacancy binding energies of different solutes to vacancies in bcc uranium. We find that the solute–vacancy binding energy in bcc uranium is significantly higher than it is in other bcc metals (e.g., Fe and W). We also find that the migration energy of molybdenum is substantially higher than the migration energy of xenon, indicating that xenon is much more mobile than molybdenum in bcc uranium. The presence of molybdenum in the nearest-neighbor shell around a xenon atom typically increases the migration energy of xenon, which indicates a reduction of xenon mobility in U–Mo alloys compared to pure bcc uranium.

Atomistic Study on the Energetic and Mechanical Behaviors of Helium Bubble Nucleation and Growth in RAFM Steel

Helium produced by neutron irradiation is a crucial inducement to bring about the property of deterioration of structural materials served in a fusion reactor. To investigate the nucleation and growth behavior of helium bubbles in reduced activation ferritic/martensitic steels, which comprise one of the most promising candidate structural materials, the Molecular Statics method and the Metropolis Monte Carlo algorithm are combined to investigate the energetic and mechanical behaviors of HenVm clusters in α-Fe. The simulation results show that the vacancy and helium atom binding energy are inclined to reach a saturation state, i.e., 4.0 eV for the vacancy and 2.4 eV for the helium atom; however, the binding energy of self-interstitial atoms decreases to minus values at high helium-to-vacancy (He/V) ratios. The crossover of the binding energy curve of the helium and vacancy indicates that the equilibrium He/V ratio is 1.68 during the nucleation of helium bubbles. Meanwhile, the dissociation energy analysis indicates that the stable He/V ratio of the clusters is 1.3 at high temperatures. Moreover, the pressure analysis of the HenVm clusters indicates that the He/V ratio corresponding to their mechanical equilibrium state varies from 0.50 to 0.65 at 0 K. Furthermore, the analysis combined with the relevant experimental data of helium density in helium bubbles indicates that the actual He/V ratio of helium bubbles in the served materials is closely relevant to the irradiation condition, such as helium production rate, temperature, etc. The investigation results in this paper contribute to elucidate the microscopic process of helium bubble nucleation and growth and provides the energetic and mechanical parameters of small-sized helium bubbles with different sizes for large-scale simulation studies.

Molecular dynamics simulations on the interactions between basal edge dislocation and point defects in magnesium at low temperature

In this work, we performed molecular dynamics (MD) simulations on the interactions between a basal edge < a > dislocation and point defects including vacancies and self-interstitial atoms in magnesium. Simulation results suggest that both self-interstitial atoms and vacancies have a blocking effect on dislocation motion, but the blocking effect of vacancies is much weaker than that of self-interstitial atoms. During interactions, self-interstitial atoms are absorbed and move with dislocation, and the edge dislocation climbs after absorbing self-interstitial atoms. In contrast, vacancies cannot be absorbed. To explain these observations, the binding energy and migration energy are calculated. The binding energy of both vacancies and self-interstitial atoms increases as they approach the dislocation core, seemingly indicating that they can be absorbed. However, the migration barrier of a vacancy is found to be about two orders in magnitude higher than that of a self-interstitial atom. Therefore, only self-interstitial atoms are absorbed, while vacancies cannot.

Effect of H on the formation of vacancy dislocation loops in α-Fe

Genetic algorithm was applied to search the energy minimization configuration of vacancy clusters in α-Fe. Molecular statics calculations and dynamics annealing relaxation were employed to calculate the formation and binding energies of vacancies and 3D vacancy or vacancy-hydrogen (H) clusters, as well as 2D vacancy or vacancy-H clusters on (111), (011) and (211) planes. Our calculations show that vacancies prefer to form 3D clusters and vacancy dislocation loops are difficult to form, while vacancy-H clusters prefer to shape into 2D clusters, especially on (211) planes. Since H prefers the directional bonding, a vacancy cluster with H atoms tend to form a vacancy dislocation loop with its slip direction along the 〈100〉 direction and on the (211) habit plane. Our results are consistent with the experimental observations, and provide a possible mechanism for the formation of vacancy dislocation loops in α-Fe. Furthermore, we have also explored how dislocation loops trap self-interstitial atoms, vacancies and H atoms. It is of interest to note that H atoms strongly bound to a 〈100〉 vacancy dislocation loop, and are able to enhance the ability for the dislocation loop to further trap vacancies and reduce its ability to absorb self-interstitial atoms, thus promoting the growth of the 〈100〉 vacancy dislocation loops.

Materials selection for nuclear applications in view of divacancy energies by comprehensive first-principles calculations

We investigated formation and binding energies of di-vacancy in 51 type pure metals with bcc, fcc, and hcp structures to understand the clustering and dissolution of vacancies in metals by means of a comprehensive first-principles study. First, mono- and di-vacancy formation energies as the first (1nn) and second (2nn) nearest neighbor were compared using three typical functionals. Comparing with other Groups, the formation energies of mono- and di-vacancy reach the maximum in Group VI metals whose d-band is half filled. The interaction between two vacancies is attractive in most metals and the energy difference between the 1nn and 2nn configurations is obvious. We predicted the relationships of di-vacancy formation energy with melting temperature and cohesive energy and discussed vacancy clustering and void formation in metals by the binding energy and dissolution energy of vacancies and their clusters. The present work provides the fundamental data of di-vacancy in metals and a theoretical guideline for nuclear materials selection and design.

Phase transformations in novel hot-deformed Al–Zn–Mg–Cu–Si–Mn–Fe(–Sc–Zr) alloys

Precipitation stages in the hot-deformed Al-5.28wt.%Zn-3.21wt.%Mg-1.48wt.%Cu-0.24wt.%Si-0.24wt.%Mn-0.14wt.%Fe(−0.23wt.%Sc-0.19wt.%Zr) alloys were investigated. Positron annihilation spectroscopy was employed for a study of vacancy assisted clustering of solutes on the atomic scale, type and concentration of open volume misfit defects and local chemical surroundings of defects. Microhardness testing, thermal analysis and microstructure observations were also used. The alloys contain the eutectic grain boundary phases: T-phase and α-Al (Mn,Fe,Si) phase. Positrons are trapped at Zn,Mg-containing Guinier-Preston (GP) zones with no open volume defects and at (precursors of) the η′-phase particles. Above ~150 °C precipitation of non-eutectic T-phase and coarsening of the S-phase particles are observed. Precipitate-matrix interfaces are characterized by vacancy-like misfit defects decorated by Si solutes. Binding energies of vacancies to various solutes (Si, Zn, Cu, Mn, Sc and Zr) obtained by ab-initio calculations were computed. Si solutes exhibit the highest vacancy binding energy among the constituents of the studied alloys. Other solutes (especially Mg, Sc, Zr) repel vacancies. The alloy with Sc,Zr-addition contains incoherent primary Al3(Sc,Zr)-phase. Layered primary particles with different morphological features such as square and polygonal shapes contain Sc,Zr-rich layers.

New insights into the mechanism of yttrium changing high-temperature oxide growth of Fe-13Cr–6Al–2Mo–0.5Nb alloy for fuel cladding

The oxidation behavior and microstructure evolution of Fe–13Cr–6Al–2Mo–0.5Nb alloys with various yttrium contents ranging from 0 to 0.23 wt.% have been investigated at 1200 °C for 4 h in steam environment. The oxide weight gain decreased with minor addition of yttrium, but this tendency was lost with excess addition. The grain boundary migration was hindered due to high binding energy of yttrium atom and vacancy. The combination of yttrium with vacancies and a large number of fine Laves phases at grain boundaries suppressed the outward diffusion of Al3+, resulting in the change of oxide growth mechanism and oxide scale adhesion.

First-principles calculations of solute–vacancy interactions in aluminum**Project supported by the National Natural Science Foundation of China (Grant Nos. 51701095 and 51771185) and the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20170798).

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 3d–4p series and the 4d–5p 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.

Effect of deep UV laser treatment on silicon‐doped Tin oxide thin film

AbstractIn this paper, the effect of deep ultraviolet (UV) laser on physical and electrical properties of amorphous Silicon‐doped tin oxide (amorphous Si‐Sn‐O, a‐STO) thin films were studied. Surface morphology, thickness, crystallinity, and optical band gap of a‐STO thin films treated by laser were investigated. Results showed that the decrease of thickness and surface roughness of a‐STO thin films after deep UV laser treatment, and the films maintained an amorphous structure, which implied that the quality of a‐STO thin films were improved. The peak position of oxygen vacancy binding energy became lower; this is caused by an increase in oxygen vacancies resulting in a decrease in coordination number. And the oxygen vacancy content of the a‐STO thin films was increased after deep UV laser treatment. In addition, the optical band gap of a‐STO films was broaden after the deep UV laser treatment. It exploits a new application of deep UV laser in oxide semiconductor.

Influence of Zn and Sn on the Precipitation Behavior of New Al-Mg-Si Alloys.

In this study, we demonstrate how Zn and Sn influence hardening behavior and cluster formation during pre-aging and paint bake treatment in Al–Mg–Si alloys via hardness tests, tensile tests, and atom probe tomography. Compared to the standard alloy, the Sn-modified variant shows reduced cluster size and yield strength in the pre-aged condition. During the paint bake cycle, the clusters start to grow very fast and the alloy exhibits the highest strength increment. This behavior is attributed to the high vacancy binding energy of Sn. Adding Zn increases the formation kinetics and the size of Mg–Si co-clusters, generating higher yield strength values for both the pre-aged and paint baked conditions. Simultaneous addition of Zn and Sn creates a synergistic effect and produces an alloy that exhibits moderate strength (and good formability) in the pre-aged condition and accelerated hardening behavior during the paint bake cycle.