Evolution Of Vacancies Research Articles

Selective linear etching of monolayer black phosphorus using electron beams**Project supported by the National Natural Science Foundation of China (Grant Nos. 11622437, 61674171, 11804247, and 11974422), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB30000000), Key Research Program of Frontier Sciences, Chinese Academy of Sciences (B.L, W.Z.), the Fundamental Research Funds for the Central Universities, China,

Point and line defects are of vital importance to the physical and chemical properties of certain two-dimensional (2D) materials. Although electron beams have been demonstrated to be capable of creating single- and multi-atom defects in 2D materials, the products are often random and difficult to predict without theoretical inputs. In this study, the thermal motion of atoms and electron incident angle were additionally considered to study the vacancy evolution in a black phosphorus (BP) monolayer by using an improved first-principles molecular dynamics method. The P atoms in monolayer BP tend to be struck away one by one under an electron beam within the displacement threshold energy range of 8.55–8.79 eV, which ultimately induces the formation of a zigzag-like chain vacancy. The chain vacancy is a thermodynamically metastable state and is difficult to obtain by conventional synthesis methods because the vacancy formation energy of 0.79 eV/edge atom is higher than the typical energy in monolayer BP. Covalent-like quasi-bonds and a charge density wave are formed along the chain vacancy, exhibiting rich electronic properties. This work proposes a theoretical protocol for simulating a complete elastic collision process of electron beams with 2D layers and will facilitate the establishment of detailed theoretical guidelines for experiments on 2D material etching using focused high-energy electron beams.

Angular dependent interatomic potential for Ti-V system for molecular dynamics simulations

Abstract An interatomic potential for the Ti-V binary alloy focusing on evolution of defects, including ones arising as a result of the irradiation process, was constructed within the Lipnitskii-Saveliev approach, which accurately takes into account three-particle interactions and the sum of all multi-particle interactions of a higher order in the framework of the centrally symmetric approximation. In the new potential, Ti-V interactions were fitted to the DFT data on set of model structures with different coordination numbers, including ones with vacancies. The properties used for fitting are accurately reproduced by the present potentials for both pure elements and alloy systems. The potential was tested on the binding energies between Ti atoms and self-point defects in bcc V, elastic moduli, thermal expansion and melting point of some alloys, and diffusion. We obtained qualitative agreement for these properties with available theoretical and experimental data. Finally, we investigated evolution of excess vacancies in the V-4 at. \% Ti alloy at 700 K, which are typical conditions of vanadium-based alloys for fusion applications. We found that no vacancy loop is formed in the alloy in contrast to the pure V, which agrees with the experimental observations. The potential is expected to be especially suitable for irradiation simulations of vanadium based V-Ti alloys.

Vacancy cluster growth and thermal recovery in hydrogen-irradiated tungsten

The thermal evolution of vacancies and vacancy clusters in tungsten (W) has been studied. W (100) single crystals were irradiated with 200 keV hydrogen (H) ions to a low damage level (5.8×10−3 dpa) at 290 K and then annealed at temperatures in the range of 500–1800 K. The resulting defects were characterized by positron annihilation lifetime spectroscopy (PALS) and positron annihilation Doppler broadening spectroscopy (DBS). Annealing at 700 K resulted in the formation of clusters containing 10–15 vacancies, while at 800 K and higher temperatures clusters containing about 20 vacancies or more were formed. Reduction of the defect concentration likely accompanied by further coarsening of the clusters started at 1300 K and ended at 1800 K with the complete defect recovery. The determined cluster sizes at 700 K and 800 K were larger than the estimated minimum cluster sizes that are thermally stable at these temperatures, indicating that the migration and ensuing coalescence of small clusters plays an important role in cluster growth.

Remarkable switching of transport properties and surface exchange kinetics in epitaxial PrBaMn2O5+δ films

Double perovskite PrBaMn2O5+δ (PBMO), which demonstrates promising application prospect as ceramic anode material for solid oxide fuel cells, was epitaxially grown on (001) LaAlO3 substrate for determining the nature of surface exchange kinetics and oxygen evolution processes. High-temperature X-ray diffraction, X-ray photoelectron spectroscopy, and transport properties measurement indicated that the oxygen vacancy evolution processes are highly dependent upon the interface strain and temperature. Especially, PBMO thin films show reversible redox reactions in the switches between oxygen and hydrogen atmospheres, and the antiphase domain boundary plays a crucial role in the redox processes. Assembled with YSZ electrolyte, the symmetrical cell shows high efficient processes of surface oxygen exchange and virtue of excellent stability, suggest that PBMO is a good candidate for solid oxide fuel cells and chemical sensors.

Study of transmutation, gas production, and displacement damage in chromium for fusion neutron spectrum

In the present work, gas production, transmutation and displacement damage in chromium are studied for fusion neutron spectra. The nuclear cross-section data are calculated with the TALYS-1.8 code. Transmutated isotopes in chromium, including the radioactive ones, are identified for typical fusion reactor neutron spectrum. Gas production per atom (He and H production) are also predicted for typical fusion neutron spectrum. Molecular dynamics (MD) simulations of damage cascade caused by the self-recoil in chromium are carried out for up to 200 keV damage energies using the LAMMPS code. Time evolution of interstitials and vacancies are studied and discussed. Based on the performed MD simulations, constant parameters of the athermal-recombination corrected dpa (arc-dpa) method are calibrated. Displacement damage cross-section of chromium are calculated using the Norgett-Robinson and Torrens (NRT) and arc-dpa approaches and later used to predict the values of displacement per atoms (dpa) in chromium for typical fusion reactor neutron environment.

Displacement damage study in tungsten and iron for fusion neutron irradiation

In the present paper, displacement damage in tungsten and iron are studied using the TALYS-1.8 code and molecular dynamics simulations. The TALYS-1.8 code is used to predict the energy spectra of recoils. The MD simulations of self-recoils of up to 200 keV damage energies are carried out to predict the number of Frenkel pairs in iron and tungsten using the LAMMPS code. Time evolution of vacancies and interstitials are studied and discussed. The results of MD simulations are used to calibrate the constant parameters of the Arc-dpa method. The displacement damage cross section of tungsten and iron are calculated for the neutron irradiation of up to 15 MeV energy with the NRT and Arc-dpa approaches. The dpa (Arc-dpa) value at the first wall location of EU DEMO reaches 1.19 dpa/FPY and 0.93 dpa/FPY in iron and tungsten, respectively. Similarly, dpa values in iron and tungsten are also predicted at different neutron fields using the NRT and Arc-dpa mechanism.

Mössbauer study of vacancy–solute pairs in iron-based binary alloys

The influence of thermal and mechanical treatment on formation and evolution of vacancies in some binary iron-based alloys (Fe-W, Fe-Ti, Fe-V, Fe-Mn, Fe-Ru, Fe-Si) were studied using the transmission Mössbauer spectroscopy (TMS). The obtained results reveal that in the case of Fe-W, Fe-Ti and Fe-V alloys, the vacancies which were generated during arc-melting and cold-rolling processes are located mainly in the vicinity of non-iron atoms. This speaks in favour of the suggestion that in the iron matrix the mentioned above solute atoms and vacancies interact attractively which supports the known from the literature theoretical calculations on the vacancy–solute interaction in α-Fe. On the other hand, in the case of Fe-Mn, Fe-Ru and Fe-Si alloys the TMS results exclude the presence of vacancy–solute pairs at room temperature.

Density evolution of atomic vacancies in copper single crystals during initial stage of fatigue deformation

ABSTRACTDensity evolutions of dislocations and atomic vacancies in copper single crystals subjected to cyclic loadings are numerically evaluated by combining theoretical models for vacancy generation and dislocations’ behaviour during plastic slip deformation. Parameters for the model of dislocation movement, accumulation and annihilation are set so as that they mimic dislocation movement in regions without or with dislocation substructure in crystals. Cyclic stress–strain curve, evolutions of statistically stored dislocations and atomic vacancies are examined in detail. Atomic vacancy densities after 15 cycles of tensile/compressive loading of ± 3% axial strain amplitude reach 1024–1025 m−3, depending on the dislocations’ mean free path and aspect ratio of dislocation loops. Results agree well with experimentally obtained data in the literature.

Layered Oxide Cathodes for Li-Ion Batteries: Oxygen Loss and Vacancy Evolution

Loss of oxygen in layered transition-metal oxides is a major reason for the structural degradation and thus the fade of electrochemical performance in the cathodes for Li-ion batteries. Via in situ...

Effect of Thermal Treatments on Sn-Alloyed Al-Mg-Si Alloys.

Sn-alloying, by deploying comparatively high vacancy binding energy, mitigates the undesired natural aging behavior of 6xxx-alloys. Targeted selection of pre-aging parameters can have a positive influence on natural aging and paint-bake performance. In this study, we aimed to combine the two approaches of Sn-alloying and pre-aging. Our results indicate that alloys modified with 100 at.-ppm Sn require altered heat treatment. In terms of solution aging and quenching, we show that the cooling rate needed depends on the types of alloy. The rate must be adapted, according to the number of intermetallic particles, to guarantee a sufficiently high level of Sn atoms in solid solution. The rather high number of intermetallic phases in alloy EN-AW-6061 means that it requires fast quenching, while the comparatively low number of precipitate-forming elements in alloy EN-AW-6016 makes it less sensitive to quenching variations. We also show that Sn reduces pre-aging kinetics. The optimal pre-aging temperature and time were consequently found to increase when Sn is added. We also studied the effect of adding a further thermal spike to the usual long-term pre-aging, at different positions within the processing route. The results we present are discussed based on a simulation of vacancy evolution in the alloy when subjected to these treatments.

Experimental Observation of Vacancy-assisted Martensitic Transformation Shift in Ni-Fe-Ga Alloys.

Positron annihilation lifetime spectroscopy is used to experimentally demonstrate the direct relationship between vacancies and the shift of the martensitic transformation temperature in a Ni_{55}Fe_{17}Ga_{28} alloy. The evolution of vacancies assisting the ordering enables shifts of the martensitic transformation up to 50K. Our results confirm the role that both vacancy concentration and different vacancy dynamics play in samples quenched from the L2_{1} and B2 phases, which dictate the martensitic transformation temperature and its subsequent evolution. Finally, by electron-positron density functional calculations V_{Ni} is identified as the most probable vacancy present in Ni_{55}Fe_{17}Ga_{28}. This work evidences the capability of vacancies for the fine-tuning of the martensitic transformation temperature, paving the way for defect engineering of multifunctional properties.

Evolution of intrinsic vacancies and prolonged lifetimes of vacancy clusters in black phosphorene

Due to the relatively low formation energies and highly mobile characteristics of atomic vacancies in phosphorene, understanding their evolution becomes crucial for its structural integrity, chemical activities and applications. Herein, by combining first-principles calculations and kinetic Monte Carlo simulation, we investigate the time evolution and formation of atomic vacancy clusters from isolated monovacancies (MVs), aiming to uncover the mechanisms of diffusion, annihilation, and reaction of these atomic vacancies. We find that while isolated MVs possess a highly mobile characteristic, they react and form MV pairs which possess much lower mobility and high stability under ambient conditions. We also show that the disappearance of MVs at the edge is quite slow due to the relatively high energy barrier, and as a result, around 80% of MVs remain even after two years under ambient conditions. Our findings on one hand provide useful information for the structural repairing of phosphorene through chemical functionalization of these vacancy clusters, and on the other hand, suggest that these rather stable vacancy clusters may be used as activated catalysts.

Microscopic Aspects of Artificial Ageing in Al-Mg-Si Alloys

Al-Mg-Si alloys with a low solute content (0.4 %Mg, 0.4 %Si) and higher contents (Mg+Si=1.4%, all in wt.%) were solutionised, quenched and then artificially aged (AA) at 180 °C, after which positron annihilation lifetime spectroscopy was applied to obtain information about precipitation and vacancy evolution during ageing. Hardness measurements were carried out to complement the measurements of positron lifetime. AA was carried out in four different heating media and allowed for varying the heating rate from 2.4 to 170 K/s. The main result of the study is that there is a competition between vacancy losses and precipitation. The more precipitation takes place before and during AA, the more vacancies are retained and act back on clustering. Higher solute content, slower heating to 180 °C, natural pre-ageing before AA and slower quenching after solutionising were found to have a comparable effect explainable by pre-precipitation before AA.

On the interfacial phase growth and vacancy evolution during accelerated electromigration in Cu/Sn/Cu microjoints

In this work, we integrate different computational tools based on multi-phase-field simulations to account for the evolution of morphologies and crystallographic defects of Cu/Sn/Cu sandwich interconnect structures that are widely used in three dimensional integrated circuits (3DICs). Specifically, this work accounts for diffusion-driven formation and disappearance of multiple intermetallic phases during accelerated electromigration and takes into account the non-equilibrium formation of vacancies due to electromigration. The work compares nucleation, growth, and coalescence of intermetallic layers during transient liquid phase bonding and virtual joint structure evolution subjected to accelerated electromigration conditions at different temperatures. The changes in the rate of dissolution of Cu from intermetallics and the differences in the evolution of intermetallic layers depending on whether they act as cathodes or anodes are accounted for and are compared favorably with experiments. The model considers non-equilibrium evolution of vacancies that form due to differences in couplings between diffusing atoms and electron flows. The significance of this work lies in understanding the vacancy transport due to the difference in intrinsic diffusion of elements in different features of the microstructure, and the severe unidirectional convective flux of atoms due to the enforced electron wind that ultimately paves the road to study nucleation of microvoids.

Deciphering the Role of Oxygen Vacancies on Structural, Electrical, and Magnetic Properties of Fe‐Substituted Strontium Titanate

A comprehensive investigation of incorporation of Fe into SrTiO3 is reported. SrFexTi1−xO3−δ (x = 0.0 – 0.3) powders are prepared via solid state route. X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS) techniques are used to analyze the phase formation, surface chemistry, and electronic structure while scanning electron microscope (SEM) is employed to investigate the surface morphology and microstructural properties. The electrical and magnetic properties are studied at room temperature (RT) to explore the effect of iron substitution on the current conduction and magnetic phase transformation. XPS results indicate the presence of mixed valence state as Fe content increased resulting in considerable enhancement of oxygen vacancies. Moreover, a continuous decrease in Fermi level (FL) is observed as Fe concentration is increased. The increment in Fe concentration leads to variation of current–voltage (I–V) characteristics from linear to non‐linear. Magnetic measurements demonstrate three behaviors; an intrinsic diamagnetic behavior (x = 0.0, 0.05) followed by ferromagnetic behavior (x = 0.1, 0.2), and finally a paramagnetic behavior (x = 0.3) due to transition metal impurities and defects. These studies have been used to establish correlation between the evolution of oxygen vacancies emanating from Fe‐substitution on structural, electrical, and magnetic properties.

Effect of atomic order/disorder on vacancy clustering in concentrated NiFe alloys

Using molecular dynamics simulations, we elucidate the effect of atomic structure on vacancy clustering in ordered (L10) and random NiFe. Based on our simulations, we predict the vacancy evolution to be in complete contrast between the two systems. While large vacancy clusters, i.e., stacking fault tetrahedra (SFT) are formed in the random structure, no clustering is observed in the ordered-L10 structure. Similar simulations are performed on L10-CuAu and L10-TiAl to understand whether SFT formation is generic in L10 structures, or is specific to NiFe. Both materials show SFT formation, thereby highlighting specific defect energetics in L10 NiFe that lead to the lack of vacancy clustering. We elucidate that L10-NiFe has unique thermodynamic and kinetic defect energetics, i.e., antisite energies, vacancy sublattice preference, and directional migration energy barriers that collectively lead to the lack of vacancy clustering. Understanding such defect energetics could open avenues to prevent defect clustering in the vision towards development of radiation-tolerant concentrated alloys for nuclear reactor applications.

Effects of Vacancy Concentration and Temperature on Mechanical Properties of Single-Crystal γ-TiAl Based on Molecular Dynamics Simulation

Abstract Molecular dynamics simulation is used to analyze tensile strength and elastic modulus under different temperatures and vacancy concentrations. The effects of temperature and vacancy concentration on the mechanical properties of γ-TiAl alloy are investigated. The results show that the ultimate stress, ultimate strain and elastic modulus decrease nonlinearly with increasing temperature and vacancy concentration. As the temperature increases, the plastic of material is reinforced. The influence of temperature on strength and elastic modulus is larger than that of vacancy concentration. The evolution process of vacancy could be observed clearly. Furthermore, vacancies with different concentrations develop into voids first as a function of external forces or other factors, micro cracks evolve from those voids, those micro cracks then converge to a macro crack, and fracture will finally occur. The vacancy evolution process cannot be observed clearly owing to the thermal motion of atoms at high temperature. In addition, potential energy is affected by both temperature and vacancy concentration.

Resistive switching behavior in oxygen ion irradiated TiO2−x films

The room temperature resistive switching behavior in 50 keV O+-ion irradiated TiO2−x layers at an ion fluence of 5 × 1016 ions cm−2 is reported. A clear transformation from columnar to layered polycrystalline films is revealed by transmission electron microscopy with increasing ion fluence, while the complementary electron energy loss spectroscopy suggests an evolution of oxygen vacancy (OV) in TiO2−x matrix. This is further verified by determining electron density with the help of x-ray reflectivity. Both local and device current–voltage measurements illustrate that the ion-beam induced OVs play a key role in bistable resistive switching mechanism.

First‐Principles Characterization of Equilibrium Vacancy Concentration in Metamagnetic Shape Memory Alloys: An Example of Ni2MnGa

Despite the fact that there is evidence for the important role that vacancies play in the martensitic transformation (MT) behavior of metamagnetic shape memory alloys (MMSMAs), little theoretical – and even experimental – work on the thermodynamics and kinetics of point defects in these systems has been carried out. Since the MT behavior of MMSMAs has a great influence on their magneto‐caloric response, investigating the vacancy evolution in MMSMAs has potentially a significant technological impact. Scarcity of studies may be due to the limited characterization capability available for studying vacancy properties as well as their impacts on the materials performance. The current work seeks to introduce the application of the grand‐canonical dilute‐solution model to the investigation of equilibrium (thermal) populations of point defects in Ni2MnGa, used as a prototypical MMSMA. The thermodynamic model is coupled to first‐principles calculations of the energetics of defect‐containing supercell structures. Such characterization capability allows for more realistic investigations of MMSMAs with the vacancy degree of freedom taken into account and subsequently opens up many interesting research topics. Here we demonstrate the capability of the first principles based characterization method by investigating the role of vacancy concentration in the kinetics of order–disorder (ODO) process and the MT temperature of Ni2MnGa.

Density evolution of atomic vacancies in HCP metal single crystals subjected to cyclic loading

Density evolution of atomic vacancies in HCP metal single crystals subjected to cyclic loading