Coexistence Of Vacancies Research Articles

The hydrogen-helium-vacancy interaction and hydrogen-vacancy clusters formation mechanisms in chromium: A first-principles study

Attributing to excellent corrosion resistance and low neutron absorption cross-section, chromium (Cr) has been considered as a promising advanced structural nuclear material. Under extreme radiation conditions, the coexistence of vacancies (Vac), hydrogen (H), and helium (He) modifies the evolution trajectory of H-defect clusters, and significantly impacts the performance of materials, all of which are under extensive investigations. Here, the occupancy mechanism of H atoms and influencing factors in Vac1/He1Vac1 systems are comprehensively studied by first-principles calculations. Based on the exhaustive investigation of H-He-Vac interactions, the equilibrium distance between H-H pairs is delineated. Serving as an effective trap center, the Vac1/He1Vac1 cluster exhibits robust attractive forces capable of luring dissociative H atoms within the crystal lattice. The accumulation of H atoms from remote regions towards the defects results in the formation of stable Hn-Vac1/Hn-He1Vac1 complexes. Moreover, it is revealed that the hydrogen-enhanced vacancy mechanism and meticulously examines the factors affecting defect-trapping capabilities and the growth of H-He-Vac complexes, providing an insight into the behavior of hydrogen-defects clusters formation mechanism and the H-He-Vac interaction in Cr.

Stress states and point defect capture radii of dislocation a-loops and c-loops in alpha-zirconium

Improving the predictive capability of microstructure evolution in irradiated α-zirconium requires a thorough understanding of the interaction rates between mobile defects and the existing, and evolving, microstructure. One of the parameters necessary to calculate these rates is the defect capture radii of dislocation loops. In order to better quantify this parameter, atomistic binding energy maps of point defects to interstitial a-loops, vacancy a-loops, and vacancy c-loops have been constructed using large-scale molecular statics simulations. Point defect capture radii have been correlated to the decay of binding energies with increasing distance using two criteria: spontaneous and thermal drift capture. Considering the spontaneous capture criteria, SIAs exhibit biased capture to all dislocation loops. However, when thermal drift capture is considered, the local stress state of dislocation loops imparts an inherent bias for the capture of same-type defects (i.e. vacancies to vacancy loops and vice versa). This phenomenon may drive the co-existence of vacancy and interstitial a-loops in irradiated α-Zr microstructure, an aspect that is poorly captured by most current models. The capture radii reported here will directly improve the predictive capability of microstructure evolution modeling in neutron-irradiated α-Zr.

Effect of coexistence of vacancy and strain on the electronic properties of NH3 adsorption on the Hf2CO2 MXene from first-principles calculations

The effect of coexistence of vacancy defect and strain on the electronic structure, adsorption properties and surface charge distribution of Hf2CO2 monolayer adsorbed by NH3 molecule is explored by the first-principles calculations. Different vacancy configurations and possible adsorption sites are considered. Less adsorption energies of HVH adsorption systems and larger recovery times of OVH adsorption systems are not conducive to adsorption and desorption, respectively. TH-CVH-1 and TH-CVH-2 systems are suitable adsorption systems because of the suitable adsorption energies and short recovery times. The properties of TH-CVH-1 and TH-CVH-2 systems under biaxial strain are further explored. TH-CVH-1 system maintains the chemical adsorption under strains, and can be used as reusable gas sensor in the strain of −7% to +1%. NH3 molecule can be desorbed from TH-CVH-2 system by applying strain. The compressive strain larger than 2% results in the transition from chemical to physical adsorption for TH-CVH-2 system, which can be used as a reusable gas sensor in the strain of −2% to +1%.

Stability of vacancy and interstitial dislocation loops inα-zirconium: atomistic calculations and continuum modelling

We combine atomistic calculations and continuum laws to model irradiation-induced vacancy and interstitial dislocation loops in α-zirconium. A comprehensive set of Burgers vectors/stacking sequences in the prismatic and basal planes, of sizes accessible to experiments, are studied by Molecular Statics (MS) simulations. Their formation energies and structural details are determined using two different interatomic potentials for α-Zr, considering dislocation loops in hexagonal and circular shapes. Molecular Dynamics annealing of dislocation loops then validates the envisioned potential energy landscape. Finally, the continuum modelling hybridly calibrated on MS results and ab initio data indicate that the coexistence of vacancy and interstitial 〈a〉 loops is supported by stability arguments. We also establish the limitations of such an approach for quantitative predictions.

Effect of tungsten on the vacancy behaviors in Ta–W alloys from first-principles calculations

Alloying elements play an important role in the design of plasma facing materials with good comprehensive properties. Based on first-principles calculations, the stability of alloying element W and its interaction with vacancy defects in Ta–W alloys have been studied. The results show that W tends to distribute dispersedly in Ta lattice, and is not likely to form precipitation even with the coexistence of vacancy. The aggregation behaviors of W and vacancy can be affected by their concentration competition. The increase of W atoms has a negative effect on the vacancy clustering, as well as delays the vacancy nucleation process, which is favorable to the recovery of point defects. Our calculations are in consistent with the defect evolution observed in irradiation experiments in Ta–W alloys. The results suggest that W is a potential repairing element that can be doped into Ta-based materials to improve their radiation resistance.

Theoretical investigation of microstructure evolution and deformation of zirconium under neutron irradiation

The radiation growth of zirconium is studied using a reaction–diffusion model which takes into account intra-cascade clustering of self-interstitial atoms and one-dimensional diffusion of interstitial clusters. The observed dose dependence of strain rates is accounted for by accumulation of sessile dislocation loops during irradiation. The computational model developed and fitted to available experimental data is applied to study deformation of Zr single crystals under irradiation up to hundred dpa. The effect of cold work and the reasons for negative prismatic strains and co-existence of vacancy and interstitial loops are elucidated.

Intrinsic point defect incorporation in silicon single crystals grown from a melt, revisited

The so-called Voronkov criterion states that the dominant intrinsic point defect in a silicon single crystal grown from a melt is determined by the ratio of pulling speed over temperature gradient near the melt/solid interface. Above a critical value of this ratio, the crystal is vacancy-rich, while for a ratio below this value, the crystal is interstitial-rich. Applying the Voronkov criterion implies, however, intrinsic point defect diffusivities and/or thermal equilibrium concentrations that can differ strongly from those experimentally determined using self- and metal-diffusion experiments. Furthermore, for a given hot zone, crystal diameter, and length, the thermal gradient itself at the melt/solid interface is a function of the pulling speed, so that the criterion in principle can be replaced by one for the thermal gradient only. There is also experimental evidence, based on crystal detaching experiments, that the growing crystal is always vacancy-rich at the solid/melt interface. In the present paper, the validity of the Voronkov criterion is critically reviewed and the impact of stress, in particular on intrinsic point defect thermal equilibrium concentrations, is taken into account and discussed. It is shown that the temperature and stress gradient near the melt-solid interface have an important impact on the intrinsic point defect incorporation and on the formation of grown-in defects that can be observed in the as-grown and thermally treated crystal. It is also likely that both types of intrinsic point defects can be present in supersaturation in different temperature windows during crystal pulling, leading to the observed coexistence of vacancy and self-interstitial clusters in the as-grown crystal. It is shown that, when taking into account stress effects, there is no need to assume intrinsic point defect diffusivities and thermal equilibrium concentrations that are different from those determined, e.g., from self- and metal-diffusion experiments and from ab initio calculations.

TEMPERATURE DEPENDENT MONOVACANCY PARAMETERS VERSUS DIVACANCIES IN ANALYZING DATA OF DEFECTS IN METALS

The appropriate procedure for analyzing experimental data of defects in metals is discussed. The following two diverse procedures have been proposed earlier: either in terms of single vacancy formation with temperature dependent enthalpy and entropy, or by assuming coexistence of vacancies and divacancies with temperature-independent parameters. Using aspects of thermodynamics of the defect formation processes in solids, we show that the former procedure leads to self-consistent parameters.

Atomistic calculations of lattice constants of mullite with its compositions

We calculate lattice constants a and b of mullite with its compositions (Al 2[Al 2+2 x Si 2−2 x ]O 10− x ) using the constant stress molecular dynamics (MD) method. The Matsui's and Winkler's interatomic models for aluminosilicate are adapted. Two sets of mullite models are considered, one with a random distribution of oxygen vacancies and Al/Si substitutions and the other with a partially ordered Al occupancy at the T * sites. All the simulation results predict the trend observed in experiments in which the lattice constant a increases while b decreases when the x-value increases. The expansion mechanism induced by Al/Si substitutions is supported by bond length analysis. The reduction of lattice constant b is supported by the locally anisotropic atom displacement in the a and b directions sensed by the oxygen vacancies. We conclude that the causes of lattice expansion and reduction are tightly interwoven with the coexistence of vacancies and Al/Si substitutions and their inherent influence on atom displacements.

Numerical evaluation of the dislocation loop bias

We have performed numerical calculation of the capture efficiency of a dislocation loop in a finite toroidal reservoir, which is a more appropriate model for a dislocation loop than a spherical or cylindrical reservoir adopted in the previous models. It allows a direct evaluation of the capture efficiency and the bias for a loop of any size with a full account of the stress field in the loop region of influence. It is shown that the loop bias depends on the loop size, dislocation density and the interstitial to vacancy dilatation ratio. With increasing loop size its bias decreases or increases to the straight dislocation bias value if the dislocation density is low or high, respectively. The bias difference of loops of different sizes is shown to be the reason of a coexistence of vacancy and interstitial loops under irradiation. In the conventional case of the dislocation bias for interstitials, interstitial loops are expected to grow to larger sizes than vacancy loops, while in a special case of dislocation bias for vacancies, the opposite tendency is expected.

Point Defects, Diffusion Mechanisms, and the Shrinkage and Growth of Extended Defects in Silicon

ABSTRACTThe paper introduces the basic point-defect models proposed for silicon, which involve either vacancies or self-interstitials only, or both types of point defects simultaneously under thermal-equilibrium conditions. The growth and shrinkage kinetics of oxidation-induced stacking faults as well as oxidation-enhanced or -retarded diffusion phenomena are discussed within the frame work of these models. Whereas no unambiguous conclusions on the dominant diffusion mechanism can be drawn from the available oxidation-related experiments, recent investigations on so-called anomalous diffusion phenomena (e.g., the ‘emitter-push effect’) and on the diffusion of gold in silicon demonstrate Si self-interstitials to be the point defects governing self- and impurity diffusion. The possibility of a coexistence of vacancies and self-interstitials in thermal equilibrium is discussed in this context. The paper concludes with speculations on how carbon in conjunction with self-interstitials may influence the nucleation process of oxygen precipitates in silicon.