Growth Of Vacancy Clusters Research Articles

Effect of transmutation elements Re and Ta on the vacancy formation and dissociation behaviors in W bulk

Transmutation elements are the essential products induced by high energy neutron radiation in plasma facing materials tungsten, which would bring the further effects on their point defects’ evolutions and service performance. Here, transmutation elements Re and Ta atom have been selected to assess the effects on vacancy and vacancy cluster in W material via first-principles calculations. The formation energy indicates mono-vacancy is more likely to form in W-Re system than pure W and W-Ta system. Both Re and Ta will reduce the diffusion barriers for the mono-vacancy migrations. The present calculations indicate that vacancy cluster is more inclined to grow up via combining the vacancy cluster rather than a single mono-vacancy. Re is favorable to the nucleation and growth of vacancy clusters, while Ta has a suppressive effect on the aggregation of small vacancy clusters. The emphasis analysis is obtained according the volumetric dependent strain. Vacancy dissociation calculations show that the dissociation of vacancy clusters is easier to begin with a single vacancy dissociation process.

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.

Evolution and role of vacancy clusters at grain boundaries of ZnO:Al during accelerated degradation of Cu(In,Ga)Se2 solar cells revealed by positron annihilation

Positron annihilation lifetime spectroscopy (PALS) and Doppler broadening positron annihilation spectroscopy DB-PAS) depth profiling demonstrate pronounced growth of vacancy clusters at the grain boundaries of as-deposited Al-doped ZnO films deposited as transparent conductive oxide (TCO) on Cu(In, Ga)Se2 (CIGS) solar cells upon accelerated degradation at 85 ◦C/85% relative humidity. Quantitative fractions of positrons trapped either in the vacancy clusters at the grain boundaries or in Zn monovacancies inside the grains of ZnO:Al were obtained by detailed analysis of the PALS data using a positron trapping model. The time and depth dependence of the positron Doppler depth profiles can be accurately described using a planar diffusion model, with an extracted diffusion coefficient of 35 nm2/hour characteristic for in-diffusion of molecules such as H2O andCO2 into ZnO:Al TCO films via the grain boundaries, where they react with the ZnO:Al. This leads to increased open volume at the grain boundaries that imposes additional transport barriers and may lead to charge carrier trapping and nonradiative recombination. Simultaneously, a pronounced increase in series resistance and a strong reduction in efficiency of the ZnO:Al capped CIGS solar cells is observed on a remarkably similar timescale. This strongly indicates that these atomic-scale processes of molecular in-diffusion and creation of open volume at the grain boundaries play a key role in the degradation of the solar cells. PhySH: Solar Cells, Positron Annihilation Spectroscopy, Grain Boundaries, Vacancies, Thin Films, Diffusion, Electrical Properties, Solid State Chemistry, Optoelectronics

Effect of W self-implantation and He plasma exposure on early-stage defect and bubble formation in tungsten

To determine the effect of pre-existing defects on helium-vacancy cluster nucleation and growth, tungsten samples were self-implanted with 1 MeV tungsten ions at varying fluences to induce radiation damage, then subsequently exposed to helium plasma in the MAGPIE linear plasma device. Positron annihilation lifetime spectroscopy was performed both immediately after self-implantation, and again after plasma exposure.After self-implantation vacancies clusters were not observed near the sample surface (<30 nm). At greater depths (30–150 nm) vacancy clusters formed, and were found to increase in size with increasing W-ion fluence. After helium plasma exposure in the MAGPIE linear plasma device at ~300 K with a fluence of 1023 He-m−2, deep (30–150 nm) vacancy clusters showed similar positron lifetimes, while shallow (<30 nm) clusters were not observed. The intensity of positron lifetime signals fell for most samples after plasma exposure, indicating that defects were filling with helium. The absence of shallow clusters indicates that helium requires pre-existing defects in order to drive vacancy cluster growth at 300 K.Further samples that had not been pre-damaged with W-ions were also exposed to helium plasma in MAGPIE across fluences from 1 × 1022 to 1.2 × 1024 He-m−2. Samples exposed to fluences up to 1 × 1023 He-m−2 showed no signs of damage. Fluences of 5 × 1023 He-m−2 and higher showed significant helium-cluster formation within the first 30 nm, with positron lifetimes in the vicinity 0.5–0.6 ns. The sample temperature was significantly higher for these higher fluence exposures (~400 K) due to plasma heating. This higher temperature likely enhanced bubble formation by significantly increasing the rate interstitial helium clusters generate vacancies, which is we suspect is the rate-limiting step for helium-vacancy cluster/bubble nucleation in the absence of pre-existing defects.

Suppression of vacancy cluster growth in concentrated solid solution alloys

Large vacancy clusters, such as stacking-fault tetrahedra, are detrimental vacancy-type defects in ion-irradiated structural alloys. Suppression of vacancy cluster formation and growth is highly desirable to improve the irradiation tolerance of these materials. In this work, we demonstrate that vacancy cluster growth can be inhibited in concentrated solid solution alloys by modifying cluster migration pathways and diffusion kinetics. The alloying effects of Fe and Cr on the migration of vacancy clusters in Ni concentrated alloys are investigated by molecular dynamics simulations and ion irradiation experiment. While the diffusion coefficients of small vacancy clusters in Ni-based binary and ternary solid solution alloys are higher than in pure Ni, they become lower for large clusters. This observation suggests that large clusters can easily migrate and grow to very large sizes in pure Ni. In contrast, cluster growth is suppressed in solid solution alloys owing to the limited mobility of large vacancy clusters. The differences in cluster sizes and mobilities in Ni and in solid solution alloys are consistent with the results from ion irradiation experiments.

Dislocation-accelerated void formation under irradiation in zirconium

Molecular dynamics simulations have been performed to model defect creation in alpha zirconium with tensile strains comparable to the elastic strains near the interfaces between zirconium and hydrides. Irradiation-induced vacancy clusters under the strain field could be initiation sites for dislocation nucleation, and the gliding of these dislocations provides channels for transporting atoms from the cascade core, significantly accelerating the growth of vacancy clusters to form a void. The critical number of vacancies in a cluster that is enhanced to grow by the external stress is estimated. The present results provide an atomic-level mechanism to explain the stress-enhanced void growth under irradiation in zirconium, as observed in experiments. The acceleration of delayed hydride cracking under irradiation might be partially attributed to the formation of these voids.

Deuterium accumulation in tungsten under low-energy high-flux plasma exposure

The accumulation of deuterium implanted in tungsten is simulated within the framework of kinetic diffusion theory. The influence of the tungsten microstructure (dislocation density and impurity concentration) on the process of deuterium capture and accumulation is considered. It is established that, under the chosen irradiation conditions, deuterium accumulation in the near-surface region is determined by capture at defects formed during implantation. The deuterium concentration gradient, together with the material microstructure, determines its accumulation in tungsten. Variation in the dislocation density and impurity concentration does not affect the simulation results, which is, first, related to the fact that the model used does not contain alternative mechanisms for the formation and growth of vacancy clusters under the subthreshold irradiation mode. The simulation results are compared with experimental data, and ways of improving the model are discussed in order to explain the deuterium-saturation effect for high fluences (more than 1023 m−2).

Effects of copper precipitates on microdefects in deformed Fe-1.5 wt%Cu alloy

As-annealed Fe–1.5 wt%Cu alloys were deformed by cold rolling. Positron annihilation lifetime spectroscopy (PALS) and Doppler broadening spectra (DBS) were used to analyze the microdefects in the alloy induced by deformation. Cu precipitates were formed uniformly in the alloy after annealing at 1173 K. To investigate the effects of Cu precipitates on the formation and migration of defects, well-annealed pure Fe samples were prepared as a reference. PALS results show that the value of positron lifetime (both long lifetime τ2 and mean lifetime τm) in the alloy is smaller than in pure Fe during deformation. This indicates that Cu precipitates restrain the growth of vacancy clusters. Results of DBS show that the S parameter in pure Fe is larger than in the alloy, and more deformation is needed for the S parameter to be saturated in the alloy.

Microstructure of Ni fatigued under neutron irradiation

The microstructure of fatigued and unfatigued Ni in the presence or absence of neutron irradiation was investigated using positron annihilation lifetime measurements and transmission electron microscopy (TEM). In irradiated unfatigued Ni, vacancy clusters were directly formed from cascades at doses of less than 10−4dpa, and started to grow at higher doses. In irradiated fatigued samples, vacancy clusters were directly formed from cascades at 1.5×10−4dpa. In unirradiated fatigued samples, single vacancies were produced. In all of the fatigued samples, dislocations introduced by the deformation acted as sinks and suppressed the growth of vacancy clusters. Following annealing at 373K, vacancy clusters were observed in irradiated fatigued Ni, but vacancies were trapped at dislocations in unirradiated fatigued Ni. No change in dislocation structure was brought about by annealing and no vacancy clusters could be observed by TEM.

Differential scanning calorimetry study on annihilation behavior of supersaturated thermal vacancies in B2-type FeAl

In this article, our recent advances on supersaturated vacancy behavior in rapidly quenched FeAl investigated by DSC measurements coupled with TEM observations are reviewed. There are two exothermic peaks only at the first heating step of DSC for the rapidly quenched FeAl. The 1st peak is small and lies in a lower temperature range, and the 2nd peak is large lying in a higher temperature range. The apparent activation energy of the exothermic reactions changes not only with Al concentration but also with quenching temperature. This indicates that the apparent activation energy is not the material constant of FeAl but a variable that changes with the concentration of supersaturated thermal vacancies. The 2nd peak corresponds to the nucleation and growth of vacancy clusters and complexes inside the bulks due to the condensation and annihilation of supersaturated vacancies, i.e. the formation and growth of cuboidal voids for rapidly solidified ribbons and of 〈100〉-type prismatic loops for water-quenched single crystals. The morphology of the vacancy complexes grown under the 2nd peak is correctly expressed by the Johnson–Mehl–Avrami (JMA) equation extended by Matusita et al. for the DSC curves: the morphology factor, m, nearly equals to 2 for the formation and growth of 〈100〉-type prismatic loops and to 3 for those of cuboidal voids. Therefore, the extended JMA method would be useful to judge the morphology of vacancy complexes.

Nucleation and growth of vacancy clusters in β-SiC during irradiation

Molecular dynamics and molecular static calculations have been performed using the empirical many-body interatomic potential to obtain the formation and binding energies of relaxed configuration of vacancy clusters in β-SiC, which are necessary when the nucleation and growth process of clusters is investigated. The formation energy of vacancy clusters in β-SiC depends on the size, vacancy composition, and vacancy configuration of clusters. When the size and vacancy composition of clusters are given, the vacancy configuration of clusters with the lowest formation energy is primarily given so as to take the smallest number of dangling bonds. Especially when the fraction of the number of silicon vacancies to the number of carbon vacancies in a cluster is quite high or quite low, the formation property of antisite defects in clusters becomes a key factor to determine the stable configuration of clusters.

Internally consistent approach for modeling solid-state aggregation. I. Atomistic calculations of vacancy clustering in silicon

A computational framework is presented for describing the nucleation and growth of vacancy clusters in crystalline silicon. The overall approach is based on a parametrically consistent comparison between two representations of the process in order to provide a systematic method for probing the details of atomic mechanisms responsible for aggregation. In this paper, the atomistic component of the overall framework is presented. First, a detailed set of targeted atomistic simulations are described that characterize fully the thermodynamic and transport properties of vacancy clusters over a wide range of sizes. It is shown that cluster diffusion is surprisingly favorable because of the availability of multiple, almost degenerate configurations. A single large-scale parallel molecular dynamics simulation is then used to compute directly the evolution of the vacancy cluster size distribution in a supersaturated system initially containing 1000 uniformly distributed vacancies in a host lattice of 216 000 Si atoms at 1600 K. The results of this simulation are interpreted in the context of mean-field scaling theory based on the observed power-law evolution of the size distribution moments. It is shown that the molecular dynamics results for aggregation of vacancy clusters, particularly the evolution of the average cluster size, can be very well represented by a highly simplified mean-field model. A direct comparison to a detailed continuum model is made in a subsequent article.

Nitrogen-doping effect in a fast-pulled Cz-Si single crystal

In the nitrogen-doped and fast-pulled crystals, the nitrogen plays a role to disturb the growth of vacancy clusters by reducing the vacancy-rich region and the vacancy concentration. Thus, the oxidation-induced stacking fault ring (OSF-ring) is formed near the edge region and its width depends on the nitrogen-doping concentration. The vacancy clustering is severely suppressed and the increased residual vacancy causes the anomalous oxygen precipitation. A conceptual model based on the reduction of point defect concentration could explain the nitrogen-doping effects.

Predicted vacancy cluster structures in MgO and their interaction with helium

Atomistic simulation calculations were performed to predict defect energies of vacancy clusters and dissociation energies of clusters with helium. The computation technique was based on pair potentials and the Mott–Littleton methodology was used as implemented in the CASCADE code. Both the full charge and the partial charge model were employed. The partial charge model shows better agreement with experimental data reported in the literature. The energetically most favourable spatial configurations for clusters composed of up to eight vacancies were determined and the associated formation energies calculated. The results indicate that the three-dimensional growth of vacancy clusters in MgO is energetically favourable. Next, the activation energies for dissociation of helium atoms from these vacancy clusters were calculated. The activation energy for dissociation of a helium atom from vacancy clusters larger than a monovacancy is approximately 3.6 eV for the partial charge model. Remarkably, the activation energy for dissociation of a helium atom from a monovacancy was found to be higher, 3.9 eV for the partial charge model. This last energy is enhanced by lattice relaxation around the monovacancy. The dissociation energies from large vacancy clusters is in good agreement with the permeation energy of 3.3 ± 0.3 eV obtained experimentally with a permeation experiment in which helium release was monitored with neutron depth profiling (NDP).

Role of dislocations in aggregation of photoproduced vacancies in crystals

Aggregation of photoproduced vacancies in a crystal in the course of irradiation is considered with allowance for recombination with interstitials simultaneously generated in the same amount. The vacancy–interstitial recombination essentially slows down the growth of vacancy clusters (pores). Therefore, the growth of pores is conditioned by outflow of interstitials. Under certain conditions, dislocations act as main sinks for interstitials. Near dislocations, pores grow fast, suppressing the growth of pores in the rest of the crystal volume. This gives rise to various growth modes depending on the ratio between the densities of dislocations and pores. For a high density of pores or a low density of dislocations, pores grow only near dislocations which can result in annihilation of the rest of pores and in a decrease of their total surface (the effect of radiation annealing). In the opposite case, pores grow almost uniformly. The fraction of photoproduced vacancies, accumulated in the form of pores, varies by several orders of magnitude depending on the dislocation-to-pore density ratio.

Growth of Vacancy Clusters During Post-Irradiation Annealing of Ion Implanted Silicon,

AbstractAnnealing experiments of deuterium implanted silicon have been performed while positron beam analysis was used for monitoring the cavity growth. The experiments indicate up to annealing temperature 500° C similar defect evolution for both the low dose of 1016 cm-2 as for a 3 times higher dose. At this temperature the deuterium stabilized vacancy clusters dissociate and only in the case of the high dose micro-cavities are formed. Monte Carlo simulations of vacancy cluster growth in silicon based on vacancy cluster dissociation energies, calculated with the Stillinger Weber potential, have been performed. The results indicate that for low initial defect concentrations vacancy clusters might be hindered to grow because the vacancy binding energy of the clusters does not increase monotonically with the cluster size. Only a high concentration guarantees that growth barriers will be overcome.

Positron annihilation studies of defects in helium-irradiated silicon

Positron annihilation studies have been carried out on Si-n irradiated with He + ions at the V.U.B. cyclotron, to a dose of 4×10 17 He/cm 2. No temperature dependence on the S-parameter and lifetimes is seen below the irradiation temperature. The positron lifetime associated to the created defects is 290 ps. During the isochronal annealing, this lifetime stays constant up to 700 K. It is attributed to the annihilation of positrons from large vacancy-clusters filled with He atoms. From the isochronal annealing results, only one annealing stage is seen. This annealing stage which extends over a long range of temperature 700–1000 K, is ascribed to the degassing of helium atoms from defects and the growth of vacancy-clusters. The lifetime of positrons in those defects reaches a value of about 530±30 ps at 1000 K, indicating that the vacancy-clusters formed have a mean size of more than 8 vacancies.

Stage II recovery in proton-irradiated aluminium studied by positrons

Defect recovery of low-temperature proton-irradiated aluminium has been studied by the positron lifetime technique. The positron lifetime parameters are observed to change significantly for isochronal annealing in stage I (30-45 K), stage II (45-190 K) and stage III (190-300 K). Stage I is the stage where interstitial atoms become mobile leading to annihilation of vacancies and interstitial atoms. In stage III the loss of vacancies at sinks and the growth of vacancy clusters, due to freely migrating vacancies left over from stages I and II, is observed. In stage II mobile interstitials form three-dimensional clusters that again rearrange into interstitial loops. Unexpectedly, the authors also observe remarkable changes in the positron lifetime parameters in this temperature region, which is explained by positron trapping at interstitial-type defects. The positron lifetime at these interstitial clusters is less than 230 ps, well below the positron lifetime in a monovacancy, but close to lifetimes deduced from recent studies on deformed aluminium. Comparison with diffuse X-ray scattering results show that the positron lifetime technique is sensitive to both the growth and the structural changes of interstitial clusters.

Characterization of Defect Structures by Perturbed Angular Correlation Technique

ABSTRACTIn applications of the perturbed angular correlation (PAC) technique to the characterization of defect structures in materials conclusions are drawn from the local electric field gradient measured at the site of a radioactive probe atom. In metals, PAC has been very successful in labeling and identifying simple probe atom-defect pairs. Based on these results, new experiments were performed on defect structures in cold-worked samples, on the dynamics of He-defect interactions and on the growth of vacancy clusters. Recent results on interactions between the acceptor atom In and different donor atoms or intrinsic defects in Si indicate that PAC may provide atomistic information also in the case of semiconductor materials.

Formation and growth of vacancy clusters in quenched dilute Al-Si alloys studied by positron annihilation

Positron annihilation Doppler-broadening spectra were measured and lineshape parameters were estimated during isochronal aging of dilute alloys of Al-1.0 at.%Si and Al-0.1 at.%Si quenched from 773 and 823K to 183K. Two peaks in the lineshape parameters were observed at about 213 and 243K in both alloys. It is suggested that the increase and subsequent decrease of the parameters around 213K are due to the growth of vacancy clusters and the collapse of their open volumes, respectively. Positron annihilation experiments showed that the collapse of vacancy clusters in quenched pure Al seems to occur in a temperature region below about 213K. The size distribution of Si precipitates was determined by electron microscopy in various aging treatments of Al-1.0 at.%Si alloys quenched from 823K. Taking into account the results from electron microscopy, it is deduced that the change in lineshape parameter around 243K seems to be related to the nucleation of Si precipitates.