Evolution Of Vacancy-type Defects Research Articles

Positron annihilation study of open volume defects and Cr segregation in deformed CoCrFeMnNi alloy

The effects of thermal annealing on the evolution of vacancy-type defects and Cr segregation in deformed CoCrFeMnNi alloys were investigated using positron annihilation spectroscopy (PAS) and scanning electron microscopy with energy dispersive spectroscopy. On microstructure observation, Cr atoms tended to segregate at the grain boundaries in the CoCrFeMnNi alloy after severe plastic deformation, and after thermal annealing, the size of the Cr segregation phase gradually decreased in the range of 300–800 K, eventually dissolving in the matrix at 1300 K. Positron annihilation spectroscopy results showed that divacancy defects were formed in the deformed samples, indicating that the movement of excess vacancies caused by deformation at room temperature increases the diffusion of Cr atoms and causes Cr segregation. The quenched alloy was annealed at 600–800 K to form vacancy clusters, and with an increase in temperature, the vacancies gradually recovered and were completed at approximately 1300 K.

Evolution of vacancy defects in heavy ion irradiated tungsten exposed to helium plasma

Evolution of vacancy-type defects has been investigated in undamaged and copper ion pre-damaged tungsten exposed to low-energy and high-flux helium plasma (60 eV, 1 × 1022 He/m2s). The results measured by Doppler broadening positron annihilation spectroscopy (DB-PAS) indicate that helium-vacancy complexes generate due to intense self-trapping in the undamaged tungsten after helium plasma exposure. In contrast, the occupation of pre-existing vacancies and vacancy clusters, caused by the presence helium atoms, plays a dominant role in the pre-damaged tungsten, but the density of vacancy-type defects in the pre-damaged case is still higher than that in the undamaged case. This means that the dominating process of helium-vacancy complexes formation in tungsten changes from self-trapping to vacancy-trapping in the case of high vacancy density. Meanwhile, the elastic recoil detection analysis (ERDA) and transmission electron microscopy (TEM) results reveal that these pre-existing defects can increase helium retention and helium nano-bubble size. However, it is surprising that in the case of pre-damaged sample, the density/volume of vacancy-type defects also decreased even outside of the helium distribution depth. We attribute this phenomenon to the interstitial dislocation loops punched by helium clusters preferentially diffusing into the pre-damage regions of tungsten far beyond the helium distribution depth, resulting in the significant recombination with vacancies or vacancy clusters. This intrinsic mechanism is further verified by TEM observations and molecular dynamics (MD) simulations.

Evolution of vacancy-type defects and hardening behaviors of T91 induced by 1.625 MeV Fe-ions at different temperatures

Irradiation temperature is one of the most important parameters which determine the evolution of structural materials’ properties. In this study, the evolution of vacancy-type defects and hardening behaviors of T91 steel induced by Fe-ion irradiation at different temperatures were investigated. The 1.625MeV Fe ions were implanted in T91 to the fluence of 2.5×1015ions/cm2 at temperatures from room temperature (RT) to 550°C. After irradiation, the samples were analyzed by positron annihilation spectroscopy (PAS) and nano-indention technology (NIT). It was found that the sample irradiated at RT possessed the largest concentration of vacancy-type defects. At higher temperatures, the variation of vacancy-type defects concentrations were in a trend of Cv550°C<Cv300°C<Cv450°C. All the samples were hardened to different extents after irradiation. The hardness values increased initially and decreased afterward with the increasing temperatures, and the peak hardening is present at 300°C.

Positron Annihilation Spectroscopic Studies of Cu&lt;sub&gt;2&lt;/sub&gt;Te Thermoelectric Material

Positron annihilation lifetime spectroscopy (PALS) and coincident Doppler-broadening spectroscopy (CDBS) have been used for investigating the evolution of vacancy-type defects in the thermoelectric material Cu2Te which annealed at different temperatures. The results of PALS show that a fraction of positrons has got annihilated at the surfaces and the sample which annealed at 450 °C has the highest concentration of surface defects. The average positron lifetime and the S parameter have the same trends which gradually increase with the increase of the annealing temperature. This change implies that the total concentration of the defects has been changed with the change of the annealed temperatures. The results of the CDBS ratio spectrum and S-W plot indicate that the defect species have no change after annealing at different temperatures.

First-cycle defect evolution of Li1−xNi1/3Mn1/3Co1/3O2 lithium ion battery electrodes investigated by positron annihilation spectroscopy

Abstract In this study the structure and evolution of vacancy type defects in lithium ion batteries are investigated in respect of crystallographic properties. The relation between positron annihilation and electronic structure is discussed in terms of structural dynamics during the lithiation process. Samples of Li1−xNi1/3Mn1/3Co1/3O2 (NMC-111) electrodes with decreasing lithium content (x = 0–0.7) covering the whole range of state of charge were electrochemically prepared for the non-destructive analysis using positron coincidence Doppler broadening spectroscopy (CDBS). The positron measurements allowed us to observe the evolution of the defect structure caused by the delithiation process in the NMC-111 electrodes. The combination of CDBS with X-ray diffraction for the characterization of the lattice structures enabled the analysis of the well-known kinetic-hindrance-effect in the first charge-discharge cycle and possible implications of vacancy ordering. In particular, CDBS revealed the highest degree of relithiation after discharge to 3.0 V at 55 °C. For the first time, we report on the successful application of CDBS on NMC-111 electrodes yielding new insights in the important role of defects caused by the delithiation process and the kinetic hindrance effect.

Defect dynamics in Li substituted nanocrystalline ZnO: A spectroscopic analysis

Very recently, vacancy-type defects have been found to play a major role in stabilizing d0 ferromagnetism in various low dimensional ZnO systems. In this context, the evolution of vacancy-type defects within the ZnO nanocrystals due to the doping of ZnO by alkali metal lithium (Li) is investigated using X-ray photoelectron (XPS), photoluminescence (PL) and positron annihilation spectroscopy (PAS). Li-doping is found to have significant effects in modifying the vacancy-type defects, especially the Zn vacancy (VZn) defects within the ZnO lattice. XPS measurement indicated that initially the Li1+ ions substitute at Zn2+ sites, but when Li concentration exceeds 7at%, excess Li starts to move through the interstitial sites. The increase in positron lifetime components and the lineshape S-parameter obtained from coincident Doppler broadening spectra with Li-doping indicated an enhancement of VZn defect concentration within the doped ZnO lattice. The vacancy type defects, initially of the predominant configuration VZn+O+Zn got reduced to neutral ZnO divacancies due to the partial recombination by the doped Li1+ ions but, when the doping concentration exceeded 7at% and Li1+ ions started migrating to the interstitials, positron diffusion is partly impeded and this results in reduced probability of annihilation. PL spectra have shown intense green and yellow-orange emission due to the stabilization of a large number of VZn defects and Li substitutional (LiZn) defects respectively. Hence Li can be a very useful dopant in stabilizing and modifying significant amount of Zn vacancy-defects which can play a useful role in determining the material behavior.

Evolution of Vacancy-Type Defects, Phase Transition, and Intrinsic Ferromagnetism during Annealing of Nanocrystalline TiO2 Studied by Positron Annihilation Spectroscopy

The evolution of vacancy-type crystalline defects across the transition from anatase to rutile structure of titanium dioxide (TiO2) nanoparticles during high-temperature annealing in oxygen and argon is studied by using positron lifetime and coincidence Doppler broadening spectroscopic measurements. The TiO2 nanoparticles were synthesized through a simple sol–gel chemical route. The changes in the crystalline phase and lattice parameters of the nanoparticles upon thermal treatment were investigated by X-ray diffraction and high-resolution transmission electron microscopy, and the results were correlated with those of photoluminescence spectroscopy and positron annihilation measurements. The structural defects, mostly 3D vacancy clusters, in the nanoparticles were found to decrease in concentration during the annealing in O2 rather than in Ar at elevated temperatures. In the case of annealing in Ar, the vacancy-type defects persisted even at the highest annealing temperature of 900 °C used in the experimen...

Generation and evolution of vacancy-type defects in nano-Cu films during plastic deformation by means molecular dynamics

Molecular dynamics simulations were performed to study the plastic deformation of Cu films under uniaxial tension. The generation and evolution mechanisms of vacancy-type defects were carefully studied. In the simulations, embedded atom method (EAM) was selected as the interatomic potential function. Simulation results indicate that the plastic deformation is due to the dislocation nucleation near free surfaces. Both the generation and evolution of vacancy-type defects are related to dislocation activities. Vacancy-type defects prefer to nucleate at the position of dislocation jogs and the intersection of stacking faults initially, and finally exist in the form of single vacancy, vacancy clusters and stacking-fault tetrahedrons.

Influence of impurities on the evolution of vacancy-type defects in neutron-irradiated nickel

In order to investigate the effect of impurities on vacancy defect evolution in nickel, specimens with high (5N) and technical (3N) purity were neutron-irradiated at ∼330K in the IVV-2M reactor (Russia) to fluencies in the range of 1×1021–1×1023n/m2 (E>0.1MeV) corresponding to displacement dose levels in the range of about 0.0001–0.01dpa and subsequently stepwise annealed to about 900K. The specimens of Ni with different purities were characterized both in as-irradiated state as well as after post-irradiation annealing by positron annihilation spectroscopy. The formation of three-dimensional vacancy clusters (3D-VCs) in cascades was observed under neutron irradiation. The density and size of 3D-VCs depended not only on dose level, but also on purity. The population of 3D-VCs in the technical Ni is lower than that in the high-purity Ni. 3D-VCs collapse into secondary-type clusters (stacking fault tetrahedra (SFTs) and vacancy loops) during stepwise annealing at 350–450K (stage III in Ni). The suppression of secondary cluster formation in 3N Ni is attributed to an effective vacancy interaction with impurity carbon atoms, which based on a relatively large vacancy–carbon atom binding energy (0.32–0.35eV). The trapping of vacancies released at the collapse of 3D-VCs by the interstitial impurity atoms dominates at low irradiation dose level (10−4dpa). Thus, we found that carbon impurity atoms have strong effects both on the primary vacancy-type defect aggregation and on the secondary vacancy clusters and complexes formation.

He and H-implanted CLAM steel investigated by positron-annihilation spectroscopy

The distribution and nature of defects induced by H ion, He ion, and mixed-beam H and He ion implantation in China low-activation martensitic (CLAM) steel were studied by positron-annihilation spectroscopy. The evolution of vacancy-type defects in the CLAM steel was detected when the ion implantation dosage was varied. Significant differences in the S parameter-positron energy profile in response to the different implanted ions were observed. The implantation-induced defects were analyzed using VEPFIT. One type of defect was found in all ion-implanted samples.

Effects of solute atoms on evolution of vacancy defects in electron-irradiated Fe–Cr-based alloys

The evolution of vacancy-type defects in Fe–Cr alloys (13–16 at.% Cr) undoped and doped with C, N, Au, or Sb and in conventional ferritic–martensitic steel (∼13% Cr) has been investigated using positron annihilation spectroscopy under electron irradiation at room temperature and subsequent stepwise annealing. Small vacancy clusters are formed in the undoped Fe–16Cr alloy, which anneal out between 320 and 550 K. It is shown that oversized substitutional solute atoms (Sb, Au) in the Fe–Cr alloy interact with vacancies and form complexes, which are stable up to 600 and 420 K, respectively. It is found that the accumulation of vacancy defects considerably increases in the alloys and the steel with an enhanced content of interstitial impurities. It is shown that this effect is related to the formation of vacancy-carbon complexes. It is known that chromium in iron decreases the diffusion mobility of carbon. Therefore, the structure of vacancy-carbon complexes and the kinetics of their annealing in Fe–Cr alloys differ from those in the Fe–C system.

The formation and evolution of vacancy-type defects in Ar-implanted silicon studied by slow-positron annihilation spectroscopy

The Doppler broadening spectrum of a silicon wafer was measured using a variable-energy positron beam to investigate the effects of vacancy-type defects induced by 180 keV Ar ion implantation. The S-parameter in the damaged layer decreases with annealing temperature up to 673 K, and then increases with annealing temperature from 673 to 1373 K. At low annealing temperatures ranging from room temperature to 673 K, argon-decorated vacancies are formed by argon atoms combining with open-volume defects at inactive positron sites. With further increase of annealing temperature, argon-decorated vacancies dissociate and subsequently migrate and coalesce, leading to an increase of S-parameter. Furthermore, the buried vacancy-layer becomes narrow with increasing annealing temperature. At 1373 K, the buried vacancy-layer moved towards the sample surface.

The formation, migration, agglomeration and annealing of vacancy-type defects in self-implanted Si

The evolution of vacancy-type defects has been studied by variable-energy positron annihilation spectroscopy (VEPAS) in samples of high-quality FZ p-type (001) silicon wafers implanted with 4 MeV Si2+ ions at room temperature to doses of 1012–1014 cm−2. The average vacancy concentration increases as (ion dose)0.70 ± 0.06. Progressive isochronal annealing measurements show that open-volume point defects (having a VEPAS signature close to that for divacancies) anneal between 500–600°C. VEPAS with enhanced depth sensitivity (via progressive etching) verified that single 30 min anneals to 550 and 600°C lead to the formation of buried clusters VN with an average N of 3.5 lying between depths of 2.2 and 3.6 μm (both ± 2 μm), close to the peak of vacancy damage just shallower than the ion range predicted by simulation. The concentration of these clusters increases as (ion dose)2.6 ± 0.1. Single anneals to higher temperatures reduce all open-volume point defect concentrations to below the limit detectable by VEPAS.