Formation Of Vacancy-type Defects Research Articles

Irradiation effects on binary tungsten alloys at elevated temperatures: Vacancy cluster formation, precipitation of alloying elements and irradiation hardening

Irradiation responses of binary W alloys were investigated systematically from the perspective of the binding energy of an alloying element with a W self-interstitial atom (W-SIA). Plates of W, W-0.3 at.% Cr, W-5 at.% Re, W-2.5 at.% Mo and W-5 at.% Ta alloys were irradiated at 1073 K with 6.4 MeV Fe ions to 0.26 dpa at the damage peak, where the binding energy of alloying element with W-SIA is in order of Cr > Re > Mo > Ta. The formation of vacancy-type defects (vacancies and vacancy clusters) was studied by using positron lifetime measurement. The precipitation of alloying elements was studied by using atom probe tomography (APT) and the hardness changes in the irradiated volumes were measured by the nanoindentation technique. The formation of vacancy-type defects was strongly suppressed by the addition of Cr and Re, while Ta and Mo had no noticeable suppression effect. The APT measurements showed fine Cr- and Re-rich precipitates in W-0.3 at.% Cr and W-5 at.% Re alloys, respectively, where the density of precipitates in the latter was clearly lower than that in the former. The distributions of Mo and Ta were uniform even after irradiation. Irradiation hardening was observed for all materials but that of W-5 at.% Re alloy was significantly smaller than the hardening of W, W-2.5 at.% Mo and W-5 at.% Ta alloys. These observations suggest that the irradiation hardening of W, W-2.5 at.% Mo, and W-5 at.% Ta alloys were mainly caused by vacancy-type defects. It was concluded that an alloying element with moderate binding energy with a W-SIA effectively suppresses vacancy formation without significantly enhanced precipitation and consequently mitigates irradiation hardening.

Formation of vacancy-type defects and hydride introduced by irradiation in pure titanium

The effect of hydrogen ion irradiation on the formation of hydride and vacancy-type defects in pure titanium was investigated by X-ray diffraction (XRD), Doppler broadening spectrum (DBS), and coincidence Doppler broadening spectrum (CDB) based on slow positron beam. The XRD results showed that γ-TiH was observed in single hydrogen ion-irradiated and hydrogen + deuterium (H + D) sequentially irradiated specimens but not in the single deuterium plasma-implanted sample. This finding indicates that the vacancy-type defects caused by H ion irradiation greatly promoted the formation of γ-TiH. The DBS results showed that for the single D plasma-implanted sample, the damage peak was closer to the surface region and lower compared to the other two samples containing γ-TiH, indicating that vacancy generated by lattice distortion didn't diffuse to the depth of the matrix, and the concentration of that was smaller, which was not conducive to the deuteride formation. For the H + D sequentially irradiated sample, the number of hydrogen-vacancy complexes (HmVn) further increased compared with the single H ion-irradiated sample. Thus, the effective volume of vacancy defects was reduced, which led to the decrease in the S parameter.

Suppression of vacancy formation and hydrogen isotope retention in irradiated tungsten by addition of chromium

To study the effect of the content of chromium (Cr) in the tungsten (W) matrix on the vacancy formation and retention of hydrogen isotopes, the samples of the W-0.3 at.% Cr alloy were irradiated with 6.4 MeV Fe ions in the temperature range of 523–1273 K to a damage level of 0.26 displacement per atom (dpa). These displacement-damaged samples were exposed to D2 gas at a temperature of 673 K and a pressure of 100 kPa to decorate ion-induced defects with deuterium. The addition of 0.3 at.% Cr into the W matrix resulted in a significant decrease in the retention of deuterium compared to pure W after irradiation especially at high temperature (≥773 K). Positron lifetime in W-0.3 at.% Cr alloy irradiated at 1073 K was almost similar to that for non-irradiated one. These facts indicate the suppression of the formation of vacancy-type defects (monovacancies and vacancy clusters) by 0.3 at.% Cr addition, which leads to the significant reduction in deuterium retention in W-0.3 at.% Cr alloy.

Formation of Vacancy-type Defects in Cold-drawn Pearlitic Steel during Tensile Deformation with Hydrogen Charging

Formation of Vacancy-type Defects in Cold-drawn Pearlitic Steel during Tensile Deformation with Hydrogen Charging

Fe effect on hydrogen interactions with intrinsic point defects and hydrogen migration in α-Al2O3 based tritium permeation barriers

α-Al2O3 based coatings are the first candidate for tritium permeation barriers (TPBs) in fusion reactors, and the impurity effect on hydrogen resistance is one of the major concerns. In the present work, Fe effect on hydrogen interactions with intrinsic point defects and hydrogen migration in α-Al2O3 under the TPB service condition has been investigated based on first-principles calculations. It is found that Fe is unfavorable for the formation of vacancy-type defects and defect complex [VAl-H] in α-Al2O3, yet beneficial for the formation of H-related defects Hi and HO. The H trapping ability by vacancy type defects in α-Al2O3 is lowered after Fe doping. Hi is the main diffusion species in Fe-doped α-Al2O3, and Fe is favorable for the H migration in α-Al2O3, which will be unfavorable for H resistance of α-Al2O3 based TPBs. During the Hi migration in Fe-doped α-Al2O3, one step of screw motion is involved, in which the forming and breaking of the H-Fe and H-O bonds occur.

Changes in the substructure of tempered martensitic steel during the application of cyclic elastic stress in the presence of hydrogen

Changes in the substructure of tempered martensitic steel during the application of cyclic elastic stress in the presence of hydrogen have been investigated. Three types of specimens were prepared. The first type was subjected to cyclic elastic stress (strain rate: 3.0×10-4 s−1, number of cycles: 70000, and stress level: 0.8±0.1 σB) without hydrogen. The second type was subjected to cyclic elastic stress with hydrogen. The third type was prepared by annealing the second type at 200 °C for 2 h. These specimens are denoted here as [(n)-cycled], [H+(n)-cycled] and [H+(n)-cycled+annealed] specimens, respectively. Tracer hydrogen was used as a probe for clarifying changes in the specimen substructure. After tracer hydrogen charging, hydrogen states present in the specimens were examined using low-temperature thermal desorption spectrometry (L-TDS). Tracer hydrogen desorption in the high-temperature range from the [H+(n)-cycled] specimen increased compared with that from the [(n)-cycled] specimen. In addition, tracer hydrogen desorption in the high-temperature range from the [H+(n)-cycled+annealed] specimen was reduced by annealing at 200 °Cto the level of the [(n)-cycled] specimen. As a result, it is inferred that the formation of vacancy type defects was enhanced under the application of cyclic elastic stress in the presence of hydrogen.

Effects of alloy composition and Si-doping on vacancy defect formation in (InxGa1–x)2O3 thin films

Various nominally undoped and Si-doped (InxGa1–x)2O3 thin films were grown by pulsed laser deposition in a continuous composition spread mode on c-plane α-sapphire and (100)-oriented MgO substrates. Positron annihilation spectroscopy in the Doppler broadening mode was used as the primary characterisation technique in order to investigate the effect of alloy composition and dopant atoms on the formation of vacancy-type defects. In the undoped samples, we observe a Ga2O3-like trend for low indium concentrations changing to In2O3-like behaviour along with the increase in the indium fraction. Increasing indium concentration is found to suppress defect formation in the undoped samples at [In] > 70 at. %. Si doping leads to positron saturation trapping in VIn-like defects, suggesting a vacancy concentration of at least mid-1018 cm−3 independent of the indium content.

Oxide Microstructural Changes Accompanying Pore Formation During Anodic Oxidation of Aluminum

Porous anodic oxide formation results from a morphological instability of uniform barrier oxide growth leading to the establishment of pores. To gain insight into this process, evidence for microstructural changes in the oxide accompanying pore initiation in anodic alumina was sought through two types of experiments: potentiodynamic anodizing and stress measurements during dissolution of the anodic films. Cyclic voltammetry during anodizing in sulfuric acid shows that pore formation coincides with the appearance of localized ohmic- conducting regions close to the oxide-solution interface of individual pores. These defects induce large current increases which are apparently responsible for growth of concave scalloped features on the metal-oxide interface at the pore base. It is argued that interface defects are generated by surface forces in the oxide that accompany flow contributing to pore initiation. Stress measurements reveal that pore initiation introduces tensile residual stress into the oxide at the pore base, suggesting the formation of vacancy-type defects in the film. Such defects are explained by increases of local conduction current density due to the concentration of anodizing current in pores. Both sets of experiments reveal that pore formation generates locally defective oxide, which may help explain the phenomenon of burning during high-rate anodizing.

Deuterium retention in W and W-Re alloy irradiated with high energy Fe and W ions: Effects of irradiation temperature

Neutron irradiation to W induces defects acting as traps against hydrogen isotopes and transmutation elements such as Re and Os. To investigate synergetic effects on radiation-induced defects and Re, deuterium (D) retention in W and W–5% Re samples were examined after irradiation with 6.4MeV Fe ions at 523–1273K followed by exposure to D2 gas at 673K. The value of D retention in W–5% Re was lower than that in W by orders of magnitude after the irradiation at high temperatures (≥1073K), while no significant effects of Re addition was observed after irradiation at 523K. Irradiation with 20MeV W ions at room temperature followed by exposure to D plasma at 443–743K also resulted in small difference in D retention between W and W–5% Re samples. The results of positron lifetime measurements showed that the reduced D retention by Re observed after high temperature irradiation was due to suppression of formation of vacancy-type defects (monovacancies and vacancy clusters) by Re.

Positron annihilation study of Fe-ion irradiated reactor pressure vessel model alloys

The degradation of reactor pressure vessel steels under irradiation, which results from the hardening and embrittlement caused by a high number density of nanometer scale damage, is of increasingly crucial concern for safe nuclear power plant operation and possible reactor lifetime prolongation. In this paper, the radiation damage in model alloys with increasing chemical complexity (Fe, Fe-Cu, Fe-Cu-Si, Fe-Cu-Ni and Fe-Cu-Ni-Mn) has been studied by Positron Annihilation Doppler Broadening spectroscopy after 1.5 MeV Fe-ion implantation at room temperature or high temperature (290 oC). It is found that the room temperature irradiation generally leads to the formation of vacancy-type defects in the Fe matrix. The high temperature irradiation exhibits an additional annealing effect for the radiation damage. Besides the Cu-rich clusters observed by the positron probe, the results show formation of vacancy-Mn complexes for implantation at low temperatures.

Formation of vacancy-type defects in titanium nickelide

In this report we briefly review the current state-of-the-art and challenges in determining point defect properties from first-principles calculations as well as from experimental measurements in titanium nickelid. . Based on the vacancy formation energy and their activation energy for vacancy migration in TiNi, vacancy mediated diffusion mechanism was examined. The behavior of vacancy defects in the TiNi structural phase transition has been described.

Charge-Induced Defect Formation in LixCoO2 Battery Cathodes: XRD and PA Spectroscopy Study

Lithium-ion batteries have developed into most advanced battery systems, e.g. laptops and mobile phones. LiCoO2 is a typical intercalation battery cathode material. However, reversible charge-discharge cycling of LiCoO2 is only possible down to 50% of the available Li-ions since further removal of Li-ions drastically reduces the capacity and cycle stability. The formation of vacancy-type defects during the charging process in LixCoO2 battery cathodes was investigated by XRD and position life-time spectroscopy and Doppler broadening of positron-electron annihilation (PA) radiation as defect specific techniques [1]. Li+-extraction, which in a battery mode corresponds to charging, was performed at 293 K under electrochemical control in a 3-electrode test-cell with a Maccor Series 4000 battery tester. The composition of the lithium-ion electrode material used was: 88wt.% LiCoO2 particles, 7 wt.% carbon black as conducting agent, 5 wt.% binder material (polyvinylidene difluoride hexafluoropropylene copolymer). Structural analysis of the electrode samples was performed by means of X-Ray diffraction using a Bruker D8 Advance diffractometer in Bragg-Brentano geometry with Cu-Kα radiation. Diffractograms were measured in the 2-Theta angle range from 150to 1300and were analysed by Rietveld refinement with the programs FULLPROF [2] and X'PertHighScorePlus (Panalytical). For positron annihilation measurements a positron source (22NaCl) was sandwiched between two identical LiCoO2 electrode samples. Positron lifetime measurements were performed with a fast-fast spectrometer with a time resolution of 221 ps. The spectra were analysed by using the program pfposfit [3]. Doppler broadening (DB) measurements were performed in a coincidence setup with two high purity Ge detectors.with energy resolution for the 511 keV annihilation γ-line in the detector system corresponds to ca. 0.88keV (FWHM). Both the Doppler broadening S parameter as well as the positron lifetime component τ1 exhibit a characteristic variation with increasing amount of Li+-extraction; the S-parameter and τ1 first increases upon decreasing x from 1 to 0.6. Further Li+-extraction causes a decrease of S and τ1 (x = 0.55), followed by a re-increase for x<0.55. Conclusions: The regime of reversible charging is dominated by vacancy-type defects on the Li+-sublattice the size of which increases with increasing Li+-extraction. Indication is found that Li+-reordering which occurs at the limit of reversible Li+-extraction (x = 0.55) causes a transition from the two-dimensional agglomerates into one-dimensional vacancy chains. Degradation upon further Li+-extraction is accompanied by the formation of vacancy complexes on the Co- and anion sublattice.

Charging-induced defect formation in LixCoO2 battery cathodes studied by positron annihilation spectroscopy

Charging-induced formation of vacancy-type defects in LixCoO2 battery cathodes was studied by the defect-specific techniques of positron lifetime spectroscopy and Doppler broadening of positron–electron annihilation radiation. The regime of reversible charging is dominated by vacancy-type defects on the Li+-sublattice the size of which increases with increasing Li+-extraction. Indication is found that Li+-reordering which occurs at the limit of reversible Li+-extraction (x = 0.55) causes a transition from two-dimensional agglomerates into one-dimensional vacancy chains. Degradation upon further Li+-extraction is accompanied by the formation of vacancy complexes on the Co- and anion sublattice.

Atomic-scale structural evolution in amorphous Nd9Fe85B6 subjected to severe plastic deformation at room temperature

Understanding the deformation-induced nanocrystallization in amorphous alloys at room temperature is proved to be a great challenge. In the present study, the formation of vacancy-type defects richly surrounded with Fe atoms in amorphous Nd9Fe85B6 subjected to room-temperature high-pressure torsion deformation is directly evidenced by positron lifetime measurements combined with a coincident Doppler broadening measurement of the positron-electron annihilation photons. This demonstrates a direct experimental evidence for shear inducing the enrichment of Fe atoms in the deformed amorphous alloy. These results presented here are of importance for understanding the deformation-induced nanocrystallization and for the technique development of nanocrystalline materials.

Hydrogen peroxide treatment induced rectifying behavior of Au∕n-ZnO contact

Conversion of the Au∕n-ZnO contact from Ohmic to rectifying with H2O2 pretreatment was studied systematically using I-V measurements, x-ray photoemission spectroscopy, positron annihilation spectroscopy, and deep level transient spectroscopy. H2O2 treatment did not affect the carbon surface contamination or the EC–0.31eV deep level, but it resulted in a significant decrease of the surface OH contamination and the formation of vacancy-type defects (Zn vacancy or vacancy cluster) close to the surface. The formation of a rectifying contact can be attributed to the reduced conductivity of the surface region due to the removal of OH and the formation of vacancy-type defects.

Gate Effect of Vacancy-type Defect of Fullerene Cages on Metal-Atom Migrations in Metallofullerenes

Metal-atom migration outside from a defective fullerene cage of a metallofullerene Gd@C(82) (Ca@C(82)), where the Gd(3+) (Ca(2+)) ion is incorporated inside the C(2)(v)()-C(82) cage, is discussed in detail at the B3LYP DFT level of theory. The metal-atom migrations are initiated by the formation of vacancy-type defects involving two coordinatively unsaturated C atoms. This step, which is assumed to proceed due to energy-particle irradiation, leads to the formation of antibonding orbitals between the two C atoms. Since the antibonding orbitals can interact with vacant d-orbitals of the Gd(3)(+)() ion in an in-phase fashion, the attractive interactions allow the Gd ion to insert into the two C atoms in the defect. As a result, the metal ion passes through the defect under energy-particle irradiation. In contrast, the Ca(2+) ion with the vacant s-orbitals does not have such orbital interactions, and thus, a C-C bond is reformed between the two C atoms, which prohibits the Ca ion from penetrating the defected C(82) cage. DFT calculations nicely demonstrate that the orbital interactions control metal-atom migration around the defect site using their orbital symmetries, and therefore, the vacancy-type defect acts as a "gate" that permits a specific atom to go out from a defected fullerene cage.

Gallium and nitrogen vacancies in GaN: Impurity decoration effects

We have performed a systematic study of magnesium and oxygen doping as well as the effect of annealing in metal organic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE) grown Gallium nitride (GaN) by positron annihilation spectroscopy. The formation of vacancy-type defects identified here depends on the presence of impurity atoms as well as their concentrations. O doping generates vacancies in the Ga sublattice with concentrations proportional to the amount of O, whereas Mg atoms introduce vacancies in the N sublattice. In addition at very high Mg doping levels ( > 10 19 cm - 3 ) vacancy clusters with inhomogeneous depth profile are formed. Annealing dissociates vacancy-impurity pairs in MOCVD GaN:Mg and the defect profile is made homogeneous by vacancy migration. Vacancies in the Ga sublattice of HVPE GaN:O are shown to be complexed with O atoms by comparing the GaN:O data with the one obtained from electron-irradiated GaN, which contains isolated Ga vacancies. This comprehensive study shows that vacancy-type defects decorated by impurities are formed in both sublattices of GaN depending on the doping of the material.

Formation of vacancy-impurity complexes in heavily Zn-doped InP

Positron annihilation spectroscopy has been applied to observe the spontaneous formation of vacancy-type defects by annealing of heavily Zn-doped InP at 500--700 K. The defect is identified as the ${V}_{\mathrm{P}}\ensuremath{-}\mathrm{Zn}$ pair by detecting the annihilation of positrons with core electrons. We conclude that the defect is formed through a diffusion process; a phosphorus vacancy migrates until trapped by a Zn impurity and forms a negatively charged ${V}_{\mathrm{P}}\ensuremath{-}\mathrm{Zn}$ pair. The kinetics of the diffusion process is investigated by measuring the average positron lifetime as a function of annealing time and by fitting a diffusion model to the experimental results. We deduce a migration energy of $1.8\ifmmode\pm\else\textpm\fi{}0.2\mathrm{eV}\mathrm{}$ for the phosphorus vacancy. Our results explain both the presence of native ${V}_{\mathrm{P}}\ensuremath{-}\mathrm{Zn}$ pairs in Zn-doped InP and their disappearance in post-growth annealings.

Positron-annihilation study of compensation defects in InP

Positron-annihilation lifetime and positron-annihilation Doppler-broadening (PADB) spectroscopies have been employed to investigate the formation of vacancy-type compensation defects in n-type undoped liquid encapsulated Czochrolski grown InP, which undergoes conduction-type conversions under high temperature annealing. N-type InP becomes p-type semiconducting by short time annealing at 700 °C, and then turns into n-type again after further annealing but with a much higher resistivity. Long time annealing at 950 °C makes the material semi-insulating. Positron lifetime measurements show that the positron average lifetime τav increases from 245 ps to a higher value of 247 ps for the first n-type to p-type conversion and decreases to 240 ps for the ensuing p-type to n-type conversion. The value of τav increases slightly to 242 ps upon further annealing and attains a value of 250 ps under 90 h annealing at 950 °C. These results together with those of PADB measurements are explained by the model proposed in our previous study. The correlation between the characteristics of positron annihilation and the conversions of conduction type indicates that the formation of vacancy-type defects and the progressive variation of their concentrations during annealing are related to the electrical properties of the bulk InP material.

A study of vacancy-related defects in (Pb,La)(Zr,Ti)O/sub 3/ thin films using positron annihilation

The formation of vacancy-type defects in La-doped lead zirconate titanate (PLZT) thin films (Zr/Ti=20/80) was studied as a function of lanthanum doping and after cooling in an oxygen-reduced ambient. The changes in the Doppler-broadening S parameter are consistent with the progressive introduction of Pb-vacancies upon La-doping. Cooling of PLZT thin films with 0 and 10% La doping in 10(-5) Torr oxygen partial pressure after growth exhibits an increase in the density of vacancy-type defects compared to films cooled in 760 Torr. It is proposed that the defects formed are likely cation-oxygen vacancy complexes.