Vacancy-type Defects Research Articles

Deuterium trapping at vacancy clusters in electron/neutron-irradiated tungsten studied by positron annihilation spectroscopy

Deuterium trapping at irradiation-induced defects in tungsten, a candidate material for plasma facing components in fusion reactors, was revealed by positron annihilation spectroscopy. Pure tungsten was electron-irradiated (8.5 MeV at ∼373 K and to a dose of ∼1 × 10−3 dpa) or neutron-irradiated (at 573 K to a dose of ∼0.3 dpa), followed by post-irradiation annealing at 573 K for 100 h in deuterium gas of ∼0.1 MPa. In both cases of electron- or neutron-irradiation, vacancy clusters were found by positron lifetime measurements. In addition, positron annihilation with deuterium electrons was demonstrated by coincidence Doppler broadening measurements, directly indicating deuterium trapping at vacancy-type defects. This is expected to cause significant increase in deuterium retention in irradiated-tungsten.

Fe2+ ion irradiated JRQ steel investigated by nanoindentation and slow-positron Doppler broadening spectroscopy

Abstract A model reactor pressure vessel (RPV) steel, known as JRQ, was manufactured in Japan for IAEA neutron embrittlement research studies in late 80 s. This model alloy belongs to the commercially used steel of A533B-1 type and shows relatively large changes in mechanical properties after a neutron irradiation due to considerable copper content (0.15 wt%). In order to simulate neutron irradiation and investigate the hardening effect, studied specimens of JRQ steel were exposed to Fe2+ ion irradiation in five different exposures calculated using the SRIM code. The ion energy of 5 MeV, temperature at 300 °C and the flux of 1.0 × 1011 cm−2 s−1 were the same during the irradiations. The hardening was investigated and observed by means of nanoindentation technique and a defect profile of irradiated steels was measured by Slow-positron Doppler broadening spectroscopy (DBS). The observed increasing trend of nanohardness as a function of fluence is in good agreement with the trend observed on the basis of Vickers hardness measured for neutron-irradiated JRQ. This confirms that Cu precipitation is most likely responsible for the observed irradiation hardening and that neutron-irradiation-induced damage can be simulated using ion irradiation in the present case. We have also excluded open volume (vacancy type) defects in the crystal lattice of JRQ steel from a responsibility for the damage arising by the Fe2+ ion irradiation.

Multiscale modelling of the interaction of hydrogen with interstitial defects and dislocations in BCC tungsten

In a fusion tokamak, the plasma of hydrogen isotopes is in contact with tungsten at the surface of a divertor. In the bulk of the material, the hydrogen concentration profile tends towards dynamic equilibrium between the flux of incident ions and their trapping and release from defects, either native or produced by ion and neutron irradiation. The dynamics of hydrogen exchange between the plasma and the material is controlled by pressure, temperature, and also by the energy barriers characterizing hydrogen diffusion in the material, trapping and de-trapping from defects. In this work, we extend the treatment of interaction of hydrogen with vacancy-type defects, and investigate how hydrogen is trapped by self-interstitial atom defects and dislocations. The accumulation of hydrogen on dislocation loops and dislocations is assessed using a combination of density functional theory (DFT), molecular dynamics with empirical potentials, and linear elasticity theory. The equilibrium configurations adopted by hydrogen atoms in the core of dislocations as well as in the elastic fields of defects, are modelled by DFT. The structure of the resulting configurations can be rationalised assuming that hydrogen atoms interact elastically with lattice distortions and that they interact between themselves through short-range repulsion. We formulate a two-shell model for hydrogen interaction with an interstitial defect of any size, which predicts how hydrogen accumulates at defects, dislocation loops and line dislocations at a finite temperature. We derive analytical formulae for the number of hydrogen atoms forming the Cottrell atmosphere of a mesoscopic dislocation loop or an edge dislocation. The solubility of hydrogen as a function of temperature, pressure and the density of dislocations exhibits three physically distinct regimes, dominated by the solubility of hydrogen in a perfect lattice, its retention at dislocation cores, and trapping by long-range elastic fields of dislocations.

The effect of ion doping at different sites on the structure, defects and multiferroic properties of BiFeO3 ceramics

The polycrystalline samples of undoped BiFeO3, Ce doped BiFeO3 (Bi0.9Ce0.1FeO3) and Zr dpoped BiFeO3 (BiFe0.9Zr0.1O3) were prepared by rapid liquid phase sintering technique. The effects of A-site substitution with Ce and B-site substitution with Zr on the structure, defects and multiferroic properties of BiFeO3 ceramics were comparative investigated. Undoped and doped BiFeO3 samples are well crystallized in perovskite structure, whilst Ce and Zr doping results in slight distortion in the lattice structure. With Ce and Zr doping, the average grain sizes decrease. Positron annihilation lifetime measurements reveal that there are vacancy-type defects in the three samples, and Bi0.9Ce0.1FeO3 sample has the maximum open volume and concentration of the vacancy defect among those three samples. Both Ce and Zr doping can improve the leakage current, ferroelectric and magnetic properties, whereas Zr doping exhibited better ability. The better electric properties in BiFe0.9Zr0.1O3 may be due to the effective reduction of oxygen vacancies, Fe2+ and impurity phases by Zr doping. The cause for better magnetic properties in BiFe0.9Zr0.1O3 sample may be that the insertion of Fe vacancies and nonmagnetically active Zr4+ ions in antiferromagnetic Fe sublattice can effectively suppress the spiral spin modulation structure.

The structure, defects, electrical and magnetic properties of BiFe1−x Zr x O3 multiferroic ceramics

BiFe1−x Zr x O3 (x = 0.00–0.30) ceramics were synthesized using solid state reaction method followed by rapid liquid phase sintering, and the microstructure, electrical and magnetic properties of the synthesized ceramics were systematically investigated. The XRD patterns show that no impurity phases exist in Zr doped samples; Zr doping induces the crystal structure distortion when x ≤ 0.10, and a structural phase transition occurs when the content of Zr varies from 0.10 to 0.20. SEM observations indicate that the average grain size is remarkably decreased by Zr doping. Positron annihilation lifetime spectra results indicate that cation vacancy-type defects exist in all samples, the cation vacancy concentration increases with increasing Zr content from 0.00 to 0.20, and then decreases with further increase of Zr content. Electrical and magnetic measurements show that enhanced leakage, ferroelectric and magnetic properties are observed in Zr doped ceramics. The analysis of microstructure and properties show that the cation vacancy defect plays an important role in modulating the electrical and magnetic properties of BiFeO3.

Microstructure, defect structure and hydrogen trapping in zirconium alloy Zr-1Nb treated by plasma immersion Ti ion implantation and deposition

The effect of low energy plasma immersion ion implantation and deposition of titanium on microstructure, defect structure and hydrogen trapping in zirconium alloy Zr-1Nb was studied. Defect structure and distribution were analyzed by Doppler broadening using slow positron beam. The surface microstructure after modification is represented by nanostructured Ti grains with random orientation. The gradient distribution of titanium as well as vacancy type defects were analyzed. The concentration of vacancy type defects is rising with increasing bias voltage. Gas-phase hydrogenation of the Ti-modified Zr-1Nb alloy was performed at 400 °C for 60 min. The strong interaction of hydrogen with vacancy type defects was demonstrated. Two different changes in the defect structure after hydrogenation were observed: when a titanium film is formed on the surface (after deposition at 500 V) hydrogen trapping occurs with the formation of titanium hydride phases, while in the implanted layer (deposition at 1000 and 1500 V) hydrogen is trapped due to interaction with vacancy type defects. The physical basis of Ti diffusion and its influence on the evolution of defect structure after surface modification and hydrogenation were discussed.

Stability of vacancy-type defect clusters in Ni based on first-principles and molecular dynamics simulations

Using first-principles calculations based on density-functional theory, the energetics of different vacancy-type defects, including voids, stacking fault tetrahedra (SFT) and vacancy loops, in Ni are investigated. It is found that voids are more stable than SFT at 0K, which is also the case after taking into account the volumetric strains. By carrying out ab initio molecular dynamics simulations at temperatures up to 1000K, direct transformations from vacancy loops and voids into SFT are observed. Our results suggest the importance of temperature effects in determining thermodynamic stability of vacancy clusters in face-centered cubic metals.

Synthesis, defect characterization and photocatalytic degradation efficiency of Tb doped CuO nanoparticles

Monoclinic undoped and Tb doped CuO are prepared by solution combustion method and annealed at different temperatures. The effect of annealing and doping on their structural and optical properties of CuO are examined using XRD, FTIR and DRS. The surface and lattice defects in CuO and Tb doped CuO is analyzed qualitatively and quantitatively using positron lifetime and Doppler broadening spectroscopy. The average positron lifetime and electron momentum (energy) S parameter increases owing to the number of vacancies in the CuO lattice upon doping and decreases with increasing temperature. The migration of vacancies from grain to grain boundary region is observed at 600°C annealed samples. At 800 °C, the overall behavior of lifetime value denotes that the vacancy type defect is recovered, cluster vacancy and microvoids exists with reducing size. The photocatalytic performance of undoped and Tb doped CuO on degradation of methylene blue (MB) and methyl orange (MO) is investigated under visible light for two different lamp power and dye concentration. The influence of annealing temperature and dopant ion on the efficiency is also elaborated. Enhanced photocatalytic efficiency in Tb doped CuO is observed upon annealing. X-ray photoelectron spectroscopy (XPS) result indicates that the valence states of Cu, O and Tb ions exist at the surface of the particles. Brunauer–Emmett–Teller N2 adsorption–desorption analyses were employed to characterize specific surface area and porosity of Tb doped CuO. The doped CuO with pore size of about ∼34nm have a surface area of 16–28m2/g. The surface area effect plays an important role in the enhanced catalytic performance on Tb doped catalysts.

Probing vacancy-type free-volume defects in Li2B4O7 single crystal by positron annihilation lifetime spectroscopy

Vacancy-type free-volume defects in lithium tetraborate Li2B4O7 single crystal, grown by the Czochralski technique, are probed with positron annihilation spectroscopy in the lifetime measuring mode. The experimental positron lifetime spectrum is reconstructed within the three-component fitting, involving channels of positron and positronium Ps trapping, as well as within the two-component fitting with a positronium-compensating source input. Structural configurations of the most efficient positron traps are considered using the crystallographic specificity of lithium tetraborate with the main accent on cation-type vacancies. Possible channels of positron trapping are visualized using the electronic structure calculations with density functional theory at the basis of structural parameters proper to Li2B4O7. Spatially-extended positron-trapping complexes involving singly-ionized lithium vacancies, with character lifetime close to 0.32 ns, are responsible for positron trapping in the nominally undoped lithium tetraborate Li2B4O7 crystal.

Electron capture by vacancy-type defects in carbon-doped GaN studied using monoenergetic positron beams

Vacancy-type defects in GaN with different carbon concentrations ([C]=2×1016−1×1018cm−3) were probed using monoenergetic positron beams. 1.5-μm-thick GaN layers were grown on Si substrates by metalorganic vapor phase epitaxy. Measurements of Doppler broadening spectra and positron lifetimes indicated that the major defect species in the GaN layers was a Ga vacancy coupled with nitrogen vacancies. The positron trapping rate of the defects decreased with increasing [C], which was attributed to the downward shift of the Fermi level position due to C acceptor and the resultant change in the defect charge state (neutral to positive). Under illumination, the defect charge state was changed from positive to neutral, which was attributed to the trapping of the excited electrons by the defects (V++e−→V0). The effect of illumination was suppressed for the GaN layer with [C]≥1018cm−3, which was ascribed to the excellent electron trapping property of C in GaN.

Vacancy-type defects in bulk GaN grown by the Na-flux method probed using positron annihilation

Defects in bulk GaN grown by the Na-flux method have been studied using a positron annihilation technique. Pyramidal bulk samples showed striation and inhomogeneous color distributions. Measurements of the Doppler broadening spectra of the annihilation radiation and lifetime spectra of positrons revealed that the concentration of vacancy-type defects increased with decreasing transparency of the samples. The major defect species was identified as a Ga vacancy coupled with nitrogen vacancies. A correlation between the oxygen incorporation and the introduction of such vacancies was observed. For c-plane GaN grown by a coalescence growth method, the concentration of vacancy-type defects was close to or under the detection limit of positron annihilation technique (≤1015cm−3), suggesting that high-quality bulk GaN can be fabricated using this method.

Effect of defect-induced carrier scattering on the thermoelectric power of graphene

The thermoelectric properties of graphene are strongly related to the defect density, and as such, these can be used to investigate carrier scattering. In this study, the defect density was controlled by the use of oxygen plasma treatment. Oxygen plasma introduces structural defects into graphene, initially introducing sp3 defects that transform into vacancy-type defects with further exposure, as indicated by XPS analysis, and these transitions cause substantial changes in both the electrical and thermoelectric properties of graphene. In this work, we estimate the effects of both defect density and species, analyzed by Raman spectroscopy, on the thermoelectric power of graphene, and find that the maximum thermoelectric power decreases with increasing defect density. We also find, from Ioffe's semiclassical approximation, that at the lower defect densities, phonons are the predominant source of carrier scattering, while at higher defect densities, the scattering is mainly caused by charged impurities, which corresponds to a change in defect population from the sp3-type to vacancies.

Empirical potential influence and effect of temperature on the mechanical properties of pristine and defective hexagonal boron nitride

The major objective of this work is to present results of a classical molecular dynamics study to investigate the effect of changing the cut-off distance in the empirical potential on the stress–strain relation and also the temperature dependent Young’s modulus of pristine and defective hexagonal boron nitride. As the temperature increases, the computed Young’s modulus shows a significant decrease along both the armchair and zigzag directions. The computed Young’s modulus shows a trend in keeping with the structural anisotropy of h-BN. The variation of Young’s modulus with system size is elucidated. The observed mechanical strength of h-BN is significantly affected by the vacancy and Stone–Wales type defects. The computed room temperature Young’s modulus of pristine h-BN is 755 GPa and 769 GPa respectively along the armchair and zigzag directions. The decrease of Young’s modulus with increase in temperature has been analyzed and the results show that the system with zigzag edge shows a higher value of Young’s modulus in comparison to that with armchair edge. As the temperature increases, the computed stiffness decreases and the system with zigzag edge possesses a higher value of stiffness as compared to the armchair counterpart and this behaviour is consistent with the variation of Young’s modulus. The defect analysis shows that presence of vacancy type defects leads to a higher Young’s modulus, in the studied range with different percentage of defect concentration, in comparison with Stone–Wales defect. The variations in the peak position of the computed radial distribution function reveals the changes in the structural features of systems with zigzag and armchair edges in the presence of applied stress.

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 lifetime studies of stainless steel under proton and Xe irradiation

Specimens of stainless steel reactor internals were irradiated with 240keV protons and 6MeV Xe ions up to 15dpa. The positron annihilation lifetime spectroscopy (PLS) was carried out to study the development of vacancy-type defects. It is indicated that positrons are trapped by vacancies, vacancy clusters and other defects (precipitate, dislocations). The short lifetime parameter τ1 decreases when specimens are irradiated to 2dpa by proton, whereas in the case of Xe irradiation, τ1 decreases when specimens are irradiated to 7dpa. The positron annihilation at small cluster with H atoms (for H irradiation) and precipitated phase (for Xe irradiation) may explain this transition. The long lifetime parameter τ2 increases continuously with damage increase. Compared with Xe irradiated specimens, a larger τ2 is obtained in proton irradiated specimens at the same damage. The gas atoms-vacancies, which are expected to be the early state of H bubbles, will develop into small bubbles with the increase of irradiation damage and thus cause long lifetime parameter increase.

Effects of stress-relief pre-annealing on deuterium trapping and diffusion in tungsten

Positron annihilation Doppler broadening spectroscopy (PA-DBS) with slow positrons was utilized to characterize the change of vacancy-type defects induced by a stress-relief annealing (1273K for 1h in vacuum) as pre-annealing can remove not only residual stresses but also most intrinsic defects of the material. Deuterium behavior after low energy (40eV/D) and high-flux (1024D/m2s) plasma exposure was analyzed by a combination of PA-DBS, elastic recoil detection analysis (ERDA) and thermal desorption spectroscopy (TDS). It is found that the vacancy defects density in tungsten is largely decreased by annealing and some intrinsic vacancies clusters to be multi-vacancies during the annealing process. With deuterium plasma exposure, a general decrease of S parameter found in both non- and pre-annealed tungsten suggests the deuterium occupation of vacancy-type defects. In combination of the S parameter change in the non- and pre-annealed tungsten, it is implied that deuterium trapping ability of vacancy-type defects is constrained due to the residual stresses. This confinement effect concentrates on the top surface and it becomes weaker with increasing depth. A similar deuterium distribution in the near surface of non- and pre-annealed tungsten supports this confinement effect. In addition, comparing the amount of deuterium retained in the near surface and in the bulk suggests that the residual stress facilitate deuterium to diffuse into a larger depth in tungsten.

Optical properties, defects, and composition of La3Ga5.5Ta0.5O14 crystals

La3Ga5.5Ta0.5O14 crystals were grown in pure argon and in an argon + 2% oxygen atmosphere. The growth atmosphere significantly effects the elemental composition of the crystals, their color, and the genesis and intensity of absorption bands in the ultraviolet, visible, and infrared spectral regions. Two types of defects were identified in the crystals: planar interstitial defects and nearly spherical vacancy-type defects.

Structure and properties of porous nickel and copper films produced by vacuum deposition from the vapour phase

The porous thick nickel and copper films were deposited by electron beam physical vapour deposition (EB-PVD) in vacuum. The microstructure and deformation behaviour of the films were investigated using scanning electron microscopy and high temperature dilatometer. It is shown that porous structure of vacuum condensates is unstable, and at heating it transforms from open-type porous structure into porous structure of closed type. It is assumed that such thermal instability of porous structure can be due to the presence of high density of vacancy-type defects in the structure of such materials. Change of structural state of vacuum condensates during heating at application of continuous load influences the conditions of their superplastic deformation. Possibility of application of such foils as interlayers in pressure welding is discussed.

Characterization the microstructure and defects of matrix graphite irradiated with Xe ions

The matrix graphite of pebble fuel elements was irradiated with 1MeV Xe ions at room temperature to fluences of 5.8×1014ions/cm2 and 2.9×1015ions/cm2, respectively. The microstructure and defects of matrix graphite samples were characterized by using scanning electron microscopy (SEM), Raman spectroscopy and slow positron beam techniques. The SEM result reveals that hundred-nanometer sized pores appear at the surface after irradiation and the density of pore increases with fluence. Raman results show that D peak (1350cm−1) and G peak (1580cm−1) are broadened after irradiation. In addition, the G peak position shifts from 1580cm−1 to 1560cm−1 with the linewidth increases from 21cm−1 to 132cm−1, corresponding to the increase in bond-angle disorder as the matrix graphite transforms from microcrystalline to amorphous carbon(a-C). The slow positron beam study shows that the defects-trapped positron S parameter increases with fluence, suggesting that the vacancy-type defects concentration or size of open volume defects increases. The analysis of Raman and slow positron beam consistently conclude that the reason for the phase transition after irradiation is the increase in irradiation-induced vacancy defects accompanied by the overlap of disordered regions.

Vacancy-Type Defects in GaN for Power Devices Probed by Positron Annihilation

Native defects and ion-implantation induced defects in GaN were studied by means of positron annihilation. Measurements of Doppler broadening spectra of the annihilation radiation for GaN layers grown on Si substrates showed that optically active vacancy-type defects were formed in the layers. Charge transition of the defects due to electron capture occurred when the layers were irradiated by photons with energy above 2.7 eV. It was found that Ti deposition and subsequent annealing introduced vacancy clusters. We also characterized vacancy-type defects in Mg-implanted GaN. The major defect species of vacancies introduced by Mg-implantation was a complex between Ga-vacancy (VGa) and nitrogen vacancies (VNs). After annealing above 1000C, these defects started to agglomerate, and the major defect species became (VGa)2 coupled with VNs. Through this work, we have demonstrated that positron annihilation spectroscopy is a powerful tool for characterizing vacancy-type defects in GaN for power devices applications.