Positron Annihilation Spectroscopy Studies Research Articles

Superior radiation resistance of ODS-RAFM steels revealed by Positron annihilation spectroscopy study

Oxide-dispersion-strengthened (ODS) reduced-activation ferritic/martensitic (RAFM) steels are considered as most promising structural materials for fusion reactors and other advanced nuclear systems due to its excellent irradiation resistance under high dose irradiations. However, the extreme complex synergistic effect between transmuted helium/hydrogen and displacement damage which most seriously affects the structure and mechanical properties of ODS-RAFM steels is very difficult to understand, especially the role of hydrogen still remains unclear. Taking advantage of very sensitive to small vacancy clusters of Positron annihilation spectroscopy (PAS) method, we conducted irradiation defect study on ODS-RAFM, RAFM and its corresponding model alloy Fe–9Cr and α-Fe to investigate their anti-swelling properties at the early stage of irradiation with hydrogen-helium synergism and the mechanism. Three distinct irradiation schemes including (1) single Fe ion beam, (2) simultaneous Fe and He (named Fe + He) ion beam, (3) subsequent H ion injection after Fe + He (named Fe + He/H) ion beam irradiation were performed. Hydrogen-helium synergistic effects on vacancy evolution within these steels were explored combining the first-principles calculation. The implantation of He was observed to significantly increase defect concentration among all four steels, while H exhibited interestingly complexity on affecting the evolution of helium bubbles. The H post-injection increased defect concentration obviously in the materials with abundant sinks, but improved poorly in ones with rare sinks. Through the systematic study of the interactions between H, He, and vacancies, it was revealed that even at the early irradiation stage, ODS-RAFM steels has already exhibited excellent irradiation resistance compared to other alloys across three irradiation schemes.

Optical, magnetic and defect studies of Ni2+ doped SrSnO3 nanostructures

The induced ferromagnetic behaviour and defect analysis of the undoped, Ni doped SrSnO3 nanostructures are examined. The undoped and Ni doped SrSnO3 (SSO) nanostructures crystallized in the orthorhombic perovskite structure, and show local structural disorders on vibrational spectroscopic analysis. The electron micrographs are witnessed rod type morphology for pure SrSnO3 and corals for Ni doped compounds. The oxidation states of elements and presence of oxygen vacancies are identified. The sharp optical absorption behaviour resulted in reduction of optical band gap. Positron annihilation spectroscopy (PAS) studies reveal the presence of site-specific intrinsic defects of considerable concentration and their consequential effect on doping. PAS significantly showed that Schottky-type VSn and VSr-XO complexes in undoped SSO. The addition of Ni2+ results in the reduction of VSr–O and the formation of Ni2+-VO-Ni2+ defect complexes. The EPR spectra show the signal with g value of 2.2432. The appearance of RT ferromagnetism could be attributed to the defects, F-center exchange (FCE) interactions in Ni-doped SSO nanostructures.

Investigation of the effect of Au2O3 dopant on elastic properties of PbO-B2O3-SeO2: Er2O3 glass ceramics by ultrasonic techniques

Various elastic coefficients of Au2O3 doped PbO-B2O3-SeO2:Er2O3 (PBSE) glass ceramics were evaluated as functions of Au2O3 content using ultrasonic velocity measurements. The elastic coefficients and micro-hardness showed a decreasing tendency with the concentration of Au2O3. Such decrease is attributed to the increasing concentration of gold metallic particles and [SeO3]2− groups that acted as modifiers and induced imperfections in these samples. Obtained results were observed to be consistent with the conclusions drawn from spectroscopic studies that include X-ray photoelectron spectroscopy (XPS), infrared (IR), photoluminescence (PL) and positron annihilation (PAL) spectroscopy studies. Overall, these studies have revealed that even though, the presence of gold metallic particles is preferable for achieving superior luminescence and electrical properties, presence of such particles caused to decrease the elastic coefficients and micro-hardness of these glass ceramics. However, when the concentration of Au2O3 is increased beyond 0.075 mol%, we have observed a slight increase of elastic coefficients and micro-hardness.

Positron Annihilation Spectroscopy Study of Metallic Materials after High-Speed Cutting.

During high-speed cutting, a white layer is often produced on the machined surfaces after mechanical machining, seriously affecting the mechanical properties. These properties are related to the material structure and the defects induced by cutting. However, there is a lack of research on the atomic-scale defects of the white layer. This paper studied the influence of cutting parameters, namely the feed rate, cutting speed and cutting depth, on atomic-scale defects induced by high-speed cutting in GCr15 steel. Positron annihilation studies showed typical plastically deformed or tempered carbon steel defects with additional vacancy cluster components. The quantity of these clusters changed with cutting parameters. Furthermore, significant changes were observed in the subsurface region up to 1 µm, occurring as a result of simultaneous phase transformations, deformation and thermal impacts. The predominant accumulation of only one type of atomic-scale defect was not observed.

High ionic conductivity and ion conduction mechanism in ZIF-8 based quasi-solid-state electrolytes: a positron annihilation and broadband dielectric spectroscopy study.

The low ionic conductivity and electrode-electrolyte interface instability issues with solid polymer electrolytes jeopardize their electrochemical performances in lithium-ion batteries (LIBs). The use of quasi-solid-state electrolytes (QSSEs) with concentrated Li salt embedded inside the pore networks of metal organic frameworks (MOFs) can successfully address the aforementioned issues. Owing to the sieve effect of zeolitic imidazolate framework-8 (ZIF-8) towards selective cation permeability over anions through the interconnected pore network, a unique QSSE with LiTFSI salt concentrated in the ZIF-8 skeleton used as a filler in poly(ethylene oxide) has been synthesized. LiTFSI gets embedded inside the interconnected pore network of ZIF-8 that furnishes unhindered pathways for Li+ ion migration leading to a very high ionic conductivity of ∼6 × 10-4 S cm-1. The higher ionic conductivity is directly related to the Li+ ion conduction through the pore network of ZIF-8 which has been experimentally evident from complementary methods viz. Positron annihilation and broadband dielectric spectroscopy. The design route towards these types of QSSEs encompassing porous MOFs paves the way for realizing Li superionic conductors suitable for practical application in commercial LIBs with high safety and stability.

Role of free volumes and segmental dynamics on ion conductivity of PEO/LiTFSI solid polymer electrolytes filled with SiO2 nanoparticles: a positron annihilation and broadband dielectric spectroscopy study.

The limited ionic conductivity of polymer electrolytes is a major issue for their industrial application. Enhancement of ionic conductivity in the poly(ethylene oxide), PEO, based electrolyte has been achieved by loading passive nanofillers such as SiO2 nanoparticles (NPs). To investigate the role of modifications in free volume characteristics and the polymer chain dynamics induced by the loading of passive fillers on the ionic conductivity of the PEO based ternary electrolyte, a systematic investigation has been carried out using positron annihilation and broadband dielectric spectroscopy. As a result of interfacial interactions, the loading of SiO2 NPs alters the semi-crystalline morphology of PEO resulting in a higher crystallinity at lower loadings due to the surface confinement of PEO chains, and the formation of smaller PEO crystallites at higher loadings due to interparticle nanoconfinement. These modifications are accompanied by a decrease in free volume fraction at the lowest loading (0.5 wt%) followed by an increase at higher loadings (≥2.0 wt%). The Almond-West formalism considering two different universalities in different temperature and frequency ranges has been used to explain the ion-conduction process at different NP loadings. The Li ion conductivity is observed to be maximum for a 5.0 wt% loading of SiO2 NPs. The enhancement in ionic conductivity is observed to be directly correlated with the free volume characteristics and segmental dynamics of the PEO matrix, confirming their role in ion transport in polymer electrolytes.

Effects of high-energetic 3He+ ion irradiation on tungsten-based composites

The interaction of plasma and its encompassing materials with one another is one of the principle designing issues of fusion reactors. Tungsten is considered one of the primary candidate materials in fusion applications due to its superior properties. In this work, we doped vanadium carbide powders to the tungsten matrix to enhance the properties of tungsten alloys. Tungsten-based composites, which irradiated with 2.5 MeV 3He+ ions at room temperature, were analyzed by atomic force microscopy (AFM) in order to obtain surface morpohologies after irradiation. Helium ion irradiated tungsten-based composites were studied by using XRD, neutron diffraction technique, raman spectroscopy, and positron annihilation spectroscopy to reveal the microstructural changes. XRD and ND analyses clarified the changes in the crystal structures of the tungsten-based materials after He ion irradiation. The simulation of radiation damage and the calculation of displacements per atom (DPA) was also determined by the SRIM code. SRIM showed that the maximum helium concentration in the specimens takes place in the depth range of 58–65 nm. The crystallite size of tungsten-based composites slightly increased after helium ion irradiation. AFM results revealed that the maximum size of bubbles on the surface of tungsten-based composites shape under 100 nm. Positron annihilation spectroscopy studies of the specimens have been discussed before and after 3He+ ion irradiation.

Corrigendum to “A detailed ellipsometric porosimetry and positron annihilation spectroscopy study of porous organosilicate-glass films with various ratios of methyl terminal and ethylene bridging groups” [Microporous Mesoporous Mater. 306 (2020) 110434

Corrigendum to “A detailed ellipsometric porosimetry and positron annihilation spectroscopy study of porous organosilicate-glass films with various ratios of methyl terminal and ethylene bridging groups” [Microporous Mesoporous Mater. 306 (2020) 110434

Positron annihilation spectroscopy study of radiation-induced defects in W and Fe irradiated with neutrons with different spectra

The paper presents new knowledge on primary defect formation in tungsten (W) and iron (Fe) irradiated by fission and high-energy neutrons at near-room temperature. Using a well-established method of positron-annihilation lifetime-spectroscopy (PALS), it was found that irradiation of W in the fission reactor and by high-energy neutrons from the p(35 MeV)-Be generator leads to the formation of small radiation-induced vacancy clusters with comparable mean size. In the case of Fe, smaller mean size of primary radiation-induced vacancy clusters was measured after irradiation with fission neutrons compared to irradiation with high-energy neutrons from the p(35 MeV)-Be generator. It was found that one of the reasons of the formation of the larger size of the defects with lower density in Fe is lower flux in the case of irradiation with high-energy neutrons from the p(35 MeV)-Be source. The second reason is enhanced defect agglomeration and recombination within the energetic displacement cascade at high energy primary knock-on-atoms (PKAs). This is consistent with the concept of the athermal recombination corrected (arc-dpa) model, although the measured dpa cross-section of both fission neutrons and wide-spectrum high-energy neutrons in W is between the conventional Norgett–Robinson–Torrens (NRT-dpa) and arc-dpa predictions. This means that the physics of the primary radiation effects in materials is still not fully known and requires further study through a combination of modeling and experimental efforts. The present data serve as a basis for the development of an improved concept of the displacement process.

Electrical and optical properties in O-polar and Zn-polar ZnO films grown by pulsed laser deposition

O-polar and Zn-polar ZnO films were grown on c-sapphire by pulsed laser deposition. Positron annihilation spectroscopy study reveals that the VZn-related defects in the ZnO films with different polarities are different in structure and their thermal evolution is different. Hall effect measurement and luminescence spectroscopy reveal that the electrical and optical properties and their corresponding thermal evolution are strongly dependent on the polarity of the film. The luminescence spectra of the as-grown Zn-polar ZnO film is signified by a negligible green defect emission (at ~ 2.4 eV) and strong near band edge emission as compared with the O-polar film. The as-grown Zn-polar film exhibited a lower electron concentration (2 × 1018 cm−3) than that of the O-polar film (6 × 1018 cm−3); this difference is attributed to their different H concentrations. For the O-polar film, the electron concentration decreased with annealing temperature Tanneal, reaching a minimum at 700°C and then increased to 4 × 1018 cm−3 at Tanneal = 900 °C. In comparison, the electron concentration of the Zn-polar ZnO film monotonically decreased with Tanneal attaining a value of ~1 × 1017 cm−3 at Tanneal = 900 °C, 40 times smaller than that of the O-polar film. The cause for the differences in the optical and electrical properties for the O-polar and Zn-polar films is explained by the presence of different defects in these films.

A detailed ellipsometric porosimetry and positron annihilation spectroscopy study of porous organosilicate-glass films with various ratios of methyl terminal and ethylene bridging groups

Abstract Organosilicate-glass films with a varying ratio of terminal methyl and bridging ethylene groups are synthesized using 1,2-bis(trimethoxysilyl)ethane/methyltrimethoxysilane mixtures and sol-gel technology. The films are characterized by Fourier Transform Infrared spectroscopy, Ellipsometric Porosimetry and Positron Annihilation Spectroscopy. The hard bake at 400 °C generates the final pore structure, which depends on the curing environment. It is shown that ethylene bridge is destructed during the hard bake in the air via formation of peroxide radicals that form ≡SiOH during the transformation. Continuous hard bake leads to condensation of silanol groups and form a structure similar to the ordinary silica. The pore size of highly porous materials (>30%) is larger in air cured films. Destruction of the ethylene bridge makes the films matrix soft and micropores collapse due to the capillary forces during the template evaporation. It leads to the film shrinkage, increases the size of internal voids. The air cured samples showed better mechanical properties than N2 cured ones although in the last case ethylene bridging groups were preserved. The reason is that the collapse of micropores increase the internal density and creates more favorable condition for condensation of silanol groups.

Insight into the spontaneous breakdown of ‘Toughened glass’: From nano- to macroscale

Instances of sudden breakdown of ‘toughened glass’ (TG) without apparent causes have grave consequences. Here we present results of investigations on two types of samples – pieces of a TG door that broke down spontaneously (STG) and pieces of a TG sheet broken down intentionally (ITG). In both cases, the broken pieces have the same morphology and were studied through X-ray Diffraction (XRD), X-ray Fluorescence Spectroscopy (XRF), X-ray Absorption Near-edge Spectroscopy (XANES) at Na K-edge, Fourier Transform Infrared Spectroscopy (FTIR), Twin Mercury Porosimetry (TMP), Atomic Pair Distribution Function (PDF) and Positron Annihilation Spectroscopy (PAS). XRD and PAS showed the same structure and high defect concentration for both the samples indicating no major difference in microstructure due to aging in STG. No trace of Ni or S could be observed from XRF, casting doubt on the presence of NiS inclusions. PAS studies indicated very large concentration of free volume cavities in the STG sample, implying that mechanical instability leading to the erosion of the structure and loss of rigidity is a result of atomic motions within the glass and the associated weakening of the molecular bonds. From XRF, a small depletion in Na content was seen in STG, which became much more apparent from low angle XANES, pointing to a surface depletion. From FTIR, a reduction in the Si–O–Si bonds and an enhancement in the O–H bonds could be identified indicating a conversion of the former to Si(OH)4. The bond distances of different pair of atoms were calculated from atomic Pair Distribution Function experiments. An increase in total porosity especially in larger pores (diameter ≥100–200 µm, from TMP) were observed in STG over ITG. Based on these results, we propose a simple model in which the Si–O network is visualized as suffering from destabilization due to the leaching out of the Na-ions from the STG over time, leading to the accumulation of large pores, increase of overall porosity and the final collapse of the structure.

Positron annihilation spectroscopy study of vacancy-type defects in He implanted polycrystalline α-SiC

The evolution of He implantation-induced defects in polycrystalline α-SiC has been investigated by a combination of positron annihilation Doppler broadening spectroscopy (DBS) and transmission electron microscopy (TEM). Samples implanted at room temperature with 230 keV He+ ions to fluences of 1 × 1015, 5 × 1015 and 1 × 1016 ions/cm2 were isochronally annealed in vacuum at temperatures up to 1400 °C with a step of 100 °C. Excellent correlation between the calculated damage profile and the positron annihilation depth profile were obtained at low annealing temperatures, after considering the positron depth distribution given by so-called Makhovian profiles.The DBS data from the peak region revealed a reasonable correlation between positron trapping at vacancy-type defects and the He-to-dpa ratio. Thermal annealing at 1400 °C resulted in the incomplete recovery of the microstructure, where large cavities distributed along grain boundaries in the damage peak region are surrounded by small agglomerations of (helium)vacancy-type defects.

An Insight into the Spontaneous Breakdown of 'Toughened Glass' from Nano- to Macroscale

Sudden breakdown of 'toughened glass' (TG) without apparent causes have grave consequences. We present results of investigations on two samples - pieces of a TG door that broke down spontaneously (STG) and pieces of a TG sheet broken down intentionally (ITG). Both samples have same morphology and X-ray Diffraction showed the same structure for both samples indicating no major difference in microstructure. No trace of Ni or S could be observed from X-ray Fluorescence Spectroscopy whereas a small depletion in Na content was seen in STG, which became much more apparent from low angle X-ray Absorption Near-edge Spectroscopy at Na K-edge, pointing to a surface depletion. Positron Annihilation Spectroscopy studies indicated very large concentration of free volume cavities in STG, implying that the erosion of the structure and loss of rigidity is a result of atomic motions within the glass and the associated weakening of the molecular bonds. From Fourier Transform Infrared Spectroscopy, a reduction in the Si-O-Si bonds and an enhancement in the O-H bonds could be identified indicating a conversion of the former to Si(OH)_4. The bond distances of different pair of atoms were calculated from Atomic Pair Distribution Function experiments. An increase in total porosity especially in larger pores were observed in STG over ITG, from Twin Mercury Porosimetry. We propose a simple model in which the Si-O network is destabilized due to leaching out of Na-ions from STG over time, leading to accumulation of large pores, increase of overall porosity and final collapse of the structure.

Investigating the Correlation of Segmental Dynamics, Free Volume Characteristics, and Ionic Conductivity in Poly(ethylene oxide)-Based Electrolyte: A Broadband Dielectric and Positron Annihilation Spectroscopy Study

Poly(ethylene oxide), PEO, based solid polymer electrolyte with different loadings of a lithium salt, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), has been investigated to study the role of...

Correlation studies between physical properties and concentration of voids entrenched in V2O5 mixed lead bismuth silicate glass system by means of positron annihilation spectroscopy

In this investigation we have estimated the fraction of free volume defects and also their size (to the nanoscale) entrenched in V2O5 doped PbO added Bi2O3–SiO2 glass system (by means of positron annihilation spectroscopy (PAS) studies) with a view to understand the influence of such defects on several physical properties of this glass system. The glasses of the chemical composition viz., 30PbO–5Bi2O3‒ (65- x) SiO2:x V2O5 (0 ≤ x ≤ 0.6, insteps of 0.1) are used in this study. Glasses were prepared by traditional melt quenching technique. PAS studies were perormed using 22Na (0.1 MBq) positron radioactive source. Positron annihilation lifetime is found to be significantly affected by the concentrations of V2O5 dopant. The third component I3 of positrons is found to be feeble in all the glasses. For this reason, initially, we have used a simple two component positron trapping model (ignoring the third component) to calculate annihilation parameters viz., the bulk average positron lifetime (τb), positron trapping rate in defect (κd), fraction of trapped positrons (η), the average lifetime for all the positrons (τavg) and defect-related time (τd). The calculations were repeated with three component positron model. The results of positron annihilation studies have indicated that concentration of free volume nano sized defects, the fraction (fv) of volume defects and their radius (R) are the lowest for the glass V3. Such minimal value of defect concentration is ascribed to dominant existence of vanadium ions in V5+ valence state that involves in glass network forming with VO5 structural units. Whereas, the observed increased value of fv with increase of V2O5 from 0.3 to 0.6 mol% is attributed to growing concentration of vanadyl complexes (VO)2+ that have acted as modifiers. Quantitative information obtained on dependence of concentration of free volume imperfections on V2O5 content in the studied glass is observed to be in good agreement with the inferences drawn from several macroscopic properties that include electrical properties reported earlier.

Defect-Engineered MoS2 Nanostructures for Reactive Oxygen Species Generation in the Dark: Antipollutant and Antifungal Performances.

Meticulous surface engineering of layered structures toward new functionalities is a demanding challenge to the scientific community. Here, we introduce defects on varied MoS2 surfaces by suitable doping of nitrogen atoms in a sulfur-rich reaction environment, resulting in stable and scalable phase conversion. The experimental characterizations along with the theoretical calculations within the framework of density functional theory establish the impact of nitrogen doping on stabilization of defects and reconstruction of the 2H to 1T phase. The as-synthesized MoS2 samples exhibit excellent dye removal capacity in the dark, facilitated by a synergistic effect of reactive oxygen species (ROS) generation and adsorption. Positron annihilation spectroscopy and electron paramagnetic resonance studies substantiate the role of defects and associated sulfur vacancies toward ROS generation in the dark. Further, on the basis of its ample ROS generation in the dark and in the light, the commendable antimicrobial activity of the prepared MoS2 samples against fungal pathogen Alternaria alternata has been demonstrated. Thus, the present study opens up a futuristic avenue to develop newer functional materials through defect engineering by suitable dopants toward superior performances in environment issues.

Positron annihilation spectroscopy study of defects in hydrogen loaded Zr-1Nb alloy

The purpose of this work is to implement the positron lifetime and Doppler broadening techniques for the study of hydrogen-induced defects in Zr–1wt% Nb (hereinafter Zr–1Nb) alloy. It was shown that spark cutting is an appropriate instrument to obtain a pair of Zr-1Nb samples with the same hydrogen content for the “sandwich” geometry and does not introduce open-volume defects in them during the cut. The ab-initio calculations were implemented to evaluate the positron annihilation characteristics for expanded crystal lattice, Zr with absorbed hydrogen, Zr-vacancy and Zr-vacancy-hydrogen complexes. The type of defects created in Zr-1Nb alloy loaded with hydrogen from the gas phase up to various hydrogen concentrations was studied. The main types of defects induced by hydrogen loading are vacancy-hydrogen complexes and dislocations. The processes of crystal lattice expansion and hydrogen dissolution in the lattice were studied.

Positron annihilation studies of irradiation-induced defects with swift heavy ions in gold

Results of positron annihilation spectroscopy studies of pure gold samples irradiated with various doses of Xe26+ have been reported. The conventional positron lifetime measurements and positron beam tests were performed. Increase of positron lifetime in irradiated samples up to ca. 170 ps is interpreted as the existence of stacking fault tetrahedras in all studied samples. The damaged zone determined in etching experiment equals 9 μm and it is in a good agreement with theoretical results. The long range effect was not found. The positron beam studies provide information about the increase of defect concentration with the growing number of displacements per atom achieved during irradiation.

Vacancy cluster in ZnO films grown by pulsed laser deposition

Undoped and Ga-doped ZnO films were grown on c-sapphire using pulsed laser deposition (PLD) at the substrate temperature of 600 °C. Positron annihilation spectroscopy study (PAS) shows that the dominant VZn-related defect in the as-grown undoped ZnO grown with relative low oxygen pressure P(O2) is a vacancy cluster (most likely a VZn-nVO complex with n = 2, 3) rather than the isolated VZn which has a lower formation energy. Annealing these samples at 900 °C induces out-diffusion of Zn from the ZnO film into the sapphire creating the VZn at the film/sapphire interface, which favors the formation of vacancy cluster containing relatively more VZn. Increasing the P(O2) during growth also lead to the formation of the vacancy cluster with relatively more VZn. For Ga-doped ZnO films, the oxygen pressure during growth has significant influence on the electron concentration and the microstructure of the VZn-related defect. Green luminescence (GL) and yellow luminescence (YL) were identified in the cathodoluminescence study (CL) study, and both emission bands were quenched after hydrogen plasma treatment. The origin of the GL is discussed.