Positron Annihilation Study Research Articles

Effect of V2O5 incorporation on the Structural, Electrical, and Conduction Properties of sulfonated PVC/PMMA Blend: Positron Annihilation Study

Effect of V2O5 incorporation on the Structural, Electrical, and Conduction Properties of sulfonated PVC/PMMA Blend: Positron Annihilation Study

Defect formation analysis in gamma-irradiated titanium nitride nanocrystals: predictions from positron annihilation studies

Defect formation analysis in gamma-irradiated titanium nitride nanocrystals: predictions from positron annihilation studies

Study of Ce3+-substitution induced defects restructuring in BiFeO3 by positron annihilation and other supportive methods

The results of Ce3+-substitution in place of Bi3+ in BiFeO3 nanocrystallites are reported in this work. Although Bi1-xCexFeO3 (x = 0, 0.025, …, 0.175) was formed with distinct crystallographic planes of X-ray diffraction, no structural transformation was observed unlike in the case of other lanthanide ion substitution of Bi3+ ions as reported earlier. The nanocrystallite sizes and the lattice parameters decreased during the increase of substitution. Transmission electron microscopic images also revealed the nanostructure formation and the decrease of the particle sizes. Energy dispersive X-ray analysis revealed appreciable incorporation of Ce into the BiFeO3 lattice. The measured band gap energies decreased owing to the creation of acceptor levels by Ce3+ ions. Distinct peaks in the X-ray photoelectron spectra at the expected binding energies indicated the presence of the two oxidation states Ce3+ and Ce4+. Positron annihilation studies proved the formation of interfacial defects with increase in substitution. Majority of the positrons diffused to the surfaces of the nanocrystallites and the mean lifetime of positrons increased. Coincidence Doppler broadened spectra, however, indicated positron trapping in the interfacial defects of the oxides of Ce and Bi overriding the annihilations of positrons at the surfaces in the range x = 0.075 to 0.125.

Defect analysis and microstructural characterization of zinc-substituted cadmium oxide nanocrystallites by positron annihilation and supplementary methods

Pure and zinc-doped cadmium oxide nanocrystallites of sizes in the range 25 nm to 16 nm are synthesized by adopting a chemical precipitation method and by varying the doping concentration from 0.0 to 0.25. The decrease in nanocrystallite sizes with increasing substitution is expected from the smaller ionic radii of Zn2+. But more revealing is the interfacial defects formation at higher concentration of doping, which is attributed to the dissimilar crystalline structure of ZnO and CdO. X-ray diffraction patterns show well defined peaks and additional characterisation is done through transmission electron microscopy. The optical band gap measurements indicate the dominance of substitution-induced disorder over the confinement of excitons, leading to a decrease in the band gap energies. The results of positron annihilation studies confirm the cancellation of the existing vacancy type defects in the initial stage, followed by the substitution. Photoluminescence spectra reveal the distinct peaks of optical plasmonic excitations and the defect population in the bandgap and the intensity variations agreed with that of the defect related positron annihilation lifetime intensity. The segregation of ZnO phase leading to the formation of interfacial boundaries is found as a strong deterrent against the success of continued substitution.

Ample Lewis Acidic Sites in Mg2B2O5 Facilitate N2 Electroreduction through Bonding-Antibonding Interactions.

Extensive research on the electrochemical nitrogen reduction reaction (NRR) has put forward a sound list of potential catalyst materials with properties inducing N2 adsorption, protonation, and reduction. However, rather than a random selection of catalysts, it is essential to understand the vitals in terms of orbital orientation and charge distribution that actually manipulate the rate-determining steps of NRR. Realizing these factors, herein we have explored a main group earth-abundant Mg-based electrocatalyst Mg2B2O5 for NRR due to the abundance of Lewis acid sites in the catalyst favoring the bonding-antibonding interactions with the N2 molecules. Positron annihilation studies indicate that the electronic charge distribution within the catalyst has shallow surface oxygen vacancies. These features in the catalyst enabled a sound Faradaic efficiency of 46.4% at -0.1 V vs reversible hydrogen electrode for the selective NH3 production in neutral electrolyte. In situ Fourier transform infrared suggests a maximum N-N bond polarization at -0.1 V and detected H-N-H and -NH2 intermediates during the course of the NRR on the catalyst surface. In a broader picture, the biocompatibility of Mg2+ diversifies the utility of this catalyst material in N2/biofuel cell applications that would certainly offer a green alternative toward our goal of a sustainable society.

Defect characteristics of cadmium oxide nanocrystallites synthesized via a chemical precipitation method

In this study, we synthesized cadmium oxide nanocrystallites measuring 16 nm–30 nm via a chemical precipitation method at synthesis temperatures ranging from 400 °C to 800 °C. X-ray diffraction analysis identified well-defined peaks with no indication of mixed or impurity phases. The samples were also characterized by transmission electron microscopy. The band gap energies measured by optical absorption varied between 2.32 eV and 2.38 eV, thereby indicating quantum confinement effects over the entire range of nanocrystallite sizes. Fourier transform-infrared spectroscopic measurements demonstrated the localized surface plasmonic resonance behavior of the samples. Positron annihilation studies were conducted to identify and quantify the vacancy type defects, sizes, and concentrations. Increased annihilation of positrons occurred on the surfaces of nanocrystallites with dimensions less than the thermal diffusion length of positrons (∼25.4 nm). In crystallites larger than this limit, positrons were trapped in the non-stoichiometric defects within the crystallites, as demonstrated by both the positron lifetime and coincidence Doppler broadening measurements. Defects played a major role in determining the favorable properties of these cadmium oxide nanocrystallites because defects and parameters such as the synthesis temperature were strongly correlated.

Positron annihilation study of the reactor pressure vessel model steels irradiated in the high flux reactor

Twelve reactor pressure vessel (RPV) model steels with parametric variation of alloying elements Ni and Mn were characterized by means of positron annihilation spectroscopy before and after irradiation at the High Flux Reactor (HFR) to ∼0.18 dpa at an average temperature of 286 °C. Two independent positron annihilation lifetime spectrometers (PALS) and one coincidence Doppler broadening spectrometer (CDBS), used in the post-irradiation examination, provided a substantial amount of experimental data used to characterize the vacancy-type defects and their agglomerations in the studied materials. The obtained results show an apparent increase in the positron trapping at vacancy-type defects after irradiation, with the concentration of the radiation-induced vacancy-type defects increasing with the Ni and Mn content. The effect of Si and Cu was found to be less pronounced, and the concentration of vacancy-type defects seems to anti-correlate with their content. In addition to open-volume vacancy-type defects, the PALS technique enabled a quantitative estimation of the dislocation-type defects, where only a modest increase in density was observed after the irradiation at the given temperature.

Freezing of water–ethanol mixture in nano-confinement: A positron annihilation study

Low temperature positron annihilation lifetime spectroscopy has been utilised to understand the freezing behaviour of water–ethanol mixture confined in silica nanopores having average pore size ∼ 8 nm. The temperature of ice nucleation from supercooled water in presence of ethanol cluster inside the pore has been explored through the change in ortho-positronium (o-Ps) lifetime at low temperature. It is revealed that the confinement plays an important role in freezing of water–ethanol mixture by reducing the freezing temperature far below the bulk freezing point of the mixture. Our finding is important in understanding the partial freezing of water in water–ethanol mixture under nano-confinement in the field of separation science.

Positron Annihilation and EBSD Studies of Subsurface Zone Created During Friction in Vanadium

Abstract Positron lifetime and electron backscatter diffraction studies were performed on pure vanadium samples subjected to the sliding test. The changes in the microstructure produced in this test are extending even to a depth of about 450 µm. As shown by the diffraction of electron backscattering, near this surface, just after starting the sliding test, a tribolayer with refinement randomly oriented grains is formed. The thickness of this layer varies from 8 µm to 19 µm depending on the sliding conditions and the mean grain size is about 0.2 µm. Its source is believed to be a continuous process of dynamic recrystallization. In this layer, the formation of cracks and debris is observed as well. Below is an intermediate layer with elongated grains and serrated grain boundaries in which a geometric dynamic recrystallization process has been recognized as well.

Evolution of the free volume during water desorption in thermoplastic starch/citric acid films: In situ positron annihilation studies

The effect of citric acid (CA) concentration and water content on the free hole volume of thermoplastic cassava starch films (TPS) was studied. To this aim, continuous in situ positron annihilation lifetime spectroscopy measurements were performed at fixed moisture content and during water desorption. The results show that the increase in CA concentration leads to wider free hole volume distributions with lower mean values. During water desorption, the mean values and width of such distributions systematically decrease with the exposure time, and the evolution of the hole volumes was well-described using the Kohlrausch-Williams-Watts function. The water vapour permeability was significantly higher in films incorporating 5 % (w/w) of CA, in line with the more open network of this material that was revealed in the hole volumes distribution. The Young's modulus of all the developed films increased significantly after partial water desorption, which was attributed to the plasticizer loss reflected in a decrease in the mean hole volume value (between 4 % and 13 %). This work evidences that the control and report of the relative humidity are essential when testing TPS-based films, as their nanostructures are strongly dependent on external conditions.

Identification and evolution of ultrafine precipitates in Fe-Cu alloys by first-principles modeling of positron annihilation

Understanding the formation and evolution of Cu precipitates in Fe-based alloys is crucial as they are key factors responsible for hardening and embrittlement. Dilute FeCu alloys are model materials for structural components in various application areas, including that of reactor pressure vessel steels for light water nuclear reactors. Positron annihilation spectroscopy (PAS) is a powerful tool to study the nucleation stage of both homogeneous and heterogeneous Cu precipitation, which are beyond the reach of most other experimental techniques. In this work, we present a first-principles study of positron annihilation in Fe-Cu systems. The positron annihilation characteristics (positron lifetimes, Doppler broadening spectra) are calculated for both homogeneous vacancy-free Cu clusters and heterogeneous vacancy-Cu complexes using two-component density functional theory. The theoretical results excellently agree with the available reference PAS experimental results. Our calculations show that the types of Cu precipitates can be clearly distinguished by positron annihilation, and the sizes of Cu precipitates can also be reasonably well estimated with our calculations. Moreover, we also successfully analyze the evolution of the experimental signals during isochronal annealing where the small Cu clusters change character. This work improves the understanding of the early-stage Cu precipitation in Fe matrix.

Positron Annihilation Study of RPV Steels Radiation Loaded by Hydrogen Ion Implantation.

Specimens of 15Kh2MFAA steel used for reactor pressure vessels V-213 (VVER-440 reactor) were studied by positron annihilation techniques in terms of their radiation resistance and structural recovery after thermal treatment. The radiation load was simulated by experimental implantation of 500 keV H+ ions. The maximum radiation damage of 1 DPA was obtained across a region of 3 µm. Radiation-induced defects were investigated by coincidence Doppler broadening spectroscopy and positron lifetime spectroscopy using a conventional positron source as well as a slow positron beam. All techniques registered an accumulation of small open-volume defects (mostly mono- and di-vacancies) due to the irradiation, with an increase of the defect volume ΔVD ≈ 2.88 × 10−8 cm−3. Finally, the irradiated specimens were gradually annealed at temperatures from 200 to 550 °C and analyzed in detail. The best defect recovery was found at a temperature between 450 and 475 °C, but the final defect concentration of about ΔCD = 0.34 ppm was still higher than in the as-received specimens.

Positron annihilation and magnetic studies of gamma irradiated nickel ferrite nanoparticles sintered at various temperature

The effects of gamma irradiation on structure and magnetic properties of nickel ferrite nanoparticles, prepared by the sol-gel method and sintered at 300, 500 and 900 °C, are studied through X-ray diffraction, positron annihilation lifetime spectroscopy, coincidence Doppler broadening spectroscopy and vibrating sample magnetometer. The prepared samples were irradiated with gamma rays from 60Co, 137Cs and 22Na for 1, 2 and 3 weeks that received 0.47, 0.94 and 1.41 rad dose of radiation. The XRD patterns showed that the prepared and irradiated samples are single phase with cubic spinel structure. The cation distribution, lattice constant, average crystallite size and lattice strain were obtained by the Rietveld refinement of XRD patterns and their variations with irradiation depend on sintering temperature and therefore the particle size. Scanning electron microscopy confirmed formation of nanoparticles. Positron annihilation lifetime and coincidence Doppler broadening measurements showed that for prepared samples with an increase in sintering temperature the size of vacancies increase while their concentrations decrease. It is observed that the effect of gamma radiation on samples depend on sintering temperature or particles size of samples. Also, the type of defects in sample sintered at 300 °C recognized different from samples sintered at 500 and 900 °C. Saturation magnetization and coercivity increased for prepared samples with increase in sintering temperature. The variations of saturation magnetization and coercivity with gamma irradiation for samples sintered at various temperature were different and explained with variation in cation distribution and surrounding environment of vacancies in samples. The measurements showed that samples with more vacancy concentration undergo more cation redistribution due to gamma radiation.

High-performance CeO2:Co nanostructures for the elimination of accidental poisoning caused by CO intoxication

Owing to the global upward trend of accidental carbon monoxide (CO) poisoning in the past 30 years, this work aimed to develop Cobalt-doped CeO2 particles by the microwave-assisted hydrothermal route under distinct conditions. Their structural, morphological, spectroscopic and electrical behaviors were investigated to correlate the influence of Co on their properties with the introduction of oxygen vacancies and their sensing capability to assist in the mitigation of CO poisoning cases. The samples were crystalline and had no secondary phases. Two distinct activation energies for the electrical conduction processes were observed due to dopant influence, corroborating the local cluster-to-cluster charge transfer (CCCT) mechanism, resulting in a response time of only 3s for the 4% Co-doped sample. On the other hand, through positron annihilation studies we showed that the oxygen vacancies are preferentially formed near Co ions, reducing the Co ion charge and leading to the formation of neutral VO-Co+2 complex clusters.

A study of Cr3+-substitution induced defects restructuring in BiFeO3 by positron annihilation and other supportive methods

The effects of Cr3+ ion substitution in place of Fe3+ ions in BiFe1-xCrxO3 nanocrystallites are reported in this work. Due to the smaller radii of Cr3+ ions, the nanocrystallite sizes and lattice parameters decreased during the substitution. The band gap energy decreased due to the increased conduction band edge tailing. The ionic concentrations, estimated from the energy-dispersive analysis of X-rays, indicated that there is only partial replacement of Fe3+ ions and a fraction of the Cr3+ ions either occupied the pre-existed Bi3+ ion vacancies or replaced some of the Bi3+ ions. Positron annihilation studies proved that, with increasing doping by Cr3+ ions, Bi3+- and O2− ion vacancies were generated. The diffusion of positrons to the surfaces of the nanocrystallites, however, continued to enhance the mean positron lifetimes. Coincidence Doppler broadened spectra also hinted at the generation of divacancies of Bi3+ and O2− ions. Dielectric measurements showed that the charge excess of the VBi3--VO2+ divacancies got balanced by a reduction of the Fe3+ ions to Fe2+ states. Magneto-electric coupling enhanced due to the quenching of the cycloidal-spiral spin structure and the increased Fe–O–Fe/Cr bond-bending. No structural transformation was observed unlike in the case of lanthanide ion substitution of Bi3+ ions.

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.

Insight into the spectroscopic behavior of Dy3+‐substituted and Li+ charge–compensated molybdate phosphors

AbstractEfficient utilization of phosphors in solid‐state lighting and phosphor‐converted light‐emitting diodes demands high emission intensity, narrow size distribution, and tunable luminescence. Here in this work, un‐doped and Li+ co‐doped CaMoO4:Dy3+ nanoparticles are synthesized via modified reflux method in ethylene glycol medium at 130°C. The host shows broad dual band emission centered in blue and green regions endowed by charge transfer and defects. Upon Dy3+ doping, the host‐to‐dopant energy transfer causes the CaMoO4:Dy3+ nanoparticles to exhibit characteristic emission lines from Dy3+. The Commission Internationale de L'Eclairage (CIE 1931) chromaticity coordinates for the prepared phosphors are estimated and found to lie in the nearly white region of color space. The light output as well as excited state lifetimes are successfully improved by lithium co‐doping by virtue of reduced charge compensating defects and sensitization via oxygen‐related defects. The positron annihilation studies show that all the lithium concentration co‐doped for charge compensation is merely helping in the removal of cations. This work shows the role of lithium co‐doping and its optimum concentration in improving the optical properties of phosphors.

Positron annihilation study of chitosan and its derived carbon/pillared montmorillonite clay stabilized Pd species nanocomposites

To establish the structure-properties relations of an organic polymer-inorganic hybrid nanocomposite, insightful investigation of the molecules packing and interfacial interactions is essential. Positron annihilation lifetime spectroscopy (PALS) has been proven as one of the most suitable technique to characterize the free volume structure (molecular packing) of polymer matrices. However, most of the attentions were paid limitedly in organic polymer-based hybrid systems rather than inorganic-based hybrid systems. Classic spherical model was the most frequently used model in free volume holes size calculation of polymer matrix. In the present study, molecular level micro-defects within inorganic-based hybrid systems encaging polymers and carbon species, i.e. novel biological macromolecules of chitosan and its derived carbon/Al pillared montmorillonite clay (CS/PILC and CS-derived C/PILC) stabilized palladium species hybrid nanocomposites, have been investigated by PALS. Besides the classic spherical model, cuboidal and cylindrical models were used to evaluate the molecules packing behavior within the interlayer space of PILC matrices encaging of polymers of CS or CS-derived carbons. The layered porous structure of the nanocomposites was also characterized with other methods, such as X-ray diffraction, N2 adsorption/desorption, and high-resolution transmission electron microscopy, etc. The correlations of the microstructure and catalytic properties applied in Sonogashira coupling reactions of the nanocomposites were discussed.

Positron annihilation study of lattice defects and nanoporous structures in Mn4+ doped K2SiF6 nanophosphors exhibiting high quantum yield

The present paper reports an application positron annihilation spectroscopy (PAS) to systematically study the formation of lattice defects, nanoporous structures and sites of Mn4+ ions doped into the K2SiF6 (KSFM) nanophosphors prepared using four different solvents. Three main techniques of PAS, including positron annihilation lifetime (PAL), Doppler broadening (DB) and electron momentum distribution (EMD) measurements, were simultaneously carried out under the same vacuum condition. The structural characteristics and quantum yield (QY) of KSFM nanophosphors were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), coupled plasma mass spectroscopy (ICP-MS), photonic multichannel analyzer-spectrophotometer (PMAS) and fluorescence spectrophotometer (PLS). The obtained results indicated that decreasing the dielectric constant of the solvents led to the reduction of the particle sizes in KSFM. The occupation of Mn4+ to the sites of Si4+ in the SiF62– octahedron structures of KSFM as well as the high concentration of lattice defects and nanopores (average size of 0.390–0.621 nm) in the doped materials that used isopropanol and diethyl ether solvents were indicated by PAS. Moreover, increasing the concentration of doped Mn4+ caused a decrease of the occupation of Mn4+ to the adjacent Si-vacancy and K-divacancy and at the same time, the number of Si-vacancies in KSFM increased. Combining PAS and QY measurements, we found that the QY of KSFM nanophosphors depended on not only the porous structures (nanopores) but also the concentration of doped Mn4+ in the sites of Si as well as the concentration of Si-vacancies.

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.