Positron Annihilation Doppler Research Articles

Dehydration of bioethanol using novel polysulfone (PSF)/nonionic surfactants hybrid membranes

This study focuses on developing a novel polysulfone (PSF) dense hybrid membrane modified with nonionic surfactants, Tween 20 (T20) and Triton X-100 (X100), for improved pervaporation (PV) dehydration of bioethanol. The modified membranes were then examined for their ethanol dehydration performance, microscopy and spectroscopy characteristics, surface charge, mechanical and roughness properties, crystallinity structure, differential scanning calorimetry (DSC), swelling, and contact angle (CA) analysis. The free volume structure of the membranes was assessed using positron annihilation lifetime spectroscopy (PALS) and Doppler broadening spectroscopy (DBS). A significant discovery from this analysis was the semi antiplasticizing effect observed when surfactants were added to the PV membranes. The PV results revealed that incorporating 0.5 wt% T20 into PSF significantly improved the total flux and PSI of PSF/T20 to 226.35 g/m2 h and 46578 g/m2 h, respectively. Similarly, the total flux and PSI of hybrid membranes containing X100 increased to 110.19 g/m2 h and 43680 g/m2 h. Consequently, the developed PSF/T20 membranes demonstrated more significant potential for bioethanol dehydration compared to PSF/X100 membranes.

Investigating the impact of gamma irradiation and temperature on vacancy formation and recombination in ZrB2 ceramics using positron annihilation spectroscopy

In the presented work, the mechanism of evolution of defects in the crystalline structure after irradiation with 1500 kGy and 3000 kGy absorption dose at room temperature in a 60Co gamma source was studied, using 43 nm sized ZrB2 crystals. The crystals were characterized by a purity of 99.9%, a powder density of 0.23 g/cm3, a specific surface area of 80 to 120 m2/g, and a hexagonal P6/mmm spatial structure. The dose rate applied was 6.05 Gy/s, with 1.17 and 1.37 MeV energy lines. The effect of particles resulting from the decay of 22Naradioactive isotope β+ with an activity of 10.5 μCi, as well as gamma quanta with an energy of 1.27 MeV, on the core and valence electrons and vacancies in the crystal structure was studied using Positron Annihilation Lifetime Spectroscopy (PALS) and Doppler broadening analysis methods. Additionally, the changes in the S and W parameters, which characterize the distribution of defects within the volume of the ZrB2 crystal, were studied using the Doppler broadening method. For unannealed material, the positron lifetime τ1 component varied between 174±2 ps and 181±1 ps. After annealing, the positron lifetime component τ2 was decreased from 290±3 ps to 226±3 ps, and the intensity component I2 increased from 17.49% to 40% depending on the radiation dose. The calculated values of the positron lifetime τ for one boron vacancy from ABINIT and MIKA packages were found to be 172 ps and 145 ps, respectively. The material was observed to satisfy the necessary functional conditions at gamma doses not exceeding 3000 kGy.

Positron annihilation lifetime and doppler broadening spectroscopies studies of defects in nano TiN crystal under gamma irradiation and high temperature

Positron annihilation lifetime and doppler broadening spectroscopies studies of defects in nano TiN crystal under gamma irradiation and high temperature

Structural, morphological, thermal, mechanical, positron annihilation, and γ-ray shielding studies of nano-tungsten oxide/polyvinyl alcohol composites

Tungsten oxide nanoparticles were synthesized. Metal/polymer composites of nano-sized WO3 and poly vinyl alcohol were blended by the casting method with concentrations of WO3. The structural, morphological, thermal, and mechanical characteristics of the prepared samples were investigated. The γ-ray attenuation properties of the synthesized composites were examined using γ-ray spectrometers, and the γ-ray mass absorption coefficients were measured using 662 keV γ-ray of 137Cs radioactive source. The defects and microstructure changes of the composites were probed using positron annihilation lifetime and Doppler broadening techniques. The shielding performance and thermal stability improved with increasing the concentration of WO3, and some of the mechanical properties, in particular the elongation at break, deteriorate. The contribution of WO3 has a strong effect on reducing the relative fractional free volume, which in turn helps to increase the γ-ray absorption of the composites. The use of a 50–70 % WO3/PVA composite is recommended.

Effect of Ag incorporation on the microstructure and properties of ZnS thin films

In this study, ZnS thin films doped with different content of Ag were deposited on quartz glass substrates by RF magnetron sputtering technique and were annealed at 600 °C in sulfur vapor. The crystal structure, grain size, surface morphology, composition, optical properties and defects of the doped ZnS thin films were analyzed by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), UV–Vis spectroscopy and positron annihilation doppler broadening spectroscopy (DBS). The results showed that all films exhibited the cubic sphalerite structure with a preferred orientation along the (111) crystal plane, and the composition of the doped films were more consistent with the stoichiometric ratio compared to the undoped film. The grain size gradually increased as the Ag content grew, indicating the improvement of crystallinity of the ZnS thin films. Besides, the bandgap values of these ZnS films with good optical transparency in the visible region were decreased from 3.87eV to 3.53eV as the Ag content grew. DBS results also showed that the defect concentration of ZnS films gradually decreased from the surface to the inner layer, and the film defects decreased with the increase of Ag content. However, the types of defects changed when the Ag content increased. Furthermore, the results showed Ag also has an effect on the porosity of ZnS thin films.

Study of irradiation-induced vacancy-type defects in tungsten using positron annihilation spectroscopy

Tungsten has been considered as a potential candidate material for the application of plasma-facing components in magnetic confinement fusion reactors. Fusion neutrons introduce severe displacement damage into the first-wall materials along with a high amount of helium which is detrimental to the desirable material properties. Ion beam implantation combined with positron annihilation spectroscopy is a well-adopted method to investigate the irradiation damage in materials without any induced radioactivity. In this study, two separate tungsten samples were individually irradiated with 9.1 MeV Au5+ ions to a dose of 2.5 × 1014 ions/cm2 and 130 keV He ions to a dose of 1 × 1016 ions/cm2 with the intention to investigate the evolution of vacancy-type defects in the absence and presence of helium. The irradiated samples were subjected to isochronal annealing and characterized using positron annihilation Doppler broadening spectroscopy. The evolution of vacancy-type defects during different stages of annealing was investigated using defect-sensitive positron S-parameter. An increase in S-parameter due to the presence of vacancy-type defects was observed upon irradiation in both samples. The S-parameter variations with respect to the temperature showed different stages of vacancy annealing corresponding to stage III from 473 K to 673 K where the migration and clustering of mono-vacancies occurs, stage IV from 673 K to 973 K due to the growth of nano-voids and stage V from 973 K to 1273 K due to the dissociation of nano-voids. Complete recovery of irradiation-induced defects occurred in both samples at 1273 K. Both the samples showed similar vacancy annealing stages and no signature on the formation and growth of He-vacancy clusters or He bubbles was observed in the present study.

Microstructural and mechanical property evolution of Ar9+ ion irradiated Zr-based alloys – effect of initial microstructure and alloying addition

This work endeavours to bring out the role of initial microstructural features and alloying additions in the irradiation response of a material. Three Zr-based binary alloys, namely Zr-0.17 wt.% O, Zr-0.33 wt.% Sn and Zr-2.5 wt.% Nb, with starting microstructures exemplifying annealing, hot-extrusion and cold-pilgering operations, respectively, were selected. The alloys were irradiated at room temperature using 315 keV energy Ar9+ heavy ion. Depth-dependent microstructural evolution of the alloys upon irradiation was assessed through positron annihilation Doppler broadening spectroscopy and grazing incidence X-ray diffraction, while nanoindentation probed the mechanical behaviour. Irradiation responses of the alloys garnered in terms of S-parameter, coherently scattering domain size, microstrain, dislocation density and hardness were compared. The observed differences were rationalized on the basis of (i) starting microstructural attributes of the alloys, which posed as sinks to the irradiation-induced point defects, and (ii) impact of alloying additions on diffusion and annihilation of these defects.

Effect of different sulfur pressure annealing on properties of sputtering-deposited ZnS thin films

ZnS, a kind of wide-band gap semiconductor, has attracted extensive attention in recent years due to its excellent photoelectric performance, which has broad application prospects in solar cells, photocatalysts and sensors. In this work, ZnS thin films are first deposited by radio-frequency (RF) magnetron sputtering, and then annealed at 600 ℃ and different sulfur pressures. The crystal structure, surface morphology, grain size, composition, transmittance and defects of ZnS thin films are analyzed by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, UV-vis transmission spectra, and positron annihilation Doppler broadening spectroscopy. The results show that the crystallinities of ZnS films can be improved by being annealed in sulfur atmosphere, and the optical band gaps of ZnS films after being annealed are in a range of 3.43–3.58 eV. When the sulfur pressure is higher than 0.49 atm, the sulfur interstitial atoms in the ZnS and the elemental sulfur on the surface reduce the transmittance of the film in the visible region. The Doppler broadening spectroscopy results also show that the defect concentrations of ZnS films decrease gradually from their surface layers to the inner layers, and the defects of ZnS films decrease with the increase of sulfur pressure. Meanwhile, the 3γ annihilation also proves that the interior of the film is relatively dense, and the open porosity of the film will increase due to vulcanization. Adsorbed sulfur occupies the position of sulfur vacancy defect in the crystal through internal diffusion, which leads the defect concentration to decrease and the film quality to be improved.

Effect of dislocation on helium irradiation defects in low-activation martensitic steel

Reduced-activation martensitic steel is a main candidate structural material for key components of advanced nuclear energy systems because of its good mechanical properties at room temperature, low neutron activation characteristics and satisfactory radiation resistance. In this work, we study the interaction between dislocations and helium-irradiation-induced defects in the steel and the effect of dislocations on the behavior of helium migration and desorption. The well-annealed samples are pre-deformed to 10% and 20% reductions in thickness by using cold rolling mill. After pre-deformation, samples are heat-treated to remove vacancy defects and retain dislocation defects. Then, the samples with reserved dislocations are irradiated with helium at room temperature (50 keV, 1×10<sup>17</sup> He/cm<sup>2</sup>). After irradiation, the samples are characterized by synchrotron radiation grazing incident X-Ray diffraction, positron annihilation Doppler broadening spectroscopy, and thermal desorption spectroscopy. The results show that dislocations hinder the diffusion of helium and helium-vacancy complexes, and reduce the accumulation of radiation damage. Such an effect becomes more significant with the increase of dislocation density. The BCC → FCC phase transition of low activation martensitic steel occurs at 1179 K. The increase of dislocation density will lead to the forward shift of helium desorption peak induced by phase transition. The retention of helium in the undeformed sample, 10% deformed sample and 20% deformed sample is 10.3%, 15.7% and 17.9%, respectively, indicating that high density dislocations promote the retention of helium.

Influence of defect dynamics on the nanoindentation hardness in NiCoCrFePd high entropy alloy under high dose Xe+3 irradiation

We report the role of irradiation-induced defects and microstructure evolution in the wake of 1.05 MeV Xe3+ ion irradiation on the structural and mechanical properties of single-phase face-centered cubic (FCC) structured NiCoCrFePd high-entropy alloy (HEA). The defect evolution was investigated using positron annihilation Doppler broadening spectroscopy (PADBS), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), and nanoindentation techniques. Single-phase FCC structured NiCoCrFePd HEA remained structurally stable even upon irradiation to a very high fluence of 9 × 1016 ions/cm2. However, an anomalous reduction in the lattice constant, along with micro-strain relaxation and crystallite fragmentation was observed at the initial ion fluence of 1 × 1016 ions/cm2, which are recovered upon irradiation at the higher ion fluence due to collapse of small-sized point defects into the large-sized dislocations and simultaneously irradiation-induced recrystallization. Ion fluence-dependent PADBS analysis revealed the formation of mono-vacancies at lower fluence which were saturated by recombination and evolution to large-size defects at successive higher ion fluence that is consistent with electron microscopic investigations. Measurement of mechanical properties showed that the hardness was initially increased by 57% upon irradiation at fluence 1 × 1016 ions/cm2 and then it was slightly reduced (∼5%) at a higher fluence of 9 × 1016 ions/cm2 irradiation due to defect recombination and recrystallization which clearly shows the resistance of this HEA towards hardening. Thus, the present study provides a deeper understanding of the defect dynamics and their relation to the mechanical behavior of NiCoCrFePd HEA, which is important for the development of radiation-resistant alloys for nuclear energy systems.

Comparison of helium ion irradiation resistance between nanocrystalline and coarse grained 304 austenitic stainless steel

Improving the radiation resistance of structural materials in the presence of helium is significant for the development of advanced nuclear power systems. Nanostructured materials reduce the grain size and significantly increase the grain boundary density. Hence, it is considered to be an effective method to improve the radiation resistance of materials. In this work, we studied the radiation resistance of nanocrystalline (NC) and coarse grained (CG) 304 austenitic stainless steel (304-SS) by helium ion irradiations. The mean grain sizes of NC and CG 304-SS are ∼45 nm and ∼30 μm, respectively. The results of positron annihilation Doppler broadening spectroscopy and transmission electron microscopy indicate that NC 304-SS not only has better swelling resistance than CG 304-SS, but also has better helium effect resistance. And the swelling of NC 304-SS is reduced by a factor of ∼5.7 compared with CG 304-SS under the irradiation of 170 keV He+ to 2 × 1020 ions m−2 at 723 K. The mechanisms for the excellent swelling resistance of NC 304-SS are discussed.

Hexagonal annular-NiO nanoarchitecture with local p-n homojunctions: Novel formation mechanism and H2S gas sensing properties

This work reported a facile and scalable route that can control the synthesis of hexagonal annular-NiO nanoarchitecture (HANN) and explore its formation mechanism as well as gas-sensing properties. The results show that the morphology and crystal structure of HANN exhibit a uniform population with a fascinating annular shape, each containing an interior hexagonal hollow. The novel formation mechanism of HANN, which is closely related to the evolution of defects and nanopores during the annealing, is examined and discussed in detail by using nuclear analytical spectroscopies such as Positron annihilation lifetime, Doppler broadening and Electron momentum distribution. In particular, we found the possible formation of the so-called local p-n homojunctions due to the presence of O vacancies with a high concentration in the vacancy clusters in HANN. The high concentration of these vacancies generates many active sites and subsequently the local p-n homojunctions, which increases the adsorption/reaction of H2S molecules and the H2S-sensing performance of HANN. Hence, HANN-based sensors show excellent response, a low limit of detection (∼12 ppb) with remarkable selectivity and high stability toward H2S. This study is, thus, particularly important since it provides a new way to understand deeply the formation mechanism and gas-sensing performance of nanostructured metal oxide semiconductors.

Hierarchical nanostructure investigation of Zeolitic Imidazolate Frameworks (ZIF-8 and ZIF-67) multilayers using depth dependent Doppler broadening spectroscopy

Zeolitic Imidazolate Frameworks (ZIFs) based gas separation is proposed to be an energy efficient process compared to conventional processes currently used in the industries. In order to achieve high gas separation factor, diffusion of gases through the size selective aperture of ZIFs has to be maximized suppressing the gas diffusion through the non-selective inter-crystal grain boundary structure. In this regard, a priori depth dependent experimental characterization of pore architecture of ZIF based multilayer membranes is quite essential. In the present study, depth dependent positron annihilation Doppler broadening spectroscopy has been used to investigate the pore interconnectivity and grain boundary structure for alternatively deposited bi- and tri-layers of ZIF-67 and ZIF-8 using fast current driven synthesis method (FCDS). The phase purity and chemical bonding of the films have been established using X-ray diffraction and Fourier transform infra red spectroscopy. The electron microscopy images of the films have shown uniform and densely packed film deposition using the present methodology. The S-E profiles of multilayers films measured using Doppler broadening spectroscopy confirm the modifications in pore interconnectivity and crystal packing as compared to seed films (ZIF-8 and ZIF-67). The S-E profiles of multilayers could not be modeled considering individual ZIF-8 and ZIF-67 films deposited over each other with distinct interface structure. The present study confirms that multilayer deposition of ZIF-8 and ZIF-67 using FCDS leads to modification in crystal packing throughout the bulk of the film. It is confirmed that grain boundary structure is significantly improved in case of multilayers as compared to pure ZIF-67 based film, which can be attributed to the better gas separation performance of multilayers compared to pure ZIF-67 observed in literature.

Characterisation of defects in porous silicon as an anode material using positron annihilation Doppler Broadening Spectroscopy

Here we present Positron Annihilation Doppler Broadening Spectroscopy (PADBS) as a powerful method to analyse the origin and development of defect processes in porous silicon structures as a result of alloying with lithium for the use in battery anode applications. Several prepared anodes were lithiated (discharged against Li+/Li) and de-lithiated (charged) with different capacities followed by a distinct treatment procedure and an analysis using the Delft Variable Energy Positron Beam. The results presented here show that we can distinguish two different processes attributed to (1) structural changes in silicon as a result of the alloying process, and (2) the formation of defects that initiate degradation of the material. The limit at which the porous material can be used for at least the first two cycles without the occurrence of damage can thus be accurately determined by using the PADBS technique.

Free‐Volume Extended Defects in Structurally Modified Ge–Ga–S/Se Glasses

Herein, transformation of free‐volume extended defects in selected chalcogenide glasses caused by thermal annealing for 10, 25, and 50 h is studied. For 80GeSe2–20Ga2Se3 as well as for GeS2–Ga2S3 glasses with different GeS2 and Ga2S3 contents (80GeS2–20Ga2S3, 82GeS2–18Ga2S3, 84GeS2–16Ga2S3), positron annihilation lifetime spectroscopy and Doppler broadening of annihilation radiation methods are applied. By analyzing the positron annihilation lifetime spectra decomposed into two components, it is shown that the observed changes in the second defect‐related component for the 80GeSe2–20Ga2Se3 glasses confirm the agglomeration of free volumes in the initial stage of annealing (10 h) with further fragmentation (25 h) and shrinkage (50 h). Increased content of Ge‐related subsystem in the GeS2–Ga2S3‐based glasses results in the agglomeration of free‐volume defects in the 82GeS2–18Ga2S3 glasses with their further expansion in the 84GeS2–16Ga2S3 matrix.

Structural tunability and origin of two-level systems in amorphous silicon

Amorphous silicon films prepared by electron beam evaporation have systematically and substantially greater atomic density for higher thickness, higher growth temperature, and slower deposition rate, reaching the density of crystalline Si when films of thickness greater than ~300 nm are grown at 425 $^{\circ}$C and at <1 $\r{A}$/sec. A combination of spectroscopic techniques provide insight into atomic disorder, local strains, dangling bonds, and nanovoids. Electron diffraction shows that the short-range order of the amorphous silicon is similar at all growth temperatures, but fluctuation electron microscopy shows that films grown above room-temperature show a form of medium-range order not previously observed in amorphous silicon. Atomic disorder and local strain obtained from Raman spectroscopy reduce with increasing growth temperature and show a non-monotonic dependence on thickness. Dangling bond density decreases with increasing growth temperature and is only mildly dependent on thickness. Positron annihilation Doppler broadening spectroscopy and electron energy loss spectroscopy show that nanovoids, and not density variations within the network, are responsible for reduced atomic density. Specific heat and mechanical loss measurements, which quantify the density of tunneling two-level systems, in combination with the structural data, suggest that two-level systems in amorphous silicon films are associated with nanovoids and their surroundings; which are in essence loosely bonded regions where atoms are less constrained.

The mechanism behind the high radiation tolerance of Fe–Cr alloys

Fe–Cr alloys are at the forefront for high radiation tolerant materials with long-standing validated performance. Yet, the detailed mechanism behind their high radiation resistance is in question and understanding the effect of varying Cr percentage is a grand challenge limiting further improvements. Here, we applied depth-resolved positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy to study the effect of Cr alloying on the formation and evolution of atomic size clusters induced by ion-irradiation in Fe. We also used atom probe tomography to investigate the possible presence of Cr clusters or α′ phase precipitates with high Cr composition. The study reveals that the well-known resistance to radiation in Fe–Cr alloys may arise from the stabilization of vacancy clusters around Cr atoms, which act as sinks for radiation-induced defects. This implies that Cr atoms do not provide a direct sink for interstitials; rather defect complexes that consist of Cr atoms and vacancies, in turn, act as sinks for irradiation-induced vacancies and interstitials. we also find that lower amounts of Cr create smaller defect clusters that act as efficient sinks for radiation damage, but larger quantities of Cr form a defect structure that is less homogenous and larger in size, resulting in less efficient damage recombination. No evidence of α′ was found before or after irradiation, which indicates that it does not contribute to the observed radiation tolerance.

Defect probing using positron annihilation and dielectric spectroscopy of PVA/Al thin films

The Casting method is used to produce nano-composite polymeric thin films (PVA/Al) with varied aluminum content (x=0.0, 0.04, 0.08, 0.1wt.%). The morphology of thin films is examined by Scanning electron microscopy (SEM), and it is used to approve the homogenous nature of the prepared thin films and to determine the grain size. The obtained nano-composite thin films reveal an enhancement of dielectric properties by adding the nano-filler to polymeric matrix. Also, the Differential Scanning Calorimetry (DSC) studies show an increase in the glass temperature (Tg) by increasing Al content. The microstructure changes of the prepared PVA/Al NPs composite films are investigated using the positron annihilation lifetime (PAL) and Doppler broadening (DB) techniques. The interactions between the PVA and Al NPs through the formation of hydrogen bonding modify the size and concentration of the free volume holes.

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.

Irradiation hardening behaviors of large-scale hot rolling potassium-doped tungsten alloy under synergistic irradiations of Fe11+ ion combined with deuterium and helium plasmas

In this study, a 25 kg potassium doped tungsten (KW) plate dispersed K bubbles was prepared by hot rolling firstly. In order to evaluate the mechanical properties of KW after irradiation, the synergistic irradiation of Fe11+ ion, He and D plasmas were performed on KW and pure tungsten (PW). 3.5-MeV Fe11+ ion irradiation was used to simulate displacement damage induced by neutrons. The total fluence and irradiation temperature were 5.31 × 1014 m−2 for 0.5 displacements per atom (dpa) damage and 400 K, respectively. Positron annihilation Doppler broadening spectrometry (PA-DBS) technique was applied to measure the type of defect in both undamaged and damaged PW & KW samples. The total ion fluence helium (He) and deuterium (D) were kept constant at 1 × 1025 He m−2 and 5 × 1024 D m−2, separately. Subsequently, the hardness value of PW and KW received from CSM nanoindentation technique was analyzed and compared. The results show that the increase of vacancy-type defects in KW is restrained after irradiation compared with PW. For un-irradiated PW and KW samples, there are almost the same pop-in events, which indicate that PW and KW a similar plastic deformation at room temperature. The KW shows a better irradiation hardening resistance than PW under the same irradiation condition. Besides, the evolution of irradiation hardening as well as their relationship with K bubbles were discussed in detail.