Search Results (182)

This work investigates the impact of an internal electric field on the annihilation characteristics of positrons implanted in a $180\left(10\right)\mathrm{nm}$ SiO₂ layer of a Metal-Oxide-Silicon (MOS) capacitor, using Positron Annihilation Lifetime Spectroscopy (PALS). By varying the gate voltage, electric fields up to $1.72\mathrm{MV}/\mathrm{cm}$ were applied. The measurements reveal a field-dependent suppression of positronium (Ps) formation by up to $64\%$, leading to an enhancement of free positron annihilation. The increase in free positrons suggests that vacancy clusters are the dominant defect type in the oxide layer. Additionally, drift towards the SiO₂/Si interface reveals not only larger void-like defects but also a distinct population of smaller traps that are less prominent when drifting to the Al/SiO₂ interface. In total, by combining positron drift with PALS, more detailed insights into the nature and spatial distribution of defects within the SiO₂ network and in particular near the SiO₂/Si interface are obtained. Full article

Background/Objectives: This study investigates the impact of high humidity (25 °C, 75% relative humidity) on gelatin and hydroxypropyl methylcellulose (HPMC) capsules used in dry powder inhalers (DPIs), focusing on moisture dynamics, structural responses, and mechanical performance, with an emphasis on understanding how different capsule types respond to prolonged exposure to humid conditions. Methods: Capsules were exposed to controlled humidity conditions, and moisture uptake was measured via thermal analysis. Visual observations of silica bead color changes were performed to assess moisture absorption, while surface wettability was measured using the sessile drop method. Hardness testing, mechanical deformation, and puncture tests were performed to evaluate structural and mechanical changes. Positron annihilation lifetime spectroscopy (PALS) was used to analyze free volume expansion. Results: HPMC capsules exhibited rapid moisture uptake, attributed to their lower equilibrium moisture content and ability to rearrange dynamically, preventing brittleness. In contrast, gelatin capsules showed slower moisture absorption but reached higher equilibrium levels, resulting in plasticization and softening. Mechanical testing showed that HPMC capsules retained structural integrity with minimal deformation, while gelatin capsules became softer and exhibited reduced puncture resistance. Structural analysis revealed greater free volume expansion in HPMC capsules, consistent with their amorphous nature, compared with gelatin’s semi-crystalline matrix. Conclusions: HPMC capsules demonstrated superior humidity resilience, making them more suitable for protecting moisture-sensitive active pharmaceutical ingredients (APIs) in DPI formulations. These findings underline the importance of appropriate storage conditions, as outlined in the Summary of Product Characteristics, to ensure optimal capsule performance throughout patient use. Full article

The microstructure and surface behavior of iron-based 304 stainless steel after temperature exposure was studied by Mössbauer spectroscopy, powder X-ray diffraction, scanning electron microscopy, energy dispersive analysis and positron annihilation. The tested specimens were in the form of cylinders produced by the casting process. The samples were annealed in air in the 600–1000 °C temperature range for 36 h. Under the influence of temperature, cast 304 stainless steel underwent austenitic–ferritic transformation and tended to form an oxide layer on the surface. The oxides were mainly found in the thin surface layer (0.3 μm) and consisted of Fe oxides and oxides of alloying elements (Cr and Mn) in the form of corundum, while, in the bulk region (10 μm), the phase transformation of austenite to ferrite occurred. Surface phase inhomogeneity was studied by Mössbauer spectroscopy. The method of positron annihilation was used to study defects and the effect of annealing on the formation and removal of a defect structure. Full article

This study systematically investigates the evolution of vacancy-type defects and heterogeneous Cu nanoprecipitates in an Fe₆₀Cr₁₂Mn₈Cu₁₅Mo₃V₂ (at%) multi-principal element alloy during thermal processing, utilizing Positron annihilation lifetime spectroscopy (PAS), coincidence Doppler broadening (CDB) spectroscopy, and transmission electron microscopy (TEM). The results show that the alloy exhibited a dual-phase coexistence structure of Body-Centered Cubic (BCC) and Face-Centered Cubic (FCC). The CDB results show that the density of heterogeneous Cu precipitates gradually increases with annealing temperature. Compared to the as-cast alloy, the precipitates annealed at 773 K exhibit a significantly reduced size (approximately 33 nm) with higher density. The PAS results demonstrate that gradual migration and aggregation of monovacancies at 573 K form vacancy clusters, while contraction and dissociation of these clusters dominate at 673 K. Within the temperature range of 773–973 K, the dynamic equilibrium between the aggregation and decomposition of vacancy clusters maintains stable annihilation characteristics with minimal lifetime changes. Full article

The urgent need for sustainable CO₂ conversion technologies has driven the development of advanced photocatalysts that harness solar energy. This study employs a CTAB-assisted solvothermal method to fabricate a Z-scheme heterojunction Fe-MOFs/V_O-Bi₂WO₆ (FM/V_O-BWO) for photocatalytic CO₂ reduction. Positron annihilation lifetime spectroscopy (PALS) was employed to confirm the existence of oxygen vacancies, while spherical aberration-corrected transmission electron microscope (STEM) characterization verified the successful construction of heterointerfaces. X-ray absorption fine structure (XAFS) spectra confirmed that the defect configuration and heterostructure changed the surface chemical valence state. The optimized 1.0FM/V_O-BWO composite demonstrated exceptional photocatalytic performance, achieving CO and CH₄ yields of 60.48 and 4.3 μmol/g, respectively, under visible-light 11.8- and 1.5-fold enhancements over pristine Bi₂WO₆. The enhanced performance is attributed to oxygen vacancy-induced active sites facilitating CO₂ adsorption/activation. In situ molecular spectroscopy confirmed the formation of critical CO₂-derived intermediates (COOH* and CHO*) through surface interactions involving four-coordinated and two-coordinated hydrogen-bonded water molecules. Furthermore, the accelerated interfacial charge transfer efficiency mediated by the Z-scheme heterojunction has been conclusively demonstrated. This work establishes a paradigm for defect-mediated heterojunction design, offering a sustainable route for solar fuel production. Full article

An analysis of defects creation in the vicinity of the selector-root connection plane in single-crystalline turbine blades made of CMSX-4 Ni-base superalloy was performed using several experimental methods. A coupling of scanning electron microscopy and X-ray diffraction topography allowed the visualization of dendritic arrays and surface defects in the root part of the blades. As a result, contrast inversions and areas where internal stresses occur were observed. The defects on a microscopic scale were characterized using positron annihilation lifetime spectroscopy and transmission electron microscopy. The registered positron lifetimes, above 0.5 ns, beyond the range characteristic for defects generally reported in metals and their alloys suggest the presence extremely large void type defects. Herein, we have identified large defects, ca. 2–5 nm in diameter, formed due to the contraction of fluid metal, captured in inter-dendritic regions during the liquid-to-solid transition. This work is a precursor to the almost untouched area of the discussion of lifetimes characteristic for positron bound states, called positronium (>0.5 ns) in relation to the morphology of void-type defects in single-crystalline superalloys. Full article

Rare earth element (Sm)-doped potassium sodium niobate (KNN)-based ceramics are fabricated using spark plasma sintering method and their properties are investigated. The results show that all the samples crystallize in a typical perovskite structure with a single orthorhombic phase. With increasing the Sm doping, the ceramics gradually shift toward the relaxor ferroelectric state and the value of dielectric loss angle tangent (tanδ) is smaller than 0.05 for x ≤ 0.003 ceramic samples. Meanwhile, the optimized piezoelectric charge coefficient d₃₃ = 128 pC/N, and piezoelectric voltage coefficient g₃₃ = 18.9 × 10⁻³ Vm/N are obtained when x = 0.001. Compared with the undoped sample, the d₃₃ of x = 0.001 ceramics has been significantly enhanced by 28%. The positron annihilation lifetime results indicate that the main defect types in the ceramics are the A-site vacancies and defect dipoles. Based on the aforementioned results, the optimized piezoelectric performance and the lowest defect dipoles concentration in x = 0.001, may be attributed to the low internal oxygen vacancy concentration in it. This work may provide insights for the further study of KNN-based piezoelectric ceramics. Full article

The type of silane coupling agent (SCA) has an important influence on carbon fiber (CF) modification efficiency and the properties of the obtained CF-based polymer composites. To quantitatively reveal the effects of SCA type, three kinds of SCA (γ-aminopropyl triethoxylsilane, γ-glycidoxypropyl trimethoxylsilane, and γ-methacryloxy propyl trimethoxylsilane)-modified CF-incorporated silicon rubber (SR) composites were prepared. The microstructure (free volume characteristic and interfacial interaction) of the obtained CF/SR composites was revealed by positron annihilation lifetime spectroscopy (PALS). Based on the results of mechanical, electrical, and thermal properties, the relationship between microstructure and performance was established. This investigation provides a powerful approach to the quantitative description of polymer composite microstructures, which will benefit the construction of structure–property relationships and high-performance polymer composites. Full article

The relationship between thermodynamic entropy generation and free volume changes during the tensile deformation of epoxy resin was investigated. Thermodynamic entropy generation was evaluated using differential scanning calorimetry (DSC) for samples at various strain levels, while free volume changes were measured with positron annihilation lifetime spectroscopy (PALS). Volumetric strain was assessed through the digital image correlation (DIC) method. The results showed that both thermodynamic entropy and free volume increase during tensile deformation, and the average free volume radius becomes more uniform. It was observed that thermodynamic entropy generation and free volume each exhibit a linear relationship with volumetric strain. Additionally, thermodynamic entropy generation increased linearly with free volume. These findings suggest that the increase in thermodynamic entropy during tensile deformation is attributed to irreversible changes, such as the expansion of free volume within the material. Full article

Defect recovery and recrystallization studies of neutron-irradiated tungsten (W) addressing the microstructural evolution in relation to the mechanical properties, provide valuable insight into defect interactions and annihilation processes. Understanding these mechanisms can aid in the development of effective healing processes, potentially extending the lifespan of fusion reactor components. Additionally, this research helps to elucidate how neutron exposure alters the behaviour of materials used in fusion reactor components, contributing to improved design and durability. Within this framework, an ITER grade forged W bar was neutron irradiated to a damage of 0.21 displacements per atom at 600 °C and subsequently isochronally annealed from 700 up to 1550 °C in 50 °C steps. Irradiation causes the formation of dislocation loops and vacancy clusters as well as the formation of Re and Os transmutation products, leading to a 35% increase in hardness and a 23% increase in resistivity. The evolution of the microstructure after isochronal annealing is investigated through positron annihilation lifetime spectroscopy, X-ray diffraction, resistivity, and Vickers hardness measurements. The total dislocation line density as well as the number density and size of voids are determined as a function of annealing temperature. Specifically, the critical resolved stresses of dislocations and voids are correlated with their densities and distinct recovery stages are identified. The kinetics of defect annihilation are discussed in relation to the annealing temperature. Nearly complete dislocation annihilation occurs after annealing at 1300 °C, followed by complete void dissolution and recrystallization at 1450 °C. Full article

This paper presents a study of brane formation in six-dimensional space. There is no a priori assumption of the existence of brane(s). However, an analysis of the generalized Einstein equations shows that there is a set of metrics describing two static branes even in the absence of matter fields. At the same time, no one-brane configurations were found. The trapping of massive particles on branes is a consequence of the metric structure, which prevents these particles from moving between branes. It is shown that communication between charged particles on different branes is provided by photons. Such positron–electron annihilation could be studied experimentally at the LHC collider. The Higgs field is distributed between the branes in such a way that it can serve as a Higgs portal connecting two worlds located on different branes. The values of the 4D physical parameters depend on the extra metric structure near the branes. We also found a non-trivial effect of the decompactification of extra space during the Hubble parameter variation. Full article

Polyamorphic transitions driven by high-energy mechanical milling (nanomilling) are studied in thioarsenide As₄Se_n-type glassy alloys obtained by melt quenching deviated from arsenic triselenide As₂Se₃ stoichiometry towards tetraarsenic pentaselenide (g-As₄Se₅) and tetraarsenic tetraselenide (g-As₄Se₄). This employs a multiexperimental approach based on powder X-ray diffraction (XRD) analysis complemented by thermophysical heat transfer, micro-Raman scattering (micro-RS) spectroscopy, and revised positron annihilation lifetime (PAL) analysis. Microstructure scenarios of these nanomilling-driven transformations in arsenoselenides are identified by quantum-chemical modeling using the authorized modeling code CINCA (the Cation Interlinked Network Cluster Approach). A straightforward interpretation of a medium-range structure response of a nanomilling-driven polyamorphism in the arsenoselenides is developed within the modified microcrystalline model. Within this model, the diffuse peak-halos arrangement in the XRD patterning is treated as a superposition of the Bragg-diffraction contribution from inter-planar correlations supplemented by the Ehrenfest-diffraction contribution from inter-atomic (inter-molecular) correlations related to derivatives of network As₂Se₃-type and molecular As₄Se₄-type conformations. Changes in the medium-range structure of examined glassy arsenoselenides subjected to nanomilling occur as an interplay between disrupted intermediate-range ordering and enhanced extended-range ordering. The domination of network-forming conformations in arsenoselenides deviated from As₂Se₃ stoichiometry (such as g-As₄Se₅) results in rather slight changes in their calorimetric heat-transfer and micro-RS responses. At the atomic-deficient level probed by PAL spectroscopy, these changes are accompanied by reduced positron trapping rate of agglomerated multiatomic vacancies and vacancy-type clusters in an amorphous As-Se network. Under an increase in As content beyond the g-As₄Se₅ composition approaching g-As₄Se₄, nanomilling-driven polyamorphic transitions, which can be classified as reamorphization (amorphous I-to-amorphous II) phase transitions, are essentially enhanced due to the higher molecularity of these glassy alloys enriched in thioarsenide-type As₄Se₄ cage-like molecular entities and their low-order network-forming derivatives. Full article

In this work, the peculiarities of microstructure changes in Cr-coated Zr1%Nb alloy under high-temperature hydrogenation and Kr ion irradiation were investigated. A comprehensive analysis revealed that Cr coating reduces the thickness of the Kr+ radiation damage zone by 15%–20% and decreases the density of radiation-induced defects compared to that of uncoated Zr1%Nb alloy. Additionally, Cr-coated samples exhibit a more uniform hydrogen distribution. According to first-principles calculations and positron annihilation spectroscopy, hydrogen-free dislocations predominate in the Cr-coated Zr1%Nb alloy after hydrogenation and irradiation. These findings emphasize the protective role of Cr coatings in mitigating radiation damage and hydrogen embrittlement in zirconium alloys. Full article

This paper presents the results of the study of the composite based on magnesium hydride with the addition of nanosized nickel powder, obtained by the method of an electric explosion of wires. The obtained MgH₂-EEWNi (20 wt.%) composite with the core-shell configuration demonstrated the development of a defect structure, which makes it possible to significantly reduce the hydrogen desorption temperature from 418 °C for pure magnesium hydride to 229 °C for hydride with the addition of nickel powder. In situ studies of the evolution of the defect structure using positron annihilation methods and diffraction methods made it possible to draw conclusions about the influence of the Mg₂NiH_0.3 and Mg₂NiH₄ phases on the sorption and desorption properties of the composite. The results obtained in this work can be used in the field of hydrogen energy in mobile or stationary hydrogen storage systems. Full article

This research explores how varying proportions of virgin polyethylene terephthalate (vPET) and recycled polyethylene terephthalate (rPET) in vPET-rPET blends, combined with preform thermal conditions during the stretch blow molding (SBM) process, influence PET bottles’ microscopic characteristics. Key metrics such as viscosity, density, crystallinity, amorphous phase relaxation, and microcavitation were assessed using response surface methodology (RSM). Statistical analysis, including Analysis of variance (ANOVA) and its power, supported the interpretation of results. The first part of the work details the experimental design and statistical methods. Positron annihilation lifetime spectroscopy (PALS) and amorphous phase density analysis revealed reduced free volume size, a substantial increase in free volume quantity, and a transformation toward ellipsoidal geometries, highlighting significant structural changes in the material. At the same time, the intrinsic viscosity (IV) and PALS studies indicate that the solid-state post-condensation effect (SSPC) is linked with microcavitation through post-condensation product diffusion. The conclusions, which resulted from the microstructure analysis, affected the material’s mechanical strength and were validated by pressure resistance tests of the bottles. Full article