Positron Lifetime Measurements Research Articles

Characterization of Vacancy Defects Using TEM in Heavy-Ion-Irradiated Tungsten Foils

The nature and type of defects formed due to heavy-ion irradiation in tungsten foils are analyzed using transmission electron microscopy. The recrystallized tungsten foils were irradiated by 80 MeV gold ions at room temperature for a fluence of 1.3 ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes $$\\end{document} 1014\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{14}$$\\end{document} ions/cm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^2$$\\end{document} that amounts to a net displacement per atom (dpa) of 0.22. The defect structures were analyzed using bright-field and weak-beam dark field imaging at two different depths to understand the depth profile of the defects. It is found that the defect clusters formed during the irradiation, both at the near-surface and at 2 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu $$\\end{document}m depth are of vacancy type that is confirmed by the local strain analysis and supports the findings of vacancy clusters in positron lifetime measurements. The analysis also shows that the dislocation-lines were of pure edge, pure screw and mixed. The fraction of mixed dislocation is found to increase during irradiation at the expense of pure edge and screw dislocations.

Subsurface zone detected in pure nickel by positron annihilation; Hall-Patch effect notification

A dry sliding test and sequential etching technique, combined with positron lifetime measurements on pure nickel, revealed an area of damage below the worn surface extending to a depth of hundreds of micrometers. Some correlation was found between the value of the mean positron lifetime measured directly on the worn surface and the coefficient of friction. The obtained dependency rather confirms the normal Hall-Patch regime. There was no correlation with the specific wear rate.

Identification of ortho-positronium annihilation sites in Y zeolites by positron lifetime measurements in different environments

The annihilation sites of ortho-positronium formed by positrons incident on Y zeolites with different Si/Al ratios are investigated by positron annihilation lifetime spectroscopy in different environments. Comparison of the measured ortho-positronium lifetimes by exposure of the zeolites to argon, nitrogen, water or vacuum enables us to determine the annihilation process and the specific annihilation site within the zeolite samples. The third lifetime component is ascribed to ortho-positronium pick-off annihilation in or around the apertures of the sodalite cage and in the connecting channels, and is reduced in water because of the physisorbed molecules that hinder the ortho-positronium mobility. The longest fourth lifetime component is attributed to ortho-positronium self-annihilation in the interparticle gaps, but is significantly suppressed in the presence of polar molecules, which suggests a contribution from ortho-positronium pick-off annihilation in the supercage.

Development of novel positron lifetime measurement system for potential liquid material inspection

β+-γ coincidence method and anti-coincidence method have great potential in positron annihilation lifetime (PAL) measurements of liquid material. Based on anti-coincidence method with the advantage of better picking out positron annihilation events in the sample, a novel PAL spectrometer, which can isolate the sample from the positron source and is suitable for liquid material inspection, is developed. The influences of the measuring geometry and the signal processing on the anti-coincidence effect are systematically studied. By comparing the PAL spectra of well annealed pure iron acquired by the new system with that acquired by the conventional system, the performance of system is evaluated. The analysis results of PAL spectra at different distances from the sample to the source and precise measurement of PAL in liquid water show that reliable values are obtained for liquid material inspection at an adjustable distance of less than 1.0 mm. This system is expected to be applied to liquid scintillators, oils or other special materials, which could easily contaminate the positron source.

Irradiation effects on binary tungsten alloys at elevated temperatures: Vacancy cluster formation, precipitation of alloying elements and irradiation hardening

Irradiation responses of binary W alloys were investigated systematically from the perspective of the binding energy of an alloying element with a W self-interstitial atom (W-SIA). Plates of W, W-0.3 at.% Cr, W-5 at.% Re, W-2.5 at.% Mo and W-5 at.% Ta alloys were irradiated at 1073 K with 6.4 MeV Fe ions to 0.26 dpa at the damage peak, where the binding energy of alloying element with W-SIA is in order of Cr > Re > Mo > Ta. The formation of vacancy-type defects (vacancies and vacancy clusters) was studied by using positron lifetime measurement. The precipitation of alloying elements was studied by using atom probe tomography (APT) and the hardness changes in the irradiated volumes were measured by the nanoindentation technique. The formation of vacancy-type defects was strongly suppressed by the addition of Cr and Re, while Ta and Mo had no noticeable suppression effect. The APT measurements showed fine Cr- and Re-rich precipitates in W-0.3 at.% Cr and W-5 at.% Re alloys, respectively, where the density of precipitates in the latter was clearly lower than that in the former. The distributions of Mo and Ta were uniform even after irradiation. Irradiation hardening was observed for all materials but that of W-5 at.% Re alloy was significantly smaller than the hardening of W, W-2.5 at.% Mo and W-5 at.% Ta alloys. These observations suggest that the irradiation hardening of W, W-2.5 at.% Mo, and W-5 at.% Ta alloys were mainly caused by vacancy-type defects. It was concluded that an alloying element with moderate binding energy with a W-SIA effectively suppresses vacancy formation without significantly enhanced precipitation and consequently mitigates irradiation hardening.

Influence of the positron implantation profile on the study of the defect depth distribution by the positron annihilation technique

The formulas obtained for deconvolution of positron mean lifetime results in the sequential etching technique for detecting defect depth profiles are presented. In this experiment, only the conventional positron lifetime measurement with radioisotope-based positrons is used. The important role of the positron implantation profile is discussed. These formulas were successfully used to describe the depth profile of defects formed in sliding contact in pure vanadium. Two different layers were detected below the surface.

Development of a small and light portable positron annihilation lifetime measurement equipment for on-site measurements

Positron annihilation lifetime measurement has been a destructive analysis method in the sense that it requires cutting the specimen in two pieces, but the anti-coincidence method we developed enables the measurement of positron annihilation lifetime without cutting. In this study, by applying this anti-coincidence method, we developed a small and light portable positron annihilation lifetime measurement equipment for on-site measurements. In order to make the equipment small and light, we used a small photomultiplier tube for the γ-ray detectors, and we determined the detector configuration by simulation and experiment. To evaluate the performance of the developed equipment, we conducted positron annihilation lifetime measurements on shot-peened stainless steel and compared the results to those by the desktop anti-coincidence equipment. Just like the desktop equipment, the developed equipment revealed the increase in the defect density by shot-peening as the change in the positron lifetime, verifying the effectiveness of this equipment.

Remarks on a source contribution in positron lifetime measurements

The process of deconvolution of a positron lifetime spectrum for searching for individual lifetime components is ill-posed, therefore the obtained values may not be unique. However, knowledge of the fraction of positrons annihilating in the 22NaCl salt and Kapton foils that consist of the positron source can partially help overcome these difficulties. Using experimental and also theoretical procedures there was found that source intensity in the spectrum is a linear function of the atomic number of the samples that sandwich the source. Several examples have shown that applying the dependency found may be useful in the deconvolution procedure. One example was the pure refractory metal, i.e., Rhenium, which was first investigated using the positron annihilation technique.

Free volume and gas transport properties of hydrolyzed polymer of intrinsic microporosity (PIM-1) membrane studied by positron annihilation spectroscopy

In this work, a series of hydrolyzed polymers of intrinsic microporosity (PIMs) membranes were successfully synthesized under alkaline conditions. Positron annihilation lifetime measurement is employed to analyze the pore structure of the hydrolyzed PIMs. In the original PIM-1, there are two kinds of pores, i.e. micropores with radius of 4.32 Å and ultramicropores with radius of 2.58 Å. The size of all the pores shows a continuous decrease with increasing hydrolysis time, which is confirmed by the reduced chain-to-chain spacing in the hydrolyzed PIMs measured by X-ray diffraction. Meanwhile, the fractional free volume also shows decrease after the hydrolysis process, while the relative number of ultramicropores increases. Benefiting from the smaller pore size in the hydrolyzed PIMs membrane, the gas molecules with smaller size (such as CO 2 ) can be separated more efficiently. The selectivity of CO 2 /CH 4 of hydrolyzed PIMs reaches up to 30.05 with a relatively high CO 2 permeability of 140.58 Barrer, which exceeds the Robeson’s 1991 upper bound. Our results indicate that the hydrolyzed polymers of intrinsic microporosity membranes are promising candidates for gas separation membrane in the future. • A series of hydrolyzed PIMs membranes were synthesized under alkaline conditions. • The process shortens the reaction time and improves the gas separation performance. • The logarithm of P shows a linear relationship with 1/FFV. • Positron lifetime measurements indicate the free volume adjusts the selectivity.

On the helium bubble swelling in nano-oxide dispersion-strengthened steels

The development of structural materials resistant to harsh radiation environments requires an in-depth understanding of the early stage of the aging processes. In radiation environments with high transmutation helium production rates such as in fusion and spallation applications, even materials with otherwise acceptable radiation stability may suffer from radiation embrittlement related to helium bubble formation. While theoretical modeling of helium-assisted cavity nucleation in pure metals and simple alloys provides some useful guidelines at the atomic scale level, these, however, do not overlap with the size resolution of available experimental techniques. In this study, we employed slow positron beam spectroscopy to characterize the nucleation and growth of nano-scale helium bubbles in martensitic steels strengthened by thermodynamically stable nano-oxide dispersoids. In combination with transmission electron microscopy, we experimentally characterized the evolution of helium bubbles from small clusters of radiation-induced vacancies to large cavities well resolvable by TEM. Superior radiation resistance of oxide-dispersion strengthened steels dominates only in the early stages of bubble evolution, where positron lifetime measurements provide a missing piece of the microstructural puzzle conventionally constructed by TEM.

Positrons as chemically sensitive probes in interfaces of multicomponent complex materials: Nanocrystalline Fe90Zr7B3

Abstract The present paper reports on a combined analytical and structural study of nanocrystalline Fe90Zr7B3 by means of positron annihilation, (analytical) high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction. Particular focus is laid on the chemical nature of the intergranular amorphous matrix which occurs between the α-Fe nanocrystallites. Energy-dispersive X-ray measurements (EDX) with an electron nanobeam reveal an increased Zr content at the interface between the nanocrystallites and the intergranular amorphous phase. According to positron lifetime measurements, the intergranular amorphous phase and the interfaces between this phase and the nanocrystallites exhibit structural free volumes of the mean size slightly smaller than a lattice vacancy as in the amorphous precursor material. Coincident Doppler broadening measurements of the positron-electron annihilation photons show that the fraction of Zr in the neighborhood of the structural free volumes is higher in nanocrystalline Fe90Zr7B3 than in the amorphous state indicating an enhanced Zr concentration in the interfaces. These results are in good agreement with the HRTEM/ EDX studies and demonstrate the potentials of the coincident Doppler broadening technique for a chemical characterization of structurally complex materials on an atomistic scale.

Harvesting Light from BaHfO3/Eu3+ through Ultraviolet, X-ray, and Heat Stimulation: An Optically Multifunctional Perovskite.

Materials with optical multifunctionality such as photoluminescence (PL), radioluminescence, and thermoluminescence (TL) are a boon for a sustainable society. BaHfO3 (barium hafnium oxide [BHO]) under UV irradiation demonstrated visible PL endowed by oxygen vacancies (OVs). Eu3+ doping in BHO (BHOE) introduces f-state impurity levels just below the conduction band for both Eu@Ba and Eu@Hf sites, causing efficient host-to-dopant energy transfer, generating intense 5D0 → 7F1 magnetic dipole transitions (MDT) with internal quantum yield of ∼70%. X-ray photoelectron spectroscopy and electron paramagnetic resonance showed the formation of OVs in both BHO and BHOE samples with more vacancies in the doped sample. The positron lifetime measurements suggested that Eu3+ ions are distributed at both Ba2+ and Hf4+ sites. The association of OVs with Hf4+ and Eu3+ ions due to high charge/radius ratio is considered to be responsible for lowering the symmetry around Eu3+ ions to C4v in BHOE. Density functional theory studies of defect formation energy justified the same. Time-resolved emission spectroscopy showed distinct spectra for Eu@Ba and Eu@Hf sites corresponding to symmetric and asymmetric environments, respectively. This could be highly relevant in designing color tunable phosphor by forcing dopant ions at one specific site because Eu@Ba displayed orange emission whereas Eu@Hf displayed red emission. We could further harness BHOE for X-ray scintillator application by designing a thin film, which showed efficient conversion of high-energy X-ray into visible light. Under beta irradiation; both BHO and BHOE showed distinct TL glow curves as shallow traps were formed in the former and deep traps in the latter, which could have long-term implications in harnessing this material for persistent luminescence. We believe that BHO/BHOE demonstrated an extraordinary credential as a perovskite for multifunctional applications in the area of defect-induced light emission, UV phosphor, X-ray scintillator, and TL crystals.

Tailoring the micropore structure of 6FDA-based polyimide membrane for gas permselectivity studied by positron annihilation

Two series of 6FDA-based co-polyimide membranes were synthesized by adding different diamine monomer containing special functional groups: APAF (containing OH) and DABA (containing -COOH). The ODA/APAF and ODA/DABA series were heat treated at 450 °C and 350 °C, respectively. Thermal rearrangement of polymer chain segments is confirmed in ODA/APAF membranes by infrared spectroscopy, while addition of DABA in ODA/DABA membranes causes decarboxylation crosslinking. Positron lifetime measurements reveal existence of ultramicropores in polyimides. The thermal rearrangement in ODA/APAF membranes induces remarkable increase in both size and number of ultramicropores and therefore increase in fractional free volume (FFV), but the crosslinking in ODA/DABA causes significant decrease in both size and number of ultramicropores. The increase in FFV is favorable for gas permeation, but too large size of the micropore drastically deteriorates the gas selectivity. Finally, the ODA/APAF(3:7) shows a high CO2 permeability of 127.4 Barrer with CO2/CH4 selectivity of 34.2, which surpasses the 1991 Robeson upper bound. Besides, the ODA/DABA(3:7) exhibits a H2 permeability of 22.6 Barrer with H2/CH4 selectivity of 256.8, which also exceeds the 1991 Robeson upper bound. Our results indicate that the gas permeability and selectivity of polymer membranes can be effectively adjusted by tailoring the ultramicropore structures, and positron can provide precise information about micropores.

Synergetic Optimization of Electrical and Thermal Transport Properties by Cu Vacancies and Nanopores in Cu2Se.

In this study, a series of Cu2+x-yInySe (-0.3 ≤ x ≤ 0.2 and 0 ≤ y ≤ 0.05) samples were prepared by melting and the spark plasma sintering method. X-ray diffraction measurements indicate that the Cu-deficient samples (x = -0.3 y = 0 and x = -0.2 y = 0) prefer to form the cubic phase (β-Cu2Se). Adding excessive Cu or introducing In atoms into the Cu2Se matrix triggers a phase transition from the β to α phase. Positron lifetime measurements confirm the reduction in Cu vacancy concentration by adding excessive Cu or introducing In atoms into Cu2Se, which causes a dramatic decrease in carrier concentration from 1.59 × 1021 to 5.0 × 1019 cm-3 at room temperature. The samples with In contents of 0.01 and 0.03 show a high power factor of about 1 mW m-1 K-2 at room temperature due to the optimization of the carrier concentration. Meanwhile, the excess Cu content and doping of In atoms also favor the formation of nanopores. These pores have strong interaction with phonons, leading to remarkable reduction in lattice thermal conductivity. Finally, a high ZT value of about 1.44 is achieved at 873 K in the Cu1.99In0.01Se (x = 0 and y = 0.01) sample, which is about twice that of the Cu-deficient sample (Cu1.7Se). Our work provides a viable insight into tuning vacancy defects to improve efficiently the electrical and thermal transport performance for copper-based thermoelectric materials.

Extremely low thermal conductivity of β−Ga2O3 with porous structure

Due to the ultrawide bandgap (4.9 eV), high carrier mobility (300 cm2V−1s−1), and high thermal stability, β−Ga2O3 can be a potential candidate for high-temperature thermoelectric materials. However, the intrinsically high thermal conductivity may hinder its application for thermoelectric conversion. In this work, porous β−Ga2O3 was prepared by the solvothermal method together with spark plasma sintering technology. Positron lifetime measurement and N2 adsorption confirm the introduction of pores by adding sucrose in the sample preparation. The sucrose-derived β−Ga2O3 sintered at a relatively low temperature of 600 °C remains highly porous, which results in an extremely low thermal conductivity of 0.45 W m−1K−1 at room temperature, and it further decreases to 0.29 W m−1K−1 at 600 °C. This is the lowest thermal conductivity for β−Ga2O3 reported so far. Our work provides an avenue to reduce the thermal conductivity for β−Ga2O3 and is believed to be widely applicable to many other thermoelectric materials.

Penetration of deuterium into neutron-irradiated tungsten under plasma exposure

Hydrogen isotope trapping at lattice defects in neutron-irradiated tungsten (W), a leading candidate as plasma facing material, is an important problem determining tritium (T) inventory in a vacuum vessel of a future fusion reactor. In this study, W samples were irradiated with neutrons in the Belgium Reactor 2 at 563 K to 0.06 or 0.016 displacement per atom (dpa). After characterizing defects by positron lifetime measurements, deuterium (D) penetration under exposure to D plasma was examined at 563–773 K. Positron lifetime showed the presence of dislocations, monovacancies and relatively large vacancy clusters. These defects trapped D atoms with different values of binding energy. Dependence of D retention on plasma exposure temperature and damage level indicated that the concentrations of weak traps with smaller binding energy increased more significantly with damage level than those of strong traps.

Reduction of background radiation effects for positron lifetime measurements in the slow positron beamline at the Kyoto University Research Reactor.

Positron annihilation lifetime spectroscopy (PALS), which is recognized as one of the major analytical methods of positron annihilation spectroscopy, can directly detect information related to the size of vacancy-type defects from lifetime values. PALS measurements performed under high background radiation have been previously reported. It is well known that coincidence techniques such as age-momentum correlation (AMOC) measurements are effective for the background reduction, but count rates decline significantly. In this study, a preliminary experiment was performed to reduce the influence of the background radiation without the coincidence technique in the pulsing system of the Kyoto University research Reactor (KUR) slow positron beamline. This experiment involved the introduction of a gate circuit for the background radiation discrimination using a dynode signal from a single scintillation detector (photomultiplier). After introducing the gate circuit, the time resolution and the lifetime value of Kapton were 308 ps and 388 ± 3 ps, respectively, with count rates of ∼400 counts/s at a KUR 5 MW operation. In the AMOC measurement, the time resolution and the lifetime value of Kapton were 297 ps and 380 ± 7 ps, respectively, with count rates of ∼40 counts/s at a KUR 5 MW operation. When the single detector with the gate circuit was used, the count rate was ∼1 order of magnitude higher than those of the AMOC measurements, while the time resolutions of the two methods were comparable.

Point and extended defects in heteroepitaxial β−Ga2O3 films

$\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ is emerging as an excellent potential semiconductor for high power and optoelectronic devices. However, the successful development of $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ in a wide range of applications requires a full understanding of the role and nature of its point and extended defects. In this work, high quality epitaxial $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ films were grown on sapphire substrates by metal-organic chemical vapor deposition and fully characterized in terms of structural, optical, and electrical properties. Then defects in the films were investigated by a combination of depth-resolved Doppler broadening and lifetime of positron annihilation spectroscopies and thermally stimulated emission (TSE). Positron annihilation techniques can provide information about the nature and concentration of defects in the films, while TSE reveals the energy level of defects in the bandgap. Despite very good structural properties, the films exhibit short positron diffusion length, which is an indication of high defect density and long positron lifetime, a sign for the formation of Ga vacancy related defects and large vacancy clusters. These defects act as deep and shallow traps for charge carriers as revealed from TSE, which explains the reason behind the difficulty of developing conductive $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ films on non-native substrates. Positron lifetime measurements also show nonuniform distribution of vacancy clusters throughout the film depth. Further, the work investigates the modification of defect nature and properties through thermal treatment in various environments. It demonstrates the sensitivity of $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ microstructures to the growth and thermal treatment environments and the significant effect of modifying defect structure on the bandgap and optical and electrical properties of $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$.

Ca2+-substitution effect on the defect structural changes in the quadruple perovskite series Ca1+xCu3-xTi4O12 studied by positron annihilation and complementary methods

A series of quadruple perovskites Ca1+xCu3-xTi4O12 was scanned by different spectroscopic and microscopic techniques. X-ray powder diffraction pattern analysis revealed that the system transformed from a monophasic (x = 0.0, 0.1 and 0.2) to biphasic (x = 0.5 and 1.0) on Ca2+ - substitution. The completely different grain morphology and the distinct signature of the UV–visible absorption band and Raman scattering peak reflect this finding well. The defects arise mainly because of the mismatch between ion and site radii. The results of positron annihilation spectroscopy to study the nature of the defects are consistent with the proposed defect-rich sample history. There are many vacancies-type defects, whose size and concentration could be characterized through the measured positron lifetimes and coincidence Doppler broadening. The results show that for x = 0.1 itself, a secondary phase may be formed. The structure-property and property-property correlations are studied.

Natural ageing clustering under different quenching conditions in an Al-Mg-Si alloy

During quenching of aluminium alloys from the solutionising temperature vacancies are partially conserved as excess vacancies, partially lost to vacancy sinks, the exact fractions depending on the cooling rate. Positron lifetime measurements in samples from interrupted quenching experiments reveal that vacancies are lost during cooling down to 200 °C, after which solute atoms start to form clusters down to 20 °C. Slow cooling leads to 1 to 2 orders of magnitude lower excess vacancies than fast cooling. Since quenched-in vacancies are crucial for natural ageing (NA) in Al-Mg-Si alloys it is surprising to find just small differences between the NA hardening kinetics after different quenches. Specifically, hardening rates differ only in the initial stage (<100 min), after which they are almost identical for NA up to ~1 year. This suggests that interactions between vacancies and early-stage solute clusters help equalising the free vacancy fractions in differently quenched samples.