Positron Annihilation Experiments Research Articles

New Route to Synthesize High-Entropy Carbide Powders by Mechanical Alloying

A new route for the synthesis of the (NbTa)0.67\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{0.67}$$\\end{document}(TiHfZr)0.33\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{0.33}$$\\end{document}C and (NbTa)0.67\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{0.67}$$\\end{document}(MoHfW)0.33\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{0.33}$$\\end{document}C high-entropy carbides powders by mechanical alloying is presented. Namely, pure metal powders were wet-milled in toluene using a planetary mill under an argon atmosphere. The synthesized materials crystallize with a rock-salt structure (space group Fm3¯m\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Fm{\\bar{3}}m$$\\end{document}). A significant reduction in structural defects with annealing of the samples was detected with X-ray diffraction and positron annihilation experiments. Simultaneously, only simple metallic properties without any trace of magnetic or superconducting phase transitions were observed.

Interface-Mediated Mechanoluminescence Enhancement from Heterojunction Phosphors: Experiment and Theory

Mechanoluminescence (ML) phosphors have made significant progress in various fields, such as artificial intelligence, the Internet of Things, and biotechnology. However, enhancing their weak ML intensity still remains a challenge. Here, we report a new series of Na1-xMgxNbO3:Pr3+ (x = 0.00, 0.10, 0.20, 0.40, 0.60, 0.80, and 1.00 mol %) heterojunction systems, which exhibit significant ML enhancement as compared with either the Pr3+-doped NaNbO3 or MgNbO3, and the physical mechanisms behind the ML enhancement have been explored comprehensively from both the experiment and theory points of view. Experimental tests, including thermoluminescence and positron annihilation lifetime measurements, combined with first-principles calculations, consistently indicate that the ML enhancement observed in these newly reported systems is due to the formation of heterojunctions, which plays a crucial role in modulating the defect configuration of the phosphors and facilitating efficient charge transfer. By controlling the Na/Mg ratio in conjunction with Pr3+ doping, continuous changes in the band offset and the concentrations of certain types of traps in the forbidden gap are achieved, leading to the optimum conditions in the 8/2 ratio samples. These findings demonstrate a novel type of ML phosphor and provide a theoretical basis for the design of high-performance ML phosphor.

Electronic structure and nanoindentation properties of electrochemical hydrogen-charged Zr-based metallic glasses

Zr55Cu30Ni5Al10 metallic glasses were treated by an electrochemical hydrogen- (H-)charging method. Samples with different H content were obtained by changing the H-charging current density and charging time. X-ray diffraction, nanoindentation, X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy, and positron annihilation experiments were used to investigate amorphous structure, nanomechanical properties, electronic structure, and positron annihilation behavior of Zr55Cu30Ni5Al10 metallic glasses after electrochemical hydrogenation treatment. The results showed that the diffraction angle corresponding to the diffuse scattering peak gradually moved to a low angle with increased H content. At the same time, the hardness and elastic modulus of the MG were significantly increased with increased H-charging content. When the H content was high, the sawtooth rheological phenomenon disappeared in the load displacement curve of the MG during nanoindentation. XPS narrow spectrum analysis showed that the Zr-3d peak in samples shifted to higher binding energies, while the other elements shifted toward lower binding energy, indicating that H addition led to the transfer of valence electrons from Zr-3d to the Zr–H bond state, resulting in hardening. Three lifetime components are observed in the uncharged and charged sample, indicating the presence of three size ranges of open volume sites. After electrochemical hydrogen charging it causes a significant decrease in the size (lifetime) of the three open volume defects, indicating that the hydrogen occupies those sites. With the increase of hydrogen content, the concentration (intensity) of the first two open volume defects gradually decreases, while the third open volume defect gradually enhances, indicating that hydrogen atom mainly occupies the first two open volume defects. Positron annihilation experiments showed that H addition reduced the average annihilation lifetime of positron vacancies in these MGs, but no new defects were produced.

Role of Vacancy Defects in Reducing the Responsivity of AlGaN Schottky Barrier Ultraviolet Detectors.

The spectral response properties of AlGaN Schottky barrier detectors with different Al content were investigated. It was found that the responsivity of AlGaN detectors decreases with increase in Al content in AlGaN. It was found that neither dislocation density nor the concentration of carbon and oxygen impurities made any remarkable difference in these AlGaN devices. However, the positron annihilation experiments showed that the concentration of Al or Ga vacancy defects (more likely Ga vacancy defects) in AlGaN active layers increased with the increase in Al content. It is assumed that the Al or Ga vacancy defects play a negative role in a detector’s performance, which increases the recombination of photogenerated carriers and reduces the detector responsivity. It is necessary to control the concentration of vacancy defects for the high performance AlGaN detectors.

Source for In Situ Positron Annihilation Spectroscopy of Thermal-And Hydrogen-Induced Defects Based on the Cu-64 Isotope.

This work aims to investigate the 64Cu isotope applicability for positron annihilation experiments in in situ mode. We determined appropriate characteristics of this isotope for defect studies and implemented them under aggressive conditions (i.e., elevated temperature, hydrogen environment) in situ to determine the sensitivity of this approach to thermal vacancies and hydrogen-induced defects investigation. Titanium samples were used as test materials. The source was obtained by the activation of copper foil in the thermal neutron flux of a research nuclear reactor. Main spectrometric characteristics (e.g., the total number of counts, fraction of good signals, peak-to-noise ratio) of this source, as well as line-shaped parameters of the Doppler broadening spectrum (DBS), were studied experimentally. These characteristics for 64Cu (in contrast to positron sources with longer half-life) were shown to vary strongly with time, owing to the rapidly changing activity. These changes are predictable and should be considered in the analysis of experimental data to reveal information about the defect structure. The investigation of samples with a controlled density of defects revealed the suitability of 64Cu positron source with an activity of 2–40 MBq for defects studies by DBS. However, greater isotope activity could also be applied. The results of testing this source at high temperatures and in hydrogen atmosphere showed its suitability to thermal vacancies and hydrogen-induced defects studies in situ. The greatest changes in the defect structure of titanium alloy during high-temperature hydrogen saturation occurred at the cooling stage, when the formation of hydrides began, and were associated with an increase in the dislocation density.

Point defect formation near the epitaxial Ge(001) growth surface and the impact on phosphorus doping activation

First-principles calculations are used to systematically investigate the impact of surface effects on the energetic cost to incorporate point defects near the growing surface [Ge(001)] and its consequence on the dopant activation in P-doped Ge layers. We illustrate the formation enthalpy ΔHf for the incorporation of a mono-vacancy, which is the dominant point defect responsible for the electrically inactive dopant incorporation in P-doped Ge. The calculated values point toward a significant lowering of ΔHf near the growing surface in comparison to the bulk cubic Ge supercell. The impact of the surface almost vanishes beyond the sixth atomic monolayer located below the surface and the calculated ΔHf values overlap with the ones computed for a bulk cubic Ge supercell. For epitaxial Ge:P layers, grown at low growth temperatures (<450°C) using the deposition method such as the Chemical Vapor Deposition, the dopant-vacancy clusters are formed within the first few monolayers close to the growing surface. The low ΔHf values for the incorporation of a mono-vacancy near the growing surface suggest that the concentration of vacancies can be significantly enhanced with respect to the bulk Ge, which can subsequently trap more dopants and deactivate them at the surface itself. Our first-principles calculation results are in line with previously reported experimental positron annihilation spectroscopy analysis on Ge:P layers grown at 440 °C using GeH4 as gas precursor. At P concentrations above 1×1020cm−3, the mono-vacancy sized open-volume defects are passivated by at least three P atoms.

Temperature Dependence of Hydrogen Solubility and Diffusivity in Hydrogen Permeable Membrane of Pd-Cu Alloy with B2-Type Crystal Structure

The temperature dependence of hydrogen solubility and diffusivity of Pd–53mol%Cu alloy membrane with the B2–type crystal structure has been investigated. The hydrogen permeation tests are performed using ultra–pure hydrogen (more than 9N) purified by a Pd–Ag alloy membrane to avoid any influences of impurities. It is found that the hydrogen permeability decreases significantly at low temperatures, especially near room temperature. The time dependence of hydrogen flux is monitored and found that the hydrogen flux decreases gradually during about 4 ~ 5 days after rapid cooling down to room temperature from 623 K.The results of the temperature dependence of the hydrogen permeability are analyzed in view of the consistent description of hydrogen permeation based on hydrogen chemical potential, where the hydrogen flux is proportional to the product of the mobility for hydrogen diffusion, B, and the PCT factor, fPCT. In this study, the pressure–composition–isotherms (PCT curves) for Pd–53Cu alloy with B2 structure are measured for the first time by the in–situ XRD–PCT method, and they are applied to estimate the PCT factors. Then, the temperature dependence of the PCT factor and the mobility for hydrogen diffusion is evaluated. It is revealed that the decrement in hydrogen permeability at low temperatures is mainly attributable to the decrement of the mobility for hydrogen diffusion.According to the positron annihilation experiments, the defects density is considered to be small in Pd–53Cu alloy with the B2 structure even at room temperature, suggesting that the excess Cu atoms in Pd–53Cu alloy occupy the positions of Pd sublattice, at which the Cu atoms form a local BCC–Cu unit. The diffusion of Cu atoms corresponds to the diffusion of BCC–Cu units in the B2 structure. Therefore the diffusion of Cu atoms and the configuration of BCC–Cu units in B2 structure could be a key to understand the gradual transition of hydrogen diffusivity at low temperatures.

Recent Progress on the Positron Annihilation Experiments on Irradiated Ferritic/Martensitic Steels Containing Helium

The well-known and often acceptable radiation tolerance of ferritic/martensitic (f/m) steels can be severely diminished when neutron irradiation is accompanied by the production of helium. The presence of helium in the irradiated materials changes the kinetics of the nucleation, recombination, and clustering of the radiation-induced defects. High production rates of helium may lead to a non-negligible volumetric bubble swelling at relatively low temperatures. Extrapolation of the knowledge gained from neutron irradiation experiments to fusion or spallation environments is additionally complicated due to the unknown and comprehensive effects of dpa rate, temperature, the presence of sinks in the crystal lattice and others. To improve the understanding of the microstructure and irradiation parameters effects, close attention must be paid to the early stages of the radiation damage. It is expected that the pre-existing vacancy-type defects, attributed to lattice distortion at the grain/subgrain boundaries and oxide-matrix interfaces, are effective sinks for primary defects and helium, i.e. they control the formation and growth of helium-vacancy agglomerations. This early-stage radiation damage, however, cannot be captured by conventional transmission electron microscopy, and thus other experimental techniques are called for. One of the most perspective experimental approaches to investigate small vacancy-type defects, with a high sensitivity to confined helium, is to utilize positron annihilation spectroscopy (PAS). In particular, two spectroscopy techniques, positron annihilation lifetime spectroscopy (PALS) and Doppler broadening spectroscopy (DBS) of the annihilation line, can be beneficially used for the characterization of helium-vacancy clusters. This paper reviews the recent positron annihilation spectroscopy characterization of various irradiation experiments involving helium. Mainly two types of irradiation experiments are addressed, helium implantation and spallation neutron source irradiation experiments. Discussion is aimed at the potential of PAS in the early-stage formation of helium bubbles and the investigation of the effects of irradiation parameters in defect production and accumulation.

Mechanical spectroscopy of atomic ordering in Fe-(16−21)Ga-RE alloys

Anelasticity of Fe-(16−21)at% Ga-RE (rare earth RE = La, Tb, Dy, Er, Yb elements) alloys at room and elevated temperatures is studied in a sub-resonance mechanical spectroscopy. Two thermally-activated and two transient effects are recorded in most of the studied alloys. To explain these phenomena, structure and phase transitions in several binary and ternary Fe-Ga alloys are investigated. The D03 ordering of rapidly cooled alloys with the A2 structure at heating at around 300 °C and the disordering at heating and the D03 ordering at cooling around 500 °C for annealed samples are proved using three in situ techniques: neutron diffraction, vibrating sample magnetometry, and internal friction and supported by positron annihilation experiments. Thermally activated transitory effects are tentatively explained by stress-induced reorientation of Ga-Ga, vacancy-vacancy pairs, and carbon atoms jumps.

Multiphoton light emission in barium stannate perovskites driven by oxygen vacancies, Eu3+ and La3+: accessing the role of defects and local structures.

Defect engineering in perovskites has been found to be the most efficient approach to manipulate their performance in ultraviolet-to-visible photon conversion. Under UV irradiation, BaSnO3 exhibited multicolor photoluminescence (MCPL) in the bluish white region. Its origin has not been well studied in the literature and has been probed in this work using synchrotron radiation, positron annihilation and density functional theory. To achieve desirable performance of doped BaSnO3 in optoelectronics, it is imperative to have correct information on the dopant local site, doping induced defect evolution and efficacy of host to dopant energy transfer (HDET). Extended X-ray absorption fine structure (EXAFS) showed that Eu3+ ions stabilize at both Ba2+ and Sn4+ sites consistent with the highly negative formation energy of around -6.26 eV. Eu3+ doping leads to an intense 5D0→7F1 orange emission and a feeble 5D0→7F2 red emission and an internal quantum yield (IQY) of ∼21% mediated by ET from the defect level of EuBa and EuSn sites to the valence band maximum (VBM). X-ray absorption near edge structure (XANES) ruled out any role of Sn2+ in the PL of BaSnO3 or Eu2+ in the PL of BaSnO3:Eu3+. Interestingly, when co-doped, Eu3+ stabilizes at Sn4+ sites whereas La3+ stabilizes at Ba2+ sites with a formation energy value of -6.44 eV. Based on the asymmetry ratio in emission spectra, it was found that La3+ ions lead to lowering of symmetry around Eu3+ due to increased vacancies and structural distortions, and also suppress the luminescence IQY. We have performed experimental positron annihilation lifetime spectroscopy (PALS) to probe the defects in BaSnO3 in pristine samples and on doping/co-doping. The positron lifetimes for saturation trapping of positrons in various kinds of defects envisaged in BaSnO3 and in the defect free system were calculated using the MIKA Doppler program. Such deep insight into the effect of local structures, dopant sites, defect evolution, ET, etc. on the optical properties of BaSnO3 is expected to provide very deep insight for material scientists into the fabrication of perovskite-based optoelectronic and light-emitting devices.

The relationship between the doping concentration and d ferromagnetism in n-type 4H-SiC

To date, research on defect-induced magnetism in SiC has been conducted to clarify the relationship between the ferromagnetic signal and the carrier concentration. It has been experimentally proven that there is an interaction between the d0 magnetic moment and the hole carrier in p-type 4H-SiC. However, for n-type SiC, the existing theoretical predictions are insufficient to explain the variation in magnetization with the doping concentration. To solve this problem, we prepared 4H-SiC epitaxial layers with different nitrogen doping concentrations and introduced defects by ion implantation. By measuring and analyzing the magnetic properties, we found that the ferromagnetic composition depends on both the implantation dose and the doping concentration. By performing first-principles calculations, we studied the magnetic moments and interactions of defects with different charge states, which is related to defect-induced ferromagnetism. These defects include not only the paramagnetic centers reported in previous studies, such as silicon vacancies and divacancies, but also the NCVSi complex defect of recent interest, which are indicated by positron annihilation experiments. Combining experimental observations with theoretical calculations, we explained the relationship between magnetic properties and the nitrogen doping concentration in the epitaxial samples. Our research will help us to better understand the physical mechanism of defect-induced magnetism in doped semiconductors and provide a potential platform for the control of defect-induced magnetism by carrier density modulation and the fabrication of SiC spintronic devices without transition metals.

Effect of free-volume holes on static mechanical properties of epoxy resins studied by positron annihilation and PVT experiments

The tensile, flexural, and fracture toughness properties of seven chemically different amine-cured epoxy resins were studied. Positron annihilation lifetime and pressure–volume–temperature (PVT) experiments were performed on each epoxy resin to characterize the average size and fraction, respectively, of free-volume holes. A negative correlation between hole fraction and hole size was revealed for these chemically different epoxy resins. Better tensile and flexural mechanical properties (higher tensile modulus and lower tensile strain at break; higher flexural modulus, higher flexural strength, and lower flexural strain at break) were clearly observed for epoxy resins with smaller hole size and higher hole fraction. However, no clear relationship between fracture toughness and hole properties was found. The correlations between (tensile and flexural) static mechanical properties and hole properties for chemically different epoxy resins should provide guidance for further improvements in the mechanical properties of carbon-fiber-reinforced polymers.

Diffusion-drift-reaction model for positron trapping and annihilation at interfaces with space charge in bulk materials

ABSTRACTThe exact solution of a diffusion-drift-reaction model for the trapping and annihilation of positrons at interfaces with adjacent space-charges in bulk materials is presented. Closed-form expressions are obtained for the mean positron lifetime and for the intensity of the positron lifetime component associated with trapping at interfaces. The exact solutions can be conveniently applied for the analysis of experimental data. Compared to available thin-film models, the present approach takes into account that drift usually occurs only in a part of the diffusion zone. The model clearly reveals that for typical drift-zone parameters, positron drift has a substantial impact on the positron annihilation characteristics, that is, disregarding existing drift could lead to misjudgement of experimental positron annihilation data.

Influence of Au ions irradiation on the deuterium permeation behavior in the oxidized Fe–Cr–Al ferritic steel

Our previous study has shown that an effective tritium permeation barrier (TPB) with Al2O3 layer found could be obtained by oxidization of a Fe–Cr–Al ferritic steel. In this study, irradiation effects on deuterium permeation behavior through oxidized Fe–Cr–Al ferritic steel (OFFS) have been investigated by Au ions irradiation followed by deuterium gas driven permeation (GDP) experiments. The deuterium permeability of the irradiated and original OFFS samples has been obtained and compared. Oxide layer has been characterized by X-ray photoelectron spectroscopy (XPS) experiment and scanning electron microscopy (SEM) experiment. The defects in the oxide layer with and without Au ions irradiation have been characterized by Doppler broadening spectrometry of positron annihilation (DBS-PA) experiments. The deuterium permeation behavior of OFFS changed owing to the Au ions irradiation, which could be attributed to the increased density of vacancy-type defects.

Experimental comparison of the (ODS)EUROFER steels implanted by helium ions and irradiated in spallation neutron target

The microstructure of EUROFER and ODS-EUROFER has been characterized by means of positron annihilation spectroscopy (PAS) with a particular focus on the effect of irradiation conditions. Two different irradiation experiments have been used to introduced helium-filled cavities in the studied materials. Materials irradiated in spallation neutron target SINQ have been investigated by conventional positron annihilation spectroscopy, while helium-implanted samples of the same materials have been investigated by slow positron beam technique. With this approach, we were able to obtain a significant amount of positron lifetime and Doppler broadening data and evaluate them with respect to the dpa, irradiation temperature and helium concentration effects. Both types of irradiation experiments confirm a more pronounced creation of growing open-volume defects for the Eurofer steel, comparing to its ODS variant. The bubble-to-void transformation was indirectly but clearly confirmed by the Eurofer specimens. In the case of the ODS Eurofer, the coalescence of helium bubbles, accompanied by saturation of positron mean-lifetime and/or S parameter, was not so evident. This present study provides the first experimental positron annihilation data, showing the onset of void formation in ODS steels.

Radiation-produced defects in germanium: Experimental data and models of defects

The problem of radiation-produced defects in n-Ge before and after n → p conversion is discussed in the light of electrical data obtained by means of Hall effect measurements as well as Deep Level Transient Spectroscopy. The picture of the dominant radiation defects in irradiated n-Ge before n → p conversion appears to be complicated, since they turn out to be neutral in n-type material and unobserved in the electrical measurements. It is argued that radiation-produced acceptors at ≈E C – 0.2 eV previously ascribed to vacancy-donor pairs (E-centers) play a minor role in the defect formation processes under irradiation. Acceptor defects at ≈E V + 0.1 eV are absolutely dominating in irradiated n-Ge after n → p conversion. All the radiation defects under consideration were found to be dependent on the chemical group-V impurities. Together with this, they are concluded to be vacancy-related, as evidenced positron annihilation experiments. A detailed consideration of experimental data on irradiated n-Ge shows that the present model of radiation-produced defects adopted in literature should be reconsidered.

Spin-Polarized Positron Annihilation Study on Some Ferromagnets

We briefly review the spin-polarized positron annihilation experiments on some ferromagnets (Fe, Co, Ni, Gd, Co2MnSi, Co2MnAl and NiMnSb) using positron beams generated with 68Ge-68Ga sources. The differential DBAR spectra between majority and minority spin electrons are well interpreted by the first principles band structure calculation. This further provides information about the half-metallicity of the Heusler alloys. The surfaces of Fe, Co and Ni are more negatively spin-polarized, that is, there are more majority than minority spin electrons. To explain the observed spin polarization quantitatively, detailed theoretical calculations and further experiments are required.

The Science Enjoyed by Professor Alec T. Stewart

Late Professor Stewart initiated and shaped the International Conference on Positron Annihilation (ICPA) series. As a first-generation experimental positron-annihilation scientist, he made full use of the angular correlation of annihilation radiation (ACAR) method. He applied this method to study Fermi surfaces of metals, positron wave-functions in crystals, positron-electron and -phonon many-body interactions, and the vacancy formation energy in solids. He also studied with this method positronium in liquids and solids (T. Hyodo, J. Phys. Conf. Series, 618 (2015) 012002). All these studies enjoyed by Professor Stewart will long be remembered by the positron study community.

Radiation-produced defects in germanium: experimental data and models of defects

The problem of radiation-produced defects in n-Ge before and after n-> p conversion is discussed in the light of electrical data obtained by means of Hall effect measurements as well as Deep Level Transient Spectroscopy. The picture of the dominant radiation defects in irradiated n-Ge before n-> p conversion appears to be complicated, since they turn out to be neutral in n-type material and unobserved in the electrical measurements. It is argued that radiation-produced acceptors at ~ EC-0.2 eV previously ascribed to vacancy-donor pairs (E-centers) play a minor role in the defect formation processes under irradiation. Acceptor defects at ~ EV+0.1 eV are absolutely dominating in irradiated n-Ge after n-> p conversion. All the radiation defects under consideration were found to be dependent on the chemical group-V impurities. Together with this, they are concluded to be vacancy-related, as evidenced positron annihilation experiments. A detailed consideration of experimental data on irradiated n-Ge shows that the present model of radiation-produced defects adopted in literature should be reconsidered. DOI: 10.21883/FTP.2017.12.45178.8599

Effect of the composition and chemical aging in tung oil-styrene networks: Free volume and dynamic-mechanical properties

A study on the effect of the composition and chemical aging in tung oil-styrene networks prepared by cationic polymerization is presented. To this aim, the experimental techniques positron annihilation lifetime spectroscopy and dynamic-mechanical analysis were used. The representative experimental parameters were the nanohole free volume, the glass transition temperature and the dynamic mechanical properties, storage and loss moduli and tanδ. The results obtained were discussed in terms of the structure of the formed network, in particular to the packing of the molecular segments and the presence of dangling chains derived from the tung oil. In the case of aged samples the results were interpreted in terms of the oxidative polymerization and degradative processes in the copolymers studied.