Search

In this article, single and doped zirconium oxide (ZrO2) with metal ions (lanthanides group), which was synthesized by the co-precipitation method, was studied. The changes in crystal structure due to metal ions as dopants were investigated by X-ray diffraction (XRD). The results of XRD show that metal ions cause a change in the chemical phase structure, in the direction of crystal growth of ZrO2. Defects, porosity, and lattice structure of pure and doped ZrO2 were investigated by positron annihilation lifetime spectrometer (PALS). The results of PALS analysis show that the metal ions Pr3+, Sm3+, Dy3+, and Er3+ occupy the interstitial sites of the zirconia lattice, and Eu2+ replaces the vacancy of the Zr atom. The results of UV–Vis show that these metal ions cause a significant reduction in the band gap. Creating an energy band by metal ions plays the role of positron trapping centers that prevent electron–hole recombination and thus affect the conductivity of ZrO2. [ABSTRACT FROM AUTHOR]

Polycrystalline recrystallized tungsten samples were irradiated with 7.5 MeV Si ions and 9 MeV Cu ions to three different damage levels (0.01, 0.1, 0.5 dpa at 200 nm depth) at 295 K. The resulting vacancy-type defects in the samples were studied using positron annihilation lifetime spectroscopy. The dependence of the average positron lifetime on the damage level is found to be non-linear: a steep increase at low damage levels with a tendency to saturation at higher damage levels (>0.1 dpa). The average positron lifetime of Si and Cu-irradiated tungsten is very similar at each damage level, suggesting similar vacancy-type defect structure. Deconvolution of the positron lifetime spectra revealed that the dominant irradiation-induced defect type is a single vacancy. The presence of small vacancy clusters is also detected. Their fraction is found to increase with increasing damage level.

Positron Annihilation Spectroscopy (PAS) is a non-destructive technique used mostly in material science for studying open spaces/free volumes in materials. In this work, several different nuclear materials originating from the nuclear fuel cycle or round-robin exercises are analyzed by PAS. Our goal was, as for a first test of applicability, to check whether the technique is potentially suitable in a nuclear forensic examination for origin assessment of nuclear materials in order to support investigation. The results, presented below are promising and can certainly be further improved by analyzing larger sets of samples. [ABSTRACT FROM AUTHOR]

Hypothesis: The use of plasma is widely used as a method to change polymer surfaces. The use of atmospheric cold plasma has more advantages than other plasma, laser and X-ray methods. This method is simple and it uses inexpensive equipment. Considering the many uses of polyethylene in industry, it can be effective to investigate its changes against cold plasma.Methods: A dielectric barrier discharge (DBD) plasma under atmospheric pressure was used to increase the hydrophobicity of low-density polyethylene (LDPE). After studying the optical emission spectrum (OES) of the produced plasma, its effects on surface and depth changes including surface morphology, chemical composition and polymer crystal structure were studied through scanning electron microscopy (SEM), attenuated total reflectance-Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry (XRD) and positron lifetime spectroscopy (PALS). Moreover, the contact angle analysis (CA) was used to examine the changes in the hydrophobicity of the polymer. Finding: Based on the data from FTIR and XRD analyses, it was found that plasma irradiation for 180 s affects the depth of a few nanometers of the polyethylene surface and does not cause significant changes in the chemical bonds and crystal structure of the polymer. In other words, plasma radiation can be used for nanometer-scale modification of the surface. On the other hand, the SEM images indicate that the plasma radiation changes the primary flat surface of the polymer into a porous surface. The results of CA analysis, while confirming this issue, show an increase in the hydrophobicity of the polymer after plasma irradiation. The results of PALS spectroscopy also reveal that at micron depth due to the sudden rise in temperature during plasma irradiation, the free volume of the material increases as a result of pore merging.

The controlled destruction of the PIN diode detectors, SIEMENS SFH 870/F170 and SFH 871/F171, by the 35 MeV beam of the 12C and by 24 MeV of the 14N, respectively, was characterized using nuclear spectroscopy, the surface profile measurements, and the positron annihilation spectroscopy technique. The beam fluence was in the range of 1012-1014 ions/cm². It has been shown that the fluence of 1012 ions/cm2 of the 12C beam did not allow it to destroy the PIN diode detector. For this purpose, one needs the fluence of at least 4 × 1012 ions/cm² for the 14N ions beam and 2:2 × 1013 ions/cm² for the 12C ions one. The presence of divacancies in the irradiated sample was detected by the positron lifetimes measurements, with the fraction significantly higher for the 12C implanted sample. Furthermore, it was found that the surface roughness changed drastically following the implantation, i.e., the arithmetic average of profile height deviations from the mean surface of the 14N beam implanted sample is significantly higher than of that irradiated with the 12C ions and the reference one, and the surface average roughness was about 2-3 times higher. [ABSTRACT FROM AUTHOR]

Positron annihilation lifetime spectroscopy was used to examine grown‐in defects in Cd1–xZnxTe mixed crystals as a function of Zn content (x = 0, 0.07, 0.11, 0.49, 0.9, 0.95, 1) and measuring temperature. All samples were prepared using the high‐pressure modified vertical Bridgman–Stockbarger method. The crystal structure and material phase were characterized by X‐ray diffraction. The positron lifetime spectra reveal the presence of both open volumes and shallow traps regardless of the sample composition. In particular, both average and bulk lifetimes are found to be much higher in ternary alloys (CdZnTe) than those in binary systems (CdTe and ZnTe). This originates from distinct differences in average electron densities and the nature of open‐volume defects between binary and ternary samples. Competition in positron trapping with increasing Zn content is observed between defects characteristic for both structural systems. Moreover, a clear correlation is shown between defects and the lattice thermal conductivity of studied samples. The applicability of the positron trapping model to CdTe‐based materials is discussed. [ABSTRACT FROM AUTHOR]

In this paper, Ta-doped Bi 2/3 Cu 3 Ti 4 O 12 (BCTO) ceramics are prepared by solid-state method. Phase structure, microstructure, optical, dielectric, and non-Ohmic properties of all samples are systematically investigated. Cu/Ta-rich phases at grain boundaries (GB) together with defects affect the microstructure, causing grain size and compactness to increase first and then decrease. Moreover, Ta doping increases the optical band gap of BCTO from 4.03 to 4.25 eV as well as the GB activation energy and GB barrier. An increase in dielectric constant (Ɛ r), nonlinear coefficient (α) and breakdown field (E b), and a decrease in the dielectric loss (tanδ) are observed in Ta-doped BCTO ceramics. The improvement in dielectric and non-Ohmic properties can be attributed to the internal barrier layer caused by GB Schottky barrier structure. In particular, Bi 2/3 Cu 3 Ti 3.95 Ta 0.05 O 12 ceramic exhibits the high Ɛ r of ∼45997 and the low tanδ of ∼0.035 at 10 kHz along with α of ∼5.62, and E b of ∼3.45 kV/cm. The temperature-independent (-110-210 °C) dielectric response confirms that temperature stability of specimens increases with doping. These results suggest that improved optical, dielectric, and non-Ohmic properties in Bi 2/3 Cu 3 Ti 4 O 12 can be achieved simultaneously via defect engineering. [Display omitted] • The high Ɛ r of ∼45997, low tanδ of ∼0.035, α of ∼5.62, and E b of ∼3.45 kV/cm are obtained in Bi 2/3 Cu 3 Ti 3.95 Ta 0.05 O 12 ceramic. • The E g increased to 4.25 eV for Bi 2/3 Cu 3 Ti 3.9 Ta 0.1 O 12 ceramic as a result of enhanced structural defects. • Cu/Ta-rich phases at grain boundary together with defects affect the electrical properties in Bi 2/3 Cu 3 Ti 4-x Ta x O 12 ceramics. • The improvement in dielectric and non-Ohmic properties via defect engineering can be applied to IBLC model caused. [ABSTRACT FROM AUTHOR]

The vacancy behavior in austenitic stainless steel 316L with high γ -phase stability in a hydrogen environment was investigated to clarify the critical defects of hydrogen embrittlement. Hydrogen was introduced in the samples by the high-pressure gas method and tensile straining was conducted at variable low temperatures. Characterization of the strain distribution and vacancy defects was performed by SEM-KAM and low-temperature positron annihilation lifetime spectroscopy measurements, respectively. Upon straining at low temperatures (<-70 °C), high-strain regions were induced locally where a high concentration of vacancies accumulated. These vacancies agglomerated into large clusters that are thought to develop into the embryos of larger voids leading to cracks. These results show that the local formation of hydrogen-induced vacancies in high-strain regions is the determining factor of hydrogen embrittlement in austenitic stainless steel 316L. • Vacancy behaviour strongly depends on straining temperature. • At around −70 °C, a non-uniformity in the strain distribution is induced. • Vacancies accumulate locally in high density in the high-strain regions. • Localization of vacancies in high-strain regions causes hydrogen embrittlement. [ABSTRACT FROM AUTHOR]

Aluminide coatings deposited by CVD method on CMSX-4 superalloy, modified by palladium and zirconium or hafnium as well as non-modified and palladium modified ones on pure nickel have been studied using positron annihilation spectroscopy. The positron lifetime values and the positron effective diffusion length in the coatings studied point out trapping of positrons in defects associated with dislocations of high density. Palladium used for modification of the aluminide coatings substitutes nickel in the β-NiAl phase. This gives rise to the increase in the positron lifetime indicating a higher number of open volume defects in the crystal lattice. Additional modification of the aluminide coating on CMSX-4 nickel superalloy with zirconium or hafnium causes only slight changes in the positron lifetime which may be related to the differences in location of precipitates in the coatings. [ABSTRACT FROM AUTHOR]

The microstructure, martensitic transformations and crystal structure defects in the Ti50Ni47.3Fe2.7 (at%) alloy after equal-channel angular pressing (ECAP, angle 90°, route BC, 1–3 passes at T = 723 K) have been investigated. A homogeneous submicrocrystalline (SMC) structure (grains/subgrains about 300 nm) is observed after 3 ECAP passes. Crystal structure defects in the Ti49.4Ni50.6 (at%) alloy (8 ECAP passes, angle 120°, BC route, T = 723 K, grains/subgrains about 300 nm) and Ti50Ni47.3Fe2.7 (at%) alloy with SMC B2 structures after ECAP were studied by positron lifetime spectroscopy at the room temperature. The single component with the positron lifetime τ1 = 132 ps and τ1 = 140 ps were observed for positron lifetime spectra (PLS) obtained from ternary and binary, correspondingly, annealed alloys with coarse-grained structures. This τ1 values correspond to the lifetime of delocalized positrons in defect-free B2 phase. The two component PLS were found for all samples exposed by ECAP. The component with τ2 = 160 ps (annihilation of positrons trapped by dislocations) is observed for all samples after 1–8 ECAP passes. The component with τ3 = 305 ps (annihilation of positrons trapped by vacancy nanoclusters) was detected only after the first ECAP pass. The component with τ3 = 200 ps (annihilation of positrons trapped by vacancies in the Ti sublattice of B2 structure) is observed for all samples after 3–8 ECAP passes.

In this paper, we present the study of local molecular motions in poly (methyl methacrylate) (PMMA) below glass transition temperature by measuring the ortho-positronium (o-Ps) intensity. Two series of experiments were performed: (1) the PMMA sample was irradiated by 22 Na positron source with elongated time at room temperature, 225 K and 16 K, respectively, and positron lifetime spectra were measured as a function of irradiation time and (2) Positron lifetime and Doppler broadening spectra were measured as a function of temperature from 16 to 350 K after positron irradiation at 16 K for more than 350 h. While the o-Ps lifetime always shows no change with elapsed time, decrease and increase of o-Ps intensity I 3 are observed at 225 K and 16 K, which are interpreted as the result of positron irradiation-induced free radicals and trapped electrons, respectively. With temperature increasing from 16 K, there is a continuous drop of I 3 beginning at around 100 K. This is due to some local group movements such as the ester and main chain methyl group rotations, which lead to the detrapping of accumulated electrons. These local motions do not need additional free volume, so we observed no change of the o-Ps lifetime. Some other structural relaxations such as β-relaxation are also observed and discussed. [ABSTRACT FROM AUTHOR]

The identification of defects in neutron damaged materials is essential for elucidating the correlation between the microstructure and the properties of a material. Positron annihilation spectroscopy (PAS) is very sensitive in open volume defects and thus a useful tool for the investigation of the radiation damage in matter. Tungsten is a critical material for the first wall and divertor of fusion reactors. In the current work, the evolution of the open volume defects in tungsten (W) materials neutron irradiated to 0.12 displacements per atom (dpa) and in the temperature range from 600 to 1200 °C in the Belgian Material Test Reactor BR2 is investigated by positron annihilation lifetime and coincidence Doppler broadening spectroscopy. Three tungsten grades were studied: W(100) single crystal, ITER grade forger bar and heavily deformed "cold"-rolled sheet. PAS results show that the neutron irradiation results in the formation of dislocations and voids of size larger 1 nm at all irradiation temperatures and in all W grades. The dislocation and void density decreases with increasing irradiation temperature. Moreover, the void size increases with the increase of the irradiation temperature. • Positron annihilation spectroscopy of neutron irradiated single crystal and polycrystalline tungsten. • Irradiation temperatures in the range from 600 to 1200 °C. • Formation of dislocations and voids at all irradiation temperatures. • The produced voids have sizes larger than 1 nm. • The dislocation and void densities decrease as the irradiation temperature increases. [ABSTRACT FROM AUTHOR]

Abstract: We present electronic structure calculations of positron lifetimes in various neutral and negative monovacancies in 3C and 6H silicon carbide. Self-consistent positron lifetime calculation schemes were used and full defect relaxation due to the creation of the vacancy and the presence of the positron was considered. Formation energies of the various possible charges of the defects were also calculated to predict their detectability in PAS. Lifetimes between 170ps and 195ps for V C and between 222ps and 227ps for V Si were obtained. Based on these results we propose new interpretations of the experimental PAS signals observed in n-type 3C and 6H-SiC samples. [Copyright &y& Elsevier]

ABSTRACT Positron annihilation lifetime spectroscopy (PALS) is a common technique used to characterize the porosity of polymers. Here, we expand its use to the study of ordered nanoporous polymer monoliths. Polystyrene (PS) monoliths with aligned cylindrical pores ranging in diameters from 15 to 35 nm were examined. Such large pores push the boundaries of the PALS technique. To achieve robust measurement, our system used larger detectors than those typically used for monolithic polymer samples. This was done to improve data rates while sacrificing timing resolution. Pore sizes determined using PALS were consistent with measurements made using small angle x-ray scattering. In addition, PALS was able to detect the collapse of the pores when the monolithic sample was heated above the Tg of PS. Because PALS measurements are not sensitive to the nature of the order within the structure nor are they, sensitive to the open or closed nature of the pores this technique could be expanded to a variety of other sample types. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1157-1161 [ABSTRACT FROM AUTHOR]

Abstract: The application of a magnetic field to the melt in the transverse direction of the wheel rotation during the solidification process induces magnetic anisotropy in amorphous magnetic ribbons. This procedure is here applied to a FeB amorphous alloy in order to check their influence on local electronic and nanoscale structure by comparing two samples (as quenched without field and field quenched). The existence of magnetic domains was showed via the Bitter technique. Mössbauer spectra analysis confirms an increase in the spin density (from 28 to 48%) in the applied field direction for the field quenched ribbon. Positron lifetime spectra analysis determines that at room temperature there is no influence on the nanoscale structure, but the annealing at 300°C provokes the beginning of the nanocrystallization process in the field quenched sample while the samples as quenched without field remain fully amorphous. [Copyright &y& Elsevier]

Abstract: A slow-positron beam system for in-situ positron lifetime measurements during ion beam irradiation is currently under development in order to obtain fundamental information on radiation-induced defects. The system uses a high-intensity positron beam generated through pair-creation by a 70 MeV electron beam from a linear accelerator. The system has typical pulsing electrodes for chopping and bunching positrons. The incident directions of positrons and ions are 0° and 45° to the surface normal of samples, respectively. H, He and inert-gas ions up to 150 keV can be introduced to a target chamber with the irradiation temperature variable from room temperature to 600°C. In a preliminary examination of the beamlines, 150 keV Ar+, electrons from a photocathode and positrons from a radioisotope were successfully transported to the target chamber from their respective sources. In addition, in-situ lifetime measurements using the existing positron beamline were demonstrated. [Copyright &y& Elsevier]

Abstract: Positron lifetime spectroscopy (PALS) is employed to study retained vacancies in Fe75Al25 and Fe70Al25X5 X= (Cr, Ni) after various thermal treatments. The PALS spectra were analyzed with the two states trapping model. The positron lifetime in defects suggests that they are Fe-monovacancies. The vacancy concentration, determined by the rates of trapping positrons to vacancies, strongly depends on the cooling rate of the sample and the ternary addition. An addition of Cr lowers, whereas an addition of Ni increases the concentration of the retained vacancies in relation to Fe75Al25 samples after the same heat treatment. [Copyright &y& Elsevier]

Erosion characteristics of high chromium (Cr, 16-19%) alloy cast iron with 5% and 10% manganese (Mn) prepared in metal and sand moulds through induction melting are investigated using jet erosion test setup in both as-cast and heat-treated conditions. The samples were characterised for hardness and microstructural properties. A new and novel non-destructive evaluation technique namely positron lifetime spectroscopy has also been used for the first time to characterise the microstructure of the material in terms of defects and their concentration. We found that the hardness decreases irrespective of the sample condition when the mould type is changed from metal to sand, On the other hand, the erosion volume loss shows an increasing trend. Since the macroscopic properties have a bearing on the microstructure, good credence is obtained from the microstructural features as seen from light and scanning electron micrographs. Faster cooling in the metal mould yielded fine carbide precipitation on the surface. The defect size and their concentration derived from positron method are higher for sand mould compared to metal mould. Lower erosion loss corresponds to smaller size defects in metal mould are the results of quicker heat transfer in the metal mould compared to the sand mould. Heat treatment effects are clearly seen as the reduced concentration of defects and spherodisation of carbides points to this. The erosion loss with respect to the defects size and concentration correlate very well. [ABSTRACT FROM AUTHOR]

Abstract: The morphology and the micro-structural changes of levonorgestrel-releasing intrauterine systems (IUSs) were studied in relation to the duration of their application. The morphology of the removed IUSs was examined without pre-treatment by scanning electron microscopy. The micro-structural changes of the different layers of IUSs were tracked by positron annihilation lifetime spectroscopy. Besides the previously found incrustation formation, the free volume of the hormone containing reservoir was remarkably increased after 3 years of application, thus increasing the real volume of the core of the systems. Although the free volume of the membrane encasing the core was not significantly changed in the course of the application, as a result of the core expansion, microcracks could be formed on the membrane surface. Along these cracks, deposits of different compositions can be formed, causing inflammatory complications and influencing the drug release of IUSs. Stability tests in combination with micro-structural screening of such IUSs could be required during their development phase to avoid the undesired side effects. [ABSTRACT FROM AUTHOR]

Abstract: A positron lifetime spectroscopy was applied to a series of samples of Nd–Fe–B based alloys prepared by hydrogenation-disproportionation-desorption-recombination (HDDR) process with various desorption-recombination (DR) times and the results were compared to the results of cross-sectional observation with scanning electron microscopy (SEM). From these studies, it is strongly suggested that microstructural changes, which causes the sudden onset of coercivity during DR stage progress abruptly in a relatively short period of time. Positron lifetime spectroscopy can detect the microstructural changes during DR treatment and furthermore, it is suggested that positron annihilation site is directly related to the region which determines the coercivity of the HDDR-processed Nd–Fe–B alloys. [Copyright &y& Elsevier]

Abstract: A slow monoenergetic pulsed positron beam at Chalmers University of Technology has been built. The system consists mainly of chopper, buncher and accelerator. The achieved positron energy range is in range between 230eV and 15keV. The FWHM of the beam resolution function is around 700ps. The beam intensity is around cps. [Copyright &y& Elsevier]

Abstract: Contamination-tolerant adhesives are of ever increasing importance in industrial applications. The possible failure mechanisms of adhesive bonds in these adhesives are however still poorly understood. Results of a series of investigations with our pulsed low energy positron beam system (PLEPS) in an epoxy-based contamination-tolerant adhesive are presented. [Copyright &y& Elsevier]

Abstract: This work demonstrates that a significant improvement of the timing resolution, simultaneously keeping coincidence count rate high enough, can be achieved by use of the state-of-the-art ultra fast 8-bit digitizers which replace the traditional analog timing modules. Performance of the digital spectrometer in conditions of a routine measurement is compared with that of the same detectors connected to the analog setup. It was found that employment of the digital setup leads to an improvement of timing resolution from 169ps for the traditional analog setup down to 146ps for the digital setup. [Copyright &y& Elsevier]

Abstract: The point defects of B2-type intermetallic compound Ni–Al have been investigated as a function of different Ni compositions ranging from 47 to 55at.% by means of positron lifetime and coincidence Doppler-broadening (CDB) measurements. The position lifetime of the annealed samples, expressed by single-component lifetime of τ =170–180ps, indicates positron saturation trapping at single vacancies in Ni–Al samples. The CDB measurements have been applied to identify these vacancy-type defects formed on the Ni sublattice in the B2-phase Ni–Al alloys. [Copyright &y& Elsevier]

Abstract: The positron lifetime spectroscopy (PLS), a non-destructive characterization method, utilizes positronium (Ps; an electron–positron bound state) as a probe and measures its lifetime in polymer free volumes. For the first time the free volumes have been estimated by PLS in polyaniline (PANI) complexes with various inorganic and organic acids. It was found that the o-Ps lifetime increases and the intensity decreases with increasing ionic radius of the counter-ions in PANI complexes. Obviously, larger counter anions result in enhanced mean size of the voids corresponding to the free volume in the bulk polymer. Electrical conductivity has been measured by conventional four-probe technique. The glass transition temperature and temperature of removal of the absorbed water have been determined by using differential scanning calorimetry. It was established fairly well correlation of the mentioned polymer parameters with the o-Ps lifetime and the free volume of PANI complexes, respectively. The greater free-volume results in a decrease of conductivity, glass transition temperature and temperature of removal of the absorbed water. [Copyright &y& Elsevier]

Abstract: 6H silicon carbide positron lifetime spectra consisting of different concentrations of carbon vacancy, silicon vacancy and divacancy were generated by Monte Carlo simulation. Although the lifetime spectra contained more than one defect, they were well fitted by the two component model with good chi squares. Artificial defect components were found to form by the merging of the different vacancy defects. For some of these cases, the invalidity of the simple trapping model could not be revealed by the commonly used τ b,mod test. This would lead to the mis-identification of virtual vacancy defects having characteristic lifetimes depending on the concentrations of the carbon vacancy, the silicon vacancy and the divacancy. [Copyright &y& Elsevier]

Abstract: In thin film electronic applications, the limiting factor, in terms of cost and usability, is generally the substrate material. As a consequence, different materials are being investigated as potential lightweight, inexpensive and flexible substrates. In this respect, we have been the first research collaboration to produce silicon-based electronics on paper substrates. Here we present structural characterisation of hydrogenated amorphous silicon (a-Si:H) layers deposited on 80gm−2 wood-free paper, with and without an intermediate metallic interlayer, using low temperature hot wire chemical vapour deposition (HW-CVD). Both pulsed positron beam profiling and X-ray diffraction studies indicate that the growth rate on the uncoated substrate is slightly higher than with prior metallization. There is no evidence of a crystalline phase or voids in the a-Si:H layers. The internal defect structure is similar, with a dominant dangling bond complex of similar size, which has a slightly longer lifetime than in layers grown at higher temperatures on conventional substrates. [Copyright &y& Elsevier]