Positron Annihilation Parameters Research Articles

Defect Evolution in Y<sub>0.5</sub>Gd<sub>0.5</sub>Ba<sub>2</sub>Cu<sub>3</sub>O<sub>7-</sub><sub>δ</sub> Layer by H Ion Irradiation

In order to further improve the superconducting current carrying capacity of REBCO coated conductor under strong magnetic field, ion irradiation is used to generate the pinning center of introduced magnetic flux in the REBCO coated conductor. In this paper, the H-ion irradiation of REBCO second generation high temperature superconductor strip was carried out by using the 320kV high charge state ion synthesis research platform. DB-SPBA combined with Raman spectroscopy was used to measure the change of microstructure in YBCO samples irradiated by H+ions within the range of 5.0×10<sup>14</sup>~1.0×10<sup>16</sup>. The positron annihilation parameters in YBCO before and after irradiation were analyzed. It is found that after 100 keV H+ion irradiation, a large number of defects including vacancy, vacancy group or dislocation group are produced in the superconducting layer. The larger the irradiation dose, the more vacancy type defects are produced, the more complex the defect types are, and the annihilation mechanism of positrons in the defects changes. Raman spectroscopy results show that with the increase of H+ion irradiation dose, the oxygen atoms in the coating rearrange, the plane spacing increases, the orthogonal phase structure of the coating is destroyed, and the degree of order decreases. The defects produced by such ion irradiation lay a foundation for the introduction of flux pinning centers. Further research can be carried out in combination with X-ray diffractometer, transmission electron microscope, superconductivity and other testing methods to provide theoretical and practical reference for the optimization of material properties.

Defect evolution in Y<sub>0.5</sub>Gd<sub>0.5</sub>Ba<sub>2</sub>Cu<sub>3</sub>O<sub>7–<i>δ</i></sub> superconducting layer irradiated by H<sup>+</sup> ions

<sec>In order to further improve the superconducting current carrying capacity of RE-Ba-Cu-O coated conductor under the action of strong magnetic field, ion irradiation is used to generate the pinning centers of introduced magnetic flux in the RE-Ba-Cu-O coated conductor. In this work, the H<sup>+</sup>-ion irradiation of second-generation high-temperature superconductor RE-Ba-Cu-O strip is carried out by using the 320 kV high charge state ion synthesis research platform. Doppler broadened slow positron beam analysis combined with Raman spectroscopy is used to measure the change of microstructure in Y<sub>0.5</sub>Gd<sub>0.5</sub>Ba<sub>2</sub>Cu<sub>3</sub>O<sub>7–<i>δ</i> </sub>(YBCO) sample irradiated by H<sup>+</sup> ions in a range of 5.0 × 10<sup>14</sup>–1.0 × 10<sup>16</sup> ions/cm<sup>2</sup>. The positron annihilation parameters in YBCO before and after irradiation are analyzed. It is found that after 100 keV H<sup>+</sup> ion irradiation, a large number of defects including vacancies, vacancy groups or dislocation groups are produced in the superconducting layer.</sec><sec>The larger the irradiation dose, the more the produced vacancy type defects are and the more complex the defect types, and the annihilation mechanism of positrons in the defects changes. Raman spectroscopy results show that with the increase of H<sup>+</sup> ion irradiation dose, the oxygen atoms in the coating rearrange, the plane spacing increases, the orthogonal phase structure of the coating is destroyed, and the degree of order decreases. The defects produced by such an ion irradiation lay a foundation for the introduction of flux pinning centers. Further research can be carried out in combination with X-ray diffractometer, transmission electron microscope, superconductivity and other testing methods to provide theoretical and practical reference for the optimization of material properties.</sec>

Effects of Interdomain and Intradomain Pores of Cu-Zn Ferrite Substituted with Nd Investigated Using Positron Annihilation Lifetime Technique

Positron annihilation lifetime spectroscopy at room temperature is used to investigate polycrystalline Cu-Zn ferrite samples substituted with Nd. Standard ceramic technique is used to prepare all investigated samples. The variation of positron annihilation parameters I1%, I2%, τ1, and τ2 are demonstrated with grain size, initial permeability, homogeneity of the samples, and electrical resistivity with Nd concentration. The important novel points are: (1) Intragranular pores are equal to the sum of intra- and interdomain pores. The lifetime τ1 is increased due to the decrease of the intradomain pores. The initial permeability and homogeneity are decreased with increasing the interdomain pores. The electrical resistivity is increased due to the increase of the interdomain pores.

Split Ga vacancies and the unusually strong anisotropy of positron annihilation spectra in β−Ga2O3

We report a systematic first principles study on positron annihilation parameters in the $\beta$-Ga$_2$O$_3$ lattice and Ga mono-vacancy defects complemented with orientation-dependent experiments of the Doppler broadening of the positron-electron annihilation. We find that both the $\beta$-Ga$_2$O$_3$ lattice and the considered defects exhibit unusually strong anisotropy in their Doppler broadening signals. This anisotropy is associated with low symmetry of the $\beta$-Ga$_2$O$_3$ crystal structure that leads to unusual kind of one-dimensional confinement of positrons even in the delocalized state in the lattice. In particular, the split Ga vacancies recently observed by scanning transmission electron microscopy produce unusually anisotropic positron annihilation signals. We show that in experiments, the positron annihilation signals in $\beta$-Ga$_2$O$_3$ samples seem to be often dominated by split Ga vacancies.

Electron irradiation induced molecular changes in PMMA: A positron spectroscopy study

To examine the positron annihilation parameters with regard to microstructural changes, the positron annihilation lifetime spectroscopy and measurements of the Doppler broadening of annihilation radiation (DBAR) of electron‐irradiated poly(methyl methacrylate) were carried out at room temperature. Trapping rate equations were applied in terms of the positron bulk lifetime as well as the positron lifetime fitting results to investigate the molecular changes due to electron irradiation effects. It was found that the mean positron lifetime shows the same trend as the o‐Ps intensity. In disordered regions, the positronium trapping rate suggests that trapping of o‐Ps increases with increasing electron irradiation fluence until electron fluence reached 5.9 × 1013 electrons/cm2, after which it begins to decrease. The line shape S‐parameter was found to have similar behavior as the o‐Ps intensity, and suggests that poly(methyl methacrylate) undergoes chain scission, followed by cross‐linking, and eventually degradation, due to the electron radiation effects. The results of positron annihilation spectroscopy were verified by X‐ray results, mechanical, and electrical measurements.

Near-surface investigation of positron diffusion length in helium-implanted Fe9Cr and its ODS variant

In the present paper, we employ the depth sensitive slow positron Doppler broadening technique to characterise the positron diffusion in various near-surface layers of He+ implanted f/m steels. One of the frequently used positron annihilation parameters, with a high sensitivity to radiation-induced defects in a wide range of displacement damage, is the positron diffusion length. This parameter expresses the probability of the back diffusion of positrons to the surface and reflects the microstructural damage. Via the application of VEPFIT program on Doppler broadening spectroscopy data obtained on helium-implanted Fe9Cr and ODS Fe9Cr steels, we determined the changes in diffusion length and thickness of corresponding layers due to implantation.

TiSr antisite: An abundant point defect in SrTiO3

We present a systematic study of the positron lifetime as a function of measurement temperature in strontium titanate (SrTiO3) single crystals grown in different conditions and by different synthesis methods. We combine our experimental results with state-of-the-art theoretical calculations of positron annihilation parameters. We find that the essentially omnipresent 180–190 ps lifetime component is most likely the TiSr antisite defect, possibly coupled with one or more oxygen vacancies, supporting the importance of the TiSr antisite related defects in SrTiO3.

Positron annihilation and electrical studies on the influence of loading magnesia nanoribbons on PVA-PVP blend

This work aims to study the effect of loading magnesia (0.5–2.0 wt%) on the positron annihilation parameters and electrical properties of the PVA-PVP blend. The films were synthesized by solution casting and checked by different techniques. XRD and HR-TEM of sol-gel prepared magnesia revealed that the average crystallite size was 14.29 nm with ribbon-like morphology with varying widths and lengths up to a few micrometers. SEM showed that the blend surface appeared smooth and homogenous and this confirmed the compatibility between PVA and PVP. However, loading magnesia increased surface roughness. TGA confirmed the thermal stability enhancement of blend film with magnesia incorporation. Ortho–positronium lifetime τ3 and the free volume Vf decrease with loading magnesia while the intensity I3 is nearly constant. These features were interpreted in view of the hole-filling mechanism, the interaction between magnesia & blend and magnesia morphology. The equilibrium swelling ratio ESR was found for the studied films and a positive correlation between ESR and Vf was reported. The current density-electric field, J-E, characteristics were of non-ohmic type. J increased with increasing magnesia levels and heating. The Richardson-Schottky effect was the dominant dc conduction mechanism at low temperature and low magnesia loading levels while it changed to Poole–Frenkel emission at higher values. Finally, a correlation between Log J and the intensity I2 was reported.

Calculation of Positron States and Annihilation Parameters in Gamma and Amorphous Al2O3

Calculation of Positron States and Annihilation Parameters in Gamma and Amorphous Al₂O₃

Simulation for the correlation of positron annihilation rate with charge density near defects in iron

Positron annihilation technology is a nondestructive detection technique to study the microscopic defects in materials. It is well known that the micro defects will redistribute the charge in materials. In this paper, the charge density of helium in iron was calculated by the Vienna Ab initio Simulation Package (VASP) and the positron annihilation parameters were obtained by Doppler program. The results demonstrated that different types of defects in materials will cause a great variety in charge density, which lead to the change of positron annihilation parameters. In a vacancy cluster containing 6 monovacancies, the positron annihilation lifetime decreases rapidly once the number of helium is greater than the number of vacancies.

Computational study of positron annihilation parameters for cation mono-vacancies and vacancy complexes in nitride semiconductor alloys

We calculate positron annihilation parameters, namely the S and W parameters from the Doppler broadening spectroscopy and the positron lifetime , for defect-free states as well as cation mono-vacancies and vacancy complexes in nitride semiconductor alloys Al0.5Ga0.5N, In0.5Ga0.5N and Al0.5In0.5N. The obtained distributions of these parameters differ from compound to compound. Especially, the S–W relation for In0.5Ga0.5N is very different from that for Al0.5Ga0.5N. For the cation mono-vacancies, introducing local structural parameters, their correlations with S, W and are investigated. The S and variations are well described with the size distributions of the vacancies while the W variation is related to the presence of localized d electrons. For the vacancy complexes as well as the cation mono-vacancies, multiple-linear-regression models to describe S, W and are successfully constructed using the local structural parameters as descriptors. The S–W and S– relations are also compared with those for AlN, GaN and InN.

Effect of Graphite and Copper Nano-Particles on Free Volume Properties of PVC/NBR Blends Studied by Pal Spectroscopy

Positron Annihilation Lifetime (PAL) spectroscopy has been used to investigate the effect of different concentrations of graphite and copper nano-particles in polyvinylchloride (PVC)/ butadiene-acrylonitrile copolymer (NBR) blends on free volume properties of PVC/NBR nano composites. However, the miscibility of PVC/NBR blends was investigated. The free-volume properties showed a negative deviation from linear additive relationship indicating miscibility of two blends. In additions, this research to confirm the results of the previous study on the same samples through the correlation between positron annihilation parameters and some of the properties studied.

Estimation of the stored energy of dislocation for plastically deformed 6082 heat treatable Al-Alloy

This work describes cold work effect on the positron annihilation parameters, which are used in determination of the stored dislocation energy of the investigated 6082 Al-alloy samples. The investigated samples were homogenized for 6 h at 723 K then annealed at room temperature and finally plastically deformed up to 23 % degree of deformation. The annihilation parameters of the alloys under investigation were determined using the trapping model after fitting with the experimental measured data of the positron annihilation lifetime. 12% thickness reduction was found to be the start of saturation trapping region of positron in defect states at which an annihilation lifetime value of about 209±4 ps is obtained.A trapping efficiency of 6×10-7 cm3s-1 gives the best fit of the experimental measurements with the theoretical mean lifetime values obtained using the trapping model. The stored dislocation energy can be determined from the data of the positron annihilation lifetime due its ability to determine the density dislocation during plastic deformation. An increase in the strain (degree of deformation) creates comparable increase in both defect and dislocation densities, hence an increase in the measured stored dislocation energy. Maximum stored dislocation energy of about 29.5 KJ/m3 was obtained at the region of saturation of dislocation.

Nonlinear Loading Effects in Oxide-filled Polyester Nanocomposites Observed by IR-spectroscopy and Lifetime Positron Spectroscopy

Impact of nanosized oxide particles of titania (rutile) (TiO2) and silica-titania fumed compound on physical properties related to inner structure of a styrene-cross-linked polyester resin have been experimentally studied. The IR-spectroscopy and the positron annihilation lifetime spectroscopy (PAL) were used to study the electronic polarizability of interatomic bonds and the positron annihilation processes with increasing the filler loading. All the nanocomposites show unmonotonous variations in the IR-absorption lines and the positron annihilation parameters. Three lifetime components, namely τ1, τ2, and τ3 with intensities I1, I2, and I3 have been determined. The lifetime loading dependences exhibit non-monotonous lowering within the low loading range (Cm ≤ 5%) with the minima values at Cm ~ 1.5 % for the unfilled resin and the nanocomposites. The free-volume portion fv (in %) in a polymer matrix decreases droningly for the nanocomposites with STO-particles upon loading while for the TiO2-nanocomposites it increases non-monotonously. The parameters describing the supramolecular structure of the nanocomposites have been calculated using the fractal approach. Some correlations between loading-dependent PAL-properties and underlying alterations in the resin's supramolecular structure have been discussed.

Positron Probing of Liquid-free Volume To Investigate Adsorption–Desorption Behavior of Water in Two-Dimensional Mesoporous SBA-3

Transport of fluids through channels and cavities of nano/mesoporous materials is of paramount importance in various fields of science and industry. The transport properties can be well derived from the adsorption–desorption behavior of fluids. Positron annihilation lifetime spectroscopy (PALS) allows probing adsorption–desorption of water from 2D mesopores of SBA-3. In situ study of the size of evolving water-free volumes during successive stages of adsorption and desorption is a sensitive way to elucidate the course of pore filling and emptying. The changes of positron annihilation parameters indicate that adsorption of water is mediated through the formation of isles on the surface of the pore walls, and these, in turn, develop into water plugs. Subsequently, these plugs grow and consecutively join together when the distance between them decreases to ca. 1 nm until the complete capillary condensation occurs. Akin to adsorption, desorption of water from the pores involves the formation of cavities cappe...

Tunable monoenergy positron annihilation spectroscopy of polyethylene glycol thin films**Project supported by the National Natural Science Foundation of China (Grant Nos. 11575205, 11475197, 11675188, and 11475193).

Doppler broadening and coincidence Doppler broadening of annihilation radiation experiments have been performed in three kinds of polyethylene glycol (PEG) membrane formed with different average molecular weight using the tunable monoenergy slow positron probe as a function of implantion energy. The obtained positron annihilation parameters are interpreted from two aspects: surface effect and differences in micro-structure or chemical environment of positron annihilation. The experimental results show that the regulation of densification of PEG molecular packing and distribution uniformity from the near surface layer to the bulk region in the film forming process can be well realized by changing its molecular weight. Combining a variable monoenergetic slow positron beam and these two positron annihilation spectroscopy methods is a powerful tool to study positron annihilation characteristics and for polymeric thin-film fine structure analysis.

Gas-phase hydrogenation influence on defect behavior in titanium-based hydrogen-storage material

Titanium and its alloys are promising materials for hydrogen storage. However, hydrogen penetration accompanies the exploitation of hydrogen storage alloys. In particular, hydrogen penetration and accumulation in titanium alloys changes their mechanical properties. Therefore, the research works of such materials are mainly focused on improving the reversibility of hydrogen absorption-liberation processes, increasing the thermodynamic characteristics of the alloys, and augmenting their hydrogen storage capacity. In the process of hydrogenation-dehydrogenation, the formed defects both significantly reduce hydrogen storage capacity and can also be used to create effective traps for hydrogen. Therefore, the investigation of hydrogen interaction with structural defects in titanium and its alloys is very important. The present work, the hydrogen-induced formation of defects in the alloys of commercially pure titanium under temperature gas-phase hydrogenation (873K) has studied by positron lifetime spectroscopy and Doppler broadening spectroscopy. Based on the evolution of positron annihilation parameters τf, τd, their corresponding intensities If, Id and relative changes of parameters S/S0 and W/W0, the peculiarities of hydrogen interaction with titanium lattice defects were investigated in a wide range of hydrogen concentrations from 0.8at% to 32.0at%.

Characterizations of the thermal decomposition of nano-magnesium hydroxide by positron annihilation lifetime spectroscopy

The thermal decomposition behavior of magnesium hydroxide (Mg(OH)2) nanopowders was investigated by positron annihilation lifetime spectroscopy (PALS), X-ray diffraction (XRD), thermogravimetric and differential scanning calorimetry (TG-DSC) analysis, high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). It was indicated that the microstructural changes started from 300°C were earlier than the phase transformation from hexagonal Mg(OH)2 nanopowders to face-centered cubic MgO occurred at about 380°C during decomposition process. The variation of positron annihilation parameters revealed the production of new vacancy defects and the aggregation of vacancy clusters in grain boundary areas and microvoids between particles due to the removal of H2O and rearrangement of interface atoms. A positron trapping model was proposed to help further understand the changes of microstructure and interfacial defects during Mg(OH)2 thermal decomposition.

Prediction of positron-annihilation parameters for vacancy-type defects in ternary alloy semiconductors by data-scientific approach

We calculated positron annihilation parameters for mono- and di-vacancies in ternary semiconductors Al0.5Ga0.5N and In0.5Ga0.5N. It has been found that the obtained annihilation parameters are well correlated with structural parameters. By constructing multiple linear regression models using selected (about 1/4 of the total) datasets as training sets in order to reduce computational cost, we could predict annihilation parameters for the rest.

The investigation of hydrogenation influence on structure changes of zirconium with nickel layer

The results of experimental investigation of hydrogenation influence on structure changes of zirconium alloy (Zr-1%Nb) with thin nickel layer have presented in this work. Nickel layer was formed by magnetron sputter deposition. Hydrogenation was carried out at gas atmosphere at constant temperature. Different hydrogen concentrations were obtained by varying time of hydrogenation. Defect and phase structure was studied by means of X-ray diffraction, glow discharge optical emission spectroscopy, positron lifetime and Doppler broadening spectroscopies. New experimental data about the evolution of the positron annihilation parameters depending on hydrogen concentration in Zr-1Nb alloy with nickel layer was obtained.