Positron Annihilation Spectroscopy Results Research Articles

On the diffusive model of C<sub>60</sub> fullerene clusterization in liquids

Purpose of research. The purpose of the study is to explain the mechanisms of clustering of fullerene molecules in liquid media observed using various structural nuclear -physical methods, as well as the interpretation of experimental data within the framework of a microscopic diffusion model. Methods. The article gives a brief overview of the results of positron annihilation spectroscopy, small-angle neutron scattering, translucent electron microscopy, with which the geometric parameters of fullerene clusters in solutions were established. The theoretical method of research is the microscopic diffusion model “Limited diffusion aggregation”, which describes the kinetic processes of clusterization. Results. The diffusion limited aggregation model displays adequately the mechanism of formation of C60 molecules in the form of fractal units in the volume of fluid, observed in experiments on positron annihilation spectroscopy and small-angle neutron scattering. The structural indicators of aggregate fullerene particles in carbon disulfide, o-Xylene, toluene, water and other solvents are analyzed. The properties of various diffusion models (the reaction limited aggregation model and the diffusion limited aggregation model) are considered and their combination in relation to the assessmentsof the kinetics of fullerene aggregation in solutions. A quantitative comparison of the results of the discussed models was carried out using the example of carbon disulfide. Conclusion. The diffusion microscopic models adequately describe the phenomenon of fullerene aggregation in polar and non-polar solvents, which are recorded by various nuclear methods (positron annihilating spectroscopy and small-angle neutron scattering), the most reliable is the diffusion limited aggregation model, more than that, it is the basis of a numerical definition of structural structures parameters of units. Compared to neutron scattering, with the annihilation of positrons, in the aggregation of fullerene the [Ps–C60] molecular complexes participate in the clusterization, but this does not affect the change in the size of the cluster and the reliability of the results.

Positron annihilation spectroscopy of thermally annealed hydrogenated amorphous carbon films

Hydrogenated amorphous carbon (a-C:H) films are prepared using a bipolar-type plasma-based ion implantation and deposition system, followed by thermal annealing of the films at 200 °C-800 °C. The structural and compositional changes are examined by Raman spectroscopy, Rutherford backscattering spectrometry, and elastic recoil detection analysis (ERDA). Positron annihilation spectroscopy (PAS) is also conducted to investigate the behavior of voids in the films. The results of Raman and ERDA measurement clearly show that the sp2 ring clusters increase, and hydrogen content apparently decreases at temperatures between 400 °C and 800 °C. Uncorrelated behaviors between positron lifetimes and line-shape parameters are observed in the PAS results, which can be attributed to several competing effects, such as hydrogen release, bond reconstruction, and electronic-state change of voids. Furthermore, it is suggested that the graphitization of the films begins from the surface to the inside, accompanied by the hydrogen release that is related to the void behavior.

Role of chemical disorder on radiation-induced defect production and damage evolution in NiFeCoCr

Understanding chemical disorder in many concentrated solid solution alloys (CSAs) at the levels of electrons and atoms has attracted increasing attention as a path forward to reveal and identify underlying mechanisms for extraordinary mechanical properties and improved radiation tolerance. Single-phase NiFeCoCr CSA is a common base for many high-entropy alloys (HEAs) that have shown improved mechanical strength and radiation tolerance. In this study, defect production and damage evolution in NiFeCoCr under ion irradiation at room temperature to dose over 20 dpa are determined using ion channeling technique along both <100> and <110> directions utilizing multiple probing beam energies. The results obtained from the multi-axial and multi-energy channeling analysis are compared with those previously obtained for Ni crystals irradiated under similar conditions. The influence of chemical complexity on defect production and clustering at early-stage under room temperature irradiation up to dose of 1 dpa is discussed based on positron annihilation spectroscopy results. Defect structure evaluation in Ni and NiFeCoCr is also discussed based on transmission electron microscopy results over a prolonged irradiation at both room and elevated temperatures. Compared with chemically complex NiFeCoCr, larger dislocation loops thus less lattice strain are expected to form in pure Ni. Moreover, the role of chemical disorder in this CSA is also investigated based on ab initio calculations using large supercells. To understand the impact of chemical complexity effect on defect structure evolution, this integrated research effort attempts to link the relatively large charge redistribution due to difference in valence electron counts resulting from alloying different 3d transition metal elements, moderate lattice distortion arising from similar adaptable atomic size, and notable suppressed or delayed damage evolution in NiFeCoCr.

Post irradiated microstructure and mechanical properties of pure V

Proton irradiation has been carried out on pure vanadium samples and the changes in the microstructure and mechanical properties have been evaluated as a function of dose. The microstructural parameters such as domain size and microstrain have been assessed along different crystallographic planes using the X-ray diffraction line profile analysis methods such as simplified breadth and the modified Rietveld technique. No significant change in the dislocation density is observed as a function of dose from the XRD results. However, the results of positron annihilation spectroscopy in terms of positron lifetime and S-parameter clearly indicated the formation of small defect clusters with irradiation. Transmission electron microscopy investigation confirmed the presence of small dislocation loops in the irradiated samples. Molecular dynamics simulations carried out to model the formation of irradiation-induced defects clusters with irradiation dose support the experimental observation of small defect clusters and increasing cluster size with dose. The yield strength and tensile strength showed systematic increase with irradiation dose corroborating with the results of microhardness measurement. The fractography of the tensile samples clearly showed the characteristics of brittle fracture in the irradiated samples.

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.

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.

Effect of Transition Metal Ion Doping on the Microstructure, Defect Evolution, and Magnetic and Magnetocaloric Properties of CuFeO2 Ceramics

The polycrystalline CuFeO2 (CFO) and CuFe1-xMxO2 (M = Ti, Hf, Zr, M-CFO) samples are synthesized by solid-state reaction method. The doping effect of tetravalent nonmagnetic transition metal M4+ ions on the microstructure, defect evolution, and magnetic and magnetocaloric properties of the CFO system are comparatively investigated. The substitution of M4+ for Fe3+ increases lattice parameters, changes the bond length of Fe/Cu-O and bond angle of Fe-O-Fe, and improves the microstructure to some extent. Positron annihilation spectroscopy results indicate that M4+ doping induces the agglomeration of small-sized vacancy defects manifested as the increase in vacancy-type defect size and the suppression of inherent defect concentration. Magnetic measurements show that the antiferromagnetic stability of CFO system is clearly affected by M4+ doping, and Ti4+ doping has a more noticeable impact than Hf4+ and Zr4+ doping. Meanwhile, the coexistence of weak ferromagnetism and antiferromagnetism is detected in all doped samples, while Ti-doped system exhibits extraordinary magnetic properties. Especially for Ti-CFO2 (x = 0.03) sample, the maximum magnetization reaches as high as 11.81 emu/g at 0.5 T, which enhanced one order of magnitude than that of undoped CFO. Isothermal M-H data at different temperatures show that entropy change (ΔSM) and refrigerant capacity (RC) for CFO and M-CFO2 samples are significantly weakening with M4+ substitution in bulk CFO. The maximum ΔSM = 4.79 J·kg−1 K−1 and RC = 12.79 J·kg−1 for undoped CFO are obtained near the transition temperature TN2 = 12 K, with the applied fields up to 6 T. The possible reasons for the above observations are discussed in detail.

Influence of Fermi level position on vacancy-assisted diffusion of aluminum in zinc oxide

The influence of Fermi level position and annealing ambient on the zinc vacancy ${V}_{\mathrm{Zn}}$ generation and Al diffusion is studied in monocrystalline zinc oxide (ZnO). From secondary-ion mass spectrometry and positron annihilation spectroscopy results, a quadratic dependence between the concentrations of ${V}_{\mathrm{Zn}}$ and Al is established, demonstrating the Fermi level dependence of the formation of the electrically compensating $\ensuremath{-}2$ charge state of ${V}_{\mathrm{Zn}}$ in conductive $n$-type ZnO crystals. In contrast, thermal treatment in the zinc-rich ambient is shown to efficiently reduce the ${V}_{\mathrm{Zn}}$ concentration and related complexes. Using a reaction-diffusion model, the diffusion characteristics of Al at different donor background concentrations are fully accounted for by mobile (${\mathrm{Al}}_{\mathrm{Zn}}{V}_{\mathrm{Zn}}{)}^{\ensuremath{-}}$ pairs. These pairs form via the migration and reaction of isolated ${V}_{\mathrm{Zn}}^{2\ensuremath{-}}$ with the essentially immobile ${\mathrm{Al}}_{\mathrm{Zn}}^{+}$. We obtain a migration barrier for the ${({\mathrm{Al}}_{\mathrm{Zn}}{V}_{\mathrm{Zn}})}^{\ensuremath{-}}$ pair of $2.4\ifmmode\pm\else\textpm\fi{}0.2$ eV, in good agreement with theoretical predictions. In addition to strongly alter the shape of the Al diffusion profiles, increasing the donor background concentration also results in an enhanced effective Al diffusivity, attributed to a reduction in the ${\mathrm{V}}_{\mathrm{Zn}}^{2\ensuremath{-}}$ formation energy as the Fermi level position increases.

Annealing effects on deuterium retention behavior in damaged tungsten

Effects of annealing after/under iron (Fe) ion irradiation on deuterium (D) retention behavior in tungsten (W) were studied. The D2 TDS spectra as a function of heating temperature for 0.1dpa damaged W showed that the D retention was clearly decreased as the annealing temperature was increased. In particular, the desorption of D trapped by voids was largely reduced by annealing at 1173K. The TEM observation indicated that the size of dislocation loops was clearly grown, and its density was decreased by the annealing above 573K. After annealing at 1173K, almost all the dislocation loops were recovered. The results of positron annihilation spectroscopy suggested that the density of vacancy-type defects such as voids, was decreased as the annealing temperature was increased, while its size was increased, indicating that the D retention was reduced by the recovery of the voids. Furthermore, it was found that the desorption temperature of D trapped by the voids for damaged W above 0.3dpa was shifted toward higher temperature side. These results lead to a conclusion that the D retention behavior is controlled by defect density. The D retention in the samples annealed during irradiation was less than that annealed after irradiation. This result shows that defects would be quickly annihilated before stabilization by annealing during irradiation.

Feasibility of classification of clay minerals by using PAS

After the nuclear power plant disaster, the evaluation of radioactive Cs kept in soil, especially in clay minerals and the elucidation of its movement are urgent subjects to promote decontamination. It is known that the extractable level of Cs depends on the sort of clay minerals. We tried to find the characteristics of clay minerals belonging to phillosilicate group using positron annihilation spectroscopy (PAS) and the relationship between the results of PAS and the amounts of substantially extracted Cs from the clay minerals. The results showed that each clay mineral was found to be distinguishable from other clay minerals by PAS and the extraction rate of Cs was different among those clay minerals, however the direct correlation between the results of PAS and the extraction rates of Cs was not found.

Effect of incorporation of deuterium on vacancy-type defects of a-C:H films prepared by plasma CVD

Amorphous deuterated carbon (a-C:D) films were prepared using plasma-enhanced chemical vapor deposition (PECVD) from CH4/CD4 and CH4/D2 source gases. For CH4/CD4, the gas flow ratio of CD4/(CD4+CH4) was varied from 0 to 100%. For CH4/D2, the additional partial gas pressure of D2 was increased from 1 to 7Pa as the flow rate increased, and the partial gas pressure and the flow rate of CH4 were maintained at 10Pa and 10sccm, respectively. The concentrations of hydrogen (H) and deuterium (D) relative to carbon (C) in the films and the film densities were determined by elastic recoil detection analysis (ERDA) and Rutherford backscattering spectroscopy (RBS). Positron annihilation spectroscopy (PAS) was performed to measure the vacancy-type defect of the film. The S value obtained from PAS measurement correlates to the vacancy-type defect in the film. The film hardness was also measured using a nanoindenter.For CH4/CD4, the D concentration in the film increased when the CD4/(CH4+CD4) gas ratio increased, whereas the concentration of H decreased. For CH4/D2, the D concentration in the film increased with increasing D2 partial pressure, whereas the concentration of H decreased. From the PAS results, the S value increased with increasing CD4/(CH4+CD4) gas ratio for CH4/CD4, whereas the S value did not change with any D2 partial pressure for CH4/D2. The hardness and the mass density of the films decreased when the CD4/(CH4+CD4) gas ratio increased for CH4/CD4, whereas the hardness and the density did not change with any D2 partial pressure for CH4/D2. A correlation among the S value, the film hardness and the film density was observed, and the S value, the film hardness and the film density did not correlate to the D concentration in the film. These findings suggest that information about the vacancy-type defect of the hydrogenated amorphous carbon films is crucial for evaluation of their mechanical properties and density.

Effects of post-irradiation annealing and re-irradiation on microstructure in surveillance test specimens of the Loviisa-1 reactor studied by atom probe tomography and positron annihilation

This paper presents a microstructural study of a surveillance test specimen from the Loviisa-1 reactor in Finland, which is a Russian-type pressurized water reactor (VVER-440), after initial irradiation to a neutron fluence of 2.5×1019n/cm2 (E>1MeV), post-irradiation annealing at 475°C for 100h and re-irradiation to three different fluences up to 2.7×1019n/cm2. Atom probe tomography (APT) and positron annihilation spectroscopy (PAS) were used to characterize the test specimens. APT results showed the formation of Cu-rich solute clusters (SCs) during the initial irradiation and their subsequent coarsening during annealing. After re-irradiation, a small number of SCs formed once again. The hardening due to the SCs was estimated using the Russell-Brown model based on the APT results, and was in good agreement with the measured hardening after the initial irradiation and post-irradiation annealing. In contrast, during the first-step of re-irradiation, the estimated hardening due to the SCs was smaller than the measured hardening. This suggested that the hardening after re-irradiation was due to some microstructure other than the observed SCs. This difference was attributed to newly-formed matrix defects during re-irradiation, which was supported by the PAS results. However in subsequent steps of re-irradiation, the hardening was almost constant.

Modelling of the implantation and the annealing stages of 800 keV 3He implanted tungsten: Formation of nanovoids in the near surface region

The formation of voids in tungsten implanted at room temperature with 800keV 3He atoms and subsequently annealed from 300K to 900K is modelled using an Object Kinetic Monte Carlo code. Different fluences are investigated ranging from 1017 to 5×1020ionsm−2 and comparisons are made with Positron Annihilation Spectroscopy results. Good agreements with the experimental results are obtained regarding the temperature range at which the vacancy clustering occurs and the dependency of the nanovoid size with fluence. Despite the small amount of He atoms in the investigated region named “track region”, their role is underlined and it is shown that they act as nuclei for the nanovoid formation. The non trivial consequence is that the higher the fluence, the smaller the nanovoids in the track region.

Effect of Mn on the Nanoprecipitation in Binary Fe-Cu alloys

Effect of the third alloying element Mn on Cu-precipitation was studied in a binary Fe-1.3% Cu alloy. Precipitation in both the alloys was investigated after homogenization treatment and subsequent artificial aging. Advanced characterization techniques such as Positron Annihilation Spectroscopy (PAS) and Tomographic Atom Probe (TAP) were used to establish the chemical composition, morphology, size and number density of the Cu-rich phases. Combined results of PAS and TAP were particularly useful in order to follow the Cu precipitation in the binary alloy. At short aging times, addition of Mn significantly increased the kinetics of hardening while its effect on the magnitude of precipitation strengthening is only marginal. It further increases the over-aging kinetics.

Microstructure of Equal-Channel Angular Pressed Cu and Cu-Zr Samples Studied by Different Methods

Polycrystalline samples of technical-purity Cu (99.95 wt pct) and Cu with 0.18 wt pct Zr have been processed at room temperature by equal-channel angular pressing (ECAP). The microstructure evolution and its fragmentation after ECAP were investigated by transmission electron microscopy (TEM), electron backscattered diffraction (EBSD), positron annihilation spectroscopy (PAS), and by X-ray diffraction (XRD) line-profile analysis. The first two techniques revealed an increase in the fraction of high-angle grain boundaries (HAGBs), with increasing strain reaching the value of 90 pct after eight ECAP passes. The increase was more pronounced for pure Cu samples. The following two kinds of defects were identified in ECAP specimens by PAS: (1) dislocations that represent the dominant kind of defects and (2) small vacancy clusters (so-called microvoids). A detailed XRD line-profile analysis was performed by the analysis of individual peaks and by total profile fitting. A slight increase in the dislocation density with the number of ECAP passes agreed with the PAS results. Variations in microstructural features obtained by TEM and EBSD can be related to the changes in the XRD line-broadening anisotropy and dislocation-correlation parameter.

Applications of positron annihilation spectroscopy to polymeric membranes

Positron annihilation spectroscopy (PAS) coupled with a slow positron beam has been used to study the freevolume depth profile (0–10 μm) in polymeric membrane systems prepared by the interfacial polymerization method. Doppler broadening energy parameters of annihilation radiation vary as a function of the depth from the surface through multi-layers in polyamide asymmetric membranes prepared under different experimental parameters, such as the time, the temperature, and pH of membrane preparation process of interfacial polymerization. Three layer structures including a skin, a transition, and the porous layers in those asymmetric membrane systems are obtained. Positron annihilation lifetime results provide additional information about free-volume size and distribution in nano- and micro-scale film membrane systems. PAS results are correlated with the pervaporation separation, flux and water wt% in permeate of 70% isopropanol aqueous solution and compared with the SEM images.

Positron lifetime studies and coincidence Doppler broadening spectroscopy of Al–6Mg–xSc (x = 0 to 0.6 wt.%) alloy

Positron annihilation spectroscopy (PAS), comprising of both positron lifetime and coincidence Doppler broadening measurements, has been employed for studying the phase decomposition behaviour of scandium doped Al–6Mg alloys. Micro structural and age hardening studies have also been conducted to substantiate the explanation of the results of PAS. Samples with scandium concentration ranging from 0 to 0.6 wt.% have been studied. The measured positron lifetimes of undoped alloy reveal that GP zones are absent in the as-prepared Al–6Mg alloy. The observed positron lifetimes and the results of coincidence Doppler broadening measurements largely stem from the entrap of positrons at the interface between aluminium rich primary dendrites and the magnesium enriched interdendritic eutectic mixture of Mg5Al8 (β) and the primary solid solution of aluminium (α). The study also provides evidence of the formation of scandium vacancy complexes in Al–6Mg alloys doped with scandium upto a concentration of 0.2 wt.%. However such complex formation ceases to continue beyond 0.2 wt.% Sc; instead, the formation of fine coherent precipitates of Al3Sc is recorded in the as prepared alloy containing 0.6 wt.% scandium. The positron annihilation studies coupled with CDBS have also corroborated with the fact that the fine coherent precipitates of Al3Sc are formed upon annealing the Al–6Mg alloys doped with scandium of concentration 0.2 wt.% and above. Transmission electron microscopic studies have provided good evidence of precipitate formation in annealed Al–6Mg–Sc alloys. Elevated temperature annealing leads to dissociation of the scandium-vacancy complexes, thereby leading to the enhancement of the mobility of magnesium atoms. This has facilitated fresh nucleation and growth of Mg5Al8 precipitates in the above alloys at 673 K.

Positron annihilation spectroscopy studies of fast neutron irradiated Czochralski silicon

In this work, the changes of irradiation defect in high dose ( 1.17 × 10 19 n cm - 3 ) neutron-irradiated Czochralski silicon (CZ–Si) at different annealing temperatures was investigated using the Positron annihilation spectroscopy (PAS) and Fourier transform infrared spectrometer (FTIR) technique. It is found that divacancy ( V 2 ) and V 4 abound in irradiated CZ-Si. With the increase of the annealing temperature the monovacancy type defect (such as VO) is annihilated. The results of PAS and FTIR show that the formation of V 4 is enhanced when the annealing temperature ran up to 400–600 °C and with the FTIR absorption peak at the wave number of 829 cm - 1 (VO) disappearing, five absorption peaks appear at the wave number of 825 cm - 1 ( V 2 O 2 ), 834 cm - 1 V 3 O 2 , 840 cm - 1 V 2 O , 720 cm - 1 and 919 cm - 1 . It can be concluded that these defect-impurity complexes prolong the lifetime of positrons.

Characterisation of defects in LiF implanted with Ar + using variable energy positron beam

Slow positron implantation spectroscopy has been used to determine the surface and near surface defect profile in LiF after implantation with 100 keV argon ions for a range of fluences from 10 13 to 10 16 cm −2. The measured S-parameter is used to characterise the radiation damage as a function of depth. The spatial extent of the defect distributions was estimated by employing the computer program VEPFIT. Maximum lattice damage is shown to occur at incident positron energy of 4.0 keV for low doses and 5.0 keV for the high dose. This energy corresponds to a sample depth of about 200 nm. The positron annihilation spectroscopy results are correlated with optical absorption measurements on the crystals.

Positron Annihilation Spectroscopy and Small Angle Neutron Scattering Characterization of Nanostructural Features in Irradiated Fe-Cu-Mn Alloys

ABSTRACTRadiation embrittlement of nuclear reactor pressure vessel steels results from a high number density of nanometer sized Cu-Mn-Ni rich precipitates (CRPs) and sub-nanometer matrix features, thought to be vacancy-solute cluster complexes (VSC). However, questions exist regarding both the composition of the precipitates and the defect character and composition of the matrix features. We present results of positron annihilation spectroscopy (PAS) and small angle neutron scattering (SANS) characterization of irradiated and thermally aged Fe-Cu and Fe-Cu-Mn alloys. These complementary techniques provide insight into the composition and character of both types of nanoscale features. The SANS measurements indicate populations of CRPs and VSCs in both alloys. The CRPs are coarser in the Fe-Cu alloy and the number densities of CRP and VSC increase with the addition of Mn. The PAS involved measuring both the positron lifetimes and the Doppler broadened annihilation spectra in the high momentum region to provide elemental sensitivity at the annihilation site. The spectra in Fe-Cu-Mn specimens thermally aged to peak hardness at 450°C and irradiated at 288°C are nearly identical to elemental Cu. Positron lifetime and spectrum measurements in Fe-Cu specimens irradiated at 288°C clearly show the existence of long lifetime (∼500 ps) open volume defects, which also contain Cu. Thus the SANS and PAS provide a self-consistent picture of nanostructures composed of CRPs and VSCs and tend to discount high Fe concentrations in the CRPs.