Crystallization of Li2OSiO2 glass studied by positron annihilation

The crystallization of 34 Li₂O65 SiO₂1 P₂O₅ glass has been studied by positron annihilation. The changes in the positron lifetime spectra have been correlated to the magnitude of the amorphous X-ray scattering. A linear relationship has been found for heat-treatment temperatures less than 700°C, above which non-linear phenomena in the lifetime spectra set in. The volume crystallinity reaches a saturation value, which varies from 70% at the heat-treatment temperature of 535°C to 80% at 700°C because of the phase separation. The dimensionality of the crystal growth in this glass is found to be n = 1.5 ± 0.1. The effect of the phase separation on the rate and dimensionality of the crystallization is discussed. The results show that positron annihilation is a sensitive and accurate method to follow crystallization in vitreous materials.     

Post-irradiation relaxation in vitreous arsenic/antimony trisulphides

Instability caused by high-energy γ-irradiation was studied in (As₂S₃)_1−x(Sb₂S₃)_x glasses (x = 0, 0.1, 0.2 and 0.3) using positron annihilation lifetime spectroscopy, the obtained results being treated within a two-state trapping model. The observed decrease in the positron trapping rate of the glasses tested just after irradiation was explained due to renovation of destroyed covalent chemical bonds. This process was governed by monomolecular relaxation kinetics agreed well with corresponding changes in fundamental optical absorption edge.     

Structural study of (As2S3)0.6(GeS2)0.4 glass

(As₂S₃)_0.6(GeS₂)_0.4 glass in non-irradiated and γ-irradiated states has been studied by using high-energy synchrotron X-ray diffraction, extended X-ray absorption fine structure spectroscopy, and positron annihilation lifetime spectroscopy. The experimental results are explained by the local changes around As and Ge atoms upon irradiation. These changes are suggested to involve chemical bonds distortion, formation of defective bonds with wrong coordination, rotation of structural units and appearance of additional free volume in the glass network.     

A research into positron annihilation of the B2F steel rolled out in step-s with constant stretch

The rolling out of step-s with constant stretch (PV-E) is a new technology developed for rolling steel in recent years. We studied the B₂F steel rolled by the new technology with the aid of the positron annihilation technique. The relationships of the average lifetime of positron annihilation with the rolling pressures, the ratio of rolling speed and the mechanical properties are obtained. The value of the lifetime shows that there are many dislocations and vacancy clusters in B₂F steel after being rolled, and which are supported by observations with a transmission electronmicroscope.     

An investigation on the disordered AlNiCo8 magnetic steel containing the higher Co and higher Ti by means of the positron annihilation

In this paper, we investigate the coercive force and magnetic properties of the disordered AlNiCo8 magnetic steel containing the higher Co and higher Ti by means of the positron annihilation spectroscopy. We found that the positron annihilation lifetime τ₂ is closely related to the manufacturing technique and disorientation of this steel, and which directly affect the coercive force and magnetic behavior of the material. It is also found that the microscopic constructional characteristics in the present sample are finer than the ones of the same kind sample of U.S.A. according to an analysis of the positron average life τ .     

On the correlation between void-species structure of vitreous arsenic selenide studied with X-ray diffraction and positron annihilation techniques

The structure of nanoscale void-species has been studied in glassy arsenic selenide g-As₂Se₃ by combining the first sharp diffraction peak in X-ray diffraction (FSDP-related XRD), treated within Elliott’s void-based model, and positron annihilation lifetime spectroscopy (PALS) within two-state positron trapping model. By suggesting that the same nanovoids are responsible for both FSDP and PALS data, the analytical correlation relationship between the FSDP position, Q₁, and nanovoid diameter, D, has been presented in the form of Q₁ = 2.3 · π/D.     

Crystallization and phase-separation of Li2OSiO2 glass

The crystallization and phase-separation phenomena in the Li₂OSiO₂ glass are studied by positron lifetime and annihilation lineshape measurements. Analysis of the kinetic data shows three-dimensional morphology of growing crystals. Phase-separation is seen to increase the density of crystal nuclei and the rate of volume crystallization, but it does not affect the morphology. In addition, surface crystallization is detected in glasses with small degrees of phase-separation. The results are consistent with scanning electron and optical micrographs.     

Dependence of positron capture rate on dislocation density in solid solutions of KCl—KBr

Generally well-annealed alkali halide crystals exhibit three lifetime components. τ₁ arises due to annihilation with anion and τ₂ due to Bloch Ps. The origin of τ₃ is not well understood. Positron lifetime and etching techniques have been performed on solid solutions of KCl_100−xBr_x with mole % of x = 0.0, 15, 30, 37, 50, 70, and 100. The positron lifetime spectra have been analysed to resolve into three components. The positron capture rate K₃ for the longest lived component and the dislocation density shows a non-linear variation with composition. The positron capture rate varies linearly with the dislocation density of the samples. The origin of τ₃ is identified as due to positronium formation in the dislocations present in the samples. The trapping probability per dislocation has been estimated to be 2.2 × 10⁻¹²/s.     

Vacancy formation enthalpy in AgZn alloys

Effective vacancy formation enthalpy (H_v) for AgZn alloy was determined by means of positron annihilation method. It was found that H_v decreases linearly with increasing Zn content for the α-phase. For the ξ and β′-phase the H_v does not change and is equal to 0.53–0.55 eV.     

Spectrometer for the study of three-photon annihilation of three-photon annihilation of positrons

A three channel spectrometer for studying three-photon annihilation is reported. It is based on a fast-fast coincidence circuit and includes NaJ(Tl) scintillation detectors, constant fraction differential discriminators, a fast coincidence circuit with variable resolving time (6 ns—60 ns), a linear gate and a multichannel analyzer. The possibility of using positron source ⁶⁴Cu in the spectrometer, its advantages and disadvantages are discussed. In this connection we describe the special units coupled to the spectrometer as well as a method which helps to avoid the influence of the short decay time of ⁶⁴Cu of the accuracy of the experimental results.     

Characterization of plasticity-induced structural changes in a Zr-based bulk metallic glass using positron annihilation spectroscopy

Changes in the free volume distribution in Zr_58.5Cu_15.6Ni_12.8Al_10.3Nb_2.8 bulk metallic glass with inhomogeneous plastic deformation and annealing were examined using positron annihilation spectroscopy. Results indicate that the size distribution of open volume sites is at least bimodal prior to deformation. The size and concentration of the larger sites, identified as flow defects, changes with processing. The size of the flow defects initially increases with deformation. More extensive deformation shifts the distribution, with a third group of much larger sites forming at the expense of flow defects. This suggests that a critical strain is required for the growth of nanovoids observed elsewhere by HRTEM. Although these observations suggest the presence of three open volume size groups, further analysis indicates that all groups have a similar distribution of chemical species around them as evidenced by the same line shape parameter. This may be due to the disordered structure of the glass and the positron affinity to particular atoms surrounding the open volume regions.     

Study of fast-neutron irradiated vitreous silica by positron annihilation methods

The nature of the glass-like structure as obtained by fast neutron irradiation of vitreous silica has been investigated by positron annihilation lifetime and Doppler broadening experiments. Comparing the annihilation parameters obtained in the unirradiated sample to those obtained in irradiated ones we observed that in the irradiated SiO₂ glasses the probability of positronium formation is reduced and the orthopositronium lifetime is increased. The decrease of the positronium formation probability is mainly attributed to the reduction of the amount of free-volume in compacted domains. The orthopositronium lifetime increase is explained by large free spaces generated along trajectories of fast knocked-on particles.     

Positron lifetime studies of X-irradiated N-acetyl-D,L-alanine

Polycrystalline samples of N-acetyl-D,L-alanine (AA) which had received an X-ray dose of approximately 6 Mrad have been studied by the positron annihilation technique. Data were obtained as a function of time following the radiation treatment, and also as a function of subsequent annealing temperature. The large decrease of the intensity of the longest lifetime component (I₃) was observed to be reversible when assisted by heat treatment.     

Solid cobalt(III) and iron(II) complexes and mercury salts investigated by positron annihilation and Mössbauer effect

Measurements of the Doppler effect of the annihilation -line and of the positron lifetime were carried out in three series of solid coordination complexes; the Mössbauer spectra of the iron-containing samples were also recorded. The lack of correlation between the Móssbauer and positron annihilation parameters suggests that electrons of the ligands rather than those of the central atoms dominate in positron annihilation processes.     

Positron annihilation study of pore size in ordered SBA-15

Ordered mesoporous materials such as SBA-15 possess a network of channels and pores with a well-defined size in the nanoscale range (2–50 nm). This particular pore architecture makes them suitable candidates for a variety of applications. Different techniques have been used to measure pore diameters. PALS (positron annihilation lifetime spectroscopy) nanoprobe has been used to investigate free volume in several materials, including mesoporous silica. PALS can be used to find out if the micropore and mesopore structures of samples prepared under different experimental conditions are different. Indeed, considering that the pores present a cylindrical shape, an equation was developed that uses specific pore volumes, theoretical density, and specific surface measurements to evaluate structural connectivity. Our goal is to determine the influence of aging temperature on the porous structure of SBA-15 samples. The structural evolution was studied by PALS, small-angle X-ray (SAXS), N₂ adsorption desorption isotherms and computational modeling to evaluate connectivity. The variation of aging temperature changes the pore structure, indicating the presence of micropores and connections between mesopores. Materials aged at high temperatures present the lowest microporosity.     

Vacancy defects in as-grown and nº-irradiated GaP and GaAs1−xPx Studied by Positrons

Measurements of positron lifetime τ and of the shape parameter S of the Doppler-broadened annihilation line are used to study bulk and defect properties in GaP and GaAs_1−xP_x. τ and S decrease linearly with the composition x of GaAs_1−xP_x layers. A second lifetime component (τ₂ = 290 ps) observed in as-grown GaP is attributed to stoichiometric P vacancies. After neutron irradiation of GaP and GaAs_0.13P_0.87 positrons are trapped by Ga vacancies (τ₂ = 250 ps). These vacancies anneal out in two stages at 200–550 °C and 550–800 °C.     

Probing the local structural changes across the amorphous to crystalline transition in iron phosphate glass: Positron annihilation and micro Raman studies

Iron phosphate glasses have been investigated using positron lifetime and micro Raman spectroscopy to study the crystallization kinetics and defect evolution across the glass and crystallization transitions. Positron lifetime parameters show distinct changes at temperatures corresponding to both the transitions. These changes have been understood as the signature of the evolution of open volume defects and their clustering across the amorphous to crystalline interfaces. Micro Raman measurements on annealed samples have identified the crystalline phases as Fe₃(P₂O₇)₂ and FePO₄. Ab-initio positron lifetime computations reveal that the positron probes the Fe/P sites in defect free crystals. The nature of open volume defects produced upon annealing is identified by computing positron lifetimes for various possible defect clusters. A combination of both experimental and theoretical techniques has provided insights in understanding the evolution of defects across the phase transition in these glasses.     

Positron annihilation study in Ca2+ and Sr2+-doped KCl

Positron lifetime and three photon annihilation measurements are reported in KCl:Ca²⁺ and KCl:Sr²⁺. The present results clearly demonstrate that the positron essentially depends on the kind of Me²⁺ and impurity concentration in these crystals.     

Investigation of radiation defects in electron irradiated Hg1−xCdxTe crystals using positron annihilation

The electron characteristics of defects in the initial and electron irradiated Hg_1−xCd_xTe (2–3 MeV, 10¹⁸ cm⁻², 300 K) crystals using the positron annihilation method have been investigated. The data of electric measurements are confirmed on connection of p-type conductivity with vacancy defects of metal sublattice initial crystals Hg_1−xCd_xTe. An analysis of correlation curves of irradiated crystals has shown a possibility of formation of associations of initial defects and radiation damages of vacancy type during radiation process. The presence of narrow component on correlation curves in the region of small angles is associated with formation of positronium states localized in the region of radiation defect complex of vacancy type. Identification of positron-sensitive defects with electrically active radiation induced ones has been carried out according to the results of isochronal annealing of irradiated crystals.     

Study of precipitates formation in Al‐4.5Wt%Cu and Al‐4.5Wt%Cu‐0.1Wt%In alloys using creep measurements and positron annihilation technique

The transient and steady state creep deformation of Al‐4.5 wt%Cu and Al‐4.5 wt%Cu‐0.1 wt%In alloys were studied using a constant stress in the aging temperatures range 403 to 703 K for various aging times under different working temperatures. Positron annihilation studies using Doppler–broadening measurements were carried out on the specimens used in creep measurements. The subsequent decrease and increase in the creep parameters n, β and ε_st as well as in Doppler lineshape parameter with increasing aging temperatures and aging times were explained on the basis of structure transformations occuring in Al‐Cu alloys. The mechanical creep measurements are very well correlated with positron annihilation characteristics. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)