Positron Annihilation Lifetime Spectra Research Articles

The Influence of the Temperature on Positron Annihilation Lifetime Spectra in Porous Sol-Gel Glass Doped with Ag Nanoparticles

Silver nanoparticles with continuous size distribution from 10 to 20 nm were incorporated into porous glass produced using sol gel technique. Positron annihilation lifetime spectroscopy was used to compare obtained material with an undoped reference glass. The positron annihilation lifetime spectroscopy measurement were made in a broad temperature range (from −180 ◦C to 200 ◦C). The experimental lifetime values were compared with the predictions of the extended Tao Eldrup model of the temperature range. Experimental results for the reference material were in agreement with the model, except for a small discrepancy at low temperatures. In doped material substantial changes and non-monotonous temperature dependences of intensity and lifetime values were observed. Heating the material to 200 ◦C resulted in reorganization of the material structure.

Formation of Quasi-Free and Bubble Positronium States in Water and Aqueous Solutions

It is shown that in aqueous solutions a positronium atom is first formed in the quasi-free state, and, after 50-100 ps, becomes localized in a nanobubble. Analysis of the annihilation spectra of NaNO3 aqueous solutions shows that the hydrated electron is not involved in the positronium (Ps) formation. PACS: 71.60.+z Positron states (electronic structure of bulk materials); 34.80.Lx Recombination, attachment, and positronium formation; 82.30.Gg Positronium chemistry Most usually, the positron annihilation lifetime (LT) spectra in liquids (below we consider aqueous solutions) are well described in terms of three exponentials (3-E analysis). It is believed that this fact indicates that 1) Ps formation is a fast process, its duration not exceeding 10 ps, the typical width of one channel of the time analyzer, and 2) the Ps atom is not involved in the non-homogeneous diffusion-controlled intratrack chemical reactions with radiolytic products. However, several facts plead against this conventional analysis, in particular: 1) The ratio of I3 to I1 (intensities of the ortho-Ps and para-Ps components) is not 3:1, as theoretically expected, but rather close to 2:1, Fig. 1 [1]; 0 20 40 60 80 100 temperature, oC 10 15 20 25 30 in te ns it ie s, % I3

Effect of NiO-doping on the microstructure and the dielectric properties of CaCu3Ti4O12 ceramics

The influence of NiO-doping on the microstructure and dielectric properties of CaCu3Ti4O12–xNiO (x=0, 0.003, 0.006, 0.010, 0.015, and 0.020) ceramics has been investigated using SEM, Raman spectra and dielectric spectrum measurements. The positron annihilation lifetime spectra (PALS) are used to investigate the influence of defects on the dielectric properties. The SEM results show that the grain morphology varies significantly with increasing NiO content. An appropriate small amount of NiO can promote the grain growth, which is beneficial to improve the dielectric properties in a CaCu3Ti4O12 (CCTO) system. Positron results show that there are vacancy-type defects in the experimental samples, and the concentration of the defect and the defect type both change with increasing NiO content. The effects of microstructure including the grain morphology and the vacancy defects on the mechanism of the dielectric properties by adding NiO are discussed. The results demonstrate the importance of the grain morphology and the characteristic of defects in controlling the electrical properties of CCTO ceramics.

Positron Annihilation Lifetime Spectroscopy Measurement of Ge<sub>5</sub>As<sub>37</sub>S<sub>58</sub> Glass

Defect configuration of Ge5As37S58chalcogenide glasses was studied by positron annihilation lifetime spectra (PALS). Size of nanovoids was determined using Liaos formula for radius of nanovoids in spherical approximation. Peculiarities of structural features were obtained from Raman spectra.

Direct Control of Defects on Positron Lifetimes and Dielectric Constant of Microwave Ceramics

It is of great importance to discover completely the correlation between the defects and positron lifetimes in positron annihilation technology (PAT) and reveal clearly the effect of defects on the dielectric constant of universal microwave ceramics. In this work, ZnNb2−xTixO6−x (0 ≤ x ≤ 0.5) dielectric ceramics with nonstoichiometric defects were prepared by solid‐state reaction. All the pores and defects in as‐prepared ceramics were accurately revealed by PAT as a convenient way. The positron lifetimes of τ3 and τ2 in positron annihilation lifetime spectra (PALS) were dependent on two kinds of pores correlated with where o‐Ps atoms and trapping positron annihilated in, whereas τ1 could be separated evidently into τb and τt which are related to bulk structure and negatively charged defects in the system. The dielectric constant of microwave ceramics ZnNb2−xTixO6−x, which increases from 26 to 49 with x increasing from 0 to 0.5 at 10 GHz, is confirmed to be directly controlled by the negatively charged defect of through cation rattling.

The influence of crystal morphology on the process of positronium formation in docosane. The accessibility of electrons for trapping

Various morphologies of docosane crystals have been obtained by crystallization of samples of the same origin with different methods. The positron annihilation lifetime spectra indicate some differences between them, which are related to the differences in the electron trapping capability. To find out the origin of these differences, some crystallographic and optical spectroscopic studies were performed. We argue that the differences in each structure are related to the ease of electron release from traps during excitation and not only to the density of the traps, as has been assumed so far.

Rearrangement of hexadecane molecules confined in the nanopores of a controlled pore glass using positron annihilation and differential scanning calorimetry

The solidification/melting of cetane confined in nine silica glasses with specific controlled pores (CPG) with different pore sizes (rp ranging from 3.7–72 nm) has been studied. Samples with variable coefficient of filling of cetane were prepared and analyzed. Combining the indication of the evolving solidification heats (via differential scanning calorimetry, DSC) with the lifetime spectra of positron annihilation, the mechanism of filling of cetane molecules in the pores of the hosting silica glass matrices has been searched. Three independent solidification effects—three independent DSC peaks—were observed in each measuring cycle, namely one crystallization exotherm Rb of the bulk-cetane (if present) and two crystallization sub-peaks Rc1 and Rc2 of the confined cetane in the case of continuous cooling. The temperatures of both Rc1 and Rc2 effects decrease on decreasing the rp of the CPG matrix. The mutual relation between Rc1 and Rc2 has been analyzed. The kinetics of all three steps of the solidification in the continuous-cooling as well as isothermal regimes have been specified.

Location and size of nanoscale free-volume holes in crosslinked- polytetrafluoroethylene-based graft-type polymer electrolyte membranes determined by positron annihilation lifetime spectroscopy

The location and size of nanoscale free-volume holes (nanoholes) in graft-type polymer electrolyte membranes (PEMs), which were prepared by radiation-induced graft polymerization (grafting) of styrene into crosslinked-polytetrafluoroethylene (cPTFE) films and subsequent sulfonation, were investigated using positron annihilation lifetime (PAL) spectroscopy. The PAL spectra of the PEMs indicated the existence of two types of ortho-positronium (o-Ps) species, corresponding to nanoholes with volumes of 0.11 and 0.38nm3. A comparison of the PAL data of the PEMs with that of the precursor original cPTFE and polystyrene-grafted films demonstrated the probability that the smaller holes were located in both the PTFE crystalline phases and the poly(styrene sulfonic acid) graft regions, whereas the larger holes are potentially localized in the PTFE amorphous phases. Taking into account both the size and the location of the nanoholes, it was concluded that gas transport through the larger holes in the amorphous PTFE phases was dominant over permeation through the smaller holes in the PTFE crystals and grafted regions.

Influence of calcination and pretreatment conditions on the activity of Co3O4 for CO oxidation

The influence of calcination and pretreatment conditions on the structure of Co3O4 and its activity for CO oxidation were studied. TG analysis indicated that the precursor of Co3O4 prepared by a precipitation method was present in the form of cobalt hydroxide carbonate, calcination of which within the temperature range of 150–400 °C in air resulted in the formation of cubic phase Co3O4. N2 adsorption-desorption, powder X-ray diffraction, transmission electron microscopy, and activity test results showed that the prepared Co3O4 samples were comprised of nanoparticles, with the specific surface area and size distribution closely related to the calcination temperature. Over these samples, a size dependence of the catalytic activity for CO oxidation was clearly observed. As identified by positron annihilation lifetime spectrum, low temperature oxygen temperature-programmed desorption, and durability testing for CO oxidation, pretreatment of Co3O4 in N2 within the temperature range 150–250 °C favors the formation of surface oxygen vacancy clusters, the occurrence of which would be beneficial for the adsorption and activation of O2, and also for the catalytic oxidation of CO. Meanwhile, the reconstruction of oxygen vacancies on the surface of Co3O4, induced by the pretreatment process, was also discussed. 采用沉淀法制备了 Co3O4, 考察了焙烧和预处理条件对其结构、低温催化氧化 CO 性能的影响. 热重分析表明, 未经焙烧的样品以 Co(OH)2CO3 的形式存在, 150–400 oC 空气气氛中焙烧后, 样品以立方相 Co3O4 的形式存在. N2 吸附-脱附法、X 射线衍射、透射电镜及活性测试结果表明, 以纳米颗粒物存在的 Co3O4 的比表面积、颗粒尺寸、催化氧化 CO 活性与焙烧温度密切相关. 正电子湮没寿命谱、O2-程序升温脱附与定温条件下 CO 氧化测试结果表明, 合适温度 (150–250 oC) 下 N2 预处理有利于 Co3O4 表面氧空穴团的形成, 它在吸附活化分子氧以及 CO 催化氧化反应中起到关键作用. 同时讨论了预处理作用下 Co3O4 表面氧空穴的再构机制.

POSITRON LIFETIME MEASUREMENTS IN NATURAL RUBBER WITH DIFFERENT FILLERS

Positron annihilation lifetime spectra (PLAS) have been measured for natural rubber polymer with different fillers (Titenium dioxide, Nanosilica and Nanoclay) as a function of filler concentration to investigate how these fillers affect the microstructure of free volume of natural rubber. The lifetime spectra is analyzed by using LT9.0 and the longest lived component(τo-Ps) is attributed to the pick- off annihilation of o-Ps in free volume sites, available mostly in the amorphous region of polymer. On the basis of the τo-Ps values the radii of the free volume holes (Rh) are calculated. The PALS results show that o-Ps lifetime as well as the size of free volume decreases with the increase of filler concentration.

Surface tension of cavities and Tolman’s length in n-alkanes. A positron study

Positron annihilation lifetime spectra were measured in n-alkanes. From the lifetime of ortho-positronium (o-Ps) one can determine the surface tension and Tolman’s length for the cavity produced by positronium in the liquid. This tension is found increased by factor of 1.4÷1.7 comparing to that of flat surface. The Tolman’s length is temperature dependent; in n-nonadecane it rises from about 0.06nm at melting point to 0.08nm at the temperature 100K higher. Near the melting point the value of surface tension of Ps cavities is practically identical for all alkanes under study, i.e. from heptane C7H16 to pentacontane C50H102.

The application of artificial neural networks to the inversion of the positron lifetime spectrum

A new method of processing positron annihilation lifetime spectra is proposed. It is based on an artificial neural network (ANN)-back propagation network (BPN). By using data from simulated positron lifetime spectra which are generated by a simulation program and tested by other analysis programs, the BPN can be trained to extract lifetime and intensity from a positron annihilation lifetime spectrum as an input. In principle, the method has the potential to unfold an unknown number of lifetimes and their intensities from a measured spectrum. So far, only a proof-of-principle type preliminary investigation was made by unfolding three or four discrete lifetimes. The present study aims to design the network. Besides, the performance of this method requires both the accurate design of the BPN structure and a long training time. In addition, the performance of the method in practical applications is dependent on the quality of the simulation model. However, the chances of satisfying the above criteria appear to be high. When appropriately developed, a trained network could be a very efficient alternative to the existing methods, with a very short identification time. We have used the artificial neural network codes to analyze data such as the positron lifetime spectra for single crystal materials and monocrystalline silicon. Some meaningful results are obtained.

Microstructural changes and effect of variation of lattice strain on positron annihilation lifetime parameters of zinc ferrite nanocomposites prepared by high enegy ball-milling

Zn-ferrite nanoparticles were synthesized at room temperature by mechanical alloying the stoichiometric (1:1 mol%) mixture of ZnO and α-Fe2O3 powder under open air. Formation of both normal and inverse spinel ferrite phases was noticed after 30 minutes and 2.5 hours ball milling respectively and the content of inverse spinel phase increased with increasing milling time. The phase transformation kinetics towards formation of ferrite phases and microstructure characterization of ball milled ZnFe2O4 phases was primarily investigated by X-ray powder diffraction pattern analysis. The relative phase abundances of different phases, crystallite size, r.m.s. strain, lattice parameter change etc. were estimated from the Rietveld powder structure refinement analysis of XRD data. Positron annihilation lifetime spectra of all ball milled samples were deconvoluted with three lifetime parameters and their variation with milling time duration was explained with microstructural changes and formation of different phases with increase of milling time duration.

Study of defects in Fe–Re and Fe–Mo alloys by the Mössbauer and positron annihilation spectroscopies

The room temperature positron annihilation lifetime spectra and 57Fe Mössbauer spectra were measured for pure Fe as well as for iron-based Fe1−xRex and Fe1−xMox solid solutions with x in the range 0.01≤x≤0.05. The measurements were performed in order to learn more about creation of structural defects during formation and further mechanical processing of the iron systems under consideration. The spectra were collected at least twice for each studied sample synthesized in an arc furnace—after cold rolling to the thickness of about 40μm as well as after subsequent annealing at 1270K for 2h. It was found that in the annealed samples positrons live much shorter than in the not annealed ones which suggest that the latter samples are more defected as it could be expected. Moreover simultaneous analysis of the positron and Mössbauer data shows that cold rolling leads to creation of two types of defects. It seems that they are dislocations and vacancies. Finally from the Mössbauer data it follows that vacancies are located mainly in the vicinity of non-iron atoms, Re or Mo. This speaks in favour of the suggestion that in iron matrix the impurities mentioned above and vacancies interact attractively which supports the known from the literature, theoretical calculations on the Mo-vacancy interaction in iron.

The Correlation Between Dislocations and Vacancy Defects Using Positron Annihilation Spectroscopy

An analysis program for positron annihilation lifetime spectra is only applicable to isolated defects, but is of no use in the presence of defective correlations. Such limitations have long caused problems for positron researchers in their studies of complicated defective systems. In order to solve this problem, we aim to take a semiconductor material, for example, to achieve a credible average lifetime of single crystal silicon under plastic deformation at different temperatures using positron life time spectroscopy. By establishing reasonable positron trapping models with defective correlations and sorting out four lifetime components with multiple parameters, as well as their respective intensities, information is obtained on the positron trapping centers, such as the positron trapping rates of defects, the density of the dislocation lines and correlation between the dislocation lines, and the vacancy defects, by fitting with the average lifetime with the aid of Matlab software. These results give strong grounds for the existence of dislocation-vacancy correlation in plastically deformed silicon, and lay a theoretical foundation for the analysis of positron lifetime spectra when the positron trapping model involves dislocation-related defects.

Positron Annihilation Lifetime Study of Steel Surface Modification by Shot Peening

Steel made machine components are often exposed to variable loads during the operation. Thus, their important characteristic is fatigue strength, which largely depends on the residual stress in the surface layer of these elements. The fatigue strength of components can be increased e.g. by exposing them to shot peening. Influence of shot peening on the defect structure in the samples observed by positron annihilation lifetime spectroscopy was studied for various steels. The differences between unprocessed and shot peened samples reflect mostly in the intensity ratio decrease of the components attributed to monovacancies and vacancy clusters. Shot peening result in uniformization of the vacancy clusters size. The carburized steel does not exhibit any changes in positron annihilation lifetime spectra caused by shot peening. On the other hand, the changes in the chromium plated steel are considerable. Their origin lies is compression of the microcracks present in the chromium layer caused by shot peening.

LT10 Program for Solving Basic Problems Connected with Defect Detection

Abstract Features of LT10 software for positron annihilation lifetime (PAL) spectra analysis are presented. It is a completely new version of well known LT9.x series with newly designed intuitive user interface. The program allows to significantly reduce the number of freedom degrees in the numerical analysis of PAL spectra. It is accomplished by simultaneous analysis of a series of spectra with regard of the correlation between related parameters of each spectrum in the series. A few examples of the program application are presented in the paper.

Probing the Phase Transition in Nanocrystalline TiO<sub>2</sub> Powders by Positron Lifetime (PAL) Technique

Positron annihilation techniques (PAT) have recently been successfully employed for the characterization of phase transitions in metals and compounds. In the present study, positron annihilation lifetime (PAL) measurements have been carried out on a nanocrystalline titania (TiO2) in the form of powders that had been heat-treated at temperatures ranging from 300 to 1273K. The PAL spectra were analyzed into two lifetime components. The shorter lifetime τ1 (185-300 ps) is attributed to positron annihilation in vacancies and the longer lifetime τ1 (400-580 ps) to positrons in microviods at interfaces. The rutile phase of TiO2 powders was utilized as a reference in order to compare their behavior with the commercially supplied and widely available anatase phase (Degussa P25). The influence of the heat-treatment upon the nanostructure during the transition of the anatase to rutile phase were also investigated by X-ray diffraction (XRD), TEM and BET surface area methods. Understanding of this effect is expected to enhance our knowledge of the morphology and nanocrystallite size of TiO2 powders and their T-dependence, and hence their physical properties.

The role of positronium decoherence in positron annihilation in matter

A small difference between the energies of the para-positronium (p-Ps) and ortho-positronium (o-Ps) states suggests the possibility of the superposition of p-Ps and o-Ps during the formation of positronium (Ps) from pre-Ps, terminating its migration in the matter in a void. It is shown that such a superposition decoheres in the basis of p-Ps and o-Ps. The decoherence time scale estimated here motivates a correction in the precise analysis of the positron annihilation lifetime spectra. More generally, the superposited Ps state should contribute to the theory of the evolution of positronium in matter.

Conceptual Approach to Renewable Barrier Film Design Based on Wood Hydrolysate

Biomass is converted to oxygen barriers through a conceptually unconventional approach involving the preservation of the biomass native interactions and macromolecular components and enhancing the effect by created interactions with a co-component. A combined calculation/assessment model is elaborated to understand, quantify, and predict which compositions that provide an intermolecular affinity high enough to mediate the molecular packing needed to create a functioning barrier. The biomass used is a wood hydrolysate, a polysaccharide-rich but not highly refined mixture where a fair amount of the native intermolecular and intramolecular hemicelluloses-lignin interactions are purposely preserved, resulting in barriers with very low oxygen permeabilities (OP) both at 50 and 80% relative humidity and considerably lower OPs than coatings based on the corresponding highly purified spruce hemicellulose, O-acetyl galactoglucomannan (AcGGM). The component interactions and mutual affinities effectively mediate an immobilization of the chain segments in a dense disordered structure, modeled through the Hansen's solubility parameter concept and quantified on the nanolength scale by positron annihilation lifetime spectrum (PALS).