Investigation of nanocrystalline CoFe 2 O 4 by positron annihilation lifetime spectroscopy

Nanoparticles of cobalt ferrite prepared by the co-precipitation method with crystallite size varying from 4.7 to 41 nm have been characterized by positron annihilation lifetime spectroscopy. Three lifetime components are fitted to the lifetime data. The shortest lifetime component is attributed to the delocalized positron lifetime shortened by defect trapping. The intermediate lifetime is assigned to the positron annihilation in diffuse vacancy clusters or microvoids at the grain boundaries and at the grain-boundary triple points. The longest component corresponds to the pick-off annihilation of ortho-positronium formed at the larger voids. The variations in these lifetimes and their relative intensities with annealing temperature and crystallite size have been studied in detail.     

Positronium in molecular substances

The progress of research into the behavior of the positronium in molecular substances in the last several years is reviewed. The Ps atom can be treated as a solute in a liquid. The repulsive exchange force is the dominant force which reacts against the van der Waals attractive force. One of the most useful general relationships found is the one between the pick-off quenching rate and the surface tension of the liquid. The pick-off quenching rate is found, in general, to be an additive property of the functional groups of a molecule. The quenching of theo-Ps atom by an active quenching agent in an inert solvent has been found to be primarily diffusion controlled. Some deviations from the simple Arrhenius equation have been found. For gases, more theoretical work on the collision problems of Ps with atoms or molecules is needed. For amorphous molecular solids, it is likely that the property of pick-off quenching ofo-Ps is similar to that in liquids. However, in crystalline molecular solids, the situation is quite different. A few definite cases have been found where a Ps atom does not form or exist. Further work in this area should be very interesting. The foregoing leads us to believe that positronium or positron annihilation can be useful in the study of fundamental properties of molecular liquids and solids.     

Annihilation rates of ortho—positronium in organic liquids: correlation with liquid compressibility and polarization ratio

The bubble model of ortho—positronium annihilation rates customarily employs the value of the planar-surface tension in the estimation of the (surface) energy required to expand a normal interstitial cavity to form the bubble. However, bubble radii are estimated to be only about 0.4 nm; the surface of such a small bubble is expected to have a tension different from that of the planar surface. We have replaced the surface tension parameter by the isothermal compressibility, which is thought not to have a nanostructure limitation. The model has been further improved by introducing the polarization ratio (n ² _D ? 1)/(n ² _D + 2) as a parameter, n _D being the refractive index of the organic liquid. Calculations are presented for 20 liquids.     

Dependence of positron lifetimes on the dielectric constants of some organic media

An attempt has been made to correlate the orthopositronium (o-Ps) pickoff annihilation lifetime with the dielectric constant for some organic liquids. A semi-empirical free volume model, developed by Thosar et al., is used.     

Investigation of the free volume and ionic conducting mechanism of poly（ethylene oxide）-LiClO₄ polymeric electrolyte by positron annihilating lifetime spectroscopy

The positron annihilation lifetime and ionic conductivity are each measured as a function of organophilic rectorite(OREC) content and temperature in a range from 160 K to 300 K.According to the variation of ortho-positronium(o-Ps) lifetime with temperature,the glassy transition temperature is determined.The continuous maximum entropy lifetime(MELT) analysis clearly shows that the OREC and temperature have important effects on o-Ps lifetime and free volume distribution.The experimental results show that the temperature dependence of ionic conductivity obeys the Vogel-Tammann-Fulcher(VTF) and Williams-Landel-Ferry(WLF) equations,implying a free-volume transport mechanism.A linear least-squares procedure is used to evaluate the apparent activation energy related to the ionic transport in the VTF equation and several important parameters in the WLF equation.It is worthwhile to notice that a direct linear relationship between the ionic conductivity and free volume fraction is established using the WLF equation based on the free volume theory for nanocomposite electrolyte,which indicates that the segmental chain migration and ionic migration and diffusion could be explained by the free volume theory.     

Investigation of the free volume and ionic conducting mechanism of poly（ethylene oxide）-LiClO_4 polymeric electrolyte by positron annihilating lifetime spectroscopy

The positron annihilation lifetime and ionic conductivity are each measured as a function of organophilic rectorite（OREC） content and temperature in a range from 160 K to 300 K.According to the variation of ortho-positronium（o-Ps） lifetime with temperature,the glassy transition temperature is determined.The continuous maximum entropy lifetime（MELT） analysis clearly shows that the OREC and temperature have important effects on o-Ps lifetime and free volume distribution.The experimental results show that the temperature dependence of ionic conductivity obeys the Vogel-Tammann-Fulcher（VTF） and Williams-Landel-Ferry（WLF） equations,implying a free-volume transport mechanism.A linear least-squares procedure is used to evaluate the apparent activation energy related to the ionic transport in the VTF equation and several important parameters in the WLF equation.It is worthwhile to notice that a direct linear relationship between the ionic conductivity and free volume fraction is established using the WLF equation based on the free volume theory for nanocomposite electrolyte,which indicates that the segmental chain migration and ionic migration and diffusion could be explained by the free volume theory.     

Positronium annihilation in silica aerogel studied by a positron age-momentum correlation technique

A high-performance positron age-momentum correlation (AMOC) spectrometer was newly developed. The counting rate is increased up to 200 cps much larger than the value 20~cps reported by other international groups. And at the same time, the time resolution still keeps at the international level of 220 ps. Furthermore, positronium (Ps) annihilation in silica aerogel was investigated by AMOC, which indicates: (1) Ps annihilation between the grains dominantly undergoes pick-off process and spin conversion from o-Ps to p-Ps; (2) Annealing below 400 ℃ changes the grain surface conditions, i. e. the desorption of hydrogen and the decrease of the defect centers concentration.     

The role of bound states in positronium annihilation

The temperature dependence of the rates of positronium reaction with diamagnetic organic molecules in liquids, and the observed complicated conversive-chemical interaction of positronium with many paramagnetic molecules in the liquid and gas phases lead to the formulation of some general concepts about positronium interaction with quenchers. The positronium and acceptor (Ps-M) complexing is a very essential step of the interaction mechanism. In the case of unstable complexes the contribution of annihilation from the bound state can change depending on the physical conditions of the reaction (pressure in the gas phase, transition from the gas to the liquid phase, heating or replacement of the liquid solvent). The dependence on temperature and on the solvent nature is accounted for by formation of a “bubble” around the positronium in many liquids. At the moment of complexing the “bubble” shrinks with a swiftness depending upon properties of the liquid (surface tension and viscosity). With unstable complexes this leads to complex decomposition and, correspondingly, either to elimination of interaction with the diamagnetic acceptor or, in the case of paramagnetic acceptors, to conversion on the latter, rather than to annihilation from the bound state. With stable complexes further positronium annihilation will occur only from the bound state.     

Particle swarm modeling of vapor-liquid equilibrium data of binary systems containing CO<Subscript>2</Subscript> + imidazolium ionic liquids based on bis[(trifluoromethyl)sulfonyl]imide anion

Based on particle swarm optimization (PSO), a thermodynamic modeling for the vapor-liquid equilibrium of binary mixtures of carbon dioxide with ionic liquids is presented. The Peng-Robinson equation of state with the Wong-Sandler mixing rules is used to evaluate the fugacity coefficient of the systems. Simulations are carried out in five systems containing 1-alkyl-3-methylimidazolium ionic liquids based on bis[(trifluoromethyl)sulfonyl]imide anion. Then, PSO algorithm was used to minimize the difference between calculated and experimental bubble pressure, and calculate the interaction parameters and the excess Gibbs free energy for all systems used. The results show that the bubble pressures were correlated with low deviations between experimental and calculated values. These deviations show that the PSO algorithm is the preferable method to optimize the interaction parameters of the phase equilibria of binary systems of supercritical carbon dioxide with ionic liquids, and can be used for other similar systems.     

正电子偶素(o-Ps)寿命困惑及其最终解决

正电子偶素o-Ps是由电子和正电子组成的纯轻子束缚系统，o-Ps在真空中的寿命可由量子电动力学进行精确的理论计算，但长期以来理论值和实验测量值之间存在较大的偏差，为了解决这一问题，理论和实验物理学经过长期共同努力，最终于近年解决了o-Ps寿命困惑．文章简要介绍了正电子偶素o-Ps寿命理论值和实验值之间不一致的历史状况和最近的实验技术的改进以及该问题的最终解决情况，并重点介绍了Tokyo实验组和Michigan实验组的最新结果．     

THE OBSERVATION ON o-Ps ANNIHILATION IN SILICA AEROGEL BY COMBINED SPECTROMETER

The yields and energy spectra of the various components of o-P_s annihilation in silica aerogel are observed using the combined spectrometer. It is shown that the processes of P_s formation and annihilation in ultrafine grains of SiO₂ aerogel are basically consistent with the diffusion model, but the grain surface effect can not be neglected. Comparing the Dopplar broadening spectra, it is understood that the pick-off process of the longest component possesses the lower annihilation momentum than that of the free positron annihilation.     

Positronium annihilation in silica aerogel studied by a positron age-momentum correlation technique

A high-performance positron age-momentum correlation (AMOC) spectrometer was newly developed.The counting rate is increased up to 200 cps much larger than the value 20 cps reported by other international groups.And at the same time,the time resolution still keeps at the international level of 220 ps.Furthermore,positronium (Ps) annihilation in silica aerogel was investigated by AMOC,which indicates:(1) Ps annihilation between the grains dominantly undergoes pick-off process and spin conversion from o-Ps to p-Ps;(2) Annealing below 400 ℃ changes the grain surface conditions,i.e.the desorption of hydrogen and the decrease of the defect centers concentration.     

Magnetic quenching of positronium in semiconductors

The positronium-determined characteristics of electron-positron annihilation in semiconductors in the presence of an external magnetic field are calculated. It is assumed that the positronium atoms can be in one of two states, delocalized or localized (at defects), and undergo not only pick-off annihilation but also orthoparaconversion (³S₁ ¹S₀) on free carriers. Numerical calculations were made for a field H=15 kG. The field has very different effects on localized and delocalized positronium, and in the first case the effect depends strongly on the nature of the defect.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No.11, pp. 75–78, November, 1980.     

A slow positron lifetime study of the annealing behaviour of an amorphous silicon layer grown by MBE

We have studied MBE grown amorphous silicon, which was recrystallized at different temperatures for one hour, with a pulsed positron beam. A positron lifetime of 538±10 ps in the as-grown state is attributed to microvoids containing at least 10 vacancies. An incompletely recrystallized sample annealed at 500°C shows an additional long lifetime from ortho-positronium (o-Ps) pick-off annihilation. The o-Ps component disappears for samples, recrystallized at 700°C and above, and the defect lifetime steadily decreases with higher annealing temperature until a value of 310 ps is reached for the layer annealed at 1200°C. This value is explained by positron trapping at dislocations or small vacancy defects stabilized by dislocations or impurities.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Electron density and electron redistribution in alloys—I: Electron density in elemental metals

The changes in electronic structure around each metal atom in an alloy are more accurately described in terms of electron redistribution than electron transfer. The important elements in a model for such electron redistribution are the electron configuration and radius of the positive ion cores and the conduction electron density and its variation in the interstitial regions. Positive ion core radii for metals are derived from an empirical relation between ion core radii and intermetallic distance with electronic configurations of the core from positron annihilation data. These derived ion core radii are in good agreement with those calculated for metal electron densities by Welch and Lynn from Hartree-Fock wave functions. Average conduction electron densities can be directly calculated from these radii and the experimental atomic volume; the variation of conduction electron density in the interstitial region is calculated by assigning electrons to octahedral and tetrahedral interstices according to the Interstitial Electron Model. The calculated electron densities and the interstitial variation agree very well with the experimental electron densities of Be as determined from X-ray diffraction data as well as with calculated electron densities for Na, Mg and Al. Ion core configurations are compared with those derived from band theory and also those of the Engle-Brewer Correlation and the Samsonov-Pryadkov-Pryadkov Configurational Model.     

表面张力对固壁旁空泡运动特性影响的理论和实验研究

表面张力是影响空泡脉动及空蚀的一个重要因素.对五种不同表面张力液体中空泡脉动（膨胀和收缩）过程进行了研究,并将实验结果与基于空泡生长和溃灭理论的计算结果进行了对比.实验中,用激光作为测试光源,采用光偏转测试系统研究了不同表面张力液体中空泡泡壁运动规律及泡壁速度的变化.结果表明：表面张力对空泡膨胀过程起抑制作用,故液体表面张力愈大,空泡能达到的最大直径越小;表面张力对空泡的收缩过程则起加速作用,液体表面张力愈大,收缩越迅速,空泡泡壁运动速度越大,其所产生的瞬时溃灭压强越大,空化效果越好. 关键词： 表面张力 空泡 光偏转     

PdAgHx金属氢化物正电子湮没研究

在77—295 K温区和氢浓度0—0.35范围采用正电子湮没寿命测量方法研究了Pd0.75Ag0.25H_x氢化物合金. 充氢后正电子湮没寿命谱可以用两个寿命成分表征. 短寿命成分τ1不随温度和氢浓度变化, 是自由正电子湮没寿命; 长寿命成分τ2及其相对强度I2不随温度变化, 但随氢浓度的增加分别增大和减小, τ2是氢气泡捕获的正电子湮没寿命, τ2增大和I2减小说明随氢浓度增大氢聚集成的气泡的尺度增大, 而浓度减小. 实验结果表明, 氢脆的微观机理是氢气泡致脆. The metal hydride PdAgHx with a hydrogen concentration x ranging from 0 to 0.35 has been investigated by positron annihilation lifetime method in the temperature region between 77 K and 295 K. The measured lifetime spectra in metal hydride PdAgHx are characterized by two lifetimes τ1 and τ2. The short lifetime τ1 is independent of both hydrogen concentration and temperature, which is ascribed to the annihilation lifetime of free positrons. The long lifetime τ2 and its intensity I2 do not change with temperature, while τ2 increases and I2 decreases with increasing of hydrogen concentration. τ2 is attributed to the lifetime of positrons trapped at the hydrogen bubble. The increase of τ2 indicates the growth of the hydrogen bubble, and the decrease of I2 shows the reduction of the hydrogen bubble concentration. The experimental result shows a microscopic mechanism that the hydrogen bubble produced causes hydrogen embrittlement.     

Characterisation of proton irradiated CR-39 detector using positron annihilation lifetime spectroscopy

The damaged produced by proton beam slowed down in CR-39 polymer results in highest degree of disorder with change of free volume proprieties. Positron annihilation lifetime spectroscopy (PALS) is nowadays well recognized as powerful tool of microstructure investigations. PALS is used in order to get more information at an atomic scale of modifications induced by irradiation of polymers particularly the variation of free volume in the irradiated region. In this paper, the mean free volume hole radii and average free volume of the micro-voids in CR-39 polymer irradiated with 200 MeV therapy proton beam at different positions in spread out Bragg Peak region (SOBP) at Ithemba proton therapy facility have been determined using PALS. © 2006 Elsevier Ltd. All rights reserved.     

On the aging characterization of poly(ethylene terephthalate) by positron spectroscopy

Positron spectroscopy was used to study the free volume properties of glassy poly(ethylene terephthalate) as a function of temperature and aging time. The experimental results indicate that the glass transition temperature increases with increasing aging time. This is confirmed by differential scanning calorimetry measurements. From the aging time dependence of the intensity of the ortho-positronium (o-Ps) component I₃ and the fractional free volume, we observe that the concentration of free volume holes and the fractional free volume show a continuous decrease at a fixed aging temperature, which suggests that structural relaxation and segmental rearrangement occur during physical aging.