Investigation of lattice impurities and photochromic centres in diamond with high resolution positron annihilation spectroscopy

Thermalized positrons in solids annihilate with core and valence electrons after the formation of well-defined configurations which represent delocalized or trapped positron states. A two-dimensional coincidence measurement of the Doppler broadening of the electron-positron annihilation quanta allows the measurement of the longitudinal component of the momentum distribution profile out to 20 keV c⁻¹, with high sensitivity and improved resolution. Positron annihilation with core electrons can thus be observed, allowing the identification of lattice defects and impurities. We report on two-dimensional Doppler broadening measurements performed on synthetic and natural diamond. The results are consistent with positron annihilation at nitrogen-related defects in the synthetic diamond and positron trapping at photochromic centres in the natural diamond.     

Applications of positron techniques to surface studies and catalysis

Positrons can be used to obtain unique information on solids. The life and annihilation of this anti-matter particle is extremely sensitive to the atomic and electronic environment. Studies of solid surfaces with monoenergetic positron beams have found increasingly interesting applications and many new techniques have been introduced with unique results. The more traditional techniques using polyenergetic positrons have been utilized in the study of internal surfaces closely related to catalysis. In this review different positron techniques are described and their applications in the field of surface science and catalysis are presented.     

正电子湮没技术用于新型硫铝酸盐基无宏观缺陷水泥的研究

探索了正电子湮没技术(positron annihilation technique,PAT)测定硫铝酸盐基无宏观缺(macro-defect-free,MDF)水泥中微细孔的实验方法。结果表明:正电子湮没中等寿命成分(τ2≈330 Ps)的强度I2随水泥中微细孔(<250 A)等的增加而有规律地增大,因此可用I2的大小来表征硫铝酸盐基MDF水泥的密实程度和结构完整性。同时,还将实验结果与水银压入法(mercury instrusion porosimeter,MIP)测试结果进行了对比。     

Nanoscale mechanism of rare‐earth doping in Ga‐codoped glassy As‐Sb selenides

The method of annihilating positrons in positron annihilation lifetime measuring mode is applied to study mechanism of rare‐earth doping in Ga‐codoped arsenic selenide As₂Se₃ glass modified with Sb. The atomic‐deficient structure of parent As₂Se₃ glass is imagined as containing positron‐trapping sites in the form of free‐volume voids within cycle‐type arrangement of corner‐sharing trigonal AsSe_3/2 pyramids, composed of atomic‐accessible geometrical holes arrested by surrounding atomic‐inaccessible Se‐based bond‐free solid angles. The Ga codoping in As₂Se₃ glass causes gradual decrease in trapping rate and fraction of trapped positrons due to agglomeration of free‐volume voids. Partial As‐to‐Sb replacement in Ga‐codoped As‐Se glasses leads to better stability against crystallization processes and possibility to further rare‐earth doping without principal changes in the type of positron‐trapping defects. Effect of 500 wppm of Pr³⁺ in Ga₂(As_0.28Sb_0.12Se_0.60)₉₈ glass is explained in terms of competitive contribution of changed occupancy sites in Ga‐modified glassy network available for rare‐earth ions and annihilating positrons.     

Characterization of polar groups in polyvinyl chloride–ethyl vinyl acetate blends using coincidence Doppler broadening spectroscopy

Coincidence Doppler broadening spectroscopy (CDBS), known for its exceptional precision, is extensively employed in examining the electronic states of polymers. In this study, polyvinyl chloride (PVC)–ethyl vinyl acetate (EVA) polymers blends were prepared using conventional techniques, following specific ratios (75:25, 50:50, and 25:75, respectively). Spectroscopic analysis reveals that the longitudinal component of linear momentum surpasses the momentum unit (i.e., 10⁻³ m₀c) when considering the ratio curve associated with the participation of oxygen core electrons. When compared to non-polymer samples, the PVA polymer and PVA–EVA polymer blend exhibit greater free volume voids in contrast to non-polymer structural defects. Furthermore, the annihilation of ortho-positronium with core electrons, likely EVA oxygen core electrons, contributes to the measured free volume voids. An increased annihilation of positrons or positroniums by oxygen in the CDBS spectrum leads to higher calculated W parameter values. Conversely, as the PVC content increases, the W parameter decreases, suggesting reduced ortho-positronium annihilation and consequently, enhanced formation of para-positronium in the overall annihilation process. In summary, this investigation demonstrates the utility of CDBS in elucidating the electronic states and free volume voids in PVC and EVA polymers.     

Positron Annihilation in Phase-Separated BaO-SiO2 Glasses

The kinetics of amorphous phase separation in BaO-SiO₂ glasses was studied by small-angle X-ray scattering and positron annihilation. Results show that the annihilation of positrons depends primarily on the average composition of the glasses; it therefore does not depend on compositional fluctuations caused by phase separation and cannot be used to determine the kinetics of phase separation in glasses.     

Positron annihilation spectroscopic studies of the influence of heat treatment on defect evolution in hybrid MWCNT-polyacrylonitrile-based carbon fibers

Polyacrylonitrile-based carbon fibers, embedded with multi-wall carbon nanotubes (MWCNTs) in different concentrations, have been prepared by an electrospinning technique. The samples were subjected to oxidative stabilization followed by carbonization and graphitization at temperatures from 1000 to 3000 °C. Scanning electron microscopy, X-ray diffraction, and positron annihilation spectroscopy were used to investigate the samples. Positron annihilation studies, viz. analysis of positron lifetime and Doppler broadened spectrum line shape, provide deeper insight on defect specific aspects of the material as a function of temperature. Positron lifetime spectra for all the samples give a best fit for two-lifetime components. The trapping of positrons at specific sites in the nanotubes embedded in the fibers indicates the exact regions of open volume defects in the samples. Carbonization and graphitization are enhanced by embedding nanotubes in the fibers and could be an effective way of tailoring the properties of this system.     

硫增感AgBrI T颗粒乳剂光电子行为研究

利用微波相敏技术，获得了硫增感AgBrI T颗粒乳剂中自由光电子和束缚光电子时间衰减信号，分析了光电子衰减时间、电子陷阱效应、光电子寿命、有效陷阱深度及束缚电子转移时间与增感时间的关系，获得了最佳的增感时间、衰减时间、电子陷阱效应、光电子寿命、有效陷阱深度和转移时间数值，     

Chemical Etching Phases of Irradiated CR-39 Track Detector by Positron Annihilation Doppler Broadening Spectroscopy

Positron annihilation Doppler broadening (DB) spectroscopy is a characterization technique that is sensitive to radiation damage in polymers. Passage of α-particles radiation through the CR-39 polymer detector produces damage or defect. It is convenient to study this defect by Doppler broadening spectroscopy. This study found that the fraction of positrons trapped depends on α-particle energies.     

Positron annihilation in C60 fullerites and fullerene-like nanovoids

C₆₀ fullerites, fatty-acid triglycerides containing nanovoids, and triglycerides with dissolved fullerenes were studied by means of positron annihilation spectroscopy. Two types of nanovoids with mean radii of 0.48 and 0.34 nm were found in the fatty-acid triglycerides. The nanovoids of the latter type change size in the course of structural relaxation and ordering at room temperature. The nanovoid radius is stabilized at the value of 0.355 nm, equal to the radius of C₆₀ fullerenes, when the latter are dissolved in triglycerides. Tetrahedral interstitials in the f.c.c. lattice are shown to be the most probable positron-annihilation sites in C₆₀ fullerites. The shell of the C₆₀ molecule is a potential barrier for positron penetration to the interior of the fullerene. Vice versa, the nanovoid in triglycerides is a potential well for positively charged particles: positrons, protons and cations.     

Positron Annihilation Study of Zirconia Nanopowders and Nanoceramics Stabilized by Magnesia and Ceria

Positron lifetime (LT) and coincidence Doppler broadening (CDB) measurements on nanopowders and ceramics of ceria‐ and magnesia‐stabilized zirconia (CeSZ and MgSZ, respectively) are presented. The nanopowders were prepared by the coprecipitation technique. Effects of nanopowder calcination and sintering at various temperatures were investigated. In the nanopowders, the two kinds of open‐volume defects associated with grain boundaries (GBs) could be identified via positron trapping: (i) vacancy‐like misfit defects situated along GBs and (ii) larger defects at the intersections of at least three GBs (triple points). CDB measurements on CeSZ compacted nanopowders indicated a segregation of Ce ions along GBs. A few percent fractions of positrons were found to form positronium localized in pores of ≈1.8 nm diameter in compacted nanopowders. Sintering of nanopowders at 1500°C appeared to be sufficient for disappearance of pores and triple point defects. In sintered ceramics, contrary to compacted nanopowders, positrons were trapped in zirconium vacancies in grain interiors.     

Positron annihilation and X-ray diffraction studies of cellulose and some derivatives

Positron lifetime measurements are reported over a temperature range from 290 to 380K on cellulose and certain of its derivatives. The lifetime spectra were resolved into three components each attributed to a different mode of positron annihilation in the polymer. The longest lifetime component is associated with the decay of ortho-positronium and is a function of the average cavity distribution in the sample. It is observed that significant differences exist in the lifetimes of positrons in cellulose and its derivatives which can be interpreted as evidence for changes in the free volume distribution in these materials. Comparison of positron decay in samples of cellulose acetate prepared at both room temperature and elevated temperature indicate that the phenomena of gelation in this material is accompanied by a reduction of free volume. The correlation of such observations with the dynamic properties of these polymers is discussed.     

Free‐volume structure of polyvinylpyrrolidone‐capped glassy As2Se3 nanocomposites prepared by mechanical milling

The method of positron annihilation lifetime (PAL) spectroscopy was first employed to study atomic‐deficient free‐volume structure of nanocomposites prepared by high‐energy mechanical milling of glassy g‐As₂Se₃ in water solution of polyvinylpyrrolidone (PVP). The formalism of x3‐x2‐CDA (coupling decomposition algorithm) describing conversion of bound positron‐electron (positronium Ps) states into positron traps was applied to identify free‐volume elements in pelletized PVP‐capped g‐As₂Se₃ nanocomposite in respect to parent dry‐milled g‐As₂Se₃. Under wet milling, the internanoparticle Ps‐decaying sites in preferential PVP environment were shown to replace free‐volume positron traps in dry‐milled g‐As₂Se₃ with defect‐specific positron lifetime of 0.352 ns, corresponding to diatomic/triatomic vacancies in g‐As‐Se matrix. POLYM. ENG. SCI., 59:2438–2442, 2019. © 2019 Society of Plastics Engineers     

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, ZnNb_2?xTi_xO_6?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 τ_b and τ_t which are related to bulk structure and negatively charged defects in the system. The dielectric constant of microwave ceramics ZnNb_2?xTi_xO_6?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.     

立方体AgBrCl硫增感光的电子行为

本文利用微波吸收相敏检测技术 ,同时获得了立方体AgBrCl乳剂中 ,在增感温度增加的条件下 ,自由光电子和浅束缚光电子的时间衰减曲线 ,分析了不同的硫增感产物随增感温度增加所起的陷阱效应。结果表明 ,在立方体AgBrCl乳剂中 ,硫增感初始阶段的增感产物主要是作为浅电子陷阱存在 ;随着增感温度增加至5 5℃时 ,增感产物浅电子陷阱效果最佳 ;如增感温度进一步增加 ,硫增感中心又变为深电子陷阱。文中还讨论了浅电子陷阱中浅束缚光电子衰减时间与阱深的依存关系。     

Observation of Vacancies by Positron Trapping in Cerium-Doped Zirconolites

Positron annihilation lifetime measurements on cerium-doped zirconolites have given strong indications of positron trapping in samples that have previously been deduced to contain cation vacancies, mainly from X-ray absorption spectroscopy, thermogravimetry, and microanalysis. The increases in lifetime of the majority lifetime component are ∼20% for samples deduced to contain a few percent of vacant cation sites. Lanthanum-doped SrTiO₃ samples, in which it is generally agreed that A-site vacancies are produced via the lanthanum doping, have been used as a model system and similar increases in lifetime have been observed for samples that contain more than ∼0.1 formula units of lanthanum.     

Defect structure transformation during substitution in quadruple perovskite CaCu3-xTi4-xFe2xO12 studied by positron annihilation spectroscopy

Positron annihilation spectroscopic studies have been performed to investigate the defect structural evolution in polycrystalline CaCu_3-xTi_4-xFe_2xO₁₂ (x = 0.0, 0.1, 0.3, 0.5 and 0.7) cubic perovskite samples. The positron lifetimes, relative intensities and Doppler-broadened annihilation lineshape parameters all indicate about the annulment of crystalline vacancies from the structure and attending porous ceramic structure at higher concentration of substitution. The compositional dependence of the intensity of the defect specific positron lifetime component clearly suggests that, in the initial stages of substitution, positron annihilates in vacancy-type defects. But, for higher concentrations of Fe³⁺, the materials are devoid of such defects and positron trapping takes place within the pores. The results of coincidence Doppler broadening spectroscopy measurements support such transformation in which the positron trapping sites are relocated to the ceramic pores leading to the modification of the material properties.     

The role of ion-induced substrate damage in thin film adhesion strength

Variable-energy positrons are combined with Auger electron spectroscopy, scanning electron microscopy, and scratch test critical load measurements to study interfacial properties in thin film adhesion. This work complements an earlier investigation of the adhesion strength enhancement produced by ion bombardment of the substrate surface before deposition. In this study, we have investigated SiO₂ films deposited by radio-frequency (RF) sputtering onto stainless steel substrates. Extended ion bombardment etching leads to three related effects: the scratch test critical load is increased significantly, Auger electron spectroscopy shows a greater penetration of the film material into the substrate; and the interfacial positron annihilation signal is dominated by large, open-volume defects. These results are interpreted as confirmation that ion bombardment leads to the formation of microvoids in the interface layer and, consequently, to an increased adhesion strength by allowing mechanical interlocking between the coating and the substrate.     

Gamma-Ray-Induced Coloration in Solids; Application to Glass Systems

General aspects of radiation-induced coloration in solids are discussed. It is considered that the most significant initial effect of the radiation is to produce free electrons and trapped holes in a track. The decay of the track from the initial condition is described in some detail, assuming (a) that the rate of diffusion of electrons from the track is small compared with the rates of other processes and (b) that primary processes which form color centers can be considered to be independent of secondary processes which cause destruction of color centers. A method of calculation is described which gives the concentration of color centers formed at various doses. With plausible assumptions for the parameters, agreement with the data of Schulman et al. on a silver phosphate glass is obtained. The treatment of glasses less simple than the silver phosphate glass is also discussed.     

Brønsted acidic strength in Y-zeolite by positron spectroscopy

Angular correlation spectra of positron annihilation in Y-zeolite adsorbed with bases (ammonia, water, and alcohol) were studied. Results by the technique successfully reveal that there exist two types of Brønsted acid sites with different acidic strengths which require temperature of desorption of the base at about 200 and 400°C.