Positron Annihilation Technology Research Articles

Structure, vacancy, and physical properties of non-stoichiometric MnWO4 ceramics

Non-stoichiometric ceramic samples of Mn0.97WO4 and MnW0.97O4 were prepared by a solid-state reaction to study the effects of vacancies at different lattice positions on the structure and properties of MnWO4 materials caused by non-stoichiometric. Positron annihilation technology was introduced to study the vacancy characteristics of the MnWO4, Mn0.97WO4, and MnW0.97O4 ceramics. The results indicated that non-stoichiometric did not affect the monoclinic lattice structure of any of the MnWO4 samples, and had no obvious effect on the lattice parameters of the samples. The analysis of positron annihilation lifetime spectroscopy showed that W vacancies were the main defects in MnW0.97O4 samples, and Mn vacancies were favored in Mn0.97WO4 samples, the vacancies concentration for non-stoichiometric MnWO4 samples was higher than that of stoichiometric MnWO4. It was observed that the magnetization and transition temperatures of the non-stoichiometric MnWO4 system were increased, and the temperature range in which AF2 phase existed was enlarged. This study demonstrated that non-stoichiometric could enhance the magnetic properties of MnWO4, the effect of Mn vacancy on the properties of MnWO4 is higher than that of W vacancy. The intimate correlation between the vacancy defects and magnetic properties provides us strong evidence that the magnetic properties of MnWO4 can be enhanced by regulating vacancy defects.

Research progress of coincidence Doppler broadening of positron annihilation measurement technology in materials

Positron annihilation technique is an atomic-scale characterization method used to analyze the defects and microstructure of materials, which is extremely sensitive to open volume defects. By examining the annihilation behaviour of positrons and electrons in open volume defects, local electron density and atomic structure information around the annihilation site can be obtained, such as the size and concentration of vacancies, and vacancy clusters. In recent years, positron annihilation spectroscopy has evolved into a superior tool for characterizing features of material compared with conventional methods. The coincident Doppler broadening technique provides unique advantages for examining the local electronic structure and chemical environment (elemental composition) information about defects due to its effectiveness describing high momentum electronic information. The low momentum portion of the quotient spectrum indicates the Doppler shift generated by the annihilation of valence electrons near the vacancy defect. Changes in the peak amplitudes and positions of the characteristic peaks in the high momentum region can reveal elemental information about the positron annihilation point. The physical mechanism of element segregation, the structural features of open volume defects and the interaction between interstitial atoms and vacancy defects are well investigated by using the coincidence Doppler broadening technology. In recent years, based on the development of Doppler broadening technology, the sensitivity of slow positron beam coincidence Doppler broadening technology with adjustable energy has been significantly enhanced at a certain depth. It is notable that slow positron beam techniques can offer surface, defect, and interface microstructural information as a function of material depth. It compensates for the fact that the traditional coincidence Doppler broadening technique can only determine the overall defect information. Positron annihilation technology has been applied to the fields of second phase evolution in irradiated materials, hydrogen/helium effect, and free volume in thin films, as a result of the continuous development of slow positron beam and the improvement of various experimental test methods based on slow positron beam. In this paper, the basic principles of the coincidence Doppler broadening technique are briefly discussed, and the application research progress of the coincidence Doppler broadening technique in various materials is reviewed by combining the reported developments: 1) the evolution behaviour of nanoscale precipitation in alloys; 2) the interaction between lattice vacancies and impurity atoms in semiconductors; 3) the changes of oxygen vacancy and metal cation concentration in oxide material. In addition, coincident Doppler broadening technology has been steadily used to estimate and quantify the sizes, quantities, and distributions of free volume holes in polymers.

Characteristics of Solar Wind Radiation Damage in Lunar Soil: PAT and TEM Study

Irradiation structural damage (e.g., radiation tracks, amorphous layers, and vesicles) is widely observed in lunar soil grains. Previous experiments have revealed that irradiation damage is caused by the injection of solar wind and solar flare energetic particles. In this study, cordierite and gabbro were selected as analogs of shallow and deep excavated lunar crust materials for proton irradiation experiments. The fluence was 1.44 ± 0.03 × 1018 H+/cm2, which is equivalent to 102 years of average solar wind proton implantation on the Moon. Before and after irradiation, structural damage in samples is detected by slow positron annihilation technology (PAT), Doppler broadening (DB) measurement, focused ion beam (FIB), and transmission electron microscopy (TEM). The DB results showed the structural damage peaks of irradiated gabbro and cordierite were located at 40 and 45 nm. Hydrogen diffused to a deeper region and it reached beyond depths of 150 and 136 nm for gabbro and cordierite, respectively. Hydrogen atoms occupied the original vacancy defects and formed vacancy sites—hydrogen atom complexes, which affected the annihilation of positrons with electrons in the vacancy defects. All of the DB results were validated by TEM. This study proves that the positron annihilation technique has an excellent performance in the detection of defects in the whole structure of the sample. In combination with TEM and other detection methods, this technology could be used for the detection of structural damage in extraterrestrial samples.

Magnetic and structural properties of Fe-implanted GaN at room temperature

GaN grown by metal-organic chemical vapour deposition on the sapphire substrate was implanted with 3 MeV Fe10+ ions to fluences of 1 × 1013 to 5 × 1015 ions/cm2 at room temperature. The change of magnetic property with the implantation fluence was measured by vibrating sample magnetometer. The structural features, including implantation-induced interstitials and vacancies, were studied by a combination of Raman spectroscopy, photoluminescence spectroscopy, transmission electron microscopy and positron annihilation technology. The ferromagnetic behaviour was observed in the as-grown sample, and the saturation magnetization reached the maximum after the Fe implantation to a fluence of 5× 1013 ions/cm2. With increasing fluence, it decreased to be even smaller than that of the as-grown sample. The possible reasons are discussed.

Research progress of hydrogen/helium effects in metal materials by positron annihilation spectroscopy

An important feature of the irradiation process in nuclear system is the formation of large displacement cascades, in which primary knock-on atoms and secondary particles formed by nuclear reactions generate a considerable number of defects such as dislocations, vacancies and transmutation gases. Predicting and mitigating the adverse effects of damage defect and transmutation hydrogen/helium produced by high-dose neutron irradiation on the mechanical properties of structural materials is the most significant challenge facing the current development of nuclear energy. To solve this problem, understanding the interaction mechanism between hydrogen/helium atoms and micro-defects is a very important breakthrough. Precursors of helium/ hydrogen bubble, small helium/hydrogen-filled vacancy complexes, may play an important role in realizing bubble nucleation, and the formation of these complexes is affected by many factors. However, only a little information about helium/hydrogen-vacancy clusters’ behavior has been obtained in metal/alloy materials. This is mainly limited by the characterization methods, such as the limited resolution of transmission electron microscope (TEM). Helium/hydrogen-vacancy clusters cannot be observed by TEM before the formation of helium bubbles. Applications of positron annihilation to the study of crystal lattice defects started around 1970s, when it was realized that positron annihilation is particularly sensitive to vacancy-type defects and that annihilation properties manifest the nature of each specific type of defect. In recent years, with the continuous development of slow positron beam and the improvement of various experimental testing methods based on slow positron beam, the application of positron annihilation technology has been extended to the research field of hydrogen/helium behavior in metal materials, which plays an important role in studying the hydrogen/helium radiation damage to metal materials. In this review, the basic principles of positron annihilation spectroscopy are briefly discussed and the three most important measurement methods used for hydrogen/helium effect studies are described (i.e. positron annihilation lifetime spectroscopy (PALS), Doppler broadening spectroscopy (DBS), coincidence Doppler broadening spectroscopy (CDBS)). In this paper, the application of positron annihilation spectroscopy to the study of hydrogen/helium behavior in metal materials is reviewed in combination with the reported relevant developments (including our research group’s achieve-ments). The advantages of three commonly used measurement methods in the following specific studies are highlighted: 1) The estimation of bubble size and concentration; 2) irradiation damage induced by hydrogen/helium; 3) the evolution behavior of irradiation-induced defects in the heat treatment process; 4) sy-nergistic effect of hydrogen and helium.

Simulation for the correlation of positron annihilation rate with charge density near defects in iron

Positron annihilation technology is a nondestructive detection technique to study the microscopic defects in materials. It is well known that the micro defects will redistribute the charge in materials. In this paper, the charge density of helium in iron was calculated by the Vienna Ab initio Simulation Package (VASP) and the positron annihilation parameters were obtained by Doppler program. The results demonstrated that different types of defects in materials will cause a great variety in charge density, which lead to the change of positron annihilation parameters. In a vacancy cluster containing 6 monovacancies, the positron annihilation lifetime decreases rapidly once the number of helium is greater than the number of vacancies.

Effects of electrochemical corrosion characteristis on electrochemical cold drawing of Q235 steel bar

In this work, the electrochemical corrosion behaviors of Q235 steel bar in tap water, NaCl, NaCl+HCl and H2SO4 aqueous solutions, and their effects on electrochemical cold drawing (ECD) were investigated, as well as on cold drawing in air for comparison. The results indicate that the higher the corrosion rate and the more uniform the corrosion are, the larger the decrement of drawing force of ECD is, and the drawing force in the H2SO4 solution is decreased by 39% compared with DIA. Previous electrochemical corrosions in these solutions can slightly soften the steel surface layer, but further obviously decrease the drawing force of ECD, and these two phenomena are all proportional to the corrosion rate. The surface softening of the previous electrochemical corrosions is verified by positron annihilation technology to be mainly attributed to the formation of vacancies. The further decrease of the drawing force should be chiefly contributed to the relaxation effect of the already formed vacancies on dislocations. But the effects of the electrochemical corrosions occurred during ECD are very limited because the time for corrosions is quite short. So the decrease in drawing force might be ascribed to the additional dislocation flux originated from the simultaneous effects of current, chemical corrosion and stress during ECD.

正电子湮没技术及其应用

正电子湮没技术是研究物质结构和电子状态的无损伤的探测分析手段。介绍了正电子湮没技术基本原理、基本实验方法，结合正电子湮没技术在材料微观结构领域的独特优势，介绍了其在材料微结构研究工作中的特点及优势，综述了近几年来正电子湮没技术的发展及应用。 Positron annihilation technology (PAT) is the noninvasive detection method of the material structure and the electronic states. In this paper, firstly the introduction of the PAT was given by describing fundamental principles and elucidating characteristic advantages of the PAT in micro structure analysis of materials. Then the features and advantages of the PAT in characterization of micro structures of materials were reviewed. In addition, the newly developments and applications in recent years of the PAT were reviewed.

Study on Enhancement Mechanism of Conductivity Induced by Graphene Oxide for Polypyrrole Nanocomposites

Polypyrrole (PPy)/graphene oxide (GO) nanosheet composites with different GO content have been successfully prepared. The morphology, microstructure, defect property and conducting mechanism were examined by Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), field-emission scanning electron microscope (FE-SEM), X-ray photoelectron spectra (XPS), positron annihilation technology (PAT), and electrical conductivity measurements for PPy/GO conducting nanocomposites, respectively. Experimental results indicated that PPy was deposited onto the GO surface homogeneously. Four orders of magnitude increase in electrical conductivity was successfully achieved with small quantities of GO compared to pristine PPy. In order to elucidate the conducting mechanism, an electron transfer model was used to describe bipolaronic formation, which can be confirmed by XPS and positron annihilation parameters measured including positron annihilation lifetimes, positron annihilation Doppler broadening spectroscopy ...

Effect of Heat Treatment on Positron Annihilation Lifetime of an Extruded Al-12.7Si-0.7Mg Alloy

Positron annihilation lifetime can reflect the information of electronic density. Therefore this technique is very sensitive to the vacancies. Therefore positron annihilation technology (PAT) is a non-destructive testing technology for the investigation on the vacancy in pure metals and alloys. The vacancy defects of as-cast pure aluminium and the extruded Al-12.7Si-0.7Mg alloy before and after heat treatment were investigated by positron annihilation lifetime spectra. The results of positron annihilation lifetime spectra show that the lifetime of free positron annihilation in the pure aluminium alloy is 155 ps. The positron annihilation lifetime of the extruded sample is longer than the lifetime in the pure aluminium due to the formation of grain boundary and dislocation during the extrusion process. The positron annihilation lifetime of the extruded sample after heat treatment is shorter than that before the heat treatment, and it is longer than that of the pure sample.

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.

Surface characteristics of titanium deuteride film implanted with deuterium ion beam

Behaviours of titanium deuteride targets have been studied experimentally by implantation with 150 kV and 210 kV deuterons extracted from an electrostatic accelerator. Surface morphologies, phase structures and irradiation defects are investigated by scanning electron microscope, X-ray diffraction analysis and slow positron annihilation technology. It is found that surfaces of titanium deuteride targets undergo a significant selective melting. Although there must be a prominent desorption of D atoms from the targets, we have not, in diagrams of X-ray diffraction analysis, observed compelling evidence of phase transformation. Running of the neutron generator is fairly stable, such that the neutron generation rate is almost not altered. Experimental profiles of irradiation defects with slow positron annihilation technique are compared with those obtained by SRIM, and the correlations between the two are discussed.

Study on Cd vacancy in CdZnTe Crystal by Positron Annihilation Technology

Cd vacancies in cadmium zinc telluride(CdZnTe) crystals have an important effect on the crystal properties. In this paper, position distribution and concentration change of Cd vacancy in CdZnTe crystal grown by the temperature gradient solution growth (TGSG) were investigated by positron annihilation technology (PAT), which was based on the potential energy distribution and probability density of the positron in the crystal. The results showed that, the density of Cd vacancy increased obviously from the first-to-freeze to stable growth of the ingots, while decreased along the radial direction of the ingots.

AMOC Positron Annihilation Study of Zwitterionic Nanofiltration Membranes: Correlation between Fine Structure and Ultrahigh Permeability

Age–momentum correlation (AMOC) positron annihilation technology was applied for the first time to composite nanofiltration membranes (NFMs) with zwitterionic terpolymers (TPs) to explain their high permeability and high salt selectivity. The NFMs showed an increase in the water flux from 32.6 to 46.3 L m–2 h–1 without sacrificing the salt selectivity when the zwitterionic monomer 3-dimethyl(methacryloyloxyethyl)ammonium propanesulfonate (DMAPS) content was increased from 0 to 4.35 mol %. The influence of DMAPS content on membrane microstructure, water permeability, and salt selectivity was evaluated by means of AMOC positron annihilation studies. Positron annihilation results showed that the cross-linked TPs selective layer of NFMs was composed of two types of pores: free volume (based on τ3) with radius of about 3.5–3.1 A and nanocavity (based on τ4) with radius of 16.6–46.6 A. The performance of NFMs depends strongly on the free volume and nanocavity size. The effect of the zwitterionic DMAPS content o...

Investigation of free volume and damping property for polycarbonate/multiwalled carbon nanotube composites by positron annihilation technology

AbstractMicrostructure and damping characteristics were investigated for polycarbonate/multiwalled carbon nanotube (PC/MWCNT) composites. Dynamic mechanical analysis results show that the damping factor and the value of the energy loss fraction w are significantly increased. Especially, nearly 300% improvement in damping factor is observed in the temperature range from 140 to 150°C. Positron annihilation lifetime measurements indicate that both the o‐Ps lifetime and the free volume increase with increasing MWCNT content, leading to decrease in the glass transition temperature and increase in the damping properties. The relationships among the damping, mechanical property, and the free volume have been first observed, that is, the increase of the free volume brings about a reduction in tensile strength and an increase in damping. The interfacial friction slipping of MWCNTs and the free volume play an important role in determining the damping property of PC/MWCNT composites. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Superconductivity Without Relation to Valence Electrons in (Fe, Zn) Doped YBCO Systems

We systematically investigated YBa2Cu3−x(Fe, Zn)xO7−δ (x=0.0∼0.5) cuprates through XRD, the test of superconductivity, oxygen contents, positron annihilation technology and simulated calculations. With the Fe and Zn doped concentration increasing, the transition temperature Tc drops from 92 K for the undoped YBa2Cu3O7−δ. The form of the Fe or Zn ion cluster in both doping samples illuminates the variation of the oxygen content. The experimental results and theoretic calculations support such a conclusion that the transition temperature Tc does not depend directly on the density ne of the valence electron in the samples.

Superconductivity without dependence on valence electron density in (Al, Zn, Co) doped YBCO systems

We have used x-ray diffraction, oxygen content, positron annihilation technology, and simulation methods to investigate systematically the cuprates YBa2Cu3−x(Al,Zn,Co)xO7−δ (x=0–0.5). The experimental results and simulation calculations support the existence of a cluster effect. Moreover, it is concluded that the cluster effect is an important factor in the suppression of superconductivity and that Tc does not depend directly on the valence electron density in the samples.

Investigation of Interfacial Interaction and Microstructure for Epoxy/CNTs Nanocomposites by PALS

Epoxy nanocomposites with multi-walled carbon nanotubes (MWNTs) were prepared. The interaction between MWNTs and epoxy matrix and microstructure were systematically investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and the positron annihilation technology (PAT) respectively. The glass transition temperatures were determined by the variation of the free volume size.

The structural properties of B–O codoped diamond films

Using Raman spectroscopy and positron annihilation technology (PAT), we investigate the structural properties of the codoped samples by implanting boron and oxygen ions into the intrinsic diamond films (called BO series) and by implanting oxygen ions into the diamond films doped with small amounts of boron in chemical vapor deposition (called CVDBO series). It is found that after 1000 °C annealing, the full width at half maximum (FWHM) value of diamond peak in Raman spectrum reduces, the amount of diamond increases above 99.6% and the stress changes from compression to tension. More important, the FWHM value in CVDBO series decreases by 1.6 cm − 1 after 1000 °C annealing, which is larger than that in the BO series with a decrease of 0.2 cm − 1, showing that the annealing prefers to more significantly reduce the defect concentration in CVDBO series. Also, the PAT measurements indicate that the S n value of CVDBO series is smaller than that of BO series after 800 °C annealing, suggesting that CVDBO series has lower defect concentrations. It is revealed that the co-doping that implanting oxygen ions into the low concentration B-doped diamond films can give a better restoration of the damaged diamond lattice by oxygen ion implantation after high temperature annealing. The intrinsic mechanism is also discussed.

Effect of Surface Decoration of CNTs on the Interfacial Interaction and Microstructure of Epoxy/MWNT Nanocomposites

Epoxy resin nanocomposites with different contents of multiwalled carbon nanotubes (MWNTs) are prepared. The interaction between MWNTs and the epoxy resin matrix and the microstructure and mechanical properties of the composites are systematically investigated by Fourier-transform infrared (FTIR) spectroscopy, transmission electron microscopy, scanning electron microscopy, and positron annihilation technology. FTIR spectra reveal that two kinds of hydrogen bonds exist at the interface for the nanocomposites modified by amine, one between the epoxy group on the side chain and the NH group, and the other between the epoxy group on the alicycle and the NH group. Compared to unmodified MWNT composites, the modified MWNT composites possess better mechanical properties, which are attributed to stronger interfacial interaction resulting from an efficient load transfer from matrix to MWNTs. Positron annihilation lifetime spectroscopy is used to characterize the microstructure of the epoxy/MWNT composites. The subtransition and glass transition temperatures are determined by finite-term positron lifetime analysis and the variation of the free-volume size as a function of temperature. Shifts of structure transition temperatures of the composites are observed with increasing MWNT weight content. Interestingly, the continuous lifetime analysis reveals the existence of two long-lived lifetime components above the glass transition temperature, which may be attributed to the formation of local ordered regions related to the packing density of chains.