Positron Annihilation Lifetime Spectra Research Articles

Solid state dispersions of MoO3 into porous γ-Al2 O3

A series of the MoO3/γ-Al2O3 samples is prepared by mechanically mixing MoO3 with γ-Al2O3 powder. The solid state dispersion of the MoO3 in to the porous γ-Al2O3 is studied by SEM, XRD, XPS, TG/DTA, and positron annihilation spectroscopy. The positron annihilation lifetime spectra of the MoO3/γ-Al2O3 are measured as a function of the mass ratio of MoO3, heating temperature, heating time and in a way that the MoO3 is loaded gradually, separately. The experimental results show the process of the thermal dispersion of the MoO3 into the γ-Al2O3 support. It is shown that the MoO3 disperses into the outer surface first, and then into the secondary pores and the micro pores. The gradual Loading can make MoO3 effectively dispersed into γ-Al2O3. Grinding can also affecte the thermal dispersion process.

A New Version of LT Program for Positron Lifetime Spectra Analysis

LT 10 is a new version of LT program designed to analysis of Positron Annihilation Lifetime Spectra (PALS). The new program is equipped with a new user interface and other features unavailable in its previous versions. It has been created again in a modern programming language and is now compatible with the latest operating systems.

Ortho-Positronium Reactions in Water Studied by Positron Annihilation Age-Momentum Correlation

Temperature dependence on S(t), positron age dependent shape parameter of annihilation gamma-rays, in water was observed. S(t) at the ortho-Ps pick off annihilation time region were higher at higher temperatures. It is indicating that there are more reactions of ortho-Ps at higher temperature. Hence it was successfully indicated that the longest lifetime, i.e. ortho-Ps component, on positron annihilation lifetime spectra for water could be shorter at higher temperatures. The radical reactions, such as PsOH formation or ortho-Ps oxidation by hydroxyl radicals, are spin dependent reactions. The geminate pair of ortho-Ps and hydroxyl radical in water can give spin dependent reaction and the enhancement of S(t) with certain time interval, i.e. quantum beats, were expected. It was observed even for water at 11°C. It is probably indicating the hyperfine coupling constants of hydroxyl radicals in water.

Positronium in binary mixture n-nonadecane + n-tricosane at high pressure

Positron annihilation lifetime spectra (PALS) in binary mixture of alkanes were measured as a function of pressure. It was found that after rapid transition from rotator to rigid phase the intensity of positronium formation is not stable in time; its relaxation contains two components differing by an order of magnitude. The appearance of transient rotator phase, analogous to that observed in temperature dependences for the same mixture was not observed. Two-stage increase of spectrum parameters at crossing the rigid–rotator transition by pressure reduction can be an indication of the appearance of an additional phase preceding the rotator.

Investigation on positron annihilation characteristics of CO 2-exposed zeolite

In this study, the lifetime spectra of positron annihilation for CO 2-exposed zeolite 4A at different conditions were measured. The effects of subjecting zeolite 4A to baking treatment and exposing it to CO 2 at different pressures on the positronium (Ps) annihilation characteristics was investigated using positron annihilation lifetime spectroscopy (PALS). Zeolite 4A was chosen as the medium because of its rigidity, stability, and well-understood structure. Its structure consists of cavities that can be classified into two types: α-cage and β-cage. Zeolite 4A at vacuum (or dehydrated) conditions gave the longest lifetime, whereas that at ambient (or hydrated) conditions, at which water molecules are sorbed in the cavities of zeolite 4A, gave the shortest lifetime. The water molecules in zeolite 4A cavities would inhibit the ortho-positronium (o-Ps) formation probability. At lower gas sorption, the CO 2 gas molecules exhibited the pore-filling effect, resulting in a decrease in the o-Ps lifetime. However, at higher CO 2 sorption, the shielding effect in the α-cage increased with CO 2 pressure and led to an increase in the o-Ps lifetime. When the α-cage cavity was filled with the CO 2 gas molecules at pressures greater than 5 atm, o-Ps tended to migrate from the α-cage to the β-cage.

Defect-Mediated Magnetism in Pure CaO Nanopowders

The intrinsic nature of ferromagnetism in CaO nanopowders has been established with the experimental observation of magnetic hysteresis loop at room temperature. On the basis of the results of X-ray diffraction, magnetic properties, positron annihilation lifetime spectra, and first-principles calculations, it is found that there are correlations among the lattice constant, the defect concentration, and the magnetization of CaO nanopowders, which suggests that Ca defects are the main reason for the magnetic order and the defect concentration is related with the ferromagnetism. Such a ferromagnet without the presence of any transition metal could be a very good option for a class of spintronics.

Defect structure and electrical conduction behavior of Bi-based pyrochlores

Abstract Pyrochlore-type quaternary systems with the generic formula (Bi1.5Zn0.5)(Nb0.5M1.5)O7 (BZNM, M=Ti, Sn, Zr, and Ce) have been synthesized by the standard solid-state reaction route. The study of positron annihilation lifetime spectra provides direct evidence for defect characterization for BZNM ceramics with different tetravalent element. The variation of the annihilation lifetime suggests that the defect structure undergoes significant changes. The defect structure is further described by means of the complex-defect model. Electrical conduction in the samples is expected to result mainly from the defects present in the lattice. Conduction measurement shows there are different conduction mechanisms. For temperatures less than 350 °C, electronic conduction dominates, while for temperatures greater than 350 °C, ionic conduction is dominant in the present system. The conduction mechanism includes the formation of charged defect dipoles, causing internal bias fields, creating oxygen vacancies that promote ionic conductivity.

Free-volume structure of fluoropolymer-based radiation-grafted electrolyte membranes investigated by positron annihilation lifetime spectroscopy

In the field of polymer-electrolyte-membrane (PEM) fuel cell technology, the structures of free-volume holes in the PEMs are very important because they are correlated to the supplied-gas crossover phenomenon, which sometimes deteriorates the cell performance. In this study, we investigated the size and location of free-volume holes in the crosslinked-polytetrafluoroethylene (cPTFE) based radiation-grafted PEMs by positron annihilation lifetime (PAL) spectroscopy. For comparison, the base cPTFE and polystyrene grafted films were also measured. From the analysis of PAL spectra, it was found that there were free-volume holes with different radius of 0.28-0.30 nm and 0.44-0.45 nm. The smaller holes should be located in both PTFE crystallites and poly(styrene sulfonic acid) grafts, while the larger holes are considered to exist in amorphous PTFE phases.

Porosity evolution of VP-DVB/MCM-41 nanocomposite

The porous structure of nanostructured vinylpyrrolidone-divinylbenzene (VP-DVB)/MCM-41 composite was characterized using N 2 adsorption at 77 K and positronium annihilation lifetime spectroscopy (PALS), atomic force microscopy (AFM) and Raman spectroscopy. Positron annihilation lifetime spectra were measured during outgassing procedure. The voids of differentiated dimensions were detected in the composite material. The number of free volumes and their dimensions depend on the degree of evacuation of volatile components from the samples.

Positronium formation in a liquid phase: Influence of intratrack reactions and temperature

AbstractThis work clarifies some discrepancies between conventional models, used in positron annihilation lifetime (PAL) spectroscopy, and experimental data. The importance of taking into account Ps interactions with primary intratrack radiolytic products is demonstrated, for example, for PAL data in water. We examine the applicability of a nonhomogeneous chemical kinetics approach and of a well‐known expression for the reaction rate constants to describe intratrack processes, and show that the prescribed diffusion approximation (PDA) is a reasonable model for this. Moreover, we have derived an analytical expression for the time‐dependent reaction rate coefficient, which previously entered PDA as an external parameter. Preliminary fittings of the PAL spectra in H2O over a wide temperature range indicate that Ps reactions in intratrack processes are diffusion controlled (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Extended free‐volume defects in chalcogenide glassy semiconductors induced by high‐energy γ‐irradiation

AbstractIt was shown that under‐coordinated topological defects induced by high‐energy γ‐irradiation can be a reason for significant changes in positron annihilation lifetime spectra of multicomponent chalcogenide glassy semiconductors within ternary Ge‐As(Sb)‐S systems. In the case of negatively‐charged sulphur and arsenic atoms, the excess of free volume is quite enough to produce additional input in the second defect‐related channel of positron trapping, while under‐coordinated germanium atoms are practically non‐detectable with this technique because of low associated free volume. Despite radiation‐induced densification, the average positron lifetime demonstrate both growing and decaying tendencies after γ‐irradiation depending on glass composition. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Defect configurations in Ge–S chalcogenide glasses studied by Raman scattering and positron annihilation technique

Abstract Defect configurations of Ge–S binary glasses have been studied systemically by Raman scattering technique and positron annihilation lifetime spectra (PALS). The correlations between the positron lifetime data, structural features, and chemical compositions of Ge–S binary glasses have been established, and also the identification of open volume originated from coordination defects has been carried out. The cognizance of defect configuration will be very helpful to further understand the unique photosensitivity of chalcogenide glasses.

Dielectric characteristics and positron annihilation study of Eu 2O 3-doped CaCu 3Ti 4O 12 ceramics

Eu 2O 3-doped CaCu 3Ti 4O 12 ceramics were sintered using traditional solid-state reaction method. The microstructure and dielectric characteristics of samples were measured and their interrelation was analyzed. The results indicate that a small quantity of Eu 2O 3-doped in CCTO can deter the formation of Cu-enriched phase and improve the structure of crystal grains, which is of help to improve the thermal and frequency stability of dielectric properties. Positron annihilation lifetime spectra measurements indicate that a small quantity of Eu 2O 3 doping in CCTO can reduce the defect concentration and thin grain boundary layers in the material. The dielectric characteristics of CCTO have been explained in terms of the defects in the material. It is concluded that the microstructure and the dielectric properties of CCTO can greatly be improved by doping with 0.2 wt% of Eu 2O 3. The ramifications of current study could be very useful in applications of this important material.

Structural analysis of rapidly solidified Mg–Cu–Y glasses during room-temperature embrittlement

The structural changes of rapidly-quenched amorphous Mg–Cu–Y alloys during time-dependent room-temperature embrittlement were investigated. A continuous reduction in enthalpic content with time, corresponding to a loss in free volume, and radial atomic distances obtained from synchrotron X-ray diffraction experiments imply an overall reduction in volume. Two distinct lifetimes were found by positron annihilation lifetime spectra, with the longer-lifetime component (attributed to areas of larger free volume) vanishing after an ageing time corresponding roughly to the time of embrittlement. Coincidence Doppler broadening suggests that the immediate environment of the annihilation sites is not altered during ageing. The mechanical behaviour correlates with the structural changes in the alloy. The densification observed is assumed to influence the sharp ductile-to-brittle transition in these Mg-based glasses, and the abrupt change in bending ductility is found to result from disappearing large free volume.

Positron study of carbon-Fe3O4 coaxial nanofibers

This paper presents the positron study on carbon-Fe3O4 coaxial nanofibers, which is one kind of new promising functional materials. According to the positron annihilation lifetime spectra and the coincidence Doppler broadening spectra, we find in this material that positrons annihilate partly in monovacancies of Fe3O4, partly in microcavities of Fe3O4 and partly in shelled carbon nanotubes. We also estimate the annihilation proportion in each part, which reveals the nanofibers’ microstructure to a certain extent.

The Effect of Anneal on PP and PA Blends Studied by Positron

The anneal effect of PP/PA/MMT blends had been studied by positron annihilation lifetime spectra (PALS). The o-Ps lifetime and intensity of unannealed blends were linearly additive basis on the PA weight fraction, which was coincident to the simple compound rule. However, the o-Ps intensity of the samples decreased after samples annealed.

A Novel Approach to the Interpretation of PAL Spectra in Glycerol

A new strategy for the treatment of positron annihilation lifetime (PAL) spectra in viscous liquids is proposed, enabling to extract values of the Ps reaction rate constants with intratrack radiolytic products as well as parameters of the free volume distribution in viscous media.

Influence of Temperature on Intratrack Processes and Ps Formation and Behaviour in Liquid Water

Positron annihilation lifetime (PAL) spectra are measured in liquid water in the temperature range 2 – 930C. The spectra are treated by taking into account intratrack reactions and assuming that radical reactions with Ps are diffusion-controlled (the respective temperature dependences obeying the Stokes-Einstein law). Equilibrium Ps bubble parameters are obtained.

Positron Annihilation in Carbon Nanotubes

Positron annihilation lifetime spectra have been measured in carbon nanotubes being pressed as a function of pressure up to 1536MPa. In addition, positron lifetime experiments for carbon nanotubes in vacuum, nitrogen and air have been performed respectively. Lifetimes have been obtained using LIFETIME program. The results display a single-component positron annihilation lifetime. Positron lifetime for carbon nanotubes decreases as the pressure increases, but lifetime is basically consistent after the pressure of 960MPa. Positron annihilation lifetime for carbon nanotubes in air is the shortest whereas the lifetime in vacuum the longest. We conclude that a positron annihilates with an electron on the external surface of carbon nanotubes.

Positron Annihilation Lifetime Study of Vacancies in Nb-Doped Rutile

Nb-doped rutile produced by sintering at 1400°C in an air atmosphere led to a monotonic increase of the shortest τ1 lifetime of the major spectral component from 0.17 to 0.25 ns in the 22Na positron annihilation lifetime spectrum as the Nb doping level was progressively increased to 0.12 formula units, at which stage TiNb2O7 was observed as a second phase. The charge compensators for Nb5+ substituting for Ti4+ are indicated as cation vacancies on the basis of the increase of τ1.