Positron Annihilation Lifetime Data Research Articles

Characterization and evaluation of Nafion HP JP as proton exchange membrane: transport properties, nanostructure, morphology, and cell performance

In this study, a novel membrane (Nafion HP JP) was studied and investigated intensively for use as a proton exchange membrane for fuel cell. A standard membrane, Nafion NRE212, was also studied under the same conditions for comparison. Fourier transform infrared spectroscopy, thermogravimetric analysis, wide-angle x-ray diffraction, atomic force microscope (AFM), proton conductivity, and mechanical test were used to study the structure and properties of the membranes. In addition, methanol permeability was measured as a function of methanol concentration. Nafion HP JP membrane exhibited a lower methanol permeability over the concentration range studied. Furthermore, Nafion HP JP has Young’s modulus of 88.3 MPa and tensile strength of 13.2 MPa which are higher than those of Nafion NRE212 (39.85 and 9.5 MPa, respectively). AFM data confirmed the higher value of water uptake for Nafion HP JP membrane. Free volume distribution data measured using positron annihilation lifetime (PAL) showed that the novel membrane has a smaller free volume hole size than that of Nafion NRE212. Moreover, the cell performance of the single cell test using the investigated membrane, hydrogen as fuel and oxygen as oxidant, was investigated at different temperatures and humidities. The power density delivered using Nafion HP JP is higher than that from Nafion NRE212 especially at 50 °C and 30% relative humidity. In addition, the membrane durability of both samples was performed for about 220 h and Nafion HP JP has an open circuit voltage reduction rate of 0.136 mV/h that nearly half that for Nafion NRE212 membrane. The optimal concentration for direct methanol fuel cell DMFC operation using both membranes was found to be 2 M which totally agrees with methanol permeability and PAL data. The obtained results reflect that Nafion HP JP shows great potential for fuel cell applications.

Defects and solarization in YAG transparent ceramics

Transparent ceramics are emerging as future materials for lasers, scintillation, and illumination. In this paper, an interesting and surprising phenomenon in YAG transparent ceramics is reported. UV light leads to significant changes in the microstructure of open volume defects and nano clusters as well as in the optical properties. Light-induced lattice relaxation is suggested as the mechanism behind this intriguing behavior. The complex F-type color center with broad absorption bands is caused by the aliovalent sintering additives (Ca2+/Mg2+) and Fe ion impurities. Two individual peaks in the thermoluminescence spectra illustrate both shallow and deep level traps. From positron annihilation lifetime data, vacancy clusters and nanovoids are detected and characterized, although these free-volume defects could not be observed by high-resolution transmission electron microscopy. The solarization induced by UV irradiation is associated with a change in the structure and size of defect clusters due to lattice relaxation. Therefore, this work shows how UV irradiation leads not only to a change in the charge state of defects, but also to a permanent change in defect structure and size. It significantly affects the optical properties of YAG ceramics and their performance in lasers and other optical applications. These results are crucial for advancing transparent ceramics technology.

Tracking free volume changes in bisphenol-a based polycarbonate sheets after treatment with liquid acetone

Free volume properties, size and distribution, in the bisphenol-a based polycarbonate (PC) sheets were investigated as a function of the treatment duration by liquid acetone using the positron annihilation lifetime (PAL) spectroscopy. Along with increasing the PC treatment duration, the free volume sizes decreased and the holes-distribution broadened, which in turns led to the polymer crystallization induction. From the PAL and WAXD data, it has been confirmed that the crystallization capability of the amorphous PC tremendously enhanced, with a rapid increase of the PC crystallinity along with increasing the treatment duration by liquid acetone. These findings were further confirmed by the surface chemistry and topography characterization techniques which revealed the formation of crystalline structure after liquid acetone treatment. The solvent-induced crystallization of PC sheets promoted restrictions to the infrared induced molecular vibrations the surface functional groups which was reflected as drop in peaks intensities and shift of the carbonyl peak of the Fourier transform infrared (FTIR) spectrum.

Microstructure characterization of multifunctional As4S4/Fe3O4 nanocomposites prepared by high-energy mechanical milling

Multifunctional As4S4/Fe3O4 nanocomposites prepared by high-energy mechanical milling are probed by complementary methods of positron annihilation lifetime (PAL) spectroscopy, supported by microstructure characterization using X-ray powder diffraction (XRPD) with analysis applied to the first sharp diffraction peak (FSDP), morphology studies by transmission electron microscopy (TEM) with energy-dispersive spectroscopy (EDS) and high-resolution TEM (HRTEM).These nanocomposites are shown to consist of Fe3O4 crystallites with particle sizes of 8–21nm, and far-separated β-As4S4 crystallites surrounded by amorphous As-S phase. In respect to PAL data, the effect of milling is identified as possible conversion from Ps traps to positron-trapping sites depending on preferential chemistry of atomic surrounding. So, the interfacial triple junctions at the intersections of Fe3O4 crystallites are identified as principal positron traps in As4S4/Fe3O4 nanocomposites with competitive influence from free-volume defects of amorphous As-S phase.

Nanostructurization effects in PVP-stabilized tetra-arsenic tetra-sulfide As4S4 nanocomposites

Nanostructurization in three types of tetra-arsenic tetra-sulfide As4S4 polymorphs composed by (1) preferential β-As4S4, (2) realgar α-As4S4 and (3) admixture of As4S4 in the form of β-realgar, pararealgar and intermediate χ-phase subjected to mechanochemical ball milling in a water solution of polyvinylpyrrolidone (PVP) are characterized with multiexperimental structure-sensitive probes. Void structure of pelletized As4S4-PVP nanocomposites is probed by positron annihilation lifetime spectroscopy complemented with atomic-sensitive techniques such as X-ray powder diffraction, Raman scattering and scanning electron microscopy with energy-dispersive X-ray spectroscopy. Appearance of arsenolite As2O3 crystallites is character for all As4S4 polymorphs affected by wet milling. Positron annihilation lifetime data considered in terms of substitution positron-positronium trapping confirm complicated nature of nanocomposites. Modified x3-x2-coupling decomposition algorithm developed in addition to unconstrained x3-term fitting procedure is applied to parameterize annihilation channels in these nanocomposites. Interfacial free-volume voids between neighboring nanoparticles in PVP environment are defined as most favorable trapping sites, they being rather loosely composed by As4S4 crystallites in full respect to variety of crystallographic polymorphs used for milling. Detected annihilation channels are ascribed preferentially to intrinsic free-volume voids insensitive to trapping states at the surface of oxidized nanocomposites.

Early processes in positron and positronium chemistry: possible scavenging of epithermal e+ by nitrate ion in aqueous solutions

Positron ionization slowing down, formation of the positron track, reactions of e+ with track species and its interaction with a scavenger on a subpicosecond timescale, including the process of the positronium formation process are discussed. Interpretation of the positron annihilation lifetime data on positronium formation in aqueous solutions of NO−3 anions, known as efficient scavengers of the presolvated track electrons, suggests that these ions may also capture epithermal (presolvated) positrons as well.

Perspective of Positron Annihilation Spectroscopy in Polymers

Positron annihilation spectroscopy (PAS) is a novel method that provides molecular-level information about complex macromolecular structure in a manner different from, but complementary to, conventional physical and chemical methodology. This paper presents a perspective of PAS in polymeric systems covering 12 aspects: historical, spacial, spherical quantum model, anisotropic structure, voids, positronium chemistry, time, positron annihilation lifetime spectroscopy and data analysis, variable monoenergetic slow positron beam techniques and depth profiling, elemental analysis, multidimensional instrumentation advances in PAS, and free volume and free-volume theories.

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.

Free volume dilatation in polymers by ortho-positronium

The possibility of positronium induced free volume cavity expansion in some polymers above the glass transition temperature was investigated using experimental positron annihilation lifetime data from the literature for polydimethylsiloxane, polyisobutylene, and polybutadiene as function of temperature. The results suggest that free volume sites can expand towards an equilibrium size, determined as the equilibrium Ps-bubble size defined earlier for low-molecular-weight liquids. The expansion can be explained by the increase of molecular mobility and hence decrease of relaxation times, which at the higher temperatures approach the o-Ps lifetimes. Nanoscale viscosities were estimated using Navier-Stokes equation and were found to be several orders of magnitude lower than the macroscopic viscosity at the same temperature.

Defect evolution during annealing of deformed FeSi alloys studied by positron annihilation spectroscopy

High silicon steel is widely used in electrical appliances. Alloying iron with silicon improves its magnetic performance. A silicon content up to 6.5 wt. % gives excellent magnetic properties such as high saturation magnetization, near zero magnetostriction and low iron loss in high frequencies. Their workability is greatly reduced by the appearance of ordered structures, namely B2 and D03, as soon as the Si content becomes higher than 3.5 wt. %. This limits the mass production by conventional rolling to this maximum percentage of Si. In this work a series of FeSi (7.5 wt. % Si) samples with different degrees of deformation are investigated with positron annihilation spectroscopy and optical microscopy (OM). The influence of annealing on the concentration of defects of different deformed FeSi alloys has been investigated by positron annihilation lifetime spectroscopy and Doppler broadening of the annihilation radiation. OM is used to investigate the microstructure of deformed samples before and after annealing. The values of the S parameter present a decrease for all studied FeSi alloys with the increase of the annealing temperature, being attributed to a decrease of the concentration of defects. A sudden increase of the S-parameter value at 600 °C was observed for all samples, which could be related to the change of the ordering of the FeSi alloys at that temperature. At 700 °C, the values of the S parameter decreased drastically and starting from 900 °C, they became constant. The microstructures of the alloys, investigated by OM, show that recrystallization is completed at 900 °C and the samples are mainly free of defects, which is in agreement with the positron annihilation lifetime data.

Study on soluble polyaniline by positron annihilation technique

The solubility, electrical conductivity, and other properties of polyaniline (PANI) are highly dependent on its oxidation state. In this work, polyaniline (PANI1) prepared by peroxodisulphate induced polymerization of aniline in acidic aqueous medium in presence of benzenediazonium chloride salt was found to exist in lower oxidation state than emeraldine form of PANI and was highly soluble in common organic solvents. This polymer was subjected to positron annihilation spectroscopic study to investigate the correlation between the oxidation state of the polymer and defect sites generated by different degrees of protonation that in turn affect its electrical conductivity. The positron annihilation lifetime data were resolved to yield a three-component fit for PANI1 subjected to different levels of protonation. The variation of positron annihilation parameters (τ1,I2) and Doppler broadening parameters (R, S) as a function of protonation level of the polymer indicate the dopant sites increase initially on protonation and reach a saturation value after a certain level of acidification. The lower value of electrical conductivity and the intensity of intermediate lifetime component (I2) for PANI1 compared to PANI in emeraldine oxidation state indicate the presence of lesser number of quinoid–imine moieties that could undergo protonation and thus yield highly enriched trapping centers. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

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)

Temperature dependence of nanoholes free volumes in amorphous polymers

Estimation of the free volume fraction in polymers from positron annihilation lifetime data is usually obtained by assuming spherical nanoholes. Although this guess is sometimes well verified, in other cases non-spherical geometries give better agreement with theoretical predictions. Furthermore, the assumption of isotropic expansion of nanoholes (implicit in these models) could be unrealistic owing to constrained movements of the molecules. Comparison of hole-lattice theory with experimental data for some oligomeric structures gives evidence of anisotropic nanoholes expansion versus temperature.

Ferro-antiferromagnetic coupling and unusual transport properties of ferro-antiferromagnetic (100−x) La0.7Pb0.3MnO3+xPr0.63Ca0.37MnO3 (x=–85wt%) composites

We report a low field (H=0.5T) magnetoresistance of colossal magnetoresistive (CMR) composites prepared with ferromagnetic (FM)—antiferromagnetic (AFM) coupled system (100−x) La0.7Pb0.3MnO3(LPMO)+xPr0.63Ca0.37MnO3 with x=0–85wt%. A large increase (∼16% maximum) of magnetoresistance (MR) (∼10% for pure LPMO) is observed at a relatively higher optimum concentration (x=70% AFM). Interesting double peaks are observed for lower AFM concentrations (x⩽35%) in the resistivity-temperature [ρ(T)] curve: one due to the usual intrinsic double exchange interaction (between Mn3+ and Mn4+) mechanism and the other due to the stronger FM-AFM coupling for lower concentration. Unlike the usual ceramic CMR material, thermoelectric power (TEP) in this system is also nonadditive in character, indicating the presence of additional defects and strain in this composites. A two-channel (conducting and semiconductor) model has been used which fit well with the ρ(T) data near the optimum concentration x=70%. Around this highest AFM concentration, showing maximum MR, the corresponding TEP is also higher than those of the individual parent compound, which indicates the presence of maximum grain boundary effect and defects. Positron annihilation lifetime data of the composites also support the presence of such defects and strain in the system with maximum around the critical concentration xC=70%.

Temperature dependence of nanoholes free volume in polypropylene glycols

Estimation of the free volume fraction in disordered systems can be obtained by combining specific volume measurements with positron annihilation lifetime data. Usually, nanoholes are supposed to expand isotropically under, e.g., temperature increase. This letter shows results for a homologous series of polypropylene glycols which are discussed in terms of anisotropic nanoholes expansion, ascribed to constraints on the molecular motions. A relationship between volume and unconstrained, average size of the cavity is obtained in the form of a power law with a noninteger exponent.

Structure, magnetic and positron lifetime studies on CaO–ZrO2–SiO2 glass system doped with vanadium oxide

The structure of CaO–ZrO2–SiO2 glasses doped with V2O5 (0.1–2 mole%) was studied using density and molar volume measurements. It was observed that the molar volume increased while the density decreased with increasing V2O5 content. Magnetic susceptibility measurements showed the diamagnetic behaviour of this glass system. The free volume was probed using ortho-positronium pick-off annihilation lifetime parameters. The mean free volume and fractional free volume have also been calculated from the positron annihilation lifetime data. The overall measured parameters from the structural and/or positron annihilation spectra indicated that the majority of the vanadium ions in the range 0–0.5 mole% of V2O5 are in the V5+ valence state which act as a glass former. On the other hand, the majority of the vanadium ions in the range 0.7–2 mole% of V2O5 are the V3+ and V4+ states acting as a glass modifier.

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We assessed usefulness of commercially available sealed 22Na positron sources for positron annihilation lifetime measurements. Three sources with different nominal activities below 1.85 MBq were adopted for measuring positron annihilation lifetimes in silicon and annealed copper. It was found that all the sources have long lifetime components over 1 ns with different relative intensities of several percents. The result suggests that care should be taken in the analysis of the positron annihilation lifetime data obtained with these sources.

Slow positron beam study of corrosion-related defects in pure iron

Corrosion-related defects of pure iron were investigated by measuring Doppler broadening energy spectra (DBES) of positron annihilation and positron annihilation lifetime (PAL). Defect profiles of the S-parameter from DBES as a function of positron incident energy up to 30 keV (i.e. ∼1 μm depth) were analyzed. The DBES data show that S-parameter increases as a function of positron incident energy (mean depth) after corrosion, and the increase in the S-parameter is larger near the surface than in the bulk due to corrosion. Furthermore, information on defect size from PAL data as a function of positron incident energy up to 10 keV (i.e. ∼0.2 μm depth) was analyzed. In the two-state trapping model, the lifetime τ 2 = 500 ps is ascribed to annihilation of positrons in voids with a size of the order of nanometer. τ 1, which decreases with depth from the surface to the bulk, is ascribed to the annihilation of positrons in dislocations and three-dimensional vacancy clusters. The corroded samples show a significant increase in τ 1 and the intensity I 2, and near the surface the corroded iron introduces both voids and large-size three-dimensional vacancy clusters. The size of vacancy clusters decreases with depth.

Gamma and positron irradiation effects on polypropylene studied by coincidence Doppler broadening spectroscopy

The coincidence Doppler broadening (CDB) technique was applied to study the positron source irradiation effect on γ-irradiated and unirradiated polypropylene. CDB results showed presence of carbonyl groups (CG) whose concentration increase with the γ-irradiation dose. Positron annihilation lifetime (PAL) spectroscopy was used to monitor the positronium (Ps) formation. The γ-irradiation effect on Ps inhibition was significantly weaker than the positron irradiation effect. The joined examination of CDB and PAL data showed that the annihilation on CGs oxygen was not the main reason for the detected Ps inhibition during the positron-irradiation.

Structural study of some divalent aluminoborate glasses using ultrasonic and positron annihilation techniques

Positron annihilation lifetime (PAL), ultrasonic techniques, and differential thermal analysis (DTA) were performed to study the structure of some aluminoborate glasses. The basic compositions of these glasses are 50 B2O3 + 10 Al2O3 + 40 RO (wt%), where RO is the divalent oxide (MgO, CaO, SrO, and CdO). The ultrasonic data show that the rigidity increases from MgO to CaO then decrease at SrO and again increases at CdO. The glass transition temperature (determined from DTA) decreases from MgO to SrO then increases at CdO. The trend of the thermal properties was attributed to thermal stability. The experimental data are correlated with the internal glass structure and its connectivity. The PAL data show that an inversely correlation between the relative fractional of the open hole volume and the density of the samples. Also, there is a good correlation between the ortho-positronium (o-Ps) lifetime (open hole volume size) and the bulk modulus of the samples (determined from ultrasonic technique). The open volume hole size distribution for the samples shows that the open volume holes expand in size for CaO, SrO, MgO, and CdO, respectively with their distribution function moving to higher volume size. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)