Doppler Broadening Energy Spectra Research Articles

Investigation on structural and optical properties of ZnSe thin films prepared by selenization

In this paper, ZnSe films were prepared on quartz substrates by selenizing as-sputtered zinc films at different temperatures for 6 h. The crystal structure, morphology, microscopic defects and optical properties of the ZnSe films were characterized by XRD, SEM, Raman spectroscopy, UV–visible spectrophotometer and slow positron beam Doppler broadening spectroscopy, respectively. The results showed that ZnSe thin films had the best structural and optical properties when the selenization temperature was 700 °C . All ZnSe thin films have a hexagonal wurtzite structure. The results of XRD、SEM and Raman spectra all showed that the crystallization and molar ratio of the films were significantly optimized with the increase of temperature. The transmission spectra of the films showed an obvious absorption edge of ZnSe and increased transmittance with the increase of temperature. The variation of defect concentration and types in the ZnSe thin films were released visually by slow positron beam Doppler broadening energy spectra.

Free volume properties of the zinc oxide nanoparticles/waterborne polyurethane coating system studied by a slow positron beam

A variable, mono-energetic positron beam was employed to study the free volume characteristics of coating system composed of zinc oxide nanoparticles dispersed in waterborne polyurethane. Positron annihilation lifetime spectroscopy, in tandem with Doppler-broadening energy spectra and derived S parameter is used to quantitatively assess the free volume and interfacial interactions between zinc oxide nanoparticle surface and waterborne polyurethane. It was found that S parameter decreases with increasing ZnO load from 0 to 5%. The decrease of S parameter is explained as a result of strong interfacial interactions between the surface of the ZnO nanoparticles and WBPU chains. Furthermore, the half depth where the change in S parameter is reduced by half from the surface to bulk was found to increase with increasing ZnO concentration. These results are presented in the framework of free volume interactions in which the S parameter is quantitatively used to characterize the free volume in the inorganic/organic nanocomposite matrix.

Preparation of ultrathin, robust membranes through reactive layer-by-layer (LbL) assembly for pervaporation dehydration

Fabricating ultrathin membranes with high separation performance and stability remains a great challenge. Tannic acid (TA) is a bio-based polyphenol and can form multifunctional coatings through catechol chemistry. In this study, polyethyleneimine (PEI) and TA were alternatively deposited on hydrolyzed polyacrylonitrile (HPAN) porous membrane via layer-by-layer (LbL) assembly driven by covalent interaction between the amine groups of PEI and polyphenols of TA. The LbL assembly process was verified by the step-wise change of water contact angles. The formation of covalent bond was confirmed by X-ray photoelectron spectroscopy (XPS). The free volume property of the membranes was revealed by the Doppler broadening energy spectra. The resultant membranes possessed ultrathin active layers with thicknesses ranging from 118 to 220nm. When utilized for pervaporation dehydration of 90wt% ethanol aqueous solution, the membrane exhibited high separation performance with permeation flux of 1343g/m2h and separation factor of 1012 with 5.5 bilayers and pH 8 at 349K, arising from the increased hydrophilicity and favorable free volume property. Moreover, the membrane showed favorable long-term stability due to the strong interfacial interactions.

The development of high-performance 6FDA-NDA/DABA/POSS/Ultem® dual-layer hollow fibers for ethanol dehydration via pervaporation

A dual-layer hollow fiber membrane consisting of copoly(1,5-naphthalene/3,5-benzoic acid-2,2′-bis(3,4-dicarboxyphenyl) hexafluoropropanedimide) (6FDA-NDA/DABA)/polyhedral oligomeric silsesquioxane (POSS) as the outer functional layer and polyetherimide (Ultem®) as the inner support layer has been developed for ethanol hydration via pervaporation. By using lower dope flow rates and optimizing inner dope concentration, the newly developed hollow fiber membranes have desirable morphology consisting of a porous inner layer with low transport resistance and a macrovoid-free selective layer for a superior separation performance. Doppler broadening energy spectra (DBES) have been employed to analyze the depth profile of morphological evolution and examine the thickness of dense selective layer as a function of spinning conditions. Incorporating a small amount (2wt%) of POSS into the selective layer increases its free volume size and diffusion selectivity and hence improves the pervaporation performance of the hybrid membranes. The diffusion mechanism of permeate molecules through polymer chains containing POSS nanoparticles was proposed and verified via FTIR spectra, XRD curves, positron annihilation lifetime (PAL) characterization and vapor sorption tests. Compared to other hollow fiber membranes, the newly developed hollow fibers have a superior flux of 1.9kg/m2h and a comparable separation factor of 166 for ethanol dehydration. The work may open up a new perspective for the development of next-generation high-performance dual-layer hollow fiber membranes with a POSS embedded selective layer for biofuel separation.

Development and positron annihilation spectroscopy (PAS) characterization of polyamide imide (PAI)–polyethersulfone (PES) based defect-free dual-layer hollow fiber membranes with an ultrathin dense-selective layer for gas separation

► Defect-free dual-layer hollow fiber membranes with ultrathin dense-selective layer. ► O 2 /N 2 selectivity of 7.73 with a dense-selective layer thickness of 63 nm. ► PAS to explore the morphology and predict the gas performance. ► PAS estimate the outer-layer thickness and demonstrate the layered structure. ► PAS fitted data agrees well with the experimental result. Defect-free polymeric dual-layer hollow fiber membranes consisting of an ultrathin dense-selective polyamide imide (PAI) layer and a polyethersulfone (PES) supporting layer have been successfully fabricated in this study for gas separation application. It is observed that a lower outer-layer dope flow rate does not necessarily result in the formation of an ultrathin dense-selective layer upon the PES supporting layer. An optimization in the velocity between the inner-layer and the outer-layer dopes at the exit of the spinneret is essential to minimize additional stresses and defect formation in the outer functional layer. The best gas separation performance of the PAI–PES dual-layer hollow fibers fabricated in this study has an O 2 /N 2 selectivity of 7.73 with a dense-selective layer thickness of 63 nm. Positron annihilation spectroscopy (PAS) has been used for the first time to explore the morphology and predict the gas separation performance of PAI–PES based dual-layer hollow fiber membranes. Doppler broadening energy spectra (DBES) from PAS accurately estimate the outer-layer thickness and demonstrate the existence of the multilayered structure of the dual-layer hollow fiber membranes. Besides, the PAS fitted data reveal that the fiber spun under the optimal condition has the densest selective layer, which agrees well with the highest gas-pair selectivity observed under this condition.

Characterization of multilayer nanofiltration membranes using positron annihilation spectroscopy

Positron annihilation spectroscopy (PAS) coupled with a slow positron beam was used to characterize in situ the layer structure and depth profile of the cavity size in thin film composite (TFC) polyamide nanofiltration (NF) membranes prepared by the interfacial polymerization method. Two techniques, using PAS coupled with a slow positron beam of Doppler broadening energy spectra (DBES) and positron annihilation lifetime spectroscopy (PALS) designed to reveal the layer structure and the cavity sizes contained in a multilayer thin film composite NF membrane, were assessed. To the best knowledge of the authors, a characterization of the depth profile of cavities in NF membranes using PAS coupled with a slow positron beam has never been reported. The membranes selected have a composite structure containing three layers: a selective polyamide layer, a transition layer, and a porous support prepared by the phase inversion technique. Furthermore, the cavity size distribution in the selective top layer plays an important role in determining the performance of the NF membranes.

Hydrogen- and Corrosion-Related Defect Characterization of Iron and Steels Using a Slow Positron Beam

Hydrogen- and corrosion -related defects of pure iron and AISI 304 and 316 stainless steels were investigated by measuring Doppler broadening energy spectra of positron annihilation. The significant increase in S-parameter after corrosion in iron was interpreted as due to the formation of defects and voids during corrosion processes. However, a decrease of the S-parameter in AISI 304 and 316 stainless steels after corrosion was interpreted as dissolution of passivity oxide film. Furthermore, similar results in iron and stainless steels after hydrogen-charging were observed. Hydrogen damage between the surface and the bulk has a significant variation with depth. The larger increase in S-parameter near the surface after hydrogen-charging mainly comes from the formation of large-size defects (e.g. voids); however the increase in S-parameter in the bulk after hydrogen-charging mainly comes from the production of small-size defects.

Polymeric membrane studied using slow positron beam

A radioisotope slow positron beam has been built at the Chung Yuan Christian University in Taiwan for the research and development in membrane science and technology. Doppler broadening energy spectra and positron annihilation lifetime have been measured as a function of positron energy up to 30 keV in a polyamide membrane prepared by the interfacial polymerization between triethylenetetraamine (TETA) and trimesoyl chloride (TMC) on modified porous polyacrylonitrile (PAN) asymmetric membrane. The multilayer structures and free-volume depth profile for this asymmetric membrane system are obtained. Positron annihilation spectroscopy coupled with a slow beam could provide new information about size selectivity of transporting molecules and guidance for molecular designs in polymeric membranes.

Further search for selectivity of positron annihilation in the skin and cancerous systems

Positronium annihilation lifetime (PAL) spectroscopy and Doppler broadening energy spectra (DBES) have been used to search for selectivity and sensitivity for cancerous skin samples with and without cancer. This study is to further explore the melanoma cancerous system and other different types of skin samples. We found that the S parameter in melanoma skin samples cut at 0.39 mm depth from the same patient's skin is smaller than near the skin surface. However in 10 melanoma samples from different patients, the S parameters vary significantly. Similarly, among 10 normal skin samples without cancer, the S parameters also vary largely among different patients. To understand the sensitivity of PAS as a tool to detect cancer formation at the early stage, we propose a controlled and systematic study of in vivo experiments using UV-induced cancer skin from living animals.

Hydrogen‐damaged defects near the surface in heavily deformed iron and steels investigated by slow positron annihilation spectroscopy

AbstractHydrogen‐damaged defects of heavily deformed iron, AISI 304‐, and 316‐stainless steels were investigated by measuring Doppler broadening energy spectra (DBES) of positron annihilation radiation. It is interestingly found that S‐parameter as a function of energy on three kinds of deformed samples falls into the similar variation, independent of chemical elements and impurities. A larger S value near the surface in deformed and hydrogen‐damaged samples than in samples without hydrogen‐damaged samples is because of the additional larger‐sized defects (e.g. voids) induced due to hydrogen damage. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Hydrogen-induced defects of AISI 316 stainless steel studied by variable energy Doppler broadening energy spectra

Hydrogen-induced defects of AISI 316 stainless steel were systemically investigated by measuring variable energy Doppler broadening energy spectra (DBES) of positron annihilation. 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. Experimental results showed that hydrogen damage between the surface and the bulk has a significant variation with depth, and strongly depends on the condition of hydrogen charging, i.e. current density and charging time. It has been suggested that the increase in the S-parameter near the surface after hydrogen charging mainly comes from the formation of hydrogen damage (voids); however, the increase in the S-parameter in the bulk after hydrogen charging mainly comes from the production of structural defects (dislocations). Defect densities induced due to hydrogen charging in some cases (e.g. dislocation density in the bulk) were estimated based on a simple trapping model. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Depth profile of free volume in a mixture and copolymers of poly(N-vinyl-pyrrolidone) and poly(ethylene glycol) studied by positron annihilation spectroscopy.

Effect of hydrogen bonding on the depth profile of the free-volume in a mixture (weight ratio of 65:35) of poly(N-vinyl-pyrrolidone) (PVP) and poly(ethylene glycol) (PEG) and the copolymers of vinyl pyrrolidone with poly(ethylene glycol) diacrylate (PVP-PEGDA) and monomethacrylate (PVP-PEGMMA) was studied using positron annihilation spectroscopy. Doppler broadening energy spectra of annihilation radiation and positron annihilation lifetime were measured as a function of positron incident energy (0-30 keV). Significant variations of the free-volume depth profile in terms of the S parameter, ortho-positronium lifetime, intensity, and lifetime distribution are observed as a result of the hydrogen-bonding replacement of covalent bonds. The polymer mixture with hydrogen bonding through two sides of PEG short chains has a larger free volume and a wider distribution than the comb-structured PVP-PEGMMA and the network structured PVP-PEGDA. A longer ortho-positronium lifetime is observed near the surface than in the bulk. This is interpreted in terms of surface effect, free volume, and hydrogen bonding for drug delivery applications of polymeric materials.

Positron annihilation studies of the effect of gamma irradiation dose in polymers

Positron annihilation lifetime (PAL) technique has been applied to study the effect of gamma irradiation dose on the structure of low-density polyethylene and polystyrene. The positron annihilation lifetime measurements were performed with a conventional fast–fast coincidence system. Gamma irradiation was carried out at room temperature with a 60Co gamma source up to a dose of 1000 kGy. The delayed lifetime spectra of irradiated and unirradiated samples were decomposed into three lifetime components. The lifetime parameters give indication of the free-volume size and concentration. It was found that the crosslinking efficiency in the polymer increases significantly with increasing irradiation dose. This appears as a decrease in the intensity of the long-lived positron lifetime components attributed to ortho-positronium formation. Measurements of the Doppler broadening energy spectra of the low-density polyethylene (LDPE) samples have also been performed using a variable mono-energetic positron beam. The trend of the Doppler broadening lineshape S-parameter as a function of the radiation dose supports the lifetime results.

Weathering durability of commercial polymeric coatings tested by positron annihilation spectroscopy

A series of commercial coatings were prepared according to the industrial specifications and were exposed to Florida natural weathering and controlled UVA irradiation. The Doppler broadening energy spectra (DBES) of positron annihilation were measured as a function of incident positron energy at different periods of weathering. A significant decrease in the S parameter was observed and it was used as an indicator to test coating durability in micro-scale. Application to weathering durability of commercial polymeric coatings under natural weathering and controlled UVA irradiation is investigated by using the S parameter from the DBES.

Surface and interfacial effect on polymer glass transition temperature studied by positron annihilation

Glass transition temperature ( T g) as a function of depth and interfacial effect in polystyrene (PS) films is investigated by using positron annihilation spectroscopy (PAS) coupled with the slow positron beam technique. We report the results of the S parameter from Doppler broadening energy spectra as a function of temperature across T g in PS films of different thickness on Si substrate. The depression of T g as the film thickness decreases is consistent with the existing data. The use of PAS to study the surface and interfacial effect on polymeric properties is discussed.

Corrosion of iron and stainless steels studied using slow positron beam technique

Corrosion-related defects of pure iron, and AISI 304 and 316 stainless steels were investigated by measuring Doppler broadening energy spectra (DBES) of positron annihilation. 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. When the sample is treated with various anodic potentials (0–4 V), a significant increase in the S parameter in pure iron is observed. This corrosion process shows the presence of defects and voids of a few nanometers near the surface. However, a significant decrease of the S parameter in AISI 304 and 316 stainless steels after corrosion is observed, which is interpreted as dissolution of passivity oxide film. Different defect results are observed between pure iron and stainless steels.

The influence of radiation and light on Ps formation in PMMA and PE studied by coincidence Doppler-broadening spectroscopy

Using two Ge detectors, the high-resolution Doppler-broadening energy spectra of positron annihilation gamma rays has been obtained by measuring the coincidences of the two photons. Light bleaching and oxygen effects on positron annihilation were investigated in this way. A large enhancement of the high-momentum part of the coincidence Doppler spectra was observed in poly(methylmethacrylate) (PMMA), which contains oxygen atoms in the polymer structure. Bleaching experiments in PMMA and in copolymer ethylene-methylmethacrylate EMMA (LDPE+MMA 3 mol%) have demonstrated that the enhancement effect may be due to the trapping of positrons by the polar C +O − groups, followed by positron annihilation with the electrons belonging to oxygen.

Durability of polymeric coatings: effects of natural and artificial weathering

The durability of three commercially available polymeric coatings are investigated using slow positron beam techniques to monitor changes in sub nanometer defects during the degradation process. Doppler broadening energy spectra (DBES) measurements were performed on the slow positron beam at the University of Missouri-Kansas City. Results from UVB-313 artificial weathering are compared to the results for natural weathering for each of the coatings. It is shown that the slow positron beam technique is very successful in detecting the very early stages of coating degradation. In addition, the depth profile of the degradation process provides important information on the durability of the coatings and the kinetics of the degradation as a function of the depth from the surface. The artificial weathering technique is discussed as a predictor of the durability to natural weathering. It is shown that for each of the weathering systems differences are observed for each of the coatings. The loss of free volume during the degradation process is interpreted as being due to photo-oxidation of the polymer, with a net result of an increase in the crosslink density of the polymer.

Characterization of photodegradation of a polyurethane coating by positron annihilation spectroscopy: correlation with cross-link density

Photodegradation of a polyurethane coating by Xe arc-lamp irradiation as a function of time is studied by positron annihilation spectroscopy. Doppler broadening energy spectra (DBES) of annihilation irradiation is measured as a function of incident positron energy (0–30 keV). A significant decrease in the S-defect parameter from DBES is observed as a function of irradiation time. This is interpreted as a loss of free-volume and holes during the photodegradation process. The cross-link density in the same system is measured by a solvent (methylene chloride) swelling method and is found to increase as a function of irradiation time. A direct correlation between the increase in cross-link density and the decrease in the S-defect parameter from the DBES data is observed. These results are used to discuss the photodegradation process in terms of chemical and physical changes due to UV-irradiation in protective polymeric systems.