Papers by Keyword: Positron

Abstract: Self-assembled amphiphile systems are utilized in a wide variety of applications including drug delivery and energy storage. Nano-scale physical and chemical interactions govern the packing of self-assembled amphiphilic molecules, resulting in thermodynamically stable phases of defined geometries. Possible phases include micellar, hexagonal, cubic, lamellar and sponge phases. The internal nano-structure of the amphiphile self-assembly materials plays an important role in the properties of these systems and their application. To date small angle x-ray scattering (SAXS) has been the most common technique used to characterise their structure. We explore positron annihilation lifetime spectroscopy (PALS) as an alternative and/or complementary technique for this purpose, using the phytantriol/water system. While PALS is a well established technique for characterising many materials, the coexistence of aqueous and hydrophobic regions in a soft self-assembled amphiphile material poses a challenge to the analysis and interpretation of the results. In order to alleviate these difficulties we developed a computer program for general-purpose PALS data analysis called PAScual. Amongst the most salient features of this new code are the possibility to perform bounded fits and the option of using advanced algorithms to provide a more robust and unbiased fit: on the one hand, it incorporates a global nonlinear optimisation routine based on the Simulated Annealing algorithm and, on the other hand it gives information on the reliability of the results by means of a Markov Chain Monte-Carlo Bayesian Inference method. In this work we present the newly developed PALS data analysis techniques as well as the results for the phytantriol/water system, comparing them with additional data obtained from complementary techniques.

Abstract: Positron annihilation age-momentum correlation measurements (AMOC) were performed for room temperature ionic liquids (IL) to investigate positronium (Ps) formation process. The solvation time of free electrons in IL is longer than in usual molecular liquids. And hence IL is very useful materials to investigate fast reactions such as Ps formation. On the other hand, Ps formation is a good tool to investigate the dry electrons. The para-Ps annihilation lifetime is about 125ps and then the effect of dry electrons should be seen on the S(t) parameters made by the age-momentum correlation measurements. The slow Ps formation was observed in the room temperature ionic liquids.

Abstract: We have employed positron annihilation lifetime spectroscopy to measure the free volumes in nanocomposites of polystyrene with addition of carbon nanofiber (CNF) and carbon dioxide as a function of composition and of temperature. We found that the glass transition temperature (Tg) increases as a function of CNF compositions but significantly decreases as the exposure of CO2. We also investigate the combination effect of CNF and CO2 and found that the CO2 significantly weakens the bonding between PS and CNF interfaces. The Tg changes are compared with DSC.

Abstract: The subject of investigation was porous glass produced using a sol-gel technique. ortho-Ps lifetimes and intensities in the material were determined as a function of temperature and pressure. Temperature was ranged from 100 to 500 K while the pressure from 0 to 1200 MPa. Nitrogen gas intercalation into pore structure was observed up to 100 MPa.

Abstract: ositron lifetime spectroscopy (PAS) and Coincidence Doppler broadening measurements have been performed in as-grown and annealing single crystal ZnO. The temperature dependence observed in annealed ZnO indicates that the defect is removed during annealing. By combining the Doppler broadening measurements, we infer that there isn′t hydrogen filling the zinc vacancy site in the as-grown ZnO due to the same characteristics in CDB for as-grown and annealed ZnO samples

Abstract: In this work, we studied a few surfaces, of which the structures have not yet been revealed, using reflection high-energy positron diffraction (RHEPD). We studied the Ge(111)/Pb and Ge(111)/Sn surfaces that exhibit the phase transition from 3×3 to √3×√3 periodicities at around 200K. We found that in both phases the equilibrium positions of Pb and Sn adatoms are conserved. That is, in the unit-cell, one of three adatoms is located upper position and two of them are located lower positions (one-up-two-down). The phase transition is interpreted in terms of an order-disorder type. We furthermore studied the quasi-one dimensional Si(111)/In surface which exhibits the metal-insulator transition at around 120K. The high temperature phase is well explained as the zigzag chain structure. We found that a dynamic atomic displacement which leads to the formation of hexagon structure occur below 120 K. We confirmed the appearance of the band gap using the surface structure determined from the RHEPD rocking curves.

Abstract: 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.

Abstract: Positron annihilation techniques were applied for irradiated Nafion-117 to investigate deterioration process in PEM. There were no significant changes in lifetime of positron/positronium, whereas the S-parameter showed fairly good correlation with proton conductivity.

Abstract: Positron annihilation lifetime measurements on polymethylmethacrylate (PMMA) at low temperature were performed. Different discrete fitting procedures have been used to analyze the experimental data. It shows that the extracted parameters depend strongly on the fitting procedure. The physical meaning of the results is discussed. The blob model seems to give the best annihilation parameters.

Abstract: The free volume property and intermolecular interaction (especially hydrogen bonding) of poly (ether-urethane) (PEU) irradiated by neutron under different atmosphere has been studied by positron annihilation lifetime spectroscopy (PALS) and Fourier transform infrared (FTIR). The results suggested that the hydrogen-bonding interactions and free volume property in PEU depended strongly on the neutron irradiation dose.