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Positron studies of the temperature-dependence of free volumes in Polydimethyls...
Marques, M. F. Ferreira; Gordo, P. M.; Kajcsos, Zs.; Gil, C. Lopes; Lima, A. P. de; Queiroz, D. P.; Pinho, M .N. deFree-volume parameters in polyurethane/urea membranes have been studied by positron annihilation lifetime and Doppler broadening measurements. The bi-soft segment membranes were obtained by varying the ratio of the structural constituents, polypropylene oxide and polydimethylsiloxane (PU/PDMS), with PDMS content from 25 to 75 wt%. The free-volume holes determined by PALS are correlated with gas permeation featu...
Positron annihilation and constant photocurrent method measurements on a-Si:H f...
Gordo, P. M.; Marques, M. F. Ferreira; Gil, C. Lopes; Lima, A. P. de; Lavareda, G.; Carvalho, C. Nunes de; Amaral, A.; Kajcsos, Zs.Defect structure of hydrogenated amorphous silicon thin-films was studied by positron annihilation spectroscopy (PAS), whereas the density of states below the Fermi level was measured by constant photocurrent method (CPM). Divacancies and large vacancy clusters were identified as the main defects present in these films, with relative concentrations strongly dependent on the rf-power. Correlation between PAS, CP...
Competitive positron and positronium trapping in porous media
Kajcsos, Zs.; Liszkay, L.; Duplâtre, G.; Lázár, K.; Lohonyai, L.; Varga, L.; Gordo, P. M.; Lima, A. P. de; Gil, C. Lopes de; Marques, M. F. FerreiraPositron annihilation acquired acceptance for structural investigation of solids but results in porous media--where positron lifetime spectroscopy (LT) reveals substantial Ps formation--were ambiguous. Data on zeolites lead to the conclusion that Ps trapping in competing "extended free volume" sites, inhomogeneous regions and grain boundaries occurs. Furthermore, positron trapping must also be considered. Syste...
Positron and positronium in porous media: zeolites
Kajcsos, Zs.; Liszkay, L.; Duplâtre, G.; Lohonyai, L.; Varga, L.; Lázár, K.; Pál-Borbély, G.; Beyer, H. K.; Caullet, P.; Patarin, J.; Lima, A. P. deCorrelations between positronium (Ps) production features and positron annihilation (PA) parameters were investigated in various zeolite samples (MFI, mordenite, Na-Y). The contribution of 3[gamma]-annihilation on data registration was closely examined. Long lifetimes up to 135 ns and triplet Ps (o-Ps) fractions up to 30% were found. Even low amounts of adsorbed gases or water drastically change the PA pattern....
Positron lifetime studies in vinyl polymers of medical importance
Marques, M. F. Ferreira; Gordo, P. M.; Gil, C. Lopes; Kajcsos, Zs.; Gil, M. H.; Mariz, M. J.; Lima, A. P. deVinyl polymers used as artificial lens implants in ophthalmology were investigated by positron lifetime spectroscopy. The structure of these polymers with free volumes offers the possibility of charging them with anti-inflammatory drugs for sustained release. A correlation between the amount of normalised free volume and the ratio of the methyl methacrylate to ethyl-hexyl-acrylate, used as polymerisation monome...
Free-volume studies in polyurethane membranes by positron annihilation spectros...
Marques, M. F. Ferreira; Gil, C. Lopes; Gordo, P. M.; Kajcsos, Zs.; Lima, A. P. de; Queiroz, D. P.; Pinho, M. N. deThe free-volume parameters in various urethane/urea membranes obtained by varying the ratio of the structural constituents, polypropylene oxide and polybutadiene, were studied by positron lifetime and Doppler broadening measurements. On bi-soft segment membranes, a correlation was found between the composition of membranes, the normalized free volume, the radii of the holes and gas permeability. However, the co...
Four-Component Analysis and Modeling of Positron Annihilation Lifetime Paramete...
Marques, M. F. Ferreira; Burrows, H. D.; Miguel, M. da Graça; Lima, A. P. de; Gil, C. Lopes; Duplâtre, G.Reverse micelles are studied in sodium AOT/water/isooctane mixtures as functions of AOT concentration (CAOT), water to AOT mole ratio (w0) and temperature (T), from 294 to 333 K, using positron annihilation lifetime spectroscopy (LS). By four-component analysis of the spectra, it is possible to extract the LS parameters (intensities, Ii, and lifetimes, τi) of the triplet positronium (o-Ps) present in the aqueou...
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