C. Lopes Gil

Free-volume studies in polyurethane membranes by positron annihilation spectroscopy

Abstract The 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 correlation is not clear when PU data are also considered, indicating that other features must also play an important role in the permeation mechanism.

Positron lifetime studies in vinyl polymers of medical importance

Abstract Vinyl 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 monomers, was found.

Influence of the a-Si:H structural defects studied by positron annihilation on the solar cells characteristics

The influence of the power density, P , on the density and structure of defects of undoped a-Si:H thin films, deposited by r.f.- D PECVD, is studied by the constant photocurrent method, (CPM), and by slow positron beam spectroscopy, respectively.Deep defect density, N , remains approximately constant at 10 cm , typical of device quality material, for P in the range 7-20 16 y3 DD D mWOcm , as calculated from CPM.Out of this range, N increases roughly one order of magnitude for both lower and higher y3 DD

Gas Permeability and Temperature-Dependent Free-Volume Studies in Polyurethane Membranes by Positron Lifetime and Doppler Spectroscopies

Positron annihilation lifetime and Doppler broadening spectroscopies were used to study the free volume parameters in polypropylene oxide/polybutadiene bi-soft segment urea/urethane membranes (PU/PBDO), with PBDO content varying from 0 up to 75 wt % in the temperature range 298–324 K. The gas permeation features appear to be correlated with the free volume sizes determined by the lifetime measurements whereas phase separation of the various soft and hard segments in the membranes is mirrored by both the lifetime and Doppler results.

Analysis and modelling of positron lifetime spectroscopy data for the characterisation of water-in-oil microemulsion systems stabilised by Aerosol OT Effect of changing the counter-ion

In previous work using positron lifetime spectroscopy, a model was proposed leading to the satisfactory description of both positronium (Ps) formation and decay in sodium AOT–water–isooctane reverse micelles. The shorter lifetime found for the Ps triplet state in the aqueous core of the micelles, compared to that found in pure water, was attributed to Ps outdiffusion from these cores, enabling the quantification of this lifetime with two fitting parameters: r0, the aqueous core radius, and h, a factor which describes the ease of passage of Ps at the water/isooctane interface. To assess the validity of the model, the experiments have been extended to the study of the Ca(AOT)2 and 2:1 mole ratio Na(AOT):Ca(AOT)2–water–isooctane systems, to examine the effect of the counter-ion on r0 as a function of the water/AOT ratio, w0 and (for Ca2+) of temperature. Using, for all systems, the value of h previously found for the Na(AOT) microemulsions, the presence of the Ca2+ counter-ion appears to result in the formation of larger reverse micelle aggregates, compared to with Na(AOT), with radii in agreement with published data. With the Ps yields, the model implies a semi-empirical parameter, Atrap, related to the probability that the energetic positrons end up in the aqueous subphase before forming Ps. As expected and experimentally verified, Atrap is proportional to r02. Unexpectedly however, with the above value for h, the variation of Atrap with r02 is found to depend on the counter-ion. Therefore, the possible variation of h was considered. Using different values of h for each system, it is possible to obtain a unique variation of Atrap with r02, independent of the counter-ion. In this case, however, the radii for the microemulsions containing Ca2+ are significantly lower than those found in previous work.

EFFECTS OF CCL4 ON POSITRONIUM FORMATION IN PURE ISOOCTANE AND IN AOT/WATER/ISOOCTANE MICROEMULSIONS

Abstract Previous work, on positron lifetime spectroscopy (LS) measurements on AOT/water/isooctane microemulsions showed that most positronium (Ps) is formed in the aqueous subphase, and predicted that a water insoluble electron scavenger should inhibit much less Ps formation than if Ps was mainly formed in the organic solvent. To test this prediction, Ps inhibition by CCl 4 is examined in pure isooctane and in the micellar system. In the former case, from the combined analysis of LS and Doppler broadening results, CCl 4 is found to strongly inhibit Ps formation ( k = 36.6 M −1 ) by the successive capture of e − , to form CCl 4 − , and of e + , to form the PsCCl 4 bound-state. As expected, CCl 4 in the micellar system displays a much lower ability to inhibit Ps formation ( k ca. 9 M −1 ). However, a very good agreement with the data is only obtained by assuming that, besides Ps inhibition in the organic solvent, CCl 4 also promotes some inhibition of that fraction of Ps formed in the aqueous subphase. Quantitatively, the low inhibition constant found for this process (5.1 M −1 ) is quite consistent with what is expected when considering the fraction of the aqueous positron spurs that overlap with the organic subphase. Another hypothesis is discussed, by which CCl 4 would decrease the probability that the positrons become trapped in the aqueous subphase, due to an increase in the electron density of the organic solvent.

Investigation of defect structures in deuteron irradiated and deformed stainless steel 316 by positron annihilation

Abstract Deuteron-irradiated and deformed stainless steel specimens were investigated by positron lifetime and Doppler broadening measurements. The evolution of the defect structures was studied as a function of the isochronal annealing temperature and for various degrees of deformation. A different behaviour was observed for deformed and irradiated stainless steel specimens. Evidence for vacancy clusters was found in the deuteron-irradiated steel. These clusters disappear after annealing around 900 K.

EFFECT OF CCL4 ON POSITRONIUM FORMATION IN ISOOCTANE

The effect of CCl4 on positronium (Ps) formation is examined in isooctane through positron lifetime spectroscopy experiments, in the framework of a series of studies on AOT-based reverse micellar systems. This solute appears as a rather strong Ps formation inhibitor, as expected in view of its well-known electron scavenging properties. The reciprocal of the triplet Ps intensity (1/I3) varies linearly with CCl4 concentration (C), up to about 0.08M, allowing to derive a high inhibition constant,k=36.6M−1. Above 0.08M, however, the inhibition slackens out, which is attributed to an anti-inhibition effect. On the basis of previous reports on halocompounds, a general scheme of reactions is proposed to occur in the positron spur, including, besides the primary Ps formation through thee+/e− reaction and the recombination reactions, electron scavenging by CCl4 as the direct inhibiting reaction, followed by the reaction of the thus formed anion with the positron. The latter reaction may lead either to a [CCl4Ps] bound-state or, if the positron picks off the electron from the anion, to some additional Ps formation. By using a simple quantitative approach to this scheme, an equation descriptive of the variation ofI3 withC is derived which agrees well with the experimental data.

Comparison of vacancy creation by nuclear and electronic processes in silicon irradiated with swift Kr and Bi ions

Silicon samples, irradiated with swift Kr (245 MeV) and Bi (710 MeV) ions at room temperature, were investigated using a continuous and a pulsed positron beam and conventional Doppler broadening and lifetime spectroscopy. In the fluence and depth ranges studied, creation of large voids and amorphization was not observed. The dominant defects were found to be divacancies, present from the near surface region all along the ion tracks. We found that the formation of divacancies from ion-induced vacancies as predicted by Monte-Carlo-calculations is higher in the case of the heavier Bi ion.

Positron annihilation studies in amorphous silicon nitride

Results of positron annihilation studies on amorphous silicon nitride thin films (a-SiNx:H) are presented. The chemical structure of the film, determined by FTIR measurements, is related to open type defects present. Samples with the lowest S parameter value exhibit the highest density and possess the best electrical performance.