Adriano P. de Lima

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.

Positron lifetime and Doppler broadening study of defects created by swift ion irradiation in sapphire

Swift ions create a defect profile penetrating deep into a solid compared to the sampling range of typical slow positron beams, which may consequently study a homogeneous zone of defected materials. To investigate the defect population created by energetic ions, we studied Al2O3 single crystals irradiated with swift Kr ions by using conventional and pulsed positron beams. Samples irradiated with krypton at 245 MeV energy in a wide fluence range show nearly saturated positron trapping above 5x10 10 ions cm -2 fluence, indicating the creation of monovacancies in high concentration. At 1x10 14 ions cm -2 irradiation a 500 ps long lifetime component appears, showing the creation of larger voids. This threshold corresponds well to the onset of the overlap of the damage zones after Bi ion irradiation along the ion trajectories observed with microscopic methods.

Near-Surface Studies on Low-Energy Ion Irradiated Ni Using a Positron Beam

Defect distributions in well-annealed polycrystalline Ni sputtered with low energy (3 and 5 keV) Ar + have been studied with a positron beam. The characteristics of the annihilation radiation were measured after the ion irradiation and after each step of an isochronal annealing up to 950o C. The simulated defect profiles consist of a close to the surface peaked distribution with a long tail into the sample. The isochronal annealing shows that the implanted species have a masking effect on the probe and the identification of open volume defects is just possible after partial annealing of the sample.

Characterisation of Ti/Al Multilayered Structures with Slow Positron Beams Applying a Simplified Positron Depth Distribution Model

In this work the depth of interfaces in multilayered structures was estimated. The fractions of positron annihilation as function of the implantation energy were estimated from an S-W plot and then converted into a function of the sample depth through the positron implantation profile in the multilayer system computed from a reduced positron profile. The results of this method in Ti/Al samples are comparable to those using the common analysis based on positron diffusion equations. The positron analyses results were compared with SIMS profiles for the same samples.

Sub-Surface Defects Induced by Low Energy Ar+ Sputtering of Silver

Induced defects in silver polycrystalline samples irradiated with 4 keV Ar+ were characterised with slow positron implantation spectroscopy. The implanted gas was found to interact with ion irradiation defects. The evolution of the defects and gas-defect interactions were followed through a multi-step isochronal annealing treatment. Two different defected regions were detected. A region near to the surface, due to a distribution of vacancy-like defects produced by irradiation, and a deeper one due to coalescence of Ar. The deeper defects evolve with thermal treatments and probably produce cavities which are not easily recovered.

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.