Z. Kajcsos

Extensive study of metallic glasses by positron annihilation

Abstract All three methods of the positron annihilation technique were applied to investigate amorphous metals. Samples of Fe40Ni40Si14B6 and Fe40Ni40P14B6 were prepared by the melt spun technique with constant ejection pressure and orifice size but at varying quenching rate and melt temperature. The influence of the production process leading to different Hc for the samples, the effect of heat treatment, cold rolling, and electron irradiation were studied. The measured dependence of positron annihilation parameters on the above treatments indicates the presence of positron-active trapping centres in the amorphous phase.

Monitoring the crystallization stages of silicalite by positron lifetime spectroscopy

In porous media, positron annihilation techniques (PAT) revealed the existence of annihilation modi with long lifetimes connected to structural and extrastructural free volumes acting as trapping centres for o-Ps. Our aim in this work was to study the influence of the synthesis applied for the production of the zeolite framework and the correlation between the trapping pattern and crystallization progress. PAT lifetime (LT) measurements were performed in silicalite-1 samples in various stages of crystallization and a large change was observed in the long components. The results are discussed in connection to the crystallization and the availability of free volume.

Micellar effects on positronium lifetime in aqueous SDS solutions

Abstract Positron lifetime measurements have been performed in aqueous SDS (sodium dodecyl sulphate) solutions. The lifetime distributions measured by fast-slow coincidence technique have been found to be influenced by surfactant concentration, which varied in the range of 1.25 × 10 −3 −3.2 × 10 −1 mol/dm 3 (i.e. 2.27 × 10 −5 −5.82 × 10 −3 mole fractions). The lifetime of the long living component connected to positronium formation and decay increases with increasing surfactant concentration. Lifetime data suggest that a direct positronium-micelle electron-exchange reaction leading to pick-off annihilation is contra-indicated.

On the application of positron annihilation for structure study of metallic glasses

Abstract A brief critical overview is given on structural models for describing metallic glasses, on positron annihilation methods and on their applications for studying metallic glasses. Results from the literature are presented, compared and discussed.

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 annihilation study of the mechanochemical reaction between Zn + Se and Zn + S

Abstract Positron lifetime spectra were recorded and evaluated in mixtures of Zn + S and Zn + Se ground for various periods. The intensity of the long-lived positron lifetime component is shown to increase with grinding time until a sudden decrease takes place at a specific grinding time thereby indicating the onset of the effective chemical reaction. The suitability of positron annihilation for investigating mechanochemical reactions is clearly demonstrated.

The influence of alkali cations on the structure of zeolite precursor gels investigated by positron lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PALS) is sensitive and proved method for material structure investigations. We used it to investigate the influence of different alkali cations on the structure of zeolite precursors in order to understand better the crystallization mechanism of zeolites. Our previous PALS investigations on Li gels show different structural features obtained for differently prepared gels, indicating the importance of the way on which cations are introduced in the gel matrix. These investigations were now extended to gels containing K, Rb and Cs cations, in correlation to their ionic radius and the way of preparation.

Structure investigations of electrodeposited nickel

Abstract Nickel, electrodeposited under different conditions and yielding different values of stress, was investigated by positron annihilation (lifetime and Doppler-broadening), Mossbauer effect and X-ray diffraction measurements. Two-component positron lifetime spectra were obtained. The first component is thought to result from bulk annihilation and trapping at single trapping centres (TC). Estimations of TC-concentrations are obtained by means of the trapping model. The second one possibly denotes annihilation at voids, the number of which is dependent on the stress in the deposit. Results of Doppler-broadening measurements support this interpretation. The Mossbauer results show differences in the magnetic orientation in the three samples examined.

O-ps lifetimes in iron containing micro- and mesoporous media

Positron annihilation lifetime (LT) measurements were used to study the influence of addition of iron on ortho-positronium (o-Ps) in micro and mesoporous media. Three types of porous samples containing iron were chosen for the investigation. LT spectra were decomposed to four components in the 0.1 - 50 ns lifetime range. An attempt is made to correlate the data with sites occupied by iron in the framework as derived from Mössbauer studies. No significant correlation could be detected in this respect. Instead, dominating role of presence or absence of iron can be deduced. Certain influence can also be attributed to the presence of adsorbed water.

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.