A comparison of different theoretical models of positron lifetime spectra for polymers

The same reference positron lifetime spectra of polyurethane, measured in temperature range 112 to 390 K, were analyzed with three theoretical models: the \conventional" model with three exponential decay component of discrete values of lifetimes, a model with one discrete exponential component and two \packages" of exponentials with log-normal distribution of annihilation rates, and a model considering positronium slow localization, positronium internal relaxation as well as \delayed" formation of positronium. It turns out that the two latter models fit the experimental data with the same excellent quality, in spite of the fact that in both models the ratio of intensities related to para-positronium and ortho-positronium was constrained to be as 1:3

Correlations between positron annihilation parameters and macroscopic properties in copolymers belonging to elastomers group

The positron annihilation lifetime spectroscopy was used to study correlations between positron annihilation parameters and macroscopic properties in two kinds of polymers from elastomers group. Two kinds of material were investigated: three samples of ethylene octane copolymers (commercial name engage) of different densities and six samples of polybutylene terephtalate-polyether glycol copolymers (hytrel) having different densities. A correlation between intensity of ortho-positronium component and the density (d) of samples was observed for both kinds of material. From the ortho-positronium pick-off lifetime the mean radii (R) of free volume centers were determined. A good linear correlation between R and d was found

LT10 program for solving basic problems connected with defect detection

Features of LT10 software for positron annihilation lifetime (PAL) spectra analysis are presented. It is a completely new version of well known LT9.x series with newly designed intuitive user interface. The program allows to significantly reduce the number of freedom degrees in the numerical analysis of PAL spectra. It is accomplished by simultaneous analysis of a series of spectra with regard of the correlation between related parameters of each spectrum in the series. A few examples of the program application are presented in the paper

Relationship between enhanced positronium formation and enhanced positron trapping in polymers at low temperature

Positron annihilation lifetime spectroscopy is used for investigation of low-density polyethylene and ethylene-methyl methacrylate copolymers of 1.45, 3.0, and 5.4 mole% of methyl methacrylate. The lifetime spectra are collected at 30 K, one by one, as a function of elapsed time. In the computer analysis a new theoretical model is de eloped, which enables separating the annihilation from positron free state, its trapped state and bound state in positronium. The positron trapping rate ¹ and the enhanced positronium formation rate · are determined. The calculated values of ¹ and · turned out to be linearly correlated. This correlation presumably originates from an in°uence of trapped electrons on the trapping of positrons. The dependences of · on measurement time are determined for low-density polyethylene and ethylene-methyl methacrylate of different methyl methacrylate content. A theoretical model describing quantitatively the dependences is proposed. The model considers the processes of electron{ion recombination, electron trapping, and electron scavenging by dipolar carbonyl groups supplied by methyl methacrylate additives

Hyperfine interaction parameters in Fe 28Al 72: 57Fe mossbauer spectroscopy and ab initio study

The paper discusses a theoretical model that associates the shape of the Mössbauer spectrum with the con guration of atoms in local surroundings of the Mössbauer nuclide. Using the model we analyse the Mössbauer spectra of Fe72Al28 alloys after various heat treatments. Basing on the model calculations the hyper ne structure parameters describing an e ect of an Al atom in the rst and the second coordination shell of 57Fe atom are evaluated. Using these parameters the values of hyper ne magnetic eld and isomers shift for the atomic con gurations present in stoichiometric Fe3Al are estimated and compared with the results of ab initio calculations

Positron annihilation lifetime in situ study of deformed polyolefin elastomers

The positron annihilation lifetime spectroscopy was used to investigate the free volume hole size of ethylene octane copolymers as a function of deformation in the range from 0 to 110% in steps of 10%. For each degree of deformation a series of 5-6 positron annihilation lifetime spectra was collected in situ. All spectra of a given series were analysed simultaneously. They were resolved into three exponential components, of lifetimes parameters ¿i and intensities Ii. The variations of ¿3 reflect three deformation regions, the elastic range, strain softening range, and plastic region

Influence of heat treatment on ordering process in Fe72Al28 alloy studied by Mössbauer Spectroscopy

The paper discusses a theoretical model that associates the shape of Mössbauer spectrum with the configuration of atoms in local surroundings of the Mössbauer nuclide. The model has been implemented to a computer program which was used to analyse the Mössbauer spectra of Fe72Al28 alloys after various heat treatments. Basing on the determined configuration of atoms, the long range ordering parameter was estimated

The Influence of Heat Treatement on Point Defect Concetration in Fe-Al and Fe-Al-Cr systems

FeAl alloys of nominal Al content 28, 38 and 45 at.% and Fe28Al5Cr with minor alloying elements added to improve their application properties are studied by positron annihilation lifetime analysis. The lifetime spectra of samples FeXAl with X ≥ 38 (both quenched and cooled with furnace) are described by a single lifetime component related to the saturated positron trapping by quenched-in vacancies of concentration much higher than 100 ppm. A very strong dependence of retained vacancy concentration on the rate of cooling is shown for Fe28Al and Fe28Al5Cr alloys. After quenching, the concentration is of the order of 10–4 whereas in samples slowly cooled it is reduced to 10–5. The chromium addition to Fe28Al5Cr dismisses the vacancies concentration in comparison to the concentration in Fe28Al after the same heat treatment

Change of the defect structure in feal alloy as a result of its aging at ambient temperature

Positron annihilation lifetime spectroscopy (PALS) was used to study the defect structure of stoichiometric FeAl samples after their slow cooling or quenching from the temperatures of 1000 and 540°C. The high value of positron lifetime in the quenched-in defects suggests that they are mainly of di-vacancy type. After long storage at ambient temperature, the concentration of defects in the samples strongly decreases and mono-vacancies predominate. Annealing at 540°C generates an additional amount of defects may be ascribed to the formation of FeAl2 phase in the FeAl matrix

Ordering process in Fe-Al28Cr5 alloys studied by Mossbauer spectroscopy

In the paper, studies of ternary Fe-Al28Cr5 alloys directed to characterization of mechanisms of the formation of ordered intermetallic phases influencing the possibilities of their practical application are shown. Also, a role of chromium as a modifier in kinetics of the formation of ordered phases is determined. Among the research method used, Mössbauer spectroscopy for the determination of hyperfine structure parameters was adopted. These parameters, sensitive to changes in spin and charge electron densities in the nearest neighbourhood of a Mössbauer isotope nucleus, caused by specific configurations of atoms, are directly connected with the degree of ordering of a compound. Spectral analysis has been carried out using the authors’ software developed based on a theoretical model relating the shape of a Mössbauer spectrum to the sample microstructure. It has been shown that Mössbauer spectroscopy enables quantitative evaluation of the degree of ordering of phases occurring in samples characterized by large graining, in the case of which it is not possible to determine the long-range order parameter by X-ray diffraction