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1
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Measurement and Analysis of the Positron Annihilation Lifetime Spectra for Mesoporous Silica

100%
Zaleski R.
|
EN
Positron annihilation lifetime spectroscopy as a technique of porous materials investigation is still under development. Both measurement conditions and lifetime spectra analysis methods have to be adapted to the requirements of positron porosimetry. If complex positron annihilation lifetime spectra of porous materials are processed, numerical methods of lifetime spectra analysis have to be used with a particular care. Among various methods of positron annihilation lifetime spectra analysis available, the most popular ones were tested implemented in programs MELT and LT. It was found, from the simulated spectra study, that a large number of counts in a spectrum is needed to obtain reliable results of analysis. Even then only an approximate solution is available. Determination of the approximation nature helps in a further interpretation of the results and in positron annihilation lifetime spectrometer setting adequate to the problem. For this purpose, a few complex spectra obtained from positron beam facility were tested. The results show a negligible influence of resolution function on long lifetime components, while the precise estimation of the background level is crucial. Also some distortions caused by imperfection of numerical analysis methods are pointed out.
2
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Positron Porosimetry Studies of Template Removal from As-Synthesized MCM-41 Silica

64%
Zaleski R., Wawryszczuk J.
Acta Physica Polonica A
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2008
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vol. 113
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issue 5
1543-1550
EN
The course of template removal from MCM-41 pores was investigated by positron annihilation lifetime spectroscopy. Two methods of removal were the object of interest: a decarbonisation after standard calcination procedure and a novel procedure - pyrolysis in vacuum. The shapes of positron lifetime spectra of decarbonised sample are determined by positronium quenching caused by presence of carbon deposit on the surface. Positron annihilation lifetime spectroscopy allows also monitoring the evolution of empty space inside pores. During pyrolysis no conductive carbon is produced, so consecutive stages of template material transformation are easy to observe. In the case of pyrolysis the effect of pore emptying is obtained at temperature 500 K, lower comparing to calcination and decarbonisation (820 K).
3
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Positron Annihilation in Steel Burnished by Vibratory Shot Peening

64%
Zaleski R., Zaleski K.
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EN
A series of C45 steel samples was burnished by shot peening with varying time of treatment. The samples were investigated by nondestructive positron annihilation techniques: angular correlations of two-quantum annihilation radiation and positron annihilation lifetime spectroscopy. To determine residual stresses present under burnished surface the same samples were studied by destructive Davidenkov method. Change of absolute value of the weighted average of residual stresses over positron range in the series of the samples is in good agreement with change of S/W ratio obtained by angular correlations of two-quantum annihilation radiation. Both parameters increase during first 10 minutes of shot peening and then stabilize. Positron annihilation lifetime spectra allowed to identify two types of structural defects: smaller ones like vacancies or dislocations and bigger - probably clusters of vacancies. Increase in shot peening time causes reduction of positron trapping rate and lifetime rise in bigger defects.
4
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Three-Quantum Annihilation in Porous Vycor Glass

51%
Jasińska B., Wawryszczuk J., Zaleski R.
Acta Physica Polonica A
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2005
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vol. 107
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issue 5
821-825
EN
Three methods of determination of 3γ o-Ps decay intensity are compared. The estimate of 3γ fraction obtained from classical lifetime measurements can be distorted due to a higher absorption of theγ quanta from the continuous spectrum of three-quantum decay, compared to two-quantum one.
5
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Controlled Porosity of MCM-41 Obtained by Partial Blocking of Pores by Silicon Oil

51%
Gorgol M., Krasucka P., Goworek J., Zaleski R.
Acta Physica Polonica A
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2017
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vol. 132
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issue 5
1559-1563
EN
Partial blocking of mesopores in the ordered MCM-41 silica by DC550 silicon oil was proposed as a preparation method of the material with controlled porosity and fixed pore size. The porosity of the samples with various content of DC550 was examined with the use of low temperature nitrogen sorption and positron annihilation lifetime spectroscopy. It was shown that the oil blocks the primary pores by forming the plugs near its entrances, but also partially locates in the interparticle spaces. The comparison of the results obtained from both investigation techniques was used to make the first attempt to obtain the calibration of ortho-positronium intensity, depending on pore volume. This is necessary to improve the utility of positron annihilation lifetime spectroscopy as a porosimetric technique. The need to take the migration of positronium to larger free volume into account is discussed.
6
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Porosity of Silica Monoliths with Tailored Mesopores of Ink-Bottle Shape Determined by Nitrogen Adsorption and Positron Annihilation Lifetime Spectroscopy

51%
Sienkiewicz A., Kierys A., Gorgol M., Zaleski R.
Acta Physica Polonica A
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2017
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vol. 132
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issue 5
1568-1571
EN
The comparison of pore size distributions obtained with the use of two porosimetric methods: low temperature nitrogen sorption (LN2) and positron annihilation lifetime spectroscopy (PALS), was performed for porous silica monoliths. Four investigated samples were prepared under various synthesis conditions. Nitrogen sorption showed the presence of bottle-shaped pores in all investigated samples. In addition, it seems that the presence of methanol during synthesis influences porosity to a greater extent than organic additives. Quite good agreement between the LN2 and PALS results was observed only for the silica monolith synthesized with β-cyclodextrins as pore directing agent. The biggest discrepancy in the results obtained from these two techniques was observed for the silica gel synthesized with no additives. The origin of such discrepancies, taking into account the shape of pore size distribution and deficiencies of data analysis techniques, is discussed.
7
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Positronium Formation in Solid Long-Chain Alkanes

51%
Zaleski R., Wawryszczuk J., Goworek T.
Acta Physica Polonica A
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2005
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vol. 107
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issue 4
635-641
EN
This paper represents a fragment of systematic studies on positron annihilation in solid saturated hydrocarbons (alkanes, paraffins) from C_{17}H_{36} to C_{30}H_{62}. The Tao-Eldrup model and its extensions allow to identify the places in the crystalline structure where positronium is trapped, and estimate respective trap sizes. In the rigid phase I positronium locates mainly in two-dimensional empty spaces between the lamellae of crystal structure. Additional free volumes exist in high temperature phases due to appearance of n-alkane conformers representing a non-planar molecule distortion. The rise of ortho-positronium intensity with the sample irradiation dose, which was seen earlier in polymers, is observed also in n-alkanes. One can find similarities between the ortho-positronium annihilation in crystalline alkanes and polymers, like exponential character of that rise, and the trap bleaching by illumination. It confirms the same origin of the intensity growth, i.e. the accumulation of trapped electrons. Two kinds of electron traps were found. The time constant of electron accumulation in the traps depends on the carbon chain length and varies with temperature in different way for even- and odd-numbered alkanes.
8
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Pressure Effects in ortho-Positronium Annihilation

51%
Goworek T., Wawryszczuk J., Zaleski R.
Acta Physica Polonica A
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2005
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vol. 107
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issue 4
608-614
EN
A short review of experiments concerning the influence of high pressure on positron annihilation in molecular solids is given. To this kind of experiments belong e.g. pressure inhibition of positronium formation, pressure induced phase transitions, and free volume swelling in the media intercalated by high pressure gas.
9
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Impact of Impulse Shot Peening Parameters on Properties of Stainless Steel Surface

45%
Wiertel M., Zaleski K., Gorgol M., Skoczylas A., Zaleski R.
Acta Physica Polonica A
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2017
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vol. 132
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issue 5
1611-1615
EN
Shot peening was applied to austenitic stainless steel 1.4541 (EN). The surface treatment was performed at various impact energies E, impact densities j and ball diameters D. This resulted in improved microhardness, which increases monotonically with the increase of E, j and 1/D. However, its changes with E and j achieve saturation at about 400 HV0.1. On the contrary, no saturation is observed in the investigated range for 1/D. In the un-shot peened 1.4541 (EN) steel, the lifetime component of low intensity was found with use of positron annihilation lifetime spectroscopy (PALS). It corresponds to positron annihilation from delocalized state of positrons in bulk. In the shot peened samples the bulk component is no longer observed. Instead, two types of defects can be identified: vacancy-like defects coupled with edge dislocations and vacancies or their small clusters (consisting 3÷5 vacancies). The results of PALS and hardness testing do not correspond very well, especially in the case of the samples shot peened with balls of varying diameters. The most probable reason for this are different depth profiles of both methods. It seems that the defects, which are responsible for the increase of static microhardness above 400 HV0.1 are located mostly below the surface layer penetrated by positrons.
10
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Gas Pressure Induced Porosity of CYTOP Polymer. A~Positron Study

45%
Zgardzińska B., Goworek T., Śniegocka M., Zaleski R., Dlubek G.
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vol. 125
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issue 3
782-784
EN
The pore formation in CYTOP polymer under high pressure of argon or nitrogen gases was investigated by positron annihilation spectroscopy. In the range 0-70 MPa, formation of large open pores was observed. After reduction of pressure to normal one, the free volumes inside the polymer structure remained enlarged. The structure of CYTOP can be restored by applying the temperature over 470 K at normal pressure.
11
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Testing the Extended Tao-Eldrup Model. Silica Gels Produced with Polymer Template

39%
Śniegocka M., Jasińska B., Wawryszczuk J., Zaleski R., Deryło-Marczewska A., Skrzypek I.
Acta Physica Polonica A
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2005
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vol. 107
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issue 5
868-873
EN
The temperature dependence of o-Ps lifetime in the pores calculated from extended Tao-Eldrup model was compared to the experimental data, collected for a set of silica gels synthesized using the polymer template technique. For the sample with average pore radii 2.0 nm rather good agreement between the model and experiment was found. For the sample with narrow pores (near 1.0 nm) the lifetime spectrum was complex. Except the range of highest temperatures the lifetimes are far from model calculations.
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