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Thermal Development of Free Volumes in Nafion Membrane

100%
Melikhova O., Čížek J., Košinová L., Hruška P.
Acta Physica Polonica A
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2017
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vol. 132
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issue 5
1538-1542
EN
In this work we employed positron lifetime spectroscopy for investigation of the thermal development of free volumes in H⁺ Nafion membrane over a broad range of temperatures from -150 to 150°C. Positron lifetime studies were combined with differential scanning calorimetry. Size distribution of free volumes was determined from ortho-positronium contribution to positron lifetime spectra. Our investigations revealed that the mean size of free volume holes strongly increases with temperature. On the other hand, the width of the size distribution and the positronium yield decreases with temperature. Transition temperatures corresponding to a change in the slope of the temperature dependence of ortho-positronium lifetime were identified. Results of positron lifetime spectroscopy agree well with the curve obtained by differential scanning calorimetry.
2
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Precipitation Effects in Mg-Zn Alloys Studied by Positron Annihilation and Hardness Testing

88%
Hruška P., Čížek J., Vlček M., Melikhova O., Procházka I.
Acta Physica Polonica A
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2014
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vol. 125
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issue 3
718-721
EN
In the present work positron annihilation spectroscopy combined with Vickers hardness testing were employed in order to investigate precipitation effects in Mg-Zn alloys. It was found that incoherent precipitates of a metastable Zn-rich phase formed in the samples isochronally annealed above 200C cause hardening of the alloy.
3
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Investigation of Precipitation Effects in Mg-Tb and Mg-Tb-Nd Alloys

88%
Vlček M., Čížek J., Melikhova O., Hruška P., Procházka I.
Acta Physica Polonica A
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2014
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vol. 125
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issue 3
744-747
EN
Investigation of precipitation effects in solution treated Mg-Tb and Mg-Tb-Nd alloy was performed. Solution treated alloys were compared with samples deformed by high pressure torsion to examine influence of deformation on precipitation effects. Dislocations present in samples processed by high pressure torsion can serve as diffusion channels for atoms and also as nucleation sites for precipitates. Therefore precipitation of some phases in high pressure torsion deformed samples was observed at lower temperatures than in solution treated ones.
4
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Characterization of Defects in Titanium Created by Hydrogen Charging

76%
Hruška P., Čížek J., Knapp J., Melikhova O., Havela L., Mašková S., Lukáč F.
Acta Physica Polonica A
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2017
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vol. 132
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issue 5
1606-1610
EN
Hydrogen interaction with vacancies in α-Ti was investigated employing positron lifetime spectroscopy combined with ab initio theoretical modeling of vacancy-hydrogen complexes. Ab initio modeling revealed that multiple hydrogen atoms up to 7 can be trapped at vacancies in the α-Ti lattice. Trapped H atoms are located close to the nearest neighbor tetrahedral sites around the centre of vacancy. Lifetimes of positrons trapped at vacancies associated with various numbers of hydrogen atoms were calculated. Positron lifetime measurement of H-loaded α-Ti samples revealed that phase transition into the hydride phase introduced dislocations. Vacancies were created by H loading as well and agglomerated into small vacancy clusters.
5
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Positronium Formation in Nanostructured Metals

64%
Čížek J., Melikhova O., Hruška P., Vlček M., Anwand W., Liedke M., Novotný M., Bulíř J.
Acta Physica Polonica A
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2017
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vol. 132
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issue 5
1579-1584
EN
Positronium formation in Au films was studied using a magnetically guided continuous slow positron beam with variable energy. Black Au film with porous morphology was compared with conventional smooth Au film. In the smooth Au film positronium is formed on the film surface only. The black Au film exhibits porous sub-surface region containing micro-cavities interconnected with surface. Positronium is formed on inner surfaces of micro-cavities in the sub-surface region and travels through interconnected cavities towards the surface. 3-γ annihilation of ortho-positronium leaves a clear signature in two-dimensional coincidence Doppler broadening spectra. Measures of 3-γ annihilation contribution calculated from single γ-ray and coincidence Doppler broadening spectra were calculated and compared.
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