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1
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Performance of a High-Resolution Digital Positron-Lifetime Spectrometer

100%
Bečvář F., Čížek J., Procházka I.
Acta Physica Polonica A
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2008
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vol. 113
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issue 5
1279-1284
EN
This work describes a digital positron-lifetime spectrometer based on two ultrafast digitizers. Performance of the digital spectrometer in conditions of a routine measurement is compared with an analogue setup connected to the same detectors. It was found that engagement of the digital setup leads to an improvement of timing resolution (FWHM for ^{22}Na) from 169 ps for the traditional analogue setup to 146 ps for the digital setup. The coincidence count rate for 1.2 MBq ^{22}Na source is 75-80 coinc./s for both configurations. These results clearly demonstrate the advantages of the digital timing over the traditional analogue setup.
2
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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.
3
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Hydrogen-Induced Defects in Palladium

100%
Melikhova O., Čížek J., Procházka I.
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vol. 125
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issue 3
752-755
EN
In the present work positron annihilation spectroscopy was employed for investigation of hydrogen-induced defects in Pd. Well annealed Pd samples were electrochemically charged with hydrogen and development of defects with increasing hydrogen concentration x_{H} was investigated. At low concentrations (α-phase, x_{H} < 0.017 H/Pd) hydrogen loading introduced vacancies, since absorbed hydrogen segregating at vacancies lowers remarkably the vacancy formation energy. When hydrogen concentration exceeds 0.017 H/Pd, particles of palladium hydride (PdH) are formed. Stress induced by growing PdH particles leads to plastic deformation which generates dislocations and vacancies in the sample.
4
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Vickers Hardness and Positron Annihilation Study of Eurofer97 and ODS Eurofer

100%
Bartošová I., Čížek J., Lukáč F., Slugeň V.
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vol. 125
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issue 3
702-705
EN
This paper describes investigations of Eurofer97 and oxide dispersion strengthened version of this steel, in as received state and after annealing. Vickers hardness test was performed on the as received specimens and after annealing at various temperatures to obtain information about development of the strength of studied materials. After identifying the temperature region where significant changes of the strength take place we applied positron annihilation spectroscopy to probe the microstructural changes in ferritic/martensitic steels. An observation of phase transformation was observed in both steels at various temperatures indicating different behaviour of the base material and its strengthened version. All applied methods proved the importance of strengthening by oxides to improve properties of steels.
5
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Influence of Ceramic Nanoparticles on Thermal Stability of Ultra Fine Grained Copper

88%
Čížek J., Procházka I., Cieslar M., Islamgaliev R., Kulyasova O.
Acta Physica Polonica A
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2008
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vol. 113
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issue 5
1285-1292
EN
A detailed study of the bulk ultra fine grained pure copper and copper with Al_2O_3 particles was carried out in the present work. The specimens were prepared by the high-pressure torsion and their microstructure was investigated by positron lifetime spectroscopy combined with transmission electron microscopy and microhardness tests. Defects in the as-deformed materials were characterized and the thermal stability of the ultra fine grained microstructure was subsequently examined in annealing experiments. An addition of Al_2O_3 nanoparticles was found to improve significantly the thermal stability of the ultra fine grained structure, the optimum content of Al_2O_3 being ≈0.5 wt.%.
6
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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.
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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.
7
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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.
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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.
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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.
9
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Positron Lifetimes in Zirconia-Based Nanomaterials

76%
Procházka I., Čížek J., Kuriplach J., Melikhova O., Konstantinova T., Danilenko I.
Acta Physica Polonica A
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2008
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vol. 113
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issue 5
1495-1499
EN
Positron lifetime spectroscopy is employed in a comparative study of several zirconia-based materials: (i) the pressure-compacted nanopowders of the three zirconia polymorphs - pure ZrO_2 (monoclinic), yttria-stabilized ZrO_2+3 mol.% Y_2O_3 (tetragonal) and yttria-stabilized ZrO_2+8 mol.% Y_2O_3 (cubic), (ii) ceramic materials obtained by sintering of the above two yttria-stabilized zirconia nanopowders and (iii) the tetragonal and cubic yttria-stabilized zirconia monocrystals. Positron lifetime data observed on the nanopowders suggest that the two shortest components, exhibiting lifetimes of ≈180 and ≈370 ps, arise from the annihilation of positrons trapped in defects associated with grain boundaries, presumably the vacancy-like defects and tripple points, respectively. Positron lifetime spectra observed on the ceramic materials resemble those found for the corresponding monocrystals, giving thus an additional support to the above interpretation of the nanopowders results.
10
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Simulation of Positron Annihilation Response to Mechanical Deformation of Nanostructured AgCo

76%
Melikhova O., Kuriplach J., Čížek J., Procházka I., Hou M., Pisov S., Zhurkin E.
Acta Physica Polonica A
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2008
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vol. 113
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issue 5
1455-1464
EN
Nanostructured materials attract nowadays a broad attention due to their specific properties. Defects play an essential role in material properties so their characterisation is very important. The evolution of the various open volume defects in AgCo nanowire modelled samples obtained using molecular dynamics was studied. Isothermal and isoenergetic deformation mechanisms are considered. General analyses of open volume defects concerning their size and their chemical environment were performed. Positron lifetimes, binding energies, and high momentum parts of the momentum distribution of annihilationγ-quanta were calculated for selected defects.
11
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Defects Studies of ZnO Single Crystals Prepared by Various Techniques

76%
Lukáč F., Čížek J., Procházka I., Melikhova O., Anwand W., Brauer G.
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EN
The aim of the present work was a comparison of defects in ZnO crystals grown by various techniques available nowadays, namely hydrothermal growth, pressurized melt, Bridgman method growth and vapor phase growth. Positron annihilation spectroscopy was employed as a principal tool for characterization of defects in ZnO crystals grown by above mentioned various techniques. ZnO crystals can be divided into two groups: (i) hydrothermal grown crystals, which exhibit positron lifetime of 179-182 ps and (ii) ZnO crystals grown by the other techniques (pressurized melt, Bridgman method, vapor phase growth) which are characterized by the lower lifetimes falling in the range of 160-173 ps. Comparison of experimental data with ab initio theoretical calculations revealed that HT grown ZnO crystals contains Zn vacancies associated with hydrogen atom in a bond-centered site. On the other hand, ZnO crystals prepared by other techniques contain most probably stacking faults created by stresses induced by temperature gradients in the melt.
12
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Content available

Defects in Ultra-Fine Grained Mg and Mg-Based Alloys Prepared by High Pressure Torsion Studied by Positron Annihilation

64%
Čížek J., Procházka I., Smola B., Stulíková I., Kužel R., Matěj Z., Cherkaska V., Islamgaliev R., Kulyasova O.
Acta Physica Polonica A
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2005
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vol. 107
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issue 5
738-744
EN
Despite the favourable strength and thermal stability, a disadvantage of the Mg-based alloys consists in a low ductility. Recently it has been demonstrated that ultra fine grained metals with grain size around 100 nm can be produced by high pressure torsion. A number of ultra fine grained metals exhibit favourable mechanical properties consisting in a combination of a very high strength and a significant ductility. For this reason, it is highly interesting to examine microstructure and physical properties of ultra fine grained Mg-based light alloys. Following this purpose, microstructure investigations and defect studies of ultra fine grained pure Mg and ultra fine grained Mg-10%Gd alloy prepared by high pressure torsion were performed in the present work using positron annihilation spectroscopy combined with X-ray diffraction, TEM observations, and microhardness measurements. Positrons are trapped at dislocations in Mg and Mg-10%Gd alloy deformed by high pressure torsion. A number of dislocations increases with the radial distance r from the centre to the margin of the sample. No microvoids (small vacancy clusters) were detected. Mg-10%Gd alloy deformed by high pressure torsion exhibits a homogeneous ultra fine grained structure with a grain size around 100 nm and high dislocations density. On the other hand, pure Mg deformed by high pressure torsion exhibits a binomial type of structure which consists of "deformed regions" with ultra fine grained structure and a high dislocation density and dislocation-free "recovered regions" with large grains. It indicates a dynamic recovery of microstructure during high pressure torsion processing.
13
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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.
14
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Content available

Ultra Fine-Grained Metals Prepared by Severe Plastic Deformation: A Positron Annihilation Study

52%
Čížek J., Procházka I., Kužel R., Matĕj Z., Cherkaska V., Cieslar M., Smola B., Stulíková I., Brauer G., Anwand W., Islamgaliev R., Kulyasova O.
Acta Physica Polonica A
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2005
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vol. 107
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issue 5
745-752
EN
Recent investigations of ultra fine-grained metals (Cu, Fe, Ni) performed within a Prague-Rossendorf-Ufa collaboration will be reviewed. The specimens were prepared by severe plastic deformation: the high-pressure torsion and equal channel angular pressing. Positron annihilation spectroscopy was used as the main method including (i) the conventional lifetime and the Doppler broadening measurements with ^{22}Na and (ii) the slow-positron implantation spectroscopy with the Doppler broadening measurement. Other methods were also involved: transmission electron microscopy, X-ray diffraction, and microhardness. First, the mean grain size was determined and defects were identified in the as-deformed materials. Defects concentration and spatial distribution were studied in detail. Dislocations situated in distorted regions along grain boundaries, and a few-vacancy clusters distributed homogeneously inside dislocations-free grains, were observed in the ultra fine-grained Cu, Fe, and Ni. Subsequently, the thermal evolution of the ultra fine-grained structures during isochronal annealing was studied.
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