Autor: Prochazka, I. - Prolib Integro

Skocz do pozycji: 1.

Tytuł:: Use of Energy Summing for Selection of Coincidence Events in Positron-Lifetime Spectroscopy
Autorzy:: Bec̆vár̆, F.
C̆iz̆ek, J.
Procházka, I.
Tematy:: 78.70.Bj; Pokaż więcej
Wydawca:: Polska Akademia Nauk. Instytut Fizyki PAN
Powiązania:: https://bibliotekanauki.pl/articles/2007883.pdf Link otwiera się w nowym oknie
Opis:: A BaF$\text{}_{2}$ positron-lifetime spectrometer equipped with a coincidence system that represents a compromise between the traditional fast-fast and fast-slow arrangements is described. The main difference of the present configuration from both the fast-slow and fast-fast ones consists in use of a sum of the energy signals from the start and stop detectors to select the coincidence events. Quality of the spectrometer response function (≈ 150 ps FWHM for $\text{}^{22}$Na) is very close to that observed for our fast-slow configuration with the equivalent detectors but throughput of the present apparatus is increased almost by a factor of two. Moreover, the electronic scheme becomes less complicated, which has also a positive impact on its cost.
Dostawca treści:: Biblioteka Nauki

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Skocz do pozycji: 2.

Tytuł:: Spatially Modulated Bragg Gratings
Autorzy:: Procházka, I.
Kotac̆ka, L.
Petrás̆, F.
Tematy:: 42.70.Dj; Pokaż więcej
Wydawca:: Polska Akademia Nauk. Instytut Fizyki PAN
Powiązania:: https://bibliotekanauki.pl/articles/2008273.pdf Link otwiera się w nowym oknie
Opis:: A set of coupled mode differential equations which describe the light propagation in an optical fiber containing a photoinduced Bragg grating is derived. Reflection spectra based on numerical solution to these equations are presented when basic grating parameters are varied. In addition, the influence of these parameters on the maximum reflectivity and 3 dB bandwidth is examined. Finally, reflection spectra of the chirped grating and the grating with spatially modulated refractive index changes are discussed.
Dostawca treści:: Biblioteka Nauki

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Skocz do pozycji: 3.

Tytuł:: Hydrogen-Induced Defects in Palladium
Autorzy:: Melikhova, O.
Čížek, J.
Procházka, I.
Tematy:: 78.70.Bj
61.72.-y; Pokaż więcej
Wydawca:: Polska Akademia Nauk. Instytut Fizyki PAN
Powiązania:: https://bibliotekanauki.pl/articles/1338026.pdf Link otwiera się w nowym oknie
Opis:: 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.
Dostawca treści:: Biblioteka Nauki

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Skocz do pozycji: 4.

Tytuł:: Performance of a High-Resolution Digital Positron-Lifetime Spectrometer
Autorzy:: Bečvář, F.
Čížek, J.
Procházka, I.
Tematy:: 78.70.Bj; Pokaż więcej
Wydawca:: Polska Akademia Nauk. Instytut Fizyki PAN
Powiązania:: https://bibliotekanauki.pl/articles/1812465.pdf Link otwiera się w nowym oknie
Opis:: 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 $\text{}^{22}Na$) from 169 ps for the traditional analogue setup to 146 ps for the digital setup. The coincidence count rate for 1.2 MBq $\text{}^{22}Na$ source is 75-80 coinc./s for both configurations. These results clearly demonstrate the advantages of the digital timing over the traditional analogue setup.
Dostawca treści:: Biblioteka Nauki

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Skocz do pozycji: 5.

Tytuł:: Defects in Zirconia Nanomaterials Doped with Rare-Earth Oxides
Autorzy:: Prochazka, I.
Cizek, J.
Melikhova, O.
Konstantinova, T.
Danilenko, I.
Tematy:: 81.07.Wx
61.46.Hk
78.70.Bj; Pokaż więcej
Wydawca:: Polska Akademia Nauk. Instytut Fizyki PAN
Powiązania:: https://bibliotekanauki.pl/articles/1338541.pdf Link otwiera się w nowym oknie
Opis:: Positron lifetime and coincidence Doppler broadening measurements on $ZrO_2$+3 mol.% $RE_2O_3$ (RE = Eu, Gd, Lu) nanopowders and ceramics obtained by sintering these nanopowders are reported. The initial nanopowders were prepared by a co-precipitation technique and exhibited a mean particle size of ≈ 15 nm. The nanopowders were calcined and pressure-compacted. All compacted nanopowders exhibited the prevailing tetragonal phase with at most 15% of the monoclinic admixture. Positrons in compacted nanopowders were found to annihilate almost exclusively at grain boundaries: (i) vacancy-like misfit defects along grain boundaries and (ii) larger defects situated at intersections of grain boundaries (triple points). In nanopowders, a small portion of positrons formed positronium in pores between crystallites. Sintering of nanopowders at 1500C caused a substantial grain growth and formation of ceramics. Sintering-induced grain growth led to a disappearance of the triple points and pores. The ceramics containing Eu and Gd dopants consist of mixture of the monoclinic and the tetragonal phase, while the ceramics with Lu dopant exhibits almost exclusively the tetragonal phase.
Dostawca treści:: Biblioteka Nauki

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Skocz do pozycji: 6.

Tytuł:: Precipitation Effects in Mg-Zn Alloys Studied by Positron Annihilation and Hardness Testing
Autorzy:: Hruška, P.
Čížek, J.
Vlček, M.
Melikhova, O.
Procházka, I.
Tematy:: 78.70.Bj
61.72.J-; Pokaż więcej
Wydawca:: Polska Akademia Nauk. Instytut Fizyki PAN
Powiązania:: https://bibliotekanauki.pl/articles/1196079.pdf Link otwiera się w nowym oknie
Opis:: 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.
Dostawca treści:: Biblioteka Nauki

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Skocz do pozycji: 7.

Tytuł:: Investigation of Precipitation Effects in Mg-Tb and Mg-Tb-Nd Alloys
Autorzy:: Vlček, M.
Čížek, J.
Melikhova, O.
Hruška, P.
Procházka, I.
Tematy:: 78.70.Bj
61.72.J-; Pokaż więcej
Wydawca:: Polska Akademia Nauk. Instytut Fizyki PAN
Powiązania:: https://bibliotekanauki.pl/articles/1196115.pdf Link otwiera się w nowym oknie
Opis:: 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.
Dostawca treści:: Biblioteka Nauki

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Skocz do pozycji: 8.

Tytuł:: Influence of Ceramic Nanoparticles on Thermal Stability of Ultra Fine Grained Copper
Autorzy:: Čížek, J.
Procházka, I.
Cieslar, M.
Islamgaliev, R.
Kulyasova, O.
Tematy:: 78.70.Bj
79.60.Jv; Pokaż więcej
Wydawca:: Polska Akademia Nauk. Instytut Fizyki PAN
Powiązania:: https://bibliotekanauki.pl/articles/1812477.pdf Link otwiera się w nowym oknie
Opis:: 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.%.
Dostawca treści:: Biblioteka Nauki

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Skocz do pozycji: 9.

Tytuł:: Positron Lifetime Spectroscopy of Nanocrystalline Copper
Autorzy:: C̆ížek, J.
Procházka, I.
Vostrý, P.
Chmelík, F.
Islamgaliev, R. K.
Tematy:: 78.70.Bj
79.60.Jv; Pokaż więcej
Wydawca:: Polska Akademia Nauk. Instytut Fizyki PAN
Powiązania:: https://bibliotekanauki.pl/articles/2007957.pdf Link otwiera się w nowym oknie
Opis:: Positron lifetime spectroscopy was applied to investigate the thermal stability of nanocrystalline copper prepared by severe plastic torsion deformation. Positrons annihilating in as prepared specimens exhibited free positron component τ$\text{}_{1}$ and two defect components τ$\text{}_{2}$=164 ps and τ$\text{}_{3}$=255 ps. Evolution of the lifetimes and relative intensities of all the three components with increasing annealing temperature during step-by-step isochronal annealing up to 630°C was studied. Behaviour of positrons in nanocrystalline copper could not be interpreted in the frame of conventional 3-state trapping model due to highly inhomogeneous defect distribution. Therefore a modified trapping model was developed and applied to explain the experimental results.
Dostawca treści:: Biblioteka Nauki

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Skocz do pozycji: 10.

Tytuł:: Defects Studies of ZnO Single Crystals Prepared by Various Techniques
Autorzy:: Lukáč, F.
Čížek, J.
Procházka, I.
Melikhova, O.
Anwand, W.
Brauer, G.
Tematy:: 78.70.Bj
61.72.J-; Pokaż więcej
Wydawca:: Polska Akademia Nauk. Instytut Fizyki PAN
Powiązania:: https://bibliotekanauki.pl/articles/1196118.pdf Link otwiera się w nowym oknie
Opis:: 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.
Dostawca treści:: Biblioteka Nauki

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