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1

Study on the effect of atmospheric gases adsorbed in MnFe2O4/MCM-41 nanocomposite on ortho-positronium annihilation

Wiertel M., Surowiec Z., Budzyński M., Gac W.

Nukleonika

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2015

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Vol. 60, No. 4, part 1

783--787

EN

In this paper, results of positron annihilation lifetime spectroscopy (PALS) studies of MnFe2O4/MCM- -41 nanocomposites in N2 and O2 atmosphere have been presented. In particular, the influence of manganese ferrite loading and gas filling on pick-off ortho-positronium (o-Ps) annihilation processes in the investigated samples was a point of interest. Disappearance of the longest-lived o-Ps component with τ5 present in the PAL spectrum of initial MCM-41 mesoporous material in the PAL spectra of MnFe2O4-impregnated MCM-41 measured in vacuum is a result of either a strong chemical o-Ps quenching or the Ps inhibition effects. The intensity I4 of the medium-lived component initially increases, reaching a maximum value for the sample with minimum manganese ferrite content, and then decreases monotonically. Analogous dependence for the intensity I3 of the shortest-lived component shows a maximum at higher MnFe2O4 content. Filling of open pores present in the studied nanocomposites by N2 or O2 at ambient pressure causes partial reappearance of the τ4 and τ5 components, except a sample with maximum ferrite content. The lifetimes of these components measured in O2 are shortened in comparison to that observed in N2 because of paramagnetic quenching. Anti-inhibition and anti-quenching effects of atmospheric gases observed in the MnFe2O4/MCM-41 samples are a result of neutralization of some surface active centers acting as inhibitors and weakening of pick-off annihilation mechanism, respectively.

2

Magnetic and structural properties of Sc(Fe1−xSix)2 Laves phases studied by Mössbauer spectroscopy and neutron diffraction

Wiertel M., Surowiec Z., Budzyński M., Sarzyński J., Beskrovnyi A. I.

Nukleonika

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2015

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Vol. 60, No. 1

155--160

EN

The investigations of iron-containing phases existing in fly ashes were performed using transmission Mössbauer spectrometry. The examined samples of fly ashes were collected from different coal combustion systems, that is, stoker-fired boiler in municipal heating plant and pulverized coal boiler in power plant. Several phases are identifi ed in the samples: iron oxides, paramagnetic aluminosilicate glass with Fe3+ ions and Al2O4- -type spinel with Fe2+ ions. It was pointed out that proportions of contents of phases strongly depend not only on the combustion temperature but also on the way of ash collection.

3

Influence of annealing temperature on structural and magnetic properties of MnFe2O4 nanoparticles

Surowiec Z., Wiertel M., Gac W., Budzyński M.

Nukleonika

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2015

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Vol. 60, No. 1

137--141

EN

Nanoparticles of manganese ferrite were obtained by the impregnation of highly ordered mesoporous MCM-41 silica support. The investigated sample contained 20% wt. Fe. The obtained nanocrystallites were strongly dispersed in silica matrix and their size was about 2 nm. The sample annealing at 500°C led to increase of particle size to about 5 nm. The Mössbauer spectroscopy investigations performed at room temperature show on occurrence of MnFe2O4 nanoparticle in superparamagnetic state for the sample annealed in all temperatures. The coexistence of superparamagnetic and ferromagnetic phase was observed at liquid nitrogen temperature. The sample annealed at 400°C and 500 °C has bigger manganese ferrite particle and better crystallized structure. One can assign them the discrete hyperfi ne magnetic field components.

4

Synthesis and characterization of iron - cobalt nanoparticles embedded in mesoporous silica MCM - 41

Surowiec Z., Wiertel M., Budzyński M., Gac W.

Nukleonika

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2013

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Vol. 58, No. 1

87-92

EN

Fe1.xCox/MCM-41 nanocomposite materials were prepared by wet impregnation of the silica mesoporous support with aqueous solution of iron and cobalt salt mixtures. Samples, after calcination, were reduced at 573 K in order to obtain homogeneous distribution of (Fe-Co)3O4 oxide species over the MCM-41 support. Bimetallic systems were achieved through high temperature reduction, carried out at 1073 K. The temperature-programmed reduction studies indicated complex nature of the oxide species. The phase analysis of the obtained samples after low temperature reduction process carried out by means of X-ray diffraction method and using 57Fe Mossbauer spectroscopy revealed the presence of nanostructured magnetite- and maghemite-like phases. The samples with higher cobalt contents (x . 0.2) contained additionally bimetallic phases. The complete transformation of oxide phases to bimetallic Fe-Co systems occurred during reduction process in hydrogen at 1073 K. It was observed that high temperature processing caused partial transformation of highly dispersed iron oxide nanocrystallites into fayalite species embedded in the silica walls.

5

Structural and magnetic properties of Sc(Fe1-xCux)2 compounds studied by means of Mössbauer effect and neutron diffraction

Wiertel M., Surowiec Z., Budzyński M., Sarzyński J.

Nukleonika

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2013

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Vol. 58, No. 1

173-178

EN

The subject of the investigations were quasibinary Sc(Fe1-xCux)2 Laves phases in which structural and magnetic properties depend on replacement of Fe atoms by Cu atoms. Powder X-ray diffraction and neutron diffraction measurements carried out at room temperature revealed that samples were a mixture of two phases: the quasibinary Sc(Fe1-xCux)2 compounds with cubic C15 structure and ScCu2 with tetragonal structure. 57Fe Mossbauer spectroscopy revealed that the Sc(Fe1-xCux)2 compounds are ferrimagnetic up to an actual concentration xact less than 0.30. A decreasing of mean values of hyperfine magnetic fields was observed. On the basis of analysis of the neutron diffraction spectra the mean values of Fe magnetic moments were determined, considering the component connected with the quasibinary Sc(Fe1-xCux)2 phases. These moments change from 1.45(29) �ĘB in the Sc(Fe0.92Cu0.08)2 to 1.18(32)mi B in Sc(Fe0.72Cu0.28)2 phase at 8 K. From the results of independent hyperfine magnetic field on 57Fe probes and magnetic Fe moments measurements, the hyperfine coupling constant in quasibinary ferrimagnetic Sc(Fe1.xCux)2 phases is estimated as equal to 13 T/mi B at room temperature.

6

Positron annihilation studies of mesoporous iron modified MCM-41 silica

Wiertel M., Surowiec Z., Budzyński M., Gac W.

Nukleonika

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2013

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Vol. 58, No. 1

245-250

EN

MCM-41 silica materials modified by iron incorporation in the stage of its synthesis were investigated. The aim of the studies was determination of the nature of iron species and the influence of its content on the structural changes of materials and following the changes of their properties. For this purpose, the N2 sorption/desorption method and positron annihilation lifetime spectroscopy (PALS) were used. Disappearance of the longest-lived ortho-positronium (o-Ps) component (tau5) present in the PALS spectra of the initial MCM-41 material in the spectra of Fe-modified MCM-41 measured in vacuum is a result of a strong chemical o-Ps quenching and/or the Ps inhibition mechanism. Filling of pores by air or N2 at ambient pressure causes reappearance of the (tau5) component with lifetime shortened in comparison to that observed in vacuum for pure MCM-41 to the extent which can be explained by usual paramagnetic quenching in air. In contrary to the tendency observed for (tau5) lifetime which is practically independent of Fe content, the relevant intensity I5 monotonically decreases. This fact suggests that only inhibition of Ps formation occurs for the samples in air. Observed anti-quenching effect of air seems to be a result of competition of two processes : neutralization of surface active centres acting as inhibitors and considerably weaker paramagnetic quenching by O2 molecules.

7

An application of Mössbauer spectroscopy in the studies of iron - molybdenum silica mesoporous materials

Surowiec Z., Wiertel M., Marynowska A., Gac W., Zawadzki W.

Nukleonika

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2013

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Vol. 58, No. 1

137-141

EN

Iron-molybdenum silica mesoporous materials were obtained by the application of direct hydrothermal method. Four samples with different metal contents relative to silica were investigated. Incorporation of iron and molybdenum ions in the synthesis stage led to structural changes of the MCM-41 support. With an increasing metals content, cylindrical pores of silica initially transformed into bottle-ink type pores, and then into the slit-like ones. Mössbauer spectroscopy investigations indicated that Fe ions were embedded in the silica walls and then formed nanosized crystallites in the pores. Superparamagnetic phase was observed at liquid nitrogen temperature.

8

Positron annihilation study of iron oxide nanoparticles in mesoporous silica MCM-41 template

Surowiec Z., Wiertel M., Zaleski R., Budzyński M., Goworek J.

Nukleonika

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2010

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Vol. 55, No. 1

91-96

EN

The subject of investigation were the samples obtained by impregnation of MCM-41 template with an aqueous solution prepared from Fe(NO3)3ź9H2O. As a result of such a procedure, iron oxides deposits on MCM-41 were formed. The Mössbauer studies revealed an occurrence of Fe3O4 nanocrystallites in the both ferri- and superparamagnetic states. Almost 80% small particles exist in a paramagnetic state. The positron annihilation lifetime spectroscopy (PALS) spectra were measured in air or in vacuum. The long-lived ortho-positronium (o-Ps) components and their intensities are time dependent due to air molecules interaction with iron oxide nanocrystallites and silica walls surfaces. The adsorption of air on the nanocrystallites surface causes a total screening of their surface and a raise of the o-Ps lifetime values. Observed anti-quenching effect is a result of competition of two phenomena: practically switching off a pick-off mechanism related to interaction of o-Ps with magnetite nanoparticles and considerably weaker usual quenching by paramagnetic oxygen molecules.

9

Transient effects in electron transport through quantum dots

Wiertel M., Parafiniuk P., Taranko R.

Optica Applicata

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2009

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Vol. 39, nr 4

959-965

EN

We investigate the transient electron transport through the quantum dot and double quantum dot systems coupled with the time-dependent barriers to infinitely large reservoirs of noninteracting electrons. Time-dependent currents and quantum dot occupancies are calculated using both nonequilibrium Green's functions formalism and the equation of motion method for appropriate correlation functions. We show that the sequence of ultrafast modulation of the tunneling amplitudes between the electron reservoirs and the quantum dots can induce quite different electron occupation of the quantum dot in comparison with the static case. We also find that the oscillations of the transient current following the sudden coupling of the electron reservoirs with the double quantum dot system have the same frequency as Rabi's oscillations of the double dot state vector.transient current.

10

Structural and magnetic investigations of Sc(Fe1-xNix)2 compounds by means of Mössbauer effect and neutron diffraction

Wiertel M., Surowiec Z., Sarzyński J., Budzyński M., Beskrovnyi I.

Nukleonika

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2007

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Vol. 52, supl. 1

71-75

EN

The structural and microscopic magnetic properties of the pseudobinary Sc(Fe1 xNix)2 system were studied by XRD and Mössbauer measurements in the range of 0.10 less-than or equal to - x less-than or equal to - 0.60. All investigated samples have the cubic C15-type structure. Both the magnetic hyperfine field on 57Fe and the Curie temperature decrease with increasing Ni content and the system becomes paramagnetic for x H 0.60 at room temperature. The form and temperature dependences of the Mössbauer spectra for 0.40 less-than or equal to - x less-than or equal to - 0.50 indicate the coexistence of paramagnetic and ferromagnetic regions in the samples and occurrence of magnetic clusters with a wide distribution of the Curie temperatures. As follows from the near to zero values of quadrupole shifts in Mössbauer spectra it results that a <100> direction is a local easy magnetization axis for x e 0.10.

11

Mössbauer study of magnetite nanowire in MCM-41 type mesoporous silica templates

Surowiec Z., Goworek J., Ryczkowski J., Budzyński M., Wiertel M., Sarzyński J.

Nukleonika

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2007

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Vol. 52, supl. 1

33-36

EN

Arrays of Fe3O4 nanowires embedded in mesoporous ordered silica were obtained in two ways: dissolving Fe3+ ions in an aqueous medium and their adsorption on internal and external surfaces, with the use of a Fe-EDTA complex. Magnetite polycrystalline nanowires were characterized by means of X-ray diffraction and 57Fe Mössbauer spectroscopy (MS). The average length of these nanowires is about 70 nm and their diameter is about 3 nm. Mössbauer studies evidenced that the composites consist of very small Fe3O4 particles. Almost 80% of particles exist in a paramagnetic state.

12

Electron transport in the multi-terminal guantum dot system

Taranko E., Taranko R., Wiertel M.

Optica Applicata

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2005

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Vol. 35, nr 3

369-374

EN

The time-dependent electron transport through a multi-terminal quantum dot system is studied. External microwave fields with arbitrary amplitudes, phases and frequencies are applied to different parts of the system considered. The dependence of the average current and average differential conductance on different parameters of the external microwave fields is analyzed. Special attention is paid to the photon-electron pump effect observed for some values of the quantum dot system parameters.

13

Effect of the substitution of Ti for Y on structural properties and hyperfine interactions in Y1-xTixFe2 compounds

Surowiec Z., Sarzyński J., Budzyński M., Wiertel M.

Nukleonika

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2004

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Vol. 49,suppl.3

17-21

EN

Abstract Room temperature 57Fe Mössbauer spectroscopy and X-ray diffraction measurements are reported for a series of Y1 xTixFe2 polycrystalline samples. Our research shows that except for x = 0 and x = 1, the samples are a mixture of two C15 and C14 pseudobinary Laves phases. Some Ti atoms occupy the crystallographic positions 8a characteristic of the Y atoms in the cubic C15 structure while the rest of them form the hexagonal C14 phase. However, the lattice constant dependencies on concentration x suggest the occurrence of Y atoms in the positions 4f characteristic of Ti atoms in the hexagonal C14 structure. The partial substitution of Ti atoms for Y atoms is responsible for the HMF changes in the cubic phase. Contrary to the situation taking place in the Zr1 xTixFe2 compounds, magnetic arrangement in the pseudobinary hexagonal phases is not observed at room temperature.

14

Effect of Sc substitution for Y on structural properties and hyperfine interactions in Y1-xScxFe2 compounds

Budzyński M., Sarzyński J., Wiertel M., Surowiec Z.

Nukleonika

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2003

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Vol. 48,suppl.1

79-83

EN

Measurements of X-ray diffraction and Mössbauer effect were made on a series of Y1-xScxFe2 polycrystalline samples (x = 0.0, 0.2, 0.5, 0.7, 0.8, 1.0). It was found that the system has the cubic MgCu2 structure except for x = 1.0 where hexagonal MgZn2 structure type is stable. The lattice constant decreases with increasing x. The results of Mössbauer effect study at room temperature show that the easy axes of magnetization remain in the <111> direction for the cubic samples and <100> for the hexagonal one. Curie temperatures TC were determined from temperature dependence of the hyperfine magnetic fields. The concentration dependence of the hyperfine fields and TC show similar trends, exhibiting a maximum at x = 0.8. The magnetic and structural behaviour show that, in spite of being isoelectronic, the substitution of Y by Sc induces clear changes in the structural and magnetic properties of the compounds under investigation.

15

Influence of adsorbed atoms on the charge transfer in atom/ionsurface collision

Wiertel M., Taranko E., Taranko R.

Optica Applicata

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2002

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Vol. 32, nr 3

301-305

EN

The ionization probability of an atom scattered on the atom adsorbed on a metal surface has been studied theoretically within the time-dependent Anderson–Newns model and the time-evolution operator technique. The influence of different parameters describing the investigated system on the dynamics of the charge transfer between the moving atom and the metal surface with an adsorbed atom has been investigated and the comparison with the results obtained for scattering on a clean metal surface has been made.

16

Rezonansowy przepływ ładunku w zderzeniach atomów z powierzchnią metalu pokrytą adsorbatem

Wiertel M., Taranko E., Taranko R.

Elektronika : konstrukcje, technologie, zastosowania

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2001

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Vol. 42, nr 8-9

45-47

EN

The interaction of ions/atoms with surfaces is a very complex process involving many fundamental phenomena. One of such phenomena is a resonant charge transfer between the incident ions/atoms and the target surfaces. Resonant charge transfer in ion scattering on the atom adsorbed on the metal surface has been studied theoretically within the time-dependent Anderson-Newns model. The effect of the metal electron density of states, the band filling as well as the relative position of the ion and adatom energy levels have been considered. The comparison with the results obtained for clean surfaces has been made.

17

Non-adiabatic effects in atom scattering on surfaces

Wiertel M., Taranko E., Taranko R.

Electron Technology

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2000

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Vol. 33, No. 3

323-328

EN

The electron occupation numbers of an atom scattered on metal surfaces within time-dependent Newns-Anderson model augmented with the correlated hopping terms are calculated. The many-body interactions are taken into account within mean-field treatment. Non-adiabatic effects in the resonant charge transfer for different values of atomic levels, work functions and atom velocities are discussed.