Tomáš Kmječ

The magnetic transition in ε-Fe2O3 nanoparticles: Magnetic properties and hyperfine interactions from Mössbauer spectroscopy

The nanoparticles of e-Fe2O3 enriched with 57Fe isotope in amorphous silica matrix were prepared by sol-gel technique starting from a single molecular precursor for both Fe2O3 and silica. From the X-ray powder diffraction pattern e-Fe2O3 was identified as the major phase and α-Fe2O3 and β-Fe2O3 were observed as minor iron oxide phases. Using the log-normal distribution for fitting the experimental data from the TEM micrographs, the characteristic size of particles d0 ∼ 25 nm was derived. The rather high coercivity of ∼2.1 T at room temperature was confirmed for our nanoparticle system. From the dependences of magnetization on temperature a two-step magnetic transition spread between 100 K and 153 K was indicated. From the 57Fe Mossbauer spectra measured in the temperature range of 4.2–300 K, the hyperfine parameters for one tetrahedral and three octahedral sites of e-Fe2O3 structure were identified. The in-field spectra in the external magnetic fields up to 6 T were taken both above and below the indicate...

Low temperature behavior of hyperfine fields in amorphous and nanocrystalline FeMoCuB

Low temperature (4.2 K) magnetic behavior of Fe76Mo8Cu1B15 metallic glass was studied by 57Fe Mossbauer spectrometry (MS) and 57Fe NMR. Distributions of hyperfine magnetic fields P(B) were determined for as-quenched and annealed (nanocrystalline) samples with relative fraction of the grains about 43%. P(B) distributions were derived for both the amorphous matrix and nanocrystalline grains. NMR of alloys with natural and 57Fe enriched Fe enabled to assess the contribution of 11B to the total NMR signal. P(B) distribution of the as-quenched alloy derived from MS matches reasonably well the one from NMR of the enriched sample. NMR signal from the sample with natural Fe exhibits contributions from 11B nuclei. The principal NMR lines of the annealed alloys at 47 MHz correspond to bcc Fe nanocrystals. Small asymmetry of the lines towards higher frequencies might be an indication of possible impurity atoms in the bcc structure. The observed differences between natural and enriched samples are attributed to highe...

FeOx/Al2O3 catalysts for high-temperature decomposition of N2O under conditions of NH3 oxidation in nitric acid production

The catalytic activity of a series of FeOx/Al2O3 prepared under various conditions was evaluated for high temperature decomposition of N2O (HT-deN2O) in a complex gas mixture produced by oxidation of ammonia. Thus the relevant step in the industrial nitric acid production process was simulated. The catalyst stability during long-term exposure (12 days) to reaction conditions relevant to HT-deN2O was studied. Both fresh and aged FeOx/Al2O3 exhibited more than 90% conversion of N2O in the 750–900 °C temperature range. Structural analysis showed that the prepared FeOx/Al2O3 catalysts contained various proportions of δ, θ and α alumina phases and up to four individual iron species. In the δ and θ Al2O3 phases, well stabilized Fe(III) in Td or Oh coordination was identified as the active species in HT-deN2O.

The Fe-Au interface: Hyperfine interactions of 57Fe by Mössbauer transmission and conversion electron spectroscopy

We studied the effect of interfaces character in 57Fe/Au thin layers on iron hyperfine parameters using Mossbauer spectroscopy. Four samples of bilayers (57Fe covered by Au) were prepared on MgO substrate by pulsed laser deposition (PLD) with varying 57Fe layer thickness (∼10–95 nm), covered by Au layer with constant thickness of ∼8.7 nm. Mossbauer spectra were acquired in transmission and conversion electron geometry. The hyperfine parameters of α–Fe layer, Fe/Au and MgO/Fe interfaces were determined.

Mössbauer spectrometry of LC 200N steel

The long-term reliability of construction materials operating in nuclear facilities under harsh conditions such as intense irradiation, high temperature, and in the presence of corrosion agents is a serious technologic, economic, and environmental demand. In this work, we elucidate the structural features of advanced corrosion-resistant LC 200N steel. High Cr and N contents provide high hardness and wear resistance of this perspective material which can be considered for possible applications in nuclear installations. Structural arrangement was checked by scanning electron microscopy and transmission electron microscopy. Chemical composition was inspected by X-ray fluorescence technique, spark optical emission spectroscopy, and neutron activation analysis. Mossbauer spectrometry was chosen as the principal method of investigation. Conversion Electron Mossbauer Spectrometry and transmission techniques were applied.

Iron meteorite fragment studied by atomic and nuclear analytical methods

Chemical and structural compositions of a fragment of Sikhote-Alin iron meteorite were investigated by X-ray fluorescence analysis (XRF), neutron activation analysis (NAA) and Mossbauer spectroscopy (MS). XRF and NAA revealed the presence of chemical elements which are characteristic for iron meteorites. XRF also showed a significant amount of Si and Al on the surface of the fragment. MS spectra revealed possible presence of α-Fe(Ni, Co) phase with different local Ni concentration. Furthermore, paramagnetic singlet was detected in Mossbauer spectra recorded at room temperature and at 4.2 K.