Bohumil David

α-Fe nanopowder synthesised in low-pressure microwave plasma and studied by Mössbauer spectroscopy

An iron-based nanocomposite has been prepared by the microwave plasma method: Fe(CO)5 vapour was introduced into an argon discharge at ~1 kPa. The synthesised nanopowder was passivated in situ with a mixture of Ar and air. The as-prepared nanopowder was characterised by XRD, TEM, Raman and M?ssbauer spectroscopies. In the XRD pattern of the nanopowder the ?-Fe (dXRD = 14 nm, 76 wt.%) and Fe3O4 (dXRD = 3 nm, 24 wt.%) phases were identified only. ?-Fe cores covered with oxide shells were observed under TEM. A huge increase of M?ssbauer absorption was observed after the sample was cooled down to 5 K. The results of magnetic and thermal properties studies at low and high temperature are presented. The heat capacity of the sample exhibited an unexpectedly high value of 599 J/kg/K.

Iron-Based Nanopowders Containing α-Fe, Fe3C, and γ-Fe Particles Synthesised in Microwave Torch Plasma and Investigated with Mössbauer Spectroscopy

Two Fe-based nanopowders were synthesised using microwave torch discharge at 1 bar. The main channel of the discharge was operating in Ar, which flowed through the central nozzle of an electrode, whereas the reactive mixture H(2)/Fe(CO)(5) was added through a concentric outer nozzle. Besides rarely observed gamma-Fe particles (for the first sample: d(XRD) = 30 nm, 35 wt %; for the second sample: d(XRD) = 9 nm, 33wt %) samples also included alpha-Fe, Fe(3)C, and Fe(3)O(4)/gamma-Fe(2)O(3) particles. The presence of gamma-Fe phase was proved by the interpretation of the XRD patterns and the Mossbauer spectra taken at 293 and 5 K.

Versatile low-pressure plasma-enhanced process for synthesis of iron and iron-based magnetic nanopowders

Using microwave low-pressure discharge, we synthesised magnetic iron-oxide nanopowder from the iron pentacarbonyl precursor. We were able to vary the size and chemical composition (especially the ratio between various iron oxides) by careful control of the process parameters. The nanoparticulate product was analysed by X-ray diffraction (XRD) and Raman spectroscopy. However, the XRD cannot reliably distinguish between the size-broadened peaks of γ-Fe2O3 (maghemite) and Fe3O4 (magnetite) due to their nearly identical crystalline structure. Hence we used a chemical method to determine the presence of Fe(II) and Fe(III) ions in the nanopowder samples. The results agree with those from the Raman spectroscopy.

Nanocrystal Growth in Thermally Treated Fe75Ni2Si8B13C2 Amorphous Alloy

Thermal treatment of amorphous Fe75Ni2Si8B13C2 alloy leads to crystallization of the stable α-Fe(Si) and Fe2B as well as to the metastable Fe3B phase. The study of the mechanism of crystal growth of the α-Fe(Si) phase revealed that the mechanism of α-Fe(Si) growth changes from two dimensional in the early stage to one dimensional in the later stage of crystallization. The Fe2B phase was found to crystallize through two independent routes: from the amorphous phase and from the metastable Fe3B phase, which leads to a different mechanism of crystal growth for each route. Both routes exhibit a change in the mechanism of crystal growth: from two dimensional to one dimensional and from three dimensional to two dimensional, respectively. The respective mechanisms of crystal growth correlate well with the observed changes in preferential orientation of the crystallites of the Fe2B phase.

gamma-Fe2O3 Nanopowders Synthesized in Microwave Plasma and Extraordinarily Strong Temperature Influence on Their Mossbauer Spectra

The article reports on two nanopowders synthesized in microwave plasma: the first sample was synthesized in a torch discharge at 1 bar and the second sample was synthesized in low-pressure plasma at 40 mbar. Morphology, composition and properties of the powders were studied by TEM, XRD, Raman and Mössbauer spectroscopies, and magnetic measurements. In the XRD patterns of the samples only gamma-Fe2O3 was identified (mean crystallite size d(XRD) was 24 nm for the first sample and 13 nm for the second sample). Based on the Mössbauer spectra measured at 5 K, the presence of other iron oxide phases was excluded in both samples. Unusually strong temperature dependence of the Lamb-Mössbauer factor was observed: I(SA)(5 K)/I(SA)(293 K) = 6 in the case of the first sample and I(SA)(5 K)/I(SA)(293 K) = 22 for the second sample (I(SA) denotes integral spectrum area). This effect is explained as the consequence of the reduced agglomeration of electrically charged nanoparticles in the plasma, i.e., particles can either move at 293 K (when they are free) or tilt (if they are a part of a chain). Superparamagnetic phase was not observed in the room-temperature Mössbauer spectra of both samples.

Powders with superparamagnetic Fe3C particles studied with Mössbauer spectrometry

Two nanopowders with superparamagnetic Fe3C particles were synthesised by the method of laser-induced pyrolysis of gaseous precursors. Both were characterised by X-ray diffraction, M?ssbauer spectrometry and standard magnetic measurements. The mean crystallite size of Fe3C was 3 nm for the first sample and 10 nm for the second sample (Scherrer formula), i.e. it was lower than in our previously studied ferromagnetic Fe3C-based sample. Fe3C phase in both present samples exhibited by ~20 K reduced Curie temperature which is interpreted as a nanosize effect. After annealing of the samples at 1073 K for 30 minutes the Curie temperature of the Fe3C phase in both samples matched its standard bulk value. Beside Fe3C phase also Fe3O4 and carbon black were present in the synthesised samples.

Kinetics of Phase Transformations in Mg2Ni-H System

Kinetics of hydrogen desorption from Mg2NiH4 was studied. Experimental material was prepared by two techniques – by melting and casting and by ball-milling and compacting into pellets. Experimental materials were hydrogen charged at elevated temperature and pressure. The pellets were charged in two different regimes resulting in structures with high fraction of twinned low-temperature phase LT2 and with low fraction of LT2. It was made an attempt to measure diffusion coefficients of hydrogen and its temperature dependence both in high-temperature (HT) and in low-temperature (LT) phases of Mg2NiH4. The measurement was carried out in temperature interval from 449 K to 576 K by the volumetric method. It was found that the LT2 slows-down the desorption rate considerably.

Iron-Based Nanocomposite Synthesised by Microwave Plasma Decomposition of Iron Pentacarbonyl

A nanocrystalline iron-based powder has been prepared by microwave plasma method: Fe(CO)5 vapor was introduced into an argon discharge at ~1 kPa. A microwave 2.45 GHz generator was operated at 430 W. The reaction took place inside a quartz tube passing through a microwave waveguide. The microwave discharge (without and with Fe(CO)5) was characterized by optical emission spectroscopy. The synthesized nanopowder was passivated in situ with air. The asprepared nanopowder was characterized by TEM, XRD, Raman and Mössbauer spectroscopies. The nanopowder included α-Fe, α-Fe2O3, and Fe3O4 phases. The core-shell nanoparticles were observed under TEM: α-Fe cores had shells formed of Fe3O4 or carbon. The mean crystallite size of α-Fe was 36 nm (Scherrer formula). The synthesized nanopowder exhibited ferromagnetic behavior.