O. Schneeweiss

Thermal behaviour of iron(II) oxalate dihydrate in the atmosphere of its conversion gases

Using a device for thermogravimetric analysis, a dynamic study of thermally induced solid-state transformations of FeC2O4·2H2O in the atmosphere allowing full participation of gaseous products (CO, CO2, H2O) in the reaction system was carried out. Solid phases formed at various temperatures between 25 and 640 °C were identified and characterized using 57Fe Mossbauer spectroscopy, TG and XRD. Up to 230 °C, evolution of two molecules of the water of crystallization takes place. Superparamagnetic nanoparticles of magnetite (Fe3O4) are formed as the primary product of the decomposition of FeC2O4, together with gaseous CO and CO2. In the next stage above 380 °C, the crystallization of magnetite is accompanied by a reduction of the remaining ferrous oxalate to iron carbide (Fe3C) by carbon monoxide. Thermally induced conversion of iron carbide into α-Fe and carbon is expected between 400 and 535 °C as the major chemical process. In the last reaction step, above 535 °C, magnetite is reduced to wustite (FeO) by carbon monoxide evolved at lower temperatures. On the grounds of quantitative Mossbauer data possible competitive reactions are discussed and a temperature dependent reaction model is suggested.

Facile fabrication of tin-doped hematite photoelectrodes – effect of doping on magnetic properties and performance for light-induced water splitting

We present a new, easily scalable method for the deposition of nanocrystalline hematite photoelectrodes based on the spin-coating of a mixed solution containing tin(II) and iron(III) chlorides followed by thermal treatment. Our facile approach does not require any additional film-forming organic species and allows simple control of the photoelectrochemical performance of the electrode by adjusting the degree of tin doping. When annealed at 650 °C a strong increase in the water oxidation photocurrent is observed with increasing tin concentration. The maximum performance (0.45 mA cm−2 at 1.43 V vs. RHE) was found at the highest possible tin loading (20u2006:u2006100, Snu2006:u2006Fe). The contrasting performance of electrodes annealed at 650 °C and 800 °C suggests different activation processes for dopant diffusion and activation. The doping of tin into the crystal structure of hematite thin films is directly evidenced by X-ray photoelectron spectroscopy and indirectly by changes in the intrinsic magnetic parameters (Morin temperature, Neel temperature) of the hematite films. The magnetization measurements thus represent a potential technique to quantify doping amounts in hematite.

Novel solid-state synthesis of α-Fe and Fe3O4 nanoparticles embedded in a MgO matrix

Thermally induced reduction of amorphous Fe2O3 nanopowder (2?3?nm) with nanocrystalline Mg (~20?nm) under a hydrogen atmosphere is presented as a novel route to obtain ?-Fe and Fe3O4 magnetic nanoparticles dispersed in a MgO matrix. The phase composition, structural and magnetic properties, size and morphology of the nanoparticles were monitored by x-ray diffraction, 57Fe M?ssbauer spectroscopy at temperatures of 24?300?K, transmission electron microscopy and magnetic measurements. Spherical magnetite nanoparticles prepared at a reaction temperature of 300??C revealed a well-defined structure, with a ratio of tetrahedral to octahedral Fe sites of 1/2 being common for the bulk material. A narrow particle size distribution (20?30?nm) and high saturation magnetization (95 ? 5?A?m2?kg?1) predispose the magnetite nanoparticles to various applications, including magnetic separation processes. The Verwey transition of Fe3O4 nanocrystals was found to be decreased to about 80?K. The deeper reduction of amorphous ferric oxide at 600??C allows ?-Fe (40?50?nm) nanoparticles to be synthesized with a coercive force of about 30?mT. They have a saturation magnetization 2.2 times higher than that of synthesized magnetite nanoparticles, which corresponds well with the ratio usually found for the pure bulk phases. The magnetic properties of ?-Fe nanocrystals combined with the high chemical and thermal stability of the MgO matrix makes the prepared nanocomposite useful for various magnetic applications.

Characterization of iron nitrides prepared by spark erosion, plasma nitriding, and plasma immersion ion implantation

Abstract The effect of the nitrogen uptake in α-iron upon spark erosion in gaseous and liquid ammonia, plasma nitriding, and plasma immersion ion implantation is studied. The resulting phases and hyperfine parameters, measured by the Mossbauer spectroscopy, are discussed from the point of view of initial conditions of their preparation and subsequent heat and/or mechanical treatment. Spark erosion in the ammonia gas produces fine particles with the dominating ferromagnetic α-Fe phase (50%). The 20% of specimen volume form α′-Fe and α′′-Fe16N2 phases. The last 30% occupy the γ′-Fe4N, ferro- and paramagnetic e phases, and γ-Fe(N). Nitriding in the liquid ammonia allows to incorporate the higher content of nitrogen into α-iron particles which results in the formation of paramagnetic e(ζ)-Fe2N phase. This phase also dominates the surface of α-iron specimen implanted by nitrogen using plasma immersion ion implantation at 300°C/3xa0h, where high uptake of nitrogen (approx. 30xa0at%) is reached. Plasma nitriding at 510°C results in the formation of γ′-Fe4N phase.

Structure sensitivity of magnetic and electrical properties of FeAl intermetallic compounds

Abstract We have investigated the structure dependence of the electrical and magnetic properties of Feue5f8Al compounds with 28 at.% Al. Local structure changes and hyperfine field parameters have been studied using Mossbauer spectroscopy. The magnetic properties and magnetoresistance exhibit strong dependence on the degree of plastic deformation. Additional alloying, e.g. by chromium, also changes the properties investigated. The results show that electrical, magnetic, and mechanical properties can be changed by controlled heat and mechanical treatment.

Fe3Si surface coating on SiFe steel

Abstract Fe3Si layers were prepared using chemical vapor deposition of Si on the surface of GO steel and its subsequent heat treatment. The changes in the structure and phase composition after different heat treatment conditions have been analyzed. The coating is characterized by high hardness, good corrosion resistance, high electrical resistivity, and the spin texture which differs from the steel substrate.

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.

Magnetoresistance in ordered and disordered Fe72Al28 alloy

Abstract On the Fe-Al alloy near the Fe 3 Al stoichiometry the magnetoresistance and hysteresis loops were measured in the ordered and disordered states. Atomic order and structure were checked by Mossbauer spetroscopy and TEM. The disordered sample shows a usual behaviour of bulk ferromagnetics. The ordered one exhibits large decrease one exhibits large decrease of reisistivity in the magnetic field and lower ability to reach the saturation. The large magnetoresistance effect is explained on the basis of similarity between naturally layered Fe 3 Al structure and artificial multilayers taking into account probable perturbation of spin orientation on the sublattices occupied by the iron atoms with lower magnetic moments.

The glassy behaviour of poorly crystalline Fe2O3 nanorods obtained by thermal decomposition of ferrous oxalate

Nanorod ferrous oxalate dihydrate (FeC2O4 × 2H2O) which had been synthesized by the microemulsion method, was used as a precursor in the thermal decomposition process performed in air atmosphere. The formation of nanocrystalline hematite as the final product was preceded by the appearence of an intermediate product. Comprehensive study comprising several complementary techniques (x-ray diffraction, transmission electron microscopy, selected area electron diffraction, thermogravimetric/differential thermal analyses and SQUID magnetometry) confirmed that the intermediate product corresponds to the poorly crystalline Fe2O3. Due to the specific nanorod shape and poorly crystalline structure, the investigated Fe2O3 showed high coercive field value of ~0.5 T at 5 K. Special attention in this study was devoted to the peculiar magnetic properties of poorly crystalline Fe2O3, which were thoroughly investigated by employing sophisticated experimental procedures such as relaxation of thermoremanent magnetization for different cooling fields, zero field and field cooled memory effects as well as aging experiments for different waiting times. At low temperatures and weak applied magnetic fields, the investigated system behaves similarly to spin glasses, manifesting slow, collective relaxation dynamics of magnetic moments through memory, rejuvenation and aging effects.

Diffusion distribution of 57Co atoms in nanocrystalline Co79Nb14B7 alloy

Abstract Distribution of 57Co atoms in a nanocrystalline Co79Nb14B7 alloy after annealing at different temperatures was studied by means of 57Co emission Mossbauer spectroscopy. After annealing below the recrystallization temperature, the diffusing 57Co atoms were found in all phases expected to be present in the nanocrystalline alloy. They do not prefer the intergranular phase. Paths of diffusing atoms are probably close to interfaces between crystallites (grain boundaries) and/or intergranular phase. With increasing annealing temperature, when the nanocrystalline state transforms into a coarse polycrystal, the 57Co atoms occupy regular sites in the bulk Co and Co3B phases.