An AES analysis of oxide films on iron

The Auger spectra of the iron oxides have been investigated. The relative amplitudes of the Auger lines from oxygen and iron have been determined for FeO, Fe₃O₄, α-Fe₂O₃, γ-Fe₂O₃, and FeOOH. It was found that the amplitude ratio s of the O (510 eV)/Fe (703 eV) lines was, to a first approximation, directly proportional to the ratio of oxygen and iron in the chemical formula. Structure was observed in the spectra resulting from the Fe M_2,3 VV transition. Different spectra were observed for Fe, FeO, Fe₃O₄, Fe₃O₃, and FeOOH; however, the same spectra were obtained for α- and γ-Fe₂O₃.     

Mössbauer spectroscopic properties of tin-doped iron oxides

The ⁵⁷Fe Mössbauer spectra recorded in situ from tin-doped Fe₃O₄ at elevated temperature in vacuo shows the Curie temperature to decrease with increasing concentrations of the dopant. Thermal treatment under oxidising conditions results in the initial formation of tin-doped γ-Fe₂O₃ which subsequently undergoes a phase transformation to tin-doped α-Fe₂O₃. ⁵⁷Fe Mössbauer spectroscopy at elevated temperatures shows the Néel temperature for tin-doped γ-Fe₂O₃ to be lower than that of pure γ-Fe₂O₃. The ¹¹⁹Sn Mössbauer spectra recorded from all the tin-doped iron oxides show the presence of a hyperfine magnetic field at the Sn⁴⁺ site which is more complex in the spectra recorded from tin-doped γ-Fe₂O₃ and α-Fe₂O₃.     

The interaction of hydrogen and carbon monoxide with oxygen on Cu(111)-Fe Surfaces

The interaction of hydrogen and carbon monoxide with oxygen adsorbed on Cu(111)-Fe surfaces containing different amounts of iron has been studied with ellipsometry, Auger electron spectroscopy and low energy electron diffraction. With carbon monoxide copper can be reduced completely and if, at larger iron deposits, γ-Fe₂O₃ is present, γ-Fe₂O₃ can be reduced to Fe₃O₄. The maximum reaction rate is proportional to the square of the total copper surface. With hydrogen all oxygen can be removed. The reduction proceeds via a number of different stages. This is explained by the subsequent occurrence of γ-Fe₂O₃, Fe₃O₄ Fe_0.95O and Fe.     

A study of nanostructures formed in the hydrogen reduction of Fe(OH)<Subscript>3</Subscript>

A complex study of the hydrogen reduction of nanosized iron hydroxide Fe(OH)₃ at 400°C was performed. It was shown that, during the reduction of Fe(OH)₃ to iron metal α-Fe, intermediate compounds such as Fe(OH)₂, α-FeOOH, β-FeOOH, γ-FeOOH, δ-FeOOH, and FeO are formed along with stable iron oxides α-Fe₂O₃, γ-Fe₂O₃, and Fe₃O₄. A scheme of chemical and structural transformations that occur in the reduction of nanosized Fe(OH)₃ is presented. The scheme takes into account the possibility of the bifurcation mechanism of reaction development.     

Partitioning and transformation behavior of selenium during coal combustion

The partitioning of selenium in coal-fired flue gas and desulfurization wastewater is of great threat to the ecological environment and human health. However, the unclear understanding of interactions between selenium vapors and fly ash hinders the emission control of selenium from coal-fired power plants. To further illuminate the mechanism of selenium partitioning and transformation, this study carefully estimated selenium distribution characteristics in the coal combustion byproducts from several industrial power plants. The effective temperature range as well as the key ash components for selenium retention by fly ash was clarified by multiple-scale experiments and theoretical perspectives. The results showed that gaseous selenium tended to be captured by fly ash at a medium-to-low temperature range (i.e. below 650 °C). The limited residence time resulted in the incomplete capture of gaseous selenium by fly ash. Mullite, quartz, iron oxides, and anhydrite in fly ash were found to be the main trappers for gaseous selenium. Among these components, iron oxides showed excellent selenium adsorption performance at a wide temperature range of 150-700 °C, which was realized by the strong chemical adsorption. By contrast, as the dominant phases in fly ash for the physical adsorption of gaseous selenium, mullite and quartz mainly captured gaseous selenium below 300 °C. On the other hand, sulfur dioxides had priority over gaseous selenium to react with calcium-containing ash components by forming anhydrite in the high-temperature region. The formed anhydrite had a limited selenium adsorption capacity, which was confirmed to capture gaseous selenium through a combination of physical adsorption and weak chemical adsorption. For the in-depth control of selenium emitted into desulfurization system and atmosphere environment, these findings provided a comprehensive insight into the behavior of selenium partitioning and transformation into fly ash during coal combustion.     

57Fe Mössbauer study of FeNx (x=0.25≈0.91) alloys

⁵⁷Fe Mössbauer measurements have been perforned at the first time on iron nitrides, FeN_x(x>0.5), films prepared by rf sputtering and the measurements for γ-FeN_0.09, γ'-Fe₄N, ε-Fe_2–3N and ζ-Fe₂N have been also performed. Two new phases for x>0.5 have been identified: One has a ZnS-type structure denoted by γ" Another phase is denoted by γ''' in which iron atoms constitute a fcc lattice but the details of occupation sites of nitrogen atoms are not yet clear. The isomer shift of γ" relative to α-Fe at 300 K is rather small in comparison with that of other well known nitrides. The roam temperature Mössbauer spectra of γ''' consist of a sum of two doublets. Mössbauer spectra of γ" at 10 K do not indicate magnetically split pattern, but those of γ''' show magnetically split patterns at 10 K with the hyperfine fields of 30 T and 48 T, respectively. The component of 48 T is due to oxides, not due to nitrides.     

Density shift and broadening of dipole transitions in antiprotonic helium

The phase composition of carbon nanotubes (CNTs) with encapsulated iron atoms was examined by ⁵⁷Fe M?ssbauer spectroscopy. It was shown that iron atoms were stabilized in thermodynamically stable iron carbide and oxide phases and phases that are not usual under synthesis conditions (γ-Fe and γ-Fe₂O₃).     

Controlled synthesis and phase characterization of Fe-based nanoparticles obtained by thermal decomposition

Iron oxide nanoparticles were synthesized by the thermal decomposition of Fe(acac)₃ and Fe(CO)₅. Three different homogeneous procedures were used for the controlled synthesis of Fe₃O₄, γ-Fe₂O₃ and Fe₃O₄/γ-Fe₂O₃ mixture nanocrystals. A combination of characterization techniques was used in order to distinguish these oxides. The controllable size, the narrow distribution and the rhombic self-assembly of the nanoparticles were revealed by the high-resolution transmission electron microscopy images and the X-ray powder diffraction results. For the quantitative analysis of the samples manganometry was used. Preliminary magnetic measurements indicated the size and composition dependence of saturation magnetization, a superparamagnetic behavior of the samples and some ferromagnetic features.     

Application of Mössbauer spectroscopy for the complex structural analysis of iron oxide-based nanomaterials

The results of complex structural investigation by XRD, TEM, and Mössbauer spectroscopy of nanomaterials based on iron oxides Fe₃O₄ and γ-Fe₂O₃ are presented. The investigated nanomaterials include nanopowders produced chemically and nanostructural powders produced mechanochemically. The magnetic properties of the nanomaterials, measured at ambient temperature, are discussed.     

An iron-57 Mössbauer spectroscopy and EXAFS study of the hydrogen pretreatment of titania-supported iron-ruthenium catalysts

The changes in composition and structure which are induced in a titania-supported iron-ruthernium catalyst following treatment in hydrogen have been investigated in situ by⁵⁷Fe Mössbauer spectroscopy and by EXAFS. The results show that ruthenium dioxide is readily reduced at temperatures below ca. 500°C to ca. 20 Å clusters of metallic ruthenium whilst α-Fe₂O₃ is partially reduced at 130°C to Fe²⁺ and Fe⁰. The Fe³⁺ which is formed by the reoxidation of Fe²⁺ under the reducing conditions at 500°C segregates to the interface of the bimetallic phase and the titania support. It is suggested that continued treatment at 700°C produces a high dispersion of iron which is coordinated to oxygen atoms of the support. The ca. 20 Å clusters of metallic ruthenium may be envisaged as being anchored to the support via iron-ruthenium bonds     

Mössbauer investigations of Fe and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> magnetic nanoparticles for hyperthermia applications

Magnetic nanoparticles of magnetite Fe₃O₄ and Fe synthesized by physical vapor deposition with a fast highly effective method using a solar energy have been studied. Targets have been prepared from tablets pressed from Fe₃O₄ or Fe powders. Relationships between the structure of nanoparticles and their magnetic properties have been investigated in order to understand principles of the control of the parameters of magnetic nanoparticles. Mössbauer investigations have revealed that the nanoparticles synthesized from tablets of both pure iron and Fe₃O₄ consist of two phases: pure iron and iron oxides (γ-Fe₂O₃ and Fe₃O₄). The high iron oxidability suggests that the synthesized nanoparticles have a core/shell structure, where the core is pure iron and the shell is an oxidized iron layer. Magnetite nanoparticles synthesized at a pressure of 80 Torr have the best parameters for hyperthermia due to their core/shell structure and core-to-shell volume ratio.     

Local atomic and magnetic structures of nanocrystalline Fe75Cr10B15 alloy

The crystal, local atomic and magnetic structures of Fe₇₅Cr₁₀B₁₅ alloys annealed at 440?C473°C for 5 min have been studied using X-ray diffraction and ⁵⁷Fe M?ssbauer spectroscopy. At the annealing temperature T _a = 440°C, nanocrystals of the ??-Fe phase (??1%) precipitate in the amorphous matrix of the alloy. The complete crystallization of the amorphous alloy occurs at T _a = 473°C with the formation of ??-Fe nanocrystals 26 ± 2 nm in size and nanocrystals of tetragonal boride t-Fe₃B 47 ± 2 nm in size. It has been found that chromium atoms are located in nanocrystals of the ??-Fe and y-Fe₃B types. The distribution functions of hyperfine fields in the nanocrystalline Fe₇₅Cr₁₀B₁₅ alloy reconstructed from the M?ssbauer spectra (at T _a = 473°C) show that there are three allowed states of iron atoms in the ??-Fe phase and three equally probable crystallographic nonequivalent states of iron in the t-(Fe,Cr)₃B phase. The chromium concentration x in the ??-Fe(Cr) phase is found to be ??10 at %. The substitution of chromium atoms for iron atoms in t-Fe₃B substantially decreases local magnetic moments of the iron atoms.     

溅射氧化铁薄膜矫顽力的研究

本文报道了有关溅射氧化铁薄膜磁性能的系统研究,特别着重于矫顽力的温度依赖关系和矫顽力随不同氧化铁相的变化。从理论分析与实验测量结果的对比中,给出了形状各向异性、磁晶各向异性及应力各向异性各自对Fe₃O₄薄膜、γ-Fe₂O₃薄膜及二者混合相薄膜的贡献,并且得到了Fe₃O₄薄膜和γ-Fe₂O₃薄膜的磁晶各向异性常数K₁的温度依赖关系曲线。 关键词：     

Mössbauer investigation of the γ→α-Fe2O3 transformation in small particles

The γ→α-Fe₂O₃ transformation in 10 nm-sized particles has been investigated with the Mössbauer effect. At 475°C the conversion was completed in about 2 days, it progressed very slowly below. Kinetic measurements at 330°C revealed a pre-transitional shift of the hyperfine field distribution. It has been attributed to the formation of interparticle >Fe?O?Fe< bridges due to the removal of chemisorbed water. Subsequent magnetic and crystalline strains probably drive the recrystallisation.     

Effect of the Calcination Atmosphere on the Structural Properties of the Reduced Fe/SiO2 System

Iron supported systems are frequently used as catalysts in the Fischer–Tropsch synthesis being the Fe⁰ the active phase for the reaction. We have studied the influence of the calcination atmosphere (air or nitrogen) on the iron oxide reducibility and the metallic iron particle size obtained in Fe/SiO₂ system. We have impregnated a silicagel with Fe(NO₃)₃·9H₂O aqueous solution and the solid obtained was calcinated in air or N₂ stream. These precursors, with 5% (wt/wt) of Fe, were characterized by Mössbauer Spectroscopy at 298 and 15 K. Amorphous Fe₂O₃ species with 3 nm diameter in the former, and α-Fe₂O₃ crystals of 48 nm diameter were detected in the last one. Both precursors were reduced in H₂ stream. Two catalysts were obtained and characterized by Mössbauer spectroscopy in controlled atmosphere at 298 and 15 K, CO chemisorption and volumetric oxidation. α-Fe⁰, Fe₃O₄ and Fe²⁺ were identified in the catalyst calcined in air. Instead, only α-Fe⁰ was detected in the catalyst calcined in N₂. The iron metallic crystal sizes were estimated as ≈2 nm for the former and ≈29 nm for the last one. The different oxide crystal sizes, obtained from the diverse calcination atmospheres, have led to different structural properties of the reduced solids. It has been possible to reduce totally the existing iron in an Fe/SiO₂ system with iron loading lower than 10% (wt/wt).     

Roles of γ-Fe2O3 in fly ash for mercury removal: Results of density functional theory study

First-principle calculations based on density function theory (DFT) are used to clarify the roles of γ-Fe₂O₃ in fly ash for removing mercury from coal-fired flue gases. In this study, the structure of key surface of γ-Fe₂O₃ is modeled and spin-polarized periodic boundary conditions with the partial relaxation of atom positions are employed. Binding energies of Hg on γ-Fe₂O₃ (0 0 1) perfect and defective surfaces are calculated for different adsorption sites and the potential adsorption sites are predicted. Additionally, electronic structure is examined to better understand the binding mechanism. It is found that mercury is preferably adsorbed on the bridge site of γ-Fe₂O₃ (0 0 1) perfect surface, with binding energy of −54.3 kJ/mol. The much stronger binding occurs at oxygen vacancy surface with binding energy of −134.6 kJ/mol. The calculations also show that the formation of hybridized orbital between Hg and Fe atom of γ-Fe₂O₃ (0 0 1) is responsible for the relatively strong interaction of mercury with the solid surface, which suggests that the presently described processes are all noncatalytic in nature. However, this is a reflection more of mercury's amalgamation ability.     

Analysis of atmospheric corrosion products of steel and coated steel by means of scattering Mössbauer spectrometry

Wearthering steels treated with and without zinc phosphate solution were exposed to atmosphere for 15 years and rust layers produced on the steels were analysed by scattering Mössbauer spectrometry (CEMS and XMS). γ-FeOOH, fine α-FeOOH, 5Fe₂O₃·9H₂O, γ-Fe₂O₃ and Fe₃O₄ were identified to be present in the rust formed on the steel without phosphate coating. Large particles of γ-Fe₂O₃ and Fe₃O₄ formed on the uncoated steel exposed to atmosphere in a position facing north on vertical plane. The layer structure of rust was affected by the position. The thin rust layer formed on the phosphate + carylite resin coated steel was considered to consist of γ-FeOOH, fine α-FeOOH, and fine γ-Fe₂O₃.     

Optical emissions of products sputtered from Fe, Fe2O3 and Fe3O4 powders

Powder iron has been bombarded by a 5 keV Kr⁺ ions in a vacuum better than 10^-7 torr and under few 10^-6 torr ultra pure oxygen partial pressure. The optical spectra of the sputtered particles were recorded between 340.0 nm and 410.0 nm. These spectra exhibit discrete lines, which are attributed to neutral excited atoms of iron. Two iron oxides, namely hematite (Fe₂O₃)_{3}) and magnetite (Fe₃O₄)_{4}), in powder form, were studied under the same experimental conditions and identical lines were observed in the obtained spectra. The absolute intensities of the spectral lines in all spectra were measured and the differences in the recorded yield photons were discussed in term of electron-transfer processes between the excited sputtered atom and the bombarded surface. In accordance with the proposed interpretation, we suggest values for the energy gaps and electronic affinities for the studied oxides and for the oxide layer that might be formed by the adsorption of oxygen atoms.     

An optimal low-temperature tartrate precursor method for the synthesis of monophasic nanosized ZnFe2O4

In this study, the synthesis of monophasic nanocrystalline zinc ferrite (ZnFe₂O₄) was achieved by controlling the thermal decomposition conditions of a zinc–iron tartrate precursor method. Differential thermal analysis/thermogravimetry (DTA/TG), X-ray diffraction (XRD), Fe²⁺ content analysis, transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET) techniques were used to investigate the effect of heat treatment conditions on the calcined powders. The thermal decomposition of the precursor led to an intermediate phase formation of ZnO, Fe₃O₄, and γ-Fe₂O₃. It was found that the Fe₃O₄ → γ-Fe₂O₃ oxidation reaction is the key step in producing monophasic nanosized ZnFe₂O₄. The monophasic nanoparticles of ZnFe₂O₄ can be obtained when the precursor is heat treated under a low temperature (300–400 °C) and long residence time (4 h) process that can prompt the Fe₃O₄ oxidation and prevent the formation of α-Fe₂O₃.     

Application of Mössbauer spectroscopy to the thermal decomposition of strontium and barium bis(CITRATO)Ferrates(III)

The thermal decomposition of M₃[Fe(cit)₂]₂ 4H₂O (M = Sr, Ba) has been studied isothermally and non-isothermally employing simultaneous (TG–DTG–DSC), XRD, IR, SEM and Mössbauer spectroscopic techniques. The anhydrous complexes decompose at 140–220°C to yield α-Fe₂O₃ and MC₅H₄O₅ (metal acetone-dicarboxylate) intermediates. The latter decomposes to respective carbonate at higher temperature. α-Fe₂O₃ and MCO₃ undergo a solid state reaction in the temperature range 400–430°C to yield Sr₂Fe₂O₅ and BaFe₂O₄ respectively. Finally above 600°C, a solid state reaction between these products and respective metal oxides leads to the formation of M₃Fe₂O_7???X showing simultaneous presence of Fe(III) and Fe(IV) species. These perovskite ferrites can exhibit electrical conductivity and dielectric properties. SEM analysis shows these ferrites to be nanosized with average particle diameter of 50–55 nm.