Metal-oxide heterophase structures formed at low temperature activation of iron. 2. IR spectroscopy

Low-temperature activation of iron is observed during its isothermal oxidation at a temperature of 300°C and an oxygen pressure of 10⁻² Torr. A 1-h treatment provides the maximum gain in the oxide layer thickness for this oxygen pressure. According to IR spectroscopic data, the amount of Fe₃O₄ in the oxide reaches the maximum in the oxygen pressure interval from 10⁻³ to 10⁻² Torr and decreases with a further increase in the oxygen pressure. In contrast, the haematite content increases with an increase in the oxygen pressure. In the latter case, first, the content of the α-Fe₂O₃ phase increases to reach its maximum at pressures from 5×10⁻³ to 10⁻² Torr, while the phase of haematite γ-Fe₂O₃ appears at 0.1 Torr. This confirms the earlier assumption that the haematite islets layer plays the decisive roles in the low-temperature activation and the active-passive transition of iron. Original Russian Text ? V.A. Kotenev, N.P. Sokolova, A.M. Gorbunov, A.Yu. Tsivadze, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 6, pp. 630–634.     

Photoelectrochemical analysis of the effects of pH and sulfate ions on the structure and the composition of the passive film formed on Fe-20Cr-15Ni alloy

The effects of pH and sulfate ions on the structure and compositions of the passive film formed on Fe-20Cr- 15Ni were examined using a photoelectrochemical technique and Mott-Schottky analysis. The photocurrent spectra for the passive film formed on Fe-20Cr-15Ni in the buffer solutions are composed of two spectral components, one of which is generated from Cr-substituted γ-Fe₂O₃ and the other of which is generated from NiO. However, the passive film formed in sulfate solutions showed only the photocurrent spectrum of Cr-substituted γ-Fe₂O₃, suggesting that the formation of NiO in the passive film is suppressed because of a severe selective dissolution of Ni in the presence of sulfate ions. Mott-Schottky plots confirmed that the base structure of the passive film on Fe-20Cr-15Ni is n-type (Cr, Ni)-substituted γ-Fe₂O₃ regardless of solution pH and sulfate ions. The photocurrent intensity, flat band potential, and donor density for the passive film varied depending on the solution pH or the presence of sulfate ions in the solution, due primarily to the Cr enrichment in the film caused by the preferential dissolution of Fe and Ni that is more appreciable in highly acidic solutions containing sulfate ions.     

Photoelectrochemical study of the growth of the passive film formed on Fe-20Cr-15Ni in a pH 8.5 buffer solution

The structural change of the passive film on Fe-20Cr-15Ni at its growth stage was examined in-situ using a photoelectrochemical technique. The photocurrent spectra showed that the passive film on Fe-20Cr-15Ni for 0.5 h ∼ 25 h in a pH 8.5 buffer solution was composed of (Cr, Ni)-substituted γ-Fe₂O₃ mixed with NiO. Photocurrent spectral analysis suggested that a crystalline film of (Cr, Ni)-substituted γ-Fe₂O₃ formed before 30 min, with its thickness exceeded that of the space charge layer after 1 h of passivation. NiO particles appeared to have gradually precipitated from (Cr, Ni)-substituted γ-Fe₂O₃ film in the early stage of passivation of 1 h.     

A study of the corrosion properties of plasma nitrocarburized and oxidized AISI 1020 steels

Plasma nitrocarburized AISI 1020 steels were oxidized for 15, 30 and 60 min to evaluate their corrosion and microstructural properties. After plasma nitrocarburizing for 3 h at 570°C in a gas mixture comprising 85 vol.% N₂, 12vol.% H₂ and 3 vol.% CH₄, the compound layer composed of ɛ-Fe_2–3(N,C) and γ’-Fe₄(N,C) phases and the diffusion layer above the matrix were observed. The top oxide layer, consisting mainly of magnetite (Fe₂O₄) and hematite (Fe₂O₃) phases, forms after post-oxidation treatment at 500°C. However, the oxide layer was severely degraded by spallation as a result of increases in post-oxidizing time. The difference in corrosion resistance should be attributed to the thickness of the top oxide layer, which was governed by post-oxidizing time.     

Magnetic properties of mechanochemically synthesized γ-Fe2O3 nanoparticles

The synthesis of mono-dispersed γ-Fe₂O₃ nanoparticles by mechanochemical processing was demonstrated for the first time, via the solid-state exchange reaction Fe₂(SO₄)₃ + 3Na₂CO₃ → Fe₂(CO₃)₃ + 3Na₂SO₄ → Fe₂O₃ + 3Na₂SO₄ + 3CO₂(g) and subsequent heat treatment at 673 K. The nanoparticles had a volume-weighted mean diameter of 6 nm and a narrow size distribution with the standard deviation of 3 nm. The particles showed a superparamagnetic nature with the superparamagnetic blocking temperature of 56.6 K. The anisotropy constant was 6.0 × 10⁶ erg/cm³, two orders of magnitude larger than the magnetocrystalline anisotropy constant of bulk γ-Fe₂O₃. The detailed analysis of the magnetic properties indicated that the γ-Fe₂O₃ nanoparticles had a core-shell structure, consisting of a ferrimagnetic core of ∼4 nm in diameter having a collinear spin configuration and a magnetically disordered shell of ∼1.2 nm in thickness.     

α-Fe2O3 nanoplates: PEG-600 assisted hydrothermal synthesis and formation mechanism

Quasi-hexagonal α-Fe₂O₃ nanoplates with lateral sizes of 40-60 nm and thickness of ca. 10 nm were fabricated by a facile poly(ethylene glycol 600) (PEG-600) assisted hydrothermal technique in combination with calcination method. The final α-Fe₂O₃ nanoplates inherited perfectly the morphology of the preliminarily hydrothermal products with phases of dominant α-Fe₂O₃ and minor α-FeOOH. The platelets could be tailored from nano- to meso- and to micro-scale via adjusting PEG-600 quantities. An adsorption-extension-attachment model was proposed to explain the formation and growth mechanism of the platelets. The as-obtained α-Fe₂O₃ nanoplates exhibited a specific Langmuir surface area of 59 m²/g and a maximum N₂ adsorption of 137.3 cm³/g at 1 atm. UV-vis measurement showed a strong absorption in a wide range from UV to visible light with a blue-shifting band gap of 2.33 eV due to the nanosize effect.     

Anodic dissolution of iron and cobalt germanides in an alkaline electrolyte

Anodic dissolution of iron and cobalt germanides and their eutectic alloys with germanium in 1 M NaOH was studied by electrochemical methods and microprobe analysis. In anodically etching the FeGe₂−Ge and CoGe₂−Ge eutectic alloys, it was the Ge phase that predominantly dissolved in the alkaline solution, while the FeGe₂ intermetallic phase was found to be more stable than both Ge and a pure iron germanide. The anodic dissolution of FeGe₂ revealed a passive range (which is absent in acidic solutions) attributed to the oxidation of iron, probably, to γ-Fe₂O₃.     

The characterization of the thin oxide film formed over Fe3C at 300°C

The oxide formed over polished iron-carbon alloys at 300°C and in a 100 Torr dry oxygen atmosphere was primarily due to the oxidation of the ferrite phase, but a thin protective oxide film ( 150 Å) was formed on the carbide phase. The protective nature was attributed to the existence of a kinetic barrier of CO and CO₂ at the carbide-oxide interface. The initial oxide film formed on the carbide phase comprised many small, randomly oriented crystallites; approximately 70 Å in size, of -Fe₂O₃. This film then underwent a grain growth process via a strain-induced grain boundary migration accompanied by a phase transformation from -Fe₂O₃ to -Fe₂O₃. This transformation produced a tensile stress in the oxide film which was later relieved by the generation of cracks along the newly grown grain boundaries. These cracks exposed fresh carbide surface, released the gas barrier at the carbide-oxide interface, and provided rapid diffusional paths for further oxidation. As a result, the oxide film (-Fe₂O₃) was reduced to Fe₃O₄ and raised ridges appeared at the crack sites. Once the cracks were healed, the film again became protective.This research was supported by grant number DA-18-035-98(A) from the Chemical Research Laboratory, Edgewood Arsenal, Department of the Army. This paper is based on work performed as part of a thesis submitted by H. J. Kim to Lehigh University in partial fulfillment of the requirements for the Ph.D. degree in Metallurgy and Materials Science.     

Fabrication of dense Al2O3-FeAl composites by using solid-state sintering

Highly densified alumina-iron aluminide (Al₂O₃-FeAl) composites consisting of ubiquitous elements were fabricated by using pulse current sintering technique under a certain uni-axial pressure. The solid-state sintering without melting FeAl was the highlight in this study. The mechanical properties of the Al₂O₃-FeAl composites were much greater than previously reported ones fabricated by reaction sintering technique. The poor wettability of FeAl against Al₂O₃ strongly influenced the mechanical properties and made it difficult to be highly densified Al₂O₃-FeAl composites by liquid phase sintering especially when volume fraction of FeAl to Al₂O₃-FeAl was high (>30.5 vol%). However, highly densified Al₂O₃-FeAl composites were obtained by solid-state sintering with control of Al₂O₃ grain size and sintering temperatures. It was concluded that highly controlled powder metallurgy made it possible to fabricate dense ceramic-metal (intermetallic) composites from the combination of materials having poor wettability.     

Synthesis, structural and magnetic characterization of nanocrystalline nickel ferrite—NiFe2O4 obtained by reactive milling

Nanocrystalline nickel ferrite (NiFe₂O₄) has been synthesized from a stoichiometric mixture of oxides NiO and α-Fe₂O₃ in a high energy planetary mill. An annealing at 350 °C, after milling, was used to improve the solid state reaction. The obtained powders were investigated by X-ray diffraction, magnetic measurements, scanning electron microscopy, X-ray microanalysis and differential scanning calorimetry. The particles size distribution was analyzed using a laser particle size analyser. The nickel ferrite begins to form after 4 h of milling and continuously form up to 16 h of milling. The obtained nickel ferrite has many inhomogeneities and a distorted spinel structure. The mean crystallites size at the final time of milling is 9 ± 2 nm and the lattice parameter increases with increase the milling time. DSC measurements revealed a large exothermic peak associated with cations reordering in the crystalline structure. The magnetization of the obtained powder depends on the milling time and annealing. After the complete reaction between the starting oxides the milling reduces the magnetization of the samples. The magnetization increases after annealing, due to the reorganization of the cations into the spinel structure.     

Electromagnetic properties of polyaniline/maghemite nanocomposites: I. The effect of re-doping time on the electromagnetic properties

This article is to carry out researches on the synthesis of the polyaniline/γ-Fe₂O₃ nanocomposites by a reverse micelle process and the effects of re-doping time on the electromagnetic properties of polyaniline/γ-Fe₂O₃ nanocomposites, investigated by Fourier transform spectrometer (FT-IR), X-ray photoelectron spectroscopy (XPS), wide angle X-ray diffraction diffractometer (WAXD), impedance analyzer, micro-ohmeter, and superconductor quantum interference device (SQUID). The nanostructure of the polyaniline/γ-Fe₂O₃ nanocomposites was characterized by micrographs of transmission electron microscopy (TEM). It was found that the dispersed γ-Fe₂O₃ phase is roughly distributed in the polyaniline matrix. Results showed that, in the presence of γ-Fe₂O₃, the growth rate of quinoid ring is markedly retarded. The crystallinity and doping level of polyaniline in the nanocomposites increase with re-doping time, the conductivity and dielectric properties (i.e., permittivity and loss factor) of nanocomposites are hence increased and the ionic polarization relaxation time become shorter from 1.71 × 10⁻⁹ to 7.48 × 10⁻¹⁰ s. The particle size and γ-Fe₂O₃ content in the nanocomposites decrease with re-doping time due to the reduction effect resulting from the doped protonic acid of polyaniline. For the room-temperature SQUID analysis, the superparamagnetic behavior is observed, suggesting the presence of the thermal activation energy contribution to the magnetic moment. The saturated magnetization was decreased with decreasing γ-Fe₂O₃ contents.     

Densification and mechanical properties of fine-grained Al2O3-ZrO2 composites consolidated by spark plasma sintering

Alumina matrix composites containing 5 and 10 wt% of ZrO₂ were sintered under 100 MPa pressure by spark plasma sintering process. Alumina powder with an average particle size of 600 nm and yttria-stabilized zirconia with 16 at% of Y₂O₃ and with a particle size of 40 nm were used as starting materials. The influence of ZrO₂ content and sintering temperature on microstructures and mechanical properties of the composites were investigated. All samples could be fully densified at a temperature lower than 1400 °C. The microstructure analysis indicated that the alumina grains had no significant growth (alumina size controlled in submicron level 0.66-0.79 μm), indicating that the zirconia particles provided a hindering effect on the grain growth of alumina. Vickers hardness and fracture toughness of composites increased with increasing ZrO₂ content, and the samples containing 10 wt% of ZrO₂ had the highest Vickers hardness of 18 GPa (5 kg load) and fracture toughness of 5.1 MPa m^1/2.     

Microstructure of Suspension Plasma Spray and Air Plasma Spray Al2O3-ZrO2 Composite Coatings

Al₂O₃-ZrO₂ coatings were deposited by the suspension plasma spray (SPS) molecularly mixed amorphous powder and the conventional air plasma spray (APS) Al₂O₃-ZrO₂ crystalline powder. The amorphous powder was produced by heat treatment of molecularly mixed chemical solution precursors below their crystallization temperatures. Phase composition and microstructure of the as-synthesized and heat-treated SPS and APS coatings were characterized by XRD and SEM. XRD analysis shows that the as-sprayed SPS coating is composed of α-Al₂O₃ and tetragonal ZrO₂ phases, while the as-sprayed APS coating consists of tetragonal ZrO₂, α-Al₂O₃, and γ-Al₂O₃ phases. Microstructure characterization revealed that the Al₂O₃ and ZrO₂ phase distribution in SPS coatings is much more homogeneous than that of APS coatings.     

Preparation and optical properties of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> powder

Y(NO3)3 and NH3·H2O were used as a raw materials,and nano-Y2O3 powder was successfully synthesized by a precipitation method.Employing TEOS as a raw material,SiO2 powder was successfully prepared by a alkoxide-hydrolysis method,and a Y2O3/SiO2 composite powder was obtained by coating.The Y2O3,SiO2,and Y2O3/SiO2 powders were characterized using X-ray diffraction(XRD),scanning electron microscopy(SEM),and Fourier transform infrared spectrophotometer(FT-IR);the Y2O3 and Y2O3/SiO2 powders were further examined ...     

Effects of [SDS]/[H2O] molar ratio on the electromagnetic properties of polyaniline/maghemite nanocomposites

Polyaniline (PANI)/maghemite (γ-Fe₂O₃) nanocomposites were prepared by using the reverse micelle polymerization, where aniline, ferrous and ferric salts and sodium dodecyl sulfate (SDS) act as monomer, precursor of γ-Fe₂O₃ and surfactant, respectively. The effect of the molar ratio of [SDS]/[H₂O] on the electromagnetic properties of PANI/γ-Fe₂O₃ nanocomposites was investigated by Fourier transform spectroscopy (FTIR), UV–visible spectroscopy (UV–vis), X-ray photoelectron spectroscopy (XPS), wide angle X-ray diffraction diffractometer (WAXD), impedance analysis analyzer, micro-ohmetry and superconductor quantum interference device (SQUID). The nanostructure of the PANI/γ-Fe₂O₃ nanocomposites was characterized by micrographs of transmission electron microscopy (TEM). Results showed that both the γ-Fe₂O₃ content and particle size in the nanocomposites decreased with molar ratio of [SDS]/[H₂O]. The γ-Fe₂O₃ phase is non-uniformly distributed in the PANI matrix, and exhibits a broader size distribution at higher [SDS]/[H₂O] molar ratio due to the reduced strength of PANI–γ-Fe₂O₃ interactions. In the presence of γ-Fe₂O₃, the growth rate of quinoid ring is markedly retarded. The retard effect is significantly reduced by increasing the [SDS]/[H₂O] molar ratio, leading to the improvement of crystallinity, conductivity and dielectric properties (i.e., permittivity and loss factor) of nanocomposites. Simultaneously, the ionic polarization relaxation time is shortened from 2.61 × 10^?9 to 1.04 × 10^?9 s. For SQUID analysis at room temperature, the typical superparamagnetic behavior is found with the saturation magnetization decreased with the [SDS]/[H₂O] molar ratio, resulting from the reduced γ-Fe₂O₃ content, smaller γ-Fe₂O₃ particle size, and the wider particle size distribution.     

Tensile Deformation of Iron Oxides at 600–1250°C

Tensile tests of virtually pure FeO, -Fe₃O₄, and -Fe₂O₃ were performed at 600–1250°C at strain rates of 2.0×10^–3–6.7×10^–5 s^–1 under controlled gas atmospheres. Mechanical properties and deformation/fracture behavior were investigated. For -Fe₂O₃, brittle fracture resulted at 1150–1250°C and the fracture strain was below 4.0% at a strain rate of 2.0×10^–4 s^–1. -Fe₃O₄ deformed plastically above 800°C. Steady-state deformation was indicated at 1200°C; elongation of 110% was obtained. Plastic deformation observed at 800 to 1100°C was considered to result from dislocation glide. Using TEM, the Burgers vector of dislocations observed in deformed -Fe₃O₄ was determined to be <110>, its slip system was estimated to be {111}<110>. FeO deformed plastically above 700°C. Steady-state deformation became predominant above 1000°C. Elongation of 160% was obtained at 1200°C. Strain rates of FeO at 1000°C and 1200°C were proportional to the fourth power of the saturated stress, indicating that plastic deformation was affected by dislocation climb.     

Heat treatment of iron oxide films

Thin, solid magnetic films of iron oxide -Fe₂O₃ and an oxide with a variable composition like Fe₃O₄--Fe₂O₃ are of practical interest for high-density information recording. This work concerns the effect of heat treatment on the changes in the structural and magnetic characteristics of iron oxide films.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 35 – 37, February, 1995.     

The Effect of Heat Treatment on the Oxidation Behavior of HVOF and VPS CoNiCrAlY Coatings

Free-standing VPS and HVOF CoNiCrAlY coatings were produced. The as-sprayed HVOF coating retained the γ/β microstructure of the feedstock powder, and the VPS coating consisted of a single (γ) phase. A 3-h, 1100 °C heat treatment in vacuum converted the single-phase VPS coating to a two-phase γ/β microstructure and coarsened the γ/β microstructure of the HVOF coating. Oxidation of free-standing as-sprayed and heat-treated coatings of each type was carried out in air at 1100 °C for a duration of 100 h. Parabolic rate constant(s), K _p, were determined for free-standing, as-sprayed VPS and HVOF coatings as well as for free-standing coatings that were heat treated prior to oxidation. The observed increase in K _p following heat treatment is attributed to a sintering effect eliminating porosity from the coating during heat treatment. The lower K _p values determined for both HVOF coatings compared to the VPS coatings is attributed to the presence of oxides in the HVOF coatings, which act as the barrier to diffusion. Oxidation of the as-sprayed coatings produced a dual-layer oxide consisting of an inner α-Al₂O₃ layer and outer spinel layer. Oxidation of the heat-treated samples resulted in a single-layer oxide, α-Al₂O₃. The formation of a thin α-Al₂O₃ layer during heat treatment appeared to prevent nucleation and growth of spinel oxides during subsequent oxidation.     

Tunneling magnetoresistance in sintered Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> samples diluted with Fe and α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>

Electric transport and magnetoresistance characteristics were investigated for Fe₃O₄-x Fe(x=0, 10, 20 wt.%) samples and Fe₃O₄-α-Fe₂O₃ samples sintered at 500°C. For composition dependence of Fe₃O₄-x Fe samples, the largest room temperature MR, 3.3% at 10 kOe, was obtained from a Fe₃O₄-10 Fe sample. For the surface heat treatment dependence of Fe₃O₄ powders, the largest room temperature MR, 4% at 10 kOe, was obtained from a Fe₃O₄-α-Fe₂O₃ sample sintered with Fe₃O₄ powders heated at 200°C in air. It was found that these enhanced MR ratios always appear together with the appropriate excess resistance which is regarded as the tunneling barrier. These enhanced MR ratios of Fe₃O₄-10 Fe and Fe₃O₄-α-Fe₂O₃ samples can be explained by the increased interparticle contact sites and the appropriate thickness of α-Fe₂O₃, respectively.     

Development of plasma nitrocarburizing and post-oxidation technology for the replacement of Cr<Superscript>6+</Superscript> plating for application to vehicle engine shafts

Plasma nitrocarburizing and post-oxidation treatments were performed to improve the wear and corrosion resistance of S45C steel. Plasma nitrocarburizing was conducted for 3 h at 570°C in a nitrogen, hydrogen and methane atmosphere to produce the ε-Fe₂₋₃(N,C) phase. It was found that the compound layer produced by plasma nitrocarburising was predominantly composed of the ∈-phase with traces of the γ′-Fe₄(N,C) phase. The thickness of the compound layer was approximately 12 μm and the diffusion layer was approximately 300 μm in thickness. Plasma post oxidation was performed on nitrocarburized samples with various oxygen/hydrogen ratios at a constant temperature of 500°C for 1 h. The very thin magnetite (Fe₃O₄) layer 1 μm to 2 μm in thickness on top of the compound layer was obtained by plasma post oxidation. It was also confirmed that further improvement of the corrosion characteristics of the nitrocarburized compound layer was possible with an application of the superficial magnetite layer. Finally, throttle valve shafts of S45C steel were treated under optimum plasma processing conditions. Accelerated life time test results using a throttle body assembled with a shaft treated by plasma nitrocarburising and post oxidation showed that plasma nitrocarburizing and plasma post-oxidation processes could be a viable technology in the very near future in place of Cr⁶ plating.