Hematite Layer Research Articles

Cathodic shift in onset potential of solar oxygen evolution on hematite by 13-group oxide overlayers

The onset potential of photoelectrochemical water oxidation on ultrathin hematite was improved by up to 200 mV by the chemical bath deposition of 13-group oxides as overlayers. It is proposed that the corundum-type overlayers released lattice strain of the ultrathin hematite layer and decreased the density of surface states. Particularly, a Ga2O3 overlayer exhibited an enhanced photocurrent attributed to stoichiometric water splitting near the onset potential. The photocurrent was sustained over a day, attesting to its outstanding performance and durability for water splitting.

Magnetic coupling parameters at an oxide-oxide interface from first principles:Fe2O3-FeTiO3

Charge mismatch at the interface between canted antiferromagnetic hematite $(\ensuremath{\alpha}{\text{-Fe}}_{2}{\text{O}}_{3})$ and antiferromagnetic ilmenite $({\text{FeTiO}}_{3})$ is accommodated by the formation of mixed ${\text{Fe}}^{2+}$ and ${\text{Fe}}^{3+}$ contact layers, leading to uncompensated magnetic moments in the system. To derive the magnetic exchange interaction parameters of the end members and interface, we map total-energy differences of collinear spin arrangements obtained from density-functional theory calculations to a Heisenberg Hamiltonian using the least-squares method. Parameters for the end members, hematite $({J}_{m}^{3+,3+})$ and ilmenite $({J}_{m}^{2+,2+})$ are in good agreement with the values obtained from inelastic neutron-scattering data. The magnetic interaction parameters between ${\text{Fe}}^{2+}$ and ${\text{Fe}}^{3+}$ $({J}_{m}^{2+,3+})$ in the contact layer show a strong antiferromagnetic coupling to the adjacent hematite layers and thus explain the ferrimagnetism in the system.

Corrosion behaviour of 16%Cr–4%Al and 16%Cr ODS ferritic steels under different metallurgical conditions in a supercritical water environment

The corrosion behaviour of 16%Cr and 16%Cr–4%Al ODS ferritic steels in different heat treatment conditions has been investigated in a supercritical water environment. The exposed coupons were analyzed using scanning electron microscopy (SEM), electron probe micro analysis (EPMA), Auger and X-ray diffraction analysis (XRD). It was found that the formation of oxides depends on the chemical composition and not on the metallurgical condition. The Al-free alloys formed a monolayer oxide film composed of (Cr, Fe) 2O 3. The Al-containing alloys formed an oxide film composed of an outer layer of hematite and magnetite and an inner layer of Al 2O 3. The oxidation mechanisms are discussed.

A nanocrystalline hematite film prepared from iron(III) chloride precursor

This paper deals with a simple and low-cost method developed to deposit hematite (α-Fe 2O 3) layers on a fluorine-doped tin oxide (FTO/F:SnO 2) substrate by thermal decomposition of solid iron(III) chloride hexahydrate (FeCl 3⋅ 6H 2O). Deposition procedure takes place through chemical intermediate iron(III) oxide chloride (FeOCl) film. A crucial influence of atmosphere dynamics involved in the calcination process of FeOCl has been observed. As-deposited films were characterized by means of Conversion Electron Mössbauer Spectroscopy (CEMS), Grazing Angle X-Ray Diffractometry (GAXRD), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) analysis. Final nanocrystalline hematite film with a cactus-field-like design consists of 20 nm thick porous crystal plates. A process of hematite doping by tin atoms from substrate coating is also discussed.

Permian–polysulphide-siderite–barite–haematite deposit Rude in Samoborska Gora Mts., Zagorje–Transdanubian zone of the Inner Dinarides

Samoborska Gora Mts. is situated within the westernmost part of the Zagorje–Mid–Transdanubian zone of the Internal Dinarides. The Samoborska Gora Mts. predominantly consists of Permian unmetamorphosed siliciclastic sediments and evaporites, overlain by Lower Triassic sediments. Rude mineralisation is hosted by Permian siliciclastic sediments, below gypsum and anhydrite strata. The central part of the deposit consists of a 1.5 km long stratabound mineralisation, grading laterally into ferruginous sandstone and protruding vertically into a gypsum–anhydrite layer. Siderite–polysulphide–barite–quartz veins are located below the stratabound mineralisation. The stratiform part ofthe deposit is situated above the stratabound and consists of haematite layer with barite concretions and veinlets. Late stage galena–barite veins overprint earlier types of mineralisation. The Rude ore deposit was generated by predominantly NaCl±CaCl2–H2O solutions. Detrital quartz from stratiform mineralisation contains fluid inclusions with salinities between 7 and 11 wt. % NaCl equ., homogenizing between 150°C to 230°C. Stratabound/siderite–polysulphide–barite–quartz vein type mineralisation was derived from solutions with salinities between 5 and 19 wt. % NaCl equ., homogenizing between 60°C and 160°C, while late stage galena–barite veins were precipitated from solutions with salinities between 11 and 16 wt. % NaCl equ., homogenizing between 100°C to 140 °C. Fluid inclusion bulk leachate chemistry recorded Na+>Mg2+>K+>Ca2+>Li+ and Cl->SO42- ions. Sulphur isotope composition of barites and overlying gypsum stems from Permian seawater sulphate, supported by increased Br- content, which follows successively the seawater evaporation line. The sulphur isotopic composition of sulphides varies between –0.2 and +12.5 ‰, as a result of thermal reduction of Permian marine sulphate. Ore–forming fl uids were produced by hydrothermal convective cells (reflux brine model), and were derived primarily from Permian seawater, modified by evaporation and interaction with Permian sedimentary rocks. Rude deposits in Samoborska Gora Mts. may be declared as a prototype of the Permian siderite–polysulphide–barite deposits (products of rifting along the passive Gondwana margin), in the Inner Dinarides, and their equivalents extending northeastward into the Zagorje–Mid–Transdanubian Zone and the Gemerides, and southeastward to the Hellenide–Albanides.

Corrosion of alumina-forming austenitic steel Fe–20Ni–14Cr–3Al–0.6Nb–0.1Ti in supercritical water

The oxidation behavior of alumina-forming austenitic (AFA) steel Fe–20Ni–14Cr–3Al–0.6Nb–0.1Ti has been examined after exposure to supercritical water (SCW) at 500 °C and 25 MPa pressure and with a dissolved oxygen content of 25 wppb, for exposure periods of 1300 h, 1700 h and 3000 h. A double layer oxide structure developed on all samples, consisting specifically of a Fe-rich outer hematite layer and an inner layer of Al–Cr–Fe-rich oxide containing a small fraction of FCC Ni-rich metal. Additionally, a thin Ni-rich layer in the bulk alloy close to the oxide/metal interface was also observed. Based on weight gain measurements, the oxidation resistance of the AFA steel was superior to other austenitic alloys, 800H, D9, and 316 stainless steel, tested under similar conditions. The formation of the protective Al–Cr–Fe-rich oxide layer may provide good long-term protection against corrosion in SCW environment.

The oxidation behaviour of the 9 % Cr steel P92in CO2- and H2O-rich gases relevant to oxyfuel environments

Abstract In oxyfuel plants metallic heat exchanging components will be subjected to service environments containing high amounts of CO2 and water vapour. In the present paper, the oxidation behaviour of the ferritic/martensitic 9 % Cr steel P92 was studied in a model gas mixture containing 70 % CO2-30 % H2O in the temperature range 550 – 650 °C. The results were compared with the behaviour in air, Ar–CO2 and Ar–H2O. In the CO2- and/or H2O-rich gases, the steel formed iron-rich oxide scales which possess substantially higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich oxide scales are formed as a result of a decreased flux of chromium in the bulk alloy toward the surface. This is the result of enhanced internal oxidation of chromium in the H2O-containing gases and carburisation in the CO2 gases. The oxide scales allow molecular transport of CO2 towards the metallic surface, resulting in carburisation of the alloy. The presence of water vapour induced buckling in the outer haematite layer, apparently as a result of compressive oxide growth stresses. Buckling did not occur in the H2O-free gas. This has been discussed in terms of the potential for H2O to increase growth stresses and accelerate crack propagation. The oxidation rates in CO2–H2O do not seem to be higher than those observed in flue gases of conventional fossil fuel fired power plants.

Oxidation of iron at 400–600 °C in dry and wet O 2

The oxidation of iron in dry and wet O 2 at 400–600 °C has been re-investigated using gravimetry, SEM/EDX, XRD and FIB. In the presence of O 2, water vapour accelerates iron oxidation at 500 and 600 °C. At 400 and 500 °C the magnetite layer is duplex and exposure to water vapour results in the formation of blades on top of a fine-grained hematite layer. At 600 °C it results in a surface without needles and blades. The increased oxidation rate at 500 and 600 °C is attributed to a smaller grain size in the hematite layer resulting in faster ion transport.

Determination of Corrosion Layers and Protective Coatings on Steels and Alloys used in Simulated Service Environment of Modern Power Plants

Abstract The development of modern power generation systems with higher thermal efficiency requires the use of constructional materials of higher strength and improved resistance to the aggressive service atmospheres. The following two examples are discussed: 1. The oxidation behaviour of 9 wt.-% Cr steels in simulated combustion gases: The effects of O2 and H2O content on the oxidation behaviour of 9 wt.-% Cr steels in the temperature range 600—800°C showed that in dry oxygen a protective scale was formed with an oxidation rate controlled by diffusion. In contrast, that in the presence of water vapour, after an incubation period, the scale became non-protective as a result of a change in the oxidation mechanism. Oxidation experiments on 10 wt.-% Cr steel in simulated steam atmosphere at 650°C have shown that a hematite layer is already formed. The investigation suggest that several independent processes such as oxide dissociation and solid state diffusion of iron cations in the scale significantly contribute to the overall oxide growth process. 2. The development of light-weight intermetallics of TiAl-basis: TiAl-based intermetallics are promising materials for future turbine components because of the combination of high temperature strength and low density. These alloys, however, posses poor oxidation resistance at temperatures above 700°C. The experimental results showed, that the oxidation behaviour of TiAl-based intermetallics can be strongly improved by minor additions of 1–2 at.-% silver.

Tailoring of inclined crystal orientation in layered cobaltite thin films for the development of off-diagonal thermoelectric effect

A highly inclined CoO2 plane orientation was realized in CaxCoO2 thin films grown on sapphire single crystal substrates. The inclination angle of the CoO2 plane orientation was controlled by changing the inclination angle of the sapphire(0001) plane with respect to the substrate surface and additionally by introducing an epitaxial buffer layer of hematite. We demonstrate that the highly inclined crystal orientation results in a unique off-diagonal thermoelectric effect with a potential to produce a voltage signal of up to 600 mV along the film in-plane direction by a unit temperature difference along the film out-of-plane direction.

Evolution of Microstructure and Internal Stresses in Multi-Phase Oxide Scales Grown on (110) Surfaces of Iron Single Crystals at 650 °C

The evolution of microstructure and growth stresses in oxide scales grown on a (110) iron single crystal surface at 650 °C was studied by electron backscatter diffraction and in situ energy-dispersive diffraction with synchrotron radiation. Within this high temperature regime, the oxidation kinetics and scale microstructure were not significantly different from those encountered in the oxidation of ferrous polycrystals. Thus, epitaxial strains did not determine the stress state within the oxide scale. Relevant sources of growth stresses were inferred to be volumetric differences between the iron oxides in the early stages, and later, inner oxide formation, scale consumption as well as pore formation. These sources caused time-dependent stress cycles in magnetite and wüstite during oxidation. In the hematite layer stress cycles did not occur and creep appeared to be the predominant stress relieving mechanism. On cooling, the differences in thermal expansion caused residual stress gradients through the oxide scale.

Effect of Substrate Grain Size on the Growth, Texture and Internal Stresses of Iron Oxide Scales Forming at 450 °C

The oxidation behavior of iron polycrystals and single crystals with (110) surface orientation was studied at 450 °C. Energy-dispersive diffraction with synchrotron radiation provided in situ information regarding the evolution of stress gradients and fiber texture in the oxide scales. Within this low-temperature regime, grain boundaries caused the oxidation kinetics of polycrystalline iron to be more rapid than iron single crystals only during the first minutes of oxidation. Epitaxial growth of iron oxides occurred only on single crystal substrates during the initial oxidation. In situ stress analyses suggested that stress relief occurred invariably in the magnetite layer due to the formation of a fine-grained seam near the iron substrates. Above the magnetite and in the hematite layer, the growth stresses depend initially on volumetric strains and later on inner oxide formation and creep of the hematite.

WO3−Fe2O3 Photoanodes for Water Splitting: A Host Scaffold, Guest Absorber Approach

Solar hydrogen production via watersplitting with hematite (Fe2O3) has been limited by poor light absorption and a small hole diffusion length. These drawbacks can be overcome by using a high-surface-area host to support a thin layer of hematite—allowing photogenerated holes to be produced in high proximity to the semiconductor-liquid junction. Here we demonstrate the effectiveness of this concept using a nanostructured host scaffold (WO3) prepared by atmospheric pressure CVD to support a thin layer of Fe2O3 nanoparticles deposited by a similar method. A 20% increase in the photocurrent was observed in host−guest electrodes as compared to control films with the same amount of hematite (equivalent to a 60 nm film) deposited without the host scaffold. The improvement is attributed to an increase in the absorbed photon conversion efficiency (APCE), especially for longer wavelengths where the photon penetration depth is large in hematite. For light with a wavelength of 565 nm, the APCE improves to 8.0%, as co...

An ESEM in situ investigation of the influence of H 2O on iron oxidation at 500 °C

The initial oxidation of iron and the effect of different O 2/H 2O mixtures have been investigated through ESEM in situ exposure and analysis at 500 °C. In dry and wet air a two-layered magnetite forms beneath a thin, fine-grained hematite layer, while only a two-layered magnetite forms in H 2O. The two-layered magnetite is separated by a straight interface (in all environments), which is suggested to be the original metal surface. The presence of low levels of water vapour (∼1% H 2O) produces a thicker hematite layer (compared to dry air), exhibiting a large number of whiskers. At least four different factors are suggested to influence the local growth rate: the surface of the metal grain, the thickness of the hematite layer, the oxide grain size and the exposure environment. The ESEM in situ exposures have in addition been shown to have high quality and reproducibility.

In situ high-temperature Mössbauer spectroscopic study of carbon nanotube–Fe–Al 2O 3 nanocomposite powder

The oxidation of a carbon nanotube–Fe–Al 2O 3 nanocomposite powder was investigated using notably thermogravimetric analysis, room temperature transmission and emission Mössbauer spectroscopy and, for the first time, in situ high-temperature transmission Mössbauer spectroscopy. The first weight gain (150–300 °C) was attributed to the oxidation into hematite of the α-Fe and Fe 3C particles located at the surface and in the open porosity of the alumina grains. The 25 nm hematite particles are superparamagnetic at 250 °C or above. A weight loss (300–540 °C) corresponds to the oxidation of carbon nanotubes and graphene layers surrounding the nanoparticles. The graphene layers surrounding γ-Fe–C particles are progressively oxidized and a very thin hematite layer is formed at the surface of the particles, preventing their complete oxidation while helping to retain the face-centered cubic structure. Finally, two weight gains (670 and 1120 °C) correspond to the oxidation of the intragranular α-Fe particles and the γ-Fe–C particles.

Study on formation of “easy to remove oxide scale” during mechanical descaling of high carbon wire rods

Oxide layer in high carbon wire rods are not completely removed by the mechanical descaling process. Formation of hematite causes poor mechanical descaling whereas magnetite and wustite are found to be non-adherent oxides which can be more readily removed. Oxide formation at high temperature has been studied using Gleeble simulation and it is found that a high laying head temperature (LHT) followed by faster cooling reduces the formation of the hematite layer. Based on this observation, plant trails have been undertaken and a lower amount of adherent oxide has been observed.

Scale formation mechanisms of martensitic steels in high CO2/H2O-containing gases simulating oxyfuel environments

In oxyfuel power plants, metallic components will be exposed to service environments containing high amounts of CO2 and water vapour. Therefore, the oxidation behaviour of a number of martensitic 9–12%Cr steels in a model gas mixture containing 70% CO2–30% H2O was studied in the temperature range 550–700°C. The results were compared with the behaviour in air, Ar–CO2 and Ar–H2O. It was found that in the CO2- and/or H2O-rich gases, the mentioned steels tended to form iron-rich oxide scales with significantly higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich scales were formed as a result of a decreased flux of chromium in the bulk alloy toward the surface because of enhanced internal oxidation of chromium in the H2O-containing gases and carbide formation in the CO2-rich gases. Additionally, the presence of water vapour in the exposure atmosphere led to buckling of the outer haematite layer, apparently as a result of compressive oxide growth stresses. The Fe-base oxide scales formed in CO2(–H2O)-rich gases appeared to be permeable to CO2 molecules resulting in substantial carburization of the steel.

Vitreous Enamel Coating on Steel Substrates

Vitreous enamel is a glassy coating formed on a metal substrate by firing at temperatures above 800 °C. The quality of vitreous enamel coating depends on the pre-treatment of the steel substrate surface and the quality of enamel slip. The main aim of this study was to characterize the composition of the steel substrate surface after firing with double finish, to explain the effect of steel substrate surface pre-treatment (blasting, acid pickling) on forming the phase interface of the steel substrate – vitreous coating system, and on its final microhardness, fracture strength and the adhesive properties of the coating. To achieve these aims, the following experimental methods were used: Mössbauer spectroscopy, X-ray diffraction, electron analyzer, and EDAX. Based on the chemical composition of the initial enamel slip and the firing technology, the metal - enamel system interface exhibited various ratios of layers of magnetite, hematite, nonstoichiometric wüstite, and crystals of faylite. The measuring results indicate that the quality and age of enamel slip influences the brittle fracture properties of vitreous enamel coating.

Scale formation mechanisms of martensitic steels in high CO2/H2O-containing gases simulating oxyfuel environments

In oxyfuel power plants, metallic components will be exposed to service environments containing high amounts of CO2 and water vapour. Therefore, the oxidation behaviour of a number of martensitic 9–12%Cr steels in a model gas mixture containing 70% CO2–30% H2O was studied in the temperature range 550–700°C. The results were compared with the behaviour in air, Ar–CO2 and Ar–H2O. It was found that in the CO2- and/or H2O-rich gases, the mentioned steels tended to form iron-rich oxide scales with significantly higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich scales were formed as a result of a decreased flux of chromium in the bulk alloy toward the surface because of enhanced internal oxidation of chromium in the H2O-containing gases and carbide formation in the CO2-rich gases. Additionally, the presence of water vapour in the exposure atmosphere led to buckling of the outer haematite layer, apparently as a result of compressive oxide growth stresses. The Fe-base oxide scales formed in CO2(–H2O)-rich gases appeared to be permeable to CO2 molecules resulting in substantial carburization of the steel.

The Influence of H<sub>2</sub>O on Iron Oxidation at 600°C: A Microstructural Study

The oxidation of iron in dry O2 and in wet O2 (40% H2O) has been studied at 600°C. The oxide microstructure was investigated by SEM/EDX, FIB and XRD. Iron forms a layered scale in dry and wet oxygen at 600°C. The scale consists of a top hematite layer, a middle magnetite layer and a wüstite layer close to the scale metal interface. All three layers grow with time, but with different growth rates, the overall growth being approximately parabolic. The presence of water vapour increases the rate of oxidation and affects the evolution of the oxide microstructure. The higher rate of oxidation in the presence of water vapour is due to an increased growth rate of the magnetite layer and, especially, of the hematite layer, while the growth of the wüstite layer is not affected. It is suggested that water vapour influences grain boundary transport in the hematite layer.