Formation of iron oxides by the oxidation of iron in Fe-MOH-H2O and Fe-MOH-H2O-O2 systems (M = Li,Na, K)

The oxidation behaviours of iron powders, ca. 100 m in diameter, in 5–50 m NaOH, 5–40 m KOH and 5–40 m LiOH solutions at 373–573 K were investigated in the absence and presence of oxygen, where m is molality. The oxidation of iron proceeded noticeably above 423 K to form Fe₃O₄, -Fe₂O₃, -NaFeO₂, -Fe₂O₃, Li _x Fe_3–xO₄ and -LiFeO₂ depending on the reaction conditions. The rate of oxidation in LiOH solutions was much slowerthan those in NaOH and KOH solutions.     

δ-FeO (OH) and its solutions

Decomposition products of -FeO(OH)-type Fe_1–x M _x O_1–x (OH)_1+x phases (M=Mg, Zn, Ca, Cd) have been studied by X-ray diffraction, electron diffraction and high-resolution transmission electron microscopy. It has been shown that the M=Mg and Cd -phases decompose to -Fe₂O₃-based solid solutions which in turn undergo exsolution to form some MgO and CdO at a higher temperature. In the case of Fe_1–x Zn _x O_1–x (OH)_1+x , the decomposition proceeds over -Fe₂O₃ ss to an unstable spinel solid solution. All decomposition products are topotactically related to their precursors in the decomposition chain. In the electron microscope some -type phases undergo in situ decomposition under intense beam bombardment with somewhat different results than obtained for thermal decomposition products under ambient conditions. The plate-like morphology and crystal size is retained in the decomposition products; however, the products have a more pitted appearance after decomposition.     

Effect of heat treatment and irradiation on electrical conductivity and crystallization in the BaO-B2O3-Fe2O3 glass system

A glass system was prepared according to the formula 75 mol % B₂O₃-(25 –x) mol % BaO –x mol % Fe₂O₃, wherex = 0, 1, 2.5, 5, 7.5 and 10. The glasses were subjected to heat treatment at 550° C for 2, 6, 12, 18 and 24 h. The glasses were also irradiated using-rays at a dose of 4.805 × 10⁴ rad h^–1 for 12, 18 and 24 h. An X-ray diffraction technique was used to identify the separated crystalline phases. The electrical conductivity and activation energy of untreated, heat-treated and irradiated samples were measured and calculated. The rate and the dimensions of crystallization were also calculated by using the Avrami equation. It was found that-Fe₂O₃ is the separated phase when a sample containing 7.5 mol% Fe₂O₃ is heat treated for 24h;-Fe₂O₃ and Fe₂O₃ are the separated phases when the sample containing 10 mol% Fe₂O₃ is heat treated for 6, 12 and 18 h, with the addition of BaO when the sample is heat treated for 24 h. A miminum value for the electrical conductivity of glass samples was found to occur around an Fe₂O₃/BaO ratio of 0.425. The rate of crystallization in the sample containing 10 mol% Fe₂O₃ is 1.30607 × 10^–3 and the geometry of crystallizationn is 1.2238, which indicates that the crystallization was in one dimension.     

Microwave flash synthesis of iron and magnetite particles by disproportionation of ferrous alcoholic solutions

This paper reports the microwave-hydrothermal treatment of alcoholic solutions of ferrous chloride (FeCl₂) and sodium ethoxide (EtONa) solutions with a microwave autoclave designed by the authors (the RAMO system). Depending on the initial concentrations, hematite (-Fe₂O₃), spinel phase (Fe_{3 – x} O₄) or iron-magnetite (Fe⁰-Fe₃O₄) nanocomposites are obtained with a lower grain size compared to conventional composites. Indeed, X-ray diffraction (XRD) analysis reveals grain sizes close to 20 nm for magnetite and 60 nm for metallic iron. However, the amount of metal is smaller (close to 11%). Furthermore, these particles are inert in the ambient atmosphere. Consequently, the RAMO (French acronym of Reacteur Autoclave MicroOnde) system appears to provide an efficient source of energy in rapidly producing inert powders of iron, magnetite and iron-magnetite composites.     

In situ conversion electron Mössbauer spectrometry (CEMS) studies on α- and γ-Fe2O3 gas sensors

Clarification of the sensing mechanism of-Fe₂O₃ and -Fe₂O₃ gas sensors was attempted. The information on the surface states of iron oxides at working temperature (400° C for-Fe₂O₃ and 250° C for-Fe₂O₃) was obtainedin situ by applying thein situ CEMS. The-Fe₂O₃ gas sensor, prepared by the precipitation of Fe(OH)₃ from a solution of iron (III) sulphate and tin (IV) chloride, was composed of fine particles (–15 nm) and was superior in sensitivity to other-Fe₂O₃. The gas sensitivity was found to depend on the amounts of remaining sulphate ion, the microstructure and a small amount of iron (II) species generated through the reduction of-Fe₂O₃. The sensing mechanism of-Fe₂O₃ gas sensor was confirmed to be due to the reduction of-Fe₂O₃ to the low resistive Fe_3-x O₄ by combustible gas and to depend on the crystal structure.     

A Mössbauer study of the oxidation of Fe-Ni alloys at 535 and 635°C

Fe⁵⁷ transmission Mössbauer spectroscopy, supported by metallography, SEM and X-ray diffraction analysis, has been employed to study the oxidation of Fe-Ni alloys at 535 and 635° C in 1 atm. of air. With increasing Ni content of the alloy, the composition of the scale changed and the oxidation rate decreased. For an alloy containing 0.9% Ni, the oxide scale produced at 535° C was Fe₃O₄ covered by a thin outer layer of-Fe₂O₃, while at 635° C FeO was additionally present as a major phase. The scale formed on a 10% Ni alloy at both 535 and 635° C was similar to that observed for the 0.9% Ni alloy oxidized at 535° C (i.e. of Fe₃O₄ and-Fe₂O₃), although the-Fe₂O₃ layer tended to be relatively thicker. For a 49% Ni alloy, the scale at both 535 and 635° C comprised an inner layer of Ni _x Fe_3–x O₄ (withx0.5, on average) and an outer layer of-Fe₂O₃, of similar thickness. Finally, on an 83% Ni alloy oxidized at 635° C, the scale consisted of roughly equally thick layers of NiO (next to the metal) and NiFe₂O₄, and a thin outer covering of-Fe₂O₃. The decrease in oxidation rate with increasing Ni content of the alloy is discussed briefly in relation to the changing composition of the scale and diffusion in the alloy.     

Electrical conductivity study of oxidation process in manganese-substituted magnetites

During oxidation in air of finely-grained manganese-substituted magnetites (Mn _0.8x ²⁺ Fe _1–0.8x ³⁺ )_A– (Fe _1+0.6x ³⁺ Fe _1–0.8x ²⁺ Mn _0.2x ³⁺ )_BO ₄ ^2– (A=tetrahedral, B=octahedral) the temperature dependence of the electrical conductivity over a temperature range of 100 to 700° C was investigated. Below 500° C the evolution of electrical conductivity might be closely associated with the position and nature of cations in the spinel lattice. The profile of the =f(t) curves show that the mechanism of electrical conduction in the temperature range 150 to 300° C can be explained in terms of the oxidation of Fe²⁺ to Fe³⁺ ions at octahedral sites. For the temperature range 300 to 400° C the conductivity involves the hopping of electrons from tetrahedral-site Mn²⁺ ions to tetrahedral-site Mn³⁺ ions. Above 500° C the oxidation of Mn²⁺ ions leads to an increase in conductivity with the generation of new phases of -Fe₂O₃, Mn₂O₃ and -(MnFe)₂O₃.     

Analytical electron microscopy of Al2O3 implanted with iron ions

Single crystals of -Al₂O₃ were implanted with iron ions at room temperature to fluences ranging from 4×10¹⁶ Fe cm^–2 to 1×10¹⁷ Fe cm^–2. The microstructure and composition in the implanted region were examined using analytical electron microscopy techniques. Special emphasis was placed on monitoring the microstructural changes which take place during post-implantation annealing. Clusters of metallic -Fe were identified in the specimen after implantation to a dose of 1×10¹⁷ Fe cm^–2. Analytical electron microscopy of implanted specimens annealed in oxygen revealed the redistribution of the implanted iron and the formation of surface precipitates of -Fe₂O₃, subsurface precipitates of various forms of spinel, and, in some cases, subsurface precipitates of iron, depending on the annealing temperature. Examination of implanted specimens annealed under reducing conditions revealed the presence of precipitates of -Fe.     

Preparation of micaceous iron oxide by the oxidation of iron with pressurized oxygen in concentrated sodium hydroxide solution at elevated temperatures

Iron powders were oxidized in NaOH solutions of 5–25 mol kg^–1 at 403–563 K and 5 MPa of oxygen partial pressure. Various types and morphologies of iron compounds such as fine particles of Fe₃O₄, micaceous -Fe₂O₃, and coagulated particles of -NaFeO₂ were formed depending on the experimental conditions. The observed critical concentrations of NaOH above which -NaFeO₂ was formed was in good agreement with those thermodynamically calculated for the hydrolysis equilibrium of -NaFeO₂.     

The influence of mechanochemical treatment of the Bi2O3–ZrO2 system on the structural and dielectric properties of the sintered ceramics

A powder mixture of -Bi₂O₃ and ZrO₂, both monoclinic, in the molar ratio 2 : 3, was mechanochemically treated in a planetary ball mill in an air atmosphere for up to 20 h, using steel vial and hardened-steel balls as the milling medium. Mechanochemical reaction leads to the gradual formation of an amorphous phase. After 5 h of milling the starting -Bi₂O₃ and ZrO₂ were transformed fully into a non-crystalline phase. After milling for various times the powders were compacted by pressing and isothermal sintering. The pressed and sintered densities depended on the milling time. Depending on the duration of the mechanochemical treatment and sintering temperature, the phases: -Bi₁₂(Zr _x Fe_1–x )O₂₀; Bi(Zr _x Fe_1–x )O₃ and Bi₂(Zr _x Fe_1–x )₄O₉ were obtained by reactive sintering, whereby the Fe originates from vial and ball debris. The dielectric permittivity of the sintered samples significantly depends on the milling time. Samples milled for 10 and 15 h and subsequently sintered at 800 °C for 24 h exhibit a hysteresis dependence of the dielectric shift (in altering electric fields higher than 10 kV/cm at room temperature), confirming that the synthesized materials possess ferroelectric properties.     

Synthesis of carbon nanotube films by thermal CVD in the presence of supported catalyst particles. Part I: The silicon substrate/nanotube film interface

The interface between the silicon substrate and a carbon nanotube film grown by thermal CVD with acetylene (C₂H₂) and hydrogen at 750 or 900 °C has been characterized by high resolution and analytical transmission electron microscopy, including electron spectroscopic imaging. Silicon (0 0 2) substrates coated with a thin (2.8 nm) iron film were heat treated in the CVD furnace at the deposition temperature in a mixture of flowing argon and hydrogen whereby nanosized particles of (Fe,Si)₃O₄ formed. These particles were reduced to catalytic iron silicides with the –(Fe, Si), ₂–Fe₂Si and ₁–Fe₂Si structures during CVD at 900 °C, and multi-wall carbon nanotubes grew from supported particles via a base-growth mechanism. A limited number of intermediate iron carbides, hexagonal and orthorhombic Fe₇C₃, were also present on the substrate surface after CVD at 900 °C. The reduction of the preformed (Fe, Si)₃O₄ particles during thermal CVD at 750 °C was accompanied by disintegration leading to the formation of a number of smaller (<5 and up to 10 nm) iron and silicon containing particles. It is believed that the formation of these small particles is a prerequisite for the growth of aligned multi-wall carbon nanotube films.     

Polymorphism in Sr3Fe2O7−x

The compound Sr₃Fe₂O_7–x , with variable iron valence, was investigated by X-ray powder techniques, both at room and at high temperatures. If the material is examined in massive form, a single phase called -Sr₃Fe₂O_7–x appears as previously reported in the literature. This -phase is tetragonal and exhibits the lattice parameters: a=3.874 and c=40.314 Å. Two other phases, called and -Sr₃Fe₂O_7–x , respectively, can be obtained on heating the finely powdered material when laid on a flat platinum support. The form is stable up to 1275° C, while the form is revealed only above 1275° C and changes always into -Sr₃Fe₂O_7–x when quenched. Both and phases are tetragonal, with a=4.001 and c= 58.251 for the form and a=4.013, c=57.092 Å for the form. The transition involves a true phase equilibrium, while the transformation is possible only by means of a suitable mechanical treatment of the material.     

Chemical and structural properties of the system Fe2O3-Cr2O3

Chemical and structural properties of the mixed metal oxides (1–x)Fe₂O₃+xCr₂O₃ were studied by different techniques. X-ray powder diffraction showed the existence of solid solutions, (Fe_1–x Cr _x )₂O₃, over the whole concentration region, 0x1. The gradual replacement of Fe³⁺ with Cr³⁺ ions in samples prepared at 900°C caused changes in unit-cell parameters; most of these changes took place in the region fromx0.3–0.9. The samples having the fraction of Cr₂O₃ in the region from 0.7–0.8, contained two closely related phases, with slightly different compositions. After an additional heat treatment at 1100°C, these samples contained only one phase.⁵⁷Fe Mössbauer spectroscopy showed a gradual decrease of hyperfine magnetic field with increasing Cr₂O₃ content. The sample having the fraction of Cr₂O₃ of 0.7, and prepared at 900°C, exhibited two separated sextets at room temperature, in comparison with other compositions showing one sextet. It was shown that Fourier transform infrared (FT-IR) spectroscopy is a powerful method for the investigation of structural changes in these solid solutions. The increase in the Cr₂O₃ content resulted in shifts of the corresponding infrared bands. In addition, a gradual transition of the spectrum typical for -Fe₂O₃ to the spectrum typical for Cr₂O₃ was shown. The transition effects observed in the FT-IR spectra were correlated with the X-ray powder diffraction and⁵⁷Fe Mössbauer spectroscopic results.     

Formation of oxide phases in the system Fe2O3-NiO

Mixed metal oxides in the system Fe₂O₃-NiO were prepared by coprecipitation of Fe(OH)₃/Ni(OH)₂ and the thermal treatment of hydroxide coprecipitates up to 800 or 1100°C. X-ray diffraction showed the presence of -Fe₂O₃, NiO and NiFe₂O₄ in samples prepared at 800°C. The oxide phases -Fe₂O₃, NiO, NiFe₂O₄ and a phase with structure similar to NiFe₂O₄ were found in samples prepared at 1100°C. Fourier transform-infrared spectra of oxide phases formed in the system Fe₂O₃-NiO are discussed. Two very strong infrared bands at 553 and 475 cm^–1, a weak intensity infrared band at 383 cm^–1 and two shoulders at 626 and 441 cm^–1 were observed for -Fe₂O₃ prepared at 1100°C. NiFe₂O₄, prepared at the same temperature, showed two broad and very strong infrared bands at 602 and 411 cm^–1, while NiO showed a broad infrared band at 466 cm^–1. Fourier transform infrared spectroscopic results were in agreement with X-ray diffraction.     

Formation and characterization of the solid solutions (CrxFe1−x)2O3, 0⩽x⩽1

The solid solutions (Cr_xFe_1–x)₂O₃, 0 x 1, were prepared by traditional ceramic procedures. The samples were characterized using X-ray diffraction, Mössbauer, Fourier transform infra-red (FT-IR) and optical spectroscopic measurements. In the whole concentration range two phases exist phase F, -(Cr_xFe_1–x)₂O₃, which is isostructural with -Fe₂O₃ and phase C, which is closely related to Cr₂O₃. Phase F exists in samples heated up to 900°C, for 0 x 0.95. Phase C exists from x0.27 to x=1 for samples heated up to 900°C and from x0.65 to x=1 for samples heated up to 1200 °C. For samples heated up to 900 °C, the solubility limits were 27.5 ± 0.5 mol% of Cr₂O₃ in -Fe₂O₃ and 4.0 ± 0.5 mol % of -Fe₂O₃ in Cr₂O₃. For the samples heated at 1200 °C the diffraction peaks for the F and C phases in the two phase region were severely overlapped and thus the solubility limits could not be determined accurately as for previous samples. ⁵⁷Fe Mössbauer spectra of the samples heated up to 1200 °C showed significant broadening of spectral lines and a gradual decrease of the hyperfine magnetic field with increase of x up to 0.50. For x0.7, a paramagnetic doublet with collapsing sextet was observed. The spectra were interpreted in terms of an electronic relaxation effect; however, an agglomeration of iron ions which would contribute to the superparamagnetic effect could not be excluded. The FT-IR spectra showed transition effects in accordance with the X-ray diffraction results. The most intense absorption bands, observed for the samples heated up to 1200 °C, were located at 460 and 370 nm (22 000 and 27 000cm^–1) for x 0.5, 500 and 360 nm for x < 0.3, and might be correlated with the strong enhancement of the pair transitions through antiferromagnetic interactions. The intensification of the ⁶A₁ 4T₁ Fe³⁺ ions in all spectra and the development of the absorption at 13000 cm^–1 due to a metal-metal charge transfer (Cr³⁺ Fe³⁺) transition, might be explained by exchange coupling which has been observed in some spinel compounds.     

Synthesis of iron zircon coral by coprecipitation routes

An iron-doped (15 mol-% of Fe₂O₃) zircon ceramic pigment has been prepared using binary (ZrO₂-Fe₂O₃ and SiO₂-Fe₂O₃) and ternary (ZrO₂-Fe₂O₃-SiO₂) colloidal gels or coprecipitates as precursors. The obtained raw powders, precalcines, and fired pigments have been characterized by XRD, thermal analysis (TGA/DTA) and SEM/EDX to analyze the effect of binary interactions (occlusion and adsorption phenomena) on the synthesis of the iron-zircon coral as well as on the coloring yield. The use of a binary raw coprecipitate as precursor prepared with colloidal silica and ferrous sulphate leads to a higher efficiency in the hematite occlusion, since a more intense coral hue (L ^* = 59.6, a ^* = 29.3 and b ^* = 25.8 at 950°C) is obtained, similar to an optimal ceramic reference. The protection or occlusion of -Fe₂O₃ in amorphous silica agglomerates of high specific surface appears to be a more effective reaction intermediate than the observed adsorption of micronic -Fe₂O₃ particles on tetragonal zirconia monoliths. The necessary transformation of tetragonal zirconia into its monoclinic form prior to zircon formation, and the higher -Fe₂O₃ segregation from the coprecipitate obtained with Zr and Fe precursors, seem to lower the efficiency of the hematite coarsening-occlusion process involved in the coral pigment formation.     

Improvements in α-Fe2O3 ceramic sensors for reducing gases by addition of Sb2O3

The microstructure, electrical properties and gas-sensing characteristics of Sb-doped -Fe₂O₃ were investigated. Powder precursors with Sb/Fe = 0–0.1 were prepared by chemical coprecipitation method. Sb-doped -Fe₂O₃ powders were characterized by means of thermal gravimetric-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), BET surface area and scanning electron microscope (SEM). It was found that the raw powders underwent crystallization into the corundum structure of -Fe₂O₃ at a temperature which increased somewhat with increasing Sb content; a proper amount of Sb doping suppressed both crystallite growth and the formation of hard agglomerates. The doping of Sb₂O₃ decreased the sensor resistance by one order of magnitude and increased the sensitivities to some hydrocarbon gases markedly. The former can be attributed to the substitution of Sb⁵⁺ for Fe³⁺ sites in -Fe₂O₃ generating more free electrons; the latter is closely related to Sb-doped samples accommodating a higher density of chemisorbed oxygen.     

Thermal decomposition of ferrous oxalate dihydrate studied by direct current electrical conductivity measurements

The feasibility of the use of direct current electrical conductivity,, measurements in the study of solid-state reactions involved in the synthesis of-Fe₂O₃ from ferrous oxalate dihydrate has been reported. The study has been carried out in static air, dynamic air, dry nitrogen and dynamic air + water vapour environments. The conductivity data determined in static air are quite complex; nevertheless, the formation of Fe₃O₄ and-Fe₂O₃ with the probable intermediate formation of-Fe₂O₃ has been indicated. In dry nitrogen the step corresponding to dehydration is well resolved in the temperature region 145–220° C; the formation of FeO and Fe₃O₄ is also well characterized. In dry dynamic air the reaction further proceeds to the formation of-Fe₂O₃. In dynamic air containing water vapour there are definite indications of the formation of-Fe₂O₃ prior to the formation of-Fe₂O₃. Definite experimental conditions have been determined for the formation of-Fe₂O₃ in dynamic air containing water vapour. The conductivity measurements have been supplemented with infra-red spectroscopy and X-ray diffraction pattern measurements. The electrical conductivity measurements were found to give additional information on the solid-state reaction to that obtained from conventional thermal analytical techniques (such as differential thermal, thermogravimetric and differential thermogravimetric analyses).Fe₂O₃, obtained from the decomposition of FeC₂O₄ · 2H₂O in dynamic air + water was found to have a coercive force of 3.142 A m^–1, a saturation magnetization value of 7.1 T kg^–1 and a ratio of remanence to saturation magnetization of 0.64.     

Influence of substrate temperature on the structural and electrical properties of α-Fe2O3 films prepared by ultrasonic spray pyrolysis

Iron oxide films were prepared by ultrasonic spray pyrolysis (USP) on SiO₂ coated Si wafers using iron acetylacetonate as an iron precursor. The crystallographic properties and surface morphologies of the films were characterized by X-ray diffraction and scanning electron microscopy, respectively. X-ray photoelectron spectroscopy (XPS) was carried out to determine the Fe oxidation states. The satellite peak associated with Fe³⁺ photoemission at a binding energy of 719 eV was detected in the XPS results for iron oxide films, which is one of the indications of the Fe₂O₃ composition. The as-deposited films exhibit a polycrystalline -Fe₂O₃ structure. In order to observe thermal stability of the films, the resistance variation with ambient temperature was measured. All the iron oxide films deposited in this experiment were found to be -Fe₂O₃ with the thermal stability lower than 2%/ °C.     

Anisotropy of electrical properties in α-Fe2O3 ceramics

Electrical conductivity and thermoelectric power measurements carried out in a heamatite ceramic showed a strong anisotropy in directions normal and parallel to the uniaxial pressing direction. This behaviour is similar to that verified in -Fe₂O₃ single crystal. The results suggest that the extended structural defects, generated during sintering, disturb the magnetic order on the (001) planes of -Fe₂O₃ and limit the mobility of n type carriers.