Formation Of α-Fe2O3 Research Articles

Microwave mediated precipitation and aging of iron oxyhydroxides at low temperature for possible hydrometallurgical applications

This preliminary study examined the role of microwave radiation on precipitation and aging of iron oxyhydroxides at 60°C. It was observed that due to rapid heating under microwave, transformation of two line ferrihydrite to α-Fe2O3 takes place within 4–10h. This is in contrast to the usual aging temperature of >100°C adopted in most conventional studies, and several days of aging. Besides rapid transformation, microwave helps forming nanoparticles, with crystallite size of ~60nm as determined from X-ray diffractograms, and <100nm particles as observed under TEM. Rapid transformation also promotes formation of α-Fe2O3 without the formation of intermediate γ-Fe2O3. This study brought out the advantages of low temperature microwave assisted precipitation/aging as a low energy process to transform iron oxyhydroxides, with likely applications in slurry rheology, synthesis of nanoparticles, and adsorption.

A Thermal Decomposition Approach for the Synthesis of Iron Oxide Microspheres

ABSTRACTIron oxide microspheres possess a wide range of applications in lithium storage batteries, sensors, photocatalysis, environmental remediation, magnetic resonance imaging and drug delivery. The most commonly used method for the preparation of iron oxide microspheres is hydrothermal synthesis. Besides this, other synthetic methods such as co-precipitation, electrostatic self- assembly, microwave and sol-gel have been reported. The reported synthetic methods usually require longer time (2 to 48 hours) and expensive experimental set up. In the present study, a novel low temperature thermal decomposition approach for the synthesis of iron oxide microspheres has been reported. Thermal decomposition of an iron-urea complex ([Fe(CON2H4)6](NO3)3) in a mixture of diphenyl ether and dimethyl formamide at 200 °C for 35 minutes leads to the formation of iron oxide microspheres. The microspheres were characterized using a variety of analytical techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS) and magnetometry. The XRD results indicated amorphous nature for the as prepared iron oxide, whereas after calcination at 500 °C, crystalline α-Fe2O3 phase is obtained. The SEM images indicated uniform spheres with an average diameter of 1.2 ± 0.3 μm. The DRS results too gave evidence for the formation of α-Fe2O3 on calcination of the microspheres at 500 oC.The field and temperature dependent magnetic measurement results indicated superparamagnetic behavior for the as prepared iron oxide microspheres indicating that the microspheres consist of iron oxide nanoparticles. On the other hand, an antiferromagnetic behavior was observed for the microspheres calcined at 500 °C. The present synthetic method is a novel method to produce magnetic materials with controlled morphologies.

Synthesis of Monolithic Hierarchically Porous Iron-Based Xerogels from Iron(III) Salts via an Epoxide-Mediated Sol–Gel Process

Various iron-based polycrystalline monoliths, Fe3O4, iron, and Fe3C, with hierarchically distributed pores have been synthesized from ionic precursors using a sol–gel process accompanied by phase separation. Propylene oxide acts as a proton scavenger to increase pH moderately and uniformly in a reaction solution, which leads to homogeneous gelation. On the other hand, poly(acrylamide) works as a phase separation inducer as well as a precipitation inhibitor. Appropriate choice of iron precursor, solvent, polymer, and epoxide allowed the formation of iron(III)-based xerogels with cocontinuous macroporous structures. The dried gels were amorphous, whereas heating in air above 300 °C led to the formation of α-Fe2O3. Calcination under an inert condition above 400 °C formed Fe3O4, iron, and Fe3C without collapse of macrostructures. Examination has been carried out using SEM, TG-DTA, FT-IR, Hg intrusion, pH measurement, X-ray diffraction, and N2 adsorption–desorption.

Study of dependence of magnetic properties on seeding temperature in fine particles of Cu0·25Co0·25Zn0·5Fe2O4

Using a combination of X-ray diffraction, room temperature/low temperature Mössbauer studies and d.c. magnetization, the structural and magnetic properties of nano-sized Cu0·25Co0·25Zn0·5Fe2O4, prepared using the coprecipitation method at different seeding temperatures, have been studied. Formation of α-Fe2O3 in these samples is observed to be very sensitive to the seeding temperature and is totally suppressed at a seeding temperature of 333 K. These results are explained in terms of the rate of coprecipitation of the different components as a function of temperature. Since α-Fe2O3 is antiferromagnetic, the presence and quantity of this impurity is also observed to deteriorate the overall magnetic properties of the spinel phase.

Synthesis and characterization of nanosized ferrites from the thermolysis of strontium and barium tris(succinato)ferrate(III) precursors

Thermal analysis of alkaline earth metal tris(succinato)ferrate(III) precursors, M3[Fe(C4H4O4)3]2.xH2O (M = Sr, Ba) has been monitored isothermally and nonisothermally employing various physico-chemical techniques viz. simultaneous thermo gravimetry-derivative thermo gravimetry-differential scanning calorimetry (TG-DTG-DSC), XRD, Mössbauer spectroscopy, VSM, and TEM to characterize the intermediates/end products. Simultaneous TG-DTG-DSC depicts the sequence of the solid-state reactions in which dehydration is the first step followed by the decomposition of anhydrous precursors to yield iron(II) oxalate, Fe(II)C2O4, and respective metal oxalate intermediate species in the temperature range 150–210°C. A subsequent oxidative decomposition of iron(II) species leads to the formation of α-Fe2O3 and respective alkaline earth metal carbonate in the temperature range of 250–350°C. Finally, the alkaline earth metal carbonates decompose into their respective oxides in the temperature range of 590–680°C, followed by a solid-state reaction (&amp;gt;700°C) with α-Fe2O3 leading to the formation of ferrites of the stoichiometry, Sr2Fe2O5 and BaFe2O4. TEM studies reveal the formation of ferrite nanoparticles with average grain size of 25–30 nm.

Formation of iron oxides nanoparticles on low carbon steel under controlled conditions

A clean route for oxide scale grown on low carbon steel alloy surface at 650°C under controlled heating rate and oxidation period, was investigated. It was found that using controlled conditions of a certain heating rate and an oxidation time led to remarkable changes in the formation of the oxide scale phases and the crystallites size. X-ray diffraction patterns show the formation of α-Fe2O3 and Fe3O4 phases with some preferred orientation in the (100) direction. The estimated average crystallites size of the oxide scale is about 30 nm after 60 hours of oxidation time at 650°C under constant heating rate of 0.1°C/min. Increasing the oxidation time slightly increases the crystallite size. The oxide scale exhibited a relatively medium saturation magnetisation of 34.59 emu/g. This indicates the presence of mixture of Fe oxides, predominately α-Fe2O3 and Fe3O4 phases, as observed in the X-ray diffraction patterns.

Sol–gel formation of γ-Fe2O3/SiO2 nanocomposites: Effects of different iron raw material

Silica coated magnetic γ-Fe2O3 nanoparticles were synthesized by the sol–gel method. The crystal phase and magnetic properties of serials of α, γ-Fe2O3/SiO2 from different iron raw materials, submitted to thermal treatment at a temperature of 400 °C were investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), Fourier-transform IR spectroscopy and magnetometry. Different amounts of TEOS, ethanol, water (media), and the different sources of iron, including ferric citrate (adjusting pH with citric acid), ferric chloride (adjusting pH with HCl), ferric nitrate (adjusting pH with nitrate) as raw material were mixed to form sol, and then gel. After drying at 60 °C in vacuum, the dry gels were sintered at the temperature of 400 °C to produce the Fe2O3 particles. Ferric citrate raw material favored the occurrence of γ-Fe2O3, while ferric chloride gave rise to α-Fe2O3 formation in SiO2 matrix.

Preparation of ferrites from the thermal decomposition of magnesium and calcium tris(succinato)ferrates(III) precursors

Thermal analysis of alkaline earth metal ferrisuccinate precursors, M3[Fe(C4H4O4)3]2·xH2O (M=Mg, Ca) has been studied isothermally and non-isothermally employing simultaneous TG-DTG-DSC, XRD, IR and Mossbauer spectroscopy to characterize the intermediates/end products. After dehydration, the anhydrous complexes decompose to yield an iron(II) oxalate intermediate, Fe(II)C2O4 in the temperature range 180–250°C. Decomposition of this iron(II) species leads to the formation of α-Fe2O3 and respective alkaline earth metal oxide/carbonate in the temperature range 250–300°C. Finally, ferrites of the stoichiometry, MgFe2O4 and Ca2Fe2O5 are formed as a result of solid-state reaction between α-Fe2O3 and MO/MCO3. A special feature of the precursor method, adopted by us, is that the formation of ferrites occurs at much lower temperature than that of conventional ceramic method.

Role of ligands in the formation, phase stabilization, structural and magnetic properties of α-Fe2O3 nanoparticles

The electrochemical synthesis of alpha Fe2O3 nanoparticles was performed using quaternary ammonium salts viz. TPAB, TBAB and TOAB in an organic medium by optimizing current density and molar concentration of the ligand. The role of ligands in the formation of α phase, structure and magnetic properties was investigated in details. The effect of increasing chain length on the particle size confirmed that as the chain length increases from propyl to octyl, the particle size decreases. X-ray diffraction spectra of as prepared samples and TEM analysis confirmed the amorphous nature of iron oxide. TEM showed beads of iron oxide joined together with a size distribution in the range of 6–30 nm. The Mossbauer studies also support this observation that for the lowest particle size, the line width is broader which successively reduces with increase in particle size. Iron oxide capped with TOAB indicated superparamagnetic nature at room temperature. The resultant internal magnetic field of 506 mm/s due to hyperfine splitting clearly established the formation of α-Fe2O3 The infrared spectroscopy and pH measurements revealed the binding of tetra alkyl ligand with iron oxide. The IR spectra and the increase in basicity of as prepared samples confirmed the formation of hydrated iron oxide. Above 800°C the spectra indicated only iron oxide. Surface area obtained by BET method was 205 m2/g.

Synthesis of ferrites

Thermal analysis of some transition metal ferrimaleate precursors, M3[Fe(mal)3]2·xH2O (M=Mn, Co, Ni, Cu) has been studied in static air atmosphere from ambient to 600°C. Various physico-chemical techniques, i.e. TG, DTG, DTA, XRD, IR, Mossbauer spectrometry, have been employed to characterize both the intermediates and final products. After dehydration the anhydrous precursors undergo decomposition to yield an iron(II) intermediate, M[FeII(mal)2] (M=Mn, Co, Ni, Cu) in the temperature range 160-275°C. A subsequent oxidative decomposition of iron(II) species leads to the formation of α-Fe2O3 and MO in the successive stages. Finally a solid-state reaction occurs between the oxides above 400°C resulting in the formation of transition metal ferrites, MFe2O4. The ferrites have been obtained at much lower temperature and in less time than in the conventional ceramic method.

XAS characterization and CO oxidation on δ-alumina supported La, Mn, Co and Fe oxides

δ-Al2O3 supported La, Mn, Co and Fe containing catalysts were prepared by impregnation of δ-Al2O3 with citrate-type precursors and calcination at 1073 K. The catalysts were characterized by X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), X-ray absorption spectroscopy (XAS), and BET specific surface area determination. XRD revealed the presence of δ-Al2O3 in all cases and, at 30 wt.% of metal loading, of other single (α-Mn2O3 and α-Fe2O3) or mixed (LaAlO3, LaMnO3, CoAl2O4 and LaFeO3) oxide phases. XAS suggested the formation of some oxide phases also at lower loading. In particular, all Mn and 10 wt.% La–Mn containing samples revealed the formation of α-Mn2O3, while the 30 wt.% La–Mn bimetallic sample showed the formation of LaMnO3 perovskite. All Co containing samples revealed the presence of CoAl2O4 spinel. Fe containing samples showed the formation of α-Fe2O3, while La–Fe containing ones, that of LaFeO3 perovskite. Catalytic tests of CO oxidation were performed in the temperature range 300–800 K. The sample containing 30 wt.% of La and Mn in the form of LaMnO3 perovskite dispersed on δ-Al2O3 was found the most active among all the examined catalysts. Most of the Co containing catalysts were found active at RT too, but they deactivated rapidly. None of the Fe-based samples was active at RT and these catalysts were found, on average, to be substantially less active than the Mn- and Co-based ones.

Iron oxidation under the influence of phosphate thin films

Iron phosphate thin films have been obtained under controlled conditions by chemical conversion deposition. The films were formed on polycrystalline α-iron by immersion in acetone, as an organic solvent. After deposition, the films were investigated by x-ray photoelectron spectroscopy (XPS) and their influence on the thermal oxidation of α-iron was studied by means of in situ low incidence x-ray diffraction of synchrotron radiation. The study reveals interesting features related to the structure of both the phosphate and thermal oxide films. The XPS data suggest the iron phosphate to be constituted of long chains of phosphate groups PO43−; these groups being interconnected by Fe2+ and Fe3+ cations. X-ray diffraction measurements have shown a significant modification of the oxidation behavior of α-iron at 400 °C and atmospheric pressure, which is derived from the presence of the thin film: α-Fe2O3 formation is clearly enhanced to the detriment of Fe3O4, compared to the oxidation of pure iron in which Fe3O4 is the dominant phase. As the growth rate of the α-Fe2O3 layer is significantly reduced compared to that of Fe3O4, thus the phosphated layers may find niche applications in low-to-moderate temperature environments.

MAGNETIC TUNNEL JUNCTIONS BASED ON PARTIALLY-OXIDIZED-IRON ELECTRODES

Partially-oxidized-iron(Fe-O)-based magnetic tunnel junctions with large magnetoresistance have been fabricated. The compositions, and the structural and transport properties of the two Fe-O electrodes are characterized. The top Fe-O layer containing 25%–28% oxygen shows a metallic characteristic with fine grain structure. No crystalline iron oxides are detected in the layer. The bottom Fe-O layer whose surface was oxidized during the oxidation process of the Al layer reveals the formation of α-Fe2O3 at the surface. The α-Fe2O3(104) is likely to be epitaxially grown on the Fe(110) plane. The layer shows a nonmetallic transport feature with negative temperature coefficient of resistance.

Infrared Spectroscopy Analysis of Oxyhydroxides as Intermediate Species in the Formation of Iron Oxides-Silica Xerogels

A detailed infrared spectroscopy analysis in the 2500 to 3800 cm−1 region has been used to study the formation of species in samples of iron oxides embedded in silica xerogel matrix. We report the presence of α, β, and γ forms of iron oxyhydroxides as intermediate species in the formation of α-Fe2O3, γ-Fe2O3 and e-Fe2O3 starting from three different iron precursors: iron nitrate, iron chloride and nanometric Fe particles prepared by chemical reduction. Our results show that under thermal dehydration α and γ forms of iron oxyhydroxides transform into hematite and maghemite, respectively, whereas the β form transform to the e-Fe2O3 without going through an intermediate iron oxide phase.

Nanocrystalline Maghemite (γ‐Fe2O3) in Silica by Mechanical Activation of Precursors

γ‐Fe2O3 nanocrystallites dispersed in an amorphous silica matrix have been successfully prepared for the first time by mechanical activation of a chemistry‐derived precursor at room temperature. The initial 10 h of mechanical activation triggered the formation of nanocrystallites of Fe3O4 in a highly activated matrix. Increasing the mechanical‐activation time led to a phase transformation from Fe3O4 to γ‐Fe2O3. The γ‐Fe2O3 phase was well established after mechanical activation of the precursor for 30 h. Further increasing the mechanical‐activation time to 40 h induced the formation of α‐Fe2O3. The mechanical‐activation‐grown γ‐Fe2O3 nanocrystallites were ∼10–12 nm in size and well dispersed in the silica matrix, as observed using TEM. They demonstrated superparamagnetic behavior at room temperature when measured using a Mössbauer spectrometer and a vibrating sample magnetometer (VSM). In addition, the γ‐Fe2O3 derived from 30 h of mechanical activation exhibited a value of saturation magnetization as high as 62.6 emu/g.

Hydrothermal formation of Ni-Zn ferrite from heavy metal co-precipitates

The hydrothermal synthesis of Ni-Zn ferrite from simulated wastewater containing Ni2+ and Zn2+ ions has been studied. The influence of co-precipitation order, the existence of Na+ in suspension, the hydrothermal reaction time and temperature on the composition, morphology and saturation magnetization (σs) of the hydrothermal products is reported. Adding the simulated wastewater to the NaOH solution can prevent the formation of α-Fe2O3 in the Ni-Zn ferrite. Increasing the hydrothermal reaction time improved the magnetization of the Ni-Zn ferrite, while the influence of temperature, stirring intensity and Na+ in suspension on the hydrothermal formation of ferrite were not obvious. Thermodynamic calculation indicated that under hydrothermal conditions (180–240°C), the order of chemical stability is as follows: NiFe2O4 > Fe2O3 > Na2Fe2O4. The high Gibbs formation energy of Na2Fe2O4 prevented the incorporation of Na+ into the ferrite lattice.

Physico-chemical studies on the thermolysis of strontium and barium ferrimaleates

The thermolysis of strontium and barium tris(maleato)ferrates(III), M3 [Fe(C2 H2 C2 O4 )3 ]2 ·x H2 O has been investigated from ambient temperature to 800 °C using simultaneous TG-DTG-DTA, XRD, Mossbauer and IR spectroscopic techniques. After dehydration the anhydrous complexes undergo decomposition to yield an iron(II)maleate/oxalate intermediate in the temperature range of 240-280 °C. An oxidative decomposition of iron(II) species leads to the formation of α-Fe2 O3 and respective alkaline earth metal carbonate in the successive stages. Finally at 540-590 °C, a solid state reaction occurs between α-Fe2 O3 and strontium/barium carbonate resulting in the formation of SrFeO2.5 and BaFe2 O4 , respectively.

Calcium ferrite formation from the thermolysis of calcium tris (maleato) ferrate(III)

For preparing calcium ferrite, calcium tris (dmaleato) ferrate(III) precursor was prepared by mixing aqueous solutions of iron(III) maleate, calcium maleate and maleic acid. Various physico-chemical techniques i.e. TG, DTG, DTA, Mössbauer, XRD, IR etc have been used to study the decomposition behaviour from ambient to 900°C and ferrite formation. Three consecutive decomposition steps leading to the formation of α-Fe2O3 and calcium carbonate have been observed at various stages of thermolysis. In the final stage the ferrite, bdCa2Fe2O5, is obtained as a result of solid state reaction between α-Fe2O3 and calcium carbonate at 788°C, a temperature much lower than for ceramic method. The results have been compared with those of the oxalate precursor.

Synthesis and reactivity of Fe nanoparticles embedded in a semi-insulating matrix

The technique of plasma enhanced chemical vapor deposition has been used to synthesize Fe nanoparticles in an amorphous boron carbide matrix. The nanoparticles range in size from approximately 0.7 to 4.5 nm. It has been demonstrated that the size of the nanoparticles is proportional to the density of the Fe precursor (ferrocene) in the vapor. The matrix films have been found to be extremely hygroscopic. Upon absorption of moisture the films buckle and delaminate from the Si substrate. The buckling is a consequence of induced strain arising from the transformation of the Fe nanoparticles into α-Fe2O3. The formation of α-Fe2O3 has been verified by electron diffraction and near edge x-ray absorption fine structure measurements. Examination of the magnetic properties of the α-Fe2O3 using the technique of magnetic circular dichroism indicate that the nanoparticles are not ferromagnetic.

Characterization of the Calcination Products of the Precipitates Obtained from the Bio-Oxidation with Thiobacillus Ferrooxidans of Sulphuric Water Pickling Liquors

The characterization of the calcination products of the precipitates obtained from the bio-oxidation with Thiobacillus ferrooxidans of sulphuric water pickling liquors has been carried out by means of Mossbauer spectroscopy, x-ray powder diffraction, infrared spectroscopy and transmission electron microscopy. The results show that a full transformation of the precipitates into α-Fe2O3 is achieved at temperatures higher than 850°C. Calcination at 700°C during two hours results in the formation of α-Fe2O3, ζ-Fe2O3 and Fe12O3(SO4)15. The Mossbauer parameters of ζ-Fe2O3 and Fe12O3(SO4)15 at 298 and 17K are reported.