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.     

Preferential X-ray line Broadening and Thermal Behavior of γ-Fe2O3

Preferential X-ray line broadenings in γ-Fe₂o₃ samples prepared from γ-FeOOH, α-FeOOH, N₂H₅Fe(N₂H₃-COO)₃-H₂O, FeOOCH₃, and colloidal Fe₃O₄ are compared. Isotropic size and small crystallites are the origin of the uniform and enhanced X-ray line broadening in samples derived from hydrazinate and colloidal Fe₃O₄. Nonuniform line broadening in ex-α-FeOOH and ex-γ-FeOOH is due to an elongated crystallite shape and the presence of stacking faults, respectively. The thermal behavior of samples with low crystallite size and uniform line broadening is characterized by an exothermal recrystallization process simultaneous to the phase transformation γ-Fe₂0₃→α-Fe₂O₃.     

Superparamagnetic Behavior of MnFe2O4 and α-Fe2O3 Precipitated from Silicate Melts

Manganese ferrite and α-Fe₂O₃ particles were precipitated within silicate melt systems to produce very unusual magnetic properties. Assemblies of particles of both kinds behaved super-paramagnetically when the particle size was small enough. As the particle size was increased, the magnetic properties of the ferrite system increased, but those of the α-Fe₂O₃ system decreased; the latter is expected from Néel's theory of a net spontaneous magnetic moment created by uncompensated magnetic sublattices at very small particle sizes. Liquid-in-liquid phase separation was pronounced in the manganese ferrite-glass systems, which may have influenced the precipitation behavior. Room-temperature initial mass susceptibilities were as high as 2 × 10 ⁻² cgs, and specific magnetizations as high as 26 gauss/g were observed. Precipitation of α-Fe₂O₃ particles exhibiting super-paramagnetic behavior was possible only with very low-viscosity melts. Initial mass susceptibility values changed by as much as a factor of 30 between 296° and 77°K.     

Studies in the System Li2O–Al2O3–Fe2O3-H2O

Subsolidus equilibrium relations in a portion of the system Li2O-Fe₂O₃-Al₂O3 in the temperature range 500° to 1400°C. have been determined near po₂ = 0.21. Of particular interest in this system is the LiFe₅O₈-LiAl₅O₈ join, which shows complete solid solution above 1180°C. Below this temperature the solid solution exsolves into two spinel phases. At 600°C. approximately 15 mole % of each compound is soluble in the other. The high-temperature solid solution and the low-temperature exsolution dome extend into the ternary system from the 1:5 join. There is no appreciable crystalline solubility of LiFeO₂ or of α-Fe₂O₃ in LiFe₅O₈. An attempt to confirm HFe₅O₈ as the correct formulation of the magnetic ferric oxide "γ-Fe₂O₃" was inconclusive, but in the absence of positive evidence, the retention of γ-Fe₂O₃ is recommended. All the metallic oxides of the Group IV elements increase the temperature of the monotropic conversion of -γ-Fe₂O₃ to α-Fe₂O₃. Silica and thoria have a greater effect on this conversion than does titania or zirconia.     

Microwave-Hydrothermal Synthesis of Monodispersed Nanophase α-Fe2O3

Synthesis of monodispersed nanophase α-Fe₂O₃ (hematite) powder to be used as a red pigment in porcelains was investigated using microwave-hydrothermal and conventional-hydrothermal reactions using 0.018 M FeCl₃·6H₂O and 0.01 M HCl solutions at 100°–160°C. Acicular and yellow β-FeOOH (akaganite) particles 300 nm in length and 40 nm in thickness were dominantly formed at 100°C after 2–3 h, while spherical α-Fe₂O₃ particles 100–180 nm in diameter were preferentially formed after 13 h using a conventional-hydrothermal reaction. However, a microwave-hydrothermal reaction at 100°C led to monodispersed and red α-Fe₂O₃ particles 30–66 nm in diameter after 2 h without the formation of β-FeOOH particles. In this paper, the effect of microwave radiation during hydrothermal treatment at 100°–160°C on the formation yield, kinetics, morphology, phase type, and color of α-Fe₂O₃ was investigated.     

Phase Diagram of a Nickel-Zinc Ferrite of Composition Ni0.685Zn0.177Fe2.138O4+γ

The oxygen content of Ni_0.685Zn_0.177Fe_2.138O_4+γ was determined gravimetrically at atmospheric pressure in varying Po₂ , 3.5 × 10⁻⁴ to 1.0 atm at 600° to 1450°C. The phase boundary associated with the precipitation of α-Fe₂O₃ was determined from the change in slope of γ vs T plots observed on heating. Metastability is particularly evident for curves observed on cooling. Isacompositional lines (0.002 < γ < 0.045) are shown on a plot of log PO₂ vs 1/T. An enthalpy of -21.6 kcal/mol is calculated for the oxidation of Fe²⁺.     

Transformation, Microstructure Development, and Densification in α-Fe2O3-Seeded Boehmite-Derived Alumina

Addition of α-Fe₂O₃ seed particles to alkoxide-derived boehmite sols resulted in a 10-fold increase in isothermal rate constants for the transformation of γ- to α-Al₂O₃. Changes in porosity and surface area with sintering temperature showed no effect of seeding on coarsening of the transition alumina gels, but the 200-fold decrease in surface area associated with transformation to α-Al₂O₃ occurred ∼ 100°C lower in seeded gels compared with unseeded materials. As a result of high nucleation frequency and reduced microstructure coarsening, fully transformed seeded alumina retained specific surface areas >22 m²/g and exhibited narrow pore size distributions, permitting development of fully dense, submicrometer α-Al₂O₃ at ∼ 1200°C.     

Anomalous X-ray Line Broadening Observed in γ-Fe2O3 Derived from γ-FeOOH

Defect structure of fine-grained γ-Fe₂O₃ powder obtained by dehydroxylation of y-FeOOH powder has been studied by X-ray diffraction and high-resolution electron micrographs. An anomalous X-ray line broadening was observed in the γ-Fe₂O₃ powder. It is found that the X-ray line breadth is dependent on the crystal structure factors of each reflection line: the reflections from net planes consisting of only metal ions on tetrahedral sites and octahedral sites of spinel structure are much broader than those from net planes including oxygen ions. The anomalous X-ray line broadening is explained on the basis of stacking faults in the spinel structure.     

Structural and Chemical Alterations of Delta Fe2O3 and Their Effect on Magnetic Properties

The defect structure, δ-Fe₂O₃, was grown epitaxially with ultrafine, activated Al₂Si₂O₅(OH)₄. More uniform particle size and consistent structure result with this process than with auto-nucleation. The low ferrimagnetic character is influenced by precipitation parameters. Magnetic properties and crystal order can be promoted by a novel thermal conversion process using a high-boiling aliphatic hydrocarbon. The process induces the formation of a more complex iron oxide. A structure is believed to form of an order higher than δ-Fe₂O₃ but lower than γ-Fe₂O₃. Analyses (C, H, CO₂, and H₂O) are correlated with magnetic properties. Specimens having a mean particle size of 20 mμ were produced with coercivities up to 290 oe and remanent inductions up to 1800 gauss/cm³. Stability tests over a five-year period showed no degradation.     

Oxygen Mobility in Polycrystalline NiCr2O4 and α-Fe2O3

Oxygen diffusion coefficients have been determined for polycrystalline samples of NiCr₂O₄ and α-Fe₂O₃ by exchange measurements with oxygen gas containing the stable isotope¹⁸O, using mass spectrometer analysis. Oxygen diffusion in NiCr₂O₄ can be represented by the equation D = 0.017 exp (-65,400/RT); oxygen diffusion in α-Fe₂O₃ can be represented by the equation D = 1 × 10¹¹ exp (-146,000/RT). The large difference between D₀ and activation energy for these materials suggests that different diffusion mechanisms are involved.     

Role of α-Fe2O3 Morphology on the Color of Red Pigment for Porcelain

To produce a new red pigment for Japanese porcelains, some hematite (α-Fe₂O₃) powders produced by different methods were investigated by mixing them with lead-free frit powders and firing them on white porcelain plates at 800°C. Commercial hematite powders and uniform α-Fe₂O₃ powders 155 and 53 nm in diameter which were prepared using conventional- and microwave-hydrothermal reactions, respectively, were used as sources of red pigments. The morphology and dispersion of the above α-Fe₂O₃ powders were found to have a significant effect on the tone of red color for porcelain pigment.     

Microwave Plasma Synthesis of Nanostructured γ-Al2O3 Powders

Nanostructured Al₂O₃ powders have been synthesized by combustion of aluminum powder in a microwave oxygen plasma, and characterized by X-ray diffraction and electron microscopy. The main phase is γ-Al₂O₃, with a small amount of δ-Al₂O₃. The particles are truncated octahedral in shape, with mean particle sizes of 21–24 nm. The effect of reaction chamber pressure on the phase composition and the particle size was studied. The γ-alumina content increases and the mean particle size decreases with decreasing pressure. No α-Al₂O₃ appears in the final particles. Electron microscopy studies find that a particle may contain more than one phase.     

Growth of α-Al2O3 Within a Transition Alumina Matrix

The growth of α-Al₂O₃ from a planar specimen of thermally grown γ-alumina on a molybdenum transmission electron microscope grid was studied. The α-Al₂O₃ grows into the transition alumina matrix and then thickens via a ledge growth mechanism. Faceted Mo crystallites cause pinning of α-Al₂O₃ ledges and are larger on α-Al₂O₃ than on the transition alumina matrix.     

DC Electrical Resistivity and Magnetic Property of Single-Phase α-Fe2O3 Nanopowder Synthesized by a Simple Chemical Method

Nanocrystalline pure α-Fe₂O₃ powder, with an average particle size of 35 nm, has been synthesized by using an aqueous solution-based synthetic route. DC electrical resistivity of the synthesized material was measured with respect to temperature by the two-probe method from 28° to 225°C. Room temperature resistivity of the nanopowder was ∼10⁸Ω·cm. Magnetic hysteresis measurement revealed that the synthesized α-Fe₂O₃ nanopowder exhibited ferromagnetic behavior at room temperature. The hysteretic features are high saturation magnetization of 5.1 emu/g, high remanence of 2.2 emu/g, and coercivity of 200.5 Oe.     

Lowering Crystallization Temperatures by Seeding in Structurally Diphasic Al2O3-MgO Xerogels

Thermal reactions in 93% Al₂O₃-7% MgO and 95.8% Al₂O₃-4.2% MgO gels seeded with α-Al₂O₃, MgAl₂O₄, α-Fe₂O₃, and SiO₂, sols were investigated by differential thermal analysis to determine the extent of nucleation catalysis of solid-state reactions. Seeding with α-Al₂O₃ lowered the α-Al₂O₃ crystallization temperature in these xerogels by 100° to 150°C. Spinel seeds have much less effect on the γ-α transition, and α-Fe₂O₃ and SiO₂ seeds do not affect it significantly. Isostructural seeding of gels may therefore permit lower ceramic processing temperatures.     

Microstructure and Humidity-Sensitive Characteristics of α-Fe2O3 Ceramic Sensor

The microstructure and humidity-sensitive characteristics of α -Fe₂O₃ porous ceramic were investigated. Microporous α -Fe₂O₃ powders were obtained by controlling the topotactic decomposition reaction of α -FeOOH. Water vapor adsorption thermogravimetrical experiments were carried out in the relative humidity (rh) range 0% to 95% on the α -Fe₂O₃ powder and the 900°C sintered compact. The microstructure was investigated by SEM, TEM, Hg intrusion, and N₂ adsorption porosimetry techniques. The humidity sensitivity was investigated by the impedance measurements technique in 0% to 95% rh on the compacts sintered at 50°C steps in the 850° to 1100°C range. Humidity response was found to be affected by the microstructure, i.e., the characteristics of the precursor powders and sintering temperatures.     

Influence of Cr and Fe on Formation of α-Al2O3 from γ-Al2O3

The effect of Cr and Fe in solid solution in γ-Al₂O₃ on its rate of conversion to α-Al₂O₃ at 1100°C was studied by X-ray diffraction. The δ form of Al₂O₃ was the principal intermediate phase produced from both pure γ-Al₂O₃ and that containing Fe³⁺ in solid solution, although addition of Fe greatly reduced crystallinity. Reflectance spectra and magnetic susceptibilities showed that Cr exists as Cr⁶⁺ in γ-Al₂O₃ and as Cr³⁺ in α-Al₂O₃, with θ-Al₂O₃ as the intermediate phase. The intermediates formed rapidly, and the rates of their conversion to α-Al₂O₃ were increased by 2 and 5 wt% additions of Fe and decreased by 2 and 4 wt% additions of Cr. An approximately linear relation observed between α-Al₂O₃ formation and decrease in specific surface area was only slightly affected by the added ions. This relation can be explained by a mechanism in which the sintering of δ- or θ-Al₂O₃, within the aggregates of their crystallites, is closely coupled with conversion of cubic to hexagonal close packing of O^2- ions by synchro-shear.     

Initial Sintering of α-Cr2O3

Since the difference between oxygen-ion and cation diffusion coefficients is greater for α-Cr₂O₃ than for α-Fe₂O₃ or α-Al₂O₃, a study of initial-sintering kinetics was undertaken to show unequivocally which species is rate controlling. Fine powders of α-Cr₂O₃, obtained by thermal decomposition of reagent-grade (NH₄)₂Cr₂O₇, were lightly compacted and their isothermal rates of shrinkage were determined between 1050° and 1300°C. Resultant data follow volume-diffusion sintering models, and calculated diffusion coefficients agree with, those measured for oxygen ions in α-Cr₂O₃. There is little evidence that oxygen diffusion along grain boundaries becomes so enhanced that chromium ions are left in control of the process.     

Atomistic Simulation of Defect Structures and Ion Transport in α-Fe2O3 and α-Cr2O3

Computer-modelling techniques are applied to the calculation of defect formation and migration energies in α-Fe₂O₃ and α-Cr₂O₃: both electronic and lattice defects are considered. The results are used to predict Arrhenius energies for cation and anion migration in different composition and temperature regimes and show reasonable agreement with experimental data where these are available.     

Nano α-Al2O3 Powder Preparation by Calcining an Emulsion Precursor

An experimental study has been conducted to evaluate the formation of nano α-Al₂O₃ under various conditions, such as different calcining temperatures and emulsion ratios of aqueous aluminum nitrate solutions and oleic acid with a high-speed stirring mixer. Four batches of the precursor powders were calcined at three different temperatures of 1000°, 1050°, and 1100°C for 2 h and a terminal product of nano α-Al₂O₃ powders was obtained. The products have been identified by X-ray diffraction (XRD), specific surface area measurement scanning electron microscope, and transmission electron microscope (TEM). The XRD results show that the phase of powders is determined to be α-Al₂O₃, indicating that the overall process has been effective. The optimum calcination temperature of the precursor powder for crystallization of nano α-Al₂O₃ was found to be 1000°C for 2 h. The TEM image indicates that the particle grains have a sub-spherical shape with a mean size of 50–100 nm.