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.     

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.     

Imaging and Diffraction Study of Continuous α-Fe2O3 Films on (0001)Al2O3

Continuous α-Fe₂O₃ films grown on bulk (0001)Al₂O₂ substrates by low-pressure chemical vapor deposition have been studied by transmission electron microscopy and the observations compared to those obtained from discontinuous films at an earlier stage of the growth process. Plan-view specimens revealed significant thermal stress in the continuous films, while cross-sectional specimens showed that cracking occurs in thicker films. The free surface of the film and the film/substrate interface appeared sharp and flat, apart from growth ledges and steps. Weak-beam imaging revealed a hexagonal misfit dislocation network consisting of perfect edge dislocations. Fine structure in the selected-area diffraction patterns which corroborates these observations is also discussed. The misfit network of partial dislocations previously observed in the discontinuous films was not observed for the continuous films, indicating an effect of film thickness, growth rate, or surface preparation on the Fe₂O₃/(0001)Al₂O₃ interface structure.     

The System Bi2O3-SiO2

The Bi₂O₃-rich side of the system Bi₂O₃-SiO₂ was studied with powder X-ray diffraction and differential thermal analysis. In the composition 6Bi₂O₃. x SiO₂, the metastable γ phase (bcc) was observed to exist over the range of 0 < x ≤ 1. In most of the compositions studied, metastable phases of water-quenched melts transformed into another metastable phase before reaching stable phases. A modification of the phase diagram is proposed.     

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.     

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.     

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.     

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.     

Synthesis of Nanosize-Necked Structure α- and γ-Fe2O3 and its Photocatalytic Activity

Highly dispersed nanometer-sized α-Fe₂O₃ (hematite) and γ-Fe₂O₃ (maghemite) iron oxide particles were synthesized by the combustion method. Ferric nitrate was used as a precursor. X-ray diffractometer study revealed the phase purity of α- and γ-Fe₂O₃. Both the products were characterized using field emission scanning electron microscope and transmission electron microscope for particle size and morphology. Necked structure particle morphology was observed for the first time in both the iron oxides. The particle size was observed in the range of 25–55 nm. Photodecomposition of H₂S for hydrogen generation was performed using α- and γ-Fe₂O₃. Good photocatalytic activity was obtained using α- and γ-Fe₂O₃ as photocatalysts under visible light irradiation.     

Thermal Expansion of the Low-Temperature Form of BaB2O4

Thermal expansion of the low-temperature form of BaB₂O₄ (β-BaB₂O₄) crystal has been measured along the principal crystallographic directions over a temperature range of 9° to 874°C by means of high-temperature X-ray powder diffraction. This crystal belongs to the trigonal system and exhibits strongly anisotropic thermal expansions. The expansion along the c axis is from 12.720 to 13.214 Å (1.2720 to 1.3214 nm), whereas it is from 12.531 to 12.578 Å (1.2531 to 1.2578 nm) along the a axis. The expansions are nonlinear. The coefficients A, B , and C in the expansion formula L _t = L ₀(1 + At + Bt ²+ Ct ³) are given as follows: a axis, A = 1.535 × 10⁻⁷, B = 6.047 × 10⁻⁹, C = -1.261 × 10⁻¹²; c axis, A = 3.256 × 10⁻⁵, B = 1.341 × 10⁻⁸, C = -1.954 × 10⁻¹²; and cell volume V, A = 3.107 × 10⁻⁵, B = 3.406 × 10⁻⁸, C = -1.197 × 10⁻¹¹. Based on α _t = (d L _t /d t )/ L ₀, the thermal expansion coefficients are also given as a function of temperature for the crystallographic axes a , c , and cell volume V.     

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.     

Oxygen Diffusion in MgO and α-Fe2O3

Oxygen-diffusion coefficients were determined in single crystals of MgO and α-Fe₂O₃ by exchanging the samples with ¹⁸O enriched gas at 1 atm and measuring ¹⁸O profiles using a proton activation technique. For MgO, in the temperature range 1580 to 1820 K, the diffusion coefficient is represented by:         

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.     

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.     

Kinetics of Solid-State Reaction of BaO2 and α-Fe2O3

The reaction kinetics of an equimolar mixture of BaO₂ and alpha-Fe₂O₃ were investigated in an ambient atmosphere under isothermal conditions. The volume fraction of products was determined using the integrated X-ray diffraction intensities of the most prominent peaks, and the rate constants of the reaction were calculated with the Jander equation. The apparent activation energy for the formation of BaFeO_{3- x} ( x = 0.02-0.08) was 82 ± 6 kJmol⁻¹, and that for the formation of BaFe₂O₄ was 82 ± 7 kJmol⁻¹.     

Thermal Analysis of a SiO2─Al2O3─CaO─CaF2 Glass

A SiO₂─Al₂O₃─CaO─CaF₂ ionomer glass was investigated using thermal analysis, X-ray diffraction, and scanning electron microscopy. The purpose of this investigation was to control the susceptibility of the glass to acid attack. The differential thermal analysis trace exhibited a sharp glass transition at about 645°C and two exotherms. The first exotherm corresponded to liquid–liquid phase separation followed by crystallization of fluorite. The second, much larger, exotherm was the result of crystallization of the remaining glass phase to form anorthite. Prolonged heat treatment below the glass-transition temperature demonstrated that crystallization of fluorite can occur without prior liquid–liquid phase separation.     

Lattice Parameters of the α-Fe2O3-Cr2O3 Solid Solution

Lattice parameter-composition relations are shown in the system α-Fe₂O₃-Cr₂O₃. This information is of some value in determining the composition of scales developed during the oxidation of alloys based on Fe-Cr, such as stainless steels.     

Sintering of Nanophase γ-Al2O3 Powder

The sintering of ultrafine γ-Al₂O₃ powder (particle size ∼10–20 nm) prepared by an inert gas condensation technique was investigated in air at a constant heating rate of 10°C/min. Qualitatively, the kinetics followed those of transition aluminas prepared by other methods. Measurable shrinkage commenced at ∼ 1000°C and showed a region of rapid sintering between ∼1125° and 1175°C followed by a transition to a much reduced sintering rate at higher temperatures. Starting from an initial density of ∼0.60 relative to the theoretical value, the powder compact reached a relative density of 0.82 after sintering to 1350°C. Compared to compacts prepared from the as-received powder, dispersion of the powder in water prior to compaction produced a drastic change in the microstructural evolution and a significant reduction in the densification rate during sintering. The incorporation of a step involving the rapid heating of the loose powder to ∼1300°C prior to compaction (which resulted in the transformation to α-Al₂O₃) provided a method for significantly increasing the density during sintering.     

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.     

High-Pressure Sintering of Nanocrystalline γAl2O3

High-pressure sintering of nanocrystalline γ-A1₂O₃ has been studied over a temperature range of 923-1323 K and at a pressure of 1 GPa. The γ-Al₂O₃ to α-Al₂O₃ transformation temperature changed from 1473 K without pressure to ∼1023 K at 1 GPa. Full density was obtained at 1273 and 1323 K in 10 min. The microhardness value of fully dense α-alumina with a grain size of 142 nm was found to be 25.3 ± 0.8 GPa. The Hall-Petch slope for the very fine grain size range is different from that of the coarse-grained alumina.