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.     

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.     

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.     

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.     

Microstructure and Mechanical Properties of Hot-Pressed α-Al2O3-Seeded γ-Alumina Ceramics

High-quality alumina ceramics were fabricated by a hot pressing with MgO and SiO₂ as additives using α-Al₂O₃-seeded nanocrystalline γ-Al₂O₃ powders as the raw material. Densification behavior, microstructure evolution, and mechanical properties of alumina were investigated from 1250°C to 1450°C. The seeded γ-Al₂O₃ sintered to 98% relative density at 1300°C. Obvious grain growth was observed at 1400°C and plate-like grains formed at 1450°C. For the 1350°C hot-pressed alumina ceramics, the grain boundary regions were generally clean. Spinel and mullite formed in the triple-grain junction regions. The bending strength and fracture toughness were 565 MPa and 4.5 MPa·m^1/2, respectively. For the 1300°C sintered alumina ceramics, the corresponding values were 492 MPa and 4.9 MPa·m^1/2.     

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.     

Preparation of High-Coercivity BaFe12O19

Particles of BaFe₁₂O₁₂ were prepared by chemical coprecipitation; their magnetic properties were studied. A coercive force of 6000 Oe, one of the highest reported for isotropic BaFe₁₂O₁₉, was obtained. X-ray and Moessbauer studies were conducted to examine the mechanism of formation. Superparamagnetic α-Fe₂O₃ was present during synthesis. Ferrites were sintered from these precipitated powders by both the usual method and a hot-press-forging method. The observed magnetic characteristics result from partial orientation of defect-free single-domain coprecipitated powders.     

Zirconium Aluminum Carbides: New Precursors for Synthesizing ZrO2–Al2O3 Composites

ZrO₂–Al₂O₃ nanocrystalline powders have been synthesized by oxidizing ternary Zr₂Al₃C₄ powders. The simultaneous oxidation of Al and Zr in Zr₂Al₃C₄ results in homogeneous mixture of ZrO₂ and Al₂O₃ at nanoscale. Bulk nano- and submicro-composites were prepared by hot-pressing as-oxidized powders at 1100°–1500°C. The composition and microstructure evolution during sintering was investigated by XRD, Raman spectroscopy, SEM, and TEM. The crystallite size of ZrO₂ in the composites increased from 7.5 nm for as-oxidized powders to about 0.5 μm at 1500°C, while the tetragonal polymorph gradually converted to monolithic one with increasing crystallite size. The Al₂O₃ in the composites transformed from an amorphous phase in as oxidized powders to θ phase at 1100°C and α phase at higher temperatures. The hardness of the composite increased from 2.0 GPa at 1100°C to 13.5 GPa at 1400°C due to the increase of density.     

Homogeneous ZrO2–Al2O3 Composite Prepared by Nano-ZrO2 Particle Multilayer-Coated Al2O3 Particles

ZrO₂–Al₂O₃ nanocomposite particles were synthesized by coating nano-ZrO₂ particles on the surface of Al₂O₃ particles via the layer-by-layer (LBL) method. Polyacrylic acid (PAA) adsorption successfully modified the Al₂O₃ surface charge. Multilayer coating was successfully implemented, which was characterized by ξ potential, particle size. X-ray diffraction patterns showed that the content of ZrO₂ in the final powders could be well controlled by the LBL method. The powders coated with three layers of nano-ZrO₂ particles, which contained about 12 wt% ZrO₂, were compacted by dry press and cold isostatically pressed methods. After sintering the compact at 1450°C for 2 h under atmosphere, a sintered body with a low pore microstructure was obtained. Scanning electron microscopy micrographs of the sintered body indicated that ZrO₂ was well dispersed in the Al₂O₃ matrix.     

Solubility of Iron in TiO2 (Rutile)

An upper limit to the solubility of iron in rutile was determined in the range 800° to 1350°C by X-ray examination of mixtures of TiO₂ and α-Fe₂O₃ powders reacted in 1 atm oxygen pressure and quenched to room temperature. The results indicate somewhat lower solubilities than would be inferred from previous measurements; the solubility ranges from 3 cation % iron at 1350°C to 1% at 800°C. There is no detectable change in rutile lattice parameters with iron content up to the limit of solubility.     

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.     

Rapid Formation and Growth of Bixbyite-Type (In0.67Fe0.33)2O3 by 28 GHz Microwave Irradiation

(In_0.67Fe_0.33)₂O₃ with the bixbyite structure was synthesized via 28 GHz microwave irradiation, using multimode microwave heating equipment. Indium sesquioxide strongly absorbs 28 GHz microwaves, and this strong coupling with microwave energy can be used to drive a reaction with iron sesquioxide. A mixture of In₂O₃ and α-Fe₂O₃ powders (In:Fe ratio of 2:1) was irradiated with microwaves at a frequency of 28 GHz. The mixture was heated to 1400°C during the microwave irradiation. The formation of a solid solution was completed within a minute, which indicated a drastic enhancement of the reaction rate. Scanning electron microscopy revealed remarkable grain growth under microwave irradiation.     

Crystallization of Beta NaFeO2 from a Glass Along the Na2SiO3-Fe2O3 Join

Fine-particle beta sodium ferrite (β-NaFeO₂), rather than α-Fe₂O₃, may be responsible for superparamagnetic behavior in a glass of composition (in mole fractions) 0.37Na₂O-0.26Fe₂O₃-0.37SiO₂. The 700°C isothermal section of the phase diagram of the Na₂O-Fe₂O₃-SiO₂ system is given, showing a three-phase field bounded by Na₂SiO₃-NaFeO₂-Fe₂O₃; there is no evidence for the existence (at 700°C) of compounds of molar composition 6Na₂O-4Fe₂O₃-5SiO₂ or 2Na₂O-Fe₂O₃-SiO₂. The Moessbauer spectrum of β-NaFeO₂ has an internal magnetic field of 487 kOe at room temperature.     

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.     

Synthesis and Properties of CaAl2O4-Coated AI2O3 Microcomposite Powders

Novel microcomposite powders, consisting of inert cores (αAL-Al₂O₃) surrounded by reactive cement-based coatings (CaAl₂O₄), were synthesized by a modified Pechini process. The evolution of the crystalline CaAl₂O₄ phase during calcination was studied using multiple analytical techniques, including DRIFTS,¹³C and ²⁷AlMAS FT-NMR, and XRD, for both pure CaAl₂O₄ and CaAl₂O₄-coated Al₂O₃ precursor powders. In both powders, decomposition proceeded via hydrocarbon chain scission and removal of ester groups at low temperatures ( T < 450°C), followed by the formation of inorganic carbonates at higher temperatures ( T > 450°C). These decomposition processes were accelerated by the underlying Al₂O₃ cores. Transmission electron microscopy (TEM) of the fully calcined powders showed that the inert αAL-Al₂O₃ particles were surrounded by relatively uniform CaAl₂O₄ coatings ranging in thickness from approximately 10 to 100 nm.     

Application of Compositionally Diphasic Xerogels for Enhanced Densification: The System Al2O3-SiO2

Stoichiometric mullite (71.38 wt% Al₂O₃-28.17 wt% SiO₂) and 80 wt% Al₂O₃-20 wt% SiO₂ gels were prepared by the single-phase and/or diphasic routes. Dense sintered bodies were prepared from both sets of gels in the Al₂O₃-SiO₂ system. Apparent densities of 96% and 97% of theoretical density were measured for the diphasic (using two sols) mullite samples sintered at 1200° and 1300°C for 100 min, respectively; this compared with 85% and 94% for the single-phase xerogels under the same conditions, and to much lower values for mullite prepared from conventional mixed powders. The microstructure of the mullite pellets from diphasic xerogel precursors is also considerably finer.     

Ion Exchange in Alkali Layers of Potassium β -Ferrite ((1 + x)K2O · 11Fe2O3) Single Crystals

The potassium ions in potassium β-ferrite ((1 + x)K₂O ·11Fe₂O₃) crystals were exchanged with Na⁺, Rb⁺, Cs⁺, Ag⁺, NH₄⁺, and H₃O⁺ in molten nitrates or in concentrated H₂SO₄. On the other hand, spinel and hexagonal ferrites were formed by soaking the crystals in the melt of divalent salts. The crystals of K⁺, Rb⁺, and Cs⁺β-ferrites decomposed to form α-Fe₂O₃ at high temperatures of 800° to 1100°C. In addition, H₃O⁺, NH₄⁺, and Ag⁺β-ferrites decomposed to form α-Fe₂O₃ at relatively low temperatures of 350° to 650°C, in accordance with the stabilities of the inserted ions. The electrical properties of some β-ferrites were measured.     

Synthesis of Transparent Nd-doped HfO2-Y2O3 Ceramics Using HIP

A Nd-doped HfO₂-Y₂O₃ ceramic having excellent transmittance was synthesized by HIPing, using high-purity powders (>99.99 wt%) of Y₂O₃, Nd₂O₃, and HfO₂. The mixed powder compacts of these powders were sintered at 1650°C for 1 h under vacuum and HIPed at 1700°C for 3 h under 196 MPa of Ar. The specimen after HIPing consisted of uniform grains measuring about 30 μm and having pore-free structure. The optical transmittance of 1 at.% Nd-doped 2.6 mol% HfO₂-Y₂O₃ ceramics ranging between visible and infrared wavelength was almost equivalent or superior to that of a Nd:Y₂O₃ single crystal grown by the Verneuil method.     

Preparation and Sintering Behavior of Fine-Grained A12O3-SiO2 Composite

A fine, uniform A1₂O₃-SiO₂ powder was prepared by heterocoagulation of narrow Al₂O₃ and SiO₂ powders. This composite powder was dispersed, compacted, and fired in air at 900° to 1580°C for 1 to 13 h. Full density was achieved at 1550°C with the formation of a mullite phase. Relative densities of 83% and 98% (0.3 μm grain size) were measured for samples sintered at 1200°C for 13 h and at 1400°C for 1 h, respectively.