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1.
Edelio Bermejo Thierry le Mercier Michel Quarton 《Journal of the American Ceramic Society》1995,78(2):365-368
The effects of sintering temperature above 1000°C and of cooling rate on the microstructual development of pure and Zn-substituted spinel lithium ferrites are discussed. The strong dependence of microstructural features on cooling rate can be explained on the basis of the precipitation of α-Fe2 O3 or the formation of a solid solution of ferrite phase with Fe3 O4 occurring during sintering above 1000°C. These two phenomena are studied by detailed characterization analyses: SEM, XRD, magnetization, and ac electrical resistivity measurements. 相似文献
2.
Manganese ferrite and α-Fe2 O3 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-Fe2 O3 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-Fe2 O3 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. 相似文献
3.
The microstructure and humidity-sensitive characteristics of α -Fe2 O3 porous ceramic were investigated. Microporous α -Fe2 O3 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 -Fe2 O3 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. 相似文献
4.
Synthesis of monodispersed nanophase α-Fe2 O3 (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 ·6H2 O 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-Fe2 O3 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-Fe2 O3 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-Fe2 O3 was investigated. 相似文献
5.
Synthesis of Pure and Manganese-, Nickel-, and Zinc-Doped Ferrite Particles in Water-in-Oil Microemulsions 总被引:1,自引:0,他引:1
In recent years, the materials research focuses toward synthesis of finer and finer microstructural features. The unique properties of nanosized particles outweigh their higher production costs. Precipitation in microemulsion is one technique, which promises to produce small particles of controlled size and morphology at reasonable cost. The present study demonstrates the synthesis of nanocrystalline α-Fe2 O3 (hematite), Mn0.5 Zn0.5 Fe2 O4, and Ni0.5 Zn0.5 Fe2 O4 particles in a reverse micellar microemulsion system [water–iso-octane–AOT (sodium di-2ethylhexylsulfosuccinate)]. The synthesis of α-Fe2 O3 is performed to obtain baseline data for the synthesis of Mn0.5 Zn0.5 Fe2 O4 and Ni0.5 Zn0.5 Fe2 O4 in the microemulsion system. Nanosized, spherical α-Fe2 O3 , Ni-Zn ferrite, and Mn-Zn ferrite particles (20–80 nm) with very narrow particle size distribution are synthesized. Crystallization of the particles is obtained at temperatures as low as 300°C. 相似文献
6.
Subsolidus equilibrium relations in a portion of the system Li2O-Fe2 O3 -Al2 O3 in the temperature range 500° to 1400°C. have been determined near po2 = 0.21. Of particular interest in this system is the LiFe5 O8 -LiAl5 O8 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-Fe2 O3 in LiFe5 O8 . An attempt to confirm HFe5 O8 as the correct formulation of the magnetic ferric oxide "γ-Fe2 O3 " was inconclusive, but in the absence of positive evidence, the retention of γ-Fe2 O3 is recommended. All the metallic oxides of the Group IV elements increase the temperature of the monotropic conversion of -γ-Fe2 O3 to α-Fe2 O3 . Silica and thoria have a greater effect on this conversion than does titania or zirconia. 相似文献
7.
Thermal reactions in 93% Al2 O3 -7% MgO and 95.8% Al2 O3 -4.2% MgO gels seeded with α-Al2 O3 , MgAl2 O4 , α-Fe2 O3 , and SiO2 , sols were investigated by differential thermal analysis to determine the extent of nucleation catalysis of solid-state reactions. Seeding with α-Al2 O3 lowered the α-Al2 O3 crystallization temperature in these xerogels by 100° to 150°C. Spinel seeds have much less effect on the γ-α transition, and α-Fe2 O3 and SiO2 seeds do not affect it significantly. Isostructural seeding of gels may therefore permit lower ceramic processing temperatures. 相似文献
8.
WILLIAM C. HAGEL P. J. JORGENSEN D. S. TOMALIN 《Journal of the American Ceramic Society》1966,49(1):23-26
Since the difference between oxygen-ion and cation diffusion coefficients is greater for α-Cr2 O3 than for α-Fe2 O3 or α-Al2 O3 , a study of initial-sintering kinetics was undertaken to show unequivocally which species is rate controlling. Fine powders of α-Cr2 O3 , obtained by thermal decomposition of reagent-grade (NH4 )2 Cr2 O7 , 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-Cr2 O3 . There is little evidence that oxygen diffusion along grain boundaries becomes so enhanced that chromium ions are left in control of the process. 相似文献
9.
The rate of nickel ferrite formation is influenced by the milling technique and in the case of wet milling by the type of liquid media used. The reactivity of different types of α-Fe2 O3 of spherical shape decreases with an increase of the surface area ratio of α-Fe2 O3 to NiO. The particle shape of the iron oxides also affects their reactivity; cubical α-Fe2 O3 has the least reactivity in the formation of nickel ferrite. The validity of Jander's equation in ferrite formation is reviewed. Based on present findings the rate of ferrite formation is best represented by the equation dx / dt = ( a - x) / t , in which x is the percentage of ferrite formed, t the time, and a the kinetic isothermal reaction rate coefficient which is related to the surface area ratio of the reacting oxides. The application of the foregoing equation to the formation of magnesium ferrites from data found in the literature is also satisfactory. 相似文献
10.
Addition of α-Fe2 O3 seed particles to alkoxide-derived boehmite sols resulted in a 10-fold increase in isothermal rate constants for the transformation of γ- to α-Al2 O3 . 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-Al2 O3 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-Al2 O3 at ∼ 1200°C. 相似文献
11.
Wen-Yu Zhao Ping Wei Hai-Bin Cheng Xin-Feng Tang Qing-Jie Zhang 《Journal of the American Ceramic Society》2007,90(7):2095-2103
Single-phase BaCoTiFe10 O19 (BaCoTi-M) nanoparticles were prepared by a modified sol–gel process, using metallic chlorides as starting materials. The physical chemistry process of BaCoTi-M formation, the interdependences between composition, technological conditions, microstructure, and magnetic properties were studied by X-ray diffraction (XRD), Fourier transform-infrared (FTIR), scanning electron microscope (SEM), transmission electron microscope (TEM), and vibrating sample magnetometer (VSM). XRD and FTIR results show that BaCoTi-M nanoparticles formed directly from γ-Fe2 O3 , spinel ferrite, and barium salts without the formation of α-Fe2 O3 and BaFe2 O4 . The lattice shrinkage of BaCoTi-M nanoparticles that occurred on increasing the calcining temperature from 973 to 1173 K under holding for 2 h or on increasing the holding time in the range 0–2 h at 1173 K was discovered by analyzing the dependences of lattice parameters on the heat-treatment conditions. The shrinkage led to a relatively higher concentration of magnetic Fe3+ cations in the unit cell, and resulted in an increase of specific saturation magnetization under the corresponding conditions. Microstructural characterization shows that the evolutions of coercivity, remnant magnetization, and squareness ratio depended on the crystal growth and the reduction of structural defect as well as a decrease of grain boundary. 相似文献
12.
The oxygen content of Ni0.685 Zn0.177 Fe2.138 O4+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-Fe2 O3 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+ . 相似文献
13.
Ultrafine (<0.1 μm) high-purity θ-Al2 O3 powder containing 3–17.5 mol%α-Al2 O3 seeds was used to investigate the kinetics and microstructural evolution of the θ-Al2 O3 to α-Al2 O3 transformation. The transformation and densification of the powder that occurred in sequence from 960° to 1100°C were characterized by quantitative X-ray diffractometry, dilatometry, mercury intrusion porosimetry, and transmission and scanning electron microscopy. The relative bulk density and the fraction of α phase increased with annealing temperature and holding time, but the crystal size of the α phase remained ∼50 nm in all cases at the transformation stage (≤1020°C). The activation energy and the time exponent of the θ to α transformation were 650 ± 50 kJ/mol and 1.5, respectively. The results implied the transformation occurred at the interface via structure rearrangement caused by the diffusion of oxygen ions in the Al2 O3 lattice. A completely transformed α matrix of uniform porosity was the result of appropriate annealing processes (1020°C for 10 h) that considerably enhanced densification and reduced grain growth in the sintering stage. The Al2 O3 sample sintered at 1490°C for 1 h had a density of 99.4% of the theoretical density and average grain size of 1.67 μm. 相似文献
14.
Fine-particle beta sodium ferrite (β-NaFeO2 ), rather than α-Fe2 O3 , may be responsible for superparamagnetic behavior in a glass of composition (in mole fractions) 0.37Na2 O-0.26Fe2 O3 -0.37SiO2 . The 700°C isothermal section of the phase diagram of the Na2 O-Fe2 O3 -SiO2 system is given, showing a three-phase field bounded by Na2 SiO3 -NaFeO2 -Fe2 O3 ; there is no evidence for the existence (at 700°C) of compounds of molar composition 6Na2 O-4Fe2 O3 -5SiO2 or 2Na2 O-Fe2 O3 -SiO2 . The Moessbauer spectrum of β-NaFeO2 has an internal magnetic field of 487 kOe at room temperature. 相似文献
15.
To produce a new red pigment for Japanese porcelains, some hematite (α-Fe2 O3 ) 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-Fe2 O3 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-Fe2 O3 powders were found to have a significant effect on the tone of red color for porcelain pigment. 相似文献
16.
Yutaka Tamaura Tatsuya Kodama Tomoyuki Itoh 《Journal of the American Ceramic Society》1990,73(8):2539-2542
High-vacancy-content magnetites with particle size in the range of 20 to 150 nm can be precipitated as stable products in the clear and strongly alkaline solution of iron(III) tartrate and dextrose at 100°C, although Fe(III) ions in alkaline solution have been reported previously to form only α-FeO(OH) and α-Fe2 O3 . When Zn(II) ions were present in the reaction solutions, high-vacancy-content Zn(II)-bearing ferrites represented by (ZnFe2 O4 )x -(Fe3 O4 )y -(γ-Fe2 O3 )z , where x + y + z = 1, were obtained in the strongly alkaline solutions. The Fe(II) ions, which were formed by the reduction of Fe(III) ions with dextrose, accelerate the crystallization of the spinel structure. 相似文献
17.
The phase relations in the region of the Fe2 O3 - and Al2 O3 -rich sides of the quaternary system SrO─Fe2 O3 –Al2 O3 –B2 O3 and the location of conjugation lines between magnetoplumbite solid solution SrO·(6 − x)Fe2 O3 ·xAl2 O3 and corundum (α-Fe2 O3 , α-Al2 O3 ) phases were determined at 1100°C in air by using the flux-growth method based on the Ostwald ripening mechanism. Activity–composition relations and the lattice parameters along the magnetoplumbite solid solutions were also obtained. 相似文献
18.
S. K. Apte S. D. Naik R. S. Sonawane B. B. Kale J. O. Baeg 《Journal of the American Ceramic Society》2007,90(2):412-414
Highly dispersed nanometer-sized α-Fe2 O3 (hematite) and γ-Fe2 O3 (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-Fe2 O3 . 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 H2 S for hydrogen generation was performed using α- and γ-Fe2 O3 . Good photocatalytic activity was obtained using α- and γ-Fe2 O3 as photocatalysts under visible light irradiation. 相似文献
19.
SHIGERU ITO NOBUO KUBO SHINYA NARIKI NOBORU YONEDA 《Journal of the American Ceramic Society》1987,70(12):874-879
The potassium ions in potassium β-ferrite ((1 + x)K2 O ·11Fe2 O3 ) crystals were exchanged with Na+ , Rb+ , Cs+ , Ag+ , NH4 + , and H3 O+ in molten nitrates or in concentrated H2 SO4 . 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-Fe2 O3 at high temperatures of 800° to 1100°C. In addition, H3 O+ , NH4 + , and Ag+ β-ferrites decomposed to form α-Fe2 O3 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. 相似文献
20.
S. C. Zanatta L. F. Cótica A. Paesano Jr. S. N. de Medeiros J. B. M. da Cunha B. Hallouche 《Journal of the American Ceramic Society》2005,88(12):3316-3321
We investigated the mechanosynthesis of gadolinium iron garnet by high-energy ball milling stoichiometry amounts of α-Fe2 O3 and Gd2 O3 , followed by short thermal annealings conducted at moderate temperatures. X-ray diffraction and Mössbauer spectroscopy results revealed, for the as-milled samples, the formation of the GdFeO3 perovskite phase, in relative amounts that depend on the milling time. The formation of the Gd3 Fe5 O12 garnet phase was observed in 1000°C/2 h or 1100°C/3 h as-annealed samples. The occurrence of a milling time was verified in which the relative amount of garnet phase formed by further annealing is maximized. 相似文献