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1.
Defect structure of fine-grained γ-Fe2 O3 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-Fe2 O3 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. 相似文献
2.
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
3.
Continuous α-Fe2 O3 films grown on bulk (0001)Al2 O2 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 Fe2 O3 /(0001)Al2 O3 interface structure. 相似文献
4.
Takeshi Takamori 《Journal of the American Ceramic Society》1990,73(1):158-160
The Bi2 O3 -rich side of the system Bi2 O3 -SiO2 was studied with powder X-ray diffraction and differential thermal analysis. In the composition 6Bi2 O3 . 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. 相似文献
5.
γ-Fe2 O3 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 Fe3 O4 in a highly activated matrix. Increasing the mechanical-activation time led to a phase transformation from Fe3 O4 to γ-Fe2 O3 . The γ-Fe2 O3 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-Fe2 O3 . The mechanical-activation-grown γ-Fe2 O3 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-Fe2 O3 derived from 30 h of mechanical activation exhibited a value of saturation magnetization as high as 62.6 emu/g. 相似文献
6.
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
7.
L. Fu D. Lynn Johnson Jian G. Zheng Vinayak P. Dravid 《Journal of the American Ceramic Society》2003,86(9):1635-1637
Nanostructured Al2 O3 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-Al2 O3 , with a small amount of δ-Al2 O3 . 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-Al2 O3 appears in the final particles. Electron microscopy studies find that a particle may contain more than one phase. 相似文献
8.
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
9.
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
10.
Dai Guiqin Lin Wei Zheng An Huang Qingzhen Liang Jingkui 《Journal of the American Ceramic Society》1990,73(8):2526-2527
Thermal expansion of the low-temperature form of BaB2 O4 (β-BaB2 O4 ) 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. 相似文献
11.
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
12.
Oxygen-diffusion coefficients were determined in single crystals of MgO and α-Fe2 O3 by exchanging the samples with 18 O enriched gas at 1 atm and measuring 18 O profiles using a proton activation technique. For MgO, in the temperature range 1580 to 1820 K, the diffusion coefficient is represented by: 相似文献
13.
Oxygen diffusion coefficients have been determined for polycrystalline samples of NiCr2 O4 and α-Fe2 O3 by exchange measurements with oxygen gas containing the stable isotope18 O, using mass spectrometer analysis. Oxygen diffusion in NiCr2 O4 can be represented by the equation D = 0.017 exp (-65,400/RT); oxygen diffusion in α-Fe2 O3 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. 相似文献
14.
C. RICHARD A. CATLOW JOHN CORISH JOHN HENNESSY WILLIAM C. MACKRODT 《Journal of the American Ceramic Society》1988,71(1):42-49
Computer-modelling techniques are applied to the calculation of defect formation and migration energies in α-Fe2 O3 and α-Cr2 O3 : 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. 相似文献
15.
Koichi Watanabe 《Journal of the American Ceramic Society》1998,81(3):733-737
The reaction kinetics of an equimolar mixture of BaO2 and alpha-Fe2 O3 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 BaFeO3- x ( x = 0.02-0.08) was 82 ± 6 kJmol−1 , and that for the formation of BaFe2 O4 was 82 ± 7 kJmol−1 . 相似文献
16.
Robert G. Hill Christopher Goat David Wood 《Journal of the American Ceramic Society》1992,75(4):778-785
A SiO2 ─Al2 O3 ─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. 相似文献
17.
Lattice parameter-composition relations are shown in the system α-Fe2 O3 -Cr2 O3 . 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. 相似文献
18.
Shun J. Wu Lutgard C. De Jonghe Mohamed N. Rahaman 《Journal of the American Ceramic Society》1996,79(8):2207-2211
The sintering of ultrafine γ-Al2 O3 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-Al2 O3 ) provided a method for significantly increasing the density during sintering. 相似文献
19.
The effect of Cr and Fe in solid solution in γ-Al2 O3 on its rate of conversion to α-Al2 O3 at 1100°C was studied by X-ray diffraction. The δ form of Al2 O3 was the principal intermediate phase produced from both pure γ-Al2 O3 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-Al2 O3 and as Cr3+ in α-Al2 O3 , with θ-Al2 O3 as the intermediate phase. The intermediates formed rapidly, and the rates of their conversion to α-Al2 O3 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-Al2 O3 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-Al2 O3 , within the aggregates of their crystallites, is closely coupled with conversion of cubic to hexagonal close packing of O2- ions by synchro-shear. 相似文献
20.
Rajiv S. Mishra Charles E. Lesher Amiya K. Mukherjee 《Journal of the American Ceramic Society》1996,79(11):2989-2992
High-pressure sintering of nanocrystalline γ-A12 O3 has been studied over a temperature range of 923-1323 K and at a pressure of 1 GPa. The γ-Al2 O3 to α-Al2 O3 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. 相似文献