共查询到20条相似文献,搜索用时 31 毫秒
1.
Julian Morales José L. Tirado Concepción Valera 《Journal of the American Ceramic Society》1989,72(7):1244-1246
Preferential X-ray line broadenings in γ-Fe2 o3 samples prepared from γ-FeOOH, α-FeOOH, N2 H5 Fe(N2 H3 -COO)3 -H2 O, FeOOCH3 , and colloidal Fe3 O4 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 Fe3 O4 . 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-Fe2 03 →α-Fe2 O3 . 相似文献
2.
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
3.
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
4.
Shang-Di Mo Yong-Nian Xu Wai-Yim Ching 《Journal of the American Ceramic Society》1997,80(5):1193-1197
γ-Al2 O3 is a defective spinel phase of alumina with cation site vacancies randomly distributed. Its structure and properties are not well understood. There has been long-standing controversy as to whether the cation vacancies are located at the tetrahedral sites or the octahedral sites. Based on an empirical pair potential calculation and first-principles electronic structure studies, we have concluded that cation vacancies are preferentially located at the octahedral sites in bulk γ-Al2 O3 . Our calculation shows that the electronic structure of γ-Al2 O3 differs from that of α-Al2 O3 in fine details. γ-Al2 O3 has a smaller band gap and wider valence bandwidths. The calculated density of states (DOS) of γ-Al2 O3 is in good agreement with recent experimental XPS and XES data. Site- and orbital-resolved partial DOS (PDOS) of Al atoms shows significant dependence on the local coordinations. The PDOS of an oxygen adjacent to a vacancy differs substantially from that of a fully coordinated anion. 相似文献
5.
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
6.
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
7.
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
8.
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
9.
Jafar F. Al-Sharab Frederic Cosandey Amit Singhal Ganesh Skandan James Bentley 《Journal of the American Ceramic Society》2006,89(7):2279-2285
Nanostructured MgAl2 O4 spinel was synthesized by a direct conversion process from cubic γ-Al2 O3 . The effect of post-annealing temperature (300°, 500°, and 800°C) on MgAl2 O4 phase formation was investigated using transmission electron microscopy, selected area electron diffraction (SAED), electron energy loss spectroscopy (EELS), and energy-dispersive spectroscopy (EDS). Relative diffraction intensities as well as lattice parameter measurements from SAED revealed that MgAl2 O4 spinel structure starts forming at temperatures as low as 300°C. EELS and EDS spectrum images also revealed an increase in elemental homogeneity with increasing annealing temperature. The degree of ordering of Mg and Al between octahedral and tetrahedral sites has been determined from relative diffraction intensities. Results show that annealing to 800°C leads to a spinel phase with an order parameter of 0.78. 相似文献
10.
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
11.
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
12.
Kiyoshi Okada Akiyoshi Hattori Yoshikazu Kameshima Atsuo Yasumori Rathindra Nath Das 《Journal of the American Ceramic Society》2000,83(5):1233-1236
The effect of monovalent cation addition on the γ-Al2 O3 -to-α-Al2 O3 phase transition was investigated by differential thermal analysis, powder X-ray diffractometry, and specific-surface-area measurements. The cations Li+ , Na+ , Ag+ , K+ , Rb+ , and Cs+ were added by an impregnation method, using the appropriate nitrate solution. β-Al2 O3 was the crystalline aluminate phase that formed by reaction between these additives and Al2 O3 in the vicinity of the γ-to-α-Al2 O3 transition temperature, with the exception of Li+ . The transition temperature increased as the ionic radii of the additive increased. The change in specific surface area of these samples after heat treatment showed a trend similar to that of the phase-transition temperature. Thus, Cs+ was concluded to be the most effective of the present monovalent additives for enhancing the thermal stability of γ-Al2 O3 . Because the order of the phase-transition temperature coincided with that of the formation temperature of β-Al2 O3 in these samples, suppression of ionic diffusion in γ-Al2 O3 by the amorphous phase containing the added cations must have played an important role in retarding the transition to α-Al2 O3 . Larger cations suppressed the diffusion reaction more effectively. 相似文献
13.
Tomohiro Yamakawa Junichi Tatami Toru Wakihara Katsutoshi Komeya Takeshi Meguro Kenneth J. D. MacKenzie Shinichi Takagi Masahiro Yokouchi 《Journal of the American Ceramic Society》2006,89(1):171-175
Aluminum nitride (AlN) powders were synthesized by gas reduction–nitridation of γ-Al2 O3 using NH3 and C3 H8 as the reactant gases. AlN was identified in the products synthesized at 1100°–1400°C for 120 min in the NH3 –C3 H8 gas flow confirming that AlN can be formed by the gas reduction–nitridation of γ-Al2 O3 . The products synthesized at 1100°C for 120 min contained unreacted γ-Al2 O3 . The 27 A1 MAS NMR spectra show that Al–N bonding in the product increases with increasing reaction temperature, the tetrahedral AlO4 resonance decreasing prior to the disappearance of the octahedral AlO6 resonance. This suggests that the tetrahedral AlO4 sites of the γ-Al2 O3 are preferentially nitrided than the AlO6 sites. AlN nanoparticles were directly formed from γ-Al2 O3 at low temperature because of this preferred nitridation of AlO4 sites in the reactant. AlN nanoparticles are formed by gas reduction–nitridation of γ-Al2 O3 not only because the reaction temperature is sufficiently low to restrict grain growth, but also because γ-Al2 O3 contains both AlO4 and AlO6 sites, by contrast with α-Al2 O3 which contains only AlO6 . 相似文献
14.
Ziqi Sun Yanchun Zhou Jingyang Wang Meishuan Li 《Journal of the American Ceramic Society》2007,90(8):2535-2541
In this paper, the mechanical properties of bulk single-phase γ-Y2 Si2 O7 ceramic are reported. γ-Y2 Si2 O7 exhibits low shear modulus, excellent damage tolerance, and thus has a good machinability ready for metal working tools. To understand the underlying mechanism of machinability, drilling test, Hertzian contact test, and density functional theory (DFT) calculation are employed. Hertzian contact test demonstrates that γ-Y2 Si2 O7 is a "quasi-plastic" ceramic and the intrinsically weak interfaces contribute to its machinability. Crystal structure characteristics and DFT calculations of γ-Y2 Si2 O7 suggest that some weakly bonded planes, which involve Y–O bonds that can be easily broken, are the sources of the low shear deformation resistance and good machinability. 相似文献
15.
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
16.
Yoshitoshi Saito Takahiro Takei Shigeo Hayashi Atsuo Yasumori Kiyoshi Okada 《Journal of the American Ceramic Society》1998,81(8):2197-2200
This paper focused on the effects of various phases of SiO2 additives on the γ-Al2 O3 -to-α-Al2 O3 phase transition. In the differential thermal analysis, the exothermic peak temperature that corresponded to the theta-to-α phase transition was elevated by adding amorphous SiO2 , such as fumed silica and silica gel obtained from the hydrolysis of tetraethyl orthosilicate. In contrast, the peak temperature was reduced by adding crystalline SiO2 , such as quartz and cristobalite. Amorphous SiO2 was considered to retard the γ-to-α phase transition by preventing γ-Al2 O3 particles from coming into contact and suppressing heterogeneous nucleation on the γ-Al2 O3 surface. On the other hand, crystalline SiO2 accelerated the α-Al2 O3 transition; thus, this SiO2 may be considered to act as heterogeneous nucleation sites. The structural difference among the various SiO2 additives, especially amorphous and crystalline phases, largely influenced the temperature of γ-Al2 O3 -to-α-Al2 O3 phase transition. 相似文献
17.
Yoshikazu Suzuki Masanobu Awano Naoki Kondo Tatsuki Ohji 《Journal of the American Ceramic Society》1999,82(9):2557-2559
Composites of BaFe12 O19 particles dispersed throughout a 3-mol%-yttria-doped zirconia (3Y-TZP) matrix have been prepared using the pressureless reactive sintering of 3Y-TZP, BaCO3 , and γ-Fe2 O3 powders. The reaction behavior of the mixed powder was studied with an in situ , high-temperature powder X-ray diffraction technique. The composite that was sintered at 1300°C consisted of submicrometer-sized 3Y-TZP grains and BaFe12 O19 particles; the size of the 3Y-TZP grains was ∼100-300 nm, and the size of the BaFe12 O19 particles was ∼200-500 nm. Based on the grain size, most of the BaFe12 O19 grains presumably had a single-magnetic-domain structure. The 3Y-TZP/20-wt%-BaFe12 O19 composite showed high magnetization and coercivity values, despite the low concentration of ferromagnetic phase. Preliminary mechanical tests revealed that the composite possessed moderately good mechanical properties. 相似文献
18.
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
19.
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
20.
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