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1.
Above 755°C, compounds along the spinel join LiFe5 O8 -Li4 Ti5 O12 form a complete solid solution and below that temperature a two-phase region separates the ordered LiFe5 O8 and the disordered spinel phase. At 800° and 900°C, cubic LiFeO2 ( ss ) and monoclinic Liz Ti03 ( ss ) exist on the monoxide join LiFeO2 -Li2 TiO3 . The distributions of cations in both the spinel and monoxide structures were calculated as a function of equilibrium temperature and composition. Sub-solidus equilibria in the system Li2 O-Fe2 O3 -TiO2 at 800° and 900°C were determined for compositions containing ∼50 mol% Li2 O. 相似文献
2.
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
3.
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
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.
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
6.
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
7.
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
8.
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
9.
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
10.
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
11.
Prita Pant Sarangi Sampat Raj Vadera Manoj Kumar Patra Chandra Prakash Narendra Nath Ghosh 《Journal of the American Ceramic Society》2009,92(10):2425-2428
Nanocrystalline pure α-Fe2 O3 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 ∼108 Ω·cm. Magnetic hysteresis measurement revealed that the synthesized α-Fe2 O3 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. 相似文献
12.
On the join LiCrO2 -LiAlO2 , solid solutions of formulas LiCri- x Al x O2 (0< x <0.6) form. On the spinel join LiAl5 O8 -"LiCr5 O8 ," solid solutions of formulas LiAl5- x Al x O8 (0< x <3) form and the order-disorder transition, observed in LiAl5 O8 at 1295°C, occurs at increasingly lower temperatures with increasing Cr content. No evidence was found for the metastable polymorphs of LiAl5 O8 reported by Datta and Roy. Ternary subsolidus equilibria at 1200° to 1300°C were determined for compositions containing <50% Li2 O. 相似文献
13.
Phase equilibria along the nonbinary join between cordierite (2MgO · 2Al2 O3 · 5SiO2 ) and spodumene (Li2 O · Al2 O3 · 4SiO2 ) were investigated in the temperature range 800° to 1550°C. using the quench technique on fourteen compositions. The phase diagram at high temperatures is characterized by a very small region of solid solution on the cordierite side, appreciable solid solution on the spodumene side, and regions of three and four phases toward the center of the system, including liquid, α-cordierite, mullite, spinel, corundum, and β-spodumene and its solid solutions. The liquidus has a flat minimum between 40 and 50% cordierite at 1347°, and rises on one side to the congruent melting point of β-spodumene (1421°) and on the other side to the temperature of complete melting of cordierite (1530°). The lowest temperature at which liquid appears is 1325°. At low temperatures a complete series of metastable solid solutions exists between μ-cordierite and β-spodumene. The significance of the data in the preparation of thermal-shock-resisting bodies is discussed. 相似文献
14.
The quenching technique has been used to determine equilibrium relations in the system manganese oxide-Cr2 O3 in air in the temperature range 600° to 1980°C. The following isobaric invariant situations have been determined: At 910°± 5°C tetragonal Mn3 O4 solid solution, cubic Mn3 O4 solid solution (=spinel), Mn2 O3 solid solution, and gas coexist in equilibrium. Cubic Mn3 O4 solid solution, Cr2 O3 solid solution, liquid, and gas are present together in equilibrium at 1970°± 20°C. The invariant situation at which cubic Mn3 O4 solid solution, Mn2 O3 solid solution, Cr2 O3 solid solution, and gas exist together in equilibrium is below 600°C. 相似文献
15.
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
16.
Jana Bezjak Aleksander Renik Botjan Janar Philippe Boullay Ivana Radosavljevi Evans Danilo Suvorov 《Journal of the American Ceramic Society》2009,92(8):1806-1812
Polymorphic phase transitions in Ba4 Nb2 O9 were studied by thermal analyses, high-temperature transmission electron microscopy and X-ray powder diffractometry. Two stable polymorphs were isolated, low-temperature α-modification and high-temperature γ-modification, with the endothermic phase transition at 1176°C. The α→γ transformation is accompanied by the formation of a 120° domain structure, which is a consequence of hexagonal→orthorhombic unit cell reconstruction. Reheating the presintered γ-Ba4 Nb2 O9 results in the formation of a metastable γ'-modification (formerly known as β-polymorph) in the temperature range between 360° and 585°C, before the γ→α transformation at 800°C. Above ∼490°C Ba4 Nb2 O9 becomes moderately sensitive to a loss of BaO. In air the surface of Ba4 Nb2 O9 grains decomposes to nanocrystalline Ba5 Nb4 O15 and BaO, which instantly reacts with atmospheric CO2 to form BaCO3 . Surface reaction delays γ→α transformation up to 866°C in air. In vacuum the loss of BaO is even more enhanced and consequently the formation of minor Ba3 Nb2 O8 phase is observed above 1150°C. 相似文献
17.
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
18.
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
19.
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
20.
The subsolidus phase equilibrium diagram for the pseudobinary join MgAl2 O4 -Ga2 O3 was determined. The shape of the exsolution boundary was obtained by heat-treating samples pre- equilibrated at 1600°C. Crystalline solubility of Ga2 O3 in MgAl2 O4 decreased from 73 mole % at 1600°C to 55 mole % at 1200°C. The crystalline solution was formed by the replacement of Mg2+ ions by Ga3+ ions to produce a cation defect spinel. The phase precipitated was the mono-clinic δ-Ga2 O3 (=δ-Al2 O3 structure). Changes in the ratios of relative X-ray diffraction intensities indicated that the crystalline solutions also disorder with temperature. 相似文献