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1.
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
2.
Noboru Yoshikawa Atsushi Kikuchi Shoji Taniguchi 《Journal of the American Ceramic Society》2002,85(7):1827-1834
An aluminum/Al2 O3 composite body is produced by a displacement reaction between SiO2 and molten aluminum. The growth rate of the reaction layer possesses negative (anomalous) temperature dependence at 1000–1300 K. This study compared reported reaction-kinetic data and investigated causes for this temperature dependence. The reaction product, Al2 O3 , changed from the γ-/θ-Al2 O3 phase to the α-Al2 O3 phase in this temperature range and α-Al2 O3 became the dominant phase at >1273 K. Isothermal transformation of the γ-/θ-Al2 O3 product phases to the α-Al2 O3 phase was also observed. Morphologies and scales of the Al2 O3 phases change drastically at 1173 K; this transition occurred in a spatially discontinuous manner. Reaction-rate retardation was interpreted in terms of occurrence of the competitive and simultaneous reactions to produce different Al2 O3 phases in this temperature range. It was also found that the hydrogen release from the raw SiO2 and the SiO2 phase transformation were not related to the negative temperature dependence. 相似文献
3.
Mechanical-Activation-Triggered Gibbsite-to-Boehmite Transition and Activation-Derived Alumina Powders 总被引:2,自引:0,他引:2
Mechanical activation of monoclinic gibbsite (Al(OH)3 ) in nitrogen led to the formation of nanocrystalline orthorhombic boehmite (AlOOH) at room temperature. The boehmite phase formed after merely 3 h of mechanical activation and developed steadily as the mechanical-activation time increased. Forty hours of mechanical activation resulted in essentially single-phase boehmite, together with α-alumina (α-Al2 O3 ) nanocrystallites 2–3 nm in size. The sequence of phase transitions in the activation-derived boehmite was as follows: boehmite to γ-Al2 O3 and then to α-Al2 O3 when flash-calcined at a heating rate of 10°C/min in air. γ-Al2 O3 formed at 520°C, and flash calcination to 1100°C led to the formation of an α-Al2 O3 phase, which exhibited a refined particle size in the range of 100–200 nm. In contrast, the gibbsite-to-boehmite transition in the unactivated gibbsite occurred over the temperature range of 220°–330°C. A flash-calcination temperature of 1400°C was required to complete the conversion to α-Al2 O3 phase, with both δ-Al2 O3 and θ-Al2 O3 as the transitional phases. The resulting alumina powder consisted of irregularly shaped particles 0.4–0.8 μm in size, together with an extensive degree of particle agglomeration. 相似文献
4.
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
5.
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
6.
Seeding of the Reaction-Bonded Aluminum Oxide Process 总被引:1,自引:0,他引:1
The effect of the initial α-Al2 O3 particle size in the reaction-bonded aluminum oxide (RBAO) process on the phase transformation of aluminum-derived γ-Al2 O3 to α-Al2 O3 , and subsequently densification, was investigated. It has been demonstrated that if the initial α-Al2 O3 particles are fine (∼0.2 μm, i.e., 2.9 × 1014 γ-Al2 O3 particles/cm3 ), then they seed the phase transformation. The fine α-Al2 O3 decreases the transformation temperature to ∼962°C and results in a finer microstructure. The smaller particle size of the seeded RBAO decreases the sintering temperature to as low as ∼1135°C. The results confirm that seeding can be utilized to improve phase transformations and densification and subsequently to tailor final microstructures in RBAO-derived ceramics. 相似文献
7.
The dehydration, transformation, and densification of boehmite (γ-AlOOH) are enhanced by addition of γ-Al2 O3 seed particles. α-Al2 O3 microstructures with uniform 1- to 2-μm grain size and sintered densities 98% of theoretical are achieved at 1300°C Thermal analysis shows that γ-Al2 O3 seed particles transform to α-Al2 O3 before the matrix, thus controllably nucleating the transformation of θ-AI2 O3 to α-Al2 O3 . 相似文献
8.
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
9.
Preparation of Nanometer-Sized α-Alumina Powders by Calcining an Emulsion of Boehmite and Oleic Acid
Chih-Peng Lin Shaw-Bing Wen Ting-Tai Lee 《Journal of the American Ceramic Society》2002,85(1):129-133
This study proposes a method to form ultrafine α-Al2 O3 powders. Oleic acid is mixed with Al(OH)3 gel. The gel is the precursor of the Al2 O3 . After it is mixed and aged, the mixture is calcined in a depleted oxygen atmosphere between 25° and 1100°C. Oleic acid evaporates and decomposes into carbon during the thermal process. Residual carbon prevents the growth of agglomerates during the formation of α-Al2 O3 . The phase transformation in this process is as follows: emulsion →γ-Al2 O3 →δ-Al2 O3 →θ-Al2 O3 →α-Al2 O3 . This process has no clear θ phase. Aging the mixed sample lowers the formation temperature of α-Al2 O3 from 1100° to 1000°C. The average crystallite diameter is 60 nm, measured using Scherrer's equation, which is consistent with TEM observations. 相似文献
10.
Daniela Domanski Guillermina Urretavizcaya Facundo J. Castro Fabiana C. Gennari 《Journal of the American Ceramic Society》2004,87(11):2020-2024
MgAl2 O4 spinel was successfully synthesized using a mechanochemical route that avoided the formation and calcination of its precursors at high temperatures. The method involved a single step in which γ-Al2 O3 –MgO, AlO(OH)–MgO, and α-Al2 O3 –MgO mixtures were milled at room temperature under air atmosphere. The formation of MgAl2 O4 occurred faster with γ-Al2 O3 than with AlO(OH) or α-Al2 O3 . After 140 h, the mechanochemical treatment of the γ-Al2 O3 –MgO mixture yielded 99% of MgAl2 O4 . 相似文献
11.
Hsing-I Hsiang Tsung-Hao Chen Chia-Che Chuang 《Journal of the American Ceramic Society》2007,90(12):4070-4072
Single-crystal α-alumina (Al2 O3 ) hexagonal platelets with a diameter of about 200 nm and 25 nm in thickness were synthesized by heating a mixture of boehmite and potassium sulfate at 1000°C for 2 h and washing with water. The potassium sulfate addition effects on the Al2 O3 phase and morphology were investigated using differential thermal analysis (DTA), X-ray diffraction (XRD), and transmission electron microscopy (TEM). It was found that potassium sulfate addition helps in the formation of single-crystal α-Al2 O3 hexagonal platelets and promotes phase transformation from intermediate γ-Al2 O3 to α-Al2 O3 . 相似文献
12.
Karen J. Morrissey Karel K. Czanderna C. B. Carter Robert P. Merrill 《Journal of the American Ceramic Society》1984,67(5):c88-c90
The growth of α-Al2 O3 from a planar specimen of thermally grown γ-alumina on a molybdenum transmission electron microscope grid was studied. The α-Al2 O3 grows into the transition alumina matrix and then thickens via a ledge growth mechanism. Faceted Mo crystallites cause pinning of α-Al2 O3 ledges and are larger on α-Al2 O3 than on the transition alumina matrix. 相似文献
13.
Arian Nijmeijer Henk Kruidhof Rune Bredesen Henk Verweij 《Journal of the American Ceramic Society》2001,84(1):136-140
Supported mesoporous γ-Al2 O3 membranes deteriorate and blister in steam-containing environments at high temperatures. This deterioration led us to the development of a new type of supported γ-Al2 O3 membrane with significantly improved stability under hostile conditions. Two measures were taken to achieve this result. First, the γ-Al2 O3 itself was stabilized by an addition of 6 mol% La2 O3 to suppress pore growth of the mesoporous structure. Second, the adherence of the γ-Al2 O3 membrane to the α-Al2 O3 support was significantly improved by application of phosphate bonding between the membrane layer and the support, using an Al(H2 PO4 )3 precursor solution. Membranes applied without phosphate bonding were separated from the α-Al2 O3 support during high-temperature steam treatment, resulting in complete loss of separative properties. The newly developed membranes could be operated for 100 h at 600°C in H2 O/CH4 = 3/1 (by volume) at 2.5 MPa total pressure with no delamination or cracking in the membrane–support interface and with no significant pore growth in the γ-Al2 O3 membrane. 相似文献
14.
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
15.
α-Al2 O3 -seeded, boehmite-derived γ-Al2 O3 was transformed in the presence of V2 O5 , resulting in a 205°C decrease in the α-Al2 O3 transformation temperature and a 74% reduction in the apparent activation energy for the γ- to α-Al2 O3 transformation at temperatures greater than 850°C. These changes are attributed to the lowered energy barrier for nucleation by seeding and the lowered activation energy for material transport through the liquid relative to the unseeded, solid-state transformation. Growth of the transforming alumina yielded fine-grained α-Al2 O3 particles which exhibited a highly faceted morphology. It is proposed that the combined control of both nucleation and growth during liquid-phase-assisted transformation provides a potentially powerful technique for tailoring powder characteristics in many material systems which undergo nucleation and growth processes. 相似文献
16.
Mari Lou Balmer Fred F. Lange Vikram Jayaram Carlos G. Levi 《Journal of the American Ceramic Society》1995,78(6):1489-1494
Aqueous mixtures of zirconium acetate and aluminum nitrate were pyrolyzed and crystallized to form a metastable solid solution, Zr1- x Al x O2− x /2 ( x < 0.57). The initial, metastable phase partitions at higher temperatures to form two metastable phases, viz., t −ZrO2 +γ-Al2 O3 with a nano-scale microstructure. The microstructural observations associated with the γ- →α-Al2 O3 phase transformation in the t -ZrO2 matrix are reported for compositions containing 10, 20, and 40 mol% A12 O3 . During this phase transformation, the α-Al2 O3 grains take the form of a colony of irregular, platelike grains, all with a common crystallographic orientation. The plates contain ZrO2 inclusions and are separated by ZrO2 grains. The volume fraction of A12 O3 and the heat treatment conditions influence the final microstructure. At lower volume fractions of A12 O3 , the colonies coarsen to single, irregular plates, surrounded by polycrystalline ZrO2 . Interpenetrating microstructures produced at high volume fractions of A12 O3 exhibit very little grain growth for periods up to 24 h at 1400°C. 相似文献
17.
Interpretation of the Kaolinite-Mullite Reaction Sequence from Infrared Absorption Spectra 总被引:2,自引:0,他引:2
The phases in the kaolinite-mullite reaction sequence were reexamined by ir absorption spectrophotometry. Particular attention was paid to the controversial intermediate Al-containing phases. Amorphous materials were leached from fired kaolinite samples with NaOH to help identify crystalline phases. Metakaolinite partially decomposes, releasing amorphous γ-Al2 O3 and SiO2 , before the "950°C" exothermic reaction in which metakaolinite is completely decomposed. The resulting spinel-type phase, which is associated with amorphous SiO2 and some poorly crystalline "primary" mullite, is γ-Al2 03 (crystalline) rather than an Al-Si spinel. There is some evidence, however, that a fraction of the γ-Al2 O3 , may be an Al-Si spinel. At ≥1100°C secondary mullite therefore forms primarily from the γ-Al2 O3 /amorphous SiO2 reaction and the recrystallization of primary mullite, whereas excess amorphous SiO2 eventually crystallizes as cristobalite. 相似文献
18.
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
19.
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
20.
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