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1.
Gradient, porous alumina ceramics were prepared with the characteristics of microsized tabular α-Al2 O3 grains grown on a surface with a fine interlocking feature. The samples were formed by spin-coating diphasic aluminosilicate sol on porous alumina substrates. The sol consisted of nano-sized pseudo-boehmite (AlOOH) and hydrolyzed tetraethyl orthosilicate [Si(OC2 H5 )4 ]. After drying and sintering at 1150°–1450°C, the crystallographic and chemical properties of the porous structures were investigated by analytical electron microscopy. The results show that the formation of tabular α-Al2 O3 grains is controlled by the dissolution of fine Al2 O3 in the diphasic material at the interface. The nucleation and growth of tabular α-Al2 O3 grains proceeds heterogeneously at the Al2 O3 /glass interface by ripening nano-sized Al2 O3 particles. 相似文献
2.
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
3.
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
4.
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
5.
Akira Kato Hisao Yamashita Hiroshi Kawagoshi Shimpei Matsuda 《Journal of the American Ceramic Society》1987,70(7):157-C
Mixtures of La2 O3 and Al2 O3 with various La contents were prepared by co-precipitation from La(NO3 )3 and Al(NO3 )3 solutions and calcined at 800° to 1400°C. The addition of small amounts of La2 O3 (2 to 10 mol%) to Al2 O3 gives rise to the formation of lanthanum β-alumina (La 2 O3 ·11–14Al2 O3 ) upon heating to above 1000°C and retards the transformation of γ-Al2 O3 to α-Al2 O3 and associated sintering. 相似文献
6.
Víctor Valcárcel Carmen Cerecedo Francisco Guitián 《Journal of the American Ceramic Society》2003,86(10):1683-1690
α-alumina (α-Al2 O3 , corundum) fibers exhibit high thermal and chemical stability, as well as good mechanical properties, even at high temperatures. Such characteristics make them good candidates for use in composites. Nevertheless, very few methods of producing α-Al2 O3 fibers are available. In the present work, we describe a method that uses aluminum pieces deposited on SiO2 powder, in an argon atmosphere, at temperatures in the range 1300°–1600°C. The α-Al2 O3 fibers are obtained via vapor-liquid-solid deposition. The novel addition of nickel and cobalt (or their oxides) allows the use of temperatures >1500°C, resulting in improved fiber production. We demonstrate that the metals do not contaminate the fibers produced in this way. Finally, we also estimate the tensile strength of the Al2 O3 fibers produced through this method. 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.
Koji Tsukuma 《Journal of the American Ceramic Society》2000,83(12):3219-3221
Composites of β-Ce2 O3 ·11Al2 O3 and tetragonal ZrO2 were fabricated by a reductive atmosphere sintering of mixed powders of CeO2 , ZrO2 (2 mol% Y2 O3 ), and Al2 O3 . The composites had microstructures composed of elongated grains of β-Ce2 O3 ·11Al2 O3 in a Y-TZP matrix. The β-Ce2 O3 ·11Al2 O3 decomposed to α-Al2 O3 and CeO2 by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce2 O3 ·11Al2 O3 divided into several grains of α-Al2 O3 and ZrO2 doped with Y2 O3 and CeO2 . High-temperature bending strength of the oxygen-annealed α-Al2 O3 composite was comparable to the β-Ce2 O3 ·11Al2 O3 composite before annealing. 相似文献
9.
Yosuke Matsumoto Ken Hirota Osamu Yamaguchi Suguru Inamura Hiroki Miyamoto Nobuaki Shiokawa Kazuhiro Tsuji 《Journal of the American Ceramic Society》1993,76(10):2677-2680
Amorphous Al2 O3 –ZrO2 composite powders with 5–30 mol% ZrO2 have been prepared by adding aqueous ammonia to the mixed solution of aqueous aluminum sulfate and zirconium alkoxide containing 2-propanol. Simultaneous crystallization of γ-Al2 O3 and t -ZrO2 occurs at 870°–980°C. The γ-Al2 O3 transforms to α-Al2 O3 at 1160°–1220°C. Hot isostatic pressing has been performed for 1 h at 1400°C under 196 MPa using α-Al2 O3 – t -ZrO2 composite powders. Dense ZrO2 -toughened Al2 O3 (ZTA) ceramics with homogeneous-dispersed ZrO2 particles show excellent mechanical properties. The toughening mechanism is discussed. The microstructures and t / m ratios of ZTA are examined, with emphasis on the relation between strength and fracture toughness. 相似文献
10.
Phase Transformation of Diphasic Aluminosilicate Gels 总被引:1,自引:0,他引:1
Aluminosilicate gels with compositions Al2 O2 /SiO2 and 2 were prepared by gelling a mixture of colloidal pseudo-boehmite and a silica sol prepared from acid-hydrolyzed Si(OC2 H5 )4 . Upon heating the pseudo-boehmite transforms to γ-Al2 O3 around 400°C, then to δ-Al2 O3 at 1050°C, and at 1200°C reacts with amorphous SiO2 to form mullite. Some twinned θ-Al2 O3 forms before mullite. Nonstoichiometric specimens have a similar transformation sequence, but form mullite grains with inclusions of either Al2 O3 or cristobalite, often associated with dislocation networks or micropores. Mullite grains are formed by nucleation and growth and have equiaxed shape. 相似文献
11.
Hiroshi Inoue Koshi Sekizawa Koichi Eguchi Hiromichi Arai 《Journal of the American Ceramic Society》1997,80(3):584-588
A coating of barium hexaaluminate (Ba0.75 Al11.0 O17.25 ) on an α-SiC substrate and the thermal stability of the formed film were investigated for a high-temperature catalytic application. The film prepared by sol coating consisted of BaAl2 Si2 O8 and α-Al2 O3 phases and always contained many cracks or exfoliations after heating at 1200C. A hexaaluminate porous film was successfully formed by slurry coating without void formation at the interface between the film and the substrate and exfoliation due to the formation of the intermediate layer after heating at 1200°C. The microstructure of the film remained unchanged, even after heating at 1300°C. 相似文献
12.
Koji Ishida Ken Hirota Osamu Yamaguchi Hideki Kume Suguru Inamura Hiroki Miyamoto 《Journal of the American Ceramic Society》1994,77(5):1391-1395
In the system ZrO2 –Al2 O3 , a new method for preparing ZrO2 solid solutions from ZrCl4 and AlCl3 using hydrazine monohydrate is investigated. c -ZrO2 solid solutions containing up to ∼40 mol% Al2 O3 crystallize at low temperatures from amorphous materials. The formation mechanism is discussed from IR spectral data. The values of the lattice parameter α increase linearly from 0.5072 to 0.5105 nm with increasing Al2 O3 content. At higher temperatures, transformation of the solid solutions proceeds as follows: c ( SS ) → t ( ss ) → t ( ss ) +α-Al2 O3 → m +α-Al2 O3 . m -ZrO2 –α-Al2 O3 composite ceramics are fabricated by hot isostatic pressing for 2 h at 1250°C and 196 MPa. Microstructures and mechanical properties are examined, in connection with increasing Al2 O3 content. 相似文献
13.
Akshoy Kr. Chakraborty 《Journal of the American Ceramic Society》2005,88(1):134-140
Mechanical mixture of γ-Al2 O3 and amorphous SiO2 , and diphasic Al2 O3 /SiO2 gels of three different compositions were synthesized. They were subjected to heat treatment to various temperatures in the range 900°–1600°C. Qualitative X-ray diffraction data show that these diphasic gels do not crystallize to a combined mixture of θ-Al2 O3 and α-Al2 O3 polymorphs at the intermediate stage, prior to mullite formation. Estimated mullite formation data show that the course of its formation from mixed oxides was different from that of diphasic gels. Results are compared with previous findings and the concept of Al–Si spinel formation in the phase transformation of stoichiometric diphasic gel system is substantiated. 相似文献
14.
Formation and Sintering of Yttria-Doped Tetragonal Zirconia with 50 mol% Alumina Prepared by the Hydrazine Method 总被引:1,自引:0,他引:1
Shinya Kimoto Ken Hirota Osamu Yamaguchi Hideki Kume Suguru Inamura Hiroki Miyamoto 《Journal of the American Ceramic Society》1994,77(6):1694-1696
Al2 O3 /Y2 O3 -doped ZrO2 composite powders with 50 mol% Al2 O3 are prepared by the hydrazine method. As-prepared powders are mixtures of AlO(OH) gel and amorphous ZrO2 solid solutions containing Y2 O3 and Al2 O3 . The formation process leading to α-Al2 O3 - t -ZrO2 composite powders is examined. Hot isostatic pressing is performed for 2 h at 1400°C under 196 MPa using θ-Al2 O3 - t -ZrO2 composite powders. The resulting dense, sintered α-Al2 O3 - t -ZrO2 composites show excellent mechanical strength. 相似文献
15.
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
16.
The sinter forging behavior of α-Al2 O3 seeded and unseeded nanocrystalline θ-Al2 O3 was investigated as a function of temperature, stress, and strain rate. Seeded samples exhibited the highest degree of plastic deformation during the θ- to α-AI2 O3 phase transformation. As a result, microstructure control, increased densification, and a higher degree of transformation were obtained. A uniform microstructure of 150 nm α-Al2 O3 grains developed, reaching 57% relative density after sintering 1.5 wt%α-Al2 O3 seeded samples for 30 min at 1060°C. When sinter forged at 0.25 mm/min to 63 MPa and 1060°C for 30 min large deformations during the phase transformation increased the relative density to 74%. When the stress was increased to 235 MPa (1060°C, 30 min), 99.7% dense α-Al2 O3 with a grain size of 230 nm was obtained. By increasing the sinter forging temperature to 1150°C, 99.5% relative density was achieved at 190 MPa for 30 min. 相似文献
17.
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
18.
Yi-Feng Su Michele A. Torzilli Justin Daniel Meyer Claudia J. Rawn Michael J. Lance Sakari Ruppi Woo Y. Lee 《Journal of the American Ceramic Society》2002,85(8):2089-2096
The feasibility of preparing a thin layer of α-Al2 O3 on the surface of a single-crystal, Ni-based superalloy was examined using a chloride-based chemical vapor deposition (CVD) process previously developed for cutting tool applications. A coating directly deposited by this method on the alloy surface consisted of ∼1 μm α-Al2 O3 crystals in a matrix of amorphous Al2 O3 . When the alloy surface was predeposited with an electroplated Pt layer, the coating was mostly α-Al2 O3 , but with the presence of fine microcracks on the coating surface. In comparison to the results observed for pure Pt substrate, the role of the Pt interlayer was apparently to promote the rapid formation of κ-Al2 O3 nuclei, which subsequently transformed to α-Al2 O3 during the CVD growth process. 相似文献
19.
Dianying Chen Eric H. Jordan Maurice Gell Xinqing Ma 《Journal of the American Ceramic Society》2008,91(2):359-365
For the first time, dense coatings have been made by the solution precursor plasma spray (SPPS) process. The conditions are described for the deposition of dense Al2 O3 –40 wt% 7YSZ (yttria-stabilized zirconia) coatings; the coatings are characterized and their thermal stability is evaluated. X-ray diffraction analysis shows that the as-sprayed coating is composed of α-Al2 O3 and tetragonal ZrO2 phases with grain sizes of 72 and 56 nm, respectively. The as-sprayed coating has a 95.6% density and consists of ultrafine splats (1–5 μm) and unmelted spherical particles (<0.5 μm). The lamellar structure, typical of conventional plasma-sprayed coatings, is absent at the same scale in the SPPS coating. The formation of a dense Al2 O3 –40 wt% 7YSZ coating is favored by the lower melting point of the eutectic composition, and resultant superheating of the molten particles. Phase and microstructural thermal stabilities were investigated by heat treatment of the as-sprayed coating at temperatures of 1000°–1500°C. No phase transformation occurs, and the grain size is still in the nanometer range after the 1500°C exposure for 2 h. The coating hardness increases from 11.8 GPa in the as-coated condition to 15.8 GPa following 1500°C exposure due to a decrease in coating porosity. 相似文献
20.
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