共查询到20条相似文献,搜索用时 31 毫秒
1.
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
2.
Rabindra N. Das Amit Bandyopadhyay Susmita Bose 《Journal of the American Ceramic Society》2001,84(10):2421-2423
Nanocrystalline α-Al2 O3 ceramic powders have been prepared from an aqueous solution of aluminum nitrate and sucrose. Soluble Al ion-sucrose solution forms the precursor material once it is completely dehydrated. Heat treatment of the dehydrated precursors at low temperature (600°C) results in the formation of porous single-phase α-Al2 O3 . The precursor and heat-treated powders have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and BET surface area analysis. The phase-pure nanocrystalline α-Al2 O3 particles had an average specific surface area of >190 m2 /g, with an average pore size between 18 and 25 nm. 相似文献
3.
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
4.
Jiang Li Yubai Pan Yusong Wu Huamin Kou Jingkun Guo 《International Journal of Applied Ceramic Technology》2007,4(3):276-284
High-quality alumina ceramics were fabricated by a hot pressing with MgO and SiO2 as additives using α-Al2 O3 -seeded nanocrystalline γ-Al2 O3 powders as the raw material. Densification behavior, microstructure evolution, and mechanical properties of alumina were investigated from 1250°C to 1450°C. The seeded γ-Al2 O3 sintered to 98% relative density at 1300°C. Obvious grain growth was observed at 1400°C and plate-like grains formed at 1450°C. For the 1350°C hot-pressed alumina ceramics, the grain boundary regions were generally clean. Spinel and mullite formed in the triple-grain junction regions. The bending strength and fracture toughness were 565 MPa and 4.5 MPa·m1/2 , respectively. For the 1300°C sintered alumina ceramics, the corresponding values were 492 MPa and 4.9 MPa·m1/2 . 相似文献
5.
Guo-Dong Zhan Joshua D. Kuntz Ren-Guan Duan Amiya K. Mukherjee 《Journal of the American Ceramic Society》2004,87(12):2297-2300
The combined effect of rapid sintering by spark-plasma-sintering (SPS) technique and mechanical milling of γ-Al2 O3 nanopowder via high-energy ball milling (HEBM) on the microstructural development and mechanical properties of nanocrystalline alumina matrix composites toughened by 20 vol% silicon carbide whiskers was investigated. SiCw /γ-Al2 O3 nanopowders processed by HEBM can be successfully consolidated to full density by SPS at a temperature as low as 1125°C and still retain a near-nanocrystalline matrix grain size (∼118 nm). However, to densify the same nanopowder mixture to full density without the benefit of HEBM procedure, the required temperature for sintering was higher than 1200°C, where one encountered excessive grain growth. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results indicated that HEBM did not lead to the transformation of γ-Al2 O3 to α-Al2 O3 of the starting powder but rather induced possible residual stress that enhances the densification at lower temperatures. The SiCw /HEBMγ-Al2 O3 nanocomposite with grain size of 118 nm has attractive mechanical properties, i.e., Vickers hardness of 26.1 GPa and fracture toughness of 6.2 MPa·m1/2 . 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.
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
8.
Nanocrystalline aluminum nitride (AlN) with surface area more than 30 m2 /g was synthesized by nitridation of nanosized δ-Al2 O3 particles using NH3 as a reacting gas. The resulting powders were characterized by CHN elemental analysis, X-ray diffraction (XRD), Fourier transform infrared spectra, X-ray photoelectron spectra, field-emission scanning electron microscopy, transmission electron microscopy, and Brunauer–Emmett–Teller surface area techniques. It was found that nanocrystalline δ-Al2 O3 was converted into AlN completely (by XRD) at 1350°–1400°C within 5.0 h in a single-step synthesis process. The complete nitridation of nanosized alumina at relatively lower temperatures was attributed to the lack of coarsening of the initial δ-Al2 O3 powder. The effect of precursor powder types on the conversion was also investigated, and it was found that α-Al2 O3 was hard to convert to AlN under the same conditions. 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.
A ∼50 nm thick alumina layer was deposited on an Ni-based superalloy substrate by a sol–gel method. α-AlOOH particles presented in the layer after drying at 140°C transformed mostly to α-Al2 O3 grains within ∼1 min at 1100°C under a low oxygen partial pressure annealing environment. During the same time period, the α-Al2 O3 grains grew significantly in the lateral direction, resulting in the aspect ratio of grain diameter to thickness of ∼20. The presence of a preferred orientation in the α-Al2 O3 layer suggested that the mechanism for the lateral growth was abnormal. The lateral growth mechanism appeared to become very slow when a critical thickness (∼100 nm) was reached. 相似文献
11.
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
12.
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
13.
Yu-Chen Lee Shaw-Bing Wen Liang Wenglin Chih-Peng Lin 《Journal of the American Ceramic Society》2007,90(6):1723-1727
An experimental study has been conducted to evaluate the formation of nano α-Al2 O3 under various conditions, such as different calcining temperatures and emulsion ratios of aqueous aluminum nitrate solutions and oleic acid with a high-speed stirring mixer. Four batches of the precursor powders were calcined at three different temperatures of 1000°, 1050°, and 1100°C for 2 h and a terminal product of nano α-Al2 O3 powders was obtained. The products have been identified by X-ray diffraction (XRD), specific surface area measurement scanning electron microscope, and transmission electron microscope (TEM). The XRD results show that the phase of powders is determined to be α-Al2 O3 , indicating that the overall process has been effective. The optimum calcination temperature of the precursor powder for crystallization of nano α-Al2 O3 was found to be 1000°C for 2 h. The TEM image indicates that the particle grains have a sub-spherical shape with a mean size of 50–100 nm. 相似文献
14.
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
15.
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
16.
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
17.
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
18.
Shinobu Hashimoto Shaowei Zhang William Edward Lee Akira Yamaguchi 《Journal of the American Ceramic Society》2003,86(11):1959-1961
Spinel platelets were formed from a powder mixture of 3–5 μm wide and 0.2–0.5 μm thick α-Al2 O3 and 1–8 μm (average 3 μm) MgSO4 heated 2 h at 1200°C. The hexagonal platelet shape of the original α-Al2 O3 platelet was maintained in the spinel, although their size was slightly increased and their surface roughened. When a mixture of α-Al2 O3 platelets and MgO powder was heated 3 h at 1400°C, the spinel formed lost the platelet morphology of the alumina. 相似文献
19.
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
20.
Preparation and Characterization of Aluminum Borate 总被引:2,自引:0,他引:2
Siba P. Ray 《Journal of the American Ceramic Society》1992,75(9):2605-2609
Aluminum borate, 9Al2 O3 ·2B2 O3 or Al18 B4 O33 , was synthesized by the reaction of stoichiometric amounts of α-Al2 O3 and B2 O3 . The Al18 B4 O33 material was formed into a dense ceramic by pressureless sintering with CaO, MgO, or CaAl2 B2 O7 additives. The material was characterized by low bulk density, moderate coefficient of thermal expansion (3 × 10−6 /°C to 5 × 10−6 /°C), moderate strength (210 to 324 MPa), and low dielectric constant. 相似文献