共查询到20条相似文献,搜索用时 15 毫秒
1.
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
2.
Isothermal transformation kinetics and coarsening rates were studied in unseeded and alpha-Al2 O3 -seeded γ-Al2 O3 powders heated in dry air and water vapor. Unseeded samples heated in dry air transformed to alpha-Al2 O3 with an activation energy of 567 kJ/mol. Seeding with alpha-Al2 O3 increased the transformation rates and reduced incubation times by providing low-energy sites for nucleation/growth of the alpha-Al2 O3 transformation. The activation energy for the transformation was reduced to 350 kJ/mol in seeded samples heated in dry air. Seeded samples completely transformed to alpha-Al2 O3 after 1 h at 1050°C when heated in dry air compared to 1 h at 925°C when heated in saturated water vapor. The combined effects of a lower nucleation barrier due to seeding and the increased diffusion due to water vapor reduced the activation energy for the transformation by 390 kJ/mol and the transformation temperature by ∼225°C compared to the unseeded samples heated in dry air. The accelerated kinetics is believed to be due to increased surface diffusion. 相似文献
3.
Altaf H. Carim Gregory S. Rohrer Neal R. Dando Shih-Ying Tzeng Cathy L. Rohrer Anthony J. Perrotta 《Journal of the American Ceramic Society》1997,80(10):2677-2680
A new two-step transformation sequence has been identified for the conversion of diaspore (α-AlOOH) to corundum (alpha-Al2 O3 ). The powder X-ray diffraction pattern of the intermediate phase, designated alpha'-Al2 O3 , has been indexed to a monoclinic (P2) cell, with a = 9.566(5) Å, b = 5.124(2) Å, c = 9.156(4) Å, and ß= 91.76(3)°. Nuclear magnetic resonance of alpha'-Al2 O3 indicates that 15-20% of the aluminum is on tetrahedral sites, with the remainder in octahedral coordination. Controlled-atmosphere high-temperature X-ray diffraction shows that water accelerates the second transformation step, alpha'-Al2 O3 →> alpha-Al2 O3 , acting as a mineralizer for the conversion process. 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.
Yong W. Bae Woo Y. Lee Theodore M. Besmann O. Burl Cavin Thomas R. Watkins 《Journal of the American Ceramic Society》1998,81(7):1945-1948
Pure nickel coupons were used as substrates in the deposition of alumina (Al2 O3 ) from the reaction of aluminum chloride (AlCl3 ) with hydrogen/carbon dioxide gas mixtures in the temperature range of 954°–1100°C and system pressures of 2.7–13.3 kPa. The apparent activation energy estimated from the coating growth rate averaged 320 kJ/mol at 13.3 kPa. At temperatures <1000°C, transition theta, kappa, and delta modifications were codeposited with alpha-Al2 O3 , whereas single-phase alpha-Al2 O3 was deposited at higher temperatures. At high AlCl3 partial pressures, nickel aluminide phases were sometimes codeposited with Al2 O3 , which was attributed to the reaction of AlCl3 with the nickel substrate in the presence of hydrogen gas. 相似文献
6.
In this paper, we describe variations in a boehmite (AlOOH) and oleic acid emulsion during the process of forming superfine α-Al2 O3 crystallites from the mixture of oleic acid and a boehmite gel precursor. We also propose that the oleic acid decomposes under calcination, generating carbon, which can effectively prevent agglomeration of Al2 O3 particles. Calcination for the present study was conducted under a reduced oxygen atmosphere, in the temperature range 25°–1100°C. Phase variations of the mixture under calcination were identified by Fourier transform infrared spectrometry (FTIR), X-ray diffractometry (XRD), and transmission electron microscopy (TEM). The FTIR spectra were used when the mixed emulsion of oleic acid and boehmite gel was heated, to investigate the adsorption reaction of the aluminum oleate; the C—O—C cross-linking structure of oxygenation, which aided in carbon formation; and the ability of the carbon generated with α-Al2 O3 during phase transformation to prevent agglomeration (vermicularity). The products were analyzed by XRD at different temperatures, and TEM was used to examine the individual diameters of the α-Al2 O3 crystallites. 相似文献
7.
Carmen Barrera-Solano Luis Esquivias Gary L. Messing 《Journal of the American Ceramic Society》1999,82(5):1318-1324
In situ development of La-ß-Al2 O3 (LBA) platelets in alpha-Al2 O3 was studied as a function of the preparation method: a conventional solid-state reaction of commercial Al2 O3 powder and La(NO3 )3 as well as a sol-gel method starting with boehmite and the same La2 O3 precursor. In both cases, homogeneous distribution of the reinforcing phase was achieved, and a noteworthy inhibition of Al2 O3 grain growth resulted. However, samples prepared by solid-state reaction densified more easily than those prepared via sol-gel, but the formation of the LBA phase occurred at a lower temperature in samples prepared by the sol-gel approach. Results on the correlation of the onset of LBA grain growth and densification to microstructure are discussed. 相似文献
8.
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
9.
Paul A. Buining Chellappah Pathmamanoharan Monique Bosboom J. Ben H. Jansen Hendrik N. W. Lekkerkerker 《Journal of the American Ceramic Society》1990,73(8):2385-2390
The synthesis of colloidal boehmite (AlOOH) is studied by heating basic aluminum chloride solutions under constant stirring. The temperature and Al2 O3 : Cl molar ratio influence the product morphology. Synthesis at 140°C generates highly fibrous polycrystalline particles that are on average 360 nm long, 30 nm broad, and 8 nm thick. They contain 0.11 mol of excess H2 O per 1 mol of AlOOH. Synthesis at temperatures between 140° and 190°C produces broader fibrils and less excess H2 O. Preparation at 220°C eventually produces fully crystalline platelike boehmite particles about 260 nm long, 95 nm broad, and 14 nm thick, without excess H2 O. Fibril synthesis requires an Al2 O3 :Cl molar ratio exceeding 1.0 to yield noncoagulated particles. The fibrils are fairly monodisperse with 20% standard deviation in their length for an Al2 O3 : Cl molar ratio about 1.0. 相似文献
10.
Marion Bartsch Bilge Saruhan Martin Schmücker Hartmut Schneider 《Journal of the American Ceramic Society》1999,82(6):1388-1392
Dense mullite ceramics were successfully produced at temperatures below 1300°C from amorphous SiO2 -coated gamma-Al2 O3 particle nanocomposites (AS-gammaA). This method reduces processing temperatures by similar/congruent300°C or more with respect to amorphous SiO2 -coated alpha-Al2 O3 particle microcomposites (AS-alphaA) and to other Al2 O3 -SiO2 reaction couples. The good densification behavior and the relatively low mullite formation temperature make AS-gammaA nanocomposites an excellent matrix raw material for polycrystalline aluminosilicate fiber-reinforced mullite composites. 相似文献
11.
Takeshi Shiono Koji Shiono Kazuyo Miyamoto Giuseppe Pezzotti 《Journal of the American Ceramic Society》2000,83(1):235-37
A precursor was synthesized from a heterogeneous alkoxide solution that contained fine MgO powder, which allowed the preparation of MgAl2 O4 spinel powder with high sinterability characteristics. The precursor consisted of a mixture of boehmite (AlO(OH)) and a mixed hydroxide (Mg4 Al2 (OH)14 · 3H2 O). The spinel phase formed through two steps: (i) decomposition of the mixed hydroxide at low temperature and (ii) solid-state reaction between MgO and γ-Al2 O3 at higher temperatures. Dense polycrystalline spinel could be obtained from the calcined powders at sintering temperatures as low as 1400°C. 相似文献
12.
Pei-Ching Yu Rung-Je Yang Ya-Ting Chang Fu-Su Yen 《Journal of the American Ceramic Society》2007,90(8):2340-2346
The possibility of eliminating finger or vermicular growth of α-Al2 O3 particles obtained by calcination of boehmite was examined. Heterogeneous precipitation of boehmite in a well-dispersed θ-Al2 O3 suspension was first prepared, in which the mass ratio of boehmite to θ-crystallite was evaluated to form agglomerates of similar sizes that will form α-Al2 O3 crystallites of <100 nm in diameter. θ- to α-phase transformation of alumina experiences a nucleation and growth mechanism, with the critical size of nucleation being ∼25 nm for θ-Al2 O3 and the size for accomplishment of transformation followed by finger growth being ∼100 nm. Hence, fabricating agglomerates that would form α-Al2 O3 crystallites with sizes <100 nm accompanied with appropriate thermal treatments can be a method for obtaining α-Al2 O3 crystallites free of finger growth. It is found that proper preparation of the agglomerate with appropriate size may initiate a simultaneous and lower temperature θ- to α-Al2 O3 phase transformation for such powder systems, substantially limiting the mass transfer among the newly formed α-Al2 O3 particles. Moreover, α-Al2 O3 crystallites free of finger growth can be obtained. 相似文献
13.
Masanori Hirano Mikimasa Imai Michio Inagaki 《Journal of the American Ceramic Society》2000,83(4):977-979
ZnGa2 O4 fine particles with a single phase of spinel were synthesized from a mixed solution of gallium sulfate and zinc sulfate in the presence of aqueous ammonia under hydrothermal conditions above 180°C. The effects of treatment temperature and ZnO/Ga2 O3 molar ratio in the starting solution on the crystallite size, morphology, lattice parameter, and chemical composition of the ZnGa2 O4 spinel particles were examined. Spinel with different morphologies, cubic nanoparticles, and elongated rodlike particles were thought to be formed based on the structure of amorphous gallium hydroxide and needlelike GaO(OH) particles, respectively. By treatment at a higher temperature, these particles with nonstoichiometric composition grew large and thick, and their composition approached ZnO/Ga2 O3 = 1. With an increase in the starting ZnO/Ga2 O3 molar ratio, the lattice parameter of the synthesized ZnGa2 O4 spinel approached the reported value for the stoichiometric composition and reached a = 0.8335 nm at ZnO/Ga2 O3 = 1.95 by treatment at 240°C for 50 h. 相似文献
14.
Preparation of Strontium Ferrite Particles by Spray Pyrolysis 总被引:4,自引:0,他引:4
Yoshihide Senzaki† James Caruso† Mark J. Hampden-Smith† Toivo T. Kodas‡ Lu-Min Wang§ 《Journal of the American Ceramic Society》1995,78(11):2973-2976
Crystalline, submicrometer strontium ferrite powders, including SrFeO2.97 , SrFe2 O4 , Sr2 FeO4 , Sr3 Fe2 O6.16 , and SrFe12 O19 , were prepared by spray pyrolysis of an aqueous solution of mixed metal nitrates. The Sr:Fe mole ratio in the precursor solution was retained in the final products. Phase-pure materials were typically obtained only at the highest temperatures investigated (>1100°C) and powders prepared at lower temperatures frequently contained crystalline Fe2 O3 . The as-prepared particles were unagglomerated, polycrystalline, and hollow at lower temperatures, but densified in the gas phase at higher temperatures to give solid particles. The strontium ferrite (SrFe12 O19 ) system was studied in detail as a representative example of the Sr-Fe-O system. At temperatures of 1200°C, dense, phase-pure magnetoplumbite-structure material, SrFe12 O19 , was obtained, while at lower temperatures, small amounts of Fe2 O3 were observed. The particles prepared at 800° and 1100°C were 0.1-1.0 μm in diameter, and consisted of crystallites <100 nm, and were nearly solid. The difficulty in forming phase-pure SrFe12 O19 was the different thermal decomposition temperatures of Sr(NO3 )2 (725°C) and Fe(NO3 )3 9H2 O (125°C) as demonstrated by thermogravimetric analysis in the SrFe12 O19 system. 相似文献
15.
Bruce A. Pint Anthony J. Garratt-Reed Linn W. Hobbs 《Journal of the American Ceramic Society》1998,81(2):305-314
Thermally grown, alpha-Al2 O3 external scales formed on alloys after oxidation in pure oxygen at temperatures between 1000° and 1500°C were analyzed using FEG-STEM/XEDS. Alloy dopants such as Y, Zr, La, Hf, and Ti were found to segregate to the alpha-Al2 O3 grain boundaries and to the alloy-scale interface. With increasing oxidation time and temperature, the amount of segregant on the oxide grain boundaries near the gas interface increases until a critical level is reached and precipitates begin to nucleate and grow. These observations are a result of the outward transport of dopants from the alloy, through the external alumina scale, to the gas interface. The apparent driving force for the dopant diffusion is the oxygen potential gradient in the growing oxide scale. 相似文献
16.
Milan Kanti Naskar 《Journal of the American Ceramic Society》2009,92(10):2392-2395
Petal-like alumina flakes were prepared by hydrothermal reaction at 170°C for 6 h with aqua-based precursor materials. The gel particles were calcined at 550°–1200°C. The gel and calcined particles were characterized by differential thermal analysis (DTA), thermogravimetry (TG), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), multipoint Brunauer–Emmett–Teller (BET) surface area, and field emission scanning electron microscopy (FESEM). Crystallization of alumina was confirmed by DTA and XRD studies. XRD results showed that boehmite particles were formed in as-prepared sample while γ-alumina (γ-Al2 O3 ) persisted up to 800°C followed by the appearance of θ-alumina (θ-Al2 O3 ) at 1000°–1200°C. The characteristics Al–O vibrations in alumina polymorphs were examined by FTIR study. FESEM confirmed the formation of petal-like alumina flakes with the assembly of alumina nanorods. 相似文献
17.
Seeding a mixture of boehmite (AIOOH) and colloidal ZrO2 with α-alumina particles and sintering at 1400°C for 100 min results in 98% density. The low sintering temperature, relative to conventional powder processing, is a result of the small alumina particle size (∼0.3 μm) obtained during the θ-to α-alumina transformation, homogeneous mixing, and the uniform structure of the sol-gel system. Complete retention of pure ZrO2 in the tetragonal phase was obtained to 14 vol% ZTA because of the low-temperature sintering. The critical grain size for tetragonal ZrO2 was determined to be ∼0.4 μm for the 14 vol% ZrO2 —Al2 O3 composite. From these results it is proposed that seeded boehmite gels offer significant advantages for process control and alumina matrix composite fabrication. 相似文献
18.
Priyadarshi G. Desai Zhengkui Xu Jennifer A. Lewis 《Journal of the American Ceramic Society》1995,78(11):2881-2888
Novel microcomposite powders, consisting of inert cores (αAL-Al2 O3 ) surrounded by reactive cement-based coatings (CaAl2 O4 ), were synthesized by a modified Pechini process. The evolution of the crystalline CaAl2 O4 phase during calcination was studied using multiple analytical techniques, including DRIFTS,13 C and 27 AlMAS FT-NMR, and XRD, for both pure CaAl2 O4 and CaAl2 O4 -coated Al2 O3 precursor powders. In both powders, decomposition proceeded via hydrocarbon chain scission and removal of ester groups at low temperatures ( T < 450°C), followed by the formation of inorganic carbonates at higher temperatures ( T > 450°C). These decomposition processes were accelerated by the underlying Al2 O3 cores. Transmission electron microscopy (TEM) of the fully calcined powders showed that the inert αAL-Al2 O3 particles were surrounded by relatively uniform CaAl2 O4 coatings ranging in thickness from approximately 10 to 100 nm. 相似文献
19.
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
20.
A coating approach for synthesizing 0.9Pb(Mg1/3 Nb2/3 )O3 –0.1PbTiO3 (0.9PMN–0.1PT) and PMN using a single calcination step was demonstrated. The pyrochlore phase was prevented by coating Mg(OH)2 on Nb2 O5 particles. Coating of Mg(OH)2 on Nb2 O5 was done by precipitating Mg(OH)2 in an aqueous Nb2 O5 suspension at pH 10. The coating was confirmed using optical micrographs and zeta-potential measurements. A single calcination treatment of the Mg(OH)2 -coated Nb2 O5 particles mixed with appropriate amounts of PbO and PbTiO3 powders at 900°C for 2 h produced pyrochlore-free perovskite 0.9PMN–0.1PT and PMN powders. The elimination of the pyrochlore phase was attributed to the separation of PbO and Nb2 O5 by the Mg(OH)2 coating. The Mg(OH)2 coating on the Nb2 O5 improved the mixing of Mg(OH)2 and Nb2 O5 and decreased the temperature for complete columbite conversion to ∼850°C. The pyrochlore-free perovskite 0.9PMN–0.1PT powders were sintered to 97% density at 1150°C. The sintered 0.9PMN–0.1PT ceramics exhibited a dielectric constant maximum of ∼24 660 at 45°C at a frequency of 1 kHz. 相似文献