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1.
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
2.
J. J. COMER N. C. TOMBS J. F. FITZGERALD 《Journal of the American Ceramic Society》1966,49(5):237-240
Single-crystal and polycrystalline films of Mg-Al2 O4 and MgFe2 O4 were formed by two methods on cleavage surfaces of MgO single crystals. In one procedure, aluminum was deposited on MgO by vacuum evaporation. Subsequent heating in air at about 510°C formed a polycrystalline γ-Al2 O8 film. Above 540°C, the γ-Al2 O, and MgO reacted to form a single-crystal MgAl2 O4 film with {001} MgAl2 O4 ‖{001} MgO. Above 590°C, an additional layer of MgAl2 O4 , which is polycrystalline, formed between the γ-Al2 O3 and the single-crystal spinel. Polycrystalline Mg-Al2 O4 formed only when diffusion of Mg2+ ions proceeded into the polycrystalline γ-Al2 O3 region. Corresponding results were obtained for Mg-Fe2 O4 . MgAl2 O4 films were also formed on cleaved MgO single-crystal substrates by direct evaporation, using an Al2 O3 crucible as a source. Very slow deposition rates were used with source temperatures of ∼1350°C and substrate temperatures of ∼800°C. Departures from single-crystal character in the films may arise through temperature gradients in the substrate. 相似文献
3.
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
4.
K. Thomas Jacob Kampurath P. Jayadevan Yoshio Waseda 《Journal of the American Ceramic Society》1998,81(1):209-212
The standard Gibbs energy of formation of the spinel MgAl2 O4 from component oxides, MgO and α-Al2 O3 , has been determined in the temperature range 900 to 1250 K using a solid-state cell incorporating single-crystal CaF2 as the solid electrolyte. The cell can be represented as—Pt,O2 ,MgO+MgF2 |CaF2 |MgF2 +MgAl2 O4 +α-Al2 O3 ,O2 ,Pt—The standard Gibbs energy of formation from binary oxides, computed from the reversible emf, can be represented by the expression—capdelta G °f,ox =−23600 − 5.91 T (±150) J/mol—The 'second-law' enthalpy of formation of MgAl2 O4 obtained in this study is in good agreement with high-temperature solution calorimetric studies reported in the literature. 相似文献
5.
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
6.
Lennart Bergstrom ers Meurk Hans Arwin David J. Rowcliffe 《Journal of the American Ceramic Society》1996,79(2):339-348
The Hamaker constants of eight different ceramic materials, 6H-SiC, tetragonal, partially stabilized ZrO2 (3% Y2 O3 ), β-Si3 N4 , α-Al2 O3 , Y2 O3 , sapphire (single-crystal α-Al2 O3 ), MgO, MgAl2 O4 , and fused silica, across air, water, and n -dodecane at room temperature and across silica at 2000 K have been calculated from optical data using the Lifshitz theory. Spectroscopic ellipsometry was used to measure the photon energy dependence of the refractive index, n , and the extinction coefficient, k , in the visible and near-UV range on several important ceramic materials. This relatively simple, nondestructive technique has proved to yield reliable optical data on sintered, polycrystalline materials such as Si3 N4 , SiC, ZrO2 , Al2 O3 , and ZnO. For the other materials, Y2 O3 , sapphire, MgO, MgAl2 O4 , and fused silica, optical data from the literature were used to calculate the Hamaker constants. The calculated Hamaker constants were estimated to be accurate within ±10%. 相似文献
7.
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
8.
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
9.
Stanislav Antolin Alan S. Nagelberg David K. Creber 《Journal of the American Ceramic Society》1992,75(2):447-454
The growth of α-Al2 O3 /metal composites by the directed oxidation of molten Al-Mg-Si alloys proceeds through four distinct stages. The first stage encompasses the early heating of the alloy ingot, melting, and continued heating to between 1123 and 1173 K. In this latter temperature range, the molten alloy surface rapidly oxidizes to form a MgO-covered MgAl2 O4 layer. During further heating and initial soak at the composite growth temperature (1373 to 1573 K), the duplex layer slowly thickens (second stage). The start of the third stage, growth initiation, is marked by the spread of a metal-rich zone over the duplex layer; this metal-rich zone is believed to be connected to the molten alloy through microcracks in the thickened MgO/MgAl2 O4 layer. Small nodules of the oxide/metal composite nucleate from the metal-rich layer. During the final rapid growth stage, the small composite nodules grow and coalesce to form a macroscopically planar growth front, which persists until growth is complete. Throughout the growth process, the external surface of the α-Al2 O3 /metal composite is covered by a thin MgO layer. Immediately under this external layer and separating it from the α-Al2 O3 is a thin layer of molten metal. 相似文献
10.
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
11.
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
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.
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
14.
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
15.
ALEXANDRA NAVROTSKY BARRY A. WECHSLER KAREN GEISINGER FRIEDRICH SEIFERT 《Journal of the American Ceramic Society》1986,69(5):418-422
Solution calorimetry of MgAl2 O4 -Al8/3 O4 solid solutions was performed in a molten 2PbO · B2 O3 solvent at 975 K. The results indicate small negative heats of mixing, relative to spinel standard states for both end-members. These data were combined with information on the energetics of the α-γ transition in Al2 O3 and on the MgAl2 O4 -Al8/3 O4 (MgO-Al2 O3 ) subsolidus phase relations to estimate the partial molar entropy of mixing of γ-Al8/3 O4 in the solid solution. This entropy is much less positive than that calculated from several models for the configurational entropy of mixing of magnesium, aluminum, and vacancies on octahedral and/or tetrahedral sites. The data suggest a good deal of local order to be present in the solid solutions, consistent with negative enthalpies of mixing and entropies of mixing far less than ideal configurational values. 相似文献
16.
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
17.
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
18.
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
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.
Analysis of Phase Distribution for Ceramic Coatings Formed by Microarc Oxidation on Aluminum Alloy 总被引:26,自引:0,他引:26
Wenbin Xue Zhiwei Deng Yonchun Lai Ruyi Chen 《Journal of the American Ceramic Society》1998,81(5):1365-1368
The phase distribution for ceramic coatings formed by microarc oxidation (MAO) on 2024 aluminum alloy was investigated using X-ray diffraction. The results showed that the ceramic coatings mainly consisted of α-Al2 O3 and γ-Al2 O3 phases. The percentage of α-Al2 O3 gradually increased from the external surface to the interface between the coating and the substrate of samples. The surface layer of coatings mainly contained the γ-Al2 O3 phase, and its fraction of the composition remained almost constant with oxidation time. It is believed that the difference in the amounts of α-Al2 O3 and γ-Al2 O3 phases in the different layers of coatings was caused by the various cooling rates of molten Al2 O3 , which temporarily existed in the microarc zone. 相似文献