共查询到20条相似文献,搜索用时 46 毫秒
1.
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
2.
α - Al2 O3 nanopowders with mean particle sizes of 10, 15, 48, and 80 nm synthesized by the doped α-Al2 O3 seed polyacrylamide gel method were used to sinter bulk Al2 O3 nanoceramics. The relative density of the Al2 O3 nanoceramics increases with increasing compaction pressure on the green compacts and decreasing mean particle size of the starting α-Al2 O3 nanopowders. The densification and fast grain growth of the Al2 O3 nanoceramics occur in different temperature ranges. The Al2 O3 nanoceramics with an average grain size of 70 nm and a relative density of 95% were obtained by a two-step sintering method. The densification and the suppression of the grain growth are achieved by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration. The densification was realized by the slower grain-boundary diffusion without promoting grain growth in second-step sintering. 相似文献
3.
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
4.
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
5.
Rajiv S. Mishra Charles E. Lesher Amiya K. Mukherjee 《Journal of the American Ceramic Society》1996,79(11):2989-2992
High-pressure sintering of nanocrystalline γ-A12 O3 has been studied over a temperature range of 923-1323 K and at a pressure of 1 GPa. The γ-Al2 O3 to α-Al2 O3 transformation temperature changed from 1473 K without pressure to ∼1023 K at 1 GPa. Full density was obtained at 1273 and 1323 K in 10 min. The microhardness value of fully dense α-alumina with a grain size of 142 nm was found to be 25.3 ± 0.8 GPa. The Hall-Petch slope for the very fine grain size range is different from that of the coarse-grained alumina. 相似文献
6.
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
7.
α-Al2 O3 platelet powders were synthesized in molten Na2 SO4 flux. The size of α-Al2 O3 platelets was significantly reduced when partially decomposed rather than pure Al2 (SO4 )3 was used as the source of Al2 O3 ; a further reduction in the platelet size was realized through additional seeding with nanosized α-Al2 O3 seeds. The addition of microsized α-Al2 O3 platelet seeds significantly influenced the platelet morphology of the final powder, as well. The platelet size of the final powder was in direct proportion to the size of the platelet seeds, and was in reverse proportion to the cube root of the platelet seed content. 相似文献
8.
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
9.
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
10.
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
11.
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
12.
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
13.
Shun J. Wu Lutgard C. De Jonghe Mohamed N. Rahaman 《Journal of the American Ceramic Society》1996,79(8):2207-2211
The sintering of ultrafine γ-Al2 O3 powder (particle size ∼10–20 nm) prepared by an inert gas condensation technique was investigated in air at a constant heating rate of 10°C/min. Qualitatively, the kinetics followed those of transition aluminas prepared by other methods. Measurable shrinkage commenced at ∼ 1000°C and showed a region of rapid sintering between ∼1125° and 1175°C followed by a transition to a much reduced sintering rate at higher temperatures. Starting from an initial density of ∼0.60 relative to the theoretical value, the powder compact reached a relative density of 0.82 after sintering to 1350°C. Compared to compacts prepared from the as-received powder, dispersion of the powder in water prior to compaction produced a drastic change in the microstructural evolution and a significant reduction in the densification rate during sintering. The incorporation of a step involving the rapid heating of the loose powder to ∼1300°C prior to compaction (which resulted in the transformation to α-Al2 O3 ) provided a method for significantly increasing the density during sintering. 相似文献
14.
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
15.
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
16.
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
17.
H. OKADA H. KAWAKAMI M. HASHIBA E. MIURA Y. NURISHI T. HIBINO 《Journal of the American Ceramic Society》1985,68(2):58-63
The rate of ZnA12 O4 formation for binary powder mixtures of ZnO and α-Al2 O3 (dense coarse particles and weak agglomerates of fine powder) fired in air or O2 atmospheres was measured and the microstructures of those systems were observed by scanning electron microscopy. With dispersed dense particles of α-Al2 O3 , the Al2 O3 surfaces were covered with ZnO and the spinel grew into the particles maintaining essentially a constant reaction interface area. Calculations based on geometric measurements and use of Jander's equation gave a similar high activation energy, 354 kJ/mol, which corresponds to the activation energy of volume diffusion of Zn2+ in ZnAl2 O4 . An oxygen atmosphere had no effect. With a matrix of fine α-Al2 O3 powder and dispersed granules of ZnO, a higher reaction rate occurred because of an increase in reaction interface area due to penetration of the powder compact matrix by ZnO vapor, which was enhanced by an O2 atmosphere. The reaction layer grew into the alumina matrix adjoining the ZnO granules with a parabolic rate law. Apparent activation energies below ∼200 kJ/mol were calculated. 相似文献
18.
KIICHIRO KAMATA TAKAO MOCHIZUKI SHIGEKI MATSUMOTO AKIFUMI YAMADA KIKUO MIYOKAWA 《Journal of the American Ceramic Society》1985,68(8):193-C
Fine A12 O3 powder was prepared by the gas-phase oxidation of aluminum acetyl-acetonate. The reaction products were amorphous material at 600° and 800°C, γ-Al2 O3 at 1000° and 1200°C, and δ-Al2 O3 at 1400°C. The powders consisted of spherical particles from 10 to 80 nm in diameter; particle size increased with increasing reaction temperature and concentration of chelate in the gas. 相似文献
19.
S. JILL GLASS PATRICK S. NICHOLSON C. BURTON CLARK A. H. HEUER 《Journal of the American Ceramic Society》1985,68(7):176-C
The interfacial reaction between Y2 O3 -partially-stabilized ZrO2 and α-Al2 O3 was studied. It was noted that α-Al2 O3 forms inside the periphery of the β-Al2 O3 grains; its formation suggests the loss of Na2 O from the p-Al2 O3 , either by evaporation or by dissolution in the ZrO2 matrix. The presence of Na2 ZrO3 is suspected. 相似文献
20.
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