共查询到20条相似文献,搜索用时 15 毫秒
1.
Zhen-Yan Deng Yu-Fu Liu Yoshihisa Tanaka Hong-Wang Zhang Jinhua Ye Yutaka Kagawa 《Journal of the American Ceramic Society》2005,88(10):2975-2977
The effect of temperature on the reaction of γ-Al2 O3 -modified Al powders with distilled water was investigated. It was found that by increasing the temperature up to 40°C, the hydrogen generation speed can be enhanced one to two orders of magnitude relative to that at room temperature (18°C). X-ray analyses and transmission electron microscopy observations revealed that the reaction by-product at 40°C is bayerite (Al(OH)3 ), which is chemically neutral. The present results imply that slightly increasing the temperature is an effective way to get the target hydrogen generation speed. 相似文献
2.
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
3.
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
4.
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
5.
Kiyoshi Okada Akiyoshi Hattori Taketoshi Taniguchi Akihiko Nukui Rathindra Nath Das 《Journal of the American Ceramic Society》2000,83(4):928-932
The effect on the γ-Al2 O3 -to-α-Al2 O3 phase transition of adding divalent cations was investigated by differential thermal analysis, X-ray diffractometry, and surface-area measurements. The cations, Cu2+ , Mn2+ , Co2+ , Ni2+ , Mg2+ , Ca2+ , Sr2+ , and Ba2+ , were added by impregnation, using the appropriate nitrate solution. These additives were classified into three groups, according to their effect: (1) those with an accelerating effect (Cu2+ and Mn2+ ), (2) those with little or no effect (Co2+ , Ni2+ , and Mg2+ ), and (3) those with a retarding effect (Ca2+ , Sr2+ , and Ba2+ ). The crystalline phase formed by reaction of the additive with γ-Al2 O3 at high temperature was a spinel-type structure in groups (1) and (2) and a magnetoplumbite-type structure in group (3). In groups (2) and (3), a clear relationship was found between the transition temperature and the difference in ionic radius of Al3+ and the additive (Δ r ): The transition temperature increased as Δ r increased. This result indicates that additives with larger ionic radii are more effective in suppressing the diffusion of Al3+ and O2− in γ-Al2 O3 , suppressing the grain growth of γ-Al2 O3 , and retarding the transformation into α-Al2 O3 . 相似文献
6.
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
7.
Marcia R. Gallas Bernard Hockey Alexander Pechenik Gasper J. Piermarini 《Journal of the American Ceramic Society》1994,77(8):2107-2112
The compaction and heat-treatment behavior of nanosize γ-Al2 O3 powder (average diameter = 20 nm) was studied. A diamond anvil high-pressure cell was used to compact the powder at pressures up to 3 GPa, both in air at room temperature and under liquid nitrogen, followed by pressureless heat treatment at 800°C. For all conditions studied, the fabricated compacts were optically transparent. X-ray diffraction confirmed retention of the γ-phase. The compacts were also characterized before and after heat treatment by microhardness measurements and by transmission electron microscopy. For both ambient and cryogenic compaction, sample hardness increased with pressure, and heat treatment resulted in about a 50% increase in hardness independent of the initial green-state value. Samples compacted in LN2 were significantly harder (up to 9.6 GPa) than those compacted in air. TEM examination revealed a random-dense-packed particle structure and interconnected porosity; interstitial void dimensions, however, were always less than the average particle diameter (20 nm). Observed effects on the increase in hardness could not be explained by microstructural changes normally attributed to increased compaction pressure or heat treatment, most notably densification. Alternative explanations are proposed. 相似文献
8.
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
9.
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
10.
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
11.
Oh-Hun Kwon C. Scott Nordahl Gary L. Messing 《Journal of the American Ceramic Society》1995,78(2):491-589
Seeding boehmite with α-Al2 O2 , followed by calcination at 600°C, results in an agglomerated alumina powder (<53 μm) that can be sinter forged to full density at 1250°C. Compressive strains as high as ɛx =−0.9, and radial flow (ɛx = 1.0) during sinter forging remove large, interagglomerate pores. The fully dense alumina has a grain size of 0.4 pm and is visually transparent. It is proposed that deformation of dense agglomerates is the primary mecha- nism responsible for large pore elimination and compact densification. The sinter forging of sol-gel-derived alumina powders offers a new technology to prepare highly transparent, optical ceramics at lower temperatures than conventional routes. 相似文献
12.
13.
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
14.
15.
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
16.
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
17.
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
18.
A water-based binder system consisting of colloidal boehmite (AlO(OH)) and 0.3 wt% poly(vinyl alcohol) was developed for low-pressure piston-type extrusion of alumina. Batches using this binder system and two different types of commerical alumina powder were characterized with regard to extrusion pressure, densification, and physical properties in the green and fired states. 相似文献
19.
Titanium-doped α-Al2 O3 exhibits a high-temperature conductivity which is ionic at high oxygen pressures and electronic at low oxygen pressures. Both are isotropic. The temperature dependence of conductivity under conditions where equilibrium with the atmosphere is not maintained indicates both the position of the energy level of titanium (TiAl x ) in the forbidden gap and the temperature dependence of the mobility of the native ionic defects (Al vacancies, V Al m ). Optical absorption responsible for the pink color of the reduced crystals is measured as a function of p o2 and is used to determine concentrations of Ti3+ and Ti4+ . Parameters for the equilibrium constants of the reactions involving electrons by which the composition of Al2 O3 :Ti and undoped Al2 O3 is varied are determined. The chemical diffusion data by Jones et al. are described quantitatively. 相似文献