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1.
Akshoy Kr. Chakraborty 《Journal of the American Ceramic Society》2005,88(1):134-140
Mechanical mixture of γ-Al2 O3 and amorphous SiO2 , and diphasic Al2 O3 /SiO2 gels of three different compositions were synthesized. They were subjected to heat treatment to various temperatures in the range 900°–1600°C. Qualitative X-ray diffraction data show that these diphasic gels do not crystallize to a combined mixture of θ-Al2 O3 and α-Al2 O3 polymorphs at the intermediate stage, prior to mullite formation. Estimated mullite formation data show that the course of its formation from mixed oxides was different from that of diphasic gels. Results are compared with previous findings and the concept of Al–Si spinel formation in the phase transformation of stoichiometric diphasic gel system is substantiated. 相似文献
2.
Stoichiometric mullite (71.38 wt% Al2 O3 -28.17 wt% SiO2 ) and 80 wt% Al2 O3 -20 wt% SiO2 gels were prepared by the single-phase and/or diphasic routes. Dense sintered bodies were prepared from both sets of gels in the Al2 O3 -SiO2 system. Apparent densities of 96% and 97% of theoretical density were measured for the diphasic (using two sols) mullite samples sintered at 1200° and 1300°C for 100 min, respectively; this compared with 85% and 94% for the single-phase xerogels under the same conditions, and to much lower values for mullite prepared from conventional mixed powders. The microstructure of the mullite pellets from diphasic xerogel precursors is also considerably finer. 相似文献
3.
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
4.
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
5.
P. L. ROEDER F. P. GLASSER† E. F. OSBORN 《Journal of the American Ceramic Society》1968,51(10):585-593
Liquidus phase equilibrium data are presented for the system Al2 O3 -Cr2 O3 -SiO2 . The liquidus diagram is dominated by a large, high-temperature, two-liquid region overlying the primary phase field of corundum solid solution. Other important features are a narrow field for mullite solid solution, a very small cristobalite field, and a ternary eutectic at 1580°C. The eutectic liquid (6Al2 O3 -ICr2 O3 -93SiO2 ) coexists with a mullite solid solution (61Al2 O3 -10Cr2 O3 -29SiO2 ), a corundum solid solution (19Al2 O3 -81Cr2 O3 ), and cristobalite (SO2 ). Diagrams are presented to show courses of fractional crystallization, courses of equilibrium crystallization, and phase relations on isothermal planes at 1800°, 1700°, and 1575°C. Tie lines were sketched to indicate the composition of coexisting mullite and corundum solid solution phases. 相似文献
6.
Fine-particle beta sodium ferrite (β-NaFeO2 ), rather than α-Fe2 O3 , may be responsible for superparamagnetic behavior in a glass of composition (in mole fractions) 0.37Na2 O-0.26Fe2 O3 -0.37SiO2 . The 700°C isothermal section of the phase diagram of the Na2 O-Fe2 O3 -SiO2 system is given, showing a three-phase field bounded by Na2 SiO3 -NaFeO2 -Fe2 O3 ; there is no evidence for the existence (at 700°C) of compounds of molar composition 6Na2 O-4Fe2 O3 -5SiO2 or 2Na2 O-Fe2 O3 -SiO2 . The Moessbauer spectrum of β-NaFeO2 has an internal magnetic field of 487 kOe at room temperature. 相似文献
7.
Akshoy Kumar Chakravorty Dilip Kumar Ghosh 《Journal of the American Ceramic Society》1987,70(3):46-C-
The independent crystallization sequence of an Al2 O3 component is modified in the presence of SiO2 and vice versa. Mixed SiO2 -Al2 O3 , gel (28 wt% SiO2 and 72 wt% Al2 O3 ) forms neither cristobalite nor γ-Al2 O3 and corundum at 1000°C but forms Si-Al spinel; an amorphous aluminosilicate phase invariably also forms after the gel is heated. However, the composition of this amorphous aluminosilicate phase is not as yet known. 相似文献
8.
Koji Tsukuma 《Journal of the American Ceramic Society》2000,83(12):3219-3221
Composites of β-Ce2 O3 ·11Al2 O3 and tetragonal ZrO2 were fabricated by a reductive atmosphere sintering of mixed powders of CeO2 , ZrO2 (2 mol% Y2 O3 ), and Al2 O3 . The composites had microstructures composed of elongated grains of β-Ce2 O3 ·11Al2 O3 in a Y-TZP matrix. The β-Ce2 O3 ·11Al2 O3 decomposed to α-Al2 O3 and CeO2 by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce2 O3 ·11Al2 O3 divided into several grains of α-Al2 O3 and ZrO2 doped with Y2 O3 and CeO2 . High-temperature bending strength of the oxygen-annealed α-Al2 O3 composite was comparable to the β-Ce2 O3 ·11Al2 O3 composite before annealing. 相似文献
9.
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
10.
The cell dimensions of pure triclinic 3CaO·SiO2 and monoclinic 3CaO·SiO2 solid solution (54CaO·16SiO2 ·Al2 O3 ·MgO) were determined and the powder diffraction patterns were indexed by the method of precise measurement of the spacings. The lattice constants are expressed in terms of triclinic or monoclinic cells corresponding to pseudo-orthorhombic cells derived from Jeffery's trigonal cell. The apparent lattice constants for pure 3CaO·SiO2 are a = 12.195 a.u., b = 7.104 au., c = 25.096 a.u., α= 90°, β= 89°44'γ= 89°44'; for 54CaO·16SiO2 .-Al2 O3 MgO, a = 12.246 a.u., b = 7.045 a.u., c = 24.985 a.u., β= 90°04'. Precise lattice constants of Jeffery's monoclinic lattice for 54CaO.-16SiO2 -Al2 O3 ·MgO are derived as a = 33.091 a.u., b = 7.045 a.u., c = 18.546 a.u., β= 94°08'. High-temperature X-ray patterns showed that pure triclinic 3CaO·SiO2 transformed to a monoclinic form at about 920°C. and then to a trigonal form at about 970°C. Monoclinic 54CaO.16SiO2 ·Al2 O3 –MgO transformed to trigonal at about 830°C. These transitions were reversible and reproducible and were accompanied by only slight deformation of the structure forms. 相似文献
11.
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
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.
The system Na3 AlF6 -AlF3 -Al2 O3 was investigated by a combination of quenching, optical microscopy, and X-ray powder diffraction techniques in order to define liquidus temperatures, univariant lines, and invariant points. Phase fields for the primary crystallization of cryolite, chiolite, aluminum fluoride, α-alumina, and η-alumina were located. A ternary peritectic point contained 28.3% AlF3 -4.4% Al2 O3 -67.3% Na3 AlF6 at 723°C. A eutectic point of composition 37.3% AlF3 -3.2% Al2 O3 -59.5% Na3 AlF6 occurs at 684°C. 相似文献
14.
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
15.
Polycrystalline Al2 O3 was chemically vapor-deposited onto sintered Al2 O3 substrates by reaction of AlCl3 with (1) H2 O, (2) CO:H2 , and (3) O2 at 1000° and 1500°C and 0.5 and 5.0 torr. Although the thermodynamics of all these reactions predict the formation of solid Al2 O3 , the deposition rate of the first reaction was considerably greater than that of the second. The third reaction was so slow that no measurable deposit was formed in 6 h at 1500°C. Formation of dense deposits of α-Al2 O3 was favored by increasing temperature and decreasing pressure. Microstructural examination of the dense deposits showed long columnar grains, the largest of which extended through the deposit from the substrate to the surface. 相似文献
16.
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
17.
WILLIAM C. HAGEL P. J. JORGENSEN D. S. TOMALIN 《Journal of the American Ceramic Society》1966,49(1):23-26
Since the difference between oxygen-ion and cation diffusion coefficients is greater for α-Cr2 O3 than for α-Fe2 O3 or α-Al2 O3 , a study of initial-sintering kinetics was undertaken to show unequivocally which species is rate controlling. Fine powders of α-Cr2 O3 , obtained by thermal decomposition of reagent-grade (NH4 )2 Cr2 O7 , were lightly compacted and their isothermal rates of shrinkage were determined between 1050° and 1300°C. Resultant data follow volume-diffusion sintering models, and calculated diffusion coefficients agree with, those measured for oxygen ions in α-Cr2 O3 . There is little evidence that oxygen diffusion along grain boundaries becomes so enhanced that chromium ions are left in control of the process. 相似文献
18.
Osamu Yamaguchi Masakazu Shirai Masaru Yoshinaka 《Journal of the American Ceramic Society》1988,71(12):510-C
In the system ZrO2 -Al2 O3 , cubic ZrO2 solid solutions containing up to 40 mol% Al2 O3 crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminum alkoxides. The values of the lattice parameter, a, increase linearly from 0.5095 to 0.5129 nm with increasing Al2 O3 content. At higher temperatures, the solid solutions transform into tetragonal ZrO2 and α-Al2 O3 . Pure ZrO2 crystallizes in the tetragonal form at 415° to 440°C. 相似文献
19.
In the system TiO2 —Al2 O3 , TiO2 (anatase, tetragonal) solid solutions crystallize at low temperatures (with up to ∼ 22 mol% Al2 O3 ) from amorphous materials prepared by the simultaneous hydrolysis of titanium and aluminum alkoxides. The lattice parameter a is relatively constant regardless of composition, whereas parameter c decreases linearly with increasing Al2 O3 . At higher temperatures, anatase solid solutions transform into TiO2 (rutile) with the formation of α-Al2 O3 . Powder characterization is studied. Pure anatase crystallizes at 220° to 360°C, and the anatase-to-rutile phase transformation occurs at 770° to 850°C. 相似文献
20.
Subsolidus equilibrium relations in a portion of the system Li2O-Fe2 O3 -Al2 O3 in the temperature range 500° to 1400°C. have been determined near po2 = 0.21. Of particular interest in this system is the LiFe5 O8 -LiAl5 O8 join, which shows complete solid solution above 1180°C. Below this temperature the solid solution exsolves into two spinel phases. At 600°C. approximately 15 mole % of each compound is soluble in the other. The high-temperature solid solution and the low-temperature exsolution dome extend into the ternary system from the 1:5 join. There is no appreciable crystalline solubility of LiFeO2 or of α-Fe2 O3 in LiFe5 O8 . An attempt to confirm HFe5 O8 as the correct formulation of the magnetic ferric oxide "γ-Fe2 O3 " was inconclusive, but in the absence of positive evidence, the retention of γ-Fe2 O3 is recommended. All the metallic oxides of the Group IV elements increase the temperature of the monotropic conversion of -γ-Fe2 O3 to α-Fe2 O3 . Silica and thoria have a greater effect on this conversion than does titania or zirconia. 相似文献