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1.
WOLFGANG A. KAYSSER MARIANNE SPRISSLER CAROL A. HANDWERKER JOHN E. BLENDELL 《Journal of the American Ceramic Society》1987,70(5):339-343
In this investigation we have studied how the presence of a liquid phase affects the grain morphology and grain growth kinetics in Al2 O3 at 1800°C using the growth of both matrix grains and large spherical single-crystal seeds growing into the matrix. The growth rates of the matrix grains were found to decrease in the following order: undoped Al2 O3 , AI2 O3 with anorthite, AI2 O3 with anorthite and MgO, and Al2 O3 with MgO. Except for the samples doped with MgO alone, the matrix grains were faceted and appeared tabular in polished sections. In samples containing anorthite both with and without MgO, the single-crystal seeds exhibit basal facets with continuous liquid films and slow growth in the 〈0001〉 relative to all other crystallographic directions. When only MgO is added, the growth of the single-crystal seeds was not isotropic; however, no faceting was observed. We discuss how anisotropic growth rates caused by the anorthite additions can stimulate discontinuous grain growth in Al2 O3 . 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.
An anhydrous alumina (Al2 O3 ) sol was prepared from aluminum isopropoxide and an organic solvent, using an acetic acid stabilizer. The complete conversion of the dried sol to α-Al2 O3 was accomplished at a temperature of 950°C by a single transition via γ-Al2 O3 . Al2 O3 that was deposited via dip coating resulted in amorphous films, even after annealing at 1100°C, because of the silicon diffusion from the substrate. This phenomenon was avoided using a rapid thermal treatment in a flame after dip coating, which resulted in uniform thin films that are converted to α-Al2 O3 via heat treatment. 相似文献
4.
Silicon carbide (SiC) porous substrates, containing alumina (Al2 O3 ) dopant levels of 3, 5, and 8 wt%, are prepared by slip casting and sintering in the temperature range of 1450°–1800°C. The linear shrinkage, bulk density, and pore size of the sintered substrate increase as the sintering temperature and the amount of dopant increase. A large amount of β-phase SiC is transformed to α-phase SiC if the dopant concentration is 5 or 8 wt%. The flexural strength of the substrate doped with 8 wt% Al2 O3 is higher than that of the substrate doped with 3 wt% Al2 O3 ; however, the Weibull modulus of the former is lower. SiC composite membranes of improved selectivity and strength are fabricated by coating the porous substrate with layers of lower Al2 O3 contents at lower sintering temperatures. 相似文献
5.
High-purity aluminum foil was used to join alumina substrates directly in air at temperatures ranging from 800° to 1200°C and soak times of 1–100 h. It was found that the bend strengths of the resulting Al2 O3 /Al/Al2 O3 joints generally increase with increasing brazing temperature and time, with a maximum bend strength of 135 MPa on average achieved in samples joined at 1200°C for 100 h. Additionally it was determined that measurable ductility is retained in the joint even after exposure under extended high-temperature conditions. During joining, an Al2 O3 scale forms along the interface between the aluminum and adjacent substrates. An increase in brazing temperature and/or time leads to intergrowth and sintering between this thermally grown oxide layer and the substrate surface, which appears to be the primary source of improved joint strength. Fracture analysis indicates that the Al2 O3 /Al/Al2 O3 joints generally fail via one of three mechanisms, (1) by de-bonding along the foil/substrate interface in specimens that were joined at low temperature or held at temperature for an insufficient period of time; (2) by ductile rupture in specimens that were joined at conditions that promoted sintering between the oxidized foil and adjacent alumina faying surfaces, but left behind a continuous residual aluminum layer within the joint; or (3) by mixed-mode fracture in specimens joined at high temperature and long exposure times, in which the thermally grown alumina that forms between the two substrates is interrupted by dispersed pockets of residual aluminum metal. 相似文献
6.
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
7.
Deok-Yong Park Jenn-Ming Yang Joseph M. Collins 《Journal of the American Ceramic Society》2001,84(12):2991-2996
Coarsening of the fine lamellar structure of a directionally solidified Y3 Al5 O12 (yttrium aluminum garnet, YAG)/Al2 O3 eutectic fiber at elevated temperatures was investigated. The fibers were grown continuously by an edge-defined film-fed growth (EFG) technique. To study the thermal stability of the lamellar structure, the fibers were heat-treated in air at 1360°–1460°C for up to 200 h. X-ray diffractometry and scanning electron microscopy were used to characterize the microstructures of the fibers. Image analysis was used to measure the length of the interface line between Y3 Al5 O12 and Al2 O3 phases. The kinetics of coarsening and the rate-controlling mechanisms were investigated. Also, the Graham and Kraft model for describing the coarsening behavior of the lamellar Al-CuAl2 eutectic alloy was used to explain the coarsening behavior of Y3 Al5 O12 /Al2 O3 eutectic fiber. 相似文献
8.
The crystallization of Al2 O3 -rich glasses in the system SiO2 -Al2 O3 which were prepared by flame-spraying and/or splat-cooling was studied by DTA, electron microscopy, and X-ray diffraction. Over a wide range of compositions, the crystallization temperature ( Tx ) remained near 1000°C, changing smoothly with composition. In all cases crystallization of mullite was detected by X-ray diffraction. In the low-Al2 O3 region, coarsening of the microstructure during crystallization was observed by electron microscopy. In the high-Al2 O3 region mullite and γ-Al2 O3 cocrystallized; this behavior may be interpreted as evidence of a cooperative process of crystallization at the respective Tx 's. The crystallite size of the mullite immediately after rapid crystallization increased continuously with increasing Al2 O3 content. In light of the Tx data, the adequacy of the evidence for the proposed metastable miscibility gap in the SiO2 -Al2 O3 system is questioned. 相似文献
9.
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
10.
Continuous α-Fe2 O3 films grown on bulk (0001)Al2 O2 substrates by low-pressure chemical vapor deposition have been studied by transmission electron microscopy and the observations compared to those obtained from discontinuous films at an earlier stage of the growth process. Plan-view specimens revealed significant thermal stress in the continuous films, while cross-sectional specimens showed that cracking occurs in thicker films. The free surface of the film and the film/substrate interface appeared sharp and flat, apart from growth ledges and steps. Weak-beam imaging revealed a hexagonal misfit dislocation network consisting of perfect edge dislocations. Fine structure in the selected-area diffraction patterns which corroborates these observations is also discussed. The misfit network of partial dislocations previously observed in the discontinuous films was not observed for the continuous films, indicating an effect of film thickness, growth rate, or surface preparation on the Fe2 O3 /(0001)Al2 O3 interface structure. 相似文献
11.
Andrey V. Zagrebelny Erica T. Lilleodden William W. Gerberich C. Barry Carter 《Journal of the American Ceramic Society》1999,82(7):1803-1808
The deformation of thin layers of glass on crystalline materials has been examined using newly developed experimental methods for nanomechanical testing. Continuous films of anorthite (CaAl2 Si2 O8 ) were deposited onto Al2 O3 surfaces by pulsed-laser deposition. Mechanical properties such as Young's modulus and hardness were probed with a high-resolution depth-sensing indentation instrument. Nanomechanical testing, combined with AFM in situ imaging of the deformed regions, allowed force-displacement measurements and imaging of the same regions of the specimen before and immediately after indentation. This new technique eliminates any uncertainty in locating the indentation after unloading. Emphasis has been placed on examining how the Al2 O3 substrate crystallographic orientation will affect mechanical composite response of silicate-glass film/Al2 O3 system. 相似文献
12.
Koji Takahashi Masahiro Yokouchi Sang-Kee Lee Kotoji Ando 《Journal of the American Ceramic Society》2003,86(12):2143-2147
Al2 O3 reinforced by SiC whiskers (Al2 O3 /SiC-W) was hot-pressed to investigate the crack-healing behavior. Semielliptical surface cracks of 100 μm in surface length were introduced using a Vickers indenter. The specimens containing precracks were crack-healed at temperatures between 1000° and 1300°C for 1 h in air, and their strengths were measured by three-point bending tests at room temperature and elevated temperatures between 400° and 1300°C. The results show that Al2 O3 /SiC-W possesses considerable crack-healing ability. The surface cracks with length of 2 c = 100 μm could be healed by crack-healing at 1200° or 1300°C for 1 h in air. Fracture toughness of the material was also determined. As expected, the SiC whiskers made their Al2 O3 tougher. 相似文献
13.
The phase diagram of the Al2 O3 –ZrO2 –Sm2 O3 system was constructed in the temperature range 1250°–2800°C. The phase transformations in the system are completed in eutectic reactions. No ternary compounds or regions of appreciable solid solution were found in the components or binaries in this ternary system. Two new ternary and one new binary eutectics were found. The minimum melting temperature is 1680°C and it corresponds to the ternary eutectic Al2 O3 +F-ZrO2 +SmAlO3 . The solidus surface projection, the schematic of the alloy crystallization path, and the vertical sections present the complete phase diagram of the Al2 O3 –ZrO2 –Sm2 O3 system. 相似文献
14.
Glasses were prepared from sintered powders ofSi3 N4 , Al2 O3 , Y2 O3 AlN, andSiO2 to study their crystallization on subsequent heat treatment. Appreciable crystallization was efected only after the glasses were doped with up to 5 wt% ZrO2 . Electron microscopy and microanalysis showed that the crystalline phase was Y2 O3 ·2SiO2 without detectable Zr. The sofening temperature is in the range 850° to 1020°C. In-situ heating in a high-voltage electron microscope at ∼12OO°C produced renucleation and growth of the crystalline phase; at higher temperatures, however, the glass phase volatilized. 相似文献
15.
Kenneth H. Sandhage Hans J. Schmutzler Robert Wheeler Hamish L. Fraser 《Journal of the American Ceramic Society》1996,79(7):1839-1850
Metallic Ba-Al-Si bonding agents have been used to produce all-ceramic, BaO-Al2 O3 -SiO2 bonds between plates of mullite (Al6 Si2 O13 ). Ba-Al-Si tapes (200 (μm thick) were fabricated by compaction and rolling of mechanically alloyed powder. The Ba-Al-Si tapes were placed between mullite plates and then oxidized by heating to a peak temperature of 1230°C in air. The oxidized tapes strongly adhered to the mullite plates at 25° and 1000°C, as indicated by the fracture morphologies obtained from compressive shear tests. Electron microscopy (EPMA, TEM) revealed that the bulk of the oxidized Ba-Al-Si tapes (away from the interfaces with mullite) consisted largely of the compound BaAl2 Si2 O8 , along with some BaSiO3 and an amorphous, barium-rich aluminosilicate. The interface between the oxidized bonding agent and bulk mullite consisted of a mixture of BaAl2 Si2 O8 , Al6 Si2 O13 , A12 O3 , BaAl2 O4 , and an amorphous, barium-bearing aluminosilicate. 相似文献
16.
Alessandro Martucci Sabrina Sartori Alessandro Muffato Lidia Armelao Elisabetta Di Bartolomeo Enrico Traversa Massimo Guglielmi 《Journal of the American Ceramic Society》2003,86(11):1965-1968
Na2 O· x Al2 O3 ( x = 9, 11)films have been obtained by sol–gel method. Crystallization processes during heat treatments have been investigated by X–ray diffraction analysis. A metastable phase with the mullite structure, λ–Na2 O· x Al2 O3 , has been observed starting from 800°C. Films remained stable after a heat treatment at 1000°C for 250 h. Impedance spectroscopy measurements showed that the films of λ-Na2 O· x Al2 O3 possess a large three–dimensional ionic conductivity at 400°C. 相似文献
17.
Byong-Taek Lee Rajat Kanti Paul Hyun-Jin Hong Kap-Ho Lee 《Journal of the American Ceramic Society》2009,92(4):911-915
Using a multipass extrusion process, continuous porous Al2 O3 body (∼41% porosity) was produced and used as a substrate to fabricate continuous porous TiO2 /Al2 O3 composite membrane. The diameter of the continuous pores of the porous Al2 O3 body was about 150 μm. The TiO2 nanopowders dip coated on the continuous pore-surface Al2 O3 body existed as rutile and anatase phases after calcination at 520°C in air. However, after aging of the fabricated continuous porous TiO2 /Al2 O3 composite membrane in 20% NaOH at 60°C for 24 h, a large number of TiO2 fibers frequently observed on the pore surface. The diameter of the TiO2 fibers was about 150 nm having a high specific surface area. However, after 48-h aging period, the diameter of the TiO2 fibers increased, which was about 3 μm. Most of the TiO2 fibers had polycrystalline structure having nanosized rutile and anatase crystals of about 20 nm. 相似文献
18.
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
19.
Robert G. Hill Christopher Goat David Wood 《Journal of the American Ceramic Society》1992,75(4):778-785
A SiO2 ─Al2 O3 ─CaO─CaF2 ionomer glass was investigated using thermal analysis, X-ray diffraction, and scanning electron microscopy. The purpose of this investigation was to control the susceptibility of the glass to acid attack. The differential thermal analysis trace exhibited a sharp glass transition at about 645°C and two exotherms. The first exotherm corresponded to liquid–liquid phase separation followed by crystallization of fluorite. The second, much larger, exotherm was the result of crystallization of the remaining glass phase to form anorthite. Prolonged heat treatment below the glass-transition temperature demonstrated that crystallization of fluorite can occur without prior liquid–liquid phase separation. 相似文献
20.
Manuel Jiménez-Melendo Carolina Clauss Arturo Donguez-Rodguez Antonio J. Sánchez-Herencia José S. Moya 《Journal of the American Ceramic Society》1997,80(8):2126-2130
Layered composites of alternate layers of pure Al2 O3 (thickness of 125 μ m) and 85 vol% Al2 O3 -15 vol% ZrO2 that was stabilized with 3 mol% Y2 O3 (thickness of 400 μ m) were obtained by sequential slip casting and then fired at either 1550° or 1700°C. Constant-strain-rate tests were conducted on these materials in air at 1400°C at an initial strain rate of 2 × 10-5 s-1 . The load axis was applied both parallel and perpendicular to the layer interfaces. Catastrophic failure occurred for the composite that was fired at 1700°C, because of the coalescence of cavities that had developed in grain boundaries of the Al2 O3 layers. In comparison, the composite that was fired at 1550°C demonstrated the ductility of the Al2 O3 +YTZP layer, but at a flow stress level that was determined by the Al2 O3 layer. 相似文献