共查询到20条相似文献,搜索用时 31 毫秒
1.
James E. Pitchford Robert J. Stearn Anthony Kelly William J. Clegg 《Journal of the American Ceramic Society》2001,84(5):1167-1168
The structure of mullite, which has a composition ranging from 3Al2 O3 ·2SiO2 to Al2 O3 ·2SiO2 , contains ordered oxygen vacancies. Sillimanite, Al2 O3 ·SiO2 , has a similar structure but with no vacancies. The indentation hardness of polycrystalline mullite (3Al2 O3 ·2SiO2 ) was measured from room temperature up to 1400°C and compared with that of single-crystal sillimanite (Al2 O3 ·SiO2 ) up to 1300°C. It was found that both materials show the same variation in hardness with temperature, suggesting that the structures have a similar resistance to plastic deformation, and therefore that the oxygen vacancies in the mullite structure are not the primary cause of mullite's resistance to high-temperature deformation. 相似文献
2.
Masayuki Imose Yoshihiko Takano Masaru Yoshinaka Ken Hirota Osamu Yamaguchi 《Journal of the American Ceramic Society》1998,81(6):1537-1540
Boehmite (AlO(OH)) solid-solution gel, which yields stoichiometric mullite (3Al2 O3 2SiO2 ) at high temperatures, has been prepared by the hydrazine method. The formation process leading to 3Al2 O3 2SiO2 is discussed. The as-prepared powder and powders heated below 1200°C consist of very fine particles showing needlelike morphology, whereas the particles of mullite powder show thin prismatic morphology. The mullite powder after heating at 1300°C has a high surface area (87 m2 /g). 相似文献
3.
Shuqiang Ding Yu-Ping Zeng Dongliang Jiang 《Journal of the American Ceramic Society》2007,90(7):2276-2279
Porous mullite (3Al2 O3 ·2SiO2 ) ceramics with an open porosity up to 92.9% were fabricated by a gel freeze-drying process. An alumina (Al2 O3 ) gel mixed with ultrafine silica (SiO2 ) was frozen and sublimation of ice crystals was carried out by drying the frozen body under a low pressure. Porous mullite ceramics were prepared in air at 1400°–1600°C due to the mullitization between Al2 O3 and SiO2 . A complex and porous microstructure was formed, where large dentritic pores with a pore size of ∼100 μm contained small cellular pores of 1–10 μm on their internal walls. Owing to the complete mullitization, a relatively high-compressive strength of 1.52 MPa was obtained at an open porosity of 88.6%. 相似文献
4.
Joaquín Aguilar-Santillán Ricardo Cuenca-Alvarez Heberto Balmori-Ramírez Richard C. Bradt 《Journal of the American Ceramic Society》2002,85(10):2425-2431
The influence of attrition milling on the thermal decomposition of kyanite (Al2 O3 ·SiO2 ) to mullite (3Al2 O3 ·2SiO2 ) and SiO2 , and its subsequent sintering, was studied. A commercial kyanite was attrition-milled for times up to 12 h. Dilatometry confirmed that as-received unmilled kyanite decomposes between 1300° and 1435°C. The decomposition reaction is slow initially and accelerates during the later stages until about one-half of the decomposition occurs in the last 35°C. For the attrition-milled kyanite, the onset decomposition temperature decreases, the transformation temperature interval is reduced, and both the decomposition reaction and subsequent sintering are accelerated. A dense microstructure of fine equiaxed mullite grains in the 1 μm size range, evenly dispersed in a glassy matrix, is obtained by sintering the attrition-milled kyanites. These results are explained in terms of the energy accumulated during attrition milling, a reduction of the milled kyanite particle size, and the presence of a liquid phase during sintering. 相似文献
5.
M. G. M. U. Ismail Zenjiro Nakai Shigeyuki Smiya 《Journal of the American Ceramic Society》1987,70(1):7-C-8
Mullite (3Al2 O3 ·2SiO2 ) of stoichiometric composition was prepared by mixing boehmite sol and silica dispersion and gelling at a pH of 3. Complete mullitization takes place at or above 1300°C. Ultrafine mullite powder prepared by calcining gel at 1400°C and attrition milling could be sintered to >98% (theoretical density) at 1650°C for 1.5 h. The flexural strength of the sintered body at room temperature was 405 MPa and 350 MPa at 1300°C. Only traces of a secondary phase were observed along the grain boundary. 相似文献
6.
Suxing Wu Helen M. Chan Martin P. Harmer 《Journal of the American Ceramic Society》1997,80(6):1579-1582
Mullite and mullite/ZrO2 ceramics were fabricated starting from Si/Al2 O3 and Si/Al2 O3 /ZrO2 powder mixtures, which were mixed and attrition milled with TZP balls in water. Isopressed powder compacts were subjected to a heat treatment in air, during which the Si was oxidized to SiO2 . At } 1410°C, reaction between Al2 O3 and SiO2 occurred, resulting in mullite (3Al2 O3 ·2SiO2 ). Depending on the composition of the starting powders, the end product was either single-phase mullite or a mullite composite. The reaction process was monitored by thermogravimetry and dilatometry. It was found that the microstructure and mechanical properties of the reaction-formed mullite ceramics were significantly improved by ZrO2 additions. 相似文献
7.
Luis A. M. Scudeller Elson Longo Jose A. Varela 《Journal of the American Ceramic Society》1990,73(5):1413-1416
A graphite chamber was used for the reaction between samples of 45 or 55 wt% alumina and a mixture of metallurgical coke and potassium carbonate. Thermal treatments were conducted at 1000°C. The results suggest that the potassium attack in silica-alumina bricks is controlled by the following reactions: K2 O + SiO2 → K2 O → SiO2 in the glassy matrix; 3(K2 O · 2SiO2 ) + 3Al2 O3 → 2SiO2 · 3(K2 O · Al2 O3 · 2SiO2 ) + 2SiO2 for short times; and K2 O → Al2 O3 · 2SiO2 + 2SiO2 · K2 O · Al2 O3 · 4SiO2 for long times. In 55 wt% alumina bricks containing corundum and tridymite, potassium also attacks those phases forming a glassy phase. The formation of kaliophilite at the matrix/mullite grain interface causes a volumetric expansion of 55.5%, resulting in cracks in the matrix. Because the kaliophilite phase is not in equilibrion with mullite, the former will react with free silica to form leucite that is more thermodynamically stable. 相似文献
8.
Kang N. Lee Dennis S. Fox Jeffrey I. Eldridge Dongming Zhu Raymond C. Robinson Narottam P. Bansal Robert A. Miller 《Journal of the American Ceramic Society》2003,86(8):1299-1306
Current state-of-the-art environmental barrier coatings (EBCs) for Si-based ceramics consist of three layers: a silicon bond coat, an intermediate mullite (3Al2 O3 ·2SiO2 ) or mullite + BSAS ((1− x )BaO· x SrO·Al2 O3 ·2SiO2 , 0 ≤ x ≤ 1) layer, and a BSAS top coat. Areas of concern for long-term durability are environmental durability, chemical compatibility, volatility, phase stability, and thermal conductivity. Variants of this family of EBC were applied onto monolithic SiC and melt-infiltrated SiC/SiC composites. Reaction between BSAS and silica results in a low-melting (∼1300°C) glass, which can cause the spallation of the EBC. At temperatures greater than ∼1400°C BSAS suffers significant recession via volatilization in water-vapor-containing atmospheres. Both reactions can be EBC life-limiting factors. BSAS undergoes a very sluggish phase transformation (hexagonal celsian to monoclinic celsian), the implications of which are not fully understood at this point. Initial rapid increase in thermal conductivity at temperatures as low as 1300°C indicates the sintering of EBC. 相似文献
9.
Yoshihiro Hirata Kazuyoshi Sakeda Yumi Matsushita Kinji Shimada Yoshimi Ishihara 《Journal of the American Ceramic Society》1989,72(6):995-1002
Submicrometer SiO2 -Al2 O3 powders with compositions of 46.5 to 76.6 wt% Al2 O3 were prepared by hydrolysis of mixed alkoxides. Phase change, mullite composition, and particle size of powders with heating were analyzed by DTA, XRD, IR, BET, and TEM. As-produced amorphous powders partially transformed to mullite and Al-Si spinel at around 980°C. The compositions of mullite produced at 1400° and 1550°C were richer in Al2 O3 than the compositions of stable mullite solid solutions predicted from the phase diagram of the SiO2 -Al2 O3 system. Particle size decreased with increasing Al2 O3 content. The sintered densities depended upon the amount of SiO2 -rich glassy phase formed during sintering and the green density expressed as a function of particle size. 相似文献
10.
A process for production of near-net-shape mullite-matrix ceramic composites at ≤1300°C has been achieved by reaction-bonding Al2 O3 , silicon, mullite seeds, and eutectics of Al2 O3 –SiO2 –mixed-rare-earth oxide. The fusion temperature of the eutectic composition utilized is 1175°C. This liquid phase facilitates silicon oxidation, mullitization, and sintering. Mullite phase develops with low residual Al2 O3 when 7.5 wt% mixed-rare-earth oxide and 5 wt% mullite seeds are used. The final sinter is >90% of theoretical density, >90% mullite (by quantitative XRD), and suffers 2.2% sintering shrinkage. 相似文献
11.
Sample disks prepared from Al2 O3 (61 wt%), SiO2 (28 wt%), and Fe2 O3 (II wt%) powders were sintered at 1270° and 1440°C and then annealed between 1300° and 1670°C. The annealed samples consisted of mullite as the main compound with minor amounts of glass and sometimes magnetite. The iron content of the mullites decreases strongly from ∼ 10.5 wt% Fe2 O3 at 1300°C to ∼ 2.5 wt% Fe2 O3 at 1670°C. A complex temperature-controlled exsolution mechanism of iron from mullite is considered. 相似文献
12.
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
13.
Marion Bartsch Bilge Saruhan Martin Schmücker Hartmut Schneider 《Journal of the American Ceramic Society》1999,82(6):1388-1392
Dense mullite ceramics were successfully produced at temperatures below 1300°C from amorphous SiO2 -coated gamma-Al2 O3 particle nanocomposites (AS-gammaA). This method reduces processing temperatures by similar/congruent300°C or more with respect to amorphous SiO2 -coated alpha-Al2 O3 particle microcomposites (AS-alphaA) and to other Al2 O3 -SiO2 reaction couples. The good densification behavior and the relatively low mullite formation temperature make AS-gammaA nanocomposites an excellent matrix raw material for polycrystalline aluminosilicate fiber-reinforced mullite composites. 相似文献
14.
James W. Palko Ali Sayir Stanislav V. Sinogeikin Waltraud M. Kriven Jay D. Bass 《Journal of the American Ceramic Society》2002,85(8):2005-2012
Directionally solidified mullite fibers have been grown by the laser-heated, float-zone method from starting materials with a nominal composition of 3Al2 O3 ·2SiO2 . The fibers used in this study have large single-crystal regions with composition 2.5Al2 O3 ·SiO2 and (001) fiber axis orientation. The complete elastic tensor of these samples has been determined by Brillouin spectroscopy at room temperature and elevated temperatures up to 1200°C. Isotropic moduli (bulk, shear, and Young's) have been calculated using the Voigt–Reuss–Hill averaging scheme. The room-temperature values obtained are K VRH = 173.5 ± 6.9 GPa, G VRH = 88.0 ± 3.5 GPa, E VRH = 225.9 ± 9.0 GPa. All moduli show gradual, linear decreases with temperature. The temperature derivatives obtained for the equivalent, isotropic moduli are d K VRH /d T =−17.5 ± 2.5 MPa/°C, d G VRH /d T =−8.8 ± 1.4 MPa/°C, d E VRH /d T =−22.6 ± 2.8 MPa/°C. Substantial differences between bulk properties calculated from the single–crystal measurements in this study and the properties reported in the literature for polycrystalline sintered mullite are identified, indicating the importance of factors such as microstructure, intergranular phases, and composition to the elasticity of mullite ceramics. 相似文献
15.
Claus H. Rüscher Holger Fritze Günter Borchardt Thomas Witke Bernd Schultrich 《Journal of the American Ceramic Society》1997,80(12):3225-3228
Mullite (3Al2 O3 ·2SiO2 ) coatings on SiC substrates and SiC precoated carbon/carbon composite (C/C-Si-SiC) substrates were produced by pulsed laser deposition (PLD) using pressed mullite powder targets. The layers can be characterized efficiently by IR reflection spectroscopy in the spectral range between 650 and 5000 cm−1 . The deposited coatings turn into mullite upon oxidation in air at temperatures between 1400° and 1600°C. Fabry-Perot interferences indicate a high quality and homogeneity of the mullite coating/SiC substrate interface. Amorphous SiO2 gradually forms during prolonged heating or at higher temperatures. 相似文献
16.
Decomposition of Mullite 总被引:2,自引:0,他引:2
ROBERT F. DAVIS ILHAN A. AKSAY JOSEPH A. PASK 《Journal of the American Ceramic Society》1972,55(2):98-101
Free surfaces of 2:1 mullite (2Al2 O3 ·SiO2 ) specimens decomposed with the evolution of SiO and O2 when they were heated at high temperatures under low partial pressures of O2 ; this reaction was analyzed thermodynamically. In addition, bubbles were observed at internal interfaces between mullite and fused-SiO2 diffusion couples. These bubbles, when formed at 1 atm ambient pressure between 1650° and 1800°C, resulted from reaction of Si particles and residual SiO2 -rich glass in the fused cast mullite. 相似文献
17.
Jalajakumari Nair Padmakumar Nair Fujio Mizukami Jan G. Van Ommen Giel B. M. Doesburg Julian R. H. Ross Anthonie J. Burggraaf 《Journal of the American Ceramic Society》2000,83(8):1942-1946
Pure Al2 O3 and different compositions of La2 O3 –Al2 O3 samples have been prepared through coprecipitation. Even after heating at 1300°C, the compositions La2 O3 ·11Al2 O3 and La2 O3 ·13Al2 O3 had higher surface area compared to the pure Al2 O3 and the La2 O3 ·Al2 O3 composition. Ethanol washing is an effective way for improving the textural stability of pure Al2 O3 and La2 O3 –Al2 O3 samples. The effect of steam on the thermal stability of La2 O3 ·11Al2 O3 has also been studied. La2 O3 ·11Al2 O3 sample is found to be stable in steam. 相似文献
18.
Hong-Yang Lu Wei-Lin Wang Wei-Hsing Tuan Ming-Hong Lin 《Journal of the American Ceramic Society》2004,87(10):1843-1847
The microstructure of vitrified kaolin ceramic tapes has been studied via scanning and transmission electron microscopy (SEM and TEM). The sintered samples contained crystalline phase of predominantly stoichiometric mullite (3Al2 O3 ·2SiO2 ), which consisted of high aspect ratio, acicular crystals that are often referred to as secondary mullite. These crystals were interlocked and embedded in an aluminosilicate glass matrix of inhomogeneous composition. The glass matrix contained an average of ∼3.63 wt% K as determined by energy-dispersive X-ray analysis (EDS), whose composition could be approximated to 5Al2 O3 ·16SiO2 ·0.1MgO·0.3K2 O·0.15TiO2 ·0.12Fe2 O3 . The acicular crystals have approximately the stoichiometric composition of Al2 O3 :SiO2 = 3:2. They have grown along a specific crystallographic orientation along the [001] axis. The crystal growth front exhibited facetting on the {110) planes with microfacetting on both the {100) and {010) planes. 相似文献
19.
The chemical composition of mullite which was termed from 2SiO2 3Al2 O3 xerogel by firing was examined by analytical TEM. Mullite formed at 950°C firing showed around 66 mol% Al2 O3 , which was fairly Al2 O3 rich compared with the bulk composition. The chemical composition of mullite gradually approached the bulk composition as the firing temperature increased to 1400°C and slightly departed again above that firing temperature. 相似文献
20.
The transformation of X-phase SiAlON (a composition of approximately Si3 Al6 O12 N2 ) to 3/2 mullite (3Al2 O3 ·2SiO2 ) plus vitreous silica (SiO2 ) at temperatures >1200°C has been studied in polycrystalline ceramics via transmission electron microscopy. The transformation of X-phase SiAlON to mullite is observed to be topotactical. Analyses of electron-diffraction patterns and lattice-fringe images reveal the following orientation relationship between X-phase SiAlON (X) and mullite (Mu): (110)Mu ∥ (100)X and [001]Mu ∥ [010]X . The observed orientation relationship between mullite and X-phase SiAlON and the close similarities between both structures-especially the occurrence of the same type of octahedral chains-suggest that these types of structural units are preserved during the transformation process. Excess noncrystalline SiO2 is exsolved between the mullite crystallites and is transported to the sample surface in later transformation stages. 相似文献