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1.
Crack-free titania composite membranes have been synthesized from colloidal titania sols (average particle size of 7–16 nm) by the pressurized sol-gel coating technique. The Knudsen permeability of N2 through the titania membrane layer heat-treated at 300°C is about 21 × 10-7 –35 × 10-7 mol/m2 ·s·Pa at room temperature. The permselectivity and the separation factor of He to O2 in the temperature range of 25°-245°C are 2.49–2.74 and 1.66–1.84, respectively. The average pore diameters of these membranes estimated from the molecular weight cutoff data (6000–9000) using poly(ethylene glycol) are 3.0–4.0 nm. By doping zirconia into titania membranes, their thermal stability could be improved up to 800°C. 相似文献
2.
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
3.
Hybrid Gels for Homoepitactic Nucleation of Mullite 总被引:2,自引:0,他引:2
Hybrid gels, defined as gels from mixtures of polymerically and colloidally derived sols, offer many opportunities for crystalline microstructure development upon heating. In this study, hybrid mullite gels are formed by mixing a colloidal boehmite—silica sol with a polymeric aluminum nitrate—tetraethoxysilane-derived sol. The polymeric gel crystallizes in situ to form mullite that acts as seed crystals for homoepitactic nucleation during the subsequent transformation of the colloidal component of the hybrid gel. Compared with the entirely colloidal gel, the introduction of a 30 wt% polymeric gel fraction results in an increase in apparent nucleation frequency from ∼5x1011 to ∼1x1014 nuclei / cm3 at 1375°C, a reduction in high-temperature grain size from 1.4 to 0.4 μm at 1550°C, and an increase in the degree of microstructural homogeneity, as evidenced by intragranular pore removal. 相似文献
4.
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
5.
Yexia Qin Wei Pan Rui Zhang Aibing Du 《International Journal of Applied Ceramic Technology》2008,5(1):105-109
A series of alumina-based ceramic cores sintered at 1300°C, 1400°C, and 1500°C for 5 h were prepared, and the phases and microstructures were characterized by X-ray diffraction and scanning electron microscopy. The effect of colloidal silica sols on the properties of ceramic core was discussed. The properties of these materials were determined. The results indicated that the microstructure of the core is characterized by the presence of substantially unreacted Al2 O3 particles having a polycrystalline composition consisting essentially of in situ synthesized 3Al2 O3 ·2SiO2 on the surface of the Al2 O3 particles. The colloidal silica sol contents do not have an appreciable effect on the densification and shrinkage of the alumina ceramic core. The ceramic cores of 5 wt% colloidal silica sol contents sintered at 1500°C for 5 h showed the smallest creep deformation in the present research. 相似文献
6.
YueFeng Tang § AiDong Li YiNong Lü ZhiDa Ling 《Journal of the American Ceramic Society》2004,87(3):520-522
Heterogeneous nucleation and growth was used to prepare composite particles with homogeneous component distribution. Composite particles consisting of α-Al2 O3 cores with an outer amorphous homogeneous silica layer were prepared by heterogeneous nucleation and growth processing using an ethanol suspension containing ammonia, tetraethylorthosilicate, and α-Al2 O3 . Fine mullite powders of average particle size 0.53 μm were fabricated by calcinating the composite particles at 1500°C for 2 h. 相似文献
7.
Flexural creep studies were conducted in a commercially available alumina matrix composite reinforced with SiC particulates (SiCp ) and aluminum metal at temperatures from 1200° to 1300°C under selected stress levels in air. The alumina composite (5 to 10 μm alumina grain size) containing 48 vol% SiC particulates and 13 vol% aluminum alloy was fabricated via a directed metal oxidation process (DIMOX(tm))† and had an external 15 μm oxide coating. Creep results indicated that the DIMOX Al2 O3 –SiCp composite exhibited creep rates that were comparable to alumina composites reinforced with 10 vol% (8 (μm grain size) and 50 vol% (1.5 μm grain size) SiC whiskers under the employed test conditions. The DIMOX Al2 O3 –SiCp composite exhibited a stress exponent of 2 at 1200°C and a higher exponent value (2.6) at ≥ 1260°C, which is associated with the enhanced creep cavitation. The creep mechanism in the DIMOX alumina composite was attributed to grain boundary sliding accommodated by diffusional processes. Creep damage observed in the DIMOX Al2 O3 -SiCp composite resulted from the cavitation at alumina two-grain facets and multiple-grain junctions where aluminum alloy was present. 相似文献
8.
Preparation of Mullite Cordierite Composite Powders by the Sol-Gel Method: Its Characteristics and Sintering 总被引:4,自引:0,他引:4
Mohamed G.M.U. Ismail Hideo Tsunatori Zenjiro Nakai 《Journal of the American Ceramic Society》1990,73(3):537-543
Mullite/cordierite composite powders containing different proportions of cordierite were prepared by the sol-gel method using boehmite, colloidal silica, and Mg(NO3 )2 ·6H2 O. Mullite and cordierite sols were prepared separately and mixed to form the composite sol. Mullitization temperature depends on the cordierite content in the composite. Also, α-cordierite crystallizes at a lower temperature in a mullite-rich (MC20) composite. The XRD patterns of the powders calcined at 1450°C for 12 h showed that mullite and cordierite exist as two different phases, and no additional phases were observed. The IR absorbance spectra of composites showed characteristic peak corresponding to both mullite and cordierite. The sintered density of the powders increases with temperature up to 1450°C and decreases beyound the melting point of cordierite (1455°C). The microstructure of MC30 sintered at 1440°C for 3 h consisted of acicular grains, whereas in MC40 and MC50 equiaxed grain morphology was observed under similar sintering conditions. The flexural strength and Vickers hardness decreases with the increase of cordierite content in the composite. Dielectric constant and thermal expansion showed a similar behavior. 相似文献
9.
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
10.
Kanoksak Luyjew Nattaporn Tonanon Varong Pavarajarn 《Journal of the American Ceramic Society》2008,91(4):1365-1368
Silicon nitride (Si3 N4 ), a high-temperature structural ceramic, was synthesized in a mesoporous form by a simple approach. At first, silica/resorcinol–formaldehyde (RF) gel was formed via sol–gel polycondensation process, using resorcinol and formaldehyde as sources for porous RF structure and amino propyl trimethoxysilane as a precursor for silica. Pyrolysis of the dried gel at 250°C for 2 h following by at 750°C for 4 h resulted in silica/carbon composite that could be converted into mesoporous Si3 N4 or Si3 N4 /silicon carbide composite via the carbothermal reduction and nitridation process at 1450°C. Significant increase in surface area of the products, comparing with that of the conventional Si3 N4 granules, was observed. The content of silica in the starting composite was found to be a critical factor influencing both phase and porosity of the obtained product. 相似文献
11.
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
12.
This paper reports on ¬¬¬a new microporous composite silica membrane prepared via acid-catalyzed polymeric route of sol-gel method with tetraethylorthosilicate (TEOS) and a bridged silsesquioxane [1, 2-bis(triethoxysilyl)ethane, BTESE] as precursors. A stable nano-sized composite silica sol with a mean volume size of ~5 nm was synthesized. A 150 nm-thick defect-free composite silica membrane was deposited on disk support consisting of macroporous α-Al2O3 and mesoporous γ-Al2O3 intermediate layer by using dip-coating ap-proach, followed by calcination under pure nitrogen atmosphere. The composite silica membranes exhibit molecular sieve properties for small gases like H2, CO2, O2, N2, CH4 and SF6 with hydrogen permeances in the range of (1-4)107 mol•m2•s1•Pa1 (measured at 200 C, 3.0×105 Pa). With respect to the membrane calcined at 500 C, it is found that the permselectivities of H2 (0.289 nm) with respect to N2 (0.365 nm), CH4 (0.384 nm) and SF6 (0.55 nm) are 22.9, 42 and >1000, respectively, which are all much higher than the corresponding Knudsen values (H2/N2 3.7, H2/CH4 2.8, and H2/SF6 8.5). 相似文献
13.
Nevenka Raji Miran eh Roman Gabrovek Veneslav Kaui 《Journal of the American Ceramic Society》2002,85(7):1719-1724
Nanocrystalline transition-metal ferrites were synthesized inside an amorphous silica matrix by the sol–gel method. The formation of spinel ferrites began above 400°C, giving fine particles of about 10 nm at 800°C. This is associated with a specific role of the silica matrix, which facilitates the diffusion of the reacting cations, enhancing the ferrite formation. Above 1000°C the MnFe2 O4 and CuFe2 O4 nanoparticles lost their fine nature. The dried gels and crystalline materials were characterized by X-ray diffraction, thermal, FTIR, and BET analyses as well as by high-resolution scanning transmission electron microscopy. 相似文献
14.
以1-甲基-2-吡咯烷酮/丙酮为混合溶剂,无纺布为支撑层,采用静电纺丝技术与溶胶-凝胶方法,制备了醋酸纤维素/二氧化硅复合纳米纤维膜,并将其浸渍于全氟烷基硅烷/正己烷分散液中进行疏水化改性。利用场发射扫描电子显微镜、红外光谱仪、孔径分析仪、接触角测量仪器等表征了改性前后复合膜表面形貌、官能团变化、孔径分布及润湿性等膜性能参数并将其应用于连续性直接接触式膜蒸馏盐浓缩过程。结果表明,静电纺丝复合膜呈三维空间网状结构,且利用正硅酸乙酯生成的纳米二氧化硅颗粒内陷于醋酸纤维素纤维内部形成微米-纳米梯级分布。经全氟烷基硅烷修饰后,红外特征峰明显,复合膜水接触角最高可达156°,且对质量分数为5%的十二烷基硫酸钠液滴也同时展示出优良的抗润湿性能(接触角125°)。以60℃、35 g·L-1的NaCl溶液为进料液进行持续性直接接触式膜蒸馏脱盐实验,当渗透温度为20℃时,各复合膜盐截留率均能达到99.99%以上,其中,CA/SiNPs-FAS膜通量可稳定在11.2 kg·(m2·h)-1。 相似文献
15.
Preparation and Characterization of High-Temperature Thermally Stable Alumina Composite Membrane 总被引:4,自引:0,他引:4
A crack- and pinhole-free composite membrane consisting of an α-alumina support and a modified γ-alumina top layer which is thermally stable up to 1100°C was prepared by the sol–gel method. The supported thermally stable top layer was made by dipcoating the support with a boehmite sol doped with lanthanum nitrate. The temperature effects on the microstructure of the (supported and unsupported) La-doped top layers were compared with those of a common γ-alumina membrane (without doping with lanthanum), using the gas permeability and nitrogen adsorption porosimetry data. After sintering at 1100°C for 30 h, the average pore diameter of the La-doped alumina top layer was 17 nm, compared to 109 nm for the common alumina top layer. Addition of poly(vinyl alcohol) to the colloid boehmite precursor solution prevented formation of defects in the γ-alumina top layer. After sintering at temperatures higher than 900°C, the common alumina top layer with addition of poly(vinyl alcohol) exhibits a bimodal pore distribution. The La-doped alumina top layer (also with addition of poly(vinyl alcohol)) retains a monopore distribution after sintering at 1200°C. 相似文献
16.
KINJIRO FUJII WAKICHI KONDO HISAHIRO UENO 《Journal of the American Ceramic Society》1986,69(4):361-364
Hydration of CaAl2 O4 (CA) was studied by calorimetry, analysis of the liquid phase, measurement of the combined water, and electron microscopy. During the induction period, the solution remains almost unchanged and is equilibrated temporarily with both superficially intrusion-hydrated CA particles and Al(OH)3 gel formed by dissociation of Al(OH)4 – ions, the solubility of the Al(OH)3 gel being 10–4.24 molkg–1 at 25°C, while the intrusion-hydrated layer on the CA particles grows following a nearly linear law to reach a critical thickness (∼3 nm at 10° to 20°C, or 12 nm at 30°C). At this point destruction of the layer occurs, nuclei of hydrous compounds are generated, and the induction period terminates. Subsequent reaction proceeds in accordance with the rate equation of Schiller based on the dissolution-crystallization mechanism. 相似文献
17.
Microstructural Characterization of Porous Silicon Carbide Membrane Support With and Without Alumina Additive 总被引:1,自引:0,他引:1
Manabu Fukushima You Zhou Hiroyuki Miyazaki Yu-ichi Yoshizawa Kiyoshi Hirao Yuji Iwamoto Satoshi Yamazaki Takayuki Nagano 《Journal of the American Ceramic Society》2006,89(5):1523-1529
Porous silicon carbide (SiC) membrane supports sintered at 1500°–1800°C were prepared by cold isostatic pressing (CIP) under different pressures and using different amounts of alumina additive (0%–4%). The relationship between processing factors and pore size and microstructure was examined. Varying the sintering temperature, the CIP pressure and the amount of additive used were found to be effective for controlling pore size and microstructure. The pore size and particle size of the membrane support prepared without alumina were found to increase with increasing sintering temperature. This was attributed to surface diffusion. Densification of the undoped support did not occur, however, because of concurrent pore development. In the SiC membrane support containing 4% alumina, small particles and a pore size of around 100 nm were retained. This was because of the formation of a limited amount of SiO2 –Al2 O3 liquid phase during sintering. 相似文献
18.
Tomoyuki Adachi Naoki Wakiya Naonori Sakamoto Osamu Sakurai Kazuo Shinozaki Hisao Suzuki 《Journal of the American Ceramic Society》2009,92(S1):S177-S180
Fe3 O4 –BaTiO3 composite particles were successfully prepared by ultrasonic spray pyrolysis. A mixture of iron(III) nitrate, barium acetate and titanium tetrachloride aqueous solution were atomized into the mist, and the mist was dried and pyrolyzed in N2 (90%) and H2 (10%) atmosphere. Fe3 O4 –BaTiO3 composite particle was obtained between 900° and 950°C while the coexistence of FeO was detected at 1000°C. Transmission electron microscope observation revealed that the composite particle is consisted of nanocrystalline having primary particle size of 35 nm. Lattice parameter of the Fe3 O4 –BaTiO3 nanocomposite particle was 0.8404 nm that is larger than that of pure Fe3 O4 . Coercivity of the nanocomposite particle (390 Oe) was much larger than that of pure Fe3 O4 (140 Oe). These results suggest that slight diffusion of Ba into Fe3 O4 occurred. 相似文献
19.
Phase Transformation of Diphasic Aluminosilicate Gels 总被引:1,自引:0,他引:1
Aluminosilicate gels with compositions Al2 O2 /SiO2 and 2 were prepared by gelling a mixture of colloidal pseudo-boehmite and a silica sol prepared from acid-hydrolyzed Si(OC2 H5 )4 . Upon heating the pseudo-boehmite transforms to γ-Al2 O3 around 400°C, then to δ-Al2 O3 at 1050°C, and at 1200°C reacts with amorphous SiO2 to form mullite. Some twinned θ-Al2 O3 forms before mullite. Nonstoichiometric specimens have a similar transformation sequence, but form mullite grains with inclusions of either Al2 O3 or cristobalite, often associated with dislocation networks or micropores. Mullite grains are formed by nucleation and growth and have equiaxed shape. 相似文献
20.
J. B. WARBURTON J. E. ANTILL R. W. M. HAWES 《Journal of the American Ceramic Society》1978,61(1-2):67-72
Oxidation of reaction-sintered silicon nitride was studied in damp air. The formation of "passive" silica films was investigated at 1 atm and 700 to 1100°C and some limited work on weight loss behavior was performed in vacuo of 10−8 to 10−5 atm at 1050 to 1200°C. Passive behavior was dominated by reaction in the pore network. Oxidation was extensive at 900 to 1000° but slight at 700 to 800°C. At 1100°C a protective skin limited reaction. Weight loss in vacuo was slight at 1050°C. The vacuum pressure required to suppress the weight loss increased from 4 to 5 × 10−7 atm at 1050° to 1.5 to 2.5 × 10−5 atm at 1200°C. 相似文献