莫来石晶须前驱体原位增强海胆状莫来石材料的性能研究

为了提高粉煤灰的高附加值利用,分别以氢氧化铝和粉煤灰漂珠作为海胆状莫来石前驱体的铝源和硅源,氢氧化铝和粉煤灰为莫来石晶须前驱体增强剂的铝源和硅源,AlF₃和V₂O₅为晶须促进剂和烧结助剂,采用固相法原位制备了莫来石陶瓷材料。主要研究了增强剂与海胆状莫来石前驱体的配比(质量比分别为3:7、4:6、5:5、6:4、7:3)对制备的莫来石陶瓷材料性能、物相组成和显微结构的影响。结果表明,随着增强剂与海胆状莫来石前驱体的配比从3:7增大至7:3,试样中形成的晶相全为莫来石,材料内部更加致密,体积密度和抗压强度逐渐增大,总气孔率逐渐减小,增强剂的引入提高了材料的线变化率和抗热震性能,但降低了材料的重烧线变化率,材料的残余抗压强度及残余强度比先增加后减小。当增强剂与海胆状莫来石前驱体的质量比为6:4时综合性能较佳。     

Formation and Densification Behavior of Mullite Aggregates from Beach Sand Sillimanite

Dense mullite aggregates with 60% and 70% Al₂O₃ have been prepared from precursor mixtures consisting of beach sand sillimanite and a high-purity aluminum hydroxide following conventional single- and double-stage firing processes. The bulk density (BD), apparent porosity (AP), and water absorption (WA) capacity of sintered mullite aggregates were found to be strongly influenced by the premullitization step of this precursor mixture. Mullite aggregates formed in a double-stage firing process exhibited higher BD and mullite content and lower AP and WA capacity in comparison with those obtained by the single-stage firing process. The values of coefficient of thermal expansion of sintered mullite aggregates are close to those found in the literature reports for high-purity stoichiometric mullite.     

Preparation of Mullite Fiber

Transparent mullite fibers have been prepared using aluminum carboxylates (ACs) and tetraethyl orthosilicate (TEOS) as starting materials. The ACs are derived from the catalyzed dissolution of elemental aluminum in a mixture of formic acid and acetic acid. The solubility of aluminum in the acids is influenced by the concentrations of the acids, water, and additives and the preparation temperature. A 1:4:3:24 molar ratio of aluminum, formic acid, acetic acid, and water dissolves the aluminum completely to give a colorless, clear solution suitable for fiber synthesis. The mixture of the ACs and TEOS, in the presence of ethyl alcohol as a mutual solvent at 50°–60°C, is concentrated to give a spinnable dope, from which mullite precursor fibers can be drawn. Heat treatment of the precursor at 1250°C yields crystallized and transparent mullite fibers.     

A Processable Mullite Precursor Prepared by Reacting Silica and Aluminum Hydroxide with Triethanolamine in Ethylene Glycol: Structural Evolution on Pyrolysis

A simple, processable precursor to mullite can be synthesized in quantities of 100 g in a few hours by direct reaction of silica, aluminum hydroxide, and triethanolamine in ethylene glycol. To delineate a processing window whereby precursor shapes can be transformed into mullite, the chemical and phase microstructural evolution of this precursor on pyrolysis to selected temperatures in air is followed by thermal gravimetric analysis, differential thermal analysis, diffuse reflectance infrared Fourier transform spectroscopy, solid-state ²⁷Al and ²⁹Si nuclear magnetic resonance, X-ray diffractometry, and Brunauer-Emmett-Teller analytical methods. The precursor behaves as a single-phase, atomically mixed material that initially transforms to a porous, amorphous aluminosilicate when heated to temperatures as high as 950°C. Above 950°C, the precursor first transforms to tetragonal mullite, based on comparison with the literature, and, on continued heating above 1200°C, to orthorhombic mullite with coincident loss of porosity.     

Infiltration and Pyrolysis of Polytitanocarbosilane in an Si-Ti-C-O Fabric/Mullite Porous Composite

Incorporating Si-Ti-C-O fabric into a mullite matrix is expected to increase the fracture energy of mullite ceramics. The present paper describes the processing of an Si-Ti-C-O fabric/mullite/polytitanocarbosilane composite. A polytitanocarbosilane (a precursor of Si-Ti-C-O fiber)/xylene solution was infiltrated into a laminated porous mullite composite with 35–37 vol% fabric and thermally decomposed to an amorphous solid at 1000°C, in an argon atmosphere, to decrease the porosity and residual stress induced by the difference in thermal and mechanical properties between the Si-Ti-C-O fabric and the mullite. The decrease in porosity of the composite with pyrolysis of the precursor polymer was analyzed theoretically, and those results were used to control the effective experimental parameters. The infiltration/pyrolysis process was repeated eight times to produce a composite of 90.4% theoretical density. The composite exhibited significant pseudoductility, with a fracture energy of 11.4 kJ/m² and a flexural strength of 290 MPa.     

Development of high strength,porous mullite ceramic hollow fiber membrane for treatment of oily wastewater

Ceramic hollow fiber membranes (CHFMs) are known for their excellent characteristics including high surface area, compact design, and good chemical, thermal, and mechanical stabilities. Despite these interesting attributes, CHFMs are also prone to certain limitations, such as brittleness and high cost that hinder them from being commercialized. To mitigate this drawback, we have developed a high strength, porous ceramic hollow fiber membrane, derived from mullite–kaolinite powder, for efficient oil–wastewater separation. The superhydrophilic, low-cost mullite-based (CHFM) was successfully fabricated through combined phase inversion and sintering techniques. Prior to the fabrication, the as-received mullite–kaolinite was characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and Brunauer–Emmett–Teller (BET) analyses. Subsequently, operational parameters such as the effect of mullite content, sintering temperature, and air gap were optimized during the fabrication of mullite ceramic hollow fiber membrane. The resulting membranes were systematically characterized and evaluated in terms of morphology, porosity, mechanical strength, water flux, and oil–water separation. Increasing the mullite content, air gap, and sintering temperature enhanced the formation of microvoid structure. It is interesting to note that the mechanical strength of 86 MPa was obtained for the membrane containing 60 wt % of mullite sintered at 1450 °C and an air gap of 5 cm. The membrane induced a stable permeate water flux and oil rejection of mullite CHFM of 182 L/m²?h and 97.1%, respectively. As compared to kaolin ceramic counterparts, this porous mullite ceramic hollow fiber membrane can be used in various water treatment applications, including for the separation of oily wastewater due to its mechanical strength and water flux.     

Coprecipitation and Processing of Mullite Precursor Phases

Ultrafine powders of aluminum-rich mullite precursor phases were prepared by spraying mixed acid solutions of silicic acid and aluminum chloride into ammonium hydroxide at different pH values. The aluminum-rich composition was chosen to avoid abnormal grain growth of the mullite grains caused by silica-rich liquid phases. The influence of coprecipitation pH on particle size, zeta potential, and the mixing degree of aluminum and silicon in the precipitated powders and on the sintered microstructures have been described. In the basic pH region, the degree of mixing of both components increased as the pH of coprecipitation decreased. Under these conditions, direct slip casting of the coprecipitated slurries was possible, and fine and equiaxed microstructures could be obtained in the sintered bodies.     

Phase-Pure Mullite from Kaolinite

Sintering of kaolinite in the presence of certain carbonate mineralizers, viz., CaCO₃, Na₂CO₃, and K₂CO₃, has been conducted at 950°–1350°C. The influence of these mineralizers at the above temperatures is evaluated using XRD and SEM techniques. A comparative study of phase formation of the above compositions shows that the sodium- and calcium-fluxed samples give rise to multiphase systems, while K₂CO₃-incorporated samples give phase-pure mullite. The observation that kaolinite in the presence of K₂CO₃ can act as a precursor material for phase-pure mullite is of great industrial significance.     

High-Temperature Reactions of Clay Mineral Mixtures and Their Ceramic Properties: I, Kaolinite-Mica-Quartz Mixtures with 25 Weight % Quartz

The mineralogy and ceramic properties (linear shrinkage and porosity) of fired compacts of quartz, kaolinite, and mica containing 25% of quartz and variable proportions of kaolinite and mica were studied systematically in relation to composition and firing temperature. A procedure for the quantitative determination of components by X-ray diffraction measurements is outlined and applied to the determination of quartz and mullite in the fired samples. For a mixture containing 25%, quartz, 75% kaolinite, the amount of mullite developed at 1300°C. is 41% and this contains 85% of the total Alsoa available. In micarich mixtures, mullite develops at lower temperatures and in smaller proportions. The shrinkage and porosity vary systematically with the percentage of mica in the samples and with firing temperature. The formation of cristobalite depends on the kaolinite content and is not related to the quartz content. Mullite and cristobalite develop at about 1100°C. from the transitory Si-Al spinel-type phase derived from kaolinite.     

机械力化学法制备单相莫来石的机理研究

以高岭土和氢氧化铝为原料,采用机械力化学法制备出了单相莫来石。用热重-差示扫描分析研究了混合粉体经高能球磨后的结构变化,并讨论了单相莫来石的形成机理。结果表明：高能球磨破坏了混合物的晶体结构;随着粉磨的进行,混合物的比表面积会增加,无序程度也会增加,键能会减小,从而导致了内部贮能的增加,反应活化能的减小,并可得到均匀混合物。粉磨30h混合物制成的烧结体的热膨胀系数要比未粉磨的低约20％。     

Fabrication and mechanical properties of SiC platelet reinforced mullite matrix composites

Hot-pressing of mullite and SiC–mullite matrix composites was performed at temperatures and pressures between 1500 and 1650°C and 5 and 15 MPa, respectively. Composites were produced using different precursors; sol–gel derived mullite and kaolinite/α-alumina. The precursor did not strongly affect the optimum density achieved, reaching 97·5% of theoretical for a 20 vol% SiC addition in both cases. The SiC platelet addition impaired densification kinetics in all composites compared to mullite monoliths. Fracture toughness, measured by the indentation strength in bending technique, was marginally higher for the sol–gel precursor material in both monolith and composite. Fracture toughness increased with SiC content for both materials. For example, for the sol-gel precursor material it increased from 2.9±0.1 MPa m^1/2 for the monolith to 3.9±0.1 MPa m^1/2 for the 20 vol% SiC composite. Similarly, hardness increased with SiC addition for both materials, but the hardness of the sol–gel material was greater than that of the kaolinite+α-alumina material for all compositions. The relationship between microstructure and mechanical properties is discussed.     

Fabrication and characterization of porous mullite ceramics with ultra-low shrinkage and high porosity via sol-gel and solid state reaction methods

Porous mullite ceramics with ultra-low shrinkage and high porosity were prepared by solid state reaction between MoO₃ and mullite precursor powders which were synthesized from tetraethylorthosilicate and aluminium nitrate nonahydrate via sol-gel methods. The synthetic process of mullite precursor powder and effects of MoO₃ amount on the phase composition, microstructure, physical properties such as firing shrinkage, open porosity, bending strength, water absorption and bulk density of porous mullite ceramics were investigated. The results indicated that the addition of MoO₃ not only lowered the mullite forming temperature from 985.4 to 853.3 °C, but also restrained densification behavior of samples due to the formation of mullite and Al₂O₃–MoO₃ solid solution, besides, MoO₃ also improves the formability, open porosity and bending strength of samples. The optimal amounts of MoO₃ is 8 wt%, and the resultant samples exhibit outstanding properties, including a low shrinkage rate of 1.86 ± 0.07%, an open porosity of 61.91 ± 0.16% and a bending strength of 9.35 ± 1.11 MPa.     

Infrared Kinetic Study of High-Temperature Reactions of Synthetic Kaolinite

An infrared frequency shift method was used to follow the kinetics of conversion of dehydroxylated synthetic kaolinite to mullite. Comparison of the kinetic and activation thermodynamic data for natural and synthetic kaolinite suggests that the mechanism of high-temperature reaction is similar, although the activation energy of synthetic kaolinite is larger than that of natural kaolinite because of the lack of nucleation-promoting impurity cations in the former. The high-temperature ir data indicate that there is little or no difference in the cation coordination of the phases intermediate to metakaolinite and mullite, but that the principal high-temperature coordination change (probably the accommodation of aluminum in well-defined tetrahedral sites) occurs during the later stages of mullite formation.     

Resolution of Thermal Peaks of Kaolinite in Thermomechanical Analysis and Differential Thermal Analysis Studies

The recent findings for the kaolinite metakaolinite, cubic-mullite, and orthorhombic-mullite reaction series have been thoroughly examined by differential thermomechanical analysis (DTMA) and differential thermal analysis (DTA). Metakaolinite shows two differential contraction peaks in the vicinity of 980°C caused by final dehydroxylation at the endothermic dip just before 980°C in DTA with expulsion of 35–37 wt% SiO₂, formation of a defect aluminosilicate phase and simultaneous contraction of the latter phase, and crystallization of cubic mullite at the 980°C exotherm in DTA. Mullitization takes place in two simultaneous reaction steps: (i) polymorphic transformation of cubic mullite to orthorhombic mullite during the ∼1250°C exotherm shown by DTA which coincides with the differential expansion peak in DTMA and (ii) nucleation followed by crystallization of orthorhombic mullite from the residual aluminosilicate compact phase during the ∼1330°C exotherm shown by DTA. The aluminosilicate formed during the large differential contraction at 1100°–1400°C as shown by DTMA. These results, obtained by the two physical techniques, corroborate earlier findings of the kaolinite transformation series.     

Low sintering shrinkage porous mullite ceramics with high strength and low thermal conductivity via foam-gelcasting

Excessive sintering shrinkage leads to severe deformation and cracking, affecting the microstructure and properties of porous ceramics. Therefore, reducing sintering shrinkage and achieving near-net-size forming is one of the effective ways to prepare high-performance porous ceramics. Herein, low-shrinkage porous mullite ceramics were prepared by foam-gelcasting using kyanite as raw material and aluminum fluoride (AlF₃) as additive, through volume expansion from phase transition and gas generated from the reaction. The effects of AlF₃ content on the shrinkage, porosity, compressive strength, and thermal conductivity of mullite-based porous ceramics were investigated. The results showed that with the increase of content, the sintering shrinkage decreased, the porosity increased, and mullite whiskers were produced. Porous mullite ceramics with 30 wt% AlF₃ content exhibited a whisker structure with the lowest shrinkage of 3.5%, porosity of 85.2%, compressive strength of 3.06 ± 0.51 MPa, and thermal conductivity of 0.23 W/(m·K) at room temperature. The temperature difference between the front and back sides of the sample reached 710°C under high temperature fire resistance test. The low sintering shrinkage preparation process effectively reduces the subsequent processing cost, which is significant for the preparation of high-performance porous ceramics.     

Synthesis of a Mullite Precursor from Aluminum Nitrate and Tetraethoxysilane via Aqueous Homogeneous Precipitation: An 27Al and 29Si Liquid- and Solid-State NMR Spectroscopic Study

A simple aqueous process is described for the preparation of aluminosilicate colloids and chemically homogeneous mullite precursor gel. Starting from a solution of aluminum nitrate and silicic acid, aluminum is slowly hydrolyzed at 80–100°C by in situ generation of ammonia. A silica gel is rapidly made, probably by a catalytic effect of urea, the base generator. This gel is then slowly digested by partially hydrolyzed aluminum species which break the Si-O-Si bonds and link to the gel by Si-O-Al bonds. Progressively a clear colloidal sol is obtained and the colloidal particle size continues to decrease toward aluminosilicate species where the silicon atoms are in a single environment and may be linked to three hexacoordinated aluminum atoms and a hydroxyl group, by reference to natural imogolite. When the hydrolysis of aluminum is nearly complete, these particles are cross-linked and a final gel precursor of mullite is obtained. This gel is chemically very homogeneous and crystallizes to mullite at 980°C. The structural evolution, from the first gel to the ceramic, has been followed by ²⁷Al and ²⁹Si liquid- and solid-state MAS NMR spectroscopy.     

Contamination Effects on Interfacial Porosity during Cyclic Oxidation of Mullite-Coated Silicon Carbide

Plasma-sprayed mullite (3Al₂O₃2SiO₂) and mullite/yttria-stabilized zirconia (mullite/YSZ) dual-layer coatings have been developed to protect silicon-based ceramics from environmental attack. The mullite/SiC system develops interfacial pores during cyclic oxidation. The development of pores at the mullite/sintered SiC interface in air has been investigated as a function of the purity of mullite at 1230°–1350°C in an atmospheric-pressure furnace. The silica scale is readily contaminated by impurities of alkali or alkaline-earth metal oxides from the mullite coating. The contamination enhances oxidation and reduces the scale viscosity by forming alkali or alkaline-earth metal silicates. The porosity increases as the temperature and contamination increase and decreases as the purity of the mullite increases.     

Electrospun mullite nanofibers derived from diphasic mullite sol

Fine‐grained mullite nanofibers derived from the diphasic mullite sol were successfully fabricated by electrospinning and subsequent pyrolysis at 1500°C. Polymethylsiloxane and aluminum tri‐sec‐butoxide were selected as the silicon and aluminum source to synthesize the diphasic sol. Results show that the weight loss of mullite precursor fibers in our work was about 60 wt.%, which is similar with that of fibers fabricated using the monophasic sol. This low weight loss was mainly attributed to the high ceramic yield of polymethylsiloxane and low introduced polyvinylpyrrolidone content, which ensures the integrity of fiber morphology during the sintering process. Mullite fibers with 216 nm average diameter were fabricated after sintered at 1500°C and the corresponding grain size was only ~100 nm, much smaller than that in mullite fibers derived from monophasic sols. Therefore, it can be predicated that mullite fibers in this work should possess a higher mechanical strength than those derived from monophasic sols when the sintering temperature was higher than 1400°C and therefore was an ideal starting materials for the fabrication of mullite nanofibrous ceramics used as the high‐temperature thermal insulation materials.     

Preparation of mullite from alumina/aluminum nitrate and kaolin clay through spark plasma sintering process

In the present study, the in-situ synthesized mullite has been prepared successfully by mixing kaolinite with alumina and aluminum nitrate nonahydrate (ANN) powders through high energy milling followed by spark plasma sintering (SPS). Using a high-energy ball-mill, the stoichiometric compositions of the starting powders, considering their final transformation to Al₂O₃ and SiO₂, have been mixed. The SPS process has been performed at 1400 and 1375?°C for the specimens containing Al₂O₃ and ANN, respectively. XRD patterns of the milled powders after 30?h showed the formation of quartz from kaolinite for both starting batches. The displacement-temperature-time (DTT) curves and the corresponded vacuum changes indicated the dehydration and phase transformation of ANN and kaolinite at different stages of the sintering process. The XRD patterns of the sintered samples revealed the formation of mullite alongside un-reacted Al₂O₃ and crystobalite for the batches containing Al₂O₃ and ANN, respectively. The results of the physical and mechanical properties tests showed higher amounts of bending strength (397?±?18?MPa), Vickers hardness (16.32?±?0.21?GPa) and fracture toughness (3.81?±?0.24?MPa?m^?1/2) alongside a lower porosity (0.070?±?0.02%) for the prepared sample containing Al₂O₃, than those of the sample containing ANN.