Preparation of elongated mullite self-reinforced porous ceramics

Elongated mullite was synthesized using mullite powder as a raw material and AlF₃·3H₂O as an additive, and characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The effects of AlF₃·3H₂O content and reaction temperature on the formation of elongated mullite were investigated, and the relevant growth mechanism was discussed based on the experimental results and density functional theory (DFT) calculations. When the optimal amount of AlF₃·3H₂O (4?wt% in the present work) was used, the length and diameter of elongated mullite increased with increasing the reaction temperature, and elongated mullite of 22.3?µm in average length and 4.6?µm in average diameter was formed after 5?h at 1873?K. Based on the results, elongated mullite self-reinforced porous ceramics were prepared by a combined foam-gelcasting and solid-reaction method, and their mechanical properties were examined. Elongated mullite in-situ formed in the porous samples evidently enhanced their mechanical strength. The flexural strength of the elongated mullite self-reinforced porous sample with 67.0% porosity (prepared using 6?wt% AlF₃·3H₂O) was as high as 13.9?MPa, which was about 26.4% higher than that of a porous sample (11.0?MPa) prepared without AlF₃·3H₂O.     

Foam-gelcasting preparation of high-strength self-reinforced porous mullite ceramics

High-strength self-reinforced porous mullite ceramics were prepared via foam-gelcasting using mullite powder as a main raw material, AlF₃·3H₂O (0–8 wt%) as an additive, Isobam-104 as a dispersing and gelling agent, sodium carboxymethyl cellulose as a foam stabilizing agent, and triethanolamine lauryl sulfate as a foaming agent. The effects of AlF₃·3H₂O content on rheological and gelling behaviors of the slurries, and porosity and mechanical properties of self-reinforced porous mullite samples were examined. Addition of AlF₃·3H₂O promoted the in-situ formation of elongated mullite in the fired porous samples, which improved considerably their mechanical properties. Compressive strength and flexural strength of 67.0% porous mullite ceramics prepared with addition of 6 wt% AlF₃·3H₂O was as high as 41.3 and 13.9 MPa, respectively. Its hot modulus rupture (HMOR) increased initially with the testing temperature, and peaked (with a maximum value of 16.6 MPa) at 800 °C above which it started to decrease with the testing temperature. Nevertheless, it was still retained as high as 6.7 and 2.8 MPa at 1200 and 1400 °C, respectively.     

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.     

Preparation of Al2O3-mullite thermal insulation materials with AlF3 and SiC as aids by microwave sintering

Al₂O₃-mullite composites were prepared under the synergy effect of AlF₃ and SiC aids by microwave heating. The phase composition, microstructure, porosity, flexural strength, thermal shock resistance, and thermal conductivity were investigated. The XRD results revealed that the content of mullite phase steadily increased with the increasing of AlF₃ content. The microstructure showed that the lower content (≤1 wt%) of AlF₃ led to the formation of granular mullite and the higher content (≥3 wt%) of AlF₃ led to the formation of mullite whiskers, which could form an interlocking structure. In addition, the SiC hot spots can also promote the generation of mullite whiskers by microwave sintering. The thermal shock resistance was significantly improved by the interlocking structure of mullite whiskers. The residual rate of flexural strength of the composite with 3 wt% AlF₃ was 86%. The composite with 3 wt% AlF₃ additives got its optimized thermal conductivity from 30°C to 950°C, the value was between 0.819 and 1.021 W/(mK), which possess excellent thermal insulation performance.     

Industrial waste silica-alumina gel recycling: Low-temperature synthesis of mullite whiskers for mass production

The increasing demand for mullite whisker-reinforced, toughened ceramic materials and mullite raw materials that meet industrial requirements has prompted the search for new and alternative sources, as well as effective technologies to obtain the target products. In this work, mullite whiskers of high purity were synthesized by a vapor-liquid–solid (V-L-S) process using industrial waste silica-alumina gel and Al₂(SO₄)₃·18H₂O as raw materials, with AlF₃·3H₂O and Na₂SO₄ as additives. The effects of sintering temperatures on the mullitization reactions and mullite morphology were investigated by XRD, TG-DTA, SEM and so forth. The results suggest that the introduction of AlF₃·3H₂O and Na₂SO₄ alters the mullitization reaction path, which leads to an initial mullitization reaction temperature of 720 °C. The SEM results demonstrate that mullite whiskers transformed from secondary growth to anisotropic growth when the sintering temperature was increased from 720 °C to 825 °C. By analyzing the experimental results, the mechanism of AlF₃·3H₂O-assisted growth of mullite whiskers with Na₂SO₄ as the liquid phase template is proposed based on the “dissolution-precipitation” process. Herein, a novel and feasible solution for the recycling of silica-rich industrial waste is proposed, which offers new and simple insights into the high value-added recycling of industrial waste, which provides new ideas for the actual mass production of mullite whiskers.     

Characteristics of porous mullite developed from clay and AlF3·3H2O

Porous mullite with interlocked needle shape microstructure was developed from China clay and aluminium fluoride trihydrate (AlF₃·3H₂O). The effects of various parameters like sintering temperature, sintering time and the amount of AlF₃·3H₂O on the phase evolution, microstructure and porosity have been studied. Quantitative analysis of mullite was carried out using X-ray diffraction combined with Rietveld-RIR method (Internal standard method). Porous mullite ceramics with 62% open porosity have been prepared at a relatively lower temperature of 1400?°C. The results show that the nucleation of mullite can be achieved from 700?°C onwards using hydrated aluminium fluoride without the formation of intermediate topaz crystals. The high amount of water vapour produced within the system during the in-situ reaction has a crucial role in deciding the reaction mechanism.     

Ultra-low cost porous mullite ceramics with excellent dielectric properties and low thermal conductivity fabricated from kaolin for radome applications

Near-net-shape mullite ceramics with high porosity were prepared from ultra-low cost natural aluminosilicate mineral kaolin as raw material and polystyrene micro-sphere (PS) as pore-forming agent. Microstructure, flexural strength, thermal conductivity and dielectric properties of the ceramics were systematically researched. Results show that the porous mullite ceramics possess fibrous skeleton structure formed by a large quantity of interlocked mullite whiskers, which results in good mechanical properties and low-to-zero sintering shrinkage. Flexural strength of the porous mullite ceramics can be up to 41.01 ± 1.12 MPa, even if the porosity is as high as 62.44%. The dielectric constant and loss tangent of the porous mullite ceramics at room temperature are lower than 2.61 and 5.9 × 10⁻³, respectively. Besides, dielectric constant is very stable with the rising of temperature, and the dielectric loss can be consistently lower than 10⁻² when the temperature is not higher than 800 °C. In addition, thermal conductivity at room temperature is as low as 0.163 W/m/K when the porosity of mullite ceramics is 80.05%. The infiltration of SiO₂ aerogels (SiO₂ AGs) can further decrease the thermal conductivity to 0.075 W/m/K, while has just little effects on the dielectric properties. Excellent mechanical, thermal and dielectric properties show that the porous mullite ceramics have potential applications in radome fields. The porous mullite ceramics prepared from kaolin not only have low cost, but also can achieve near-net-shape.     

Fibrous ceramic membrane constructed by mullite whiskers for the treatment of oil-in-water emulsions

Ceramic membranes with high porosity and excellent separation efficiency are necessary for the efficient treatment of large-scale wastewaters. However, the conventional ceramic membranes are usually prepared by particles-packing, which inhibits the advances of separation efficiency because of the low porosity and connectivity. Here, a fibrous ceramic membrane with mullite whiskers-interlocked structure was prepared by gas-solid reaction. The effects of aluminum fluoride (AlF₃) on the formation and growth of mullite whiskers, and then the permeability and selectivity of the ceramic membranes were investigated. With the increase of AlF₃ contents, the mullite phase evolved from needle-like, rod-like to flake-like structure, thus the catalyst accelerated the growth of mullite whiskers in the diameter direction. For the ceramic membrane sintered at 1400°C, the porosity increased from 58% to 76% while the average pore sizes increased from 0.65 to 3.93 μm because of the whisker-constructed structures. For the ceramic membrane sintered at 1450°C, the emulsion flux increased stably from 295 L/(m²·h) to 992 L/(m²·h) with the increase of trans-membrane pressure, and the oil rejection exceeded 98%. Thus, this study provides a feasible strategy for the preparation of ceramic membranes with high porosity and excellent separation performances.     

Fibrous porous mullite ceramics modified by mullite whiskers for thermal insulation and sound absorption

In order to meet the demand for thermal insulation and sound absorption, fibrous porous mullite ceramics (FPMC) with high porosity and an interconnected pore structure were prepared, followed by a pore structure modification with in situ grown mullite whiskers on the three-dimensional framework of the FPMC. The resultant hierarchical material exhibited superior sound absorption performance in the low-to-medium frequency to most reported sound-absorbing materials, as well as a sufficient compressive strength of 1.26 MPa with low thermal conductivity of 0.117 W·m^?1·K^?1. Moreover, the effects of solid content and mullite whiskers on the microstructure and physical properties of the material were analyzed. The increase of solid content led to increased compressive strength and thermal conductivity and decreased frequency corresponding to the first sound absorption peak. The thermal conductivity and compressive strength of the material increased as the mullite whiskers grew, while the median pore size decreased.     

Recycling of waste fly ash for production of porous mullite ceramic membrane supports with increased porosity

Low-cost porous mullite ceramic membrane supports were fabricated from recycling coal fly ash with addition of natural bauxite. V₂O₅ and AlF₃ were used as additives to cause the growth of mullite crystals with various morphologies via an in situ reaction sintering. Dynamic sintering, microstructure and phase evolution of the membrane supports were characterized in detail and open porosity, pore size, gas permeation and mechanical properties were determined. It showed the membrane support with 3 wt.% V₂O₅ and 4 wt.% AlF₃ addition exhibits an open porosity of ∼50%, mechanical strength of 69.8 ± 7.2 MPa, an interlocking microstructure composed of anisotropically grown mullite whiskers with an aspect ratio of 18.2 ± 3.6 at 1300 °C. Addition of more V₂O₅ lowered the secondary mullitization temperature, resulting in more mullite formation at lower temperatures. The fabricated membrane supports feature high porosity without mechanical strength degradation, possible strengthening mechanism of the mullite whiskers was further discussed.     

Preparation and characterization of mullite whisker reinforced ceramics made from coal fly ash

Ceramics with mullite whiskers were prepared from coal fly ash and Al₂O₃ raw materials, with AlF₃ used as an additive. The phase structures and microstructures of the ceramics were identified via X-ray diffraction and scanning electron microscopy, respectively. The results show that pickling of coal fly ash is an effective method for enhancing the flexural strength of ceramics. Sintering temperature and AlF₃ addition were also key factors influencing the creation of ideal ceramics. The ceramic made from pickled coal fly ash, 6?wt% AlF₃, and sintered at 1200?°C, exhibited the highest flexural strength of 59.1?MPa, and had a bulk density of 1.32?g/cm³ and porosity of 26.8%. The results show that ceramic materials made under these conditions are ideal candidates for manufacturing ceramic proppants for the exploitation of unconventional oil and gas resources.     

In-situ growth of mullite whiskers and their effect on the microstructure and properties of porous mullite ceramics with an open/closed pore structure

Porous mullite ceramics with an open/closed pore structure were prepared by protein foaming method combined with fly ash hollow spheres. Both the open porosity and total porosity of samples were enhanced by increasing the hollow sphere content. Mullite whiskers with a diameter of 0.2–4 μm were grown in-situ in the porous mullite ceramics with an AlF₃ catalyst, conforming to a vapor-solid growth mechanism. The pore structure of the porous mullite ceramics was significantly affected by the mullite whiskers which increased the open porosity and total porosity. Moreover, the median pore size was reduced from 65.05 μm to 36.92 μm after the introduction of mullite whiskers. The flexural strength and the thermal conductivity of the samples decreased with increasing total porosity. The porosity dependence of the thermal conductivity was well described by the universal model, providing a reference for the prediction of thermal conductivity of porous ceramics with open/closed pores.     

Constructing secondary-pore structure by in-situ synthesized mullite whiskers to prepare whiskers aerogels with ultralow thermal conductivity

Ultralow thermal conductivity and ultralight mullite fibers/mullite whiskers composite aerogels (MF/MW) with secondary-pore structure have been prepared via vacuum impregnation and high-temperature treatment. The in-situ generation of mullite whiskers during vapour-solid reaction process and the mechanism of improving thermal stability have been discussed in detail. Under catalysis condition at 1200 °C, the zero-dimensional nanoparticles of SiO₂-Al₂O₃ aerogels are guided to in-situ transform into one-dimensional mullite whiskers. The secondary-pore structure formed by the overlapped fibers and whiskers in MF/MW reduces the thermal conductivity [as low as 0.0488 W/m^?1 K^?1 compared with that of MF preform (0.0698 W/m^?1 K^?1)] and exhibits excellent thermal stability after 1400 °C heat treatment (0.0503 W/m^?1 K^?1) due to the macropores are decreased and gaseous heat transfer being further weakened effectively. Moreover, the MF/MW exhibits good mechanical performance with high critical compressive stress of 0.2809 MPa, which is more than 317% higher than that of MF preform (0.0673 MPa) at room temperature.     

Preparation of self-reinforcement of porous mullite ceramics through in situ synthesis of mullite whisker in flyash body

Self-reinforced porous mullite ceramics were fabricated by a starch consolidation method with flyash, different aluminium sources (Al(OH)₃ and Al₂O₃) and the additive AlF₃ as raw materials. The reinforcement mechanism of needle-like mullite whiskers through in situ synthesis in ceramic body was investigated. The bulk density, apparent porosity and bending strength of the samples were tested. Phase compositions and microstructures of the sintered samples were measured by XRD and SEM, respectively. It showed that AlF₃ as additive was helpful to the formation of mullite whiskers at a low temperature. As the aluminium sources, Al(OH)₃ was more suitable for the preparation of mullite whiskers than Al₂O₃. The in situ synthesized mullite whiskers formed an interlocking structure, which enhanced the mechanical strength of the porous mullite ceramics. Porous mullite ceramics with bending strength of about 100 MPa and apparent porosity of about 55% were made at 1550 °C.     

Microstructure and properties of porous anorthite/mullite whiskers ceramics with high porosity

Porous anorthite/mullite whiskers ceramics with high porosity (>91%) and high strength (>0.45 MPa) have been successfully prepared by foam gel-casting method. Effects of extra mullite whiskers on properties including thermal conductivity and compressive strength at different temperatures were investigated and discussed in terms of microstructure observed through SEM and TEM. The results showed that the addition of extra mullite whiskers in certain content could effectively reduce thermal conductivity, improve the compressive strength both at room and high temperature at same time. When the mullite whiskers content was 20 mol%, the porosity was as high as 91.6 ± 0.19%, the thermal conductivity was low to 0.034 ± 0.003 W/(m·K), and the compressive strength at 1000°C was high to 0.64 ± 0.11 MPa three times to the pure one. Small pores, small grains, and more phase interface or grain boundary caused by the addition of extra mullite whiskers were the main factors for low thermal conductivity. Meanwhile, small pores, closely bonded small grains, and the stable three-dimension network formed by mullite whiskers helped to improve strength.     

Effects of AlF3 and MoO3 on properties of Mullite whisker reinforced porous ceramics fabricated from construction waste

Mullite-whisker-reinforced anorthite-mullite-corundum porous ceramics were prepared from construction waste and Al₂O₃ powder by adding AlF₃ and MoO₃ as the additive and crystallization catalyst, respectively. The effects of AlF₃ and MoO₃ content on the properties of mullite whiskers, such as open porosity, mechanical properties, pore size distribution, microstructure and phase structure, were investigated in detail. The results showed that the morphology of the mullite whiskers and properties of the porous ceramics were greatly influenced by the AlF₃ and MoO₃ content. The specimen obtained by co-adding 12 wt% AlF₃ and 3 wt% MoO₃, and sintering at 1350 °C for 1 h, exhibited excellent properties, including an open porosity of 67.4±0.5% and biaxial flexural strength of 24.0±0.8 MPa. The mullite whiskers were uniformly distributed; the whiskers had a diameter of 0.05–0.5 µm, length of 8–10 µm, and aspect ratios (length to diameter ratio) of 20–30 on average.     

Effects of V2O5 addition on the synthesis of columnar self-reinforced mullite porous ceramics

Spearhead columnar mullite was synthesized by in-situ reaction with V₂O₅ as additive. When the content of V₂O₅ was 7 wt%, the length of the spearhead columnar mullite was the longest with an aspect ratio of about 3.5. Furthermore, columnar self-reinforced mullite porous ceramics were prepared by a foam-gelcasting method, and the effects of V₂O₅ content on the rheological and gelling properties of mullite slurries as well as the microstructure, physical property and thermal insulation property of the prepared mullite porous ceramics were studied. The results showed that the flexural strength and compressive strength of the porous ceramics with 63% porosity prepared by using 2 wt% V₂O₅ additive were respectively as high as 13.9 and 41.3 MPa, and the thermal conductivity was about 1.04 W m^?1 K^?1.     

Novel cordierite-acicular mullite composite for diesel particulate filters

Cordierite-acicular mullite composites containing 0, 25, 50, 75 and 100 wt% of mullite were fabricated from waste MoSi₂ and commercial powders of Al₂O₃ and spinel (MgAl₂O₄). Careful oxidation of pulverised waste MoSi₂ rendered a precursor mixture of MoO₃ and amorphous SiO₂, which served as pore forming agent and SiO₂ source, respectively. Evaporation of MoO₃ at ～750 °C allowed production of highly porous cordierite-mullite ceramic composite after sintering in air at 1350 °C for 4 h. The combination of equiaxed cordierite grains and elongated (prism-like) mullite grains, resulted in unique microstructure with open porosity between 53.3 and 55.6 vol% which makes the obtained composite convenient for application as diesel particulate filter material. The presence of mullite affected four key thermo-mechanical properties which determine the thermal shock resistance of cordierite-mullite composite. The best thermal shock resistance was measured in composite containing 75 wt% of mullite. It was a result of improved thermal conductivity (1.081 W/mK) and bending strength (3.62 MPa) and relatively low values of coefficient of thermal expansion (3.8 × 10^?6 K^?1) and elastic modulus (2.27 GPa).     

Preparation and performance study of cordierite/mullite composite ceramics for solar thermal energy storage

The employment of solar energy in recent years has reached a remarkable edge. It has become even more popular as the cost of fossil fuel continues to rise. Energy storage system improves an adjustability and marketability of solar thermal and allowing it to produce electricity in demand. This study attempted to prepare cordierite/mullite composite ceramics used as solar thermal storage material from calcined bauxite, talcum, soda feldspar, potassium feldspar, quartz, and mullite. The thermal physical performances were evaluated and characterized by XRD, SEM, EPMA, and EDS. It was found that the optimum sintering temperature was 1280°C for preparing, and the corresponding water adsorption was 11.25%, apparent porosity was 23.59%, bulk density was 2.10 mg·cm^?3, bending strength was 88.52 MPa. The residual bending strength of specimen sintered at 1280°C after thermal shock of 30 times decreased to be 57 MPa that was 36% lower than that before. The thermal conductivity of samples sintered at 1280°C was tested to be 2.20 W·(m·K)^?1 (26°C), and after wrapped a PCM (phase change materials) of K₂SO₄, the thermal storage density was 933 kJ·kg^?1 with the temperature difference (ΔT) ranged in 0‐800°C. The prepared cordierite/mullite composite ceramic was proved to be a promising material for solar thermal energy storage.     

High gas permeability and directional channels of mullite porous ceramics prepared by pectin-based freeze-drying method

New gel system for preparing mullite porous ceramics by gel-casting freeze-drying was proposed, using pectin as gel source and alumina and silica as raw materials. Directional channels were formed due to sublimation of water during freeze-drying and decomposition of pectin during high temperature sintering to prepare porous mullite ceramic membranes. Effects of solid content on the properties of mullite ceramics in terms of phase composition, microstructure, apparent porosity, bulk density, pore size distribution, compressive strength, thermal conductivity, pressure drop, and gas permeability were investigated. It was found that prepared porous mullite possessed high apparent porosity (56.04%–75.34%), low bulk density (.77–1.37 g/cm³), uniform pore size distribution, relatively high compressive strength (.61–3.03 MPa), low thermal conductivity (.224–.329 W/(m·K)), high gas permeability coefficient (1.11 × 10⁻¹⁰–4.73 × 10⁻¹¹ m²), and gas permeance (2.18 × 10⁻²–9.32 × 10⁻³ mol⋅m⁻²⋅s⁻¹⋅Pa⁻¹). These properties make prepared lightweight mullite ceramic membranes promising for application in high temperature flue gas filtration. Proposed gel system is expected to provide a new route to prepare porous ceramics with high porosity and directional channels.