A nitride whisker template for growth of mullite in SiC reticulated porous ceramics

Silicon carbide reticulated porous ceramics (SiC RPCs) were fabricated by polymer replica technique. The effects of nitride whisker template on the growth of mullite, the strut structure and mechanical properties of SiC RPCs were investigated. Prepolyurethane (PU) open-cell sponge was first coated by SiC slurry consisting of SiC, reactive Al₂O₃, microsilica and Si powder, then it was nitridized at 1400 °C in a flowing N₂ atmosphere to prepare SiC preforms. Subsequently, these preforms were treated by vacuum infiltration of alumina slurry and fired at 1450 °C in air. The results showed that Si₂N₂O whiskers grew on the surface and in the matrix of SiC preforms after nitridation. The diameter of struts in SiC RPCs increased after vacuum infiltration process because alumina slurry was easily adhered by the surface nitride whiskers. In addition, such whiskers inside the strut of SiC preforms acted as the template to promote the growth of column-liked mullite in SiC RPCs. The mechanical properties and thermal shock resistance of SiC RPCs were greatly improved due to the special interfacial characteristics of multi-layered struts as well as better interlocked column-liked mullite in SiC skeleton.     

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.     

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.     

Seed assisted in-situ synthesis of porous anorthite/mullite whisker ceramics by foam-freeze casting

Porous anorthite/mullite whisker ceramics with both high strength and low thermal conductivity have been successfully prepared by combining seed-assisted in situ synthesis and foam-freeze casting techniques. The addition of mullite seed was conducive to a reduction in the sintering shrinkage, pore size, and anorthite grain size. This increased the high aspect ratio of mullite whiskers, which enhanced the strength and diminished the thermal conductivity. Mullite whiskers overlapped to form a stable three-dimensional network structure similar to the bird's nest, which was also beneficial to heighten the mechanical properties of the prepared porous ceramics. Through this method, the prepared materials had a high apparent porosity of 87.7–90.2%, a low bulk density of 0.29–0.36 g/cm³, a high compressive strength of 0.65–3.31 MPa, and low thermal conductivity of 0.067–0.112 W/m·K. The results indicated that the method described here can fabricate porous ceramics with excellent properties for further thermal insulating applications.     

原位合成莫来石晶须增强氧化铝基陶瓷

采用原位反应合成法研制了自生莫来石晶须增强氧化铝基陶瓷材料。研究了莫来石晶须的形成过程、微观形貌、化学成分以及复合材料的显微结构和力学性能。结果表明,添加适量的AlF3可以促进原位反应合成进程和莫来石晶须的生成。莫来石晶须的直径为0．2～1.0μm,长径比为8～30,可以显著提高复合材料的强度、韧性和抗热震性。     

多孔陶瓷材料的制备研究进展

介绍了多孔陶瓷材料的分类、性能和应用,综述了多孔陶瓷的制备方法,并分析了不同制备方法的特点。     

In situ formation of mullite strengthened SiC porous ceramics via gelcasting with high strength and good alkali resistance

Mullite-bonded porous SiC ceramics sintered in air by gelcasting are still challenges due to the high porosity induced severe oxidation of SiC, which results in the formation of large amount of detrimental cristobalite phase. Here in this work, small amounts of Y₂O₃ and CaF₂ were added in SiC and Al(OH)₃ raw materials as sintering additives for the in situ growth of mullite reinforcement. This additive system promoted the reaction between oxidation-derived SiO₂ from SiC and Al₂O₃ decomposed from Al(OH)₃ to mullite phase. Almost no cristobalite phase was detected when sintered at 1450℃/2 h with CaF₂ addition of more than 2.0 wt%. Mullite whisker reinforcement was in situ formed due to the gas reaction mechanism caused by CaF₂ addition. Thus obtained porous SiC ceramics exhibited a flexural strength of 67.6 MPa at porosity of 41.3%, which maintained exceeding 36 MPa after 8 h corrosion in 10 wt% NaOH 80℃ solution, being the best performance up to now. This high performance of porous SiC was attributed to the additive induces proper phase control and in situ formation of whisker-like mullite reinforcement.     

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.     

Fabrication and properties of porous anorthite/mullite ceramics with both high porosity and high strength

Porous anorthite/mullite ceramics with both high porosity and high strength have been successfully fabricated by foam-gelcasting and pressureless sintering technology, using α-Al₂O₃, SiO₂, and CaCO₃ as starting materials and MnO₂ as sintering aids. The porous mullite ceramics prepared in this study had 83.3% porosity and 0.3 W/m·K thermal conductivity, exhibited compressive strength value as high as 6.1 MPa. The samples fabricated with mullite content of 30 mol% possessed 79.4% porosity and 5.9 MPa compressive strength showed thermal conductivity as low as 0.19 W/m·K. With the addition of MnO₂, the properties of the prepared materials varied slightly when mullite content changed in a large scale. The results showed that the addition of MnO₂ promoted the reaction, affected sintering and grain growth, and contributed to high strength and low-thermal conductivity.     

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.     

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.     

The preparation and properties of high-strength porous mullite ceramics by a novel non-toxic gelcasting process

A novel approach to fabricate porous mullite ceramics with homogeneous pore size and high-strength using green non-toxic and cost-effective poly-γ-glutamic acid (γ-PGA) gelling system was reported for the first time. Effect of γ-PGA addition, additive amount and solid loading on rheological behavior of the slurries, and microstructure and properties of samples were investigated systematically. By optimizing the solid loading of mullite samples, we are able to get the sample with small pores (< 200 µm) dominating (93.3% of the total pores), and compressive strength of the sample reaches up to 26.62 MPa. In addition, the mullite ceramics exhibited high porosity of 75.7% with low thermal conductivity of 0.279 W/(m·K) at room temperature. This study not only provides a green and non-toxic gelling system but also offers porous mullite ceramics with low thermal conductivity and excellent mechanical strength as an energy-saving thermal insulation material.     

The fabrication of porous mullite ceramics based on the different sizes of aluminum hydroxide

In this research, for studying the influence of size and heat treatment temperature of initial Al(OH)₃ on the physical properties of porous mullite ceramics, porous mullite ceramics were prepared by in situ reaction sintering of amorphous silica and treated Al(OH)₃. The transition phases χ-Al₂O₃, к-Al₂O₃, and stable phase α-Al₂O₃ can be obtained in turn when the treatment temperature of raw Al(OH)₃ is 500, 1000, and 1500°C, respectively. The coarser the raw Al(OH)₃, the higher the strength of porous mullite ceramics. When the sintering temperature is 1500°C, the bending strengths of PS500-C, PS1000-C, and PS1500-C (PSx-C represents that the specimen was prepared by the coarse grade Al(OH)_3, which was previously treated at x°C) are 40.3 ± 2.1, 54.9 ± 5.2, and 64.8 ± 4.8 MPa, respectively. In addition, although the activated Al₂O₃ can decrease the formation temperature (∼100°C) of porous mullite ceramics, the strength and density of porous mullite ceramics prepared by activated Al₂O₃ will decrease at the same sintering temperature. It is believed that the increase of defects and pores during the phase transformation should be responsible for this phenomenon.     

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.     

Multi-functional mullite fiber-based porous ceramics with a multilevel pore structure assembled by alumina platelets and mullite whiskers

Mullite fiber-based porous ceramics have been widely used in the field of heat insulation. To further broaden their applications in other fields, such as filtration and sound absorption, mullite whiskers and alumina platelets were introduced as the secondary structural materials in mullite fiber-based porous ceramics by a sol-gel combining heat-treating method, and new fiber-based porous ceramics with a unique multilevel pore structure were developed. By adjusting the molar ratios of aluminium tri-sec-butoxide to aluminium fluoride and calcination temperature, these fiber-based porous ceramics not only presented the characteristics of lightweight (maximum density of 0.38 g/cm³) and good heat insulation (minimum thermal conductivity of 0.11 W/mK) comparable to traditional fiber-based porous ceramics, but also showed a superior specific surface area (up to 11.5 g/m²) and excellent sound absorption performance (average sound absorption coefficient as high as 0.728). Owing to these outstanding characteristics, the corresponding porous ceramics are expected to be promising multifunctional materials in diverse fields, especially thermal insulation and sound absorption.     

Preparation and characterization of porous mullite ceramics via foam-gelcasting

Porous mullite ceramics were prepared via foam-gelcasting using industrial grade mullite powder as the main raw materials, Isobam-104 as the dispersing and gelling agent, sodium carboxymethyl cellulose as the foam stabilizing agent, and triethanolamine lauryl sulfate as the foaming agent. The effects of processing parameters such as type and amount of additive, solid loading level and gelling temperature on rheological properties and gelling behaviors of the slurries were investigated. The green samples after drying at 100 °C for 24 h were fired at 1600 °C for 2 h, and the microstructures and properties of the resultant porous ceramic samples were characterized. Based on the results, the effects of foaming agent on the porosity level, pore structure and size and mechanical properties of the as-prepared porous mullite ceramics were examined. Porosity levels and pore sizes of the as-prepared samples increased with increasing the foaming agent content up to 1.0%, above which both porosity levels and pore sizes did not change. The compressive strength and flexural strength of the as-prepared sample with porosity of 76% and average pore size of 313 μm remained as high as 15.3±0.3 MPa and 3.7±0.2 MPa, respectively, and permeability increased exponentially with increasing the porosity.     

木炭添加量对多孔莫来石陶瓷性能的影响

为了改善以木炭为造孔剂的多孔莫来石陶瓷的性能,以烧结莫来石(0.25~0.3 mm)、SiO2微粉、Al2O3微粉、滑石粉、球黏土、膨润土、甲基纤维素、木炭粉(≤0.044 mm)为原料,研究了不同量的木炭粉(外加质量分数分别为0、2%、4%、6%、8%、10%、12%)对多孔莫来石坯体的成型外观、烘干后的常温耐压强度及1 400℃保温3 h热处理后的显气孔率和常温耐压强度的影响,并对不同木炭添加量的多孔莫来石试样进行了显微结构分析。结果表明:外加质量分数≤8%的木炭,制成的多孔莫来石坯体可较好成型;外加质量分数2%~8%木炭的莫来石坯体与不添加木炭的相比,烘干后试样的常温耐压强度明显提高;多孔莫来石热处理后的显气孔率随着木炭添加量的增加而增加,常温耐压强度随之降低。综合考虑多孔莫来石陶瓷各项性能,木炭外加质量分数不宜超过8%。     

粉煤灰细粉添加量对多孔莫来石材料的影响

为了提高粉煤灰的资源化利用,在m(漂珠):m(氢氧化铝):m(V2O5):m(AlF3)=45:55:4:3的混合料中分别添加不同量(每100 g混合料分别添加0、5、10、20、30 g)的粉煤灰细粉,以PVA溶液为结合剂,经干料混匀、泥料搅拌、泥料陈腐、挤制成型、1100℃保温2 h烧成制备莫来石试样,然后检测其致...     

Transient liquid phase diffusion process for porous mullite ceramics with excellent mechanical properties

Porous mullite ceramics were fabricated by the transient liquid phase diffusion process, using quartz and fly-ash floating bead (FABA) particles and corundum fines as starting materials. The effects of sintering temperatures on the evolution of phase composition and microstructure, linear shrinkage, porosity and compressive strength of ceramics were investigated. It is found that a large amount of quartz and FABA particles can be transformed into SiO₂-rich liquid phase during the sintering process, and the liquid phase is transient in the Al₂O₃-SiO₂ system, which can accelerate the mullitization rate and promote the growth of mullite grains. A large number of closed pores in the mullite ceramics are formed due to the transient liquid phase diffusion at elevated temperatures. The porous mullite ceramics with high closed porosity (about 30%) and excellent compressive strength (maximum 105?MPa) have been obtained after fried at 1700?°C.     

多孔陶瓷制备技术的进展

多孔陶瓷是一种含有较多孔隙的无机功能材料,在化工、冶金、运输等领域有着广泛的应用前景.对多孔陶瓷的制备技术及最新研究进展进行了详细的综述,报道了一种在自蔓延高温合成法和发泡法基础上创新的制备方法--自蔓延高温合成喷射法,并对未来研究方向进行了展望,认为今后对多孔陶瓷制备技术的研究将沿着对现有技术进行改进及结合其他种类材料的制备技术进行创新等两个方向开展.