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Mullite fiber felt is a promising material that may fulfill the demands of advanced flexible external thermal insulation blankets. However, research on the fabrication and performance of mullite fiber felt with high-temperature resistance and thermal stability is still lacking. In this work, mullite fibers were selected as raw materials for the fabrication of mullite fibrous porous materials with a three-dimensional net structure. Said materials' high-temperature resistance and thermal stability were investigated by assessing the effects of various heat treatment temperatures (1100 °C, 1300 °C, and 1500 °C) on the phase composition, microstructure, and performance of their products. When the heat treatment temperature was below 1300 °C, both the phase compositions and microstructures of products exhibited stability. The compressive rebound rate of the product before and after 1100 °C reached 92.9% and 84.5%, respectively. The backside temperature of the as-prepared products was 361.6 °C when tested at 1500 °C for 4000 s. The as-prepared mullite fibrous porous materials demonstrated excellent high-temperature resistance, thermal stability, thermal insulation performance, and compressive rebound capacity, thereby indicating the great potential of the as-prepared mullite fibrous porous materials in the form of mullite fiber felt within advanced flexible external thermal insulation blankets. [ABSTRACT FROM AUTHOR]

Abstract Highly porous mullite fiber-based ceramics with a three-dimensional cross structure were successfully designed and fabricated by filtration method. To address the thermal migration of silica sol, polysiloxane was introduced in the bonding system as high temperature binder to form more stable nods in the skeleton as opposed to silica sol which would migrate to the surface of fibers during the sintering process. Due to the lapped structure of mullite fibers, all products showed high porosity(74.1%∼80.9%), low density(0.554g/cm3∼0.608g/cm3) and relatively low thermal conductivity(<0.14 W/(m*K)). The reaction process of in situ porolysis and fracture mechanisms were illustrated, respectively. Besides, results showed that both sintering temperature and binder content had significant influence on the microstructure and mechanical properties of the products. [ABSTRACT FROM AUTHOR]

Abstract: Silica sol is one of the frequently used binders in high‐temperature resisting fibrous porous ceramics, but in the drying process, it can diffuse with water and influence the uniformity of ceramics. To solve this diffusion problem and fabricate homogeneous fibrous porous ceramics, cationic starch was firstly introduced in mullite fibrous system. The effects of starch content and high‐temperature binder content on microstructure, physical, and mechanical properties were also investigated. The results indicate that starch consolidated mullite fibrous ceramics owned a homogeneous 3D skeleton structure, since the introduced starch can absorb both water and silica particles by gelatinization and ensure the even distribution of binders. Compared with the mullite fibrous ceramics fabricated without starch addition, starch consolidated ceramics owned better microstructure and higher interior compression strength. Furthermore, both the starch and silica sol content had great impact on the microstructure, density, porosity, thermal conductivity, and compressive strength of the fibrous ceramic. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The mullite ceramic/fiber brick system was bonded by two kinds of phosphate adhesives. The specimens were treated from 200 to 1400°C. The mechanical properties were tested at room temperature and at high temperature, and the relevant bonding mechanism was also discussed. The results show that the addition of silicon can greatly improve the adhesive's mechanical properties. The room-temperature shear strength of the component bonded by adhesive with the silicon calcined at 800°C can reach 6.58 MPa. The shear strength of the adhesive with silicon tested at 800°C can reach 0.42 MPa. [ABSTRACT FROM AUTHOR]

Abstract: A novel technique of flyash utilization was presented and mullite ceramics were prepared from flyash and desilication-flyash. The effect of desilication process was analyzed and the result shows that the only difference between flyash and desilication-flyash lies in the silica content, which is caused by the removal of silicate glassy phase during the desilication process. The physical and mechanical properties of flyash sample and desilication-flyash samples were measured. All results indicate that the desilication-flyash is more suitable to be used to prepare the mullite ceramics than flyash. The inner structure of flyash particle was first discovered, which can help us form a new description of the flyash particle and finally benefit the utilization of flyash. Lath-like mullite crystals appeared in high-iron samples and needle-like mullite crystals were observed in low-iron samples. The flexural strength of high-iron sample was higher than that of low-iron sample because of the formation of network structure consisting of many interlocked lath-like mullite crystals in high-iron samples. [Copyright &y& Elsevier]

Mullite fiber-based porous ceramics is one of the most commonly used high-temperature insulation materials. However, how to further reduce the thermal conductivity and improve the strength of such materials has been a challenge in the field of high-temperature insulation. In this paper, mullite micro/nano fiber-based porous ceramic was prepared by introducing mullite nanofibers into mullite microfiber-based porous ceramics and the effect of microfiber/nanofiber weight ratio on the samples properties were characterized and analyzed. Results showed that the compressive strength and high-temperature thermal insulation properties of the porous ceramics increased significantly with an increase in the nanofiber content. For the porous ceramics with similar density (about 0.50 g/cm3), the compressive strength of the sample with a microfibers/nanofibers weight ratio of 1:2 was 1.43 MPa, which was much higher than that of the porous ceramics prepared with only microfibers (0.26 MPa). Meanwhile, the backside thermal equilibrium temperature of the sample with a microfiber/nanofiber weight ratio of 1:2 at 900 °C environment was 355.8 °C, which was much lower than that of the porous ceramic prepared with only microfibers (476.5 °C). In addition, in order to investigate the temperature resistance of mullite micro/nano fiber-based porous ceramics, the sintering temperatures was increased from 1400 °C to 1500 °C. Results shows that the high-temperature thermal insulation performance of mullite micro/nano fiber-based porous ceramics with high mullite nanofiber content decays more seriously. The back-temperature test showed that the thermal equilibrium temperature of the samples with a mullite micron/nanofiber weight ratio of 1:1 in 900 °C environment increased rapidly from 386.3 °C to 454.2 °C when the sintering temperature was increased from 1400 °C to 1500 °C. The preparation of mullite micro/nanofiber-based porous ceramics provides a new strategy to improve the performance of fiber-based porous ceramics. [ABSTRACT FROM AUTHOR]

The mechanical behaviors of single mullite fiber and binder/mullite fiber interface were experimentally investigated. The growth of the mullite grains of the fiber with the increasing calcination temperature greatly affected the tensile and compression strengths of the mullite fibers. As the calcination temperature went up from 1000 °C to 1500 °C, the average tensile strength decreased from 0.954 GPa to 0.191 GPa, and the Weibull modulus decreased from 5.1 to 1.8. The compressive strength was 50–60% of the average tensile strength under the same calcination temperature. The bonding strength of the binder/fiber interface increased with the increase of the calcination temperature. The mechanical properties and fracture mechanism of the mullite fiber-based porous ceramics with different binder contents were comprehensively analyzed based on the mechanical behavior of the single mullite fiber and the binder/fiber interface. [ABSTRACT FROM AUTHOR]

Mullite fibrous ceramics with different types of binders (silica particles, SiO 2 -B 2 O 3 mixture and silicon particles) were successfully fabricated by a TBA-based gel-casting. The influence of the above three types of binder on the microstructure and properties of the ceramics were investigated. The results show the resource of the binders played an important role in tailoring the inner structure of the ceramics, consequently influencing the product properties. All products showed high porosity, low density and low thermal conductivity, attributed to the special 3D fibrous skeleton structure. The products with silica as binder sintered at 1500 °C exhibited relatively low compressive strength because of the high melting point of the silica particles. On the contrary, the products with silicon and SiO 2 -B 2 O 3 as the binder sintered at 1500 °C showed higher compressive strength owing to the low viscosity of the binders during the sintering process. Compared with the products SiO 2 -B 2 O 3 as the binder, the products with silicon as the binder was more suitable for being used in the high-temperature environment (above 1500 °C), since the silicon could transform into the silica phase with high melting point. Besides, with the increase of the silicon content, the compressive strength and the thermal conductivity of the products increased gradually. [ABSTRACT FROM AUTHOR]

How to improve the strength of fibrous porous ceramics dramatically under the premise of no sacrificing its low density and thermal conductivity has remained a challenge in the high-temperature thermal insulation field. In this paper, a new kind of high-strength mullite fiber-based ceramics composed of interlocked porous mullite fibers was prepared by nanoemulsion electrospinning and dry pressing method. Results show that as to the porous ceramics with the same density (∼ 0.8 g/cm3), the three-dimensional skeleton structure composed of porous mullite fibers was much denser than that composed of solid mullite fibers. Therefore, porous mullite fiber-based ceramics exhibited a higher compressive strength (5.53 MPa) than that of solid mullite fiber-based ceramics (3.21 MPa). In addition, porous mullite fiber-based ceramics exhibited a superior high-temperature heat insulation property because the porous structure in fibers could reduce the radiant heat conduction. This work provides new insight into the development of high-temperature thermal insulators. [ABSTRACT FROM AUTHOR]

A novel mullite fibers-based hierarchical heterostructure were fabricated using a sol–gel method. The porous performs were fabricated by mixing the mullite fibers with the aluminum borate xerogel which is derived from the gelation of Al(OC 4 H 9 ) 3 and boric acid. After calcination, the preforms with different mole ratio of Al/B, transformed into the composites with hierarchical structures of the aluminum borate nanowhiskers/mullite fiber and alumina plate/aluminum borate nanowhiskers/mullite fiber. The formed aluminum borate nanowhiskers showed a uniform distribution on the mullite fibers, and the growth process may follow a self-catalysis mechanism. The fabricated hierarchical mullite fiber-based products possessed low density of 0.448 g/cm 3 , low thermal conductivity of 0.132 W/mK, high specific surface area of 230.7 m 2 /g and excellent elastic property with high compression-resilience of 92.7%, and were promising flexible insulant materials for the application in high-temperature heat sealing field. [ABSTRACT FROM AUTHOR]

Abstract: 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 100MPa and apparent porosity of about 55% were made at 1550°C. [Copyright &y& Elsevier]

• Complex-shaped mullite components were DLP printed based on a hydroxysiloxane/Al 2 O 3 preceramic precursor strategy. • The introduction of hydroxysiloxane can improve the homogeneity and rheological properties of mullite slurry. • The introduction of hydroxysiloxane can reduce the sintering temperature of 3D-mullite precursor. • A sintering aid AlF 3 was employed to promote the sintering reaction between the hydroxysiloxane and the Al 2 O 3 filler. By taking advantage of the low sintering temperatures of the preceramic polymers, stereolithography printed mullite components derived from preceramic polymer precursor containing alumina particles can be sintered at low temperatures. However, due to their high specific surface, nano alumina particles are difficult to be dispersed into the photocurable polysiloxane. Herein, to prepare mullite slurry, a photosensitive hydroxysiloxane was employed as the preceramic polymer matrix while γ-Al 2 O 3 nanoparticle was added as the active filler. The introduction of photocurable hydroxysiloxane not only improved the homogeneity and rheological properties of mullite slurry but also shorted the ionic diffusion distance of Si-ion and Al-ion during the sintering process. Therefore, 3D mullite preceramic precursor stereolithography printed from hydroxysiloxane-Al 2 O 3 slurry was endowed with a low sintering temperature around 1400 °C. During the sintering process of preceramic precursor, sintering aid AlF 3 can participate in the reaction and further promote the formulation of mullite grains. [ABSTRACT FROM AUTHOR]

Mullite fibrous porous ceramics is one of the most commonly used high temperature insulation materials. However, how to improve the strength of the mullite fibrous porous ceramics dramatically under the premise of no sacrificing the low sample density has always been a difficult scientific problem. In this study, the strategy of using mullite nanofibers to replace the mullite micron-fibers was proposed to fabricate the mullite nanofibrous porous ceramics by the gel-casting method. Results show that mullite nanofibrous porous ceramics present a much higher compressive strength (0.837 MPa) than that of mullite micron-fibrous porous ceramics (0.515 MPa) even when the density of the mullite nanofibrous porous ceramics (0.202 g/cm3) is only around three quarters of that of the mullite micron-fibrous porous ceramics (0.266 g/cm3). The obtained materials that present the best combination of mechanical and thermal properties can be regarded as potential high-temperature thermal insulators in various thermal protection systems. [ABSTRACT FROM AUTHOR]

Mullite nanofibrous aerogels with low density, low thermal conductivity and high service temperature (as high as 1400 °C) was fabricated by the freeze-casting method, and two different kinds of ice crystal growth inhibitors, glycerol as the cryoprotector and agarose as the gel-former, were adopted respectively to influence the growth of ice crystal, thereby adjusting the sample microstructure (including the pore size and the pore distribution). The addition of glycerol could influence the growth of the ice crystal, leading to the pore structure changing from the cellular pores to the spherical pores and the decrease of the large pore size, consequently resulting in the increase of the sample density and compressive strength. The addition of agarose could inhibit the growth of the ice crystal due to the formation of gel network, resulting in the disappearance of the large pores (cellular pores). The sample with high agarose content exhibited a relatively high compressive strength of 252.41 kPa due to high density and uniform pore structure. The research of this work provides an effective method to regulate the properties of nanofibrous aerogel to meet the demand of different applications. [ABSTRACT FROM AUTHOR]

Abstract Here, we developed silica/mullite fiber composite membranes with double-layer structure by a simple vacuum procedure for the removal of sub-micrometer dust. The support with three-dimensional skeleton structure exhibited high porosity (higher than 90%), low density (lower than 0.25 g/cm3) and high compressive strength (higher than 0.55 MPa) at 1000 °C. By controlling the mass ratio of silica sol to mullite fiber, we can obtain uniform and complete filtering layers with different thicknesses. The composite membranes exhibited high PM filtration efficiency with 99% for 1–10 µm, 97% for 0.5 µm and 90% for 0.3 µm. These samples had high air flow with very low pressure drop (lower than 600 Pa when airflow velocity reached 1 m/s). These results indicated that the silica/mullite fiber composite membranes were very promising for PM pollution control in the field of hot gas filtration. [ABSTRACT FROM AUTHOR]

Fly ash cenosphere/mullite (FACs/M) composites with excellent high-temperature resistance were prepared through a tert-butyl alcohol gelcasting process, using the mullite and FACs as the matrix and the filler, respectively, and the matrix of mullite was derived from phase transformation of the raw material of kaolin. Results show that the FACs/M exhibited low density and high specific compressive strength and the thermal conductivity was distributed over in the range of values predicted from Maxwell-Eucken model 2 and Effective Medium Theory (EMT) equation. Exponent m describing the relationship between compressive strength and porosity was 2.37. Young’s moduli of the composites were lower than that predicted from the Hashin-Shtrikman model. The high-temperature testing results revealed that the FACs/M composite exhibited high compressive strength and the 50% FACs/M composite displayed compressive strength of 14.11 MPa after testing at 800 °C. The FACs/M composites with excellent mechanical properties both at room and high temperatures may possibly be used in the deep sea and other fields in the near future. [ABSTRACT FROM AUTHOR]

Mullite fibrous ceramics with a two-dimensional planar random structure was prepared via a pressure infiltration method. The in-plane and out-of-plane mechanical properties, thermal expansion coefficients and thermal conductivities were compared. MoSi 2 -borosilicate glass coatings were applied on two adjacent surfaces of the as-prepared mullite fibrous ceramics via an in-situ reaction method. The bonding strength and thermal shock resistance of the two coatings were compared. Results show that higher crosslinking degree and more fiber orientating in the planes resulted in the fact that the in-plane compressive and tensile strength of the ceramics were 4.6 times and 2.1 times higher than the out-of-planes values, respectively. The in-plane CTE (Coefficient of Thermal Expansion), thermal conductivity and permeability were also higher than the out-of-plane ones. The interface bonding strength of Coating B (coated perpendicular to the planes) was higher than that of Coating A (coated parallel to the planes), because of the better mechanical locking at the interface and the higher tensile strength of the substrate. Besides, Coating B presented a better thermal shock resistance because the CTE of the substrate parallel to Coating B was closer to the CTE of the coating, and the substrate was more complaint parallel to Coating B so that could absorb some of the thermal stress by deformation. [ABSTRACT FROM AUTHOR]

Porous mullite fiber/silica composite was successfully fabricated by a TBA-based gel-casting, and the influence of mullite fiber content on the composite microstructure and properties were investigated. The results show that the mullite fiber content played an important role in tailoring the microstructure of the composite, consequently influencing the sample properties. The composite fabricated with high fiber content showed a porous fibrous structure with mullite fibers as the fiber skeleton and silica as the binder, and exhibited a low density, high porosity, excellent elasticity and low thermal conductivity. On the contrary, the sample fabricated with low fiber content possessed a high density and high strength due to its dense microstructure which is similar with that of the fiber reinforced ceramic matrix composite. [ABSTRACT FROM AUTHOR]

Mullite fibrous ceramics were successfully prepared by a TBA-based gel-casting with mullite fibers as the main matrix. The effects of the fiber length and the gel-casting solid loading on the composite properties and microstructure were investigated. The 3D structure of the composite was constructed by the randomly arranged mullite fibers with the fixed crossing point, and therefore the fiber length was the most important factor influencing the microstructure of the composition. Further analyses indicate that long fibers were more suitable for the fabrication of high porosity composite. Compared with controlling the fiber length, adjusting the gel-casting solid loading was an easy method of tailoring the properties of the composite. The composite fabricated with the low solid loading and long fibers exhibited a high porosity, a low thermal conductivity, and an excellent elastic property, and can be regarded as a potential high-temperature thermal insulator applied in the industrial or aerospace thermal protection system. [ABSTRACT FROM AUTHOR]

Fibrous ceramics with the mullite fibers as the matrix and silicon resin as the binder was fabricated by a molding method. The effect of sintering temperature, especially 1500 °C, on the properties of the products, such as porosity, microstructure, mechanical behavior and fracture mechanism, were investigated. This fibrous ceramics exhibited low densities (0.425–0.441 g/cm 3 ), high porosities (81.6–82.3%), low thermal conductivities (0.083–0.089 W/mK), relatively high compressive strength (1.21–1.58 MPa), and high rebound-resilience ratios (70–85%), which made it a promising high-temperature insulation materials. When the sintering temperature increased above 1300 °C, the fracture behavior of the products was mainly attributed to the fibers break and the binders fracture. As the sintering temperature increased to 1500 °C, the grains in the mullite fibers grew up, and the strength of fibers decreased, finally leading to the degradation of the sample mechanical performance. What's important, the compressive strength of the products sintered at 1500 °C was 1.36 MPa and the elastic strain could reach 3.2%, which means that the products can still be put to use in the high-temperature fields. [ABSTRACT FROM AUTHOR]

Abstract: Mullite fiber-based hierarchical structures were prepared by mixed slurry filtration and heat-treating method. Mullite fibers׳ framework served as the main structure of the material, and mullite fibers acted as curved substrates for the secondary structures׳ growth. The secondary structures formed on the fibers through fluorine-catalyzed gas phase reaction, and their morphology could be adjusted by tailoring (Al/Si)mol of the raw material powders (mixed AlF3–SiO2 powders). Inter-locked corundum hexagon plates as secondary structures grew on the mullite fibers when (Al/Si)mol of the raw material powders was 3/0. Mullite whiskers became the only secondary structures on the fibers when (Al/Si)mol of the raw material powders was 3/2–3/3. The inserted corundum plates or the intersected mullite whiskers acted as the unfixed lap-jointing points of the mullite fibers in the frameworks. The compression–resilience performance of the frameworks was tested at 1MPa, and the resilience ratio of the frameworks was higher than 90%. [Copyright &y& Elsevier]

Fiber-reinforced aerogel composites are extensively used in thermal insulation and energy-saving fields, while the inherently weak interface combination between fiber and aerogel undermines the effect of fiber reinforcement. To deal with this problem, mullite whiskers were in-situ synthesized on the surface of alumina silicate fiber through a facile and environmentally benign method. The nanoscale mullite whiskers growing on the fiber strengthen the interfacial bonding, thereby improving the mechanical properties of the composite. The alumina silicate fiber/mullite whisker/silica aerogel composites delivered ultrahigh compression stress and modulus of 6.45 MPa and 69.74 MPa at 50% strain, which is 6.14 times and 9.07 times higher than those of bare fiber reinforced silica aerogel. Meanwhile, the composite maintained a high retention rate of maximum stress of 97.8% after being compressed at 30% strain for 200 cycles. Moreover, the composite exhibits an ultralow thermal conductivity of 27.5 mW m−1 K−1 and a low density of 0.26 g cm−3. The strong, resilient, lightweight composite with ultralow thermal conductivity is applicable for heat preservation and energy saving. [Display omitted] • Mullite nanowhiskers were in-situ synthesized on alumina silicate fiber without F catalysts contaminating environment. • The stress (6.45 MPa) and modulus (69.74 MPa) at 50% strain increased dramatically due to interfacial enhancement. • The tough and resilient composite can be compressed at 30% strain for 200 cycles. • The composite also exhibited a low density (0.26 g cm−3) and a low thermal conductivity (27.5 mW m−1 K−1). [ABSTRACT FROM AUTHOR]