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The Applications of silica aerogel are limited due to its brittleness and low strength. As a result, it is essential to strengthen and toughen it. Organic nanofibers are one of the preferred reinforcement materials. In this work, we designed and fabricated flexible and compressible nanostructure-assembled aramid nanofiber/silica composites aerogel (ANF/SiO2 aerogel) to improve the mechanical strength and flexibility of silica aerogel without compromising thermal insulation properties. The aramid nanofiber/silica composite aerogels were prepared by immersing the aramid nanofiber wet gel into the silica sol for a certain period of time followed by freeze drying without solvent replacement. The surface modifier 3-aminopropyltriethoxysilane (APTES) was used as a coupling agent to form chemical linkage between the ANF fiber and silica gel. It was observed that APTES can effectively drive the silica sol to infuse into ANF hydrogel, promoting the assembly of silica gel onto the fiber surface and a uniform distribution in the network of ANF. The compressive resilience, thermal stability, and thermal insulation properties of the composite aerogels were evaluated by inducing the silica aerogel into the ANF network to form a protective layer on the fiber and change the pore structure in the ANF network. [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]

The method of in situ synthesis of mullite whiskers by gas-phase deposition and reaction was applied to improve the compressive strength of the mullite fiber brick. During the preparation process, silica sol, Al(NO 3 ) 3 solution and NH 4 F solution were introduced into the fibrous brick in the form of ions or sol through vacuum impregnation and freeze drying, and the silica sol, Al(NO 3 ) 3 and NH 4 F served as the silica sources, aluminum source and catalyst, respectively. Effects of process parameters (concentration of impregnation solutions, holding time, sintering temperature) on compressive strength and elastic modulus of the fibrous brick during the in situ toughening process were analyzed. SEM and XRD analysis results demonstrated that the mullite whiskers were synthesized on the surface of mullite fibers based on the reaction of AlOF and SiF 4 . What is more, the whiskers on adjacent fibers intersected with each other and formed many unfixed lap-jointing points, resulting in the increase of compressive strength and elastic modulus. Although the density and thermal conductivity of the sample after the generation of mullite whiskers fabricated with the optimum process were 0.406 g/cm 3 and 0.1262 W/(m K), respectively, which were slightly higher than that of the raw fibrous brick (0.375 g/cm 3 density and 0.1069 W/(m K) thermal conductivity, respectively), the corresponding compressive strength and elastic modulus of the sample reinforced with the whiskers increased to 1.45 MPa and 42.03 MPa, respectively, which were much higher than that of the raw fibrous brick (0.39 MPa compressive strength and 6.5 MPa elastic modulus). [ABSTRACT FROM AUTHOR]

Abstract: Porous silica ceramics were successfully prepared from silica spinning solution and introduced silica particles by electrospinning. After sintering, the sample was mainly composed of the introduced silica particles instead of the electrospun ceramic fibers. Therefore, the role of the introduced particles in this work was quite different from that of the introduced fillers in other electrospinning works. The porosity of the samples sintered at 1000°C and 1200°C were 62.4% and 55.6%, respectively. Furthermore, the porosity of the sintered sample can be adjusted by changing the content of the introduced silica particles. This method can be extended to prepare other porous ceramics just by changing the kinds of introduced particles. [Copyright &y& Elsevier]

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]

Abstract: Silica ceramic foams with controllable microstructure were produced by direct foaming method. The silica foams were prepared through three different systems: individual cationic surfactant stabilized system, individual anionic surfactant stabilized system, and the mixed cationic/anionic surfactant stabilized system. The foaming ability and foam stability of the three systems were investigated and the related mechanisms were discussed. The results showed that the mixed system had both long-term stability and good foaming ability compared with the others. In addition, the role of the anionic surfactant (SDS) on microstructure of the ceramic foams was studied. The results showed that the foam structure changed from closed pores to open pores with the increasing concentration of SDS. [Copyright &y& Elsevier]

• A high solid loading polysiloxane (PSO)/SiO 2 slurry had been developed and DLP printed into 3D-PSO/SiO 2 precursor. • More ceramic content can be preserved in the PSO/SiO 2 precursor through a two-step argon-air sintering process. • Si-based composite ceramics can be obtained through the two-step argon-air sintering process. • Bulk 3D-silica ceramics can be DLP fabricated from the PSO/SiO 2 slurry. • The introduction of sintering aid NaF can promote the sintering between the PSO and SiO 2 powders. Nowadays, 3D polymer-derived silicon oxycarbide ceramics (SiOC) can be fabricated by the stereolithography method successfully. However, due to their intrinsically poor ceramic content and the large thermal shrinkage during the pyrolysis, it is difficult for the bulk 3D polysiloxane precursors to be pyrolyzed into dense 3D-ceramics. Herein, the ceramic content of the photocurable polysiloxane precursors was increased by adding a large amount of SiO 2 powders into the low solid content epoxy-acrylic siloxane. After being added with proper dispersant, the viscosity of the high solid loading polysiloxane/SiO 2 slurry can reach a proper level. Bulk 3D-silica ceramics with the wall-thickness around 8 mm can be conveniently fabricated from the polysiloxane/SiO 2 slurry by stereolithography and a two-step sintering process. The addition of a proper sintering aid NaF can promote the 3D-PSO/SiO 2 precursor to be converted into dense and crack-free 3D-silica ceramics with good mechanical proprieties. [ABSTRACT FROM AUTHOR]

The multiscale architecture of the mullite fiber/mullite whisker was fabricated and applied for reinforcing silica aerogel. The mullite whisker could successfully grow on the mullite fiber, with a length of 5–10 μm and a diameter of about 100 nm. The mullite fiber/mullite whisker could be well-embedded in silica aerogel, which hold a mesoporous structure and a high specific surface area of 916.67 m2/g. The as-prepared mullite fiber/mullite whisker/silica aerogel possessed a low thermal conductivity of 0.034 W/(m·K) and a low density of 0.270 g/cm3. Thanks to the interface enhancement and the stress dispersion from the introduction of mullite whisker, the compressive stress of mullite fiber/mullite whisker/silica aerogel composite was 1.32 MPa (30% compression strain), 187% higher than that of the composite without mullite whiskers, meanwhile, a high resilience rate of 88.5% was maintained. The lightweight, heat-insulating, compression resilient composite exhibits huge prospects as the thermal barrier in the heat preservation field, especially in aerospace and aviation insulation. [ABSTRACT FROM AUTHOR]

High emissivity coating plays a critical role in thermal protective system, which can radiate a large amount of aero-convective heat. Silica sol bonded MoSi 2 -SiC-Al 2 O 3 (S-MSA) coating was proved to be promising for mullite fibrous insulation. However, the bonding mechanisms of the coating at elevated temperatures are not clear. In this work, the S-MSA coatings were heat-treated at temperatures from 600 °C to 1500 °C to reveal the bonding mechanisms at elevated temperatures. The S-MSA coatings go through a relatively stable stage (600 °C–1000 °C), a crystallization stage (1100 °C–1200 °C), and a densification stage (1300 °C–1500 °C) at ever increasing temperatures. Results show that both the contact damage resistance and the bonding strength of the calcined coatings exhibit a decrease followed by an increase at elevated calcination temperatures, with the inflection point at 1200 °C, corresponding to the transition temperature of the bonding mechanisms from 600 °C to 1500 °C. [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]

A silica-sol-bonded flexible high emissivity coating, with MoSi 2 and SiC as emittance agents, was prepared on quartz fiber fabrics to improve surface radiant heat loss. The high-temperature behaviors of quartz fiber fabrics and the influence of silica sol dipping and alumina sol dipping on the fabrics were studied prior to coating preparation. The emissivity, tensile strength and bending properties of the coated fiber fabrics as functions of MoSi 2 contents were investigated. Results show that the tensile strength of quartz fiber fabrics decreased sharply as temperature increased because of cristobalite crystallization. Silica sol dipping increased the tensile strength of the fabrics and therefore was selected as the binder of the high emissivity coating. The coating emissivity was higher than 0.85. The coated fiber fabrics had higher tensile strength than raw fiber fabrics and showed high bending deflection and good flexibility, resulted from the abundant microcracks formed during sol drying. The tensile strength decreased after the coated fiber fabrics were calcined at 800 °C, and the coated fabrics with higher MoSi 2 contents had higher tensile strength, higher toughness and better flexibility, benefited from the pesting oxidation of MoSi 2 , which formed an interfacial gap at the fiber–coating boundary, and promoted interfacial debonding and fiber pull out. [ABSTRACT FROM AUTHOR]

A type of buoyancy material with excellent temperature resistance was successfully prepared through a molding method, and the hollow glass microspheres (HGMs) and silicon oxide (SiO 2 ) were used as the filler and the matrix, respectively. The effect of sintering temperature on mechanical performance, microstructure, and high-temperature performance of the HGMs/SiO 2 composite were investigated in the study. The HGMs/SiO 2 composite exhibited low density (0.47–0.53 g/cm 3 ), high porosity (66.7–75.6%), low thermal conductivity (0.18–0.27 W/(m·K)), and relatively high compressive strength (3.8–5.2 MPa) properties. The values of Weibull moduli m used to predict the variation degree of compressive strength during the compression process was in the range of 5–15 and its values for the samples sintered at 850 °C was 14.64. Two stages namely elastic stage and fracture stage were shown in the stress-strain curves and in the elastic stage, the stress increased linearly with increasing of the strain until reaching the maximum stress. In the fracture stage, the sample gradually destroyed with further increasing of the strain until complete failure. The compressive strength of the HGMs/SiO 2 composites tested at high-temperature was larger than that of the one tested at room temperature. The HGMs/SiO 2 composite exhibited low density, low thermal conductivity and excellent temperature resistance properties and can be used as the structural material or buoyancy material in the deep sea field in the near future. [ABSTRACT FROM AUTHOR]

In this study, two kinds of alumina based fibrous ceramics, conventional alumina fibrous ceramics and the designed alumina/silica composite fibrous ceramic, were fabricated with different high temperature binders and the effect of the kind of high temperature binder and the binder content on the sample properties were analyzed. Furthermore, comparative analyses of the microstructure and compression and rebound properties of two kinds of alumina based fibrous ceramics were conducted. Conventional alumina fibrous ceramics were fabricated with the silica sol as the high temperature binder and the introduction of silica sol endow the sample a high compressive strength; however, the introduced silica phase would fill the pores constructed by the alumina fibers, consequently increasing the sample porosity. The alumina/silica composite fibrous ceramics were fabricated with glass fibers as the high temperature binder. The alumina fibers lapped with each other and acted as the main matrix of structure, while the molten silica binders bonded the junction of matrix fibers. Therefore, the high temperature binder would not influence the sample porous structure. The alumina/silica composite fibrous ceramic exhibited a higher compression and rebound ratio than the alumina fibrous ceramic due to its excellent skeletal structure. [ABSTRACT FROM AUTHOR]

Abstract: Effects of fly ash cenospheres on the microstructure and properties of silica-based composites were studied in this paper. It was found that the density of the samples decreased as the content of fly ash cenospheres increased, while the strength increased inversely when the content was lower than 50wt%. An interlaced microstructure of lath-like and needle-like mullite crystals was discovered in the composites, which was a beneficial factor for strengthening the silica matrix. Furthermore, the results indicated that fly ash cenospheres can promote the densification of silica as a sintering aid and restrain the crystallization of cristobalite as an inhibitor. After sintered at 1200°C for 2h, the density and flexural strength of silica-based composites with 50wt% fly ash cenospheres were 0.92g/cm3 and 14.5MPa separately, and the strength was as high as that of the silica ceramics without fly ash cenospheres. [Copyright &y& Elsevier]

Abstract: Porous silica ceramics with relatively high mechanical strength can be successfully fabricated via a novel tert-butyl alcohol-based gel-casting method. The microstructure similar to stone arch bridges and the sintering neck can be easily observed due to the evaporation of tert-butyl alcohol (TBA) during the drying process. The porosity of samples with different solid loading sintered at 1150°C varied within the range of 56.4% and 70.1%, as the bulk density from 0.67 to 1.04g/cm3. The compressive strength of sintered silica ceramics increased from 4.32MPa to 8.82MPa, which was attributed to the 3D-framework structure and the connection of silica particles. By setting the solid loading (15wt.%) and changing the sintering temperature from 1050 to 1200°C, the porosity of the sintered samples varied within the range of 51% and 72.4%, as the bulk density remarkably varied from 0.59 to 1.18g/cm3 and pore sizes from 3.31 to 2.89nm. [Copyright &y& Elsevier]

A new practical phosphate adhesive was prepared based on aluminum and phosphate as matrix and SiO 2 and B 4 C as additives, which can be applied to join oxide and non-oxide ceramics under air and argon atmospheres. After being cured at room temperature, the as-received joints do not need subsequent processing and can keep stable shear strength within the whole heat treatment process. Its bonding effect for mullite under air was better than that for SiC under argon. The shear strength of mullite joints could be up to 14.5 MPa after treatment at 1300 °C, while that of SiC joints was about 9 MPa. [ABSTRACT FROM AUTHOR]