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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]

Fibers crystallize and become brittle at high temperatures for a long time, so the surface coating must maintain long-lasting emission performance, which requires superior antioxidant properties of the high-emissivity fillers. To improve the radiation performance of the coating and the tensile strength of the fiber fabric, a double-layer coating with high emissivity was prepared on the surface of flexible aluminum silicate fiber fabric (ASFF) using MoSi2 and SiC as emissive agents. The incorporation of borosilicate glass into the outer coating during high-temperature oxidation of ZrB2 results in superior encapsulation of emitter particles, effectively filling the pores of the coating and significantly reducing the oxidation rate of MoSi2 and SiC. Furthermore, the addition of an intermediate ZrO2 layer enhances the fiber bundle's toughness. The obtained double-coated ASFF exhibits an exceptionally high tensile strength of 57.6 MPa and a high bond strength of 156.2 kPa. After being subjected to a 3 h heating process, the emissivity exhibits a minimal decrease of only 0.032, while still maintaining a high value above 0.9. The thermal insulation composites, consisting of a flexible ASFF matrix and a ZrB2-modified double-layer coating, exhibit significant potential for broad applications in the field of thermal protection. [ABSTRACT FROM AUTHOR]

Ceramic fiber thread is one of the key components in flexible external thermal insulation blankets, and it has been applied in various fields as a flexible ceramic fibrous material with excellent deformability and high-temperature resistance. However, ceramic fiber threads are often subjected to reciprocating friction motion at specific bending angles, making them highly susceptible to abrade and fracture. Enhancing the abrasion resistance performance of ceramic fiber threads under bending conditions is the future trend and remains a significant challenge. Hence, we design and construct a novel polyurethane-modified coating on the ceramic fiber threads to improve their abrasion resistance performance. The effects of the types and concentrations of modifiers on the microstructure, abrasion resistance property, and tensile property of ceramic fiber threads are systematically investigated. The ceramic fiber threads, after modification with hexamethylene diisocyanate waterborne polyurethane (HDI-WPU) with a concentration of 3%, exhibit excellent abrasion resistance properties. The number of friction cycles at fracture of the modified ceramic fiber thread is more than three times, and the tensile strength is more than one and a half times, that of the original ceramic fiber thread, demonstrating the great potential of the HDI-WPU modifier for enhancing the abrasion resistance performance of ceramic fiber threads. [ABSTRACT FROM AUTHOR]

High emissivity coatings with sol as the binder have the advantages of room temperature curing, good thermal shock resistance, and high emissivity; however, only silica sol has been used in the current systems. In this study, aluminum sol was used as the binder for the first time, and MoSi2 and SiC were used as emittance agents to prepare a high emissivity MoSi2-SiC-Al2O3 coating on mullite insulation tiles. The evolution of structure and composition at 1000–1400 °C, the spectral emissivity from 200 nm to 25 μm, and the insulation performance were studied. Compared with the coating with silica sol as a binder, the MoSi2-SiC-Al2O3 coating has better structural uniformity and greater surface roughness and can generate mullite whiskers at lower temperatures. The total emissivity is 0.922 and 0.897, respectively, at the wavelength range of 200–2500 nm and 2.5–25 μm, and the superior emissivity at a low wavelength (<10 μm) is related to a higher surface roughness and reduced feature absorption. The emissivity reduction related to the oxidation of emittance agents at a high temperature (−10.2%) is smaller than that of the silica-sol-bonded coating (−18.6%). The cold surface temperature of the coated substrate is 215 °C lower than the bare substrate, suggesting excellent thermal insulation performance of the coating. [ABSTRACT FROM AUTHOR]

Fibrous porous materials are one of the most commonly used high-temperature insulation materials because of their high porosity and low thermal conductivity. Due to their wide applications in the aerospace and energy industries, the investigation of high-elastic thermally insulating porous materials has attracted increasing attention. In order to improve the elasticity of fibrous porous materials, quartz fibers with high aspect ratio were used as matrix, sodium hexametaphosphate (SHMP) was selected as dispersant. We innovatively reported that a unique three-dimensional skeleton structure was constructed by adjusting the dispersion of fibers in the slurry, and the lightweight, thermal insulating and elastic SiO2 fibrous porous material was then prepared by the compression molding method. The characterization results of zeta potential and absorbance showed that the addition of SHMP was an effective method to enhance the dispersibility of quartz fibers in the slurry. SiO2 fibrous porous materials with 0.4 wt% SHMP content exhibited an ideal three-dimensional skeleton structure, which endowed the porous material with high porosity (89.39%), low density (0.04751 g/cm3), and low thermal conductivity (0.0356 W·m−1·K−1). The three-dimensional skeleton structure formed by overlapping fibers with high aspect ratios endowed the porous material with excellent elasticity. SiO2 fibrous porous materials with 0.4 wt% SHMP content could undergo large strains of 30% and achieved a resilience ratio of 81.69% under the 30th compression cycle. Moreover, after heat treatment at 800 °C, SiO2 fibrous porous materials also maintained good elasticity with a resilience ratio of more than 80%. [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]

Abstract: Near-net-shaped hierarchical structure-adjustable short mullite fibers/mullite whiskers frameworks (MF/MW frameworks) were prepared by slurry-filtration and heat-treating method. The main structure of MF/MW framework was constituted by lap-jointed mullite fibers. Every single fiber in the framework was densely covered by mullite whiskers which formed through fluorine-catalyzed gas-phase reaction, and the fibers actually served as curved substrates for the mullite whiskers' growth. The lap-jointing points of the fibers were served by movable intersected mullite whiskers. Moreover, the microstructure of the frameworks could be adjusted by tailoring the raw materials mass ratio. The volume densities, the apparent porosities and the thermal conductivities of the MF/MW frameworks in different raw materials mass ratios were 0.459–0.487g/cm3, 79.7–82.8% and 0.1356–0.1965W/km, respectively. The compression–resilience property of the samples was tested under 0.4MPa at room temperature. The compression ratio and resilience ratio of the MF/MW frameworks in different raw materials mass ratios were 1.63–2.25% and 92.67–98.16%, respectively. The MF/MW frameworks with advanced thermal and mechanical properties were considered to be promising high-temperature heat-insulation material. [Copyright &y& Elsevier]

High emissivity coatings on hypersonic vehicle surfaces dissipate heat by emitting thermal radiation. While the difficulty of increasing emissivity limits the performance of coatings to improve heat-dissipation of the thermal insulation felt. Herein, a novel high-reflectivity and high-emissivity integrated double-layer coating was developed to enhance the thermal insulation performance of the alumina fiber fabric. The coating comprised of TiO 2 @SiO 2 (T@S) as reflective agent and ZrO 2 /SiC w (ZSC) as emittance agents, was applied on the surface of alumina fiber fabric (AFF). The reflecting effect of radiation heat from the high reflective agent enhanced the thermal insulation properties of the double-layer coating. In the wavelength range of 0.4–2.5 μm, the average emissivity of the ZSC coating was 0.83 and the average reflectivity of the T@S coating was 0.63. With continuous heating at 1400 °C for 10 min with a butane torch, the temperature on the backside of the T@S–ZSC-coated AFF remained relatively stable at 225 °C, which is notably decreased by 80 °C compared to that of the single-layer ZSC-coated AFF. The tensile strength of AFF with T@S–ZSC coating reached 35 MPa after being heat treated at 1200 °C. Moreover, the bonding strength between the coating and the AFF substrate was measured to be 0.15 MPa. The double-layer coating with high emissivity and reflectivity, along with a flexible AFF substrate, offers a promising solution for addressing the challenging of surface heat-dissipation in hypersonic vehicles. [ABSTRACT FROM AUTHOR]

Integration of thermal insulation and electromagnetic wave absorption was crucial in stealth technology and hypersonic vehicles but challenging. Herein, silicon carbide nanofibers were synthesized by in situ chemical vapor deposition within a carbon foam skeleton to form a wave absorbing and heat insulating integrated composite aerogel with a hierarchical porous structure. The unique hierarchical porous structure provided an excellent platform for the absorption of electromagnetic waves, achieving a minimum reflection loss (RL min) of −51.46 dB at 17.20 GHz. Meanwhile, the C/SiC nanofiber aerogel maintained a low density of 0.013 g/cm3, an ultra-low thermal conductivity of 0.030 W/(m⋅K) and a high thermal stability up to 1000 °C. Furthermore, to enhance the electromagnetic wave (EMW) absorption property of the aerogel, polypyrrole (PPy) was coated on the surface of C/SiC nanofiber aerogel by facile impregnation and chemical polymerization. The multiple conductor-semiconductor-conductor structure of C/SiC-PPy composite aerogel leads to various EMW loss mechanisms and excellent impedance match, resulting in an outstanding EMW absorption performance, with a RL min of −60.31 dB at 2.96 GHz. [ABSTRACT FROM AUTHOR]

Mullite fibers have been widely used in thermal protection materials, but they still face challenges related to inefficient wave absorption performance. SiBCN aerogel has the advantage of tunable dielectric properties, outstanding microwave absorption performance, low density and low thermal conductivity, making it an ideal candidate for enhancing mullite fibers. Herein, a novel SiBCN aerogel/mullite fiber composite (SiBCN/MF composite) was prepared by sol-gel, freeze-drying and high temperature thermal treatment for the first time. By tuning the concentration of polyborosilazane (PBSZ), the density, microwave absorption performance, and thermal insulation properties were regulated. The obtained composites displayed low density (0.151–0.190 g/cm3), excellent microwave absorption performance (the minimum reflection loss as low as −67.83 dB and effective absorption bandwidth of 5.40 GHz) and low thermal conductivity (0.056–0.081 W/(m·K)). The outstanding comprehensive performance of the fabricated composites makes them promising candidates for use in the thermal protection system of aerospace vehicles. [ABSTRACT FROM AUTHOR]

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]

Carbon fiber (CF) reinforced SiBCN composites were generally considered as key candidates for thermal protection under severe aerodynamic heating in aerospace due to their excellent high-temperature properties, but they were also faced with challenges in stably monitoring the structural integrity of themselves under extreme conditions. Herein, with the introduction of SiC coating, multifunctional CF-SiC/SiBCN composites were fabricated which integrated functions of the thermal protection and the structural health monitoring. Compared to the untreated CF/SiBCN composites, the stability of sensing of CF-SiC/SiBCN composites was greatly improved and the sensitivity of CF-SiC/SiBCN composites maintained at a high level with a gauge factor of 652.65. Furthermore, additional researches revealed that the CF-SiC/SiBCN composites enjoyed a high compressive strength (155.33 MPa), a light weight (1.07–1.61 g/cm3), and a relatively low thermal conductivity (4.02 W/(m·K)), which showed a potentiality of CF-SiC/SiBCN composites to be applied as multifunctional structural components in thermal protection systems. [ABSTRACT FROM AUTHOR]

To prepare a coating with a high emissivity and low solar absorption ratio on the surface of aluminum silicate fiber cloth, the sol-gel method was used with silica sol as a binder. In this study, the effects of two white fillers, titanium oxide and zirconia, on the substrate and radiation properties of coatings were compared after heat treatment at 800–1200 °C. The ZrO 2 -based coating had good stability after heat treatment at 800–1200 °C, and maintained its white color, exhibiting solar absorption ratio of approximately 0.14 and emissivity of 0.84. ZrO 2 -based coatings prevented harmful cracking, as observed for the TiO 2 -based coatings after heat treatment, because of the ZrO 2 phase transformation. The ZrO 2 -based coating did not have strong bonding with the substrate, and the substrate strength was improved by maintaining a weak bonding interface. Compared with the TiO 2 -based coatings, which exhibit reduced substrate strength above 1000 °C, the ZrO 2 -based coating was more suitable for the surface of flexible fiber cloths as a temperature-resistant coating with good development prospects. [ABSTRACT FROM AUTHOR]

To improve the oxidation inhibition of TaSi 2 -based high-emissivity coatings at high temperatures, TaSi 2 –SiC coating modified by SiB 6 was prepared on the surface of alumina fibre fabrics. The effects of the SiB 6 content on the surface appearance and emissivity of the coating were investigated, and the mechanical properties of the coated fabrics were compared. When the SiB 6 content in the coating was 2.5%, the borosilicate glass liquid phase generated by SiB 6 oxidation effectively prevented the oxidation of TaSi 2. The bond strength between the coatings and fibre fabric was 207 kPa after calcination at 1200 °C, which was 39% higher than that of the coated fabric without SiB 6. The emissivity of the TaSi 2 –SiC coating, modified by a SiB 6 content of 2.5%, reached above 0.92 after calcination at 1200 °C for 5 h. Therefore, the TaSi 2 –SiC high-emissivity coating modified by SiB 6 has good application prospects in the field of thermal protection. [ABSTRACT FROM AUTHOR]

Thermal protection has always been an important issue in the energy, environment and aerospace fields. Porous ceramics produced by the particle-stabilized foaming method have become a competitive material for thermal protection because of their low density and low thermal conductivity. However, the study of porous ceramics for composite systems using particle-stabilized foaming method was relatively rare. Here, silica-alumina composite porous ceramics were prepared by particle-stabilized foaming method, which was achieved by tailoring the surface charges of silica and alumina through adjustment of the pH. Porous ceramics exhibited porosity as high as 97.49% and thermal conductivity (25 °C) as low as 0.063 W m−1 K−1. The compressive strength of porous ceramics sintered at 1500 °C with a solid content of 30 wt% could reach 0.765 MPa. Based on the light weight and excellent thermal insulation properties, the composite porous ceramic could be used as a potential thermal insulation material in the spacecraft industry. [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]

Porous Al 2 TiO 5 ceramics have drawn increasing attention in diverse applications owing to their low thermal expansion coefficient, and good resistance to thermal shock; however, Al 2 TiO 5 ceramics prepared in previous studies still exhibited a relatively high density, which limited their further application in the field of high-temperature thermal insulation. Herein, we reported the successful fabrication of hierarchically porous Al 2 TiO 5 ceramics with a unique three-dimensional skeleton structure formed by overlapped ceramic nanofibers. Results show that the introduction of SiO 2 into Al 2 TiO 5 nanofibers could significantly improve their thermal stability and meanwhile reduce their formation temperature. Al 2 TiO 5 nanofibers with 10 mol% SiO 2 exhibited a superior microstructure and excellent thermal stability, and were selected as the raw material in the subsequent preparation of corresponding porous ceramics. Porous Al 2 TiO 5 ceramics prepared by the freeze-gel casting exhibited a unique hierarchical pore structure with large pores originating from the sublimation of ice crystal and small pores formed by the overlapped of fibers. This unique microstructure endowed porous ceramics with an ultra-high porosity (97.59–95.59%), ultra-low thermal conductivity (0.0456–0.0754 W m−1 K−1) and excellent thermal stability. Owing to these outstanding characteristics, the resulting porous Al 2 TiO 5 ceramics are expected to be employed as the high-temperature thermal insulation material in a wide spectrum of advanced applications. [ABSTRACT FROM AUTHOR]

In recent years, the need for robust thermal protection for reusable spacecraft and vehicles has spurred strong demand for high-performance lightweight thermal insulation materials that exhibit high strength. Herein, we report silica porous ceramics prepared via the direct foaming technique with lightweight, ultra-low thermal conductivity and enhanced compressive strength. Silica particles (particle size: 500 nm and 2 μm) were used as the raw materials. The nano-sized silica particles were easily sintered, thereby improving the compressive strength of the ceramics, whereas the micro-sized silica particles maintained the pore structure integrity without deformation. The addition of nano-silica enhanced the compressive strength by 764% (from 0.039 to 0.337 MPa). In addition, the thermal conductivity of the ceramics was as low as 0.039 W m−1 K−1. Owing to these outstanding characteristics, these porous silica ceramics are expected to be employed as thermal insulation material in diverse fields, especially aerospace and space where weight is an important constraint. [ABSTRACT FROM AUTHOR]

[Display omitted] • SiBCN/SiC nanowire aerogel was prepared by a simple process. • The enhanced electromagnetic absorbing performance is achieved. • Superior thermal insulation property with low thermal conductivity is obtained. • The resulted aerogel is highly promising for defense and high-speed vehicle. Electromagnetic wave (EMW) absorption materials with lightweight and thermal insulation properties are highly sought after in aircraft and military application, because they can simultaneously resist aerodynamic heating and display EMW stealth function. Herein, a polymer-derived SiBCN/SiC nanowire composite aerogel was prepared through in-situ synthesis of SiC nanowires in SiBCN ceramic aerogel by catalyst-assisted annealing. The phase composition, nanoscale architectures, and EMW absorbing property of the as-obtained aerogels could be regulated by tuning annealing temperature. The resulting composite ceramic aerogel annealed at 1400 °C, which is composed of amorphous SiBCN, SiC nanowires, and free carbon, exhibits excellent microwave absorbing performance (minimum reflection loss of −50.1 dB and effective bandwidth of 6.4 GHz) and low thermal conductivity of 0.052 W/mK with low density (0.142 g/cm3). The superior EMW absorption capability is primarily ascribed to the improved impedance matching, multi-reflection, as well as enhanced interfacial polarization caused by SiC nanowires. Owing to the integration of EMW absorption and thermal insulation properties, the SiBCN/SiC nanowire aerogels pave the way for the construction of thermal insulation material for EMW absorption in the application of military and daily life. [ABSTRACT FROM AUTHOR]

• A protective double-layer SiC-ZrO 2 coating was fabricated on the surface of flexible aluminum silicate fiber fabric. • SiC-ZrO 2 coated aluminum silicate fiber fabric possesses high emissivity, tensile strength and interface shear strength. • ZrO 2 transition layer improves the toughness of the double-layer coating based on transformation toughening. • The introduction of the double-layer SiC-ZrO 2 coating greatly improves the thermal insulation performance of fiber fabric. A high emissivity double-layer coating with SiC and ZrO 2 as emittance agents was prepared on the surface of flexible aluminum silicate fiber fabric (ASFF) to improve the thermal insulation performance of the fiber fabric. The tensile strength and interfacial bonding strength of raw ASFF, ASFF with single-layer ZrO 2 coating and ASFF with double-layer SiC-ZrO 2 coating were compared and studied. The ASFF with SiC-ZrO 2 coating still exhibits a high tensile strength of 75 MPa after heat treated at 1100 °C. The shear bond strength of the coating/substrate interface and the coating/coating interface is 0.08 MPa and 0.18 MPa, respectively. The spectral emissivity of the SiC-ZrO 2 coating is as high as 0.93, which is about 1.6 times that of the raw ASFF. The heat-proof and thermal insulation integrated composite consisting of flexible ASFF substrate and double-layer SiC-ZrO 2 high emissivity coating demonstrated potential application as large area of thermal protection material. [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]

Highlights • Mullite-based nanofibrous aerogels were firstly fabricated. • The aerogels show a unique multilevel pore structure. • The aerogels exhibit an ultralow density and ultralow thermal conductivity. • The aerogels show an excellent high-temperature thermal stability. Abstract The fabrication of insulation materials with ultralow thermal conductivity and excellent thermal stability at high temperatures (higher than 1200 °C) has remained an extremely challenging. In this study, we reported the manufacturing of mullite-based nanofibrous aerogels via the gel-casting and freeze drying methods using the electrospun nanofibers with different alumina/silica molar ratios (3:2, 3:1 and 3:0) as the matrix and silica sols as the high temperature binders. The formation process of the mullite-based fibrous aerogel and effect of aerogel composition on the sample physical and mechanical properties were investigated. All mullite-based nanofibrous aerogel show a similar multilevel pore structure. The minor pores were formed by the overlaps of nanofibers and were the fundamental porous structure of the aerogel, while the major pores was caused by the sublimation of the ice crystal. This unique multilevel pore structure make the mullite-based nanofibrous aerogels exhibit an ultralow density (34.64–48.89 mg/cm3) and low thermal conductivity (0.03274–0.04317 Wm−1 K−1) although the sintering temperature was as high as 1400 °C much higher than the service temperature of the traditional nanoparticle aerogel. In addition, besides controlling the fabrication parameters, the physical and mechanical properties could be also tuned by adjusting the composition of the nanofibers. The research of this work provides a new insight into the development of high efficient thermal insulation materials used at high temperatures. [ABSTRACT FROM AUTHOR]

• The electrostatic repulsion and Debye radius of the sol-gel particles were affected by the addition of N-hexane. • Modulation of gelation time and microstructure of silica aerogels by addition N-hexane to solvent. • Thermal conductivity of silica aerogel is as low as 0.012 W/m·K by modulating microstructure. • The variation of thermal conductivity in the experiment is verified by introducing theoretical calculations. In this work, a kind of superhydrophobic silica aerogels was prepared at ambient pressure using tetraethoxysilane (TEOS) as precursor. Particularly, N-hexane was added in the solvent to modulation pore microstructure and effect on the insulation properties of samples were analyzed. It is discovered that when the volume ratio of N-hexane to TEOS is 1:2.24 the thermal conductivity of samples can reach the lowest of 0.012 W/m·K. Compared with that of samples no addition of N-hexane, the density is decreased, the theoretical average pore size increases and the total pore volume increases. Due to N-hexane it leads cross-linking between the sol particles stronger and inhomogeneous. Thus gelation process is speeded up resulting in a change in pore structure. Low density can reduce the solid-phase heat transfer, perfect pore microstructure reduces the contribution of gas-phase heat transfer, and only when the two aspects forms a suitable combination it can exhibit optimal thermal insulation performance. [Display omitted] [ABSTRACT FROM AUTHOR]

In our report, a fibrous ceramics with the mullite fibers as the matrix and SiO 2 –AlPO 4 sol as the high-temperature binder were fabricated by the infiltration method. The effects of sintering temperature on the properties, such as porosity, phase transition, microstructure, compressive strength and fracture mechanism, were investigated. Compared with the fibrous ceramics with the SiO 2 –B 2 O 3 binder, the fibrous ceramics with the SiO 2 –AlPO 4 binder exhibited lower densities (0.560–0.595 g/cm 3 ), lower thermal conductivities (0.157–0.165 W/mK), high strain (7–8%), high rebound-resilience (77–90%) and the same level of compressive strengths (1.1–2.1 MPa). During the cooling stage of the heat treatment process, α→β-cristobalite displacive phase transition in the SiO 2 –AlPO 4 binder was successfully suppressed. The fracture behavior of the ceramics tends to be a layer-by-layer failure, which is quite different from that of the traditional ceramics. The non-brittle fracture mode and the excellent elasticity made this fibrous ceramics promising high-temperature elastic insulation materials. [ABSTRACT FROM AUTHOR]