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Water-glass-based pastes show good prospect for high-temperature applications, but they are subject to substantial dry-shrinkage. Silicon and aluminum powders were added to prevent the dry-shrinkage. The influence of them on compressive strength, bonding strength and volume of the pastes was discussed. SEM, XRD, and TG/ DSC were employed to study the influencing mechanisms. The results show that Si and Al effectively prevented the dry-shrinkage; Al improved compressive strength and bonding strength, while Si was bad for bonding strength. When Al dosage was 6%, the pastes presented mild expansion of 1.05%, the compressive strength increased by 8.9%, and the bonding strength by 33.4%. [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]

A temperature resistance buoyancy material was fabricated through a tert-butyl alcohol gelcasting process with borosilicate glass (BG) and hollow glass microspheres (HGMs) as the matrix and filler, respectively. The effects of the mass ratio of HGMs to BG and sintering temperature on the microstructure, thermal conductivity, and mechanical properties of the composite were studied. The results show that HGMs were bonded together by the BG, and the sample sintered at 750 °C exhibited a broad pore size distribution, from several microns to more than one hundred microns. The thermal conductivity experimental values of all the samples were less than that of Hashin-Shtrikman upper bound (HS + ) prediction but agreed well with that predicted from effective medium percolation theory. The relationship between compressive strength and relative density was predicted by the Gibson-Ashby model, with the calibration factor phi below 0.7. Young′s modulus values obtained from the experiment were below that of HS + prediction. The modulus values of the four types of samples sintered at 650 °C agreed well with the Pabst model prediction, while the values of the samples sintered at 700 °C and 750 °C were distributed in a zone between Ashby-Gibson model and HS + prediction. [ABSTRACT FROM AUTHOR]

Heat-resistant syntactic foams with relative high compressive strength, elastic modulus and low density were successfully fabricated through binding hollow glass microsphere by phosphate adhesive. The effects of additive B 4 C on the mechanical behavior of syntactic foams were investigated. The bulk density, compressive strength and elastic modulus of the syntactic foams could be tailored by varying the content of additive and the sintering schedule. The results revealed that the additive B 4 C was useful in regulating the mechanical behavior of the syntactic foams due to the volume compensation from B 4 C's oxidation and the formation of glassy state B 2 O 3 , which promoted the bonding of hollow glass microspheres. The compressive strength, bulk density, and elastic modulus of the syntactic foams were 10.80–20.38 MPa, 0.525–1.198 g/cm 3 and 0.81–1.23 GPa, respectively, when the samples containing 5–15 wt.% B 4 C sintered at 700–900 °C for 2 h. Therefore, the heat-resistant syntactic foams possess the potential application in aerospace, aviation and oil exploitation. [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]

The anisotropy of compression response and energy absorption of fiber enforced porous ceramics by directional freeze-casting have been investigated to infer the failure mechanism. An enhanced porous ceramic structured with fiber-bridging-lamellar walls was synthesized by directionally freezing nano spherical alumina suspension with different fiction of alumina microfibers. Due to the anisotropic microstructure, the mechanical properties of the freeze-casting ceramics are closely related to the orientation of the applied stress. Different orientations of loads correspond to the different deformation mechanisms under the load. The compression experiments under different orientations were carried out, and the failure mechanism was judged by the energy absorption characteristics and compression response. In contrast to traditional freeze-casting ceramics, the scaffolds supported with fiber bridges have higher mechanical properties (up to 15 MPa in compressive strength and density up to 0.76 g/cm3), and their failure behavior tends to plastic fracture, which increases toughness and availability without decreasing porosity and pore size. [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]

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]

The heat resistant buoyancy materials were fabricated by the molding method using the aluminum dihydric phosphate as the matrix and hollow glass microspheres as the filler, respectively. In order to improve the waterproof performance of the buoyancy, two strategies, adding the B 4 C additives and introducing the silica-based coatings, were adopted. The study showed that the samples with the B 4 C content of 10 wt% possessed the optimal comprehensive performance, such as low density (0.540 g/cm3) and low water absorption (29.6%). The effect of the coating types and coating times on the density, water absorption, compressive strength and specific strength of the buoyancy materials were also studied. After 4 times of coating, the density and the compressive strength of the sample increased to 0.62 g/cm3 and 6.0 MPa, respectively; the water absorption of the samples greatly decreased to 10%. Besides, all samples exhibited significant pseudoductility. The buoyancy materials with the above superior properties can be potentially used in the deep see fields. [ABSTRACT FROM AUTHOR]

Abstract Silica sol, one of the most commonly used binders in the fabrication of fibrous porous ceramics, can diffuse as the solvent evaporates during the drying stage, leading to an uneven structure of ceramics. To prevent the diffusion of silica particles and prepare homogeneous mullite fibrous porous ceramics, propylene oxide (PO) was introduced into the fabricating procedures. The effects of solid content of silica sol and the molar ratio of PO /silica phase (PO/SiO 2) on the microstructure and physical properties of mullite fibrous porous ceramics were also investigated. The results demonstrated that PO addition strongly consolidated the mullite fibrous skeleton and helped to obtain a homogenous property, because silica phase had been consolidated in situ around the fibers during the gelation stage. Furthermore, the mullite fibrous ceramics prepared with PO addition showed an even microstructure and a much better compressive strength (1.489 MPa), compared with the ones prepared without PO addition (0.868 MPa). Finally, the solid content of silica sol showed a very strong relevance to the properties of the ceramics: the compressive strength raised from 0.731 MPa to 1.715 MPa with the solid content of silica sols increasing from 9 wt% to 18 wt%; while the molar ratio of PO/SiO 2 had little: the compressive strength was around 1.480 MPa, with the corresponding molar ratios of PO/SiO 2 ranging from 0.43 to 1.72. [ABSTRACT FROM AUTHOR]

Directional ice-templated ceramics have unique lamellar porous channels between ceramic walls which run from bottom to top of the samples. This highly oriented morphology results in the anisotropy in mechanical properties both parallel and perpendicular to freezing direction. In this research, fiber-enhanced ice-templated porous ceramics were fabricated by introducing fibers into alumina slurry for freezing. The trans-lamellar fiber bridges connecting adjacent ceramic walls were formed by adjusting the aspect ratio of fibers and freezing velocity, which greatly enhanced the compressive response in two directions and weakened the anisotropy in mechanical properties of the directional ice-templated ceramics. The results showed that the of fiber bridges increased with the increase of fiber aspect ratio and the longitudinal and transverse mechanical properties increased by 73.5% and 232.6%, respectively. [ABSTRACT FROM AUTHOR]

A temperature resistant buoyant material, namely, fly ash cenospheres-hollow glass microspheres/borosilicate glass (FACs-HGMs/BG) for high temperature application, was prepared through a tert-butyl alcohol-based gelcasting process, with FACs and HGMs as the fillers and BG as the matrix, respectively. The effect of FACs addition on the mechanical and high-temperature performance of the FACs-HGMs/BG composites was studied. The composites containing different amounts of FACs displayed low density (0.44–0.89 g/cm 3 ), low Young's modulus (0.05–1.10 GPa), and relatively high compressive strength (2.78–9.75 MPa). Three stages, namely, elastic, plateau, and fracture stages, could be observed in the stress-strain curves. The compressive strength of the samples at high temperature was lower than the ones at room temperature, and the 10%FACs-HGMs/BG composite tested at 500 °C displayed a compressive strength of 6.03 MPa. The FACs-HGMs/BG composites demonstrating excellent mechanical properties both at room and high temperatures may be used as promising candidates for structural materials or temperature resistant buoyant materials in the near future. [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]

Here a facile method to fabricate closed-cell alumina ceramics by binding hollow alumina spheres with high-temperature binder was introduced. Hollow alumina spheres as the pore-forming agent were bonded together by silica and phosphate binder to form closed-cell structure at the high temperatures. The mechanical properties of as-received closed-cell alumina ceramic can be tailored by the volume fraction of hollow alumina spheres and sintering schedule. Besides, the effects of fusing agent B 2 O 3 and chopped mullite fiber on bulk density and compressive strength of closed-cell alumina ceramics were investigated, respectively. The results indicated that the addition of chopped mullite fiber can effectively enhance the compressive strength of the closed-cell alumina ceramic without significant change in bulk density. The compressive strength of the closed-cell alumina ceramics with 62.5% total porosity was 11.97 MPa when 5 wt% B 2 O 3 and 6 wt% chopped mullite fibers were added in the sample. [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: New fibrous ceramics with polycrystalline mullite fibers as the matrix and silica–boron sols as the high temperature binder, which was inspired by the bird's nest structure in nature, were synthesized. The most important structure characteristic of this fibrous material is that the silica–boron binder only fixed the fibers at the crossing points rather than filled the pores among the fibers. The elastic behavior was investigated, both at room temperature and elevated temperature. Compared to conventional ceramic matrix composites, the samples show a much higher degree of elasticity because of the bending of the fibers. The rebound resilience decreased slowly with the increase of the temperature, but it still remained 86% of that at ambient temperature at 1000°C. The sample exhibits good elasticity performance, relatively high strength (2.25MPa) and high porosity (83%) indicating it is a potential high-temperature seal material. [Copyright &y& Elsevier]

Abstract: Porous Y2SiO5 ceramics with relative high compressive strength (as high as 24.45MPa) and ultra-low thermal conductivity (∼0.08W/mK) were successfully fabricated by a tert-butyl alcohol based gel-casting method. The formation mechanism of the 3D interconnected pores and the properties of the green body are discussed. The porosity, pore size, compressive strength and thermal conductivity could be controlled by varying the initial solid loading and the sintering temperature. When regulating the initial solid loading (from 20 to 50wt%) and sintering temperature (from 1200 to 1500°C), the porosity can be controlled between 47.74% and 73.93%, and the compressive strength and the thermal conductivity of porous Y2SiO5 ceramics varied from 3.34 to 24.45MPa and from 0.08 to 0.55W/mK, respectively. It should be noted that the porous Y2SiO5 ceramics with 30wt% solid loading and sintering at 1400°C had an open porosity of 61.80%, a pore size of 2.24μm, a low room-temperature thermal conductivity of 0.17W/mK and a relatively high compressive strength of 13.91MPa, which make this porous Y2SiO5 ceramics suitable for applications in high-temperature thermal insulators. [Copyright &y& Elsevier]